Candlesticks turned from 2-1/4" aluminum round (plans from Machinist's Workshop magazine a few years ago) in a four-jaw chuck.
A cutoff tool I ground from a high-speed steel blank with what looks to me like appropriate clearances. It's on center and, to my eye, rotated to feed with the clearances even. The cutting tip is about 3/16" across, cut straight (i.e, it cuts into the workpiece parallel to the rotating axis). The tool is mounted on a quick-change tool holder on the compound.
I ran a continuous oil drip into the progressing groove.
One fair cutoff and three really dreadful ones resulting in ruined candlesticks suggest to me that my technique could be better. Too bad; the candlesticks looked quite nice otherwise. I was especially pleased at how they came out because the specified taper is longer than the compound's movement and had to be done in two parts.
The bad ones all had chatter and wound up "dished" with the cutoff piece convex. The chip was small flakes; the turnings and facings on this metal all produced nice continous ribbons.
If your cutoff face isn't flat you forgot to lock your carriage or else the bolt holding down your toolholder on your compound is loose and it's sliding sideways.
On small lathes you may well need to really crank down the gibs on the compound and on the cross-slide, until they are quite difficult to move. This adds a lot to rigidity.
You may well need to run faster rather than slower for aluminum.
I never had much luck with parting on my 9" SB until I got the T-slot cross-slide table and rear toolpost from MLA and machined them. It can still be tricky but now I can do it.
Of course, you should have minimum overhang on both the part and on the tool, and you need to make sure you aren't experiencing aluminum buildup on the cutoff tool. The fluid I use on aluminum is kerosene, and I apply it from a squeeze bottle I got from my kitchen. It once held Huy Fong Sriracha sauce, if that has any meaning for you. Anyway, it can do nice slow drips or a steady stream, and it is wonderful for dispensing kerosene. I've never had much luck using oil on kerosene.
You may be able to save your parts by using Cerrobend or similar in a cup chuck to grip them.
Start with the width of your tool. 3/16" is way too wide. When machining, it's always a good idea to keep tool contact to a minimum----and in this case, you haven't. For parting only 1-1/8" deep, you should be able to get by with a 1/8" wide tool, and that makes a significant difference.
Speed. Aluminum will machine much faster, so unless your machine is very light duty, running faster, and parting near the chuck, is much better than running at your slowest speed.
When you get chatter, once you've adjusted tool contact area, feed rate and speed need to be balanced. Increasing feed rate often eliminates chatter. I think in your case, especially considering you're talking about what appears to be an 8" lathe (I'm not familiar with your machine) that it may be somewhat too light to be able to do what I'm talking about. Rigidity is everything when it comes to parting.
Make sure that your compound is not well extended, so there's spring under the overhanging portion. Having it rest mostly on its ways will help make your setup more rigid. Have it extended only as much as is necessary. Is your compound creeping on you? Keep it locked if possible, or keep the gib snugged up so it's hard to move. You can back it off for easy movement when you need the compound. That's not very convenient, but it often makes a big difference in how well a machine resists chatter. I practice that with my lathe.
Don't use oil on aluminum. It doesn't work nearly as well as other things. Kerosene works wonders, or you can even use WD-40, but oil is a poor choice. Keep the cut wet at all times. The occasional drop may not be enough, and you'll get gulling of the aluminum on the tool. That, of course, depends on the grade and condition of the aluminum. 2024-T3 will part without problems, but you're not likely to be machining that alloy.
Did you hand grind your parting tool? Are you familiar enough with side, front and back clearances that the tool is making contact only with the cutting edge? Is the cutting edge perfectly parallel with the part? Does your tool have any back rake (Positive, never negative)? Is the edge sharp? On center? Generally, if a parting tool dishes, it's either dull on a corner, or the relief angle isn't adequate and it's dragging. Once it starts cutting a dish, the dish shape encourages more and more error. Also, make dead sure that the parting tool is set up such that it *never* makes contact with the sides, assuming it parted straight. Any contact with the sides will encourage dishing. Check that your tool, when mounted, addresses the part such that it still has side relief. While it may look fine in your hands, once it's mounted in the lathe, it may be leaning to one side or the other, dragging at the base of the tool. That could be the cause of the dishing condition you spoke of.
Too bad I'm not there to take a look! I may have overlooked something here. I've always hand ground parting tools and have pretty good luck with them.
Ecellent answers from both Grant and Harold. I have tried numerous different squeeze bottles from the kitchen myself, but, like Grant mentioned, Huy Fong Sriracha has worked best for me, too. Bill
Dont cut off aluminum slow. You can do it at least 500- 800 rpm. And make sure EVERYTHING on your lathe it locked down, including the carraige. Aluminum can also..sometimes..benifit by putting a bit of a positive rake on the end of the tool. Not a lot..but some. Couple degrees worth. and grind a smidge of relief just behind the cutting edge so its a very small inverted V with the wide ends of the V doing the cutting.
Take a piece of scrap, and start cranking up the rpms as you cut and you will find a magic speed it comes off like butter. Aluminum is sometimes odd.. but its soft and if you try cutting it too slow..it trears rather than peels.
I run my HLV-H normally about 1000 rpm for just about everything..and that includes cutting off aluminum with homemade cut off tools.
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
What Grant, Harold and Gunner stated.......Just to lazy to type tonight.........There was a good poiece posted here by a member that is now deceased by the name of T Nut. It laid it all out pretty darn dead on the money for parting with smaller lathes which seem to give most folks a problem. His way of grinding the parting tool as much different than I have ever been taught both by old timers and in tech school, and it worked 100 times better......if you have the time do a google search for posts on parting by T Nut (aka Robert Bastow) I have it tucked away somewhere, just have to find it.... Regards
Thank you, everyone. I will print out all the replies and try some scrap turning. There are several suggestions that would not have occurred to me. I'm off work tomorrow and should be able to report back.
I very much appreciate everyone's time. I've been following this newsgroup for a long time and have learned a great deal from all of you.
Another issue is feed. The OP did not say if he was power feeding or hand feeding the cutoff.
I've found that chatter will happen if the feed rates are too slow, so I like to do perform cutoff operations by hand typically, so I can easily change the feed rate adaptively, to get the smoothest cut.
Often it's a lot faster than one would imagine.
If he's getting a dished cut that implies something either shifted during the cut or was misaligned beforehand. As you say, lock everything down. ON smaller machines snugging the gibs helps too.
The biggest problem I've experienced on small machines (even with the cross slide gibs tightened) is that even a little hogging will take up the lash on the crossfeed screw, resulting in a lot of hogging... so raking the bit is out for my little 6" lathe. The 14" seems not to care.
Not necessarily. If the tool isn't ground properly, or isn't set up properly (leaning to the side, for example) it is possible it's touching (ever so slightly) somewhere besides the cutting edge. It takes very little to steer the blade at an angle. Considering I've always hand ground my parting tools, I've learned that even irregular side relief *can* be the source of dishing. Same goes for top rake, which I always grind by using the radius of the wheel. If it isn't parallel to the ways, that alone can encourage the parting tool to drift. Unless a parting tool is very short (grooving tool, really) all surfaces should be square with the cut, otherwise you encourage poor performance.
True enough. When I had my old tired Logan 10, I somehow or another managed to cut off with a slightly negative tool. Mostly because I kept burning up cut off tools, before I had learned anything. It also gave me a decent enough finish with negative rake tooking, if I did press too hard. Back carragie "ways" were pretty tight, and it didnt lift the back of the carraige, so I could get away with it.
But aluminum is pretty easy to cut off. Now that 3" chunk of chromemolley I cut off last night..boy howdy..that was interesting getting the cut started...then I had to keep pouring the feed to it..cut pretty nice once I found the magic feed rate..which of course kept increasing the deeper I got.
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
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