Question for Lathe Yoda Chris

[LatakiaLover]

I can manage to get good results by test-cutting and tweaking each time I chuck up a new piece of material, but rarely DON'T have tear-out UNLESS I fuss with cutter height, feed speed, and rotational speed.

Is that normal?

If not, how do you guys avoid playing the "Hi! Can you guess what will work with me?" game with every damn piece of stem stock in your shop?

Here's proof that the tools I have can do the job in the "intrinsically capable" sense of alignment, adquate sharpness, etc. (22mm NYH as it came straight off the machine. No polishing.)

[caskwith]

I never mess with cutter height, all turning tools are on centre height (or as close as I can get), parting blade is a tiny bit above centre height, same for boring bars. Once set I never move them ( I have quick change holders).

Tool needs to be super sharp and for most stem stock you need some decent top relief/rake to get the best results. Zero rake tools will take a nice fine finishing cut (it's what I use for facing cuts) but will not take a heavy turning cut, it will tear.

Turning speed is pretty fast, 1000rpm for me. Feed speed however does need to be different depending on the material and I do that by feel mostly. I do have a power feed but I only use it for roughing when tear out isn't an issue, for finishing cuts I do them by hand so I get it just right, also I am using turning tapers/curves so power feed isn't useful anyway.

Hope there is some useful info there.

[LatakiaLover]

I just looked up what top relief is, and it's definitely something I've been missing. All my cutters are flat.

All righty, then. I'll give that a go and report back. (Only going to change one thing at a time so I'll know what's responsible for performance changes when they occur.)

Muchacho gracioso, Senor Chris.

[caskwith]

Flat top cutters will work for most things but technically they scrape rather than cut. This is fine when the edge is fresh and razor sharp (especially if you have micro burr on the edge) but once that edge dulls you start to get problems as you are presenting a very obtuse angle to the material. When cutting a "grabby" material like brass this is a good thing, but metals like aluminium will smear and gall and make a right mess. Different plastics will behave in different ways to this. I find ebonite does ok with a scraping cut (while it's sharp) but will tear when it's dull.

I use flat top tools for certain jobs on stem stock and bowls but I sharpen them very regularly. I have a bench grinder set up at the correct angle and because the tools are flat top sharpening never changes the centre height so I can just pop them out, quick run on the grinder and back on the lathe in less than 30 seconds. Flat top tools are great for facing cuts (sharpened after every 2 or 3 cuts for best results) but no good for roughing/turning down stock as they simply can't take a heavy cut and leave a decent finish.

For most of my stem work (excluding facing cuts) I use sintered carbide cutters which I hone myself to a razor sharp edge as the factory edge is often too dull for working on plastics. Tips designed for aluminium are best if you can get them but they also benefit from a honing.

[LatakiaLover]

For most of my stem work (excluding facing cuts) I use sintered carbide cutters which I hone myself to a razor sharp edge as the factory edge is often too dull for working on plastics.

I'm assuming the only way to cut carbide is with diamond, and that means one of those honeycomb-looking grit board things.

https://www.amazon.com/s?k=diamond+hone ... _sb_noss_1

If so, how do you control the angle of use precisely by hand, plus, don't they leave scratches in the carbide?

[caskwith]

Carbide can be cut with other materials, even standard aluminium oxide will cut it but it's very slow and will wear quickly. Diamond is the best.

I have a dedicated diamond "grinder" for honing my carbide tools but you can use hand hones as well, it's just slower. For hand honing I find it best to clamp the carbide tool in the vice and then use the hone in my hand, keeping things aligned by eye. Scratches etc are just the same as with any sharpening, work through the grits to something very fine.

[LatakiaLover]

What a fussy PITA... Everything working perfectly, then started on a piece of 22mm stem stock that was 3/4" longer than the previous one and it chattered hopelessly while cutting a tenon. Turns out it needed a live center to keep it from flexing.

Everything else identical to before, only the length changed, and she no work.

Lathes are evil.

[caskwith]

I knew you would find something to moan about

[LatakiaLover]

Welcome to gettin' old. Moaning accompanies pretty much everything.

I still find it hard to believe that a stick of 22mm ebonite that's protruding from a lathe chuck about 2.75" rather than 2.0" is ENOUGH longer to flex from the pressure of a finishing cut, but if adding a live center to the equation stopped the chatter I can't imagine what else it might have been.

In other news, I thought of a way to touch up the edge of a flat-topped (finishing) cutter that's fast and foolproof:

Put a piece of 1200 grit wet-or-dry on surface plate or piece of heavy glass (anything as long as it's dead-flat), and remove a few microns of material from the TOP of the cutter by smoothly dragging it with even pressure.

No idea if it's "correct" or whatever, but works great to re-set the edge without worrying about retaining the proper angle of undercut on a rounded cutter.

[caskwith]

If your cutter was not on centre height that could cause chatter too as it the more flexible part (in this case the workpiece) gets pushed or pulled up or down. The live centre would instead make the workpiece the stable part and stop this movement. On a lighter cut you know wouldn't have enough pressure to move the tool so the cut is smooth.

If you have a flexible material like thin delrin you can actually see this happen because the delrin will flex out of the way of the tool and stop cutting rather than chatter. This happens to me when parting off delrin tenons and I my setup needs adjustment.

[DocAitch]

I find that when I am using a bit with a relatively large contact area (such as one of several that I have ground to cut flares on freehand stems), any flexible material tends to get “sucked into” the bit. I have to use a live center.
DocAitch

[LatakiaLover]

I'd love to see what one of those 50,000 frames-per-second cameras with 20X magnification would show is happening at the point of contact between cutter and rubber...

My guess is that on a small scale ebonite is actually more "stretchy" than we IMAGINE it is because of how rigid a rod feels in hand.

It is just a bunch of tinye "rubber capsules" smooshed together, after all.

[caskwith]

That would be fascinating to see.