Losing the battle with parting tools

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Glenn Brooks
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Losing the battle with parting tools

Post by Glenn Brooks »

Well, broke -shattered is a better word- three parting tools tonight, trying to part off some 2 1/2" round stock. The blades shattered about 3/4" into the cut. Obviously catching and shattering out the bottom of the blade from the lower front edge back to the tool holder. The last one still has a big scallop piece missing from the lower part of the blade.

Lathe is a fairly big old iron 12 x48 Standard Modern Utilathe.
65 RPM , 2.225" finish diameter mild steel part
Hand feed less than .003" Depth /rev
Good HSS blades - old stock from England, unfortunately
11/32" x 3/4" blades, protruding from the holder about 2"
Set up on center of the work.

Any idea what iam doing wrong??

Thanks
Glenn
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Harold_V
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Re: Losing the battle with parting tools

Post by Harold_V »

11/32" wide? If that's not a typo, you may now understand what the problem is. That's WAY, WAY, WAY to wide for a parting cut on a reasonably small machine.
Hand feeding a parting cut is generally a mistake. Lack of proper lubrication, especially in deep parting cuts, is a mistake, as if there's any tip welding, it generally results in a broken tool. Adequate lubrication with the proper lubricant limits the occurrence of welding. Parting speed is way too slow for the remaining diameter (only about 20 sfpm), although it may be acceptable for the beginning of the cut.

Deep parting cuts require that the parting tool be properly aligned, so the sides don't shave the resulting groove. If they do, cutting pressure slowly increases to the point where it overcomes the tensile strength of the slender parting tool. It also often causes the tool to wander in the cut, favoring one side or the other, generally breaking instead of conforming to the resulting groove. That said, the taller the parting tool, the greater the strength, and the narrower the cut, the lower the cutting pressure. Problem is, for deep cuts, a tool can be too slender and wander in the cut.

If you have no other means of separating the material (a horizontal band saw, for example), you might consider parting the material to a given depth, then finish the cut with a hack saw, with the lathe turning at a low speed. It's not as good as parting all the way, but the reduced diameter will cut reasonably quickly as compared to sawing the entire diameter.

Harold
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Glenn Brooks
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Re: Loosing the battle with parting tools

Post by Glenn Brooks »

Hi Harold, Whoops- try 3/32" x3/4" - just shy of 1/8" wide blade. Not a typo, I just made up the wrong math in my head. I think my alignmet was good, as I didn't see any shaving coming off the sides of the cut.

Hard to get cutting oil down into the grove though. 30 degrees in the shop and the cutting oil was thickened a bit by the cold, didn't flow well. maybe I was having some tip welding.

If speed to slow, what's a better speed for this size diameter round stock?

Eventually I did pull the part out of the lathe and finish it with a power hack saw. Messy and brute force, but will clean up the cut face in the mill tomorrow.

Thanks
Glenn
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

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Harold_V
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Re: Loosing the battle with parting tools

Post by Harold_V »

Ahhh! That's very different!
Assuming your lathe is rigid enough, a wider blade would likely serve you better for that depth of cut. The problem manifests itself as chatter, which can often be controlled by slowing the spindle speed.

Low carbon steel simply won't part dry without problems. Leaded steel often does allow that, however. By some means, you have to get lubrication to the tool, as that stuff is miserable to machine under good conditions. Really sucks under bad conditions. Flood works really well in these circumstances, but that's likely not an option for you. Drips of oil from a dispenser (oil can?) can help---even running a well soaked acid brush in the groove will help. Anything but running dry.

When I part larger diameters, I often increase speed as the cut deepens. In my case, I can select speeds in steps, without stopping the spindle. That may not be an option for you, though, but nothing prevents you from stopping the spindle as the cut progresses, selecting a higher speed. Disengage feed when you do this, then re-engage when the spindle is up to speed. When you get down to the 1" diameter (calculated from your comments), you could safely run @ 350 rpm, so look for speeds in that vicinity. If you get severe chatter, slow down some, and if the chip flows well (requires good lubrication) you can often increase feed rate to offset the chatter problem. Again, with a light duty machine, none of this may be true, however.

Note that I have changed the title of this thread twice now. The word you should be using is losing (as in "I lost something", not loosing (as in it is loose, or not tight). I made the changes so when others do a search they find the desired results.

Harold

edit: To ensure proper alignment, set the blade on a face that you have machined in place. For that deep of a cut, if the blade is either wandering, or is not set properly, you can expect problems. Don't guess, especially with insert type blades, as they have no back relief, so the slightest misalignment causes dragging, whether it produces chips, or not. Really raises hell in a cut. It also helps if the cutting edge addresses the part without lead. If one side enters before the other, that's often enough to steer the blade in an arc, which is a sure way to break the blade in a deep cut.
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earlgo
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Re: Losing the battle with parting tools

Post by earlgo »

Never having been a pro machinist, I can only offer what works for me on a flexible hobby lathe. When making deep cuts with a cutoff blade I plunge in about 1/4", back up and move over about 1/2 the width of the blade, plunge in the original 1/4" + another 1/4", back up, move over to the original place, plunge in. Repeat. This seems to take a lot of pressure off the blade, and it also makes more room for the cutting fluid.
I also align the cutoff blade by placing a 123 block against the face of the chuck and run the side of the cutoff blade flat against the block.
Just my methods.

--earlgo
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SteveHGraham
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Re: Losing the battle with parting tools

Post by SteveHGraham »

You really have to look out for work hardening when parting. If I have to stop for any reason, I pull back immediately so the tool won't ride on the work and harden it.

If you part manually, you have to be a man about it and avoid slowing down or stopping.
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Dave_C
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Re: Losing the battle with parting tools

Post by Dave_C »

I can relate to this post as well having had problems in the past with parting.

Harold has covered all the basics but what I found was that without coolant, the chips fouled the cut and broke my tool! To make parting cuts that deep you have to get the chips out and at low RPM they tend to stay in the cut and on top of the tool to boot.

I was cutting way to slow. I sped my cutoff speeds up, stopped feeding by hand, added coolant and I've not had any issues since. (I did have to perfect my tool alignment though. I just thought it was straight!)

For example: 1.5" aluminum stock, 800 rpm with a .003" depth of cut and flood it with coolant to wash out the chips. The finished cut doesn't even need to be faced off!

Dave C.
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Harold_V
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Re: Losing the battle with parting tools

Post by Harold_V »

I made mention of back relief. May not have been clear on my thoughts, however. I now realize that I made mention of insert tools, but that just offers confusion, as there's a myriad of insert (carbide) tools on the market, and that's not what I had in mind. The insert in question would be as described, a long, slender HSS tool that fits the appropriate holder. Such tools are ground straight, so they don't vary in thickness (side to side), thus they do not provide any relief towards the holder. For that reason, it's very important that they be held at a precise right angle to the rotation of the spindle.

I hand grind parting tools, so I have the option of providing back relief, which ensures that the tool doesn't make contact at any place but the cutting edge. I also grind the top of the tool, using the periphery of the wheel, to create modest positive rake. With such a tool, the chip will often generate in the slot, winding a large coil, like a clock spring.

Dave_C's comments are spot on! Don't be timid with parting. Slowing down, playing it "safe" is generally the source of problems, not the solution to the problems that can be so easily avoided.

Harold
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Richard_W
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Re: Losing the battle with parting tools

Post by Richard_W »

I always use the acid brush with cutting oil, unless I have flood coolant available. A 1/8" parting blade would have been a better choice for that parting depth. I assume you kept the parting tool mounted over the cross slide and not hanging off to the side?
spro
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Re: Losing the battle with parting tools

Post by spro »

That is true as is all the advise of this thread. Many of us use acid brushes but remember the older way. My 1911 mill has a cast iron container for cutting fluid arranged on a swinging arm. There is a sensitive valve and flexible copper tube at output. That arrangement allows drip-drip along any area it would be needed.
John Hasler
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Re: Losing the battle with parting tools

Post by John Hasler »

Harold_V writes:
> I hand grind parting tools, so I have the option of providing back relief, which ensures that the tool doesn't make contact at any
> place but the cutting edge.

I also grind a back relief. A few thou makes a lot of difference. I don't try to taper the sides all the way back: I just make sure that the tool is widest at the front corners. The goal is to make sure that the cutting edge is wider (if only by .005) than any other part of the blade that gets into the slot. This allows for the fact that nothing is ever truly square or perfectly straight.

I also grind a side relief, making sure that the top surface of the tool is wider than any part lower down and the tool narrows toward the bottom. This makes it less likely that chips will get wedged.
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SteveHGraham
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Re: Losing the battle with parting tools

Post by SteveHGraham »

That thing spro mentioned is something I've been wishing I had for a long time.
Every hard-fried egg began life sunny-side up.
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