Boring Bar Sings

[epanzella]

The boring bar pictured below sings like a canary and I can't seem to stop it. It bores "OK" but although I can't feel the chatter with my fingernail in the bored surface I can see the herringbone. I've got a project coming up that requires boring to a square shoulder that has to seal against 40,000 psi and I know if that bar is singing now when it gets to that shoulder it's gonna dance. I've tried lots of feed/speed and tool height combinations with no luck. The machine is a Grizzly G4003G and it parts like a champ so I know it's rigid. My other bars cut great but won't fit in a .458 hole. It sings with the amount of projection shown in the photos and that is approximately the depth I'll need for the project. The bar looks to have 15 degree negative rake built into it via flats on the top & bottom. If the rake angle is the problem I can make a holder that will allow changing the angle. Any thoughts?

 

[Fabrickator]

What material are you working with now, and what will it be with the upcoming project? I'm currently doing some boring with a carbide insert-5/8" bar (w/zero rake) and I'm cutting aluminum. It has a very slight bit of a sing to it but I'm getting a very good finish.

 

[epanzella]

I'll be machining a barrel blank that will become a .458 smokeless muzzle loader. It is 4140 steel. The 10 week lead time is up and it should ship anytime now. So far I've used it on various pieces of scrapyard whoknowsium steel that have cut well with my other bars.

 

[JimDawson]

Try wrapping some lead wire (or solder) around the back end of the boring bar. That will change it's resonant frequency. The other trick I have done is to clamp a piece of metal onto the bar, maybe a drill chuck or something. Anything to change the mass. Then play with the cutting angles, feeds and speeds.

 

[chips&more]

If the part you are boring is protruding from the chuck enough. I would try putting “Duck Seal” on the part and bar. Duck Seal is used by HVAC folks. It’s similar in appearance to clay. Yes, I know it could be thrown off when you turn the lathe on. Use some Yankee ingenuity on than issue. But it works, and very well, it deadens the vibrations. I keep a handy glob of it close to the lathe and mill when chatter pops up.

 

[Don B]

You could also try some elastics around the boring bare, I keep a couple of lengths of rubber tarp straps to wind around the work + a little duct tape for fastening.)

 

[pineyfolks]

If you reload shotshells hang a bag of shot on the back of the bar. Maybe even wrapping some rubber bands around the bar might do it.

 

[epanzella]

I've tried various wraps on both ends of the bar to no avail. A vicegrip clamped on towards the work seems to help a bit but won't work in practice. Pressing my thumb against the bar really hard right behind the insert seems to do the most but that's not practical either. What do you guys think about the 15 degree negative rake?

 

[Don B]

I've tried various wraps on both ends of the bar to no avail. A vicegrip clamped on towards the work seems to help a bit but won't work in practice. Pressing my thumb against the bar really hard right behind the insert seems to do the most but that's not practical either. What do you guys think about the 15 degree negative rake?

For the length you have that bar stuck out is an option to go with as big a piece of HSS you have and will fit, stuck out like a boring bar and on a bit of an angle, with the end ground for clearance and such.
Also once the chatter has started usually you need to slow the speed down to clean it up before you can really tell if any of you dampening tricks really worked.

Edit: one other thing you could try is get a shim as thick as you can under that bar, let it stick out as far as you can towards the insert end.

 

[JimDawson]

I've tried various wraps on both ends of the bar to no avail. A vicegrip clamped on towards the work seems to help a bit but won't work in practice. Pressing my thumb against the bar really hard right behind the insert seems to do the most but that's not practical either. What do you guys think about the 15 degree negative rake?

It sounds like you have tried all of the tricks. I guess I would start grinding the insert to find a combination that will work. Another option would be to buy a set carbide tipped or HHS boring bars, and grind to something that will work.

I don't know how deep you have to bore, but sometimes I use an endmill as a boring tool on the lathe. Set up one of the points just like you would any boring tool.

EDIT: I just re-read your original post, a 3/8 or 7/16 endmill would probably work pretty well in this application.