Octagon to Round with Transtion Advise

Tim Mahony · Jun 4, 2018

I am a hobbyist looking for advise or tips on turning an octagon barrel half round with a wedding band transition.

I have an old barrel that I will use as a test piece and I really want to do this on my South Bend 9" lathe....smile. Lathe bed is long enough to mount 32" barrel on centers and I have a steady rest and a follower rest.

So ideas or thoughts regarding setup, tooling and how to approach this project is what I am hopping for. Thanks in advance for any thoughts or ideas. tim

Rob F. · Jun 4, 2018

So you are not sure how to mount the octagon in a steady rest?
Get a round piece of tubing the octagon fits very loosly through, drill some threaded holes for soft adjusting screws to center octagon in tube and then the tube will run in the steady. Octagon needs 4 screws, 4 on each side of steady for 8 total

Tim Mahony · Jun 4, 2018

Rob F. said:
So you are not sure how to mount the octagon in a steady rest?

Rob, thank you. Simple solution that solves that consideration for me.

Looking to use a face plate and dog with a center on a collet and a live center on the tail stock end. It would be convenient to mount the barrel to cut the round on the tail stock side. Does it matter what side I cut from considering the barrel is mounted on centers?

Again thanks for ideas and suggestions to my newbie questions. tim

Paolo_MD · Jun 4, 2018

Just a couple of quick thoughts:
1) cathead (i.e. the piece of tubing with the eight screws: do not waste too much time aligning it perfectly: once it is in place, you can take a couple of skim cuts to improve the contact surface for the fingers of the steady rest and that will take care of the final alignment too.
2) Cutting direction: for light cuts, really it doesn't matter too much. However, for heavy and interrupted cuts you should cut in the direction of the most solid end, like when cutting a gear on the mill: cut from talistock to headstock, like you would cut the gear from footstock to dividing head.

I hope that both make sense to you.

Good luck!

Paolo

Rob F. · Jun 5, 2018

Thanks Paolo, I was trying to remember the "cathead", I have a few that dont get used too often...
Can't you chuck the octagon in a 4 jaw and put the muzzle end in a center and not need the steady/cathead? Or is the bore not centered in the octagon so you need to do between centers?

Paolo_MD · Jun 5, 2018

If the barrel is virgin and not yet fit to the receiver, and/or is skinny enough to fit inside at least the nose taper of the tiny spindle of a 9", yes, you could chuck it with a 4-jaw chuck, although it's sub-optimal to cutting between centers.
Unless you're looking for tons of chatter and poor finish, with 32" you cannot get away from using the steady rest close to you most critical turning (i.e. wedding band).
Again, if the barrel is still virgin, have you thought yet about how you cut the chamber and fit to the receiver? I'd bet that you need to hold the muzzle in a chuck (or center and dog plate, when fitting to the receiver) and have the cathead mounted as close as possible to the breach end. One of the major advantages of mounting between centers while cutting threads is that you can take the part out for test-fitting and, as long as you don't disturb the dog, it's still perfectly indexed for the next cut.

Paolo

Tim Mahony · Jun 5, 2018

Paolo, thank you for your time and input.

This barrel project is for an Under-Hammer muzzle loader build, so no cutting a chamber or threading an external barrel thread. My S-B 9" only has a 3/4" head stock thru hole and regardless I have had more accurate setups using collet's and centers. Just never attempted something so long.

Light cuts it will be, and I will play with both carbide and HSS tools to see what works best....any thoughts on that?

Also it would be most convenient to cut round on the tail stock side due to the dog attachment but maybe that is a mistake in regards to stiffness? Ultimately I will lop 3 or 4" off the barrel so I guess I could turn round up to the dog attachment and do the barrels round cut on the head stock side? Even with the work mounted on centers that is likely still the stiffest side?

Again, I am appreciative for all the reply's. Thx tim

ulav8r · Jun 9, 2018

HSS tooling is less likely to chip when doing interupted cuts taking the octagon to round. Muzzle to tailstock, cut toward headstock so that the headstock bearings take the thrust as they are much more robust than the bearings of a live center.

HSS can be easily ground to cut the wedding band with straight infeed.

Tim Mahony · Jun 11, 2018

ulav8r said:
HSS can be easily ground to cut the wedding band with straight infeed.

Thanks ulav8r, I basically get the concept. Another neophyte question ....could I shape a 1/2" HSS tool to do the whole band in one straight in cut?

I'm truly grateful for the responses. I hope to post a photo of a successfully contoured barrel. I also have a dead barrels to practice on.

Paolo_MD · Jun 12, 2018

Tim,
Likely, you will receive contrasting answers on this point. Personally, I think that it is like looking for trouble on your lathe. It's true that a SB 9" is a nice, relatively well-built lathe for its category. However, it's not rigid enough and you're already dealing with a not too sturdy setup. The best that it can happen while doing it would be terrible chatter.
Form tools generally are flat on the top (i.e. no top rake angle) in order to cut exactly the profile ground into the tool (if they had a rake angle, it would cut a different shape, since the back of the profile would be below center). Therefore, they require more pressure to make the cut.
You will be better off cutting it with a relatively narrow and very sharp tool and possibly touching it up with a nice file.

Paolo

MilGunsmith · Jun 12, 2018

You could use the form tool for a final light finish cut, after removing most of the metal with a pointed tool. Finish with a file to remove any chatter.

ulav8r · Jun 12, 2018

Reply 11 has the answer.

Tim Mahony · Jun 12, 2018

Guys,again thanks for the advise and tips. I feel that I have decent clarity on my path forward. So I will match these thoughts with a bit of practice on my test barrel and determine my trajectory...smile.

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