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What causes this pattern?
- Thread starter BWS Arms
- Start date
I can't tell you which item it is, but some items there are getting into a harmonic situation. (Chatter essentially, even if it doesn't actually chatter).
If that were me, even though you're doing the headstock thing, I'd probably drill a center and support the end, just long enough to see if that fixes the problem with the finish. Not to measure that specific cut, just to rule out variables.
I'd also probably play with the speed a little, as going that slow probably isn't helping. It might be a much faster speed that you need and you do have room for that as far as the materials go. (Don't push until you're worried about the part coming loose..... It sure isn't worth that....), or it might be a slight speed adjustment to get between resonant frequencies so it'll settle down a little. Generally speaking, that is pretty much an unacceptable stickout. There's always an exception to every generallization, but there's kind of a reason that it's generally frowned upon.
With that said... When I went through my lathe, grinding high speed steel was new to me, and it took me many go-rounds to get a tool ground that would cut smoothly with that kind of stickout. Can you grind a tool? Do you have alternate inserts you could try? Just to pick on one more variable?
Actually, the FIRST thing I would probably do... I'm sure you've studied up on doing the cuts/measurements that you're doing, if this is redundant I apologize, but I'd hate to see you go down a rabbit hole, when I believe you have evidence that there's a rabbit hole right there for you to fall into.
I'd attach a micrometer to the carriage/cross slide/comopound slide somewhere, somehow, doesn't matter. I'd take that indicator and watch the test bar as you move the carriage across both test cut areas. If you CUT the bar from a tool mounted on the carriage, then the indicator mounted somehow to the carriage will read the same reading across both of them, regardless of any taper induced by a misaligned spindle. If you DO get a different reading on that test, then you have some deflection in the equation that's going to have to be addressed before the diameter measurement will actually matter too much. If you've got chatter like that, ODDS ARE it came from the bar, and/or it came from somewhere else, but either way, cause or effect, the bar participated in it. That chatter will have a different degree of freedom away from the chuck than it will near the chuck.
If that indicator sweep mounted to the carriage somewhere gives a good enough result to get a needle reading you can work with... You could go ahead and use that undesirable finish as a basis for as it is for these measurements. I "suspect" that it's not as bad dimensionally as it looks by eye. If it tickles the indicator more coarsely than the accuracy you're aiming to hit, then you'll a little more obligated to sort the finish out first.
- Joined
- Oct 11, 2016
- Messages
- 3,857
It is one kind of resonance or other, but with 12" stick out, you have to expect it.
You would be better off using a larger diameter work piece and using 6" of stick out instead. When trying to do anything with high accuracy, everything acts like it is made of warm rubber, if you measure accurately enough.
By trying for longer measuring surface, 12", you are introducing variables that are much harder to control. If you can align you headstock to .001 or .0005 over 6 inches on a 3" aluminum test cut, you are far exceeding all but the most demanding requirements.
Remember that your accuracy depends on the entire loop, from cutting surface, through the work piece, through the headstock, along the ways and all the parts of the saddle, including your compound. Every single part has error. That long loop has deflections in the order of x to the third power (sorry for the engineering detail).
At 12" of stick out without support, you can expect to have similar markings or worse on almost any lathe to speak of.
On my smaller lathes I try for less than .001 over 6" (using a 3" steel test bar). On my big lathe I can get it to .0002 over 10" of a 4" steel test bar - that's a 4000lb slant bed (or sort of) tool room lathe, without any perceptible wear (less than 400 hours use, perhaps way less).
When I am doing my final finish test cuts I use carbide inserts ground for aluminum, taking .0005 finish cuts, and only measuring after the chip, finish and sound of the cut is consistent across the 6".
All the best in your alignment task!
- Joined
- Dec 23, 2012
- Messages
- 771
First start with increasing the rake angle.
Next use a rubber belt around the shafts. This trick is for brake drum turning.
Dave