I have a real nice DGR in the making with a .950 muzzel and have always wanted to make a one piece barrel band front sight. I've tried to do this on a rotary table set up for plunge milling into the round stock followed by rotating the table to create the circular part of the band. This approach looks like it will work except that the surface finish is really rough. I suspect its a function of the milling cutter and speeds I'm using. However, I've never played with this set-up before. If any of you guys have done this before, I'd appreciate any tips.-Rob

Rod, Sounds like you are on the right track. I don't know what equipment you have in your shop so its hard to recomend a different approach.
I would turn the ID on a lathe from oversize stock. You could use a 4 jaw to get the ID off center but steel is cheap and time is not. Then I would contour a round boss in an aluminum block on a CNC vertical mill, drill and tap a hole in the center. The boss height needs to be around .010 shallower than the band width. The fit should be snug. Use a washer and clamp the steel on to the boss. I would then contour the profile of the band. Use two end mills to do the profiling one for rough cut (leave .005/.010 per side) then a finish cut. For the finish I like the high helix carbide endmills. They have a high shear area and give a better surface finish than the standard helix mills. The only down side is there is alot of upward force so the washer and screw should be sturdy no smaller than 5/16-24. I assume you want a small corner radius so you may need to go slowly as you approach the corners to avoid chatter but use the largest diameter cutter you can (more ridgid). A .002 chip load per flute and surface speed of 50fpm should work well on the finish cut. Depth of cut should be no more than 20% of endmill dia. Use a good cutting oil. I find tapping fluids work well for this. You can substute a vertically mounted 5c collet with an expanding mandrel for the boss idea but I think you will add flex and some chatter.
From reading your posts machining is not a hobby for you and you probably already know most or all of this already
I hope some of it is usefull. John

fritz4545- Thanks for the info and complement. As you can tell, I worship at the alter of accuracy and precision and what better way worship than by attempting to become a competent machinist. It goes hand and hand with firearms!
I particularily like your info on cutter size and speed. I'll try it tonight! I was reasonably successful with a trial run in aluminum last night. The concept works quite well, but surface finish is still sub-optimum. There is no doubt, however, that I can make the part in one piece using this approach. However, the idea of a aluminum boss would improve the ridgidity of the set-up significantly. I am using a high helix carbide endmill and you are correct about the quality of the finish they provide. I'm using a 6 inch mill right now due to the hight of the rotary table. Perhaps with a better more ridgid set-up, I can reduce that and also improve the ridgidity. Thank you very much for your thoughtfull reply.-Rob

Fritz,
I don't think I could have said it better myself. Rob, sounds like you've aquired the right prescription for your problem. Also, don't be too reluctant to use those high helix endmills for roughing either. They are nothing short of awesome. I push the 1/2" endmills through 416 stainless at 3000 RPM and 30 IPM, taking a .750 depth of cut and .300" on a side with these things. I'll get tool life out of those endmills that far surpass any rougher that I've ever used. The good and bad of them is the fact that they produce a lot of upward pressure that Fritz pointed out. While they will tend to test your fixturing methods, they're great for chip evacuation that will lead to less tool breakage.

I've made the barrel band swivel bases before in the past, and Fritz idea is pretty much right on the money. If you really want to nail it down, put a finished bore through the round bar that is not to finished size for your barrel. Leave about .050" remaining in the I.D.. Machine out a spud on your lathe that is .001" smaller in diameter than your bore in your swivel base and is drilled and tapped in the center. Then, place a counterbore in the end of it that 45-60 deg. by about .200" deep. Take your run-of-the-mill torx head or allen head cap screw and machine a taper on the head that matches the counterbore in your spud. This screw that you've turned down needs to have a high polish to reduce friction. The next thing that you do is drill and tap two holes in the bottom of your spud on the opposite end that you put the counterbore in. These screw holes will be used to hold the spud down onto a plate that you can either mount in a vice or place directly onto your machine table. Remember, you'll have to countersink the capscrews that you use for holding the spud on. The next thing that you need to do is take a hacksaw and make 4 equal cuts, half-way through the spud, cutting from the counterbored side. You have now made an expanding mandrel. As the screw in the middle is forced into the counterbore of the spud, it forces the spud to expand into the I.D. of the swivel base. This system has an enormous amount of holding power. I've used this system on countless other applications. Once you've have completed profiling your swivel base on the mill, machine out a collet for your lathe that will hold the swivel base on the outside of the ring. By doing this, it will allow you to take a boring bar and machine a taper into the ring to whatever you want, allowing for a much easier installation. This system also lends itself to being quite accurate as far as machining the i.d. to the o.d. consistently from one op to the next.

Good Luck on your project.

Rob,
Is your rotary table setup vertically (axial rotation parallel to mill table) or horizontally (rotation parallel to the spindle)? Sounds like you have the table vertically and have a long end mill so you can clear the rotary table with the spindle. If this is the case I can see why you're getting a poor finish. Set-up the rotary table so the axis of rotation is in line with the spindle. You should be able to contour the OD of the band and then dial the mill table out to generate the tang for sling mounting. You will have to trig out the start and stop points to get a perfect blend where the tang begins. One radial cut to form the OD and then back off and pick up the points where the arc stopped to mill a parallel tang.
This would be a much more rigid set up than the other. Your finish should improve substantially. Be sure to use as short a cutter as you can.
I may have completely misunderstood your set-up but the above would be the best way to go. John

[ 07-19-2002, 19:20: Message edited by: fritz454 ]

I did just this a few years ago on a 20 gauge Win pump.Sounds like good advice on the milling however the lathe work can make the milling easier if done this way.Take a 4-6" long piece of barstock and machine your OD,ID and length to whatever you require.Go in with a grooving/partoff tool to a point a bit below where you are going to mill to.Leave the piece on the end of the barstock.Grab the whole works in your 3 jaw on the rotary table.Do your milling,deburring and polishing and then put it back in the lathe and part it off.Much easier to hold it in the rotary table this way.I built a muzzle brake the same way.A lot of small parts that require milling are easily built this way.Solves all the work holding problems in the rotary table/dividing head.

Fritz454- I may have not been clear, that I'm trying to make a one piece barrel band with an integral front sight. Kinda like the masterpiece sights, but with a larger bore than commercially offered. My 8 inch rotary table is set up vertically with a 3 jaw on the table holding the part which is 1.5 inch roundstock at a right angle to the mill cutter. I have already bored the correct ID for the muzzel on the lathe. I am plunging with the mill about .5 inches to make a sharp shoulder in order to make the rib of the front sight. This is the Big problem I'm trying to deal with. After plunging, I am rotating the table counterclockwise ( as viewed from the back of the rotary table) into the mill cutter to form the circular portion of the barrel band. I then rotate the table back to the starting point , advance the milling table 25 thousanths and repeat this entire procedure. Total length is about 2 inches to make the high front sight and enough band . This way I can make essentially an offset high front sight and band at the same time leaving the whole thing still attached to the bar stock. In the end, I'll mill out the un-needed part of the band and contour the rear of the sight base on a grinder, then part the whole thing off on the lathe. If this is still confusing, I'll take some pictures and post so you can see what I'm trying to do. I now believe that the best approach is to rough things out as I am describing, then do a final clean-up by setting up the rotary table in the horizontal position and cleaning up the band with the band and mill cutter on the same axis. This set-up is actually pretty ridgid and I think the finish problem is due to my choice of cutter size, speed and relief.-Rob

Rob i am VERY interested in your process & am awaiting photos.

Rob,

It's going to be real tough to get a good finish with 6 inches of end mill poking out of the collet. I know exactly what you are trying to machine, and I can't think of any reason why you should need one that long.

Think it through, and I bet you'll find that you can do almost all your axial work with the rotary table positioned horizontally.

You want to use the shortest end mills possible. You also want your work piece to be as rigid as possible. I'd have done the ID after I'd completed all the OD work.

I've quite a bit of experience doing aerospace and high tech machining and I'm also going to be visiting my folks near Palo Alto this week. If you are interested in talking over your dilemma, shoot me an email at sthornley@mindspring.com and we can swap phone numbers if you'd like.

Regards,
Scott

[ 07-22-2002, 14:19: Message edited by: Scott Thornley ]