Parting off in a Lathe

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KellyJones
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Parting off in a Lathe

Post by KellyJones »

I got my first machine in about 2001, and have attempted to teach myself all I can as a hobbyist. One type of job that has always scared the willies out of me is parting off, mainly because one of my first attempts ended in disaster, and if not for the face shield, could have ended with a serious injury. I have looked in all the usual sources for information about how to part safety and correctly, and there is little to be found. I thought I would share my observations with others who may be new to the hobby and looking for some help. I also thought Spro, Harold, and other more experienced folks could chime in and either correct anything I mis-state (thus providing me additional education), or add to the general discussion.

Parting disaster seem to come in two flavors: The first due to the tool being pinched in the work, and the second due to the work riding up on the tool. Taking the second issue first, parting tools need to be set perfectly on the center line of the work. If the tool is too low, the work will run up the tool (particularly near the end of the cut), and something will give, probably catastrophically. Too high, and the tool won;t cut at all and you will be tempted to "push it", again with catastrophic results.

The tool also needs to be mounted perfectly perpendicular to the centerline. I found this difficult to do at first. I tried setting up a dial indicator and running the tool back and forth on the cross slide, and adjusting until it came into zero. I found a much easier method since then. I simply loosen the tool post and place a parallel between the parting tool and the chuck face and ensure everything is flat to each other, then tighten the tool post. This method requires a flat chuck face and a flat parting tool.

Another reason the tool might get pinched is lack of rigidity in the set up. I saw this the other day (which is what made me think about writing this). I was parting a .75" bar with about 3 inches sticking out of the chuck and only about 1 inch in the jaws. I saw the work piece start to "move" and immediately stopped the cut. Had I kept going, I am sure the tool would have gotten pinch and I would be dodging shrapnel.

I also use the slowest possible speed when parting. When the tool pinches, things happen fast. More experienced folks are able to run at higher speeds, but I try to take it slow. This also means a very slow feed. Since I am a hobbyist, not in production, no harm is done. Take your time.

Use lubricant. Lots of lubricant. We are trying to prevent the tool from getting stuck, remember? In addition to lots of lubricant, I also withdraw the tool every few thousandths and clear all the chips. The chips has a nasty habit of trying to get stuck between the tool and the work, thereby pinching the tool.

Finally, listen and watch. More than any other operation, you will likely hear or see disaster before it happens. If the sound of the cut changes, or you start to see visible "runout" of the work when there wasn't before, stop the cut immediately and figure out what went wrong.

I welcome comments and insights from folks who are more practiced at this than myself.
Kelly Jones, PE
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NP317
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Re: Parting off in a Lathe

Post by NP317 »

I always dress the cutoff tool cutting surface with a diamond hone, before cutting.
It always makes the cut much easier.
But most important: Rigidity and setup.
A sharp, properly aligned tool in a rigid setup allows cutting at advanced turning rpms.
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BadDog
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Re: Parting off in a Lathe

Post by BadDog »

Talking about slow, in my limited experience, that doesn't always help with success. Slower surface speed for parting doesn't seem to matter a whole lot, particularly since it varies tremendously from start to finish just to physical geometry. But slow infeed is another thing. I found that there is too small an infeed, and that results in poor chip formation, potential packing, chatter, and potentially pushing as the edge wears away. So a reasonable infeed, preferably managed by power feed, is the option I would take. I also start at the highest speed (rpm) I can without hurting the tool so that the drop in speed as I get closer to center is still as fast as I can manage. I don't do work where speed is really important, but I'm quite impatient to get on with whatever task I'm working on. Sitting and waiting isn't in my nature.

Beyond that, my experience with parting has been very good since I upgraded to a Rockwell 11" lathe. Before that, my little 9" limp-noodle lathe didn't like it AT ALL. My current 17" parts 2" steel without any drama at all, but that's getting outside the fat part of the curve for the majority home shop machines, and I also use Sandvik insert parting tools. And that's a sure combo that takes all the drama out o the operation even with minimal machinist skill.
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SteveM
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Re: Parting off in a Lathe

Post by SteveM »

BadDog wrote: Fri Nov 27, 2020 8:24 pm But slow infeed is another thing. I found that there is too small an infeed, and that results in poor chip formation, potential packing, chatter, and potentially pushing as the edge wears away. So a reasonable infeed, preferably managed by power feed, is the option I would take.
My Atlas is too flexible for power infeed, but when you are cranking you can definitely feel a sweet spot in the feed rate.

I parted off disks off a 3-1/2" bar of steel on the Atlas. Did it with back gear and no more than 1/4" of overhang beyond what needed to be in the part. This is where the Aloris-type parting toolholder sucks - because the tool is on an angle, if you pull the bit out, you have to re-adjust the hight.

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Steggy
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Re: Parting off in a Lathe

Post by Steggy »

KellyJones wrote: Fri Nov 27, 2020 6:06 pmI also use the slowest possible speed when parting.
Back in the day (way back, 60-odd years, to be exact), I was taught to use a spindle speed for doing cutoff work that was in the same range as ordinary turning and facing.

An unnecessarily slow spindle may encourage you to feed your cutoff tool too hard to speed up the process. Tool loading will be excessive, and work-piece deflection and/or tool breakage may occur. Conversely, running the spindle too fast may overheat the cutoff tool and dull it. With some experience, you will develop a feel for this and will know how much your lathe can tolerate without getting wonky.

When I part something in the lathe I power-feed my cutoff tool, even in tougher materials, such as austenitic stainless. Doing so frees my left hand to stop the cross-feed at the right time, and frees my right hand to inject plenty of cutting oil into the kerf and keep everything cool. The procedure works well enough to produce a smooth finish comparable to a facing finish-cut.

The two tapping plugs in the below photo were made from 304 stainless. The face of the one on the left is the face produced with the cutoff tool—that tool is HSS, by the way. That's what it looked like when it came out of the lathe.

bolster_tapping_plug01.jpg
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V »

SteveM wrote: Fri Nov 27, 2020 11:35 pm
BadDog wrote: Fri Nov 27, 2020 8:24 pm But slow infeed is another thing. I found that there is too small an infeed, and that results in poor chip formation, potential packing, chatter, and potentially pushing as the edge wears away. So a reasonable infeed, preferably managed by power feed, is the option I would take.
My Atlas is too flexible for power infeed, but when you are cranking you can definitely feel a sweet spot in the feed rate.
That sweet spot can be duplicated by power feed with a net result of a better parting experience. That's true until something changes in the operation, often with bad results. There's nothing magical about hand feeding, and, in fact, it's not nearly as good as power feeding. That doesn't prevent me from doing it on a fairly regular basis, however. I'm not critical of the method, just want readers to understand that the problem with parting isn't with how the tool is fed, although it certainly can be.
I parted off disks off a 3-1/2" bar of steel on the Atlas. Did it with back gear and no more than 1/4" of overhang beyond what needed to be in the part.
Need I say more? You achieved success because you didn't violate any of the simple rules. Keeping the amount of material between the parting tool and the chuck jaws to an absolute minimum is very much a part of that.

H
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armscor 1
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Re: Parting off in a Lathe

Post by armscor 1 »

I use Iscar and Kennametal inserts in my parting blades with minimal stickout, on centre, same speed as normal cutting speed, cutting oil or coolant to lengthen the life of the inserts, not cheap.
Watched one of my mates on a large Le Blond years ago when Iscar was just producing inserts, he set the Lathe on its highest speed and just ploughed the part off feeding as fast as he could.
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V »

KellyJones wrote: Fri Nov 27, 2020 6:06 pm I was parting a .75" bar with about 3 inches sticking out of the chuck and only about 1 inch in the jaws. I saw the work piece start to "move" and immediately stopped the cut. Had I kept going, I am sure the tool would have gotten pinch and I would be dodging shrapnel.
My thoughts follow, for what they're worth! :wink:
Where you made the cut is all important. If the cut was very near the jaws, that's reasonable. If you had a good distance between the parting tool and the chuck jaws, it isn't. Also, if your chuck isn't gripping full length (loose at the outside end, tight at the back), the part isn't supported well enough to expect a reasonable outcome. In that circumstance, the part is free to oscillate because of the limited support, which also is beyond the face of the jaws. Make sure your chuck is holding at the tips. Part as close to the jaws as you can, even if you must relocate the part for the cut. If you own a light duty machine, this is ultra critical, as you face circumstances that are difficult to overcome without adding to the problems. Parting is a relatively heavy operation and is not forgiving of a flexible machine.
I also use the slowest possible speed when parting. When the tool pinches, things happen fast. More experienced folks are able to run at higher speeds, but I try to take it slow. This also means a very slow feed.
What should dictate spindle speed more than anything is the rigidity of your machine, although you generally should observe recommended surface speeds, so you don't destroy the parting tool. There are circumstances where one can part at blinding speed, such as when parting the right grades of aluminum, or free machining (leaded) brass. If your lathe has a top speed of 2,000 rpm, there's no reason why you can't part 1" diameter leaded brass at that speed.

It's no secret that slowing spindle speed is a way of limiting or stopping chatter, which is an issue with parting tools because of the broad surface presented to the cut, but spindle speed should be faster than the lowest spindle speed on the machine, assuming you have reasonable rigidity. Chatter is generally exacerbated by the slender parting tool, often extended beyond a reasonable amount. When parting, you should limit the amount of overhang of all elements. That includes the distance between the parting tool and the holding device. You should part as close as you can to the jaws, or if you're using a collet, as close as you can to the collet.
Use lubricant. Lots of lubricant. We are trying to prevent the tool from getting stuck, remember?
I agree. It's generally an invitation to a broken parting tool if one parts dry. That's particularly true of the typical HSS parting blade, which has no relief towards the shank. As I don't use that type of parting tool, that's not one of the issues I face. While cooling is a side benefit, what lubrication does is prevent chip welding, and that's really important when parting most materials; especially mild steel, which has a propensity for galling.
In addition to lots of lubricant, I also withdraw the tool every few thousandths and clear all the chips. The chips has a nasty habit of trying to get stuck between the tool and the work, thereby pinching the tool.
If you happen to gain enough confidence to part under power, using the correct feed rate, that's not a problem you face. You should be generating a ribbon, and, with luck, it will coil in the generated cut. You end up with a "watch spring", tightly coiled, for lack of a better description. That is likely not true if you're using insert carbide tools, however. They most likely do not generate such a chip. Hand ground tools can and will. The material being parted plays a huge role in this, too (my comments about mild steel, above, as an example).

H
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liveaboard
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Re: Parting off in a Lathe

Post by liveaboard »

I've noticed that parting tools for smaller machines are narrower [and shorter].
I assume that will reduce the loading and chatter.

I bought a 3mm wide HSS blade type, and after gaining some experience with getting the right grind it works ok for me.
It's probably a bit thick for my machine, but I really do need the length it has.

Maybe I'll dare try power feed parting one day. Maybe not on the lathe I have now.
SteveM
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Re: Parting off in a Lathe

Post by SteveM »

What I do for lube is use a brush to insert lubricant at the top of the slot so it is right before the cutter. It lets me brush it into the cut instead of trying to use the tip on the can to drip it in, which tends to get more on the sides than on the bottom.

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Re: Parting off in a Lathe

Post by earlgo »

One procedure I use to prevent tool chip "pinching" is to cut about 2x the tool width in depth, back out the tool, move it about 1/3 the tool width to one side and cut again but another 2x the tool width past the previous depth. Back out the tool, move it over to the previous cut and repeat as required using lots of cutting oil. Bear in mind I am using a 12" Atlas commonly known as a "flexible flyer". Before utilizing of the above procedure, I had all the oopsies and crashes that Mr Jones describes.
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John Hasler
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Re: Parting off in a Lathe

Post by John Hasler »

I grind my parting tools with side relief so that they taper from top to bottom all the way back. This pretty much eliminates chip wedging. I also provide a few thou of back relief so that the tip is the widest point. This helps prevent pinching and also allows for the fact that it is impossible for the tool to be *perfectly* perpendicular to the work.

On deep cuts I also use earlgo's method, though I only move over about ten thou. I generally get pretty good results using my 9x18 Logan.
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