Need Help to Measure a Thin Gap

[extropic]

I want to measure the change in gap between two planar surface resulting from a change to the force pushing the surfaces together. I need practical suggestions. No LASER scanners. Just conventional measuring tools.

These are steel surfaces with fine surface texture (probably 8 or better). The parts can not be modified for this test. The surfaces are vertical. The surfaces are circular. For each measurement, I want to measure the gaps at 3 places, equally spaced around a diameter. The magnitude of the gap is .001" to .005". I want to measure the gap to .0001" resolution.

Simple explanation: Imagine, with the surfaces separated, I stick a small bead of Silly Putty, at 3 places, equally spaced around a diameter on one surface. Then I bring the second surface to bear and apply the lesser force. The beads of Silly Putty will be compressed to the thickness of the gap.

The problem is, how do I measure the thickness of the compressed Silly Putty without altering it's thickness. I don't think Silly Putty is an appropriate material.

I thought of using 3 drops of a quick curing epoxy. Spray the steel surfaces first with a light oil to prevent the epoxy from adhering. Sounds reasonable to me. I'm thinking the flattened cured epoxy beads would be tough enough to survive their thickness being measured with a micrometer.

After measuring the gap at the lesser force, I would reset and perform the process again at the greater force.

My questions to the group are: Do you think the epoxy will be OK? What material would you recommend to replace the epoxy? Is there a much better way to measure the gaps?

Thanks for reading.

 

[Jubil]

Just a guess but could Plastigage be used?

Chuck
Sorry. I didn’t read all the questions. I’m unsure about the epoxy.

 

[extropic]

Just a guess but could Plastigage be used?

Chuck
Sorry. I didn’t read all the questions. I’m unsure about the epoxy.

I haven't used Plastigage in 50 years. I think resolution to .0001" will be uncertain at best, but for about $10 at the auto parts store it's affordable.

I'm also concerned that my low force scenario may be too low for Plastigage to work properly. The thing about epoxy (or similar) is that it is liquidous and requires essentially no pressure to conform to the gap.

It's on the list of options and I do like the fact that I could put Plastigage across the full width of the annulus, rather than a small bead at one (or a few) radial spots. I sure don't want to epoxy the surfaces together.

 

[fixit]

When working on large compressors, we insert a piece of solid lead solder through a valve port and roll the compressor over. The piston mashes the solder, which can be measured with a micrometer. This was how we set the piston to head clearance in the forward and reverse positions.

 

[redgrouse]

Use three pieces of small lead shot, compress them and measure, however I don't think you can really be certain of 0.0001" resolution ! You could consider other malleable materials according to to the pressure/force applied, maybe tin/lead, [wheel weights?] aluminium, or WHY?
This method is / was used to the ascertain the pressure developed buy ammunition on firing using lead for shotgun ammo and copper for centre fire metallic ammo thus the pressure can be calculated.
Edit= spelling correction --Ascertain was assertion Autocorrect ????

 

[ChazzC]

I also immediately thought of Plastigage; have only used it once decades ago (for its original purpose) and don’t remember the relationship between width & thickness so can’t address the resolution of the results.

I like the idea of using lead or similar soft shot, measuring the pressed thickness with a 0.0001” drop indicator on a comparator stand to minimize force that would further compress the shot.

 

[francist]

Silicone moulding putty may be your ticket here. Two part, consistency of warm silly putty when kneaded together, but will not adhere to most materials. Resolution is extremely good — ie: it picks up fingerprint whorls — and will hold its shape when cured. My only concern would be whether it might be a little too squishy to get tenth accuracy with a mic. Not cheap, but available in small pots so you don’t have to buy a ton.

 

[Flyinfool]

The smallest gap that can be measured with Plastigauge is .001.
The epoxy should work but be careful, compressed to .001 ore less it can spread a long way and get into places you really don't want it. You would need a really tiny dot. It would also help to find it if you could add a colorant to it. When down around a thou or less it will be extremely fragile.
Even 5 minute epoxy will take a long time to cure in such thin films. at least an overnight cure. If you read the instructions (Yea right who does that) many will be pretty hard in 5 minutes but full cure is still 24 hours.
The oil film will also add some thickness. Maybe a good coat of wax would be better?

 

[Cadillac]

Maybe play doe. I remember it dries when left out. Has the same consistency as silly putty.

 

[Harry Knutz]

I want to measure the change in gap between two planar surface resulting from a change to the force pushing the surfaces together. I need practical suggestions. No LASER scanners. Just conventional measuring tools.

These are steel surfaces with fine surface texture (probably 8 or better). The parts can not be modified for this test. The surfaces are vertical. The surfaces are circular. For each measurement, I want to measure the gaps at 3 places, equally spaced around a diameter. The magnitude of the gap is .001" to .005". I want to measure the gap to .0001" resolution.

Simple explanation: Imagine, with the surfaces separated, I stick a small bead of Silly Putty, at 3 places, equally spaced around a diameter on one surface. Then I bring the second surface to bear and apply the lesser force. The beads of Silly Putty will be compressed to the thickness of the gap.

The problem is, how do I measure the thickness of the compressed Silly Putty without altering it's thickness. I don't think Silly Putty is an appropriate material.

I thought of using 3 drops of a quick curing epoxy. Spray the steel surfaces first with a light oil to prevent the epoxy from adhering. Sounds reasonable to me. I'm thinking the flattened cured epoxy beads would be tough enough to survive their thickness being measured with a micrometer.

After measuring the gap at the lesser force, I would reset and perform the process again at the greater force.

My questions to the group are: Do you think the epoxy will be OK? What material would you recommend to replace the epoxy? Is there a much better way to measure the gaps?

Thanks for reading.

Soft solder is what guys building 2 stroke saws use to measure what's called squish when you pull the cord and the cylinder gets to top dead center it compresses the soft solder and then an accurate measurement can be taken to determine the clearance between the piston and cylinder head.

This may work for you, But without really seeing what you are working with it's hard to tell.