Parting 6061

Hi All,
I'm fairly new to this group but I've had a CNC Shoptask for about 6 years.

I'm having a bad time parting off 6061 on the lathe. I'm using a very solid tool mount, a sharp HSS 1/8 X 1" blade. The problem is chatter. I get the cuts made but the chatter is fierce. The diameter is 2" down to 3/4".

I've tried top rake angles from +7 to -7 degrees, tool positions from center to .015" above center, speeds from 60 t0 1100 RPM and amazingly none of this seems to make much difference. The chatter is the same under all these conditions which leads me to think I'm way off in something I'm doing.

Facing, turning, and drilling are no problem. I do get some chatter Boring.

Appreciate any suggestions.

Jerry

I have a little Harbor Freight 9x20 and have similar problems with parting, and not just aluminum. It seems so hard on my lathe that I tend to avoid parting and use a hacksaw blade in a little one hand holder instead. Or I will cut it down by facing from the front with a regular turning tool. I dont know, I wish I had a good answer for both of us.

If you were trying to part off steel, I'd tell you store that HSS tool in the round file :D

But for aluminum, you should be able to make it work. I'd try grinding about a 1/4 to 1/2" radius in the top of the blade, attempting to create a "hollow ground" hook. For aluminum, this top rake "hook" can be very positive, because the heat is not going to hurt the tool, and positive rakes cut more easily.

The tool tip needs to be on center height.

Watch closely the front clearance that you give to the blade. It only needs 2 or 3 degrees of clearance, because in lathe work, the roundness of the work itself creates adequate clearance. Too much front clearance will contribute greatly to the chatter. With reduced front clearance, the front of the tool helps stabilize the workpiece, and dampen vibration.

If you have constant surface speed available on your cnc, this is a good time to make use of it. You can use all the rpm that you dare, but you must keep the feed up at a steady, aggressive pace.

Use flood coolant if possible. Plan to retract a couple of times to apply lube manually, if that is your method.

At 1100 rpm, your feedrate should be about .002 to .004 per revolution. If the lathe cannot handle that, then maybe try a 3/32" wide blade.

There is an advantage to using commercially made part off tooling with carbide inserts. These inserts have chip formers on top that will help the chip roll up in a coil, and at the same time, they also narrow the width of the chip so that it comes more easily out of the groove.

One guy I used to know, used big old power hacksaw blades as "tool blanks" for parting off. These are quite narrow, and don't take too much power. He also ground a pronounced hook in the top of the blade to improve the cutting action.

i would suggest a thinner blade than 1/8" and if that doesnt work you may want to try taking a relief cut down to about 3/4 and pull out and cut again and leave about .010" to clean up chatter, i canned all the hss blades and went to inserted carbide tools, it makes the job easier

Thanks to All of you.
The problem is definately not side binding. The stock is a 1.75" round piece being run .350" off center in a four jaw chuck. The chatter starts fronm the first touch of the tool.
I'm not sure the width is a problem because I think that is a power question and there is plenty of horsepower on this machine; 1 HP DC motor.
Since posting I've done some research and I'm inclined to think that HU's suggestion of a high rake is correct; and maybe an aggressive feed; although that sounds scary.
I think my next try will be with a 35 degree rake!!!! and a .002/rev feed. Both blow my mind, but I know what I've tried so far doesn't work.
Jerry

I agree you do need high rake. Never run your tool above centre on OD turning though.
You should be slightly below centre. When the tool is above centre and it deflects, it gouges MORE material off causing undo stress and chatter, if it is slightly below the centre it only deflects into air and will remain more stable.

I re-read the post's so I would not re-state something already mentioned but did not see any suggestion to make the cut-off tool sticking out of the tool holder no further then necessary, also that there be as minimal amount play in the compound slide as possible.

Ken

Thanks Ken,
The blade sticks out of the holder only 1/8" beyond what's needed, there is no compound and the whole setup seems rock solid and over-engineered.
I can cut steel even stainless easily with the same setup.

Also I'm not new to using a parting tool, my first time was 45 years ago although I'm not a machinist. It really sounds to me like I need to change the top rake is the answer.
I'm finished with that little job and going into milling mode but when I get back to lathe I'll try a new grind and report what I find. I'll also reduce the clearance angle as suggested. That may be a biggie. The carbide tips that work so well for external turning have no clearance angle.

Jerry

Thanks DareBee,
I'll also check out my height. The reason I raised it a little was that it increases the effective rake and reduces the clearance at the same time, but I do see your point.

Jerry

I have been hesistant about using a parting tool, but on this last project I picked up a 1/8 in wide Cobalt parting tool....on aluminum no problems even though the tool was set a bit above center....then tried the same on 12L14 and some chatter...probably from the rubbing...lowered the tool to just below center....and it cuts well most of my cuts have been around .003 to .004....since I'm using a QCTP, the parting tool is held as some angle...I'll check that angle....it appears to be 30 degrees or less.