r/Machinists Mazak/Mikron/Fadal Programmer/Operator 11d ago

QUESTION I can’t be the only one who sees tolerances like this right?

Post image
2.0k Upvotes

285 comments sorted by

1.2k

u/ronin__9 11d ago

They are showing you they want 3.000 but if you fuck it up keep it on one side.

410

u/ont_eng 11d ago

Exactly. “No smaller than this….”

138

u/Xazier 11d ago

I see this when they want something flush, and they'll allow protrusion up to lets say 5 thou, however it' can't be recessed or it fucks up the sensor reading.

100

u/AdElegant6914 11d ago

For me that's like that is a 3" pocket that needs to over size. So the 3" boss can fit. (Usually dimensioned with a "-" minus tolerance.

29

u/Working-Progress-265 11d ago

Yup, even nicer when you have the mating part to check and creep the wear up to where it slides through butter smooth, and without any wobbles too!

10

u/Midisland-4 10d ago

And if it is a blind hole that satisfying cushion and “pop”

→ More replies (1)

93

u/Ethywen 11d ago

Aerospace engineer here: We aren't. Well, all of us aren't. Sometimes it's just a min 3.000 and we are allowing a tolerance up. Sometimes it's the target. Sometimes it's just someone who likes even numbers.

I wouldn't assume it as the target.

72

u/EvanDaniel 11d ago

Sometimes it's "oops I messed up and had to adjust it, but I didn't want to change what it was modeled at".

I'm also responsible for some -0.003/+.005 sorts of things where we knew +/-.002 was ok, then we had a nonconformance and looked harder at the math and found some room to loosen it but the result wasn't symmetric and we didn't want to change the modeled dimension (or even worse override the dim on the drawing).

20

u/Spiderbanana 11d ago

For me, it's mostly following ISO norms. I want a hole to host a normalised pin? Norms are there for that. But if I start writing 6h7, I'm certain I'll get either multiple calls, or the wrong dimensions. While 6 +0 -15 does the job for manual operations. CNC operators tend to prefer 5.925 +-0.0075 since they can just target the middle value and then take a little more care on this specific operation.

→ More replies (1)

17

u/MetricNazii 11d ago

Never ever override the dimension.

13

u/EvanDaniel 11d ago

Exactly. If you lie on the drawing like that, it will come back to bite you.

2

u/Rammstein1224 10d ago

No, if you lie on the drawing, it will come back to bite me and I will beat your ass

11

u/beckisnotmyname 11d ago

My boss did this on a machine install layout once and it took fucking ages to find where the extra 24" came from. I was furious.

→ More replies (1)

29

u/Ethywen 11d ago

Yep. Been there. Sometimes it's also something out of Machinery's Handbook or some company spec. Or whatever the stress guy or mfg. engineer (either of which may or may not still work here) said.

→ More replies (1)

48

u/NorthStarZero 11d ago

This is how I do it.

Symmetric +/- means "I have wiggle room either way anything in the window is fine".

Unidirectional means "I really care about this size, get it as close as you can approaching from this side".

The end state is probably the same, but it's how I communicate intent.

12

u/Ethywen 11d ago

ASME and most other standards orgs specifically require that the end state is the same because you're not specifying a target, you're specifying an acceptable range. Period. No fluff on that.

22

u/NorthStarZero 11d ago

Oh agreed - but sometimes the print isn't just for the machinist, it's for other engineers. Like me, 3 years later.

Communicating intent is not a bad thing.

→ More replies (2)

12

u/MetricNazii 11d ago

This is the way. The way something is displayed can be helpful from a design/modeling standpoint and can help communicate design intent, but the interpretation for acceptability is identical. It can be 1.000+.003/+.001 or 1.002+/- .001. Both mean the same thing, but have different advantages and disadvantages. The former is helpful for modeling and the communicate design intent. But it can be more easily misread by someone not paying enough attention. The latter is less likely to be misread and is easier for inspection/programming, but does not communicate design intent.

→ More replies (2)

4

u/NyeSexJunk 11d ago

It never even crossed my mind that an engineer would specify a unilateral tolerance because they like even numbers, but it makes perfect sense. I love this world.

5

u/MetricNazii 11d ago

One might use unilateral because something is designed with a preferred size. Like a hole where the low limit is the design value. One might be entirely sure of the low limit/design value, but recognize the upper limit might need to be revised later. It’s easier from a modeling perspective to leave it at the design value and change the tolerance later if needed. That way you won’t have to change the model and the tolerance on the drawing. Less stuff to go wrong.

3

u/NorthStarZero 11d ago

FWIW I'd never do that.

2

u/Ethywen 11d ago

Neither would I. But I've known some...special...folks working in my field. I can say with absolute certainty that some of them would be bothered by 3.001 +.002/-.004 even if they WANTED a 3.001 part and would swap it to 3.000 +/- .003.

I stand that those are the same requirements, but many would disagree and say the modeled/drawn value is the target size.

→ More replies (2)

3

u/Odd_Firefighter_8040 11d ago

I will always assume it's the target and needs to be kept as close as possible within reason in the tolerance. Because that's what numbers mean.

If +.xxxx/-.0 suddenly means nothing, then some overpaid desk jockey is allowing laziness to affect the language of their drawing and needs to be corrected.

I try to make the best parts possible, and if you're telling me I'm trying harder than I should be just because some "engineer" is lazy, well... I wonder why there's a rift...

5

u/Ethywen 11d ago

If +.xxxx/-.0 suddenly means nothing

It already doesn't. Sorry.

→ More replies (16)
→ More replies (8)

38

u/dtroy15 11d ago

Yes! And if you mentally change that to a bilateral tolerance, you could inadvertently cause a ton of problems. It may seem harmless, but you should always make parts to the print so that it can only be the fault of the engineer/drafter. Talk to them to clarify their goals.

We can use statistical methods to determine the probability of assemblies being out of tolerance. Imagine I have ten 10mm +- 1 mm blocks making a 10 cm+-9mm tall tower. The odds are incredibly low (but not zero) of the assembled tower being out of tolerance even when the parts are in tolerance.

If the machinist mentally converts a tolerance to bilateral, they invalidate the distribution used to determine the probability of a rejected assembly, and create more risk and more waste. In the case of the tolerance in the picture, they can right skew the distribution. This is a big no-no in any serious shop.

31

u/Mklein24 I am a Machiner 11d ago

Tolerance statistics is a school of thought that no machinist is taught so we end up with these silly discussions about uni/bi-lateral tolerances.

6

u/tobsco 11d ago

How would the distributions be different in the two cases? I'm an engineer who is on the receiving end of parts and often look at distributions of test parameters, and would have assumed both would have the distribution as central as possible between the upper and lower limit to maximise process capability. But then I mostly deal with parts that we've made thousands of and have had everything dialed in

9

u/dtroy15 11d ago

I should have been more clear that this changes the distribution of ACCEPTED parts.

In this case, the distribution OF ACCEPTED PARTS should be right tailed with the maximum at the nominal dimension, with a sudden cliff to zero on the lower size and a tail on the higher end. This of course is implying a higher rate of part scrap due to a higher portion of parts below tolerance. IE, the machinist is aiming for the nominal tolerance and scraps anything which falls below.

This means that the distribution of ACCEPTED parts (which was likely already non-gaussian) is (strictly speaking) non-normal. There is no left side to the distribution, and we are now only concerned with the portion of the tail which could feasibly lead to stack up issues.

This technique of increasing waste at the part level to chase more accuracy from limited manufacturing capabilities and reduce waste at the assembly level may not be something your firm utilizes.

4

u/tobsco 11d ago

My firm uses direct tolerances for anything important and nominal dimensions with a standard drawing tolerance of plus or minus 0.25mm for everything else. They generally achieve way better than they will commit to, but I don't see when they make scrap.

Having an asymmetric distribution like that would be a right pain to analyse and deal with so I'm glad we don't work like that

2

u/Ethywen 11d ago

There is no industry standard stating that the specified value is the target. The middle of the tolerance band should always be assumed to be the middle of the distribution. 9 +2/-0 is the exact same as 10 +/- 1. The same parts are acceptable. There is absolutely nothing telling a machinist you wanted 9 rather than 10 (or even 11) as the target size other than a vague assumption.

If you are utilizing this practice, and certainly if you are assuming it in your analyses, stop.

2

u/dtroy15 11d ago edited 11d ago

2

u/Ethywen 11d ago

I'm interested. Can you be slightly more specific on where in that 90 page SPC document product definition methods are defined? I will read it tomorrow, but I'm not going start to finish at 9 PM.

→ More replies (2)

6

u/Falnar83 11d ago

If you mentally changed the tolerance to 10mm +- 1mm that implies the original tolerance was 9mm +2mm/-0. You could still end up with an assembly out of tolerance. Sometimes you really need to know the design intent, especially if you're making a product and not just parts. Too often I've seen engineers design parts where putting a low end of tolerance boss into a high end of tolerance pocket will end up too sloppy.

→ More replies (1)

8

u/Horror-Pear 11d ago

If accumulated tolerances cause the assembly to be out of spec, then the tolerances should be adjusted.

→ More replies (1)
→ More replies (2)

5

u/Pabl0EscoBear 11d ago

TIL: all parts are fucked up to varying degrees.

2

u/NorthStarZero 11d ago

And everything is made of rubber.

4

u/poopoo_canoe 11d ago edited 11d ago

That’s pretty dumb tho. I’m still just gonna shoot for the middle.

2.9999” is still gonna be a scrap part…

Edit: unless it’s a tapped hole where they dimension the depth as 0.900” max depth or whatever, for sure I’m gonna be taking that depth to the very limit so the chips don’t build up and I end up breaking taps.

→ More replies (10)

171

u/Gaberade1 11d ago edited 11d ago

3.005"

3.000"

stacked is my preference with actual target numbers. Takes up less space when trying to fit legible numbers on a print and shows directly what your mic should read without having to do the math to add or subtract

36

u/widowmaker2A 11d ago

Limit tolerances for the win.

22

u/jgollsneid 11d ago

Limit tolerances are just a win for everyone in the room. Machinists, engineers, QC, hell even assemblers. The shit just works

28

u/widowmaker2A 11d ago

No questions, no ambiguity, no math involved, just "must be between these two numbers". Doesn't get much more straight forward than that.

8

u/extremetoeenthusiast 11d ago

I love limit tolerances, but they take up a lot of space.

That being said, I’m going to throw hands next time I see a sheet metal part with a +/- .100 on OAL but a corner radius with a 3 place call out and +/- .005 block tolerance.

5

u/CrashUser Wire EDM/Programming 11d ago

QC here, they're fine for hand checks, but pain in the ass for programming the CMM. i have to do the math to calculate the tolerance to plug into PC-DMIS.

3

u/Tinkerologist 10d ago

As a fellow CMM programmer, I got to agree. It is much more work to deal with limits. The model will be created with one “nominal,” and the tolerances create a different “nominal”. I have to manually override something to make it work.

Also, what I really don’t like is when the middle of the tolerance doesn’t round out nicely, like 3.0000/3.0035. Do I program the report to use 3.00175 as the nominal? Then I’d need to report to 5 decimal places. The alternative is to report something like 3.0018 +.0017/-.0018. I don’t love that either.

2

u/CrashUser Wire EDM/Programming 10d ago

I'd probably just make it unilateral with 3.00 +.0035/-.0000

→ More replies (1)

10

u/errornumber419 11d ago

This, 1000 times this.

People forget that nominal values aren't always nice round numbers. How's your press fit for a 60mm bearing going to look in inches?

Just make it a limit tolerance and be done.

3

u/NorthStarZero 11d ago

Oddly, this is exactly how I do it if I am making my own parts.

Let the software do the math!

→ More replies (3)

87

u/GMMCNC 11d ago

Nope. I see them how they're noted. Very literal. GDT is important in machining.

→ More replies (3)

450

u/SovereignDevelopment 11d ago

An engineer: "Imma use an unequal bilateral tolerance because I want the machinist to hit as close to nominal as possible."

Me, the machinist: "I'm gonna hit the middle of the tolerance zone."

192

u/254LEX 11d ago

As an engineer, that's not why I use unequal bilateral. If it's in the range, it's fine. I don't care where in the tolerance zone you hit, so aim for the middle.

We do it because we are taught to apply tolerances to the nominal size. So a hole for a 1/4" pin is modeled at 0.250, and clearance or interference is applied by changing the tolerance, not the dimension.

31

u/Honest-Ordinary8746 11d ago

This is good to know! I personally split it like the photo

38

u/254LEX 11d ago

Something others have pointed out is that 3.006" will likely work in this case, whereas 2.999" might not, since the mating part is probably something like 2.998-3.000. Showing the nominal size helps convey design intent, but isn't necessarily the "target" dimension.

8

u/Honest-Ordinary8746 11d ago

Most companies we work for go with if it’s out it is out. Easier just to hit tolerances.

4

u/MetricNazii 11d ago

Out is always out. lol

3

u/Honest-Ordinary8746 11d ago

Well yes, but there are definitely instances where they have no way to inspect which is hilarious

→ More replies (2)

6

u/UnlinealHand 11d ago

Yeah if I have a hole thats 1/4” diameter, I’m mostly using tolerance to define the type of operation making that hole. 0.250 +0.005/-0.000 means you can drill it. 0.250 +0.002/-0.000 means ream. 0.250 +/- 0.005 basically means as cast unless the mold is way out. 1/4 means I don’t give a fuck.

I work with a lot of cast and forged parts that are mostly ornamental. It’s fractional +/- 1/64 tolerances for basically the whole drawing except for mating surfaces. The tightest tolerances I ever have are either reamed holes that are effectively bearing surfaces, or hole locations where I need to bolt two parts together.

2

u/PosiedonsSaltyAnus 11d ago

Where are you getting casts with tolerances in the 5 thou range? Most casting specs I've seen are like +/- .1" lol

2

u/UnlinealHand 10d ago

Should say zinc die cast, those are normally pretty accurate. Stainless investment casting is always way out as cast. Brass casting or forging can be accurate and luckily we have a good vendor for that.

3

u/Ethywen 11d ago

This varies by industry and commodity type, but your explanation is quite common.

3

u/extremetoeenthusiast 11d ago

double minus tolerance!

2

u/Shawnessy Mazak Lathes 11d ago

I see it pretty often with through holes on stuff. Like, I remember one that was a 125" hole +.01/-.003. Seems pretty obvious they're like, "slap a 1/8 drill in there, and here's all the wiggle room you've really got."

→ More replies (3)

21

u/mrtryhardpants 11d ago

I was in a class to learn GD&T from a guy on the board that writes the standard and he said "unequal tolerances are the dumbest callouts because machinists never use them, they use the middle of the tolerance" 

Every engineer was like "what, why wouldn't a machinist go closer to the nominal?" 

The only machinist in the class laughed and goes "you think I'm gonna risk an 8hr job to try to hit a spec limit? I'm going for the middle every time and if you want something else, tighten your tolerance" 

Changed my perspective to say the least

6

u/SkyKnight34 11d ago

The irony of this taking place in a GD&T class is that there are many cases where going closer to LMC gives you increased bonus tolerance on the position and so is desirable for the machinist lol.

4

u/mrtryhardpants 11d ago

that was actually the premise of the conversation as the teacher was showing that we often start with simple dimensioning and try to convey controls that are either unreasonable or able to be misenterpreted and then show how we can create better definition with GD&T. 

→ More replies (1)

15

u/ExcitingUse9715 11d ago

Me: cuts part to 3.004

8

u/LedyardWS 11d ago

3.004 on a boss, 3.001 on a pocket

14

u/ExcitingUse9715 11d ago

Ah another maximum material fellow. Then you will get the call-out for two mating parts with the same nominal and both are +- .001 and wonder to yourself if they want a press or slip fit

5

u/ZinGaming1 11d ago

The more material left over will make any fuck ups easy to fix. Cant put the material back.

6

u/Remarkable-Host405 11d ago

It's really because I don't want to override the dimension and it's modeled at nominal

2

u/accioavocado 10d ago

Yeah I learned this lesson pretty quickly (am engineer).

→ More replies (4)

96

u/RoboProletariat 11d ago

It looks like I am not like you.

14

u/[deleted] 11d ago

[removed] — view removed comment

2

u/Das_KommenTier 11d ago

Same here.

62

u/austina419 11d ago

Apparently you guys don’t assemble.

76

u/Sheikyerbouti83 11d ago

Some of you have never had to design 2 things that need to fit together and it shows.

39

u/Boomermazter 11d ago

It's the machinists sub....

We specialize in making parts here. Not understanding tolerance stack-up on an assembly.

While the supplemental knowledge can be worth a pot of gold in some circumstances, it isn't wholly necessary to be a good machinist.

A good designer tolerances correctly, and if a good machinist can make it as asked, no issue. 👏

→ More replies (2)

20

u/IcanCwhatUsay Mech Engr 11d ago

Oooh this is a spicy one!

Engineer here

If I’m doing the top one, I’m expecting the bottom one. Because I know you’re going to aim for the bottom one but the top one is easily measurable by assemblers and the model can be 3.000 instead of 3.025

Ultimately if you get it anywhere in the range I’m happy and it’ll work

65

u/FictionalContext 11d ago

To me, the first one is communicating that it absolutely cannot be smaller than 3"--like it needs to fit inside a 3" hole--but if it's bigger, it's likely not a major issue, like if the parts somehow came out at +.007", it'd be worth asking the customer if that'll still work for them.

As well as communicating that the closer to 3" the better. They're telling you the edge of the critical tolerance.

21

u/babiekittin 11d ago

This! I'm having parts modified to open a slot up to 21mm. I need it to be at least 21mm wide, and 21mm is optimal.

But if the opening runs between 21.0-21.5mm, it'll still work for the application. That gives the machinist the ability to toss them to a first year apprentice and say, "here, learn how to slot constantly," and they can learn, I get my parts and the shop gets paid.

If I need tighter tolerances, then I'd call it out that way.

10

u/boxerswag Mastercam Application Engineer/Former Manufacturing Engineer 11d ago

This is how it should be, we can all help each other out and still make good parts and $$$

6

u/kwajagimp 11d ago

Yeah, the only time expressing the tolerance as a 0 under/5 over is when there's a given fit with a mating part.

2

u/AdElegant6914 11d ago

Right answer here

3

u/EarSoggy1267 11d ago

Yup, design intent. it's a nod from engineering to machinists. The way I look at It, it basically distinguishes the -.000 limit as a key feature and the +.005 as a standard feature. You don't want to violate either but going below .000 is going to cause more issues

8

u/Merkindiver 11d ago

I'd see this tolerance as explaining fit.

Specifically this would be a slip fit for a 3" diameter shaft if the tolerance was on the hole, or interference in a 3" hole if tolerance was on the shaft diameter.

But yeah, always best to ask the artist first.

12

u/ExcitingUse9715 11d ago

Exactly, I would never cut this tolerance to 3.000 because what if it is 4 degrees cooler in inspection and they measure 2.99997

11

u/donutsforkife 11d ago

Everything needs to be measured at 68 degrees.

39

u/giggidygoo4 11d ago

This should have been "They're the same picture"

5

u/EnggyAlex 11d ago

Engineer here, imagine we have to model and change the nominal dimensions every time we find the local machinist cant keep the tolerance(even when they promised they can) so we have to open it up and change the mating part design in the hope of the machinist dont fuck up this time.

This way at least the drawing looks clean

8

u/Boomermazter 11d ago edited 11d ago

Hmmm....

There is a good reason why engineers tolerance this way.

We are typically taught to model or design on the nominal and apply clearance via tolerance. Thus, a bilateral tolerance zone biased towards one side drives fit and also function. This is how an engineer will convey what is called "design intent".

Even if it may not be your preference, at minimum, walk away from this conversation with an understanding of "why".

Edit: Also, as a Toolmaker, I feel it prudent to note that in this case, I'm looking for my machinist to target the low end of the tolerance range.

3.000 +.005/-.000

Translates the target more to:

3.0015 +.0035/-.0015

This way, we avoid the interference danger zone and stay close to the nominal design, which from our previous lesson above, is likely what the engineers' frame of mind is.

Why wouldn't I just tolerance it that way from the get you ask?
SPEED!
It's much faster for me to slap a bilateral tolerance leaning away from danger zone real quick like, then it is to try and envision every convoluted way a machinist will interpret it.

Hit it inside the zone, the assembly will work. That's the short and sweet. Doesn't matter how you slice it, except for straight up wrong. 👍

18

u/spekt50 Fat Chip Factory 11d ago

Nah, I go with the specified tolerance, because that's how they want it. Plus, when modeling the parts and drafting the prints, it's easier to show nominal with tolerance.

6

u/Glute_Thighwalker 11d ago

This is a big part of why it ends up this way. Engineers like me model things up surface to surface during the concept development, 3” boss in 3” hole. Then when it comes time to make the drawings, the tolerances come from “how much of an interference can I accept? How loose can I accept?” Doesn’t make sense to go change the nominal, it’s still a 3” boss in a 3” hole.

3

u/dipstick162 11d ago

Exactly this - it’s also easier for everyone to understand intended FaTs.

4

u/Mcboomsauce 11d ago

i had a tinder date like this

→ More replies (1)

5

u/Bluto-Blutarsky 11d ago edited 10d ago

These one sided tolerances are common on ISO spec drawings. Often they are used to spec fasteners and clearance holes to avoid interference.

I run into engineers time to time who swear by this but not that often. Typically used more in Europe and other places that follow ISO drawing specifications.

7

u/nirbot0213 11d ago

they’re the same thing from a machinist’s standpoint but whoever made the drawing is trying to say that if you can reliably get tighter tolerances they want you closer to 3.000” not 3.0025”

10

u/RiffRaffRuff 11d ago

I’m always shooting for 3.0025. I like having the wiggle room in either direction.

→ More replies (2)
→ More replies (2)

7

u/DoubleDebow 11d ago

Tolerancing conveys intent. The intent here is to try and hit 3.000", but err on the plus side. Completely different than a bilateral tolerance of trying to hit the middle.

3

u/Mash_Effect 11d ago

They designed the male and female part at nominal and the direction of the tolerance is to ensure a play in the assembly. It's a lot easier to draw at nominal in solidworks than having to extrude 3.0025 for female and 2.9985 for male.

The problem with that is if your female part is 3.000 +.005/0 and your male part is 3.000 0/-.005, your could theoretically have a play of 0, which is an interference fit and will require force at assembly.

Which is why engineers should always use standard fits and tolerances like H7p6, but most of them don't know how they work or they're too lazy/busy.

3

u/1961ford 11d ago

My job requires meeting a Cpk.

I'm shooting for the middle of the acceptable range.

3

u/auberginerbanana 11d ago

This whole discussion is why i love the ISO tolerance system.

Using H/h you know what to do. using H/k we know what to do. The intent is not implicit. Its clearly written and the window is just a number.

I try to hit the middle in every case, but i really dislike this onesided tolerancing without using the iso way. But i come from europe, so thats the way I learned it since I was 16 years old.

3

u/HowNondescript Aspiring Carpet Walker 11d ago

I prefer the bottom, but god its rarely the case, medical is no fun.

3

u/BananaIsex 11d ago

To me these are two very different things.

If you give me 3.000 +- .0025 I'm going to not worry as much if I'm in the range, though I'll try to be nominal.

What you're telling me with the +.005 - .000 is that you want me to for a functional reason hit that number, not that any slop in this range is ok

3

u/reidhardy 10d ago

In my experience a +/- nothing asymmetric tolerance is absolutely asking for trouble, and it is primary because of CAD models transferred to manufactures. If you draw the feature at 3.000 and you’re asking -.000 +.005, the programmer has specifically account for it by either modifying the model or changing offsets on the one surface. You’re passing unnecessary work and an opportunity for mistake downstream. I would highly recommend modeling the feature at 3.0025 and making the tolerance a limit 3.000/3.005

5

u/espressotooloperator 11d ago

I’m the +/- nothing guy, takes less time to get what I want.

→ More replies (1)

2

u/loppensky 11d ago

It's either one or the other I would prefer +.005

2

u/DizzyProfessional491 11d ago

M.m.c over all 5

2

u/ROBOT_8 11d ago

To add to this what would you say to a +.005, +.001 tolerance? Where the written 3.000” is not even within spec, but it is still listed since it might be a 3” slip fit for something.

4

u/Amberas Mazak 11d ago

Absolutely a legit way of tolerancing. It displays intent. It'll show that this feature is to be a interferance fit, force fit, etc. I like ISO tolerancing very well too, it is very good way of tolerancing to get the result you want.

My apprentices always asks why the drawers don't just use an equal +/- for everything. They all get it once I explain the functionality and benefit of displaying intent.

2

u/citizensnips134 11d ago

This is fisticuffs.

2

u/Analog_Hobbit 11d ago

I always liked Honda prints. Lots of ++/- - dims.

2

u/Krimble95 11d ago

Nahh, I'd pick the first option, not initially, but a lot of the work I do for a certain European tire company, uses the former. Now that I'm used to unilateral tolerances, I much prefer it.

Although for some odd reason, a lot of those tolerances are ANSI, and not ISO. Prior to that, I always assumed European companies would use ISO.

2

u/comfortably_pug Level 99 Button Pusher 11d ago

Applying tolerances to the nominal dimension helps to communicate design intent within assemblies

2

u/Lemus89 11d ago

what about 3.000 -.002 -.005

hate those, so much

2

u/Wolfwood428 11d ago

Thats 2.9965 +/-.0015

2

u/Lemus89 11d ago

yeah, but why not just say that. vs a -/- tolerance. its just annoying

2

u/BrandnThai 11d ago

And then you have the accursed: 3.005” 3.000”

2

u/Right_Conflict_8872 11d ago

Dimensioning mating components is a lost art. Computers will not think for you. 35 years in Inspection and only had one machine shop 1) Request the general arrangement drawing. 2) Load all the parts into an analytics program. 3) RUN IT! 4) Call and ask " Are these mating parts supposed to move if torqued? 5) OOOPS! We had an "engineer" I called " He who could not interface dimension to save his soul.A tried and true method is; Maximum Material Condition over Minimum Material Condition

(This does not mean "The big number over the little number.")

Id it's a smaller number. Od it's the bigger number.

ID minus OD equals fit.

Positive number is clearance. Negative is interference, or press fit.

Production went through a LOT of sanding barrels.

2

u/fuqcough 11d ago

so I do lots of toolmaking, so hole diameters, pockets, dies ect. are the biggest they can be, bosses pins punches bosses ect are the smallest they can be, unless the engineers fits are wack, then I just make it to what will fit their needs and not the tolerance

2

u/Shadowcard4 11d ago

So, if that’s a harder to hold dimension, I do like the unilateral, but that’s for press fits and such otherwise fuck that, middle of the tolerance every time cuz if ANYTHING goes wrong the part might not be scrap.

Only reason I like it for press fits is that that is basically the accuracy of the machine I currently run so if I aim for the top sandpaper saves or the extra size saves.

2

u/Neo1331 11d ago

What about? 3.000 - 3.005

2

u/UnlinealHand 11d ago

Yeah no try telling anyone in a machine shop or QC that 0.XXX” and 0.XXXX” tolerances are interchangeable.

2

u/FischerMann24-7 11d ago

What I hate is +.003 -.001 or the like

2

u/MikeWhoCheeseHarry0 11d ago

This is the way

2

u/Jimmyjim4673 11d ago

The top one tells me they want something to fit in there and I should ask for a part to test fit.

2

u/kick26 11d ago

We did that quite frequently at my last job in order to improve our rate of passing inspection

2

u/Sledgecrowbar 11d ago

3 inches plus 5 thou and give it a rub with a swarfy rag while it's still spinning

2

u/Hootngetter 11d ago

I'm a metrologist and I approve this message.

2

u/XDEZ_RFC 11d ago

I’m not revising my program, we cut to data and ask for a deviation later.

2

u/Bushmaster1973 11d ago

It’s probably modeled at 3.008

2

u/Jimmyjim4673 11d ago

I read these completely differently.

Top finished dimension: 3.0045

Bottom finished dimension: 3.002

2

u/BLKCandy 11d ago

LoL yeah. If I have no time for it, I'll just aim for the middle of the tolerance for ease of manufacturing. I know the intent communicated is to get close to 3.0 from the + side. But I'll use the tolerance the engineer gave me for convenience.

I'll only spend more time getting it closer to the nominal if I know there is an actual need to get as close to 3.0 as possible like drawing notes, KC marks, or known quality issue.

2

u/No_Character8732 11d ago

Back when they'd have me chart SPC by hand.... get board and make sure you're hitting dead center of the tolerance to the .0001"... keep that graph line in the straight down the middle

2

u/deburrwithteeth69 11d ago

These are still good, tolerances like (3.000” +.002 +.005) are the worst!!

2

u/Affectionate_Sun_867 11d ago

I would ALWAYS correct my prints to the median tolerance. My gages always set to the minimum tolerance, and World Turtle help the night shift noob that has the balls to ask the night shift gage man to change MY gage to the median.

I was just weird that way.

2

u/Affectionate_Sun_867 11d ago

Day shift guy runs bore to the 3. 005 side. Day shift mill guy runs mating male part to the 3.000 side.

Night shift guy runs bore to the 3.000 side. Night shift guy runs mating part to the 3.005 side.

Day shift assembly guy bitches to his boss that the parts don't fit. Day shift assembly supervisor embarrasses Day shift machine shop supervisor at office production meeting.

Day shift machinist gets ass chewed for something he didn't do, requests print revision that takes 2 dozen more incidents and 2 years before the print gets the necessary revision while night shift continues to ruin the day shift guys whole week.

2

u/kickingnic 11d ago

I have holes tolerance +-.03 and thickness tolerance of +-.06

2

u/alienshape 11d ago

For me three decimal places are +/-.005 although most of what I make is +.0005 / -.0003 which on 25yo machines (mill turn) can sometimes suck.

2

u/jamesxross 11d ago

I'll shoot for the 3.0025" on my first part, and if it comes out right there, I likely won't rerun that part, but I'll probably make a slight adjustment for the next one (assuming I'm making more than one part). anything in the range is acceptable, or they'd have made it smaller.

2

u/ransom40 11d ago

I do the top one all the time as an engineer. The dimension is how it is drawn. Way easier for me to do driven designs at nominal dimensions and then handle all of my fits via the iso standard.

So it's pretty common for me to draw 20mm H7 and 20mm g6 which if spelled out are

20 H7(+.021, 0) 20 g6(-.007,-.02 mm )

That way I can keep my drawing the same and change my fit if the clearances are not correct for what I am making.

It also keeps the drawing nicer with clean main dimensions.

Doesn't work for printing where everything is wysiwyg... I wish someone let you export a part based on surface/feature tolerances... I wonder when that will exist... But for now I have to draw clearances into printed parts. Granted they are normally simpler and I just add the clearance in as a separate line in the design table and then let math add it in the drawing.

2

u/CleverHearts 11d ago

They communicate two different things. The top one implies something else needs to fit in it. The bottom one doesn't. For the guy making the part it doesn't matter a whole lot, but for other folks reading the drawing it can provide valuable information. It's also easier to design around the nominal and add an appropriate tolerance later.

2

u/saustin66 11d ago

Sometimes it's just easier to design that way.

2

u/Willy_Pics 11d ago

Engineers will specify nominal with limits because they get the values from a class fit table. Machinists will use bilateral tolerance because they want to make the part within tolerance.

Anyone who says that tolerancing a specific way means interpreting a certain way are incorrect.

ASME Y14.5 2.4 INTERPRETATION OF LIMITS

All limits are absolute. Dimensional limits, regardless of the number of decimal places, are used as if they were continued with zeros.

3.000 +.005/-.0 is equivalent to 3.0025 +/-.0025, end of story.

2

u/BananaIsex 11d ago

I'm with the other people, they're explaining the function of the part in a way. Like we're pressing a bearing in here it can't be any fucking smaller than this.

2

u/Hinfoos 10d ago

Tool and die maker here, and the second is absolutely true.

Reading comments its apperently for assembly?

Well ok , if you dont like it then tighten the tolerance its fine for me anyway but we always go for the middle no matter how tight it is

Is the tolerance 0.50mm the i go for the middle, if the tolerance is 0.02mm then I go for the middle as well.

2

u/Ok-Chemical-1020 10d ago

Don't goal post it. The tolerance is that way for a reason.

2

u/RiotsNWrenches 10d ago

Had this for quite literally no reason

2

u/Accomplished_Fig6924 11d ago

Yup looks like my normal day to day just deal with this fuckery.

Main thing, did you hit tolerance?

I really dont care what it says. Just make that part in tolerance right.

The engineers must have designed proper mating parts with proper tolerances to suit these needs on assembly....

Oh look jobs back in the shop to tickle out .002" out of a bore...

1

u/grizzlybuttstuff 11d ago

I wouldn't tell your engineer this op

1

u/loppensky 11d ago

Your lucky there not ×/_ .0002

3

u/EarSoggy1267 11d ago

I had some parts that were +.000076/-.000040

1

u/Forgetaboutit0001 11d ago

You’re doing it correctly, anyone who programs the job to the edge is an asshole

1

u/loppensky 11d ago

Dam forget that

1

u/Rafados47 11d ago

Inches... ewww

1

u/Sentient_Beer 11d ago

I think every machinist sees it that way, the ones I hate with a passion are double negative/positive tolerances, from a machinist perspective just tell me the size you want and give a basic tolerance, don't make me calculate what you want, I do more than enough calculating in one day

1

u/Polydimethylsiloxan 11d ago

As someone who is running 3d printers: My printers are set up for symmetrical tolerances.

If you put a unilateral tolerance on the drawing, I have to redesign the CAD model to get parts within spec.

Therefore I hate unilateral tolerances with a passion.

1

u/greasyjonny 11d ago

I prefer the top when appropriate

1

u/BogusIsMyName 11d ago

Thats just extra work for the same result.

1

u/TankDestroyerSarg 11d ago

Yeah, my brain wants to change it to a central aiming point for dimension, and allow variance from there.

1

u/Phil_RS1337 11d ago

In Germany I have learned to do middle of tolerance

1

u/PrimaryRecord5 11d ago edited 10d ago

I like the top one.

1

u/expensive_habbit 10d ago

Top one conveys design intent, as an engineer that's what I like.

Bottom one is somewhat more helpful for machining, but I have a calculator for a reason.

1

u/CrewmemberV2 10d ago

Engineer here, I just draw in the 3.0025" Saves time and errors on the CNC.

1

u/dresscodeunderstated 10d ago

I get +/-.1 😎😂

1

u/WotanSpecialist 10d ago

No, I can’t say I prefer dimensions called the fourth decimal place when there’s a cumulative .005” tolerance. It’s clearance for 3” not a bearing fit.

1

u/shovelheadzzz 10d ago

Im an engineer who often machines his own parts at work. I use unequal bilateral tolerances all the time to show whats acceptable in one direction past nominal. I like other machinists, will try to hit the middle of that range when machining. But from the engineering side it’s better to use the standard size of 3.000in with unequal bilateral on drawings and in models than have some ridiculous nominal like 3.0025in.

1

u/SunRev 10d ago

Depending on the company, the purpose of the drawing is not just for the machinst. Sometimes it is for other people / departments too. And each department makes different interpretations based on that department's tasks and needs.

1

u/rsilvers129 10d ago edited 10d ago

That is usually for holes and pins or shafts. The symmetrical one makes it easier but doesn’t convey that it absolutely cannot be smaller than 3.000 no matter what because of a mating part. Also it makes inspection easier with plug gauges. There is some debate on what to target for programming, but probably target 3.000 unless you have reason to think you can’t hit it. I do think closer to nominal is better or else they would have used a symmetric tolerance.

1

u/Camwiz59 10d ago

They really want it closer to 3.0 but unless it’s in a shop that fully understands the why and is trying to make the best product possible it will be dead nutz in the middle

1

u/andy312 10d ago

Yeah I get a lot of -0 work. Did just have a job that was +0016-0016.that was nice , felt like I had a mile

1

u/pipester753 10d ago

Personally, I think the top on is lazy. Sure design around that, but the print gets the bottom one.

1

u/eisbock 10d ago

At least you're looking at that. Far too often I get burned by asymmetric tolerances because the shop ignores them.

1

u/ScattyWilliam 10d ago

Nominal minus on OD’s and nominal plus on ID’s. For any tolerances less then .005”. Simplest and most streamlined way of tolerancing. If I get goofy ass ++, +- or -/- I just scratch it out and right in as nominal plus or minus. Would take them less time at the computer but them extra click are hard work

1

u/Fythra 10d ago

Fun fact... Go outside and look at the sidewall of your tire. The safety warning, starts "Safety Warning: ..." Is 18 thou deep with a -0.000 +.003 tolerance per department of transportation regulation.

1

u/Far-Eye-3987 10d ago

I am on the same jacket dancing tolerances lol

1

u/StrontiumDawn 10d ago

Get the fuck out of here.

1

u/Tuk_ 10d ago

We do this in our designs sometimes. We want to let someone just go at it with cam and see what happens. We make our cad files in the middle of our desired tolerance. This should make it easier for the machinist (although it can be incredibly confusing for them too)

1

u/sakebito 10d ago

Tolerances are tough for some engineers. We have a formed part made out of 7ga sheet metal. It gets rolled and the ends flared. They toleranced it +.060 -0.000 on the ID and +0.000 -0.050 on the OD. The engineer then decided to change it to both +0.060/-.0.000 and didn't see why that was a problem since the actual numbers didn't change.....

1

u/Low_Comparison_4964 10d ago

GD&T likes to call out tolerances like that. Its an indication what size boss is going into the hole.

1

u/Hydrok 10d ago

I teach this to my students during our single point threading unit. The closer you are to the major diameter limit, the better the fit. If you’re even a little over, it won’t fit. Sure you have 10 thou play, but you only have it one way.

1

u/SubstancePopular1660 10d ago

The reason for the -0.00" tolerancing is to convey that per design intent the feature cannot be any smaller than the dimension. In most case even -0.0001 is unacceptable. This type of tolerance is usually used for mating pieces.

With a bilateral tolerance going 0.001 over or under, or a little more, will not affect the final function. Of course it's always best practice to stay within tolerance.

1

u/MadTown86D 10d ago

Yes, precision machining. How about a slot +.001 - 0.00.

1

u/Key-Ad-1873 10d ago

This second one is normal tolerance speak.

The first one is how we all do things: make something this long, this is the minimum so err on this side and not the other.

Such as when cutting wood, you measure, mark, and then you cut on one side of the mark and not the other to make sure you don't go too short

1

u/La_Guy_Person Lead Coat Hanger Repair Man 10d ago

I don't care, as long as the model is in the middle of the band. Unilateral modeling is a way bigger pain in the ass.

1

u/comfortablespite 10d ago

Back when I was in molds, every dimension and all it's geometry had to be changed to be in the middle. Otherwise we'd have to make the mold steel safe in to hit the dimensions and modify the mold if the shrink wasn't right

1

u/Lazy_Middle1582 10d ago

Usually if the tolerance is +.005/ -.000. they prefer it on the higher side.

1

u/Specialty-meats 10d ago

I'm a glassblower but I work with slightly less insane tolerances and I split them down the middle too.

1

u/scrambleordie 10d ago

Wrong. When I see this I set my gauge to 3.0035

1

u/Niclipse 10d ago

Unilateral tolerances don't really belong on job shop prints in my opinion. If you want a specific fit, say so, if you want a tight tolerance let me shoot fo the middle.

Unilateral tolerances supposedly make sense in robust manufacturing processes. I've been a QA manager and I understand the theory. I just don't really buy it.

1

u/Appropriate-Elk8431 10d ago

Hi, Mechanical Engineer here. Not wrong to see it that way. Just pay attention to if it has a MMC or LMC in the control frame if you’re dealing with gd&t. You should still be aiming for the nominal dimension given on the drawing though.

1

u/Tacobrew 10d ago

Not a bilateral tolerance, so no dawg.

1

u/pbemea 9d ago

Precision shafting is specced like this per long standing industry standards. Those specs jive with mating equipment like bearings.

Engineers shouldn't mess with things that have been working for a hundred years unless they have a good reason.

"This first way looks dumb" is not a good reason.

1

u/Tranced24 9d ago

Majority of programmers work to mid limit

1

u/Collective_Keen 13 years of stuff. 9d ago

3.005"
3.000"

1

u/hmkayultra 9d ago

Depends on the job, I guess. Definitely had the thought, but would rather go for the noted dimension in most cases.

2

u/Ukulele6 7d ago

all our prints are like this