Whats the largest diameter you've ever parted on the lathe?

I'm talking bar not some big ass pipe. How big did you go and what tricks got it done?

Nothing over 3-1/2" for me. Bandsaw for bigger for safety reasons.

48" diameter X 48" long rubber extrusion piston. No real tricks. Harder part was rigging it for the Pettibone mobile crane. Some of the turbines were larger in diameter, but the shaft to be undercut was only around 8".

WTF kind of tool did you use to cut off a 48 inch piece? What happened when it parted?

I would guess the original was bandsaw cut. This was a spray welding shop. We undercut the O.D., spray welded it, and then machined it back to the original diameter. The largest new part I made was roughly 24" diameter which came in cut to length plus a 1/4". Some kind of three bladed shredder for Omni Source, a local scrap yard that was bought by Steel Dynamics.

I think I needed more clarity in the original question. I'm asking about parting off large diameters, not turning large diameters.

I've parted 3" solid a few times on a 10K SB in back gear. Maybe 4" when it also had a decent size center bore. Not really fun. Had a few wrecks on smaller bars, learning, too.
Mostly it is about a good slightly hollow ground end that the points are square across the face, and the tool dead square to the work.

Kyle, I'm not understanding your posts. How long was the parting tool extension on the 48" part, and on the 24" part? What the heck size was the cross section of the tool? Was there some kind of support rigged under the parting tool as it progressed??? Or were these hollow parts?

smt

Parting off is one of my least favorite operations. Two inches thick off hand. I've done 24" rings, maybe .75 thick wall stainless. If whatever your parting off has any thickness to it, you can side step the cut to give clearance for chips.

Tom

I haven't parted more than 3 inch. I'm not sure it's practical to go much bigger unless you had some way to support the piece thats going to drop.

I'm sure someone out there has parted off a mile deep with a special set up.

Parting works great when you've got a rigid machine and good set up. I think a lot of the hobby guys and others with small lathes have trouble because they have such shit for rigidity. If they had good equipment, the'd have no problem.

I've parted ~2 1/2" steel on a 8" plain turning bench lathe. Excellent tool, started with the blade choked up in the holder and then fed it out about halfway into the cut. Backgear, slow as hell. Would have been faster to saw, but no saw. Have done similar diameter 316 on the same machine but only had to part about halfway through.

Kyle Smith said:

48" diameter X 48" long rubber extrusion piston. No real tricks. Harder part was rigging it for the Pettibone mobile crane. Some of the turbines were larger in diameter, but the shaft to be undercut was only around 8".

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I'm really tempted to call BS on this. I have turned and cut a lot of rubber compounds and parting off a 48" diameter seems impossible to me. Even with the best coolant and the sharpest and widest tool you would have so much heat to contend with that the cut would close up and you would be screwed. I can imagine using a cutoff tool that is braced to maintain rigidity, and somehow holding the cutoff section on rollers or something similar to keep it from tightening on the tool but really the concept of parting off a radius of 24 inches in a lathe in any material is ridiculous.

I've parted 4" in aluminum, although the workpiece did have about a 1" ID so not quite like parting solid. In steel, I routinely part 2" 1045 solids. I never try to part all the way through workpieces that need a tail center, and always use power cross feed.

GTN inserts are certainly not the latest thing, but they work for me and I would not be shy about running an SGIH 26-x blade through 3" solid or a SGIH 32-x blade through 4" solid.

jkruger said:

I'm really tempted to call BS on this. I have turned and cut a lot of rubber compounds and parting off a 48" diameter seems impossible to me. Even with the best coolant and the sharpest and widest tool you would have so much heat to contend with that the cut would close up and you would be screwed. I can imagine using a cutoff tool that is braced to maintain rigidity, and somehow holding the cutoff section on rollers or something similar to keep it from tightening on the tool but really the concept of parting off a radius of 24 inches in a lathe in any material is ridiculous.

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I read that as a piston for making rubber extrusions. Still, don't see how parting 48" is possible.

Has anyone found any part-off blades that are longer than the typical 6"? I have searched in the past, and come up with nothing, except what appeared to be some extreme blades maybe 24" long and 1/2" wide.

I have made parting tools from 12" long, 1/2" wide and 1" tall hardened tool steel parallels. The side clearance was ground to 5 degrees and the cutting edge was ground to form a chip that curled back in on itself so that the chip did not drag on the side of the cut (the chip roll was .010 narrower than the tool tip). This was in 6061 and we were starting at 18" diameter (the finished part was 17.75 dia) with a bore of 2.2". We had to try various tool heights to find the right combination of speed, feed and downward tool deflection to get it just right. These parts were 9" long and it was cheaper and faster to cut them to 18.75" in the band saw and turn one length into two parts. The parting operation was faster than the band saw. (less than 6 minutes) We had three speed changes, one at 16" one at 13" and one at 8". We stopped at 3"dia and finished the cut in the band saw, that made it much easier to control the cutoff parts. Hope I never have to do those again.

Hu. Get much larger than the standard lathe parting tools and you're pretty much into grind your own territory. There's still a few HSS makers that supply 3/ x 1 14 x 8 M2 and MAYbe M42 HSS planks but you have to hunt for it. The shipyard I retired from stocked them in the tool room for use in bulkhead cutters, etc. The drillers in the ship fitter shop had cutting blanks from steel, aluminum, and stainlees plate down to an art form. I used to snitch their trepanning tools used in their whopper circle cutters for the heavy tool section in the machine shop.

Making larger tools from solid HSS isn't economical. SOme of it is exxpensive as silver pount for pound. I've welded HSS blanks to alloy steel shanks many times (stainless filler and 800 F degrees preheat,) Be sure to use M42 or the T series. M2 loses edge durability at temps over 800 F or so.

Stoody sells HSS TIG filler rod in a couple alloys that if puddled in a prep and ground makes a fine cutting edge. As welded deposit is Rc 62. It AINT cheap.

I've seen a few factory deep grooving tools but they were old. I suppose now all that is designed and made from scratch. I know enginers get their butts kicked if they design stuff that requires too much special tooling. No profit in a shelf full of "used once" tooling.

3” Ø shafting was the largest I’ve parted. But I have a good story…

I was teaching a Precision Machining night course at the Community College back home. Several of the older “students” were there strictly to use the well-equipped shop and some were newbies. At the beginning of class the old hands used the shop while the others accompanied me up to the mezzanine classroom for lecture. One topic that evening was lathe tooling and operation, including parting, and when we finished in the classroom we walked down the stairs, passing directly by the Colchester 17” lathe at the base of the stairs. “Big John” had a parting tool sticking WAY out of the toolholder – around 4” of overhang IIRC – and was advancing the tool toward a wheel spindle/disc rotor to part off the rotor so he could use the spindle on his Allison V-12-powered hot rod tractor.

I asked him why the tool was sticking out so far and he indicated that it had to stick out that far to clear the rotor. I asked the other students, “What do you think of this setup?”

No one spoke up so I suggested to them – and John – that the setup was too flimsy and was sure to chatter and probably break. Big John didn't miss a beat as he reached down into the chip pan and said, “Doggone it, I sure wish you would have told me before I did this one.”… and he proceeded to pull out another spindle and rotor, neatly parted in two. Big John was a BIG man (hopefully still is) but he had the smoothest touch with machining and micrometers and such. And that evening he certainly taught the instructor a thing or two about parting!

-jmcvo

Biggest I've parted off was 10" or 10.75" i can't remember exactly. I had started with a piece of 11" x 8" 4140 about 10" or 12" long on a 22" Kingston with the jaws flipped. I had to make about 4 or 5 parts out of it and it was to big for our bandsaw. I was always taught to use a drill or a rod in the tail stock to catch a part or the quill if the ID was big enough, for these parts I used the quill and I had wrapped it in a bunch of rags to increase the diameter and to cushion the dropped part other wise it they would have dropped on the cross slide. I had used a piece of 2x4 for a make shift center to support the first parts that i had to turn and thread. It was a scary job hanging on about 5/8 of material with the rest of it free hanging. I still had to bore the ID and the steady rest didn't fit around the OD, but I took my time and it worked out. The things you have to do for the oil field..

I part 4 5/8" 6061 T6 aluminum in production. There are two main requirements, first you need a blade with steps, preferably on each side to make a center chip and two very narrow ones. That way, the main chip is narrower than the groove and does not jam. Next you need oil squirting down into the groove continuously. Stop it for a moment and you are done. As Ray Behner can testify, having a 15" Sheldon lathe doesn't hurt. I have attached a picture of a chip.

Bill

In the first year of my apprenticeship, another apprentice and I were given the job of parting-in-half a cast-iron "chill" about 4 feet diameter and 5 feet long.

The lathe was utterly ancient.

The parting tool was a carbide insert brazed to the corner of a large L-shaped "square" cut from about 3/8" plate; one leg of the "square" was set in the toolpost and the end of the other leg was set on the lathe bed for support. We may have put a piece of steel "crossways" under the end of the "leg"; I can't remember the details after rather more than half-a-century.

It took, I think, about 3 days of slowly-winding the cross feed; the chill was supported in a set of rollers like a steady rest and, when the cutoff section broke free, it was something of an anti-climax.

It just stayed on the supporting rollers which had been set so as to balance it with a small bias towards the cut.

We apprentices knew nothing; our foreman supervised the setup and he was a man of infinite knowledge.