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Offline jls

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three grooves
« on: October 08, 2010, 04:43:00 AM »
trying to measure the groove diameter on a 1868 sharps 50-70 with the 3 groove liner in it. any tips on getting an accurate read with my mikes? a long time ago I came across an article using a v-block and a formula but that went the way of the buffalo. the odd # of grooves makes it a bit tough.  ???     .
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Deceased due to a 3rd stroke on Dec. 12, 2011.

RIP John.

Offline tacklebury

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Re: three grooves
« Reply #1 on: October 08, 2010, 02:55:04 PM »
Well, there is such a thing as a 3-point inside micrometer/caliper.  These aren't cheap, but if you've got a Metalsmith shop near you they might have one, or a university metal shop.  8)  They are pretty range specific, like .2" to .4" and kinda pricey.  Some have interchangeable shafts for different diameters.  8)

Just an example picture:
http://image.made-in-china.com/2f0j00AemQTEWwMCoy/Three-Point-Inside-Micrometer-Blind-Hole-.jpg
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Tacklebury --}>>>>>    Multi-Barrel: .223 Superlite, 7mm-08 22", .30-40 Krag M158, .357 Maximum 16-1/4 HB, .45 Colt, .45-70 22" irons, 32" .45-70 Peeps, 12 Ga. 3-1/2 w/ Chokes, .410 Smooth slugger, .45 Cal Muzzy, .50 Cal Muzzy, .58 Cal Muzzy

also classics: M903 9-shot Target .22 Revolver, 1926 .410 Single, 1915 38 S&W Break top Revolver and 7-shot H&R Trapper .22 6" bbl.

Offline jls

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Re: three grooves
« Reply #2 on: October 08, 2010, 05:19:21 PM »
thanx TB, theres a gunsmith college near me and I'll see if they have something. I don't want to spend that kind of dough on something I likely won't use but once.
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Deceased due to a 3rd stroke on Dec. 12, 2011.

RIP John.

Offline tacklebury

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Re: three grooves
« Reply #3 on: October 08, 2010, 05:33:37 PM »
Well, you could get really close by using the calculation for a circumscribed triangle geometrically.  If you measure from the center of 1 of the lands to the center of another land that would be 1 leg of a triangle (call this S for side) connecting all three land's centers.  If you use the formula: S/SQRT(3)*2, you will have the diameter of your lands.  ;)  This will get you very close depending on how accurately you eyeball the center of the land.  Maybe using a fine tip sharpie and a divider, you could determine the dead center pretty accurately.  8)

***edit***

I actually mispoke in the above post, you really want groove diameter, not land.  Use the calculation above with the center of the grooves.  Sorry, getting tired I guess.  ;)
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Tacklebury --}>>>>>    Multi-Barrel: .223 Superlite, 7mm-08 22", .30-40 Krag M158, .357 Maximum 16-1/4 HB, .45 Colt, .45-70 22" irons, 32" .45-70 Peeps, 12 Ga. 3-1/2 w/ Chokes, .410 Smooth slugger, .45 Cal Muzzy, .50 Cal Muzzy, .58 Cal Muzzy

also classics: M903 9-shot Target .22 Revolver, 1926 .410 Single, 1915 38 S&W Break top Revolver and 7-shot H&R Trapper .22 6" bbl.

Offline 26-t

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Re: three grooves
« Reply #4 on: October 09, 2010, 04:46:17 AM »
Use a set of thread wires, 3 wires of a known dia. the same way that thread grooves are measured. Do a google search to see how it is done. It is not as hard as it sounds. 26-t
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Offline gunnut69

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Re: three grooves
« Reply #5 on: October 11, 2010, 07:04:01 AM »
A fairly cheap plug guage set will get you the bore diameter. To find the groove diamter measure one of the grooves between two of the lands. Then measure the adjacent land. Measure both from the outside of the barrel. For accuracy's sake measure the lands on both side of the groove, then do all three grooves. The average of these measurements will be pretty close..
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Offline goofyoldfart

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Re: three grooves
« Reply #6 on: October 31, 2010, 07:15:52 PM »
on an odd number of grooves--try wrapping a 1 or 2 thousands feeler gauge around the circumference of the slug and mike with a regular micrometer getting the overall diameter then subtract the thickness of feeler gauge X2. that will get you very close ;D
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Offline Screwbolts

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Re: three grooves
« Reply #7 on: November 01, 2010, 01:54:14 AM »
There used to be an add in the "Fouling Shot"  from a guy that would measure slugs from the bore for you and let you know what the bore was.  You just have to send him some slugs to measure. Look at some of the back issues. He specialized in the odd #s. I prefer the Odd numbers for rifling.   :)

Ken
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