Why? (re: Checking Clearance of Diameter responses)

[keith]

After reading the responses to the thread for "Checking Clearance of Diameter" and seeing the consistency in those response, I need to know why? If the machine is supposed to be accurate within 3.1 microns, why can't it check .0002"?

I have a Global Image 555 calibrated to ISO-10360-2. The calibration certificate shows:

Maximum Permissible Maximum Observed Error
Error Specification (um) (Actual Recordings at Customer)

Length Meas 1.7 + L/K 3.1 at 460 3.1 @ 460
(E A + L/K)

Probing Test 1.9 1.8
(R rmax - rmin)

Some of the tolerances on the dimensions being measured are +/- .0002" (mostly true position). Is this machine NOT the right one for this application?

Thanks,
Keith

[#2#]

CMM's just aren't the best for measuring diameters. I'll let someone else explain why.

[bw_bob]

On diameters pc-dmis is like a virtual gage. It measures the smallest circle(min_cirsc) that would fit over a stud or the largest stud (max_insc) that would fit in a diameter. Least_sqr is a average of all points.
Dmis is not the best at measuring diameters with tight tolerances.

[\v/inston]

it depends on the hole, is it sheet metal punch and button? or is it a perfectly round hole created on a swiss lathe? the more hits you take the better chance you have for high acuracy, and your probe vector needs to be perfectly square to the axis of the hole.

try checking a ring gage, check it with 3 hits, dimension size, roundness, circularity.... then change your hits to 999 or whatever you have time for just make it a lot and see what you get compared to what is on the gage.

[bboyd]

I believe that your asking 2 questions,
Is the machine capable of measuring a hole dia to .0002?
Yes if your hole is perfectly round clean etc. but I would use a pin because it removes all uncertainy and because you culd be calling a good part bad and a bad part good.
Is the machine capable of measuring TP off .0002?
Yes, as long as form is not skewing your points.
All measuring instruments have limitations, it is a matter of using the most accurate tool for the most accurate results. Can't beat a go/nogo pin on tight toleranced diameters.

[Jan d.]

Issue with tunneling: big balls and small holes...seriously!

What CMM's are not very good at is measuring small holes with big balls (just go-ahead now, have at it!). This has to do with a phenomenum called tunneling.

Machine tools are absolute champs at following a prescribed path. They have extremely stiff drives/guides and if you tell them to follow a certain path, they will.

CMM's on the other hand, have very low stiffness drives (belts, small rack/pinion) that are designed to create smooth motion. NOT to exactly follow a path. Therefore, as they approach a part, the wander off the ideal line quite far. Some machines are known to be off by several thousands!

The way to test this: put a gauge block in the machine volume so that it makes a 45 degree angle to ALL axes. Now align on the block and call the normal vector to the plane Z. You will see that the repeatability for Z is going to be great. Indeed, within a few micron. However, if you print out X and Y, you will see that they can wander a lot. You want them to hit the bulls eye. You'll find that they may miss the center by a large amount.

Now why should you care? If you measure a bore that is real big with a relatively small ball (IE 2" ring gauge with a 3 mm stylus ball), it does not really make much difference if you are off by a few thousands on the vector because the cosine error for not hitting the exact spot will be small.

Now imagine measuring a 3/16" hole with a 3 mm ruby. If you are off by even a few tenths, the cosine error will become significant immediately!

Location will be good, but due to tunneling, your diameter WILL vary.

I ALWAYS test my CMM's for tunneling. It is in our company spec when we buy one. I have to measure many small holes with (yes, yes yes, funny funny funny) big balls. If my machine doesn't tunnel, no way I am going to find decent bore sizes.

Manufacturers do not like to talk about tunneling because it makes their machines look bad. If they correct for tunneling, they typically loose single point repeatability. So its a catch 22 for them. Therefore they keep quiet about tunneling and tell us to be careful when measuring size.



Jan.

[Tested]

I believe that your asking 2 questions,
Is the machine capable of measuring a hole dia to .0002?
Yes if your hole is perfectly round clean etc. but I would use a pin because it removes all uncertainy and because you culd be calling a good part bad and a bad part good.
Is the machine capable of measuring TP off .0002?
Yes, as long as form is not skewing your points.
All measuring instruments have limitations, it is a matter of using the most accurate tool for the most accurate results. Can't beat a go/nogo pin on tight toleranced diameters.

I think .0002" tolerance is a bit tight for pins.

http://www.engineersedge.com/dowel_pin.htm

[RussL]

I must say I find this thread rather interesting.
Back in June I posted a thread noting my concern with getting a proper true position bonus if I can not rely on the CMM to measure holes accurately and was made to believe you can measure holes, that if I could not, the error was due to the technique I used.

[#2#]

I must say I find this thread rather interesting.
Back in June I posted a thread noting my concern with getting a proper true position bonus if I can not rely on the CMM to measure holes accurately and was made to believe you can measure holes, that if I could not, the error was due to the technique I used.

You can measure holes. You just have to be very carefull. I personally wouldn't measure a hole with a tolerance of ±.0002. Most of my tolerances on diameters are ±.005 so I feel comfortable measuring these with the CMM. When I do run across a tighter tolerance I will measure it with the CMM and double check it with another method. Of course, I am not usually working for speed and have plenty of time to get things done and make sure they are done right.

[\v/inston]

I must say I find this thread rather interesting.
Back in June I posted a thread noting my concern with getting a proper true position bonus if I can not rely on the CMM to measure holes accurately and was made to believe you can measure holes, that if I could not, the error was due to the technique I used.

thats a good point. i don't think you can get accurate bonus on TP without checking the holes manually and then editing the report especially on sheet metal.

[Tested]

Tests

I must say I find this thread rather interesting.
Back in June I posted a thread noting my concern with getting a proper true position bonus if I can not rely on the CMM to measure holes accurately and was made to believe you can measure holes, that if I could not, the error was due to the technique I used.

Just my 2 cents
The only way any of us are going to know what we and our machine are capable of is to do some testing. You need to compare what you get and what technique you use to a standards. And you can't just do it once and get good readings and say you've got it. You need to run some cap. studies. On different techniques, sizes, operators, .....

[bboyd]

I think .0002" tolerance is a bit tight for pins.

http://www.engineersedge.com/dowel_pin.htm

I wouldn't use dowel pins either.
I purchased certified go no/go pins.

http://www.frankcox.com/pages/plaingauges.htm

[Scott Appleyard]

Let's not forget that the type of probing we use will also affect the outcome of your measurement. Probing systems like TP-2 or TP-20 touch trigger systems will introduce lobing error. TP-200 and SP-600 systems use strain gauge technology and are much more accurate. If you need to measure holes with the CMM, you may want to look at using these options instead.

[Wes Cisco]

I ALWAYS test my CMM's for tunneling. It is in our company spec when we buy one. I have to measure many small holes with (yes, yes yes, funny funny funny) big balls. If my machine doesn't tunnel, no way I am going to find decent bore sizes.

Manufacturers do not like to talk about tunneling because it makes their machines look bad. If they correct for tunneling, they typically loose single point repeatability. So its a catch 22 for them. Therefore they keep quiet about tunneling and tell us to be careful when measuring size.



Jan.

I understand cosine error causing tunneling. What I am curious about is your claim that your company spec requires you to test for this. Would you, could you, *Please*, share with us some specific details about the type of tests you do and what standards you evalute them to with regards to tunneling and/or, (even better) your full prepurchase CMM evaluation. TIA. This is a good thread.

[keith]

fascinating!

So it's not the accuracy of the machine but the technique being used that is in question. I understand. Thank you all.

Keith