Parting off in a Lathe

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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V »

I should have made mention of one thing. When turning the corners off, as we're discussing, do keep in mind, the only thing restraining the plate is tailstock pressure. It stands to reason that the cuts you take are limited. The fine feed required helps keep that pressure low, as does the width of the cut. There is no need to take the full width of the plate in one pass. If you have the least bit of difficulty in driving the plate, reduce the width of the cut. In all cases, however, the feed rate should be light.

Liveaboard---you mentioned exciting. Was it successful? If not, care to discuss the issues you faced? Power parting is almost always better than manual parting.

H
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liveaboard
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Re: Parting off in a Lathe

Post by liveaboard »

It worked; the pressure on the tool was very high though, much more than I would have exerted by hand.
I realize this is probably due to poor tip grinding. I tried but my grinding skills are poor.

Also, the 3mm thick parting blade I have may be more than my lathe is good for rigidity wise. Motor power is ample.
There was chattering for the first half of the cut.

First try, the toolpost rotated due to high force on the tool and low force on the clamp nut.

After regrindng the tip and tightening the post nut, it went ok. but the star nut started to slip because of the high torque on the drive.
Still, the cut itself was quite ok. material curled and chipped.
I was just 2mm from the chuck jaws. I used plenty of coolant.

40mm mild steel rod with a 19mm center hole.
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V »

I understand. Bear in mind, what you parted is difficult to machine. Mild steel is soft, but its behavior leaves a lot to be desired, as it chip welds readily, even when lubricated. Had you tried that cut in aluminum, you'd have been totally dazzled, assuming you chose an adequate speed. You'd have enjoyed better success with one of the free machining stainless alloys, too. They tend to part nicely.

Chatter is typically controlled by spindle speed, especially if you have already loaded the cut via heavy feed. However, assuming feed is light, increasing feed rate will limit, or often eliminate, chatter. It's a balancing act ----rigidity of the machine, the quality of the tool grind, the feed rate, the extension of the parting tool, the distance from support, spindle speed.

I rarely suggest that parting can be accomplished without slowing the spindle. Not that it can't be done, but the parting tool generally introduces a lot more area to the cut than does a turning tool, thus parting tends to chatter when machining may not. Again, slowing the spindle (slowing surface cutting speed) is one of the reliable ways to control chatter.

I've made mention that I hand grind all my parting tools. I start with square stock. One of my objectives, always, is to create good chip flow. As is almost always the case, positive rake improves performance immeasurably. Even with those commercial HSS parting tools that are so commonly used, if you grind the top of the tool using the periphery of the wheel, you create some rake, plus a little extra side relief is created. The negative in doing this is that the cutting edge can be narrower than the width of the blade behind the area in which the rake was ground. That limits the depth of cut, and, likewise, the useful life of the blade, as it reaches a point where it is no longer able to be sharpened. At that point, the entire length that was ground on the top must be removed in order for the blade to be used again. That may or may not be an issue for each individual, but the benefits of better performance of the tool may be great enough to render the consideration a non-issue.

H
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Re: Parting off in a Lathe

Post by liveaboard »

Any chance of a picture o drawing of the grind you describe?
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V »

liveaboard wrote: Mon Dec 07, 2020 3:01 am Any chance of a picture o drawing of the grind you describe?
Take a look at this image.

TOOLS.jpg

The tool second from the right, is a parting tool, as is the one on the right, which has not been ground on top yet.
You see the second from the right tool from the side. If you look closely, you'll see that the top is ground with a large radius. The centerline of the radius is behind the tip, so that it is ground deepest at the centerline. That creates slight positive rake, and provides a nice path for the generated chip to follow. As the grind marks parallel the centerline of the tool, they present a minimum of friction.

It stands to reason that the tool tapers top to bottom. When the top is ground, it yields a little more clearance on the sides, which are already tapered towards the shank. When properly ground, the tool makes contact with the part only at the tip. They perform exceedingly well, assuming you do your work properly.

A hint.

You may think a work rest is helpful for grinding tool bits. Most folks do. I, too, was in that camp until I hired in at United Precision way back in '66. The only grinder they had was a small motor mounted to a column of the building. It was about chest height, and had no work rest. I hated the grinder at first, but as I mastered the art of grinding free hand, I came to understand the value of not being hindered by a work rest.

Grinding by this method demands that the wheel runs true, and that you choose the proper wheel. They are not all made alike. When you have the proper wheel for grinding HSS, it's amazing how quickly a tool can be roughed and finished. Note the finish on the side of the parting tool in the image. That was created by hand, with a ½" wide wheel. It would border on the impossible to grind with a rest, which would just get in the way of proper wheel access.

If you are not well acquainted with grinding wheels (few people are), it might pay for you to read my comments in that regard in the Resource Library. There's a couple threads that discuss wheels and grinders. Easy to find, as they're pinned to the top by request of readers.

If the grind of the top of the tool is not clear to you, ask and I'll take a photo of the top of a parting tool I use and add it to this post. It will show, clearly, the grind I describe. I did not have one in my collection of photos--I have only the one I posted.

H
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John Hasler
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Re: Parting off in a Lathe

Post by John Hasler »

That tool looks exactly like mine with the exception of the grind on the top. I'd appreciate seeing a better photo of that.
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Re: Parting off in a Lathe

Post by liveaboard »

I attempted to grind my cutoff tool more like Harold does; I had to make another cut the same as yesterday, 40mm mild steel rod.

It seemed to have less pressure on the tool; the chips were consistent too.
Chatter was still pretty bad. I slowed it down and that helped a little.

I only have 2 feed speeds without swapping change gears.
cutoff tool grind 1.jpg
cutoff tool grind 2.jpg
Cutting off is what gives me the most trouble of anything I do on the lathe.
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Harold_V
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Re: Parting off in a Lathe

Post by Harold_V »

The second photo is very revealing. The radius on your tool is ground in the wrong place. The net result is negative rake, which is a deal killer. That's one of the reasons you have high cutting pressure. With the cutting edge below the top of the tool, the result of a mislocated grind, the tip is narrower than the top of the tool. You most likely are touching on the sides, raising cutting pressure unreasonably.

I'll try to remember to take a picture of the top of a tool to clearly show the grind. If I don't forget, I'll be able to post late tonight.

Mean time, consider that the top grind should NOT remove anything from the cutting edge. The grind begins back of the edge and is continued (by going deeper) until it meets the cutting edge, and is parallel with the top. By grinding the rake this way, the tip becomes the widest point of the tool, until it has reached the shank. NOTHING should contact the work piece except the very tip, the cutting edge.

H
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Re: Parting off in a Lathe

Post by Harold_V »

Ok, here's a couple photos of a parting tool with the top ground rake.

DSC00011.JPG
DSC00012.JPG


Take note that the area immediately behind the cutting edge is not straight---it's the profile of the wheel periphery. I start with 7" wheels and use them until they're about 5½" diameter, so the radius isn't constant from tool to tool, but that doesn't make a significant difference. So long as you start behind the cutting edge and plunge the grind straight down, so the centerline of the wheel remains behind the cutting edge, you end up with positive rake, albeit a small amount. It's enough to spell a huge difference in performance.

If this grind isn't making sense to you, just think of placing the top of the tool on the wheel periphery such that it makes contact half way between the cutting edge and the end of the blade, where it meets the shank. Keep it perpendicular to the blade, side to side, so it ends up meeting the cutting edge without grinding away any of the edge. If you don't keep the edge relatively straight, so it doesn't dip on one side or the other, the tool will have a tendency to cut at an angle, and can result in the tool breaking in the cut. It's a lot easier to do than it sounds.

The wheel should be an acceptable grade (for HSS), and dressed well, but NOT with a diamond. A diamond dressed wheel tends to cut hot and require greater pressure. A well dressed wheel, using either the corner of a sintered silicon carbide dressing stick, or an impact dresser, will cut much cooler and require less pressure in the grind. It MUST run smooth, and it's a real good idea to have it flat, not crowned or hollowed.

Clear? If not, tell me where you're not getting it.

Take note that the actual cutting edge displays a bit of a radius (notice it's a little shiny at the edge?)--it's not sharp. It is well used and in need of resharpening, but it still cuts.

H
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Re: Parting off in a Lathe

Post by liveaboard »

Thanks for the explanation.
I made a little grind adjustment today and made a very nice part on a stainless tube.
curly chips slid away and all...
But it was a very small piece, just 10mm diameter.
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Re: Parting off in a Lathe

Post by Harold_V »

Unless it's free machining, stainless is an excellent test for a parting tool. It's tough (although usually not hard) material, and resists the cut quite nicely. Real sharp tools help.

Well done!

H
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John Evans
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Making tinsel !! Parting off

Post by John Evans »

Ran out of tinsel decorating the tree !! LOL Decided to make a larger drive pulley for my Hamilton sensitive DP to get the RPM up for under 1/8 bits. Had a piece of 5" aluminum bar so had at it. To short to cut off in my horizontal saw ,so parting was the answer. Use one of my insert style parting blades and decided to hand feed. dripped in some aluminum cutting fluid every once in awhile and no issues . Off it came. Had to stop several times to remove the built up tinsel !!

Merry Christmas & Happy New Year to all,
John Evans
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