ID's

[Ken379128]

I am having a proble with ID's again :-) being new I was wondering if I could get some advice. I am measuring a 9.21mm ID with 7 hits in Least Sq.they are coming out ULL 9.188 I have rechecked with a validator hand driven and a plug gage class Z that are coming out 9.209 and the 9.19 go flys right in. So any suggetion as to what be the problem.
I am running 3.7 mr2 on a global image

[#2#]

I am having a proble with ID's again :-) being new I was wondering if I could get some advice. I am measuring a 9.21mm ID with 7 hits in Least Sq.they are coming out ULL 9.188 I have rechecked with a validator hand driven and a plug gage class Z that are coming out 9.209 and the 9.19 go flys right in. So any suggetion as to what be the problem.
I am running 3.7 mr2 on a global image

The problem is that a CMM isn't very good at checking diameters. I would try measuring with more hits (I usually go with a minimum of 11). Also change the measurement method to "maximum inscribed" That will give you the largest circle that will fit within the hits (kind of like a simulated plug gage).

[Underspec]

Basically the more hits you take the more accurate the best fit circle will be. I usually take 24 hits (every 15 degrees) on circles.

[Shipwreck]

I have checked bores for years large and small, trust me you should not use the CMM to rely on the size.. You need to buy a dedicated bore gage, and if the volume is too low, than use the plug gage, 3 pnt, or some other way... On your cmm report put a statement that the diameter is for reference only, the corrdintates will be accurate tho. so you can use them. Most arguments from the customer comes when they have a CMM and we have a more accurate bore gage, we have to prove to them that they are wrong

[Ravenmaniac]

With close diameter readings, I find that I first clean and calibrate my tip to be used very carefully. I check the possible error in the calibration macro and try and get a tip that measures very precise. Second, I use alot of points spaced as equally as possible. Thirdly, check your form...sometimes you'll get erroneous readings as the hits you took fall into the worse form areas of the diameter. Also, as a check, verify your machine's capability by testing out your tip/diameter program on a known set ring that is precisely round and to size at the room's ambient temperature. Hope this helps....B

[Ken379128]

problem is we have been using it and getting good reading consistant with plugs gages. It has been in the last week or so it has been out. Tried what has been suggested so far to no avail. I was thinking of a few thinks on the way home if it works I will let you know if not I will never admit I tried them :-) at least to my boss

[John Kingston]

Did the machine have a crash? Did you have a major swing in room temp? It sounds as if something in the machine itself has changed. You may need to get it serviced (lasered square). I would start checking a "standard" part on a daily or weekly basis. This will give you an idea if the machine drifts. Chart the information for easier tracking. If you search posts by Hilton Roberts, you will find a wealth of useful information on this concept.

[craiger_ny]

What kind of form are you dealing with? Do you get the same results when you measure a ring gage of similar size or something whose form is known to be quite good. When you run into form issues with diameters your reported size can get funky. We had parts here with 25mm IDs I was getting size all over the place on and it took a little investigating to find out that not only was roundness way out but the holes were also barrel shaped so that diferent depths reported diferent numbers in their own funky way. None of which coincided with a plug gage even when I asked PCDMIS to do the math as MAX INSC. Ravenmaniac is offering the cheapest thing to start with, check a ring gage and go from there. Might as well start cheap, no sense calling a tech if you have bad parts. That would be not only expensive but embarassing. Check that part with anything other than the CMM if you can, preferably you have a 2 point contact bore gage you can get in there with.

Craig

[Ken379128]

Working on it this morning and we have a tip cal program that ring gage measure fine with in .00006. But another ring gage is out .0004 problem is I did not write the tip cal prog trying to see if there is something I am doing wrong but everything looks good. Boy do I have a headache :-) Oh and just calibrated on 6/06 with no known crashes or bent tips.

[Tested]

Ken,
Unlike some here my machine works great for diameters, like you I've been using it and it works. You may try looking at the hole visually, there may be something of an out of round condition, noncleanup, or even a ding in the part. Also check to be sure you are not shanking out.

[#2#]

Working on it this morning and we have a tip cal program that ring gage measure fine with in .00006. But another ring gage is out .0004 problem is I did not write the tip cal prog trying to see if there is something I am doing wrong but everything looks good. Boy do I have a headache :-) Oh and just calibrated on 6/06 with no known crashes or bent tips.

When you say "calibrated on 6/06" do you mean that someone came in and calibrated your machine? I would certainly hope that you have calibrated your probe/tip since 6/06.

Can you try a different probe/tip just to eliminate that from the equation?

[Ken379128]

:-) B&S cam and calibrated the whole machine. We do the tips every time they are changed we have a tp200 even then I like to calibrate at any sign of trouble just to make sure. And yes I tryed a different tip. On something else now so my headache will go away

[John Kingston]

On something else now so my headache will go away

This seems like a time for the Cleveadian Law to be invoked.

BEER

I'm leaning towards something has changed in the machine or the environment it is in. I would look closely at that. I know it costs, but a service visit is less costly than shipping nonconforming product.

[Shipwreck]

Ok here is how you can prove that CMM's are crap for checking bores.... with tight tolerances. You need to do a short or long form gage r%r study... you will be agassed..... It will be very bad... typically on a .0005 inch r%r on a small bore with an air bore gage I get about a 5 ot 7% gage r%r... If you can match that with your CMM I will eat crow...

hehe

[Ken379128]

Funny it looks like the left side of the cmm is measuring the ring gages ok but the right side is still ULL by .0006 so looks like something happened.