Small feeds on the lathe

I put an motor-driven feed on my lathe

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which I built to give a minimum feed of .003. I don't know why I chose that value - it's probably the smallest that the change box will do.

I was just watching a Tubal Cain video on YouTube about the power feed he put on an Atlas lathe. The Atlas gears only went down to .0046 (IIRC) and he wanted much finer. (His other lathes went to .0016 & .00076.)

My first thought was to modify my feed to go much finer/slower. But I got to thinking that there must be some limit. It seems that if the feed was too fine the bit would just rub until enough feed had accumulated and then the bit would bite and jump. And the bit's edge would be critical - razor sharp or better.

How fine do you feed? What are the considerations in doing a very fine feed?

Thanks, Bob

Reply to
Bob Engelhardt
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Bob Engelhardt fired this volley in news: snipped-for-privacy@news4.newsguy.com:

You can feed as fine as you wish. A lot of industrial precision cutters take off mere microns per pass.

Your concern about "skating, then hogging" is only a legitimate concern with positive-rake, fairly dull tooling.

Do the same with a zero or negative rake tool _that_is_sharp_enough_, and you can take a 0.0001 cut (or a 0.000001 cut), if you wish.

Basically, if the cutter edge has a radius of a small fraction of the feed distance, and has a small enough rake to NOT pull itself into the work, it will do just fine.

If you're using HSS tooling, I'd keep the feed above 1.5 thousandths.

Lloyd

Reply to
Lloyd E. Sponenburgh

My Logan is geared right now so that it feeds .0022 on the finest feed . Stock setup feeds .0044 , but sometimes that isn't fine enough . Lloyd , re-read your statement about feed rate vs tool radius . I find I get the best finish if the tip radius is larger than the feed rate ... -- Snag

Reply to
Snag

"Snag" fired this volley in news:lkuru.147492 $ snipped-for-privacy@fx21.iad:

You mis-read my statement. I did not say "tip radius", I said "EDGE" radius. Even a 'perfectly' sharp edge doesn't come to a perfect angle.

If it did, it wouldn't withstand any cutting forces. That tiny radius on the edge has to be a small fraction of the cut depth, or skating will occur.

LLoyd

Reply to
Lloyd E. Sponenburgh

"Snag" fired this volley in news:lkuru.147492 $ snipped-for-privacy@fx21.iad:

And yes, you do require a larger tip radius than the feed rate.

The thing about the edge radius is important, because most folks don't have the ability to hand sharpen and hone a tool well enough to take off a mere tenth. I don't think I do anymore, and certainly not at the cost/time of decent insert.

LLoyd

Reply to
Lloyd E. Sponenburgh

Gunner Asch fired this volley in news: snipped-for-privacy@4ax.com:

I said, by hand... A lot of hobby shops don't have anything but a bench grinder, and don't even _know_ about honing an edge. (some do... I'm not casting everyone in the mold, Gunner).

Lloyd

Reply to
Lloyd E. Sponenburgh

I keep a piece of hard Arkansas stone right next to my bits . Amazing how much difference a little polish on the edge can do . And Lloyd , I get what you meant about edge radius . A controlled breaking of the edge makes a smoother cut than the random breakage on a microscopic level you can get when a perfectly sharp edge contacts the work . -- Snag

Reply to
Snag

I'd venture to say that it applies in spades if cutting stainless steel, which work-hardens as you cut it - you can't sneak up on a final dimension there, IME.

Reply to
Ecnerwal

I think I would use a nice sharp variable radius corner cutter.

Reply to
Bob La Londe

Cheap screw solution

Reply to
F. George McDuffee

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Mostly (for hss) you just need to use an exceptionally hard and fine grit polishing stick..

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Reply to
PrecisionmachinisT

"PrecisionmachinisT" fired this volley in news:QvqdnVI_fvUeETLPnZ2dnUVZ snipped-for-privacy@scnresearch.com:

Good stuff! Thanks.

Lloyd

Reply to
Lloyd E. Sponenburgh

"PrecisionmachinisT" fired this volley in news:QvqdnVI_fvUeETLPnZ2dnUVZ snipped-for-privacy@scnresearch.com:

True enough, but most amateur folks (and more than I'd like to say of professionals) don't have it in their dialog to hone the edge they've just ground on a fairly coarse wheel.

You know that "feels sharp" isn't quite the same as "is uniformly sharp across the edge".

I'm lucky to have a microscope with a good measuring graticule. If I don't get what I need from a cutter, I _can_ examine it. Seldom is that necessary, and it's a waste of time for most jobs. But if I want to take off just swarf, it can be worthwhile to do. (and, completely eliminated by buying good inserts).

Lloyd

Reply to
Lloyd E. Sponenburgh

And fewer still know what ScarySharp(tm) is.

Absolutely right.

An extra minute or three on the sharpening end can give you a lot more time on the cutting end, so it's usually worthwhile.

Reply to
Larry Jaques

Hmm , I have to disagree there , a lot depends on how fast you turn the work . I've had times when I had to take less than a thou off of SS , and it can be done with a very sharp properly ground 8% cobalt steel cutter .

Reply to
Snag

"Snag" fired this volley in news:6tOru.42$ snipped-for-privacy@fx09.iad:

I have to agree, there. I spent the first 2/3 of my machining experience with shop-sharpened HSS tooling. I do a LOT of 18-8 and 316 SS stuff, because of the corrosive qualities of the materials my machines must process.

I still like high quality carbide inserts the best. (not for interrupted cuts, though, NO!).

Lloyd

Reply to
Lloyd E. Sponenburgh

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