Turning a thin walled barrel almost chatter free

[lloyd-ss]

I don't have a decent steady rest or follower for my lathe and turning long thin walled sections has always ended up being an exercise in how to remove the chatter marks after I was finished. (And I do have a good method for that!)

But this time it worked a lot better.  I was turning a 24" long .410 barrel to remove all the taper from the 10" closest to the breech.

I wrapped a 4' long 3/8" oak dowel with duct tape in a spiral pattern for a tight fit into the barrel.  I had the breech end in a live center in the tail stock.  Lightly chucked (Barely tight enough to hold) on the barrel with thick copper pads on the jaws.  10" of barrel were exposed.  The portion of the  barrel in the chuck had a little taper, so tightening up the tail stock taper-locked the barrel in the chuck.  Excess length of the dowel was sticking out through the head stock.  I found that by pushing or pulling on the end of the spinning dowel (bending it) I could control the chatter.  I slipped a loop of rope around the end of the dowel, pulled on it to get about 4" of bend in the dowel, and tied the rope off to the bench to hold the tension.  Finish cut was 300 rpm, .0025 IPR, and .003 DOC.  I was using a carbide insert set to have about a 45 degree lead angle that I lightly diamond honed to get a crisp edge.

I'd like to say it came out perfect.  Not quite, but almost.  Next time it will!

If anyone has other tricks on getting a nice finish on thin tubes, please chime in!

Lloyd

[gene_sc]

Wow Lloyd that was some trick to fix da chatter. Thanks for sharing with us. I often make barrel shrouds from thin wall aluminum. Some are 24" long. When these shrouds are this long I need to make them in three sections. Yes they will chatter once you reach a certain spot and continue. It will ruin a project.

What I did to stop the chatter.. Either a rubber hose or dowel inserted in the tube stops the chatter. Has to be a good fit and not loose.

Works fer me....

Gene

[chronic]

Thats cool, you loaded your work piece
I've done similar things like that at work, I used a broom handle inside a work piece supported on rubber bands to try and knock out some of the noise and it did work a bit! Still had to use super positive carbide inserts and low rpms.

Good old trick is to touch a lead hammer on your work piece near the cutter as you're feeding to cancel harmonics. I also wrap thin walled material with rubber on the od and feed up to that point, pull out then move the rubber over to the finished size, feed back in and keep going.
But if you're doing long thin work pieces, sometimes there is nothing you can do but section by section unless you get a follower rest.

Good work Chatter not worthy! Just cross hatch it when you polish and know one will know what you went through

[Jawbreaker 38]

What nose radius was your insert? I fight these battles every day and have found that burying your nose radius completely is one of the best solutions, but in some cases it's not always possible. I also try to run the RPM's slow and feed as heavy as possible to stay ahead of the chatter, a positive rake makes a major difference as well. I've stopped in the middle of a cut and wrapped rubber bands on the material behind the tool to help absorb vibration. When boring deep holes I've wrapped rubber bands around the shank of the boring bar and around the outside of the part also.

[lloyd-ss]

Thanks for the input everyone,  seems like its a universal problem

What nose radius was your insert? I fight these battles every day and have found that burying your nose radius completely is one of the best solutions, but in some cases it's not always possible. I also try to run the RPM's slow and feed as heavy as possible to stay ahead of the chatter, a positive rake makes a major difference as well. I've stopped in the middle of a cut and wrapped rubber bands on the material behind the tool to help absorb vibration. When boring deep holes I've wrapped rubber bands around the shank of the boring bar and around the outside of the part also.

I used .016 radius triangular insert and honed it with a diamond hone to get a sharp edge on the top.  I also used a right hand holder so that the trailing edge was perpendicular to the part and the leading edge was at an acute angle to the OD of the part.

  What also helped a lot was some old fashioned maintenance: I cranked the compound back to eliminate overhang, tightened up the gib screws on the compound and cross slide, and fixed the excess clearance that I had on the bar under the far side of the carriage that hooks under the rear way.  That was long overdue anyhow.

Lloyd

[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

[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..

[chronic]

Yeah there are so many people involved in air guns that are talented with their hands, I love it.

[lloyd-ss]

Right guys.  The fact that you can do so many different things with airguns is what I love about them.  You can go plinking or hunting or target shooting, but for me, its the time I spend working on them that makes them so great.  They are hard enough to work on so that's its a challenge, but you can still be successful at making them better or faster or nicer or quieter or more accurate.  Not like golf clubs where you might get them out once a week if the weather is good.

Plus meeting great people and exchanging ideas.  The GTA adds greatly to the hobby!  Thanks to all!
Lloyd

[Rocker1]

 On car engines especially the high performance ones we stress the block with  plates if we have a chatter on the bar we torgue or tighten certain bolts on the plates, lots of time chatter in this application comes from hard spots kinda different from what you guys speak of, but  a chatter none the less. Thanks David

[BenjiHunter]

Wrapping a long item in the lathe with 1 lead electrical wire(the stiff kind) works great when you do ID machining.
Same if you have a long tool in the lathe.It works just like a damped tool.

[sixshootertexan]

When I turn something like a shroud I take a scrap piece of material and chuck it in the lathe. Turn it to about .001" smaller than the tube  ID 1"- 2" long. Leaving a shoulder for the tube to rest against. Slide the tube over it and insert live center in the other end. Use the smallest radius I have available on my tool and cut above center. Making a mandrel like this also decreases the chances of the part spinning on you versus between centers.