# Small bore gauges

• posted

There are T-shaped telescopic bore gauges to measure hole diameters as small as 3/8". Is there anything suitable for holes that are considerably less? I have a tapered wedge-shaped steel gauge for measuring the end of the hole but nothing that measures the ID further in.

Cliff Coggin Kent England

Je suis Charlie

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I'm not sure how one is supposed to do it but I've used the following method provided the hole is round. If you have a set of small numbered drills (unused or at least with shanks in good condition), find the shank which just fits and use a micrometer to measure its diameter.

Alan

• posted

I'm not sure how one is supposed to do it but I've used the following method provided the hole is round. If you have a set of small numbered drills (unused or at least with shanks in good condition), find the shank which just fits and use a micrometer to measure its diameter.

Alan

-------------------------------------------------------------------------------- I have done this on occasions Alan, but it really only tells me the smallest ID of the hole and assumes the hole is round. I would like to measure the ID at various depths and orientations to test whether the hole is truly round and whether the diameter is constant.

Cliff.

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The link shows a 4 piece set of expanding ball guages. Range from .125" to .500". Many companies make these types of gauges. Just like a telescoping gauge you must expand the gauge in a hole and then remove the gauge so you can measure it with a micrometer. And just like a telescoping bore gauge you will need to develop a "feel". This is not hard to master and with practice you should be able to reliably measure hole sizes and be correct within .0002". Eric

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Eric, in his OP Cliff is asking for devices to measure 'considerably less' than 3/8. Knowing Cliff is into horology I expect we're talking a millimeter or two at best :)

Cliff, an off the wall thought fill the hole with a liquid, measure the length and volume accurately and you have the 'average' diameter and will know if it deviates from your expectation having measured at the (accessible) mouth of the hole. Messy but possible.

Or using 'drill blanks' approaching from the smaller sizes upwards 'blue' them and see what internal contact they are making with the bore.

Andrew

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Andrew, I didn't know Cliff meant THAT small. If I had to measure accurately a

1mm diameter hole to a .0025mm tolerance, and had lots of money I would use an air gauge. If I didn't have lots of money I would make my own expanding ball gauge. The hardest part would be splitting it. My thinnest slitting saw blade is .2mm thick and I know thinner ones can be had so even a 1mm expanding ball gauge could be split in a home workshop. I wold think that a horologist would have the drills to put a hole through the center of the gauge and maybe even the slitting saw. Just ruminating about this makes me want to make one. Eric
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Andrew, I didn't know Cliff meant THAT small. If I had to measure accurately a

1mm diameter hole to a .0025mm tolerance, and had lots of money I would use an air gauge. If I didn't have lots of money I would make my own expanding ball gauge. The hardest part would be splitting it. My thinnest slitting saw blade is .2mm thick and I know thinner ones can be had so even a 1mm expanding ball gauge could be split in a home workshop. I wold think that a horologist would have the drills to put a hole through the center of the gauge and maybe even the slitting saw. Just ruminating about this makes me want to make one. Eric

-------------------------------------------------------------------------------- Be my guest Eric . Small holes I can drill, but turning then halving a 1 mm steel ball is beyond my skills.

Just out of curiosity, as I don't have lots of money, what is an air gauge in this context?

Cliff.

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It also made me curious. The following pdf has a detailed description from p5 onwards but will require more study for me to fully understand!

Alan

• posted

It also made me curious. The following pdf has a detailed description from p5 onwards but will require more study for me to fully understand!

Alan

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Thanks Alan. It looks to be a versatile system to measure a variety of small apertures using the flow/pressure relationship, however I don't see that it can measure whether a hole is uniform or even round.

The search continues...

Cliff.

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Greetings Cliff, An air gauge, if rotated in the hole, will show out of roundness. The gauge probe in this case is not a perfect cylinder but instead is either slightly rectangular or has the side opposite the air hole relieved so that the gauge uses three contact points. Two points are solid and actually contact the surface while the third point is the air jet coming out of the probe. Like using VEE block and an indicator to measure out of roundness. Eric

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Greetings Cliff, To make a small gauge like this is no that hard. I just made one with a file and some sand paper. I didn't spilt it but I could. I just wanted to see if my idea would work. I used a #60 drill shank. The drill was dull so I cut the soft shank off and used a file to taper it back about 5mm. So the shank is .039" diameter that necks down to .030" and then tapers out to .038" at the end. I used the file then to put a radius on the .038" end and then used 600 grit sandpaper and

1500 grit sandpaper to polish the radiused end. The radius is small, maybe .004". Then I drilled it with a .30mm drill. Now all that need to be done is split it and make a tapered rod that I can pull through to expand the end into the hole. But I'm not gonna go that far. Eric
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Look on p8 "Straightness": "As the plug is passed through the part both diameter and any lack of straightness are clearly diplayed on their respective meters."

I'm up to p31 out of 72 so far- still no mention of price but as "Affordable" crops up under features I assume they are horrendously expensive.

Alan

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A variation on that would be to use something that solidifies - maybe a low temp alloy melted and poured in then the plug pulled out and measured. Does assume the heart won't damage the item

Guess you could use polymorph plastic possibly

Dave

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