# Some imitational flattery



## mklotz (Sep 11, 2009)

I was so impressed with George Britnell's gage for measuring bore sizes

http://www.homemodelenginemachinist.com/index.php?topic=5015.msg51407#msg51407

that I knew I had to make one of my own.

[Aside: Actually "impressed" is far too mild a term. After seeing it, it's just so obvious. We use a DTI that way on a height gage. How could I have been so dumb as to not see it myself. I'm annoyed with myself for not having thought of it myself.  ]

I'm a real connoisseur of fine tools but I go absolutely gaga over homemade tools that aren't available in the market place. In fact, I was cutting metal the same day I saw George's post.

The first photo shows my efforts. Not quite as pretty as George's but functional nevertheless.








and a side view of the gage.






This tool cried out to me for a vernier fine adjustment and, feeling the need to personalize the project, I added one. It acts similar to the sort used on beam compasses but, instead of a simple threaded shaft uses a differential screw.

As is apparent in the next picture, the differential screw is threaded with two different threads. The body that threads into the vernier guide is 1/4-40 and the end which threads into the movable arm is threaded 10-32.

With this arrangement, one turn of the vernier adjustment knob will move the movable arm by:

1/32 - 1/40 = 1/160 = 0.00625"

the equivalent of a 160 tpi thread. With six notches on the knob, a one notch movement is very close to one thousandth of an inch movement.






The next photo shows the dovetail that secures the DTI in the end of the rod. To make it, I slotted with a 3/16" endmill to the correct depth and then cut the dovetail with a 1/4" dovetail cutter that I purchased from Brownells, the gunsmith suppliers in Iowa. Such tiny cutters are used to cut the dovetails used to secure the (adjustable) front sight on some types of firearms.

http://www.brownells.com/






[Another aside: As made, the adjusting screw is backwards. A clockwise turn of the screw will move the movable arm toward rather than away from the vernier guide. Were the 32 and 40 tpi sections reversed, this would be rectified but I was using dies and wanted a one piece screw so accepted the reversed movement which doesn't annoy me anyway. However, be warned if you make your own differential screws - think it through first.

If you want help with the mathematics of designing differential screws, there's a program on my page titled, surprisingly, DIFFTHRD that will be of assistance.]

Thanks for the idea, George. It's a real winner.


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## gbritnell (Sep 11, 2009)

Thanks Marv,
I don't know if it's unique today but at the time I built it there wasn't anything like it available. I often think about going back and doing what you did and make an adjusting screw to move the arm but you know how that goes. You did a fine job on yours and you'll be very pleased with all the applications you can use it for. 
George


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## GailInNM (Sep 11, 2009)

Very nice Marv, and a well done to GB also.

As a hint, for calibration to a standard size, both inch and fractional, ball bearings work well. Even the lowest class,ABEC1, has a bore tolerance of +0/-0.0004 in ID's of less than an inch. I know you can calibrate with a micrometer, but it always seems to take one more hand than I have.  

Ball bearings also make a good practice piece for persons learning how to use split ball and telescoping gages. Pick one and practice with it until you start to get repeatable readings. Practice is the only way to get the "feel" of how to use them.

Gail in NM


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## lugnut (Sep 11, 2009)

Both you guys deserve a big hand for your measuring tools! I guess I missed George Britnell's gage post. 
Something else to build. I think building tools is one of my greatest pleasures (there are others, but not here) Thanks to you both
Mel


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## d-m (Sep 11, 2009)

Hi all :-[
OK either I missed some thing or I'm a D. A. could you plz explain the method you use with this tool to get the end results. I would think you would set the ends into the hole and move till you see movement on the DI but were do you measure the tool to get your total dim.


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## Lew Hartswick (Sep 11, 2009)

Another toy to occupy the time between classes at school. 
If I can get all the projects the teacher has lined up for me done.
  ...lew...


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## Deanofid (Sep 12, 2009)

Good work, Marv. The practice of imitational flattery is a good way to end up with a shop full of great tools. 
I like your use of the differential thread! Something else for me to imitate, along with George's idea.

Dean


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## gbritnell (Sep 12, 2009)

Hi d-m, 
My design was strictly a reference gauge, put it in the hole, adjust to a reading, withdraw tool and mike over the balls. The original intent was for measuring small slots and holes that were to shallow to get a telescoping gauge into. Marv has refined it to the point that once the tool is set to a given diameter the accuracy of the threads can be relied upon for taking a direct reading.
By the way, no DA's around here. If you don't ask you'll never know why.
George


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## d-m (Sep 12, 2009)

Thanks George for clearing that up . And a nice tool build, to you and Marv
Dave


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## kvom (Oct 13, 2014)

If I read the description correctly, to use it you extend until the DI moves slightly, then set the dial to 0.  Then when measuring with a micrometer, the anvil moves the DI until it's back to 0.  Correct?


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## gbritnell (Oct 13, 2014)

That's correct.
gbritnell


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