Turning finish

[kmanuele]

More newbie questions:

I am turning a 1.5" piece of 12L14, and have problems with finish.

The tool geometry seems about right, and the edges are all honed. It is M42 HSS --- or is supposedly M42. It has a radius on the nose.

Turning about 500 RPM, and even at 0.0025" feed rate the finish is rather poor, and inconsistent along the length. The first 1/8" or so at the start seems to be pretty good, then it goes down hill.

Using Oatey Dark Thread Cutting Oil as lube -- because that's what I had in the shop.

There is material build-up on the tool (photo).

Any help/comments appreciated.

 

[Parlo]

Try a feed rate of at the very least 0.005" rev, finer feeds are not always the answer to better finishes.

 

[benmychree]

500 rpm may be a bit fast for HSS, the built up edge may testify to that, especially with free cutting steel. At a cutting speed of 100 FPM, it would be 250 RPM, at 120 RPM, it would be about 300 RPM, at 500 RPM, it would be 1300 FPM; way too fast.
!!!!!! Oops my mistake, read the calculator wrong, 500 RPM is (only) 200 FPM, still too fast. 100 FPM would be about 260 RPM, 130 FPM would be about 330 RPM

 

[SLK001]

How are you determining that your tool is sharp? Also, are you sure that the material is 12L14? That should cut like butter with a sharp tool.

 

[Cadillac]

From the pictures that tool doesn’t look sharp

 

[KevinM]

When I cut 12L14 I use aluminum tool geometry.

 

[mikey]

It looks like your nose radius is pretty big but I'm not sure. If you could take a pic that shows the top of the tool, that would help.

Your tool has side rake but no back rake. This causes the tool to plow through the cut. I would grind some back rake into the tool, maybe 12-15 degrees.

Make sure to hone the edges. Sharp edges will reflect no light so if you see any bright spots or highlights on the edges, it isn't sharp.

It would also help if you show us your set up at the lathe. Show us how the tool is positioned relative to the work piece.

 

[pontiac428]

Are you presenting that tool with the leading flank at 90 to the work? From the pic of the build-up, it appears that your presentation angle is acute or your depth of cut is gigantic. Changing the presentation angle to suit the cut is fine if it is within the tool's geometry. With your .0025 feed rate and that nose radius, you should be getting a nice finish, but your depth of cut should be light, maybe .020 or so. That's 1mm dia. reduction per pass, so it's not glacier slow, but it's not pushing your lathe to the edge, either.

 

[Illinoyance]

One photo of the tool definitely showed a built up edge.

 

[graham-xrf]

One photo of the tool definitely showed a built up edge.

Is that what it is? I did wonder.

In any event, the third picture shows the tool honed up to a polish, but with a blunt edge. This kind of happens if the one tries to hone up using abrasive paper block, because of the "give" in the paper. It can be done that way, but needs care. It needs to be on a stone, or wheel hone, or one of those low-cost diamond flat discs, and care taken not to round over the edge. In final stoning by hand, do not wobble. Put a tiny radius on the sharp point. I can't stretch the budget to a Deckel grinder, but HSS tools that cut beautifully can be done by hand on a bench grinder, with a final touch-up on a slip stone.

Then get the speeds and feeds right.

There are great YT videos all about tool sharpening. I am thinking Stefan G, or This Old Tony, or Quinn Dunki.