tool steel

[seamonster]

opening caveat: there will be heap-tons of ignorance in this question.....

I'm working on a bunch of stems back to back, and I'm starting to notice some real differences in surface finish. at first I thought it was a difference between how SEM and NYH machines (and I still think that this is part of it....)
and I've been playing around with slightly tweeking the tool profiles I'm grinding.
But this isn't really fixing the problem.

Then I noticed that a tool blank that is giving me a really nice smooth surface finish is cobalt and a blank that is cutting powder instead of nice ribbons is HSS. Both tools are ground very similarly (not identical, but as closer as I can by hand). Sharpened and honed the same way.

Is this a thing? I'm using automatic drive, so feeds and speeds are the same.

Anyone got ANYTHING to tell me about this?

Hmmmmmm?
thanks,
Jeremy.

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[sandahlpipe]

I’m not sure which surface you’re describing is it the surface of the tenon, or something else? And what about the finish is different from one stem to the next?


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[seamonster]

Sorry Jeremiah, thanks for the clarification. I'm talking about turning the ebonite, (I use delrin tenons) either down to size, or shaping a flare, etc.

Sometimes I get nice clean ribbons coming off the tool, and the surface is smooth and even. Other times, I'm getting more of a powder coming off the tool and the surface is rougher and sometimes pock-marked. It's seems like a dull tool leaving tear out, but this is fresh from the grinder, touched up and honed.

Here are the variables I can think of:
- slight differences in cutting angle, not finding that sweet spot
- slightly different tool geometry
-cobalt vs. HSS
-NYH vs. SEM (though this isn't always true)

I'm using a rocker style tool post, and am always careful to center the tool.

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[Sasquatch]

Geometry is my thought - one is not set at the right height, try a shim or something, get that cutter exactly where it needs to be. It's acting dull so it sounds like it's low (acting like a scraper not a cutter).

[sandahlpipe]

Definitely not a problem of NYH vs SEM. Tool sharpness, burr or lack of burr on the tool edge, and tool height (I have mine slightly below center) and relief angle on the tool, plus the distance of the material from the headstock causing runout, not to mention depth of cut. All of these are variables when turning. If you’re getting pock marks, it’s tearing the material out, not cutting. My most likely guess is your tool is set too high.


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[KurtHuhn]

Different steels will have different grain structure, and can cut differently. It doesn't seem like they would, all other things being equal, but that's why different alloys are available. For my purposes, I tend to use HSS blanks (M2 alloy) which works a treat, cutting fine ribbons and leaving a surface that only needs minor sanding. I also tend to leave a burr on the top edge - a very fine wire of metal that acts like a razor and is what does the actual cutting. Because of this, I tend to set the cutting edge just below center and take light cuts.

But that's what works for me on my lathe. Every lathe has its own personality.

[LatakiaLover]

Another variable is rotational speed. The manufacturers of ebonite say that (for all practical purposes) faster is better, almost without limit. They go into material "structural" reasons that I can't follow, but boil it boils down to skimming vs. digging.

Leave everything as it's currently set and run a cut down a section of rod at 4K rpm and report back.

[seamonster]

Alright fellas, Sasquatch says too low, Jeremiah says too high, Kurt says put a burr on it, and George says too slow.....

I'm gonna send you all out back to the cage-smack-down-pit, let y'all fight it out, let me know who's the last man standing, huh, will ya?

And while you're doing that, I'm gonna take me out to the shop and try all seven of you're ideas.... just as soon as that tenon glue up is done.

Thanks all for some things to chew on, I'll let you know what I figure out.

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[KurtHuhn]

/me looks at cage match brewing, shrugs shoulders, wanders away to make some anachronistic thingy....

Out of all that posted above is going to be something that will work. You get to figure out which one it is.

[seamonster]

I had a little time to run some some tests, and these are by no means exhaustive.....

Aided by a bowl of Presbyterian, I reshaped a tool, honed it and set it how I normally do, ran a pass down a piece of rod, raised the tool a hair, ran another pass, lowered it a hair lower than I would normally set it, ran a pass..... and the results: virtually no difference. I used the automatic feed so variables we're limited to too height. Likely, the difference of height was so slight it made no impact.

Then I got the tool with a diamond hone just enough to raise a burr, and DING DING DING a much nicer finish (go Kurt, it's yer birthday, have a party).

Then I tried some different speeds. Without excessive funking around, my little Atlas will do 500, 805, 1270 and 2072. I normally cut wood at 805, and if I remember to switch, stems at 1270 (though prolly leave it at 8 two-thirds of the time).
Not much difference, other than 2072 is scary scary fast......

So, for now, I'll be sure to hit the tool with a diamond lap to raise a burr, and see how that works in my normal work flow.

.... and now back to making stems. I'm tryna get ready for Vegas...... hope to see you all there....



cheers,
Jeremy.

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[DocAitch]

Jeremy,
You might try the bits that Premal has on RawKrafted
https://rawkrafted.com/tools/carbide-turning-inserts/
They cut very nicely.
DocAitch