I HATE parting!!!

[jgarrett]

I had just finished turning a piece of 1" CRS to .750" for the crankshaft on my "Mill Engine" and had gone maybe .125" into the cut parting it off. I was using an Aloris QCTP with a HSS tapered cutoff blade. RPM was 60 and was just using light preasure on the feed when BAM! the whole thing exploded!!!
This was a almost new 6" blade and I managed to find 4 pieces about the size of a dime each. Good thing I was off to the side. Have no idea what happened..Usually when I have problems it is near the end of the cut.
Yes, Rick I did have my glasses on...I did find one more piece buried in the cut.
Julian

 

[tattoomike68]

Parting can be such a pain, about all I do to make it happen easy is pump oil on it like crazy the whole time.

 

[rake60]

Parting off is a learned skill to be sure.
For 1" CR I run about 300RPM and use a home ground HSS bit.
The most important thing to watch is the chip. As Mike says, cutting oil
goes a long way toward keeping that chip moving. If that chip stops
moving it will gall up quickly and snap the tool off. The tool position
to the center line of the stock is also critical. If it's too high the tool will
tend to climb and rub rather than cut. Too low and it will grab causing
enough down pressure to break the tool.
Mini-Lathe.com has a good tutorial on parting in their web pages.
Parting Operations
Having all that information, it is still a "Seat of the Pants" operation.
When it starts to chatter you don't have much time to react.

Rick

 

[Cedge]

I must be among the blessed. Parting has not been a problem. I regularly part off up to 3 inches in diameter on a 7 inch Chinese lathe. I've dulled a few cut off blades, but only twice have I managed to shatter one. Those were my fault for getting overly aggressive while plunging. Dunno what I'm doing differently but I sometimes do a cut off as high as 250 rpm with no major difficulty beyond a slight tendency toward chatter.

The only materials that have been a problem were some types of brass and common drill rod. Both seem to want to dig in, even with no rake on the blade. Soft aluminum collecting on the blade can be annoying but lubrication seems to stop even that.

Bumble bee syndrome, perhaps?

Steve

 

[lugnut]

Someone on the HSM forum suggested turning the blade upside down and running the lathe in reverse. The chips tend to fall away from the tool and help keep the cut clear. I have had real good success using this method.
Mel

 

[Bogstandard]

Parting off seems to be a personal thing, you either love it or hate it.
If you hate it you will have trouble. If you are scared of it you will have trouble. The reason being you tend to be too cautious, slow machine speeds, very light cuts etc, waiting for the 'bang'. The metal has time to 'cool down' and will gall and then stick.
I have no inhibitions about parting off, especially non ferrous. I can't use a hacksaw, so I use the lathe for cutting all my billets, even 1/4" ss rod. In industry if you were seen to have trouble with it you were out of the door, it is just another machining operation.
Here is how I do mine. Razor sharp tooling. Tool set a 'smidge' (two or three thou) high to allow for downward forces pushing the tool a little lower. For brass, somewhere between 500 & 1000rpm, whatever the lathe is at, for ali just a bit slower. Set the width I want, with the cut very close to the chuck to keep down vibration and to take the side loads. For ali a bit of cutting oil, and put on power cross feed. A few seconds later the bit falls off the end.
For ferrous I just feed in by hand and it is feel that gets it thru, but again none of these slow speeds and feeds and the tool is again set slightly high.

Very sorry gents, but for you who have trouble there are no golden rules for parting off, it is a matter of finding a regime that suits you where you get the results you want. All you can do is look at the tips that the people have put in these posts and try it until you get good results.

Just to prove it is standard practice here is a vid of it being done this morning.

<embed src="http://www.youtube.com/v/aS6Ct502S7g" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed>


John

 

[jgarrett]

Good info guys, Thanks, Here are some of my thoughts.

I was at 60 RPM which is too slow.

I used no lubricant.

I had the blade really clamped down tight in the holder,maybe enough to stress it because it did not just break the tip the whole blade shattered.

I am going to try again today but with a ground bit since I don't have another blade.

Thanks again,
Julian

 

[georgeseal]

Julian,
The cutoff bit that came with my lathe ( 12" X 36" Atlas ) was just a little over .125 thick. No matter how it was ground, what feed , what lube, it would either chatter or break. Every once in awhile I would would get good results just enough to keep me from taking up gardening.

I switched to one that is .040 thick and have had no further problems.

One other trick is to widen the groove if posiable The chip has to get out of the groove.
Never part off between centers

George

 

[mklotz]

Supplying cutting oil greatly improves the chance of a successful parting-off event. Since I only do light hobby work, I don't have a pressurized feed on my lathe so I cobbled this 'dripper'

together from plumbing bits available at the local DIY store. The 'cup' at the top is a stepdown copper plumbing union. The valve is a commercial type used on the water lines to refrigerators with icemakers. The rest is small copper pipe and sundry fittings turned from brass. The whole thing is soft-soldered.

The back of the lathe carriage came with some tapped holes (presumably for a taper attachment which I don't have) so I cobbled a holder to hold the dripper above the cut. (Since it's attached to the carriage, the dripper will follow the tool and always be above it.)

I fill the dripper cup with sulfurized cutting oil and adjust the valve so it drips about a drop every five seconds or so into the developing parting-off slot.

It's also useful for applying oil when trying to make a really nice finish cut on a part.

When it's not needed, loosening two thumbscrews removes the dripper and support from the lathe.

 

[Sprocket]

If you use a "T" shaped parting tool in a quick change tool holder, you may have to shim out the bottom edge so the tool stays vertical in the holder. Also, when you square the blade to the lathe axis, square to the blade, not the tool holder. I usually use a parallel against the chuck face, and set the blade to that.

Doug

 

[jgarrett]

Marv, drip idea is pretty neat.
Doug, It was a flat blade.
Made another one today and used a handground tool and every thing went fine. Upped speed to 240 and used gobs of oil and only had small amount of galling. Now on to next part.
Thanks,
Julian

 

[Tin Falcon]

Parting is so sweet sorrow.
William Shakespeare

Was he a machinist???
Tin

 

[mklotz]

Parting is so sweet sorrow.
William Shakespeare

Was he a machinist???
Tin

No, but dragging his elegant poetry into a metalworking discussion probably has him turning in his grave.

 

[rake60]

Here's an automatic parting story for you.
The CNC machine I run at work can part off pieces up to 3" in diameter
without a bit of chatter. It maintains the surface speed as the tool advances
so once you find what the material wants, it will adjust that SFPM
speed up to whatever maximum speed you set.

The pins we were making had a 1/8" chamfer on the back side where it
was to be parted off. The young man who wrote the program selected
the wrong parting sequence in the program, causing the machine to part
the 4 pound 3" diameter pin to a diameter of .150" and THEN cut the
chamfer. Once the coolant is flying inside the machine you can't see what
the tool is doing. My first indication that something wasn't right, was a
sound that could be described as a cannon blast. The machine
overloaded and immediately shut it's self down. An access door at the end
of the machine was blown open but it closed as it should have.
After a change of pants and SOCKS the program was changed to cut the
chamfer THEN part the pin off the other 8 were machined without incident.

Operator error isn't limited to manual machines when it come to parting. :?

Rick

 

[alan2525]

On my little Taig Lathe I've had problems Parting off, suffering a fair bit of chatter and always have to face off the surface afterwards. The parting tool was one of the T shape, hollow ground ones, and had CHINA written on it...

That was good enough reason not to use it! It also had HSS written on it as they obviously put that on as the one they copied probably had HSS written on it! It seemed a little bendy for High Speed Steel too...

I purchased a Cut-Off blade from A.R.Warner Co from the Little Machine Shop and the quality is superb. The tool has nice sharp edges, precision groud and looks to be nice quality Steel too. Parting is now fun! I also purchased some of the narrower .040 x 1/2" A.R. Warne cut off blades which work an absolute treat!

;D

 

[Cedge]

I found myself working with copper today. As anyone who has turned copper knows, it's a gummy metal that wants plenty of lubricant and a razor sharp cutting tool. I can count myself among those who know these things now, but only after I'd found just about every way, that it didn't like, to torture that poor piece of metal.

When it came time to do a cut off, I figured I'd get some real education and a perfect chance to use all those words I keep stored away when the grandkids are around. To my surprise, it went quite smoothly, as long as I "watched the chip" to gauge how things were progressing. A regular shot of WD40 and the cut kept moving along nicely.

I actually found it a pleasure to work with the copper once I figured out "what it wanted". I'll post a photo of the finished piece, later, in the Water pressure engine thread.

Steve

 

[zeusrekning]

The big PITA with parting is it is almost always done after a bunch of other operations have been completed. If you are just parting off blacks it usually goes fine. I have never used one of these smaller lathes but does a negative insert help the issue of chatter and tool pulling?
Tim

 

[BobWarfield]

Tim, should be the other way around: more positive is more likely to help.

I started out having a terrible time with parting. Much of it was my technique, some of it was my tooling. I wound up buying an Aloris indexable cutoff tool because I found one cheap on eBay and I like carbide tools. ;D

It was a big step up for me. Part of the big step up also came because I already knew carbide wants to be more aggressive, so I was a lot less tentative with this tool. The biggest problem it has is size. It wants to knock an 1/8" cut out of whatever you part, and sometimes that's too much. So, I got some nifty HSS parting blades that fit in a regular QCTP holder. They're ground with the same 3/8" or 1/2" square shank with a blade on the end. These too respond well if you feed them aggressively.

My tactic has become:

- Check center height and squareness really carefully before starting. It's easy to miss that the blade is slightly angled or that you're not on center. Make sure!

- Feed a little easy for just a small cut to test how well things cut. If you can't even get started at first, you have something seriously out of whack. Increase rigidity, change speeds, add coolant, but do something! BTW, I don't automatically assume coolant is needed, but it seems to help in maybe half the cases. I have a bunch of squeeze bottles (plastic condiment bottles) handy that have my coolants available: heavy sulfurized cutting oil, kerosene, and liquid WD-40 which is sort of in between.

- When that initial feed is cutting freely, I step up the feedrate to take as healthy a cut as I can. I let the chips be my guide. In other words, I feed harder so long as the chips peel off well, but you can see when you've just gone too far.

- If chatter still sets in, I first increase the spindle rpm slightly (1/4 turn on my speed control). If that makes it worse, I try decreasing. One direction or the other is nearly always helpful, and usually the increase is the better.

This last point relates to that idea of Constant Speed turning that Rick mentions. On a CNC, your spindle speeds up as you cut to smaller diameters so the surface speed on the tool stays the same. Have you ever been facing or parting (operations where you cut a lot of different diamters) and noted the cutting action suddenly got better? Your spindle speed may have been too slow for the larger diameter. If it gets worse, the opposite may be true. If you don't have variable speed, this may not help much. But if you do, a little tweak on the control may make things much nicer. If I take a lot of material off turning, I may even increase the speed a tad as I get to smaller diameters. You'd be surprised at how effective this can be.

Cheers,

BW

 

[chuck foster]

hi guys ;D
i hated parting until a good friend of mine showed me how to make and use a rear mounted parting tool.
the tool is mounted upside down and is fed in from the back side of the work.
i don't know why it works so good but it does!!
i can part off any type of metal and any shape, i have parted square cast iron (2" x 2" ) and i have parted the same in stainless.
i have never had the tool dig or dive in to the work piece, the only time i have had trouble is when the tool becomes dull.
i would guess when parting i can run the lathe (9" south bend) at about 600 to 700 rpm and you can feed the tool in very fast (compaired to parting from the front). 1" diameter cold rolled takes about 30 to 60 seconds to part off.
i have pictures of the set up if anyone is interested in seeing what i have.

cheers
chuck