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Using CMM To Inspect Ground Pind

CMMguy0216

Plastic
Joined
Mar 19, 2018
Anyone have a good method for checking the diameter and length of small diameter pins on a manual CMM? The pins are steel and around 5/8 inch diameter x 5 inches long, and they have been centerless ground. I check the diameter on each end and about half way along it's length. I've tried standing them up in a vee block, but I have to flip the pins over to check one end.
 
Any reason just using a micrometer isnt an option? A cmm uses calculations to get diameter. I find it is often times easier to just basic measuring equipment for simple things like this.
 
I run into this ALL of the time at work. Everyone wants me to run their widget thru the CMM. Most items go to the lab and never get touched by the CMM but are measured with conventional measuring equipment (mics., digital height stand, indicator etc.).
 
A 2 point mic won't show lobing which is the most likely defect off a centreless ,however a dial indicator and Vee block and the mic will do the job.
 
The rationale for using the CMM is the need for full dimensional reporting and the quantity of pins. Hand-writing reports is time consuming
 
I have no idea if it would work ,but could the pin be stood up on a magnetic chuck such that the dimensional checks could be made all around?
 
The mag chuck (or something similar) might be a good idea. I have no idea whether there might be some oddball interaction of the probe/head innards with a magnetic field, though.

OR, you could get a couple of small matched V-blocks, and set them up so you can lay the pin down, and have access to both ends, and the center between the blocks. This presumes you are using a CMM with an indexable probe head, obviously.
 
I don't know how you check a ground pin tolerance in a CMM unless you have one of the million dollar plus guys.
You do know your basic gauge R&R on such a check?
If not you are pissing in the wind.
Bob
 
I don't know how you check a ground pin tolerance in a CMM unless you have one of the million dollar plus guys.
You do know your basic gauge R&R on such a check?
If not you are pissing in the wind.
Bob

I made a couple of pins and my customer sent them to Mitutoyo for inspection on a CMM ,they did a fancy 3D image of it ,remarkably considering the state of my old 1966 grinder the numbers were amazing ,it cost about 50 times what I charged for the pin to measure it.
 
I am curious, how small were the deviations from round and straight?

It was a while ago but roundness was a couple of microns ,straight around 5 ,these were only 8.5mm and 25mm long. My machine is rough ,not like your Studer so it is surprising what can be done ,does take a fair bit of messing around on a poor machine though.
 
The rationale for using the CMM is the need for full dimensional reporting and the quantity of pins. Hand-writing reports is time consuming

If you're making these pins en masse it would make much more sense to invest in the proper equipment for the job. At minimum a bench mic with data output and recording software. At the high end a Sylvac Scan or equivalent. Both are better tools for the job and both will provide generated reports.

Even a vee block fixture with an LVDT gauge could be hooked up to a computer to automatically record and print reports. Probably the fastest method too.

CMM, even assuming the accuracy is acceptable, is a highly inefficient means to do what you want.
 
What are your tolerances? How many data points are you taking to guarantee compliance? Are you reporting average diameter, min circumscribed diameter, max inscribed diameter? How accurate is your cmm? How accurate and repeatable are YOU? How are you evaluating lobing?

It seems a 0-1" mic, a v-mic and a spreadsheet would be far faster even if working to tenths.
 
The rationale for using the CMM is the need for full dimensional reporting and the quantity of pins. Hand-writing reports is time consuming

I had a customer that had the same issue.
Turned out that their CMM report showed each and every Grade-5 balls we've supplied to be out of round between .0003-.0005.

Unfortunately for them, the balls were certified.
Then after a bit of back and forth they were re-certified by a calibration lab of their choosing.

Let's just say they are no longer using a CMM to measure such things.
( and yes, they were a close to $1M thingies )
 
I am looking at investing in a bench mic. Thanks...

I do use a CMM for such work. Most of my work is in the micron range. CMMs speced at +/-5 microns. I do use what looks like a machinist clamp and a 'V' notched on one side. Then clamp that in a vise. I also do use some magnetic blocks. They are ground with a 'V' in them. Some of my parts are non metallic. So magnetic 'V's don't work. I have not noticed any appreciable effects of the magnetism.
Advantages of a CMM over a mic, and I do have a Super Mic, if I can report roundness. Also, measure as a cylinder. I can do straightness. Also, I can report a graph on the roundness for form fault. Also, I could measure it as a cylinder. Then I could also report straightness.
With a magnetic 'V' block. I can measure most as a circle. In the 'V' portion, if needed, then I could measure a partial circle. This can throw off results to an extent. I can also do cylinders, and space out linear traces, or remove that trace.

No, I am not what I would consider a CMM jockey. I do have 9 years as a machinist and 15 years as a drafter. Also, some production experience. I will say under 5 micron, then it goes to a form tester.
 








 
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