Measuring to a large radius, short arc

[Jonathan S]

I need to measure a dimension to the centerline of a radius, but the radius is 2" and only covers an arc of about 25 degrees. Measured on the optical comparator, the radius is nearly nominal, but on the CMM it measures 2.117". This throws my centerline over .100" out of position. How can I create a centerline from points on the radius using the nominal radius? Can I force PC-DMIS to best fit a 2" radius to points on the surface?

[Dick Helm]

My experience is that you need at least 110 degrees of the arc to get any reasonable accuracy.

[Sean Harris]

Use a fixed Rad algorythem to find center of the rad. then origin on that circle. Now measure points on the radius and report the PR. It gives more of a profile but it will do your trick.

[dfiola66]

Could you type in the theoritical location of the first radius (ie create a point) or measure it, and in polar coordinates, take a few profile hits on the smaller radius and tolerance out the profile relative to the first rad?

Keep in mind that I've not tried this but sounds like it should work.

[bw_bob]

There is no way to actually measure that large of a radius with that small of an angle. At least with PC-DMIS.

[Alan B. Metzel]

You might have one shot at making this work... the problem is, that as prev. stated, the measurement algorithm breaks down on small arcs, but also due to that breakdown should give you nonrepeatable results. As the error is a "normal" error, it is subject to statistical analysis. So, if you do get such "nonrepeatable" measurements, run the part twenty or thirty times and look for the x-bars on size and position. I have successfully used this method on other systems.

[Devaney]

Use a fixed Rad algorythem to find center of the rad. then origin on that circle. Now measure points on the radius and report the PR. It gives more of a profile but it will do your trick.

Sean is on the right track. We just discussed this in the user group. And there is a way to get an accurate reading for a small arc segment. I have to find my notes but I think Sean is correct.

When asked about it the "experts" quickly rattled of the sequence to achieve an accurate reading. They should put a step by step set of instructions together for everyone to go by. Every time someone says that the CMM doesn't measure worth a **** they bring up the segment of an arc. I frickin hate that! That's why I wanted know how to do it. So I could stick it right up their azz!!

[Bill **********]

Pull the nominal CIRCLE from the radius in CAD, I'm assuming that you have CAD, set your origin at the center of the circle and then take polar radius hits on the arc. This is the only way I've ever gotten PCDmis to work for small cicrle segments.

Bill

[jjewell]

Sean has my agreement also. It is NOT a Pc-dmis thing, or even a cmm thing, its a mathematical issue. Polar coordinates are the best way, from the known radus center. I wish ppl would say its not possible, if I don't know for sure, I just say that, not, "It can't be done with Pc-dmis".

[BOB PARSONS]

BIG RADIUS SMALL ARC

I AGREE W/ THE ABOVE POST. WE GO WITH A KNOWN CENTER LOCATION THEN
USE POINTS. THIS METHOD IS SIMPLE AND VERY ACCURATE AND QUICK. YOU WILL
GET AN ACCURATE RADIUS.

[VONDY]

Do you just take one individual point or many? If many, do you construct a circle out of all the points? If using only one individual point how can you do a True position call out?

[Mike N.]

steps I use for location and diameter of small arc/radius

At a recent users group meeting this topic was discussed and the following steps were given to achieve both the location and diameter of a small arc/radius

To measure a partial arc/radius:

1. create a "measured" circle
2. edit (F9) the circle and add additional hits and edit the "best fit math type" to "FIXED Radius", edit the NOMINAL DIAMETER, check box to "regenerate hits", when prompted "equal spacing" answer YES
3. ctrl E the feature to execute it and re-capture the measured hit points
4. dimension the "location only" of the fixed radius circle/arc
5. edit dimension tol's to print
6. create an alignment and set origin on fixed radius circle
7. (if using CAD skip this step) F10/Parameters>Probing> enable polar compensation for the active workplane
8. with joystick measure more individual/separate points on arc radius (if using CAD use "vector point" skipping step #7)
9. dimension individual points using the "PR" polar radius function
10. F10/Parameters>Probing> turn off or disable polar compensation

[dccfreak]

I have used this method in my travels and it works pretty well, however, I'm not sure that I can say I have ever seen a difference in doing this and scanning the area. I sometimes scan these areas then generate the scan to points and get the polar radius of the points that way.

It seems to correlate rather well with the measured feature option, but, seems to be a little faster IMO.

[VONDY]

I work 95% off-line. So using the jog box and taking extra hits is somewhat out of the questions unless I use auto vector points. ctrl+e and recapture all seem to be on-line programming. Is there another procedure for off-line programming.
I tried using an auto circle with 30 hits on 2 small radius, then I also did a scan with 30 hits that were then created into vector points. I created a circle out of them. These were two different radius, one on top, one on bottom. Both types came out extremely lousy.
Reporting out individual points still doesn't give a true position reporting. Is what you are saying is that it is very unlikely that I will ever be able to create a circle out of these hits and get accurate true position results.

[Mike N.]

I work 95% off-line. So using the jog box and taking extra hits is somewhat out of the questions unless I use auto vector points. ctrl+e and recapture all seem to be on-line programming. Is there another procedure for off-line programming.
I tried using an auto circle with 30 hits on 2 small radius, then I also did a scan with 30 hits that were then created into vector points. I created a circle out of them. These were two different radius, one on top, one on bottom. Both types came out extremely lousy.
Reporting out individual points still doesn't give a true position reporting. Is what you are saying is that it is very unlikely that I will ever be able to create a circle out of these hits and get accurate true position results.

No, that not what I said at all
these steps were given to me from a B&S AE and work fine ONLINE
you could try to modify them for use offline and test it online

The true pos could be obtained in the step #4
diameter in step #9