Making a cast of a blind hole

I have an aluminum block with a 11mm dia. hole, 30mm deep machined in it.

The problem is the hole is tapered towards the bottom of the hole which I am unable to measure.

I am thinking of making a 'cast' of the hole so I can then remove it and measure it.

Has anyone had experience of suitable casting material?

I had thought of spraying the inside the hole with silicone spray, then pour in molten candle wax and hoping it will pull out in one piece!

2 part urethane casting resin would be an excellent choice, and would give a rigid plug for accurate measurements. See www.smooth-on.com for this. Their Smooth Cast 310 pint kit has given me excellent results. I don't have any connection to this firm except as a satisfied customer for many years.

The other possibility that comes to mind is dental impression material. If you know a dentist he may be able to give you a syringe of this two part polymer system used to make moulds of teeth and gum structures. Very safe material and the impressions are high resolution detail. I don't think suppliers will sell this resin system to individuals but a dentist could maybe help you out,

Wax would be OK but may deform or crumble when removed. Beeswax rather than paraffin wax as in candles may be better.

Whatever resin you use, to help get it out, you could embed a wooden or metal or plastic square rod with grooves cut around it to enable twist and pull of the impression to remove it. Just food for thought.

Polymorph

You can buy dental impression compounds on ebay.

There are all sorts of 2-part epoxy putties about so long as you use some form of release agent to stop it bonding and remove before fully cured.

Modelling clay might be enough too so long as you're gentle with the mesurements or 'fimo' which come in a whole slew of varieties and you can bake the impression to harden if you want to get fancy and cut it in half after.

pgk

You could use a pair of internal calipers as the hole tapers inwards towards the bottom. I mean ones like this:

Not the measuring sort.

Or ordinary silicone bath/kitchen sealant, after a light oiling as a release agent. How accurate does it need to be?

John

Apart from the above:

Measure bottom and use these with the depth measurement to the taper and calculate the taper? Is it an important feature?

Alternatively, use the depth to taper and bottom, then fill with a syringe to find the volume. Use that to check the taper?

Finding the diameter at the bottom of the hole could be done by test with different sized drill bits, as could the depth where the 11mm diameter reduces.

Bore it out and sleeve it, with a taper that you do know?

I’m trying hard to imagine a use for such a hole, but failing miserably. No spare imagination today.

Edited By not done it yet on 05/01/2021 17:35:32

Drop 2 different sized balls down it and measure the difference in ball depths to the surface. Calculate the taper this way.

Thanks for all your ideas on measuring this blind hole.

Last night I found my self in the local supermarket buying a tub of Play dough and a packet of 'icing clay'

I got 'average' results after trying to stuff these products down a blind hole and removing it in one piece.

Today I was allowed to machine a hole in half which has made life a lot easier.

To give an idea of size, top of hole is 11mm. It now reveals that the hole was drilled in two, maybe three operations. I am going to see my local tool grinder company and ask if they can re grind a drill to do it in one operation.

Make a D-bit out of tool steel. Its what I do to make tapered holes for plug valves.

Search the internet for how to make a D-bit. They are not hard to make.

Paul

Edited By Paul Lousick on 07/01/2021 05:44:21

A company called Rubert used to make a product that was intended to make a replica of the surface of difficult to reach places so that the surface finish could be checked.

The company is still in business but I don't know if they still supply this material or what the name of it was. Perhaps it is still available from other makers of surface measuring equipment.

Brian

Andy

I am afraid that a drill, because of the way it cuts will not produce what you are looking for. A drill does not cut on the flutes which is what you want to produce a tapered hole. A 'D' bit is the way to go. Step drill the hole to remove most of the metal, then finish with the 'D'bit, make sure the component is securely bolted down as the cutting forces will be astronomical.

Edited By Dusty on 07/01/2021 09:59:00

Andy

Question, what goes into the hole? it is easier to measure the component that goes into the hole than the hole itself in most cases.

Is this project related to ammunition manufacturing?

When the end detail is not critical, casting with a tube in the centre helps. Then some low pressure air can be put down the tube to remove the casting.

More interesting suggestions! Many thanks.

I will have a go at making a D bit.

The hole is used to hold a type of plastic test tube.

An interesting approach. Drop balls into a tapered blind hole. What could possibly go wrong?

George B.