Diagnosing Mill Chatter?

[cdhknives]

First off, I know I have a poor excuse for a mill. This is the milling attachment for my Atlas 10x36 lathe, and I'm trying to hog out a cavity in 7075-T6 Aluminum. I predrilled 5/16" holes to get some of the bulk out, and now I'm trying to go around with a (brand new) 3/8" ball end 4 flute carbide mill @ 685 RPM to hog out the web between drilled holes. I plunged about 1/2", locked the carriage to set depth, and started traversing. Chatter and vibration is horrible! Swarf is thin needle like slivers about 1/4 to 1/2" long, not really chips. I upped the RPM to about 800 and it mostly helped, but when the vibration hits it is worse.

I tried to lean on the workpiece to compensate for possible looseness in the leadscrew. No real help besides maybe the extra mass of my hand dampening the vibration a bit. Milling attachment and carriage are locked, so the only play is the cross slide.

RPM problem? Online calcs seem to lean towards CNC, recommending RPM's well in excess of my lathe's capability.

Fact of life with lathe milling attachments?

Cut depth?

??????

 

[JimDawson]

The only thing I can suggest is to back off the cut depth. That is not the most stable setup so anything you can do to reduce the cutting forces will be helpful.

 

[Paul in OKC]

Not used a milling attachment, but my thought has always been that it is not the sturdiest of things to do. But folks use them. Even with a carbide end mill I would be tempted to slow down the rpm a bit, and maybe take shallower depth of cut. The ball end will increase the amount of tool doing the cutting. Using some thing to lubricate, I assume?

 

[cdhknives]

"Conventional Wisdom" says carbide should be run fast and dry...so I was.

I can try cutting oil and shallower cuts...I was trying to keep cutting edge in contact to prevent this chatter by cutting deep and slow.

I can also try a standard square cutting endmill...just trying to use what I had before putting in the long flute mill for finishing the sides.

 

[JimDawson]

I would use WD-40 or kerosene as a cutting fluid on aluminum. It will make a world of difference. It could be that the aluminum is ''welding'' to the cutter without any lube.

 

[Andre]

I would stay away from carbide with your admitadly less-than-ideal setup. Lack of rigidity can cause the carbide to chip.

I think a picture of your setup would be good to have. FWIW, I never get "chips" off my mill, only slivers, because that's how helical flute endmills form them

 

[cdhknives]

Unfortunately I don't have the budget or space for a real mill, and even a benchtop unit is not going to happen this year at best...so I'm trying to 'make do' and I understand this is less than ideal.

Here is the best pic I have of my current setup, when I was drilling the starter holes. There is a thread in the gunsmithing forum on this project.

 

[Bob Korves]

Make sure you are feeding against the cross slide backlash, not with it.

 

[cdhknives]

This is the initial plunge and traverse cut...basically if it wasn't for the pre drilled holes I'm intersecting the mill would be cutting with the full width of the bit. So it's cutting both with and against the backlash. I physically leaned into the mill with my off hand (heavily) to try and prevent the backlash from causing this, with generally poor results.

From looking at it, I think it is a combination of the lightweight mill head lacking rigidity and the large overhang of the workpiece. The vibration looks like the whole assembly is flexing. Since a heavier machine is not in the works, I guess very light cuts and lots of patience is in order.

 

[stupoty]

I would try a square edge end mill 2 or 3 flute maybe 1/4 inch diamitor. You could pre plunge the round edges out with the end mill then do a full depth clean up with a shallow side cut. Or try 1/4 inch depth passes, it will probably grab a bit and judder as it enters and exits the pre drilled areas though.

Stuart