Coup De Grace measurement... how?

[LVLAaron]

How would a person reliably measure "2.0217-.2887 x DimA"

 

[Tucker65]

Make a tube that is bored to seat up against the barrel shoulder on one end and big enough for the ball to fit in on the other end. Make the tube longer than dimension A. Then you can use a depth mic to measure from the face of the tube to the ball and do the math to come up with dimension A.

 

[LVLAaron]

Good idea. Gonna have to be a stepped collar/tube

 

[Doug Beach]

Would simply cutting a partial cone, then threading the action on, bolt included, until it stops, work? Then you just measure the gap between receiver face and action face, add your desired clearance, and cut the cone that much deeper. Works on any other cone breech I’ve seen, but I’ve never handled a Coup de Grace.

 

[WSnyder]

If you have a DRO on your lathe touch off both surfaces.

 

[Rank Amateur]

Assuming that is the diagram of a barrel chamber/tenon? If so, why not cast the chamber and just measure?

 

[DaveTooley]

I don't see how A has anything to do with a 1.250" radiused CB. Just an evil engineer.

 

[LVLAaron]

Would simply cutting a partial cone, then threading the action on, bolt included, until it stops, work? Then you just measure the gap between receiver face and action face, add your desired clearance, and cut the cone that much deeper. Works on any other cone breech I’ve seen, but I’ve never handled a Coup de Grace.

Works great, if you have the action in hand.

 

[LVLAaron]

I don't see how A has anything to do with a 1.250" radiused CB. Just an evil engineer.

You're right. I'm running on fumes today. I'm looking for the "2.0217-.2887 x DimA" measurement, not DimA.

 

[DaveTooley]

Do the math. Not much change for a few thou change in A. How are you cutting the radius? Err on the side of more clearance.

 

[LVLAaron]

They actually give the exact numbers needed. I just saw the ball bearing and wondered if I was missing out on some neat machinists trick I hadn't seen before.

 

[Doug Beach]

Works great, if you have the action in hand.

Aha. Yes, yes it does.

 

[DaveTooley]

If you have a DRO on your lathe touch off both surfaces.

I have kept a 2" travel indicator on every manual machine I've owned

 

[DaveTooley]

.822"-.760= .062"
.147"-.062"= .085" at 30'
Am I missing something

 

[V35]

When I did my CDG in July, I touched my DTI on the shoulder, zeroed the DRO and a dial ind on the apron, backed the crosslide off, and went to the ball and ran the DTI up on the ball and touched it. Both the DRO and dial ind agreed. As for DIM A, I just used sammi chamber spec for 223, I cut a 223AI chamber. Worked out good. Great action, and I have one more to do a 6Dasher build on.

 

[jackieschmidt]

Make a tube that is bored to seat up against the barrel shoulder on one end and big enough for the ball to fit in on the other end. Make the tube longer than dimension A. Then you can use a depth mic to measure from the face of the tube to the ball and do the math to come up with dimension A.

This seems like the simple solution.. or at least that’s the way I would do it.

 

[WSnyder]

I have kept a 2" travel indicator on every manual machine I've owned

Same I do have a DRO on the larger machine.

 

[GenePoole]

.822"-.760= .062"
.147"-.062"= .085" at 30'
Am I missing something

I'm sure you've done many more cone breeches than I can ever even imagine doing, so maybe you have some insight into how to reliably gauge that 0.085 distance. There's no square surface to mic against. I think that's why they supply the gauge ball drawing as well. The ball will seat solidly against the cone and can then be measured by conventional mics.

(full disclosure, I just set that barrel on the bench up for photo)

 

[LVLAaron]

Move in .085 from the face of the breech end... use compound to cut the cone.





The problem with the 1 inch ball bearing method - The ball doesn't make contact with the cone, it rests on edge of the hole... so if there's a chamfer on your chamber... the measurement will be off.

I assume that's why ARC suggests a larger ball... but I'm bad at that kind of math.

 

[GenePoole]

Move in .085 from the face of the breech end... use compound to cut the cone.

View attachment 1483484



The problem with the 1 inch ball bearing method - The ball doesn't make contact with the cone, it rests on edge of the hole... so if there's a chamfer on your chamber... the measurement will be off.

I assume that's why ARC suggests a larger ball... but I'm bad at that kind of math.

Well, the example I showed was for a Panda barrel in 6.5x47, and I did the CAD sketch to make sure that the ball was resting against the breech cone before I did the measurement. I've confirmed it before with some dykem as well, but you are probably correct for the ARC dimensions, hence the 1.25 ball.