Questions for you skilled machine guys

[Lucky Shooter]

When measuring hole diameters with small hole gages or telescoping gages----how close can you measure to actual size ? I like the idea of pin gages but don't have a full set.

Small hole gages are a wild card for me----can make them read just about anything.

I've thought about getting a few ring gages to help me develop the feel to get the right answer.

Any suggestions on how I can learn this skill ?

Thanks.

A. Weldy

 

[Toolbreaker]

With practice, telescopic gages can used with confidence to very close tolerances (.0002"-.0003" depending on bore size). Taking multiple readings at different angles and depths within a hole is good practice. Also, it's much easier to get a consistent reading on a hole with a good finish (bored, ground, lapped, ect) vs. a rough (drilled, cast) finish. I've never really trusted the split-ball type small hole gages as they spring so easily. Somewhere around .0015"-.002" is as close as I'll trust myself with them. Any smaller than about 1/2", I usually just check with gage pins.

 

[lb-ft]

Practice with the T gauges, you’ll get there. A good exercise is to take a bunch of readings on a known bore until you’re able to replicate again and again.

Alternately, you can make a go/no-go pin gauge on your lathe if you have one available.

Lastly, a tri-mic will read with resold to .0001 with practice. They are pricey though.

 

[lb-ft]

“resolution” not “resold”

 

[ridgeway]

Inside micrometer? They get called many things... Bore mic, intramic, hole micrometer. They get you pretty darn close on a hole that is pretty round. Smaller ones usually read in tenths and bigger, 2 tenths.

 

[liljoe]

You can use a neck bushing to practice with also.

Joe

 

[AccelR8]

Small hole gauges, expanding ball type, I can get within .001" I guess.

Practice and developing a feel is important. You need to develop your feel for the gauge in the hole, and then feel for measuring the gauge with the micrometer. Get some things with known hole diameters, the neck bushing suggestion is a great idea.

"Light drag" is the term I use, you slide the gauge in the hole, start out too small and increase the gauge size little by little until you can detect the slightest friction or drag. As you know, it's easy to deform the gauge by making it too big and forcing it. They're springy. Same thing with the gauge and mic, you can easily squash the gauge a little bit with the mic. One guy who taught me insisted on not using the mic's clutch but turning the thimble directly and going by feel only. He believed a friction or especially a ratchet clutch would over tighten the mic.

I was an instructor for a time. I would pass around the class of 12 guys a part with a bored and honed little valve bore in it, a small hole gauge and a mic. Ask them to measure the hole and write down their findings. Invariably, there would be 12 different measurements. The point is, these things are difficult to use and require finesse.

 

[Lucky Shooter]

Thanks or all the replies.

Its been my concern that if I am to assemble something with any degree of precision, I must have confidence in my measurements.

Its winter now and improving the accuracy of my measurements is an indoor project.

A. Weldy

 

[gunsandgunsmithing]

https://www.amazon.com/dp/B082TW8GH2/?tag=accuratescom-20

 

[Roeder]

Inside micrometer? They get called many things... Bore mic, intramic, hole micrometer. They get you pretty darn close on a hole that is pretty round. Smaller ones usually read in tenths and bigger, 2 tenths. View attachment 1155295

Intramikes or Tri-Mic; they are probably the best and most accurate small hole Mics. The one pictured is pretty big, probably about 2", but they can be had down to under .200. Also very pricey.

 

[butchlambert]

I think pin gauges are lesser expensive, but remember a .250 pin will not fit a .250 hole.

 

[TAJ45]

Thanks or all the replies.

Its been my concern that if I am to assemble something with any degree of precision, I must have confidence in my measurements.

Its winter now and improving the accuracy of my measurements is an indoor project.

A. Weldy

Allen, Allen - stop obsessing, send me THE 223 and put a rear end in the recliner. That's all the practice you need this winter. It will be OK, I promise.

 

[Lucky Shooter]

OK Tom.

I'd laugh at you but that might be bad manners------she ain't a 223.

Here's hoping your medicine works.

A. Weldy

 

[TAJ45]

Dadgum CRS....yeah, it ain't. But, you can laugh, just as long as you don't point.

 

[Lucky Shooter]

Come on down Tom, you can shoot it and I'll load for you.

We might even get our buddy to pay for lunch.

A. Weldy

 

[TAJ45]

That is an offer I have to refuse for the nonce. However, I would like to take a dee luxe rain check.

 

[jdh47]

Pin gages come in plus and minus sets so if you get them you need two sets. They will come marked with size and tolerance. For barrel work Deltronic Pin Gage sets are really nice. It just amounts to how anal you want to get with it.

 

[tobnpr]

If you're just looking for a measurement (and not needing to indicate), Deltronic makes gage handles that get fitted with bushings, looks similar to reamer pilots. The bushings are only a couple of bucks each, available in tenths:

https://www.deltronic.com/literature/Deltronic-Gage-Guide-2018.pdf

 

[hogpatrol]

CMM, take a minimum of three points.

 

[TAJ45]

CMM, take a minimum of three points.

Yah, I always take my portable suit case Range CMM w me to check neck expansion coefficients when I'm working up loads.