Low Force Lathe Tool Grind?

[epanzella]

I have a heck of a time tapering barrels due to chatter. I would like to just set over the TS for the taper but the cut comes out looking like I did it with a hatchet. I end up choking up on the barrel, turning it 6 inches at a time, and then blending it just before it goes in the bluing tank. This is reeeely tedious and I wondering if there is a tool bit geometry that would impart minimum force onto the barrel and allow me to turn the whole taper at once without chatter. I have a steady and a follower rest but they just don't work with a taper.

 

[mikey]

Have you contacted @Bamban? He does this kind of work and might be a good resource.

 

[f350ca]

You could try a shear tool. They only take light cuts, 2 to 3 thou at a time and need slow speed and low feed rate. BUT put very little force on the part. I've cut 2 inch schd 40 pipe 4 feet long with no hour glass shape.
Do a search "lathe shear tool"

Greg

 

[mikey]

In order to be of more help to you, especially with regard to tool geometry, you need to tell us what the barrel is made of, what lathe you're using and can you grind a decent tool. I ground a tool for @Bamban for use on 416R SS with the intent of minimizing cutting forces and improving the finish. I don't know a lot about the results he got but I think it worked okay. PM him and you guys can PM me and we'll work on a design together.

 

[epanzella]

In order to be of more help to you, especially with regard to tool geometry, you need to tell us what the barrel is made of, what lathe you're using and can you grind a decent tool. I ground a tool for @Bamban for use on 416R SS with the intent of minimizing cutting forces and improving the finish. I don't know a lot about the results he got but I think it worked okay. PM him and you guys can PM me and we'll work on a design together.

The barrel will be 4140 chrome moly. It's ordered but not yet received. The lathe is a Grizzly G4003G. For bluing I have to go over the whole barrel with 320 anyway so the finish doesn't have to be that great as long as it's free of chatter. I get a good fish when I choke up on the barrel but I can't turn an unsupported 26 inch blank without chatter. Attached is a pic of my last build. You can plainly see the difference in the finish between turning full length on centers and turning smaller smaller sections.

 

[john.k]

Gunsmiths doing this work use self compensating hydraulic steadies......the other way round is to leave a series of collars on the barrel for steadies,then shift the steadies and turn off the collars......If you look at pics of barrels being ground between centres ,the use of slip or solder on collars is prevalent....The old method was that of spinning,but you re going to need a 7 foot dia grindstone,and you ll probably die young in a wheel burst ,or or of grinders lung.....silicosis.

 

[mikey]

You can turn 4140 readily with inserted carbide tools. Positive rake tools work well with lighter depths of cut, negative rake tools finish nicely but with higher cutting forces.

If you prefer HSS, I would keep your side and end relief angles at about 10 degrees, use about 14-15 degrees of side rake and 10-12 degrees of back rake. I would keep the nose radius at about 1/64" max and hone the tool well. These angles have essentially stock recommended relief angles and a bit more side and back rake to reduce cutting forces. The smaller nose radius will reduce deflection as much as possible, although it may require slower feed rates to maintain your finish.

I don't turn barrels so I'm not the best guy to address this issue but it seems to me that when turning a 26" long work piece, you sort of need some kind of additional support. The support collars that @john.k suggests make sense to me.

 

[derf]

You need to have a tool pointed like this "^", not a general turning/facing tool. Run .015" doc and .015-.018" per rev. Set the tool height as high as you can get it and still cut. If it squeals some at the start, that's ok. You may not get rid of all the chatter, but that's what a belt sander is for.
Some of your problem is probably the lathe, being not rigid enough. I have a 1340 Jet that is not the best at turning barrels, but my 13" South Bend will do it with no trouble at all.
The biggest problem with all the asian clones is the tailstock does not have a big enough foot print for the size of swing.

 

[john.k]

The catch with high rake carbide is the slightest chatter chips the edge............One old dodge for controll of chatter is lead ....fill the barrel with fine lead shot,and plug it..........or even cast it full of a LMP alloy like cerrometal...........one time we had to thread a long thin wall tube ,and the solution was cerrometal.....also wrap the barrel with lead wire ,and let the tool push it aside as it feeds.....another method was to be a human steady with a heavy lead block against the tool push.......trouble with chatter ,once it starts ,its nearly impossible to stop on subsequent cuts.

 

[epanzella]

Thanks for the info guys. A lot of this stuff I've tried already but the lead shot and the shear tool are new territory worth trying. I typically turn .250 or more off a blank so I have plenty of meat to experiment with. Below is a pic of the part of the last barrel that was "surplus".