CMM-type precision measurment in very narrow deep gap

Are you trying to measure the cut straightness, along the vertical length of the cut, or are you trying to measure multiple points around the cut profile?

Do I understand correctly that the work and scrap piece have not been separated, and you want to measure inside of the kerf?

By "an inch and a half deep" do you mean a through-cut in a workpiece that's 1-1/2" thick?

Hi, Ed:-

Yes, and at multiple positions along the cut. There is only a small length along the cut that is critical.

They can't ever be (non destructively) separated in this application, so yes.

Correct. It's a through cut, obviously, but I'd like to do it from one side if possible.

Phew. That's a challenge. I hope you get some useful ideas.

The only thing I can think of is an electronic bench gage with a special probe. The capacitive type barely touches the workpiece, and we used one at Wasino that had a resolution of 2 microinches (1/20 micron). That was on a good day, if you didn't breathe on it, and if you wore insulating gloves or if your body temperature was 68 deg. F. d8-)

A probe that thin, that would slip all the way down into a 0.009" kerf through an inch or so of depth sounds awfully iffy, because the probe would bend if you looked at it cross-eyed.

Those bench gages, though, are basically height gages (although they can work sideways), so you'd have a problem moving the workpiece to gage it.

Mitutoyo and others make profilometers (profile gages), but I can't think of one that could use a probe that thin. It would, however, give you either the vertical or the profile measurements that you want. The instrument is designed for exactly that kind of thing, but without the space restriction. They do make a lot of different probes for them, however.

This is a case where I'd get on the phone with Mitutoyo, or Federal, or Starrett, and start asking their application engineers for technical help.

Good luck!

Toolmaker's microscope would be the first thing to try. The resolution on mine is half a tenth (~1 micron), but you need sharp edges and high magnification to acutally see that small an increment. I'd have to check what the standoff distance is for the higher power objectives --

If this is a matter of many parts to inspect, Mitutoyo makes some amazing automated inspection systems. One of my customers is using them to check dozens of features on a small part for form and location with micron resolution.

Hi, Ed:-

That's an excellent suggestion. Mitutoyo has a lab only a few miles away.

Interesting-- I've never seen those before! Thanks.

Unfortunately, we need to know exactly what kind of a curve the gap has, not just the width at any given point, as that has an enormous effect on the characteristics in our application.

Well that won't work, will it. I imagined measuring the locations of points in the bottom of the kerf.

I wonder if you could air gauge it using a probe with a right angle orifice. But as Ed says that will be a VERY thin probe.

A 34 gauge hypodermic needle is 0.00725". I won't use anything smaller than 30 gauge because they bend over when I try to stick them into my leathery hide. They be very small. d8-)

In that case you have a research project on your hands. You might experiment with Reprorubber or putty:

You could also make a custom tip for an electronic indicator from a steel shim. The shim would have a small bump on the end to make contact.

Also, talk to the gap sensor company. I beleive they work by measuring distanced on both sides separately. The only issue is the probe is not very stiff. If you could stiffen the probe and insert it carefully from the top, it might be possible to get a good profile.

Is it possible to just try it in your application and see if it works, or are you attempting to diagnose bad performance?

I once developed a process involving step polymerization where I could not calibrate my flow systems to the accuracy required in the polymerization, but the performance was fine. Also, any slight deviation in a batch of raw material showed up as a big variation in final polymer.

Solution was start up, run analysis and make the correction. This was a continuous process and A-B step polymerizations approach infinite molecular weight at a mole ratio of A:B::1:1. Actual calculated flow changes were miniscule, below 0.05%. Couldn't see it on the screen through the measurement noise, couldn't calibrate it, but you could damn sure see the change in the product, and it'd hold as long as that batch of raw material held out. So we just ran it.

Pete Keillor

Hi, Pete:-

We're seeing more part-to-part variation in measurable (indirect) parameters than what would be expected from what we can currently dimensionally measure. I want to get to the bottom of it.

I generally like that kind of method. Measure something closely related to what matters, and control that rather than some arbitrary other parameter. It makes it difficult to improve the process, which is where I'd like to go, since I have time and money to do it properly (how rare is that?).

Best regards, Spehro Pefhany

Yep, if you need to improve the process, you need information. Hope one of the measurement companies can help. Good luck.

Pete Keillor