Tolerance ranges when calibrating Fixtures

[TCeckels]

Another question on fixtures....I know someone out there besides me has to be accountable for annual fixture calibrations......

For example: I'm taking vector hits on the fixture block that the part sits on to validate that the form of the block is correct. On the print form contours are 2.00 to A,B,C. or + / -1.00 in each direction. However that is the tolerance zone for the part itself.
I was told years ago that for the fixture a part tolerance of + / - 1.00 would transfer to + / - 0.100 or 10% of the part tolerance. My question is would you agree with this or do you use another method for calibrating fixtures with a part print???
In the case I have right now, most the vector points are outside a + / - 0.100Tolerance. I have a hard time believing that all these points are out of tolerance simply cuz if they'd been that far out (.2 to .4) the fixuture would never been put in place at the time of the PPAP.
Because I'm blaming myself at the moment for these out of tolerance points I've aligned this fixture several different ways ( ABC from the tooling balls, An interative alignment off the tooling balls and an interative alignment on the fixture features and in all 3 cases the end results is basically the same thing!!
So, I dunno, maybe I am right and the fixture is screwed....wouldn't be the first time but still back to the original question...what tolerance do you use when calibrating fixtures??? is 1/10 of part tolerances correct???

Help if you can...I'm on my own here....

Tc

[DeWain Hodge]

We normally use 1/2 of 10% of the print tolerance (5%) when checking check fixtures.

[Matthew D. Hoedeman]

FoMoCo standard USED TO BE (I don't know if it still is or not) 10% of part tolerance to a MAXIMUM of +/-0.050 for the fixture, SO, the max fixture tolerance would be +/-0.05mm (or +/-0.002"). If the part had a tighter tolerance than +/-0.50, then the fixture tolerance would be tighter.

[Obelix]

Print from gage manufacturer should have tolerance, to what they built gage.
If you don’t have print 10% is good rule.

[pniven]

I use the 10% rule myself. If some areas do not meet this, I check with the engineer and get there approval on it. Then place it within the report noted. Plus I let anyone dealing with the fixture checking parts be aware of it. I have never had a fixture check the same as it was when first certified by someone else. Sometimes better but most of the times worst, but still within spec.

[TCeckels]

FoMoCo standard USED TO BE (I don't know if it still is or not) 10% of part tolerance to a MAXIMUM of +/-0.050 for the fixture, SO, the max fixture tolerance would be +/-0.05mm (or +/-0.002"). If the part had a tighter tolerance than +/-0.50, then the fixture tolerance would be tighter.

So, just to be perfectly clear......if my form contours are 2.00 to ABC and 10%of that would be 0.200 or 4 times the latitude that you say is the maximum???

Geez I can't even get half my points within tolerance at 0.20, 0.050 would put almost 100% of the points out of tolerance.

I have a hard time believing that the fixture is that far out but I did a simple ABC alignment to the tooling balls with the CAD file and a second alignment using the tooling balls for an iterative alignment and both reports are different only by a couple microns......

this kinda **** drives me insane but I guess it is what it is until someone else comes along and proves me wrong.......I don't know what else to do to prove I'm right.

Thanks for the feedback.

TC

[TCeckels]

Print from gage manufacturer should have tolerance, to what they built gage.
If you don’t have print 10% is good rule.

Print from gage manufacturer?? I don't believe I've ever seen one of those....I don't even think they exist around here. I tried to call the builder of this fixture the other day only to find out they went outa business back in '05. Maybe fixtures like this one are why they're gone??

Tc

[Obelix]

Manufacturer should provide print and instructions how to use gage.
Check your PPAP folder.
I have seen attribute gages that are .006”-.007” out of tolerance.

[TCeckels]

Manufacturer should provide print and instructions how to use gage.
Check your PPAP folder.
I have seen attribute gages that are .006”-.007” out of tolerance.

The PPAP folder is one of the first things I grab when I start off down this road....I've finished this gage now and it is indeed at least .008" out, up .012"!!

this is a U Shaped part and The block itself is .548" too thin !!! No doubt it's been like this since the beginning....someone before me obviously did some creative number crunching to get this one by.......geez.

Tc

[gsepan]

10% rule is what we use if needed. Chrysler standards are +/-0.05mm for nets, +/-0.12mm for form check areas, +/-0.10mm for hole location bushings, +/-0.002mm for pin sizes and +/-0.15mm for flush checks. We are in the sheetmetal industry and have used these tolerances for all other manufactures without complaint. HTH


Greg (gsepan)

[pniven]

I do not remenber where I got this from but I will post it.

Ford Checking Fixtures General Construction Tolerances.
These tolerances are to be used in the construction and certification of stamping and Assembly checking fixtures
unless otherwise specified by Ford Motor Company Dimensional Control Department.
1) CMM starts to machined edge of base = +/- 0.125mm.
2) Surface locators to CMM starts = +/- 0.05mm controlling direction.
3) Surface locators to CMM starts = +/- 0.2mm non-controlling direction.
4) Positional tolerance of Pin locator to CMM starts = +/- 0.05mm.
5) Nominal Pin diameter is .07mm undersized from product design.
6) Pin diameter tolerance is +/- 0.0127mm.
7) Check rails profile to CMM starts = +/- 0.15mm.
8) -4- way (H) Locator Pin to -2- way (h) Locator Pin relationship = +/- 0.05mm controlling direction
9) -4- way (H) Locator Pin to -2- way (h) Locator Pin relationship = +/- 0.1mm non-controlling direction
10) Template (Flipper details) surface profile = +/- 0.15mm
11) Probe bushing for Data Myte measuring points from CMM starts +/- 0.20mm non-controlling direction.
12) Probe bushing for Data Myte measuring points from CMM starts +/- 0.10mm controlling direction
13) Base must have a flat and parallel to +/- 0.10mm
14) Base must be level to 0.l25 total overall tolerance.
15) Generic holding fixture - Columns machined flatness and parallel hole to hole = +/- 0.05mm.
16) Generic holding fixture - Columns square to base within 0.075mm
17) Master Body Gage (Blue Buck) overall tolerance from front to back = +/- 0.13mm all directions.
18) Master Body Gage locating pin relationships, Pin to Pin = +/- 0.13mm all directions.
19) Check rail Flush check trim line 90 Degrees to surface of metal when angle is over 7 Degrees.
20) Check rail Flush check trim line 90 Degrees to surface of base when angle is under 7 Degrees.
21) Scribe line details to CMM starts = +/- 0.125mm
Originator: Thomas P. Burns/tburns1 Page 1 of 1 Date Issued: Feb 09, 2000
fixt Date Revised: 12/20/00