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Thread: Need tips on milling an octagon

  1. #1
    Boolit Bub
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    Mar 2013
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    Callahan CA
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    Need tips on milling an octagon

    I've got a .308 bull bbl that I'm considering turning into an octagon sporter. But, my books are not helpful and I know nothing. I'm not even sure what questions to ask. End milling or side milling? Up milling or down milling? Lots of light passes, or fewer medium passes? Temperature control ?Last thing I want to do is induce warpage, it is a pretty good bbl. How many holdown fixtures to get good rigidity? My machine is a small but very heavy, combination horizontal and vertical, American made mill (too small for the job, really, but time means nothing on this project). If it wasn't a rifle bbl it would be a pretty straightforward machining operation, but having some experience straightening bbls I am fully aware that bbls want to warp, and any mistreatment will let them have their way. Any tips would be greatly appreciated.

  2. #2
    Boolit Grand Master

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    A decent dividing head or means of indexing with a tail stock. Dead center in head and live center or dead center in tail stock. A machinist jack in center and a strap clamp to help with support. The vertical spindle will do fine. The big part of this job is going to be the set up especially if you want a tapered octagon contour. A way to make set up easier is to make a heavy plate 1/2-3/4" thick 4"-6" wide and long enough for the barrel dividing head and tail stock plus some extra. On the bottom side center drill and bore a pin hole to match you mills t slot. On tip side mill the key slots on center for the dividing head and tails stock. Drill and tap holes to clamp them down. Drill and tap a line of holes center each way 6" every 1"-2", these are for the strap clamp over the machinists jack. The pin will set in the T slot and help hold it when indicating it in. This set up makes it much easier to get aligned. I would take light cuts opposite each other to finish size. .005 -.010 passes. your machine will determine direction a solid machine with ball leads does very good climb milling. standard screws and a little looser conventional is better. Choose an end mill that gives the desired radius at the tulip. A higher helix mill will give a better finish, and require less cutting force.A flood coolant would be best here to control heat and chips,a spray mister will work well as will a dripper type applicator. The later 2 should be mounted on the machine ram so they "follow" or "track" with the cutter. The lighter passes will help a lot on finish and cutting opposites will help maintain straight.
    Step up is the big thing and needs to be right dont get in a hurry, when you think you have it walk away for a few minutes and recheck it.
    Most blanks are stress relieved now and that helps a lot with reliving warp-age. On the tapered octagon the cut will get longer and taper out. Some barrels ma do better with 2 machinists jacks an clamps spaced out. A 30-34" BPCR barrel usually will. With the side milling and proper clamp placement you dont have to re-position clamps thru the cut. A piece of cold roll to practice on is a good idea.

  3. #3
    Boolit Bub
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    Country Gent, I don't want to make a nuisance of my self and I really appreciate your detailed response. I am not too interested in laying out the money for a dividing head so: can you envision a 1" thick, say 3" by 3" , mild steel block with milled octagon flats around the perimeter and a dead true threaded center to engage the barrel shank threads, kind of like a large eight sided nut...providing a substitute for a dividing head?
    I've got plenty of length, 28" and hoping to get a finished length of 22 or 23 inches. Other than "doing it the hard way", can it work?

  4. #4
    Boolit Grand Master

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    Better would be a block with center and a reamed pin hole dead center below it. then a plate threaded to barrel shank with the same center lone and 8 hole pattern. this will maintain set up and slide pin out rotate insert pin and lock down. The bigger the hole circle the more accurate. You can get the numbers to do the hole pattern from the machinists handbook. And do it clamped down flat using X Y coordinates. This can be a pain if you dont have dro since it means taking play out every move.
    The bock will work but with out the dividing head becomes much harder. Also once cut and mounted maintaining alignment is much harder with out it on centers.

  5. #5
    Boolit Master Moleman-'s Avatar
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    The horizontal mill I have has a shorter X axis than most of the barrels I want to do so they get side milled in a vertical mill. I've tried face milling but have had better luck with side milling. I usually use a similar setup to what country gent spoke of only since it's side milling using an angle plate in the middle to support the barrel. Nothing fancy, a 5C indexer mounted to a 3/4"x4" plate long enough to fit the indexer, barrel and tailstock on. Nice compact setup that works well on smaller machines. Also have used a dividing head and tailstock on a bridgeport clone with a 49" table but actually prefer the 5C indexer and plate combo. Makes a handy holding fixture for draw filing also. Making a .050" pass, index and repeat 7 more times and readjust the angle plate quickly makes a lot of chips and flood coolant helps wash them away and keep everything cool. Here's a pic of the indexer setup set up for draw filing and one of a similar barrel getting milled on a larger bench mill. The chips are covering up the T-slots, the 3/4" plate and are halfway up the 3/4" angle plate. Don't think I've got a pic of the bridgeport setup milling a barrel with the dividing head but it's a similar side milling setup with the angle plate. The face milling one you can angle the dividing head and raise up the dead center in an adjustable tailstock.
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  6. #6
    Boolit Bub
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    Thanks, guys, I'll start with some cold rolled round and see what kind of accuracy I get.

  7. #7
    Boolit Grand Master

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    side milling fas another advantage to it over horizontal or end milling. Side milling helps negate the table rock on older machines as weight distribution changes with the table movement, between knee y axis and x axis this can .008 or more. making the flat a shallow radius.

  8. #8
    Boolit Bub
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    Just as a heads up, cold rolled will warp like crazy due to all the internal stresses from the cold rolling process. If you can have the cold rolled stock stress relieved or start with hot rolled you stand a much better chance of success. I'm not sure how accurately that will represent your barrel but I assume (***-u-me) your barrel was also stress relived. Do you have any idea what material you barrel is or who made it?
    quando omni flunkus moritati

  9. #9
    Boolit Bub
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    Bill Holmes book "A Master Gunmaker's Guide to Building Bolt-Action Rifles" has a section covering how he milled octagon barrels, if you have that book in your library.
    quando omni flunkus moritati

  10. #10
    Boolit Bub
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    Forgot about stress in cold rolled. In any case I'll find something in the steel rack, the "test" is not going to be a full duplication...mostly I just want to see if I can get a decent looking, uniform tulip at the transition. I never won any draw filing awards. The subject barrel is a Wilson match, probably 416 R. It chambered and threaded nicely. I have heard the Lothar Walther's are pretty tough, and have never bought one.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
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