Neck Turning - Too Deep - Pics

[Martin52]

Just for information, I thought some might like to see these pics.

One batch of cases started to lose necks on firing - left pic - on the 4th reload.

I sectioned a case to show what happens if the neck turning process cuts too deep on the neck shoulder junction. In the right pic you will see a groove has developed on the inside that creates a weak spot resulting in the neck being either partially or fully dislocated from the shoulder on firing.

My solution was to change the cutter blade to closely match the shoulder angle. This has produced a better result for me although I will probably section a case from the latest batch sometime soon just to check.

 

[Nick Caprinolo]

What are you turning the necks down to? How does that compare to the thicklness of the brass shown above?

 

[Martin52]

Yes, they have, I anneal after every firing for 6 seconds using a drill mounted case holder and a single flame propane torch. The time was initially set with Tempilaq.

I do use an expander after FLS'ing in a custom die giving a neck o/d of 0.333 (fired cases are .340) then expand up with a .3065 mandrel to .3345 and the finished round is .336.

 

[Martin52]

What are you turning the necks down to? How does that compare to the thicklness of the brass shown above?

The necks are turned to 0.140 and that is the thickness of the neck in the pic - unless you're talking about the thinned section at the groove, I have not measured that.

 

[Nick Caprinolo]

I shoot a lot of 6PPC and the necks are turned to .0075. Never had a problem. Did you maybe get a bad batch of brass. I generally turn all my other necks to .013 in order to make sure they are uniform. Again no problem..

 

[Martin52]

No, I don't use an expander ball in the die, for this rifle/chamber I resize with a custom FLS die with a neck honed to .333 then expand up with a mandrel.

This particular batch of cases was neck turned out of the box after trimming and expanding and had a slightly unusual appearance at the shoulder, almost a bull nose instead of a straight line to the shoulder. This could have been a bad batch of brass. On reflection I should have perhaps FLS'd the cases before turning, but I followed my usual process The neck turner left a very small lip that could be felt with a fingernail at the top of the shoulder and slightly below the base of the neck. All my other cases go through the same prep, this batch is the only batch that have had the necks dislocate. I do agree that if annealing is overdone that this is a contributor to the result - combined also with the possibility the expander mandrel might be slightly grabbing the inside of the neck causing a push down/pull up action resulting in the neck shoulder junction weakening. These 2 possible additional causes, in themselves, don't appear to be doing damage to other cases but I think do highlight secondary areas for improvement in case prep. that would otherwise go unnoticed.

I sectioned a case yesterday from a 11 x fired batch that has been though the same prep process and found no weakening so I think the primary cause is cutting too deep - with possibly over-annealing and the neck expander mandrel doing the final damage.

 

[bugman]

In my humble opinion, no way they are turned "too deep", especially when your turner was set for .014.
Check the difference between the fired versus sized headspace dimensions.

 

[joshb]

“My solution was to change the cutter blade to closely match the shoulder angle.”
I think that’s a good idea. One can cut a neck clean off using the wrong angled cutter. Ask me how I know.

 

[Martin52]

In my humble opinion, no way they are turned "too deep", especially when your turner was set for .014.
Check the difference between the fired versus sized headspace dimensions.

The issue is more the depth of cut into the shoulder as opposed to the depth of cut on the neck wall thickness. The cut on the outer surface is weakening the junction.

 

[FeMan]

I bought some once fired brass that was neck turned and it did the same thing. The issue there was the same thing - too deep of a cut into the shoulder. 300 pieces of junk.......

 

[bugman]

The issue is more the depth of cut into the shoulder as opposed to the depth of cut on the neck wall thickness. The cut on the outer surface is weakening the junction.

I have 6PPC brass that is 40 to 50 time fired. Those necks are .0082 and the brass at the neck/shoulder junction is way, way thinner than that on your photo. I have 6 dasher brass that is 12 to 15 times fired, with turned necks to .01080 and the 40 degree cutter has the brass thickness at the neck /shoulder junction to the same .01080 thickness. Both of these calibers are run at pressures exceeding those .308 cases in the photos, and zero neck separations occurred.
The point here is what you are calling a "groove" that resulted from the shoulder cut is often seen when you over headspace the case and the resultant neck stretching upon firing. Combine that with over annealing that is almost half way down the body, and you get a case neck and shoulder that moves too much and is too soft to resist separation.
I am not going to argue this point. I simply asked that you compare the fired headspace dimension to how much you are bumping that shoulder back. If you are over 5 thou, and you keep annealing that aggressively, neck separations are going to happen because of case stretch........ and that continued stretching then over- bumping will result in the "groove" prior to the actual separation.

 

[Martin52]

......The point here is what you are calling a "groove" that resulted from the shoulder cut is often seen when you over headspace the case and the resultant neck stretching upon firing. Combine that with over annealing that is almost half way down the body, and you get a case neck and shoulder that moves too much and is too soft to resist separation.

Thanks, and I too am not looking to argue over the point and I do accept there can be a difference of opinion.

I initially thought the pic's would simply be a good illustration of what happens when things don't work out the way intended.

Regarding the brass, it is fired in a tight chamber and only set back by 0.001 - 0.0015 in a custom die. However, the neck is reduced from .340 o/d to .333 o/d in one step and that could be a contributor.

Annealing time is only 6 seconds in a single propane flame. Maybe the flame could be directed higher on the neck rather than at the shoulder/neck junction.

I do agree generally that over-annealing combined with excessive shoulder bump could very well result in neck separation from overworking the neck/shoulder junction - but I am not certain these 2 points are the main issue here because I have another 300 cases that have gone through the same prep process (except the over- cut) and have been fired the same number of times or more and are OK. I tend to think the cut into the shoulder is the fundamental problem since it has created a weakness with the brass then being inconsistent in thickness at a critical point. Possible over-annealing and a 'sticky' mandrel has likely dealt the final blows.

 

[rogina]

The necks are turned to 0.140 and that is the thickness of the neck in the pic - unless you're talking about the thinned section at the groove, I have not measured that.

I hope you are more careful with your loading than you are with typing numbers. Necks .140 thick?

 

[243winxb]

Reminded me of this fire forming thread. https://benchrest.com/showthread.php?97490-Lapua-6BR-Case-Problem

 

[47WillysGuy]

The issue is more the depth of cut into the shoulder as opposed to the depth of cut on the neck wall thickness. The cut on the outer surface is weakening the junction.

Yep, been there, done that. Lost one neck out the end of the barrel, the second neck was removes with a cleaning rod and bronze brush.

 

[Martin52]

I hope you are more careful with your loading than you are with typing numbers. Necks .140 thick?

lol, yup, missed the O......0.0140.

 

[Martin52]

Reminded me of this fire forming thread. https://benchrest.com/showthread.php?97490-Lapua-6BR-Case-Problem

View attachment 1281297

Interesting read. Post #9 says can't feel the line on the inside but its a definite stress point.
Maybe after another firing or two the line would have been felt.