Vee block for measuring odd numberof lands and grooves

[Mk42gunner]

One of the Gun Digests from the mid sixties had a small note about making a vee block, with the algebraic formula for measuring bore slugs from a barrel with an odd number of grooves such as a S&W revolver or an Enfield rifle.

I thought it would be useful to make one for a few reasons:

1. It would see occasional use.

2. I'm frugal, (frugal sounds so much better than cheap).

3. I have a vise, files and some scrap steel to file one out of; plus I enjoy making my own tools when possible.

Unfortunately I cannot find the book. I have it, and I know I have it, I just can't find it[smilie=b:.

From what I remember, the angle of the vee is supposed to be 108 degrees, true? It shouldn't be too hard to layout with a protractor and a square.

Does anyone know the formula, and would you mind posting it?

Or does anyone know for sure which year it was in? I think it was somewhere around 1965 or so, give or take a few years.

Also, is one block at 108 degrees useable for multiple numbers of grooves such as 3,5,7, or do you need a different angle for a different number of grooves?

Thanks for the help,

Robert

[Nueces]

I think I have that article and made a little v-block based on it years ago. I'll see about scanning and posting the page later.

As for the proper angle, the requirement is that the lands of the measured slug be tangent to both of the anvil faces. Since these lands are generally narrow, that means a unique angle is needed for each number of grooves.

I found the little block fiddly to use and finally ponied up for a Mitutoyo v-anvil mike from ebay. Even with the mike, lead slugs are soft and easily accumulate nicks and bruises, which makes getting repeatable measurements difficult. The nicest setup would be a v block under a drop indicator.

[RedHawk357Mag]

This is what I have.

103015

thought i had more literature than what is in the jpg, will have to look around my hard drive.

[Mooseman]

A simple way is with .001 or .002 metal sheet or feeler gauge strip cut so you wrap it around the slug forming a snug tube and measure the OD, then subtract 2x the thickness of the sheet used from the reading on the micrometer...
Rich

[Sig]

Here's some info I found a while back on the subject. I managed to work with a 90 degree "V" block.103042

[Sig]

Just found this.
http://castboolits.gunloads.com/showthread.php?119952-how-I-measured-a-5-groove-bore

[Nueces]

Dass it, Redhawk! It's in Handloader's Digest #6.

[Mk42gunner]

Hey thanks for posting the .jpg Redhawk, that is the info I was looking for.

Nueces, good to know it is also in the Handloader's Digest #6. I think I have a dial indicator and stand, if it didn't walk off before I retired.

I read it several times and thought it would be handy before I decided to make one. Then when I went to find the book to verify everything, I couldn't find it.

Thanks for the help guys,

Robert

[RedHawk357Mag]

No problem. Thanks Sig for that link. Been looking for a math/calc program that is mentioned in the link. Have a great day everyone:)

[OuchHot!]

A fellow in the cba used to build 72 degree blocks for the 5 land measurement. I found by calculation that you could use a standard vee block and calibrate against a plug gage near the expected bore diameter....the amount of error from a wrong angle in the vee is trivial when you fudge your measurement from a cylinder near to expected. I don't suppose any of that made sense?

[fouronesix]

Actually it made quite a bit of sense. Fudging, estimating is fine until you run into an "engineer" or bullet caster or reloader who insists on measuring the diameter of a cinder block with a mic to the nearest .0001".

The thin metal or shim wrap-around method is good (matter of fact very precise) if you're careful with the technique. You can also measure from top of one land to bottom of opposing groove then add the height of one land (depth of one groove). Not hard to measure the groove depth by using a depth gauge while working under a loupe or magnifier.

[Mooseman]

Another trick...use 2 slugs butted up together with one turned so the lands line up with the other ones grooves, then mic at the joint. Quick and effective !
Rich

[leftiye]

Push your slugs through sizing dies until you find one that drags but still goes through. I know there are varying opinions about whether a micrometer works, but I go with the method of measuring for largest result by measuring the front of one groove (land on slug) and the rear of the opposite groove. If the lands and grooves are of the same width, these two points are opposite each other.

[leftiye]

A fellow in the cba used to build 72 degree blocks for the 5 land measurement. I found by calculation that you could use a standard vee block and calibrate against a plug gage near the expected bore diameter....the amount of error from a wrong angle in the vee is trivial when you fudge your measurement from a cylinder near to expected. I don't suppose any of that made sense?

Compare your measurement (height gauge the slug) to pin gauges - the pin gauge that gives the same measurement is the same size as the slug. (Then read size from the gauge) Since boolits of even .003 over bore size work fine (as long as the chamber mouths or freebore will acomodate them), .0001s probly aren't necessary. Could even say - don't matter.

[W.R.Buchanan]

The Vee block for 5 grooves is 72 degrees included or 36 degrees off the top surface on both sides. For 3 grooves it is 120 degrees included. The entire purpose of having these Odd Vee blocks is so the bullet contacts the Vee block at such a place that a line that is drawn thru that tangent point and the center of the bullet is exactly perpendicular to the surface of the Vee.

If you used a 90 degree Vee Block the line thru the center of the bullet and the tangent point would not be perpendicular and as a result some stupid formula would have to be used to determine the diameter of the bullet.

What you do is make the Vee first, then you put a .500 gage pin in the block and mill the opposite side down until it measures 1.000 over the pin.

Then any bullet you put in it with five grooves will be measured by measuring over the bullet to the bottom of the block, and subtracting .500. You can verify that this works by measuring other gage pins with known dimensions by the same method.

Much simpler and more accurate than a bunch of formulas you don't understand. :mrgreen:

Randy

[Mk42gunner]

What you do is make the Vee first, then you put a .500 gage pin in the block and mill the opposite side down until it measures 1.000 over the pin.

That makes sense to this non-machinist. I'm all for doing things the easy way if I can.

Robert

[OuchHot!]

If you use the "incorrect angle" you will indeed have error because you are not contacting the center of the groove impression. With this I agree and understand. If you use the nearest pin gage and measure the stack height and then the stack height with a boolit and simply add or subtract the apparent difference from the nearest pin gage...the error is trivial.

[W.R.Buchanan]

Ouch: the whole point of using the .500 pin gage or whatever size is so that you can cut the Vee block down until it is a known height. A 1.000 stack height over a .500 pin would yield a block that is .500 high.

Then by subtracting that known amount (.500) from your measurement over the boolit you get an accurate measurement of the boolit.

Example: The boolit in the block measures .812. total .812 -.500 = .312 simple.

Randy

[aap2]

Randy: That's a very clear explanation. Is there somewhere that I can buy a 72 and 120 degree block in a size useful for measuring boolits? I looked at online sources thay are available but not in the proper size. Thanks.

[W.R.Buchanan]

Probably have to be made for you. I have never seen anything like this in machine shop land.

There are specialty Micrometers made specifically to do this type of measurement. It is how they measure odd number fluted cutters.

I have two 3 flute end mills, and they are supposed to be .750,,, I have no idea what size they are in reality because I have no way to accurately measure them.

Randy

[bruce drake]

speaking of measuring 3-groove barrels. I have to slug an older 50-70 Govt Trapdoor this week to get a proper bore dimensions for a sizing die purchase later on. Anyone know the angle for the v-block for that odd-groove barrel?

[M-Tecs]

speaking of measuring 3-groove barrels. I have to slug an older 50-70 Govt Trapdoor this week to get a proper bore dimensions for a sizing die purchase later on. Anyone know the angle for the v-block for that odd-groove barrel?

120 degree

[bruce drake]

Thanks! Seems almost too simple after you consider it! Thanks for that knowledge!

[gnoahhh]

All good info, and food for the inquiring mind! But I submit, if I may, that it is a useless exercise to measure groove diameters. I quit that long ago and simply rely on throat dimensions when fitting a cast bullet to a gun. I size to .0005" under throat diameter and ignore whatever the grooves are. Life is easier and good, accuracy likewise as a result. Frank Marshall disabused me of measuring groove diameters 30 years ago, and as usual he was right.

[John 242]

I quit that long ago and simply rely on throat dimensions when fitting a cast bullet to a gun. I size to .0005" under throat diameter and ignore whatever the grooves are.

Not arguing, but could you explain the theory behind this, because it seems to run counter to most of the bullet fit theories I'm familiar with.

If the groove diameter in the last, i dunno... say 6-inches, from the muzzle in a nominal .308 barrel, opens up to .311, this doesn't affect accuracy or doesn't lead to leading with cast?

If a revolver throat is .352 and the groove diameter is .357, this isn't going to affect accuracy or cause leading?

Again, not trying to start an argument. I love learning and this is something I haven't heard before. I know of a few rifle builders that will slug a bore and either reject it if it isn't consistent along it's length or will at least make the tight end the muzzle end.

[gnoahhh]

If a revolver's throats were that grossly undersize, I would have them reamed. If the last 6" of a rifle barrel were as egregious as your example, it would indeed be conducive to leading there, not to mention I would be surprised if it were accurate. In an instance like that I would be making tracks toward a re-barreling.

A rifle's throat will be at least groove diameter, and in most instances a skinch bigger. Everybody has their own idea as to what constitutes a perfect throat. There are few bad ideas, really, as long as the bullet is matched to it. What you must remember is the throat (and leade) is what starts the bullet straight into the rifling (assuming straight seating in the case to begin with)- more important with cast bullets than with jacketed, and it's pretty darn important with jacketed too. If the bullet fits the throat (which also in turn means the bullet will also be at least groove diameter, for a good seal in the bore), and better yet with a nose that conforms to the leade of the rifling, it can't help but start down the bore straight- or reasonably so. There are a bunch of other factors to take into consideration such as lead hardness/alloy, velocity, chamber pressure/burning rate of the powder charge, bullet shape, condition of throat/rifling, etc. etc. I merely maintain that if the bullet is correctly fitted to the throat then you're well on your way toward accurate shooting.

My idea of a perfect .30 throat for cast bullet shooting, given a .308 groove diameter: a .310 throat of around 1/4" followed by a 1½º included angle leade. (If the magazine will tolerate a loaded round of the length this would likely dictate. If not, adjust back toward a "standard" throat- if magazine feeding is needed.) I have a throating reamer of those exact dimensions, and have treated a bunch of my .30's with it. I also used the reamer to make a sizing die to taper the noses of bore-riding designs to perfectly match the leade taper. Kind of surprisingly, jacketed ammo seems to shoot a bit better through such an arrangement too, but don't take that as gospel.

I hope I clarified where I'm coming from regarding my statement about knowing throat diameter versus knowing groove diameter.

[jsn]

Pee Dee wires in one of the lands on the slug. No fixtures, excessive time and cost, just center it and don't push the wire into the soft slug.