Measuring an ID with gage blocks (not gage pins)

B

Bill Kahn

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I have found measuring an ID is way less precise than measuring an OD. I have used my calipers, and snap gages followed with calipers or micrometer. I can reliably get within .005", but not the .001" I can with OD.

I guess folks will use a gage pin set to do better. I do not have a set.

BUT, I do have an (economy) set of gage blocks. So, I have clamped two of them together with a small parallel clamp, twisted them in the calipers to measure the diagonal (the nicely chamfered edges ensures the Pythagorean Theorem doesn't work), and slide them into the hole to be measured.

I guess I am mis-treating the edges of the gage blocks, but, this approach seems to be a frugal path to better ID measurement.

Is this a standard hack? Any reason not to use? Any other suggestions for better ID measurement?

Thanks.

Cordially,

Bill
 
Bill, I have always just used telescopic gauges (assume what you are calling a snap gauge) for smaller holes and an inside micrometer for anything larger than 1" dia. and have had good luck getting a good reading. When using the telescopic snap it open on an angle in the hole, then tighten and "pull" it around to get it to the hole size. Feel how tight you had to pull it around based on how much you tightened it and that is how hard you need to pull it through your mic when reading to get the size. With the right feel it should be the same. With the inside mic set your mic and move it through the bore the same way going side to side ensuring you have it truly centered then pulling it through, need to get the same feel in the bore and in the mic for an accurate reading. Hope that helps.
 
If you tighten the telescoping gage quite snugly and rock it over center in the hole only once and do not try to "feel around" with it and then mike the result, you should be able to get measurements within tenths of a thousandth. I had to educate several employees who had been mistakenly taught the "feel around" method by supposedly knowlegeable apprentice teachers at the local Navy Yard.
The way to measure bore using gage blocks is to get an accessory set with circular projections that overhand the blocks and can be inserted into the bore a short distance.
If you have a decent dial or other caliper, you should be able to get reliable bore measurements within a couple of thou. Insert the nibs into the hole and put pressure against the movable jaw and wiggle it a bit to find the maximum measurement. For this use, I like dial calipers best as the measurement does not "hunt" around like with electronic caliper does. I have had several Mitutoyo dial calipers (and others) that I had to carefuly stone the nibs off to get inside and outside readings to coincide, it seems like the makers make them oversize to allow for wear, by perhaps a couple of thou.
 
I don't recommend you clamping gauge blocks together - they should 'ring' -that is by twisting and pressing they hold together quite well on their own. (it isn't by vacuum pressure, but by a surface effect on a molecular level, much like how a gecko holds to a wall)

Bu once you get two blocks together you lose all accuracy doing this on the diagonal. they can slip, changing the diagonal distance. Even when clomped (not recommended) with parallel clamps...

Learning to use snap gauges is an essential skill that is worth developing. Everybody, including professional tool and die makers, needs to take several readings using snap gauges until you are convinced that you reading is accurate. I usually take 3, but sometimes up to 6 readings. They may all vary, but by doing this and practicing you can easily use snap gauges to 2 tenths of a thousandth of an inch.
 
It's something you get a feel for. I can work to a couple of thou with decent verniers, tenths with snap gauges. Did take me a while to get good technique, though.

If you want some pointers on excellent snap gauge technique, look at some of Abom79's old videos on YouTube.

Once you get good technique, it's really hard to not be accurate and quick with snaps, but I applaud the ingenuity with the blocks!
 
Bill Kahn said:
I have found measuring an ID is way less precise than measuring an OD. I have used my calipers, and snap gages followed with calipers or micrometer. I can reliably get within .005", but not the .001" I can with OD.

I guess folks will use a gage pin set to do better. I do not have a set.

BUT, I do have an (economy) set of gage blocks. So, I have clamped two of them together with a small parallel clamp, twisted them in the calipers to measure the diagonal (the nicely chamfered edges ensures the Pythagorean Theorem doesn't work), and slide them into the hole to be measured.

I guess I am mis-treating the edges of the gage blocks, but, this approach seems to be a frugal path to better ID measurement.

Is this a standard hack? Any reason not to use? Any other suggestions for better ID measurement?

Thanks.

Cordially,

Bill
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Bill,
I feel your frustration, we all go through it, but don't give up on the telescoping gauges. Like any of the precision measuring tools we use, it takes practice and a learned feel to get consistent readings. Not something that comes from the tool, it is us.
Practice and develop that feel, you will be glad that you spent the time and frustration to get there. There is no better feeling than taking a measurement twice and get the same answer.
 
When I know I'll be boring several same-sized holes I make a tapered plug gage, then use my outside calipers (or mike) to mark it with the location of the ID I'm shooting for. It doesn't take much time or material to make one. Two marks for your acceptable min/max ID and you have a go/no-go tester. I'm talking a Sharpie mark, not scribe lines, so there won't be raised edges from the scribing.
 
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