Milling Octagon flats

[oldred]

When milling the flats for an octagon barrel on a mill (J-head BP in this case) should it be side milled or milled from the top with the end of the mill? Prior to buying the mill the others I have done were done on my lathe using a home-made milling head, basically just a belt driven jack shaft-motor arrangement mounted on the cross slide and utilizing a collet to hold the end mill, and while this setup does work ok allowing for plenty of room to go the full length of the barrel it's a bit finicky about feeds&speeds to avoid chatter. I thought this time I would try it on my mill and was just wondering which method (top or side milling) would be best?

[Moleman-]

I've done both and like side milling better. Seems like the finish is easier to draw file clean. I add an angle plate with a piece of plywood as a backer in the middle and use a 7/8" to 1-1/8" endmill with flood coolant to wash away the pile of chips it makes.

[bob208]

I have always done mine in the horizonal mill.

[country gent]

side milling will leave a cut mark straight across the flat 90* to it and a radius shoulder . end milling will leave a square shoulder and a swirled cut mark. Depends on what you want. I set it up I dividing head and center with a jack screw and strap clamp in the middle ( on longer BPCR barrels I use 2 screw jacks and strap clamps). Also if you want a tapered octagon its easier to set up side milling it. I think controlling heat and chips is easier side milling also.
The horizontal mill is good for this but few have access to one or the tooling.
A good coolant mist or flow helps a lot with heat and chips. a sir line blowing can help also if the others arnt available. Be careful with brushes and around as the cutter will suck anything it grabs in. Also cleaning up and cutting watch those chips they are sharp little razors waiting to stick you.

[oldred]

side milling will leave a cut mark straight across the flat 90* to it and a radius shoulder . end milling will leave a square shoulder and a swirled cut mark. Depends on what you want. I set it up I dividing head and center with a jack screw and strap clamp in the middle ( on longer BPCR barrels I use 2 screw jacks and strap clamps). Also if you want a tapered octagon its easier to set up side milling it. I think controlling heat and chips is easier side milling also.

The horizontal mill is good for this but few have access to one or the tooling. A good coolant mist or flow helps a lot with heat and chips. a sir line blowing can help also if the others arnt available. Be careful with brushes and around as the cutter will suck anything it grabs in. Also cleaning up and cutting watch those chips they are sharp little razors waiting to stick you.

All good points and the kind of info needed to make the decision. The lathe setup worked very similar to a horizontal mill but was limited to using only the mill end since the shaft was set up to center on the "Z" axis so I did indeed end up with the usual swirl pattern that can sometimes be difficult to draw file smooth, that draw filing part alone I suppose is reason enough to side mill. I was pretty much stuck with using the mill end on that lathe set up since building a milling attachment that would have allowed for the required travel in two axis would have been far more complicated, for that reason I opted for the much simpler system that only required the horizontal movement of the cross slide. Also, unlike a horizontal mill, center support for the barrel became a problem leading to chatter which was expected from the start of the project. I found that filling that fairly large 45 caliber bore with lead shot acted as a sufficient damper to the point that careful selection of feed and speed rates eliminated the chatter, still it was a pain to get just right so this is why I am determined to do it on the mill this time. Since using the mill end method was my only octagon milling experience and then on a grand total of just two barrels I thought it to be a good idea to ask about the methods before just jumping in and trying it, I kind of thought side milling would be better but having no experience with milling barrel flats using that method and never even seeing it discussed before I just felt it better to ask first.

All things considered here I suppose I am going to set this thing up today to side mill (yep I'm all to familiar with those evil little "razor" chips that side milling produces! ) even though with the fixtures I have end milling would probably be simpler. Cooling shouldn't be a problem since I have flood cooling on both machines and the plan is to take only light cuts before rotating the barrel to cut the opposite side after each cut, maybe over kill on the light cuts but it seems like that would be better to deal with any stress that might be lurking in that long steel tube.

Even with my 42" table I am also going to run out of travel about an inch and a half shy of full length so this is going to run into having to move the set-up. This is the main reason I did my last barrel on the lathe despite having the mill that time, with this barrel however I decided that dealing with that problem will be easier than avoiding the chatter when using the lathe.

[country gent]

Old Red, If your mill is a true Bridgeport style or Bridgeport the easiest way to "extend travel" is to rotate head off center to one side and cut the flats then rotate other way off-center and finish. In this way you don't have a second set up only recatching / blending the first cuts. One problem with the Bridgeport style mills is the table rock when going over center much, the play in the table cross feed and knee all combine to allow the table to cut a radius more than straight. Even on a good machine this can be up tp .008-.010 each way. You might consider a couple roller stands to help block the table up. Side milling also will negate most of this since the rock will be parallel to the cutter. The DPMS mills were a little better here since the head was z axis and only 2 gibs for table play X and Y.

On a side note those fine side mill chips seem to "jump" right into my fingers hands and forearms LOL.

Another thing that might help with finish and holding size is to use a high helix mill. this gives a better smoother shear angle and less jump between cutting edges.

[Moleman-]

He's a pic of the setup I've used for side milling tapered barrels. The indexer, angle plate and tailstock are all bolted to a 3/4"x4" x32" plate which is in turn bolted to the table. The angle plate with a plywood backer is snugged up to the barrel still light enough that the plywood can be slid out of the way while indexing and pushed back in for cutting. The angle plate needs repositioned every full index to keep it snug to the barrel. Ran the mill around 300rpm for the 7/8"-1-1/8" 4-6 flute end mills I've used and the speed at 2.0"-2.65" per minute at a .050" DOC depending on the number of flutes. Didn't cut opposite sides, just indexed 1/8 turn and recut. Flood coolant stream on both sides of the bit and plexiglass splash shields to keep it and the chips on the tray and off of me and the floor. Final pass was smaller of course.

[kootne]

Some thoughts from an old but not yet dead guy. Side milling is better in every way. The cutter holds up better. You can get good use out of a cutter you have already wore the end off, so it's like free. The helical grind is smoother entering and exiting a cut, less hammering on your drive train. Lots of old Bridgeports out there have some bow in the table from a combination of wear, overhanging weight and peening by hammering things down in the vise or onto the table. If you don't think this is an issue, run your table all the way one way, put a precision level on the hung out end and run the table back all the way the other way. Shim one end of the level to get it back to level. Then you can do some math to see how much bow you will have induced into each flat if you are end milling. Side milling this issue goes completely away, the table still rocks end to end but the cutter is parallel instead of perpendicular to the error.
If you don't have enough travel on your BP table to get your full barrel at one pass you can do this;
1. Set up however works for you to get your desired taper and indexing for your flats.
2. Cut what your travel allows, (I would cut a flat, cut the opposite flat, then make it a square, then an oct.)
3. Put your vise in, dial it in to the table travel. Now you can finish the flats by just rotating the barrel in the vise for each pass. You will have to compensate for the taper on the MOVABLE jaw, there are several ways, I made a jaw with a large arc and an insert so it can rotate to fit any angle. Doing this, you will obviously need to be cutting on the side clamped to the FIXED jaw. And you will need to support the out board end of the barrel if there is much overhang. Obviously this is only going to work side milling, not end milling.

[oldred]

Wow lots of good info here and not just for octagon barrels!


Country Gent, swinging the head hadn't even occurred to me but as soon as I saw that it was one of those "Duh, of course, now why didn't I think of that already?" moments! As far as the drooping table I have already had to deal with that one and I already have a support rig that I can use if needed, my table is in good shape with little wear but my understanding is this is just one of those things inherent to pretty much all these old mills of this type.

Even worse than having those slivers "jump out" to get me is carrying them into the house and even worse than that is having a bunch of them stuck in my clothes when my wife does the laundry, my getting stuck with them is bad enough but having to sleep in the doghouse is really bad!

Moleman, the old saying that "a picture is worth a thousand words" is a wise saying indeed, thanks for the pic and description. I especially liked that center support.

Kootne, good ideas there also and even us old guys barely hanging on can still learn something by listening to those who did different things in life!

Thanks everyone, I have learned some new tricks here which in my case is quite valuable since I have had no formal machine shop training. The sum of my experience is trial and error (lots of error!) and listening to folks like you guys who have "been there and done that". I bought my lathe years ago out of necessity for rebuilding my farm equipment and just had to learn by doing, naturally my love of all things firearms related just fell right into place leading to expanding my shop and tooling for way more than just farm needs. Now it seems that building and working on my rifles has become about the most fun pastime I have ever undertaken, I only wish I had started sooner when I was younger and learned the finer points of the machining skills. As the old saying goes better late than never so if I ask a few of what should be elementary questions it's because the one thing I have learned more than anything else is that if unsure then ask before doing no matter how simple it might seem!