Parting Off ridiculousness!!

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marcopolo

Newly Enlightened
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Jan 14, 2008
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Why is it so difficult to part off on the mini lathe? I read in books that it can be difficult and thought to myself - what's all the fuss about? Stupid me.

Parting off tool 3mm wide, on 38mm OD Aluminium tubing taking off roughly 2mm deep. It's taken me 15min per slot!! I'm using Rocol Mutlisol 1:15 water.

It's squeals and whines throughout the job and stalls the lathe if I dig too fast. I've tried dead centre, slightly below centre with tool angled down a few deg, above centre a few degrees - nothing makes it easier?? As far as I can tell the tool is very tightly mounted and square to the job. Help - any tips??

The tool is tungsten carbide.

Many thanks.

Marco.
 
Parting off tool 3mm wide
Click to expand...
Therein lies the problem ... even on a larger lathe, like my 10" South Bend, 3mm (.118") is too wide for the available power & mass of the machine. I wouldn't try anything larger than 2.2mm (.087"), and you'll get better results by going thinner. For your machine, 1.5mm (.062") should be perfect, and it is a common insert width from lots of suppliers.

Set the tool about .010" below center & infeed at a constant speed that produces a continuous curl. If using tailstock support, start withdrawing the tailstock ram as the tool nears the break through point. Dry or wet cutting should make no difference with carbide tooling, although aluminum is gummy & may want to stick to the insert resulting in a BUE (Built Up Edge). Coated inserts are less prone to BUE, and aluminum specific inserts do the best job, but are not yet really common.

The thin insert will solve & avoid most of your problems.
 
Until I got my Hardinge which is rigid enough to make parting a sweet sorrow, I almost always cheated when it came time to parting. This admission will likely show me for the true hack that I am but then if the shoe fits...

I would use the parting tool to put a groove in the part where I wanted it parted and give me a reference. At that point, with the spindle turning, I would use a hack saw with caution and enough pressure to get chips coming off but not to the point that I would have it bind and want to do something nasty. I would let the hack saw generate a kerf more in keeping with the parting tool's width and not as narrow as just the blade. In some cases if it didn't feel right. I would stop the spindle before the parting cut was done and finish off the parting just cutting through with the hacksaw by hand.

EDIT I should add that my parting tool has a max of 1" OD. Even now with my Hardinge if the stock is greater in diameter, I do what I can with the parting tool and finish off with the hack saw. :eek:
 
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Many thanks for the info.

I've orderd a 1.45mm Parting tool with holder (8mm shank). I take it the pressure on the face is too great at 3mm, regardless of how slow I feed the blade in?

Meanwhile McGizmo, i'll give your hacksaw method a careful try!!

Marco.
 
In addition to above, can you lock your saddle? And your carriage most probably has a lock as well. Obviously don't lock the carriage but tighten it a little just for the parting off operation, to get rid of any play there might be.
Part as close to the chuck as you can. Minimum of overhang. You need everything as rigid as possible to stop chatter etc.
 
I use a simple HSS parting blade, and have no problems with most parting jobs. The blade is 1/16 inch (.066) wide and 1/2 inch tall. I ground a little relief on the sides, and some top rake. I set it at dead center. 1 inch 6061 in a few minutes ( though I never time it, so it may really be an hour :) )

I usually use tap-magic gold. It smells good and keeps it from binding. I use a small brush the keep the chips clear if necessary.

Real thick pieces, I go in 1/2 inch, back out and move the carriage .030 to make the groove wider so chips don't cause a problem.

Too little infeed and it work hardens. Too much and it can seize.

Good luck.

Daniel
 
the pressure on the face is too great at 3mm, regardless of how slow I feed
Click to expand...
That's pretty much it. Smaller machines cannot resist the push of a wide tool, so the tool chatters, sometimes the part jumps up on the tip of the tool (OUCH!), etc. The 1.45mm should be perfect ... you will not believe how little force is needed as you advance the tool into the work.

I would use a hack saw with caution
Click to expand...
We've all done that at one time or another, but most will not admit to it:D The secret to hacksaw parting is a really slow spindle speed. Using a HSS blade, even a bimetal blade, the rpm should be set at a speed to give about 100 sfpm. That means 382 rpm for sawing a 1" OD cylinder.

I worked in a shop that had mostly larger lathes, 18" was the little one, 72" was the biggest (coal mine repair shop). We would often part mechanical tubing that was 6" OD, 1/2" wall, for a contract job that ran about monthly. The tubing was first cut on the band saw, to yield a length slightly more than twice the length of the finished piece. That got chucked in the lathe & the parting tool was run in about .490". You really didn't have to watch the infeed too closely, as the pitch changes when you're almost through. Also, if you shine a light inside the tube, you'll see a raised band when the tool is about to come through. Stop the lathe before breaking through, take a 5# brass hammer & sharply strike the top of the tube, and off comes the part, leaving the other half still in the chuck jaws. If you just paid $30k for a nice CNC lathe, you probably don't want to try this:crackup:
 
For aluminum use kerosene as a lubricant to keep the buildup on the tool to a minimum. One of those pump-can type oil cans works well for applying the Kerosene.
 
I have been using a .040 parting tool for all my aluminum parting. I use a brush with kerosene. I try to part items as close to the chuck as possible.
 
This is the tool I use to part off. It cuts through aluminum like butter. In titanium it's pretty sweet too. In 1" ti stock I usually set the power feed at around .012"/rev and the spindle speed about 180-260rpm. Seems like the higher speed actually works better than slower. Maybe one of these days I'll make a short video clip of this tool parting off a piece. It's really quite a site compared to how I used to do it on the old South Bend. :D

Edit: Just parted of a 1" piece of ti @ 370rpm and .012"/rev... :naughty:

parting_tool2.jpg


parting_tool1.jpg
 
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One of the problems with the mini lathes you dont get into the horsepower band until you get into the high RPM range but you get a lot of chatter. low rpm less chatter but low horsepower, you can do it at a low rpm but grind your bit to a negative rake and use cutting oil. I run my tool tip just above center. the negative rake will allow you to feed harder but will not allow it to bite and stall. Hope this helps you!:thumbsup:
 
Having dedicated grinding wheels for carbide and steel tooling helps a lot. One of those big lighted magnifiers and adjustable rests helps also.
 
sport69 said:
One of the problems with the mini lathes you dont get into the horsepower band until you get into the high RPM range but you get a lot of chatter. low rpm less chatter but low horsepower, you can do it at a low rpm but grind your bit to a negative rake and use cutting oil. I run my tool tip just above center. the negative rake will allow you to feed harder but will not allow it to bite and stall. Hope this helps you!:thumbsup:
Click to expand...

Sport69. Interesting observation on negative rake for low power lathes. You have taken a position opposite to the generally accepted principle of very sharp, positive rake tooling for small lathes. Can you elaborate a little?
 
Very sharp tooling has it place but my experience with small lathes is that with the lack of horsepower in the low rpm range a positive rake tooling has a tendency to want to dig in causing the lathe to stall, now if you have a geared head and good torque at low rpm you may get away with it. The negative rake at low rpm helps keep the tool from wanting to dig in letting you push the tool a little harder also if all goes well you get less chatter thus a better finish. another method is if you are not just grooving use a part off bit which is not a flat bottom tool. My past experience is 20+ years journeyman machinist, now full time LEO.....BY the way Kerosene was a good call for Aluminum! *Old School* Anglepoise goog advise on locking the saddle as well!! There appears to be a lot of experience out here.
 
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Chatter and digging in / biting and stalling are all signs that you don't have enough stiffness. How far away from the chuck are you trying to part off? Is the part you're trying to part supported by a steady / live center?
 
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