Spotting medium thickness measurement

I have been making a repeat-o-meter for checking surface plates using an ebay $31 Chinese electronic half tenth/1 um gauge.

It seems to work properly so I decided to try measuring the thickness of my typical application of Canode using a Speedball soft rubber printer's brayer.

Repeated measurements consistently showed a thickness of about a tenth which is actually not a lot more than the repeatability for the indicator.

Have Fun!
Reg

What tip did you use? Using a ball tip (or "dog" tip with smallish radius) may seriously underestimate the film thickness. Try a disk tip that's actually flat and test to make sure the disk is aligned parallel to the surface plate (i.e., not contacting first at one side on a point or narrow oval).
What I'm getting at is that point contact can shove the spotting medium out of the way.

Similarly a wide tip might plough up the spotting medium and result in an increased thickness.

The photos clearly show that I'm using a ball tip (came with the indicator) bearing on a flexure (0.020" feeler gauge )to spread the load. The carbide feet and flexure contact surfaces have been lapped on a 1200 grit diamond lap. The flexure has less than 1 mm total travel in use.

There isn't enough pressure to adhere any Canode to the feeler gauge. It barely marks the surface of the layer.

rhb said:

The photos clearly show that I'm using a ball tip (came with the indicator) bearing on a flexure

Click to expand...

My apologies. The thumbnail shows in my browser with a black bar obscuring the bottom part of the photo, and I did not open it up to see the larger image. The photo is quite clear, and you have a very nice broad contact surface.

Repeatability and cheap Chinese crap are usually not synonymous.

Digital displays inherently have a 1 digit ambiguity, so this is not actually bad. The Mitutoyo 543-793 is spec'd at +/- a tenth which is the same performance as this. The Chinese now do so much contract manufacturing that the quality is much higher than 30 years ago.

It will be interesting to compare it to my used Mitutoyo. Unfortunately, it has a shorter stem similar to my Baker tenth indicator. So it won't fit this holder. I plan to make a new holder that fits the two of those.

I plan to test all my tenth indicators with gauge blocks on a plate, but I'll need to make a multi-indicator stand so I can move the blocks from one to the next easily. The real question is how linear are the indicators over their ranges.

I'd previously considered measuring the layer thickness as very difficult. This was completely accidental. I looked at it and asked,"what will that show?". I didn't believe it at first, but the impression it made on the surface of the Canode was very shallow. So shallow that repeated measurments at the same spot didn't change things until I'd done 5-6 repetitions.

The actual contact area (on a hard surface) on the underside is about 1/8" x 5/16" and about 1/4" farther from the block than the indicator. It's very usable as is and I want insulating wooden grips on it more than an improved flexure. At 6 millionths per degree F it only takes a 12 degree increase for a 1" block to grow by a tenth.

At the price, I'm going to make a case to hold this indicator and stand for measuring film thickness and some improved stands for testing surface plates and similar.

Here's a photo of the underside showing the contact area in relation to the spindle.