positional tolerance

[UGMENTALCASE]

Good Evening,

I'm looking at a drawing of two parts. One is a component with a diameter flange with a series of holes in with a positional tolerance of 0.1 on them. Our plate which the component sits on has the same pattern of holes in and we locate with pins through the holes.

Now I've been given this job and I think I've confused myself. The guy has put a positional tolerance on our holes of 0.025. Now is it just me or does that have a potential to not fit? Should our holes not be the same as those in the component, I.E positional to 0.1. Then obviously the location pins down size to suit the component holes tolerance.

Thanks in advance

 

[greenimi]

As long as the virtual boundaries are the same, the parts will fit.

 

[Belanger]

It depends on the size of the holes in the two parts. What you're asking about is a classic case of the floating fastener formula (or maybe fixed fastener; it isn't clear from the description if the pins get pressed/threaded into the holes).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[UGMENTALCASE]

Hi, thanks for the reply. Our plate has a number of holes which pins are pressed into. The component is then lowered over the pins. Pins have a lead on them to guide component down, then the component locates on the pins when it comes to rest. Holes are approximately 12mm diameter. We've been told that each pin is to be a full diameter. Normally we would locate on a central diameter and time with a diamond pin, but the customer doesn't want that so the pin diameter is to be clearance on the component hole size

 

[Woosang]

As JP said, this is a typical fixed fastener case. If you have a copy of ASME Y14.5-1994 or 2009, have a look at FIXED FASTENER FORMULA in the appendix. Then you would find what you want, relationship between the holes size, pin size, positional tolerances of holes in both parts, etc. If you don't have the standard, you can google with "FIXED FASTENER FORMULA"

 

[greenimi]

Are you dimensioning the plate before or after the pins are pressed in?
If the former the projected tolerance zone should be used.

 

[UGMENTALCASE]

Yes it's before the pins are pressed in. Thanks for the responses, I have found said formula etc. and worked things out. However I've been told the positional tolerance of the components' holes and our holes are not to be the same....

 

[greenimi]

Re:"However I've been told the positional tolerance of the components' holes and our holes are not to be the same...."

I would agree with that,but still I my opinion the virtual boundaries (VC) should be the same for the upper plate to fit.
Am I right? Are the VC's the same?

 

[UGMENTALCASE]

Sorry I think I'm getting lost here, but I don't think the VC's are the same no Ok, in the examples I've worked with in the past, and looked at recently both positional tolerances should be the same. I've attached a sketch of what is happening in this instance. For me I can forget the pin being pressed in it's more about the holes lining up. I've put on the document the hole sizes being used and positional tolerance, the 11mm is max hole/pin size.

I think I'm getting confused with previous work done, and examples I've looked at recently, especially that formula that basically states one must equal the other, so I've gone over previous tools and mine follow this formula. However this one isn't. I've just gone over it again and I'm getting errors.

 

[drawoh]

UGMENTALCASE,

Are you working in inches or millimeters? 0.025mm is a very accurate positional tolerance. Your call-out may be good. Your math may be good. If the shop cannot fabricate it, this all is a waste of time. This is why you specify diamond pins, or slots.

--
JHG

 

[JNieman]

We sometimes use multiple round pins in plates, and machine them before assembly, and they work out fine. It's very, very difficult to make it simply bolt up nice on multiple round pins, but not impossible.

However, have you considered a change in process? This is one of those times that it may be an exceptional circumstance that the standards just don't adequately cover. If you specify that the pin holes are to be match-drilled/reamed together, you eliminate a lot of hair pulling. Then you just have to chase the clearance hole with a slightly larger reamer to get it to size. Or make a spring cut with the interpolated end mill, or whatever suits the manufacturer's preference.

It's not uncommon practice in tooling and die work, but it obviously does not result in completely interchangeable components.

 

[greenimi]

If you do the math correctly (as per the standard) and the VC’s are the same (VC=material free zone) then you will get interchangeable parts with no need for .025mm = .0009 (inch) tolerance zone.