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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Turning a thin walled barrel almost chatter free

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Pete:
The easyer way is to cut low...
tip below centre...
that way the shaft "barrel" is
being pulled towards the tip...
instead of trying to move away from it..
But not to low so the barrel wants to climb
on to the tip...
turn at a slow speed..

Dosen't always work... give it a try next time

pete

Jawbreaker 38:
Sounds like the combinitation worked out well, I'm sure that shortening up the over hang and increasing the ridgity was a major help.

Here's my process of turning shafts between centers, if you have any tips or pointers I'll gladly take them, but this is what has worked out best for me.

First if there are more than one diameter I chuck on the largest, but I always bore my jaws or collet first and I prefer to locate against a surface then advance my tail stock and pin the work piece to the locating surface. This keeps the weight of the shaft from drooping and then pullng straight when the tail stock engages which could gaul the the jaws or collet.

I prefer to have all of my rough cuts done before I put it between centers, sometimes depending on the tolerances I will take one rough cut between centers.  The one most important part of the whole process is the finishing allowance. I always prefer the smallest nose radius possible, which is usually .008 and take off atleast twice that amount (in diameter). So, I generally leave the O.D. .020 O/S. This seems to be some sort of major secret, as generally people think the least amount of removal creates the least amount of tool pressure, but that isn't true. The geometry of the insert will actually hook and pull the material into the cut. I've actually seen the center of a shaft run smaller than the ends because of the insert's pulling effect.

In most finishing applications with a tight tolerance or a fine surface finish I generally don't like to feed more than 50% of the the tool radius. So, most of the time with a .008r insert I'll feed at .004 I.P.R. at a relatively slow R.P.M. If chatter occurs I'll decrease the speed before increasing the feed, if it continues I'll increase the feed. I can usually tell by looking at the chip if I need to adjust anything, with a full cut it should coil and roll around the the tip and eject nicely, but I don't want them long or else they'll wrap around the work piece. I've actually seen them pull chips out of the pan and wrap around the shaft and cause a very bad day or a serious injury. If they don't break nicely I'll increase the feed until they come off looking like little springs. Hard materials will be dis-colored if taking a full cut as the heat is leaving with the chip if the surface speed is appropriate.

Most of my work is done on a very ridgid CNC lathe, I can't adjust the angle of the tool, but that hasn't ever been something I felt I needed to do. This is also where your method and mine would vary. If I were to stop a tool in the cut you'd see an obtuse angle from the work piece (in the direction of feed) to the rake of the insert, this is what eliminates tool pressure and deflection ( assuming the nose radius is burried, other wise the radius tries to climb around the material and won't roll the chip).

While my method and yours may be different, the proof is in the pudding of your turned barrel. While I have a firm grasp of turning and boring they really aren't my area of expertice. I'm really more comfortable in the areas of deep hole drilling, threading and milling and generally in a production enviroment. If there's something I'm off track on please enlighten me, my boss would greatly appreciate it, as would I. This is my occupation, so if I can gain some knowledge it will help to put food on my table and for that I'd be very greatful.

chronic:
lloyd I'll do that too with boring sometimes, I'll back feed so I am cutting with my cutters trailing edge. Chatter loves surface contact from your cutter, but its funny how sometimes a nice shallow cutting edge helps reduce the cutting pressure and gives you a sweet finish.

Can you vary the rpms while your lathe is running? If its a geared head try switching gears every cut, it can help with harmonics. You may get noise at one rpm, but to change it the different frequency sometimes has trouble laying into your part when its already touched by another.

This doesn't really apply do your project... but most of my chatter experience comes from boring deep holes. One old russian lathe I run sometimes for big parts actually aids chatter, just the belts running and vibrating can be transfered into my bore. You touch the bed ways and feel that humming.
So my boss taught me sometimes keeping as little contact with the machine its self can help. And I've had it work, chuck my part by only like an inch on the hard jaws, keeping the face of my port sub or what ever I am working on away from the chuck face. Then a steady of course... has yielded great results with all the other typical boring tricks, cutting above center, small tool nose rad, devibe bar, or the good old fashioned 8foot long boring bar sticking out way past your tool post with a spare 4 jaw chuck clamped on the end to load the bar :D

lloyd-ss:
Wow, its always nice to get advice from the pros.  I've now got plenty of new ideas to try next time I am faced with a chatter problem. I'll have some new ideas to try.
Thanks again!
Lloyd

gene_sc:
This is the kind of stuff dat makes our hobby/sport more enjoyable. Love it when folks share ideas. Keep it up guys. I am learning something new every day here in this forum. Sure hope others are gaining ideas and knowledge as well..

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