Lathe Boring Brass Bullet Mould-RPM?

[Southron Sanders]

Tonight I was lathe boring a brass bullet mould. Normally I run my lathe at 1,000 RPM and don't use any coolant.

Tonight I ruined a mould because of clatter marks.

Any suggestions for the ideal speed when boring brass (360 Brass) bullet moulds?

THANKS!

[R.M.]

Probably not your speed. I'd take a guess and look at your tool height, and sharpness.

[JIMinPHX]

Proper speed depends on diameter. I should probably look up the actual "correct" spindle speeds for 360 before quoting what right, but just off the top of my head, if the diameter that you are cutting is much above 3/8", then 1,000rpm may be a little quick. If you google "speeds & feeds for cutting tools" you will probably find a bunch of charts that list the proper spindle speeds & feed rates for various tools in various materials.

Boring bars are sensitive to a lot of things. As has already been mentioned, tool height & sharpness are two of them. The amount of stick out is another big one. If you are more than 4 or 5 bar diameters out, then you need to take lite cuts. If you are more than 7 or 8 diameters out & the shank of the bar is not carbide, then you may be asking for trouble, even in a cooperative material like 360. The rigidity of your lathe, the tightness of your gibbs & the amount of end play in your spindle bearings are other potential sources of trouble. Cutting edge geometry is also important. Too much contact surface or too little clearance angle are classic causes of chatter.

[leftiye]

Faster is nice, but it is like faster boolits , they tend to be problematical. All of the recommendations in Machinery's Handbook - if I have it right are optimum production speeds (good machine, top of the line tool, good coolant and all of the tricks - just like high vel boolits). The analogy holds concerning slower cutting and slower boolits. Less heat, smoother cutting, less chatter, coolant not as critical, etc. Not at all to be contesting what Jim and R.M. said. BaBore cuts his final cuts on his molds by hand. At least that's what he said in one post way back. With some exceptions, slower is almost always fine, and has fewer problems. Especially with cherrying and reaming, where the tool doesn't have twist to dig in.

[Cap'n Morgan]

Check your tool height. It's better to keep the tool somewhat over the centerline when boring (and below when outside turning) when cutting brass which machines better with a negative rake angle. Breaking the cutting edge ever so lightly with an Arkansas stone or diamond file also help stopping clatter. If you have room for it try wrapping a rubber band around the tool shank - it's great for stopping bad vibes :wink:

[Southron Sanders]

THANKS EVERYONE!

I REALLY APPRECIATE YOUR RESPONSES AND WILL NOW TRY THEM OUT!!!

My "Cutting Tool" is a "Spoon" (that I will now sharpen again!) I have made out of a piece of Hi Speed Steel-so no Boring Bar or anything like that.

[white eagle]

how are you on center ???
I could run brass all day long without coolant

[Southron Sanders]

I was cutting "On Center" but will raise the tool height just a tad.

I am making .578 Minie Ball moulds-so I guess that I will slow down my speed some.

[Dutchman]

THANKS EVERYONE!

I REALLY APPRECIATE YOUR RESPONSES AND WILL NOW TRY THEM OUT!!!

My "Cutting Tool" is a "Spoon" (that I will now sharpen again!) I have made out of a piece of Hi Speed Steel-so no Boring Bar or anything like that.

A "spoon" is not a single point cutting tool as in normal lathe turning. Using 1,000 rpm for a spoon is exceedingly high. Try 200-350 rpm to reduce chatter (not clatter).

Chatter is caused by vibration. Too high a speed is one cause. Too much cutting tool in contact with the workpiece is another and I suspect both those factors are at work in this case.

You're not really "boring" with a spoon. A spoon is a form tool. The tool edge geometry is much different for a form tool than a single point boring cutter. You need much less edge clearance and a much slower feed (and speed).


Dutch

[JIMinPHX]

Spoons are tough because they usually have a VERY large contact surface. When surface contact area goes up, then everything else needs to be more conservative.

[Southron Sanders]

Again, THANKS EVERYONE!

I will be trying out your suggestions tomorrow!

[Southron Sanders]

THANKS EVERYONE! I raised my tool height just a "tad," 350 RPM, S-L-O-W feed and was rewarded with a beautiful bullet cavity in the mould I am making.

I hit .578 perfectly as confirmed by my .578 plug gauge!