View Full Version : Technical: lathe RPM formula for crowning barrel
July 25, 2000, 18:03
Hi folks, I am the proud owner of a brand new mini-lathe. I bought it for a completely different project but I see no reason why it can't do double duty as a gun lathe. Since it's been about 20 years since I took shop class I've spent the last week playing with it making different shapes to reacquaint myself.
My questions are: What is the optimum RPM for the lathe to cut an 11° crown (assuming I'm using carbide tipped bits)?
And after I've cut the crown, what's the best way to protect the end of the barrel without having to repark it?
Thanks in advance.
[This message has been edited by Pete2 (edited July 30, 2000).]
[ July 30, 2001: Message edited by: gary.jeter ]
July 30, 2000, 02:15
August 01, 2000, 11:31
RPM = SFPM * 3.82 / diameter
SFPM is surface feet per minute
SFPM for carbide on 4140 Q&T = 150
3.82 is constant (12/pi)
diameter is diameter of workpiece on lathe
RPM = 150 * 3.82 / outer barrel diameter
Hope this helps.
August 22, 2006, 23:46
When I worked in a machine shop our rule was always spin it as fast as you can and take off as much as you can get away with. Before your finishing cut make sure you leave about .035 stock so you can get a smooth cut with your feed geared for a 250 finish and your tool sharpened with 1/32nd radius.
Oops I just thought about something. To get the 11 degree angle in the crown you're gonna hafta use the manual feed on the tool rest, so just spin it as fast as you can safely get away with, touch off on the outside edge as near the end of the barrel as you can get, then smoothly feed your tool in.
November 26, 2006, 10:10
I always run crowns in at around 250 RPM. Swing the lathe compound rest to 11 degrees, use lots of oil dabbed on with a brush and try to make the motion continuous rather than turn stop turn stop. I would also recommend leaving no more than .010 for a finish cut, .035 is too much and will affect the quality of the finish. Use a new carbide insert with .015 radius for doing crowns.
November 26, 2006, 13:57
500 rpm ..5 degree carbide insert feed in for rough cut ....z- .010 for finish ...(backfeed out)....oo3 feed .....per revolution if you have feed capabiitys ....ad make dam sure its lined up chamber and bore ...or ya might just as well use a hacksaw ....or a dummel
December 03, 2006, 09:48
Dont use oil with carbide bits.
December 03, 2006, 10:58
Pete, if you have a lathe, you likely have a bench grinder, too, but I'll bet you do not have a wheel on it for grinding carbide tools.
High speed tool steel will last almost forever, on 4140 barrels, as they are soft, usually 24-28 Rc.
Carbide will limit you to commercially available shapes(read that expensive) compared to anything you can wish for, if you do the grinding, out of high speed.
Sone of my lathe tools are 30 years old, of MoMax, and still do fine work.
Just a thought. Terry
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