Probing threaded hole

[skymoco]

I need to probe a number of 1/4 threaded holes to find the X-Y coordinates on a metal plate. I'm thinking it might be more accurate to use a piece of rod with one end threaded so that 1/4 inch or so sticks up and probe that instead of probing a threaded hole.

My probe has a .1 diameter tip if that matters at all.

Advice from someone who does this more than I do..

[DaveCVI]

Hi,
Yep, MSM's hole probing routines do assume that the hole has smooth, vertical (Z parallel) sides. Let's consider the case where the probe tip is small compared to the pitch of the thread... on one side the probe tip might contact at a thread crest, and at the other at a thread root.... that would throw off the calculated center location.

If the probe tip was large in diam compared to the thread pitch, then the above would not matter... but to be that relatively large, the tip probably does not get into hole to begin with...

I like the idea of using the threaded rod. Of course the results will depend on the fit of the threads (less slop the better), the perpendicularity of the hole, the straightness of the rod... etc.

If it were me (I'm not an expert here), I think I'd start by experimenting with probing the top of a pin gage that just fits in the threaded hole. I'd be sanity checking the results as we're talking about .1 diam tips probing roughly .2 diam posts here...

I suspect you'll also end up needing to fine tune various probing parameters: things like back off distance. If you do try to probe the threaded hole, you'll be finding it hard back off a .100 tip by .100" when inside a nominal .201 (the typical #7 tap drill size fore a .250 hole) diam hole...

I'd guess it all depends on the needed accuracy of the hole measurement.

I'll be interested to hear how it goes for you.

Dave

[skymoco]

I did a little experiment. I am using a homemade CNC router, it has an all steel frame, ballscrews, and a aluminum table. The gantry is fixed, with the table moving in the Y and the router moves in the X.

The probe is a scratch built based on an article in Digital machinist with a tip purchased somewhere.... Considerable time spent getting it zero'ed using the lathe so it's pretty on.

I first tried to find the center of the 1/4 inch threaded hole by probing .15 deep, and then at .25 deep. Results almost exactly the same.

I then took an ordinary shoulder bolt, cut the head off so there was about 1/4 unthreaded rod at the top, screwed on a nut up to the shoulder and screwed it into the hole. I used the nut to snug the bolt so it didn't move. This time the center averaged .0015 and -.0031 in the X/Y compared to my reference.

I then backed off the bolt a little bit and re-tightened the nut. My X was worse at .005 but the Y was almost exact.

First off, I am pleased with how repeatable the measurements are. I just do a single move probe operation and the biggest std deviation for each set of measurements was .0004. It also seems like just probing the inside of a threaded hole works well, at least for my specific circumstances. Not too shabby for something cobbled up....

I did find one minor thing. Since my probe tool is not my master tool. (probe is 250, master is 100). I cannot probe Z with the probe mounted. Gives me an error about probe not mounted, I never ran into this before.

All in all it was fun in a geeky way.

All measurements are relative to the first, as I did the "probe only" after than.

The Raw Data
Threaded Hole .15 deep
X Y
1 0 0.0000
2 0 0.0000
3 0.0001 -0.0001
4 -0.0001 -0.0001
5 0.0001 0.0003
AVG 0.0000 0.0000 <--- Average
STD 0.0001 0.0002 <--- Standard Deviation

Threaded Hole .25 deep
X Y
1 0 0.0000
2 0 0.0000
3 0.0001 -0.0001
4 -0.0001 -0.0001
5 0.0001 0.0003
STD 0.0001 0.0002

Bolt #1
X Y
1 0.0016 -0.0032
2 0.0015 -0.0031
3 0.0018 -0.0034
4 0.0007 -0.0031
5 0.0017 -0.0029
AVG 0.0015 -0.0031
STD 0.0004 0.0002


Bolt #2
X Y
1 0.0051 -0.0001
2 0.005 -0.0001
3 0.0049 0.0000
4 0.0051 -0.0001
5 0.005 -0.0001
AVG 0.0050 -0.0001
STD 0.0001 0.0000