Turning inside the neck, again

[bikemutt]

I'd given up on inside neck turning for so many reasons I've lost track but, here we go again  .

I got to thinking about how good a job conventional neck turners do with respect to surface finish; bright, shiny and almost devoid of evidence they've been turned. I casted a wide net to see what common lathe turning tools, specifically inside boring tools, might be similar in geometry.

The picture below illustrates a good match.



Please ignore the glitter on the face of the O-ring cutter, it's a cellphone camera artifact. The important dimension, IMO, is the width of the cutting edge. The two are quite similar, both are as far from the typical single-point cutting tools used for lathe turning as possible. The 21st Century cutter is proven, it will work as expected. The O-ring cutting tool loses all advantage because it's presented parallel to the work versus perpendicular, think tool deflection, this is where feed and speed come into play.

The result on virgin Lapua 6.5 Creedmoor brass is quite good, the seating feel is excellent.

I'm working on having both inside and outside cutters applied in one pass. While that may be an elegant solution to zero net force, and net deflection issues, it has it's own set of issues, geometry management among them.

As the lead dead horse beater here, I feel the need to proceed 😁. 

[LeadHammer]

That's looking real nice Chris 

[autoxforfun]

That turned out really clean.  Well done.  I tried doing some brass turning using standard single point lathe tooling and it doesn't really work that well.  Way to keep beating that horse.

[DHuffman]

I'd agree it looks very nice. Obviously that can't be done in a mandrel and I can't remember how you support the case to work inside the neck other than the case head.

After working inside of two 3/4" 24 tpi muzzle brakes recently all I can say is very nice!

I truly believe stuff like this "matters" then you realize there are many high level competitors running no turn chambers.
When refurbing 730 ish pieces of BRA recently I culled 90 pieces off because I didn't like the inconsistent thickness on them. I have to assume the error was mine either when originally turned as new or in the re turn.
There was a time when I really enjoyed neck turning because I thought I was really doing some high tech stuff that was giving me an advantage. Anymore I'm thinking about no turn LOL

Seriously considering no turn one one gun to test it and at the same time seriously considering paying someone like DJ's to do my next set and see if I can tell a difference over 5-7 firings.

Again nice work Chris.

[bikemutt]

Obviously that can't be done in a mandrel and I can't remember how you support the case to work inside the neck other than the case head.

I'm using an ER20 straight-shank collet chuck to hold the brass. The upper clamping range for ER20 is about 0.51", just enough to hold onto 6.5 Creedmoor brass. What I like about the ER20 collet size is the collets are very flexible, this lets me get a real good hold of the tapered body without risking deformation. The straight shank has a split PVC sleeve which electrically isolates it from the rest of the lathe, that assembly is held in a 4-jaw independent chuck which facilitates getting the neck to spin with minimal to zero runout. Being electrically isolated, I can use a simple continuity tester to "beep" when the cutter touches the inside of the neck, from there I can dial in the offset for the cut. It sounds like a Rube Goldberg contraption but it's really pretty easy to operate.



I'm using an aluminum collar to prevent the neck running away from the cutter, especially as things get close to the case mouth, still not positive it really makes a difference, but it can't hurt.

[autoxforfun]

Chris, I went looking for tooling similar to what you are using.  The smallest I found had a minimum hole ID of 0.25".  That is a bit big for a 6mm case.  Have you found anything smaller?

[bikemutt]

Chris, I went looking for tooling similar to what you are using.  The smallest I found had a minimum hole ID of 0.25".  That is a bit big for a 6mm case.  Have you found anything smaller?

Bob, I think this one from Sandvik-Coromant has promise:

CXS-05G100-5220R 1025 CoroTurn® XS solid carbide tool for grooving

The minimum bore diameter is 5.2mm. I determined the flat width is 0.0362", that number excludes the radiused edges. The cutter I'm using is a Micro-100 OR-096-8, it's flat width is 0.0760". I will say the CoroTurn tool did not do a good job for me when run at the same feed and speed as the Micro-100, that doesn't mean it can't or won't work, it's just going to take time to find out what it's favorite food is. The other thing I don't care for with Sandvik-Coromant boring tools is they have no flat, instead they sport a 30° rear end for properly registering the tool. Needless to say, only their tool holders have such a registration scheme, and they are priced for profit. I figured a way to mount and register the tool in a standard OXA holder.

All that being said, the theoretical minimum bore for the Micr0-100 cutter is 0.272" which is obviously greater than 0.264". Micro-100 derives their minimum bore based on a chip load which has to go somewhere, I think that may be supremely relevant for blind holes which a case neck is not, so I ignored the constraint. It is a really tight squeeze though, I thought I might have to expand the neck past 0.264" but with proper tool alignment, it fits  .

If you want to try the Sandvik cutter PM me, I'll send it your way, along with the registration workaround I made.