# Any tricks for measuring the inside dia of a shallow bore or pocket?



## dbb-the-bruce (Feb 9, 2021)

I'm stumped. There must be some tricks to that make it easy (or easier) to get an accurate inside diameter of a shallow bore.
I've got a part that I want to turn a shallow 1" dia. bore into. The depth is only about .050 and it's too shallow for the telescoping bore measuring tools.

When I try to use the pointy tips of calipers I get readings that vary +/- .003 because it's hard to find the true diameter with the caliper tips.

Is there some special tool or a clever trick for getting an accurate measurement in this case?

I did try a short piece of 1" drill rod and was able to use little bits of plastic shim to determine that I over shot it to 1.003, but there has to be a better way to do this while turning it.


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## mmcmdl (Feb 9, 2021)

Three prong mics . They are quite expensive but they can measure bores almost as accurate as dial bore gages . They may show up as tri mics on the Bay sometimes . You should be able to get within .0005 with a good set of calipers though .


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## dbb-the-bruce (Feb 9, 2021)

mmcmdl said:


> You should be able to get within .0005 with a good set of calipers though .


I find using a typical dial/digital caliper is just way to fussy in regards to position and not loosing the measurement. It's really hard to get a repeatable measurement.

I didn't have much luck searching for "three prong mic or indicator" there must be a better name for them.

I did find dial bore gages, Mitutoyo has a special probe tip that can measure with in 2 mm of the bottom of a hole. Also pricy and limited range. The other speciality thing I found was shallow bore gages - very expensive and seems like they are for large 12" holes at the smallest.

The shallow bore gage gave me the idea of two pins, one fixed on movable in a slot and lockable that I could make. The pins would be used to transfer the measurement and make it easy to measure with calipers.

I'll keep looking. 

I did recently by a full set of pin gages from .25 to .06 - They have made checking and matching smaller holes *MUCH* easier. really glad I got them.

Maybe I'll splurge on and inside dial caliper.

Thanks for the suggestion.


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## dbb-the-bruce (Feb 9, 2021)

This looks interesting: https://www.mscdirect.com/product/details/06453013

Looks like I could use an indicator I already have. I could also make tips/probes that would go to the bottom.


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## mmcmdl (Feb 9, 2021)

You need it ? $75.00 It's called an indical . I'm holding one in my hand .


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## dbb-the-bruce (Feb 9, 2021)

mmcmdl said:


> You need it ? $75.00 It's called an indical . I'm holding one in my hand .


Yeah! that would be awesome. Thanks!


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## mmcmdl (Feb 9, 2021)

Stay put . I'll get a pic ! You do have a dovetail indicator , correct ?


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## dbb-the-bruce (Feb 9, 2021)

Yes, I've got couple.


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## mmcmdl (Feb 9, 2021)

OK , hold on a minute . Pics to follow .


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## Chewy (Feb 9, 2021)

Turn a piece of rod to a couple of dimensions, like .998, .999 & 1.  Take your time and get them perfect.  When the .999 goes in and the 1 is hard to start, then you are there.  Basically you are making gauge pins.  If you have money to burn, you could buy a set or use a 1" round gauge block.  My round gauge block came from Ebay in a junk assortment.  The Indicol mentioned above took me a couple of months to get at $60.  New is around $180.


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## mmcmdl (Feb 9, 2021)

Kinda look like these ?


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## dbb-the-bruce (Feb 9, 2021)

Chewy said:


> Turn a piece of rod to a couple of dimensions, like .998, .999 & 1.....


I tried this (sort of) as I had a chunk of 1" drill rod that measured at .999. But I overshot and then it was hard to figure out by how much other than using plastic shim once it was off the lathe. I think a tool that will let me cover a range easily is the way to go.


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## dbb-the-bruce (Feb 9, 2021)

mmcmdl said:


> Kinda look like these ?


Looks great to me. How do we exchange addresses?


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## mmcmdl (Feb 9, 2021)

Incoming pm . They will go out tomorrow if PP is recieved before 8 am . I have a few more boxes going out . How's that for service ?


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## RJSakowski (Feb 9, 2021)

For a .050 deep counterbore, I would make a disk around .75" in diameter and .050" thick to sit in the bore.  Then I would fit gage pins to the gap.  Th3e bore diameter would be the sum of the two diameter.  It should get you within a thousandth.  For deeper bores, I just use two  gage pins but it would be a pain to try to manage two pins in a shallow bore,; hence, the disk.


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## dbb-the-bruce (Feb 9, 2021)

RJSakowski said:


> For a .050 deep counterbore, I would make a disk around .75" in diameter and .050" thick to sit in the bore.  Then I would fit gage pins to the gap.  Th3e bore diameter would be the sum of the two diameter.  It should get you within a thousandth.  For deeper bores, I just use two  gage pins but it would be a pain to try to manage two pins in a shallow bore,; hence, the disk.


Yeah that could work nicely. Maybe I'll make a few disks in 1/4 increments and keep them with my pins.


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## benmychree (Feb 9, 2021)

Ask yourself why does the feature need to be overly accurate?


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## Winegrower (Feb 9, 2021)

You can’t use a cheap inside caliper to make a transfer measurement?


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## erikmannie (Feb 10, 2021)

What about pin gauges?


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## Larry$ (Feb 10, 2021)

dbb-the-bruce said:


> I did recently by a full set of pin gages from .25 to .06 - They have made checking and matching smaller holes *MUCH* easier. really glad I got them.


A couple of years ago I bought sets from .011 to .500 in under sized by up to .0002. I use them more often than I had expected.  By using two pins in a bore I can get to .999 - .0000 to -.0004.   I just made a "sort of" gage pin @ .900 +- a fuzz by turning it a bit over size then using my home made tool post grinder bringing it to as close as I could measure. So now I can bore measure accurately (enough) to 1.4". All done by the feel of sliding two pins into the bore. Since using the smallest pins is very difficult I decided not to go to a full 1", that and my smallest pin is.011 which is really fragile and would make the first step up from the new gage pin jump too much. I'm thinking now I should make several more larger pins. You can also buy individual large size (to 1") pins.  Class Z ( 0.0001) about $14 each @ McMaster-Carr.  Or you can get a pair of go-no go pins for 1" @ about $100 for the pair. Ring gages are even more expensive.


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## Technical Ted (Feb 10, 2021)

Winegrower said:


> You can’t use a cheap inside caliper to make a transfer measurement?
> 
> 
> View attachment 354941



I have had very good luck using calipers was winegrower posted. They can be surprisingly accurate! Well, let's put it this way... they will be as accurate as the person using them.

Ted


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## dbb-the-bruce (Feb 10, 2021)

Technical Ted said:


> I have had very good luck using calipers was winegrower posted. They can be surprisingly accurate! Well, let's put it this way... they will be as accurate as the person using them.
> 
> Ted


And @Winegrower 

I did try this method also.
It was definitely better than using standard dial/digital calipers, but still fussy. In addition to getting the diameter points exact, the outward spring of the calipers makes it hard to determine exactly where you have diameter and NO spring.

Too be clear, it's the shallowness of the hole that is the problem here. Inside calipers typically have a radius end that exceeds the .05 depth or at least makes so you are trying to accurately engage the top .01 or less of the walls. I guess grinding the tips to be pointier would help but then it's harder to make the transfer measurement from the tips.

I'm really fighting the shallowness of the hole and lack of wall to measure.


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## Technical Ted (Feb 10, 2021)

Here's a trick I've used. Leave the part longer than needed on the end so you can bore a deeper hole. Leave it enough longer than needed so you can measure the bore more easily. When finished with the bore, face off to desired finish length.

Ted


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## ChazzC (Feb 10, 2021)

There's a couple on eBay right now: one for $90 (eBay item 274670776150) and one for $140 (eBay item 193858684259), both plus s/h.


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## BGHansen (Feb 10, 2021)

How about an inside micrometer?  You can pick up an import 0.2" - 1.2" for around $50.

Bruce


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## Iceberg86300 (Feb 11, 2021)

Here is a three pronged, i.e. "Tri-mic" / "Tri-mik" / "Intri-mik" / "Holtest" 

I really love these for repeatability b/c it's tough to screw up. Not all will bottom though, so be careful of that fact.





(Photo is brown sharp NOT the spi in link)



			https://www.mscdirect.com/product/details/00342865
		


And here's one of the type Bruce posted a pic of:



			https://www.mscdirect.com/product/details/38169108
		




Edit: for your specific application I'd go with the latter. The tri mics are awesome but it's gonna be tough to find one that will go within .05" at anything approaching reasonable cost. Their main use is repeatable measurements deep & DEEPER with extensions, like feet deeper, into bores. 

The latter style is generally for the top portion, say ~1/4"-1/2" deep max, of bores less than 1" in diameter, and ~1"-2" deep max in bores larger than 1" in diameter. Depending on brand, of course.

Regards,

Steve

Sent from my SM-N975U using Tapatalk


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