TP Bonus tol.

[RussL]

We never use the CMM to measure hole diameters,
we have found it less than reliable,
I have seen entries on this forum that indicate we are not alone here.
Yet if I dimension a true location, the bonus tolerance is calculated using
the measured diameter we usually ignore.
How can I rely on TP tolerancing in PC D-MIS when it uses a measurement I consider less than reliable.
I'd be interested in some opinions in regards to this issue.
Thanks.

[Schrocknroll]

Hole accuracy

It depends on the tolerance of the hole. If the hole has a tolerance of +/- .005" then I'd use the CMM with no concern. If the hole had a tolerance of +/- .0005" then I'd be leary of the readings.

In my experience holes are rarely off more than 1 or 2 tenths of an inch. At least on the CMMs that I have used. One was a Zeiss retrofitted with PC-DMIS, and the other is a Brown and Sharpe Global.

If I needed the readings better than that I'd use our Zeiss Prismo VAST running UMESS. I am usually within fifty millionths with it. Sorry B&S.

[craiger_ny]

You could use an input comment or keyin dimension to have a bore gage reading entered in. Set a variable, construct a generic feature then TP that.

[bw_bob]

Everything is based off a good alignment and measuring feature techniques used for PC-DMIS. If that is sound then your TP dimensions should be repeatable and acceptable to use.

[Perry Fisher]

Everything is based off a good alignment and measuring feature techniques used for PC-DMIS. If that is sound then your TP dimensions should be repeatable and acceptable to use.

I agree with Bob. I'm checking pierced holes in most instances and have found an odd number of hits and max_inscribed give me the closest result to a gage pin check. The number of hits varies depending on the hole diameter. For larger holes I use more hits.

[Shelley Liebman]

Working in a machine shop, with cast iron, steel, aluminum, and brass, and some very akward shaped parts with thin wall sections at times, I can tell you that we have seen just about every type of shape that a supposed "round" feature can be.......the accuracy of the cmm versus the gage used on the shop floor directly relates to this. A three lobed hole will measure completely different with a 2 pt gage versus the cmm.

Keep all of these things in mind.

We usually work with +/- .0005" tolerances (bearing bores) on many of our parts, and if we have a pretty stable part, with a typical .0002-.0003" roundness/form, then we trust the cmm within a few tenths of what our gages on the floor are telling us.

My question though, is why are you are having to use the bonus tolerance so often? I would question the capability of the process being used to make the feature, if I had to rely on the bonus allowable for a part to be in spec.

There are some engineers that dimension true position at 0, then your only allowable deviation is the bonus, but I don't often see that myself. (this would be the only time I would approve a job to run if it requires the use of the bonus tolerance very often.)

We tend to shy away from using bonus tolerances unless we are in a pickle with a job that is almost over, or a machine that is just about to be repaired, or a fixture that is not worth salvaging etc, etc,....

What type of process/part/tolerances are you looking to hold?

[cmmguy]

You could use an input comment or keyin dimension to have a bore gage reading entered in. Set a variable, construct a generic feature then TP that.

Craig is correct - if the positional tolerance is too tight or the form is questionable on the diameter then use the alternate method that he suggested. Usually diametrical tolerance is much tighter than positional tolerance and that is why it is acceptable to use the cmm's calculation of diameter for position.

The Max-inscribed method is good for getting the functional diameter as long as you understand what you are getting(using only three points to calculate diameter) and that you take a lot of points(at least every 10-15 degrees or more if you are scanning.

The "bonus" tolerance is not about accepting a bad process or questionable parts. The purpose is to lower manufacturing costs by not being so restrictive on location. It is part of the tolerance not a cheater tolerance. Maybe the word BONUS is not a good word. So I dont understand the idea of not using the additional tolerance and restricting manufacturing. That is not Quality's function.

[kbotta]

Maybe the word BONUS is not a good word. So I dont understand the idea of not using the additional tolerance and restricting manufacturing. That is not Quality's function.

CMMGUY is correct (again ).

[No where will you find the term 'bonus' in Y14.5 - as he stated "additional tolerance" ....]

[Tested]

Shelley you are right, for setting up the machining process you should not use Bonus Tolerance. Bonus Tolerance should only be used as a last resort. For machine setup. This tolerance should be given to production only for process variation, and even that should be only when you really need it.

[VADER]

We never use the CMM to measure hole diameters,
we have found it less than reliable,
I have seen entries on this forum that indicate we are not alone here.
Yet if I dimension a true location, the bonus tolerance is calculated using
the measured diameter we usually ignore.
How can I rely on TP tolerancing in PC D-MIS when it uses a measurement I consider less than reliable.
I'd be interested in some opinions in regards to this issue.
Thanks.

UHH. MMMMM.

I have been using pc dmis (several different versions) for 5 yrs. I check hundreds of sizes of I.D. 's that are toleranced +/-.0002 with a T.P. of .001. This data has to correlate with the customers inspection data perfectly. I have yet to ever have a major discrepency that was not some how due to a programming issue on my part. I find it to be quite accurate.

[Jan d.]

You could use an input comment or keyin dimension to have a bore gage reading entered in. Set a variable, construct a generic feature then TP that.

This is very good. I agree, if you do not trust your CMM, use whatever your hard gauge tells you.

I find that even at the 0.0001" range, I do get good correlation with the hard gauges we use to check the diameter. So I also typically use the CMM diameter. However, I am always looking for correlation.

One thing I learned to do is that when I have to TP a hole, I measure it as a series of circles. They can be individually analyzed, just in case. Then construct a cylinder. This has proven to be a more reliable way to measure holes.


Jan.

[craiger_ny]

......One thing I learned to do is that when I have to TP a hole, I measure it as a series of circles. They can be individually analyzed, just in case.....

I do the same, it comes in handy for trouble shooting time. Sometimes form problems are not just limited to roundness. We have made holes that taper this way, that way, barell shaped, hour glass shaped, you name it we have done it and it usually takes about three hours of argueing and 3 different gages to convince manufacturing that what you told them the first time is the way it is.

I am going to differ with the folks that say bonus should not be considered. If it is there use it. Anyone that says not to is not considering $, that is what GD&T is all (not quite all but almost all) about. Conveying a logical, econmical intent for a part.

[RussL]

I appreciate all the input, should have known it was a matter of technique.
Since I have been placed in the CMM room about 2-1/2 years ago, I have been told it's common knowledge that you can not measure hole diameters on the CMM (by people who have been here alot longer than I).
I will let management know some people have success measuring hole diameters on the CMM.

[JIM HARRIS]

I have 2 b&s machines 1 global & 1 mistral that set on the shop floor,this is a pretty clean shop.Most of are dia. are a tol. of .0005 total for size,double checking with air gage and dial bores the cmm are usally no more then .0001 differance.I have no problem trusting the cmm for size on holes.I hear that statement all the time about not trusting the cmm for size.I have never had a problem as long as you keep everything clean and your techniques are right.

[Shelley Liebman]

The reason I say not to make it a practice to use the bonus tolerance is based on the general mentality that "the more you make, the more you spend"
If you tell production that their first part is ok, but needs to be improved, they say--why, its in spec?

I have also found that once you tell production, it is ok with the bonus, they start to assume that that they get the same bonus with all holes etc, and they start to forget to actually measure the hole they are making to see what bonus they can use........