lathe cutting bits

I'm so very confused.

I have 3/8" shank holders with HSS inserts. I have 1/32 and 1/64 radius inserts.

I was testing the cutting straightness of the lathe with a

1" steel bar over 5". The results I got were 2 thou bigger at the headstock than at the free end.

I then had a professional in to look at my issue. He said the bit I was using wouldn't give me proper results. I was using the 1/32, and he said it needed to be at the biggest 1/64. He ground a HSS blank with a knife edge. Then he tested using 1" aluminum. He then aligned the headstock, which he said wasn't that far off. (On a previous visit he straightened/levelled the bed). When he got done it was straight within a few thou over 4" or so.

After he left I tried my test again using the 1/64 bit. With the steel bar it was still bigger at the headstock, but only by .001. Using the aluminum bar it was off by .0005.

On both bars after cutting toward the headstock, I left the bit where it was, and reversed the feed toward the tailstock. More material was cut off. When I measured them, it was only off a few thou.

I don't know how it could be larger at the headstock, and then come out straight on the 2nd cut. And what good are indexable inserts if they don't cut straight?

Once again I'm so confused. Wayne

Reply to
Wayne
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Different turning bits may require more or less cutting pressure. The difference may lessen your problem or make it worse, depending on which bit you use. Same goes for steel vs. alum. The fact that your lathe turns a taper is still there, just gets worse depending on the circumstances.

Reply to
Tony

snipped-for-privacy@operamail.com (Wayne) wrote in news:51701a28.0407271631.2782c0c9 @posting.google.com:

So if you are referring to being off by 0.001 diameter, it is out of alignment by 0.0005 over 5"

The extra material removal is from the flex in the tool/toolpost/set-up when taking the inital cut. The information about taper when doing this is irrelevant and unuseful due to that fact. You would need to be able to measure the differences in the flex of the tool tip in both directions in order to obtain any taper information.

Reply to
Anthony

Anthony wrote in news:Xns9533D73003623acziparle3sp835@216.77.188.18:

Ohh..One other thing: a piece of 1" bar stock, freely ended 5x Ø out of the chuck *is* going to deflect under cutting load. You may not have any taper in the machine at all.

Reply to
Anthony

Anthony wrote in news:Xns9533D89259918acziparle3sp835@216.77.188.18:

Oops...failed to notice you said *larger* at the headstock. That would not be material flex. It could be the slide running uphill or downhill to the spindle though.

Reply to
Anthony

Consider the possibility that you have a little slop in your headstock. The larger radius tool will push the stock more out-of-center giving you a taper which is larger at the headstock.

Reply to
Jim

Just a big of trivia..that last final pass is usually called the "spring pass"

Gunner

"In my humble opinion, the petty carping levied against Bush by the Democrats proves again, it is better to have your eye plucked out by an eagle than to be nibbled to death by ducks." - Norman Liebmann

Reply to
Gunner

Wrong. Work flexes away from the tool resulting in smaller diameter as you go away from the headstock.

BeamOut ''(30E6,I_tube 1 .5)Beam 0 (5 35) pos'n pt. couples shear bend. slope def'l force stress stress 0 -35 -175 -59.42 1783 0 0 5 35 0 0 0 0.0002971 0.0009903

It only takes 35 pounds force to deflect a cantilevered 1" steel bar a thou at 5 inches from the support.

Ted

Reply to
Ted Edwards

You might want to consider positive/negative rake carbide inserts. See my article at

Also, see my reply elswhere in this thread re deflection.

Ted

Reply to
Ted Edwards

||Anthony wrote: || ||> Oops...failed to notice you said *larger* at the headstock. That would ||> not be material flex. || ||Wrong. Work flexes away from the tool resulting in smaller diameter as ||you go away from the headstock.

Now, as far as being a machinist goes, I'm not. But this doesn't make sense to me. If the work is solidly supported close to the chuck, then the flex is minimal and the bit cuts to the intended depth. As the cutter moves away from the chuck, deflection grows, the cutter cuts less deeply as the work moves away from the tool. That will result in a larger diameter as it moves away from the chuck, because less material is being removed. Am I missing something here? Texas Parts Guy

Reply to
Rex B

Nope. I was missing my second cup of coffee. The deflection calculation is correct - I just blew the direction.

Ted

Reply to
Ted Edwards

Ted Edwards wrote in news: snipped-for-privacy@telus.net:

Umm...... Ted, if the part is flexing AWAY from the tool as you go away from the headstock, it is cutting LESS material off, and therefore the diameter of the turned part will be larger, not smaller.

Reply to
Anthony

This is why I never do these tests with a cutting tool. Instead, I use a piece of hardened and ground shafting of known straightness and roundness. I put this in the chuck and wiggle a little until it turns as true as I can get it. I then read at two distances from the chuck with a dial test indicator. Lately I've been using a surplus Federal Maxxum electronic indicator that reads to .00005" although that last 5 is pretty iffy. I turn the chuck manually and average the readings. This thing only puts a couple of grams of pressure on the bar, so the deflection is pretty small (but it is still there, of course). You bypass all the errors caused by heating, built-up edge, cutting tool wear, surface roughness changes, etc. that you would also have when actually cutting a bar. Also, you can use the carriage handle to rock the carriage left and right, and see if the reading changes. This can detect wear in the bedways or carriage.

Before doing this, I level the bed with the best level available, for front-back level, at both the headstock and tailstock end. You want to eliminate as much twist in the bed as you can, as this can foul up all other measurements.

Jon

Reply to
Jon Elson

i don't know, I usually do them with a cutting tool. I am only interested in how the lathe cuts, after all...If I am trying for a thou or less over some distance, I sneak up on it, learning how my bit is cutting at the depth and feed I will use for the finish cuts, and if I have to I tweak the lathe bed to get what I need. Obviously this will differ between OD cutting and boring. Thankfully I very rarely need to bore or cut over more than an inch to that level of accuracy. My lathe can do a couple of inches to less than a thou no problem.

Brian

Reply to
Brian

Brian,........How do you "tweak the lathe bed" in this particular case ? regards.....Jan ter Doest.

Reply to
j. ter doest

Tweak the lathe bed - for example if I am cutting big on the OD on the chuck end of the work-piece, I raise the rear tailstock leg a bit. Intoduce a tiny bit of twist to the bed, so that the bit is a tiny fraction farther away from the work at the tail-stock end of the piece. I use aluminium wedges to level the lathe, I just tap one. I've only done this once or twice at most. To me a lathe isn't really designed to cut to a half-thou over five inches, or bore to similar accuracy, that's in the realm of grinding and honing.

Brian

Reply to
Brian

Right. I posted a correction.

Ted

Reply to
Ted Edwards

Your not asking for unreasonable accuracy. My okuma manual has a test that you use a piece of 2inch diameter cast iron or aluminum bar 7+ inches long. You chuck it with no tailstock take a roughing pass and then a finish pass. In 7 inches it can have no more than .0002 taper. Mine will do that without a problem. Charlie

Reply to
Charlie

Ahh, the infamous test bar. I've searched, but can find nowhere to get one. I'm getting the impression you have to make your own. But since I can't make straight cuts yet, I wouldn't know how to do it.

Reply to
Wayne

Now I know the name for it. I've seen that happen on my mill/drill. I kinda thought a much sturdier lathe taking only .002 diameter wouldn't do that.

Reply to
Wayne

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