donuts to go

[ntsqd]

The way that I see it an ID reamer will follow the existing hole. Most of the time in machining we want this to happen, but not in this situation. If that hole happens to be eccentric to the neck OD then the result will also be eccentric to the neck OD. Holding both the reamer and the case in a die could result in a consistent neck thickness, but that's subject to the operational clearances required by the tools. To the best of it's ability the tool is still going to try to follow the existing hole.

Turning the neck's OD while piloted by a mandrel in the ID just seems like a consistent neck thickness is a more easily obtained result. It is still subject to the operational clearance required by the mandrel in the neck ID, but in this situation the tendency of the tool to follow the hole results in an OD that is more concentric to the ID.

 

[bluedog69]

Here is the K&M cutter that cuts the donut but not the inside of the neck wall.
You can see how it will only cut something that is "in the way" of the front of the cutter.

Carbide Cutting Pilots - K+M Precision Shooting Products

Carbide Cutting Pilots for use w/ our neck turning body has four cutting flutes used to remove the “dough-nut” at the inside neck wall and shoulder junction

kmshooting.com

 

[Bob Wright]

Yes, reamers will follow the existing hole. The only way to get a consistent wall is to ream and turn in the same operation. One can expect less than .0002 runout as my measurements have shown. Some variation is most likely non-cleanup if the wall thickness is trying to be maintained to a minimum condition. That could be a result of thin brass necks 'as new'.

 

[Mikecr]

I guess I could see it if there were something like a 24cal reaming kit, with enough reamer sizes to select what will just cut donut area -without munging up the whole neck.
Otherwise, I would think you'd need a matching expansion mandrel to set neck IDs at a proper value for a given reamer diameter.
My perspective from so much outside neck turning I guess.

 

[ntsqd]

A key goal, then, from where I'm sitting would be for the neck turning tool to accept gauge pins as it's turning mandrel because then you could get a mandrel that is exactly the correct dimension for any particular case neck ID.

 

[QuietTexan]

IMO if you're turning with a hand tool it doesn't really matter what the diameter is in absolute measurements so long as the expander mandrel and turning arbor are matched and sized correctly relative to each other. Which K+M generally does well, I use their tool set for most of my neck turning and they match toghether nicely. Even if the neck rode on a gauge pin there would still be internal inconsistencies that aren't fully pushed outward by the expanding mandrel, sizing button, case blowing against the wall, etc.

The solution is what Bob alluded in cutting both inside and outside at the same time, which is what the IDOD and AUTODOD from F Class Products does. I finally had to break down and get one to make 30BR cases. Despite having great results with K+M in the past, I wasn't able to get the consistency I needed because the K+M reamer pilot only comes in one dimension per caliber. With the AUTODOD I can set how much is taken off from each side of the case neck, and I get better internal clean up from the single point cutting tool rather than a four-tooth carbide pilot. Or the LE Wilson tool either for that matter.

This is the bottom of the bottomless pit though, 30BR at 100 yards will show you everything you've ever worried about doing wrong. Not something that would show up anywhere other than in the most anal-retentive benchrest competition. The 0.0077" 100 yard record 30BR group is 100 times smaller than the groupings where I stop worrying about most rifles, and 10x smaller than what I can do on my best days And yes that's point ZERO ZERO seven, not a typo! I've shot in the .1s with the 6.5-284 Norma I cut donuts out of with the K+M kit and I thought that was the 's until I made the mistake of gettting a 30 BR.

Otherwise, I would think you'd need a matching expansion mandrel to set neck IDs at a proper value for a given reamer diameter.

This is basically what I did, except since I had the K+M expanding mandrel and turning arbor (pilot) already, I called those the fixed measurements. Then I used gauge pins to measure the exact size of the donut after expanding with the mandrel, and ordered a reamer to that spec from LE Wilson - the size of the pin that DIDN'T pass the donut, but did pass that upper portion of the neck.

 

[QuietTexan]

Now that we've gotten to the end of neck turning, I think we only need to say Hammer and Prometheus in this thread to get the LRH Trifecta of anal-retentiveness, fan-boy-ism, and stupid-expensive tools

 

[ntsqd]

Using a gauge pin as the turning mandrel was my idea for handling an ID that didn't exactly fit the common turning mandrel sizes. With the Porter's Precision expanding die you can use those same gauge pins to do the neck expanding as well. Say that you use a .3080" gauge pin to expand those .30BR cases, but they spring back to .3065" once the case is pulled off the pin. Use a .3065" or maybe a .3063" gauge pin for the turning mandrel instead of the .304" or whatever normal turning mandrel.

<edited for clarity, I hope>

 

[QuietTexan]

That's what K+M does, the expanding mandrel used before turning is larger than the turning arbor, so the brass rides on the arbor correctly as it springs back. They're a matched set, not two randomly sized tools. The neck should be somewhat tight on the arbor to ride concentricly and not deflect under the cutter so it's not a big deal if the arbor is slightly larger than the neck ID.

 

[Airedale56]

Airedale56, here are the pictures. The first is a drawing of my reticle hold on the 1/2 inch dot and the second is the group with that hold.

Nicely done.