Case neck turning

[chas442]

I apologize if this has been discussed. I was unable to locate any info on the site.

I am converting 7.62x39 brass to 6.5 grendel. I have sized and ready for firing. I measured neck wall thickness to vary between 0.010-0.014" on all cases measured. Not surprising since it was necked down. Have started turning the necks to a consistent 0.009-0.010". I have attached a photo that shows how far down the neck I have turned down. Is it adequate or do I go down to the radius of the neck and shoulder interface?

TIA

 

[Walkalong]

Cut ever so slightly into the shoulder

 

[243winxb]

The cutter angle should match the shoulder angle, when removing a lot of brass. https://www.accurateshooter.com/gear-reviews/neck-turning-lathe-from-21st-century/

 

[Varminterror]

That photo does not appear to have advanced enough down the neck.

It’s great when the cutter angle matches the shoulder angle, certainly, but it’s not a pure necessity. The difference is a smoother appearance, but we end up facing part of the shoulder to get the neck cut fully.

In a manner of describing this, the Neck turning cut needs to advance sufficiently down the neck and slightly into the shoulder that the entirety of the neck, relative to the inside surface, has been turned to the target thickness. So when the OUTSIDE surface of the brass is making that 30-40degree bend at the shoulder on the OUTSIDE, recognize the INSIDE surface of the neck is still tracking straight for several thousandths farther, so stopping where the OUTSIDE meets the shoulder will leave a thickened doughnut at the base of the neck, lurking on the outer dimension, which will move to the INSIDE upon firing and resizing.

 

[Blue68f100]

Here is an example, one I did for 224V.

 

[Walkalong]

An outstanding sample pic

 

[jmorris]

Cut ever so slightly into the shoulder

^ this because if you don’t go low enough the extra material on the outside, after firing will migrate to the inside of the neck, as the case is blown out to fit the chamber. So it has a smaller ID at the neck/shoulder junction, than the rest of the neck.

Called a “donut”.

 

[AK Hunter]

Wouldn't it be better to ream from the inside that cut from the outside?

 

[JFrank]

I have a tight neck (268) chamber that requires turning 6 br virgin brass 30* prior to forming into 6 br-Ackley 40* which is speced out .004 longer at the neck shoulder junction to create a natural crush fit, I cut about one revolution into the case shoulder junction, to go any further the cut becomes part of the shoulder.
I’m not sure if the OP used a cutter angle that matches the existing shoulder angle, it just doesn’t look quite right. I shoot the first one and make sure that’s what I want before cutting the rest.

 

[chas442]

Thanks for the input.
I searched the internet and decided to check with the experts. I was concerned that hitting the shoulder would weaken the brass excessively.

 

[Walkalong]

Wouldn't it be better to ream from the inside that cut from the outside?

I used to ream, then turn when making 6PPC cases.

Reaming donuts is the way to go, but it's best to avoid donuts if you can.

 

[Varminterror]

Wouldn't it be better to ream from the inside that cut from the outside?

Reaming is better in theory, but has complicating aspects in practice.

Neck turning, we are better able to ensure consistent thickness against the mandrel, whereas when reaming, we have more work to do to ensure the case is pressed against the supporting die - and naturally, turning and confirming a mandrel is a lot easier than doing so for a die bore, and then also hope the reamer is supported concentrically…

So it’s easier to neck turn, and it’s not so difficult to pay close enough attention to avoid creating a doughnut by not turning far enough.

 

[Walkalong]

Not so complicated as to be impossible.

 

[JFrank]

From my perspective it appears the cutter angle doesn’t match the existing shoulder angle. I wonder if this is a good candidate for the dreaded doughnut to form ?

 

[Varminterror]

View attachment 1124202 From my perspective it appears the cutter angle doesn’t match the existing shoulder angle. I wonder if this is a good candidate for the dreaded doughnut to form ?

The OP’s picture is EXACTLY how doughnuts are created.

Lower neck doughnuts can also appear when thicker shoulder brass migrates to the neck via body stretch, but neck turning as pictured above is a surefire way to create a doughnut immediately.

 

[chas442]

Here is a photo of my most recent neck turning effort

 

[JFrank]

Here is a photo of my most recent neck turning effort

Those look better

 

[chas442]

Thanks

Does it look adequate?

 

[JFrank]

Thanks

Does it look adequate?

I’m no expert , I would say yes but not to advance any further.

 

[chas442]

I guess it's time to hit the range and run off a few. Going to start with low powder charges/
Is there anything I should look for besides blown cases?

 

[jmorris]

You know, if you have a gauge pin set it might be a learning experience if you shoot the first one along with the others.

 

[243winxb]

Is there anything I should look for

Hard chambering if the necks outside diameter is to large for the chamber.

 

[AJC1]

^ this because if you don’t go low enough the extra material on the outside, after firing will migrate to the inside of the neck, as the case is blown out to fit the chamber. So it has a smaller ID at the neck/shoulder junction, than the rest of the neck.

Called a “donut”.

View attachment 1124080

Will natural brass migration cause donuts regardless if one turns brass or not. Seems like those that do the work of measuring find the problem and those that don't remain oblivious.

 

[jmorris]

Will natural brass migration cause donuts regardless if one turns brass or not. Seems like those that do the work of measuring find the problem and those that don't remain oblivious.

It certainly can, lots of variables at play.

No doubt some are oblivious and just know that their “load” seems to fall apart as the number of firings increase. The velocity might go up, ES becomes larger, measured run out might grow. Then they replace the brass and everything comes back together.

 

[AJC1]

It certainly can, lots of variables at play.

No doubt some are oblivious and just know that their “load” seems to fall apart as the number of firings increase. The velocity might go up, ES becomes larger, measured run out might grow. Then they replace the brass and everything comes back together.

It seems that there would be senerios that generate this problem more or more often. Any situation that causes growth of the case faster would be one that's easy to monitor. The easy poke would be gas guns, or sizing combinations that move a lot of brass and result in more or faster growth. This would be discovered or tracked in trimming.