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01-24-2017, 03:02 PM #21
I will measure the pitch tonight, since based on the information above....the pitch might be a good indication of where the problem resides.
johansen - Thank you for your detailed explanations, everything you said makes perfect sense to me. When I looked at the rack and pinion fit last night with a flash light, looking for a gap....it was easy to see that the teeth on the pinion gear were worn in the area that contacts the rack. Based on your explanation of things, it likely won't be as simple as shimming the rack down, because the wear pattern of the pinion teeth are causing a change in forward motion as they rotate. Simply put, I effectively have the wrong size pinion gear now (slightly small, with an incorrect tooth shape) to mate properly to the rack.
I believe that explanation, also lines up with why the surface finish improved somewhat when I "helped" the rotation with slight pressure on the hand wheel.
I hope to have time to do some additional testing tonight, based on the ideas and feedback from everyone.
Thanks again for everyone's help!
01-24-2017, 07:19 PM #22
I managed to do a little testing tonight. I do have roughly .001 of diagonal from the front left to the rear right of the saddle.
I can make the pattern both better and worse by the pressure I put on the hand wheel. As stated earlier, it improves when I help the hand wheel turn...if I apply a little drag to the hand wheel the pattern gets significantly worse. While putting drag on the hand wheel I could clearly feel the uneven motion and meshing in the rack and pinion.
I believe I need to find a new pinion gear shaft and shim the rack down to create a better mesh.
I plan to address the saddle as well. On a semi related note, what is the general opinion on warn ways? There is wear on sides of the ways as expected, leaving a ridge on top...should the ridge be filed off? For what it's worth I'm able to hold acceptable tolerances with the lathe, outside of the obvious surface finish issue.
01-24-2017, 07:29 PM #23
Is there play in the rack and pinion shaft? That may amplify the issue.
01-24-2017, 07:43 PM #24
I will check for play in the pinion shaft when I take the apron off. There is some play in the hand wheel shaft, not sure how much if any effect that could have.
I did check for the thread pitch on the pattern and couldn't get an exact match with my cheap thread gage set. I would say it's just under 24 tpi, which is the gage shown below. My next lower gage is 20 tpi and it's too wide of a pitch.
01-24-2017, 08:35 PM #25
Don't worry about bed wear...don't touch it.
do look carefully at the worm and key like Allan said, 24tpi is 3 times 8tpi, which happens to be the exact reduction the apron gearing gives the 8tpi leadscrew...so it sound like your primary problem is there and is being amplified by the other issues...probably more in the key itself if you didn't notice any roughness in the rotation of the worm- the key will wear on one side into a groove that also gets angled like a dovetail and can hang up...IIRC 3/16"x 5/16-3/8" key stock will do the trick once you mill or file out the center leaving the hooks on the ends.
BTW- to shim the rack down some you do not even have to remove it, just use one of those hand impact drivers to loosen the screws,slide shim stock in between them and tighten back.
Last edited by iwananew10K; 01-25-2017 at 04:30 AM.
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01-25-2017, 06:52 AM #26
Is there considered a best practice for ensuring the lead screw is perfectly aligned? I suppose now that I think about it, you can only really adjust the front to back alignment, not the top to bottom. Like someone mentioned before, if my saddle has dropped to the point that I have 0.060 gap between the rack and pinion....my lead screw is likely 0.060 higher than the worm gear.
I'm assuming I should try to replace the pinion gear prior to doing any shimming, so I'm not over shimming everything down as a result of worn pinion gear teeth.
01-25-2017, 09:44 AM #27
*best* practice for alignment is a rebuild...not however practical or even necessary most of the time.
There is no adjustment for leadscrew alingnmnet, BUT you can lower the gearbox,leadscrew bracket, and rack to account for saddle drop via shimming them down..SBs are unique in this regard since it all attaches through the top of the bed and is not doweled in place.
Additionally SBs can take quite a bit of misalignment before issues start really affecting work.
Unless it is *really* bad or bent bad I doubt you need to replace the pinion simply improving the engagement will likely net satisfactory results.
For now just try loosing the rack and sticking some shim material in between the bed and rack to lower it, you may be surprised how much it helps.
Costs nothing but a few minutes of time to find out.
01-25-2017, 07:08 PM #28
Slices of index card make nice shin stock in a pinch. Perfect for a test fit.
01-25-2017, 07:17 PM #29
Thanks for everyone's input, tomorrow evening I'll be working on things tomorrow evening...and will give an update with what I find.
First step is to shim the rack down then go from there. Luckily I bought a brass shim pack when I started to restore the lathe. Looks like I'm going to be using more of it.
01-25-2017, 10:35 PM #30
01-27-2017, 11:14 AM #31
Ok....so I managed to do a little work on things last night and today. I shimmed the rack down to get a better mesh with the pinion gear.
I did get some improvement, but not as much as I hoped for. The ridges or high spots left behind seem to be leaving about half the material left as before. Looks like I still need to do more work.
01-27-2017, 01:02 PM #32
Shimming the rack would correct a problem that repeats at 4.45 tpi, or 14/pi.
You have a problem that repeats once for every leadscrew rotation. I would put an indicator on the carriage and measure off various surfaces and see what is moving relative to what.
If your finish here is a surface finish issue then you could have a periodic change in velocity causing this trouble, and that could be caused by the worm gear not meshing correctly with the floating worm screw. But it almost looks like I can see the change in diameter.
While putting drag on the hand wheel I could clearly feel the uneven motion and meshing in the rack and pinion.
01-27-2017, 01:21 PM #33
I've placed a dial indicator on the machined surface on the cross slide, in an attempt to measure vertical movement of the saddle during power feed. Prior to shimming the gearbox and bracket, I showed roughly 0.001 of vertical movement. After shimming, I would say roughly half of that movement remains. I need to keep shimming / measuring and adjusting....
01-27-2017, 05:24 PM #34
So after spending all afternoon messing around with things, shimming the rack, shimming the gearbox...etc. I'm fairly convinced that I either have a slightly bent leadscrew or worn bushings on the worm gear.
I'll probably have time to work on it more tomorrow.
01-27-2017, 10:11 PM #35
If you drop out the second tumbler and disengage the feeds on the apron, does the screw turn freely? I initially got the two jam nuts on mine set too tight which was causing the lead screw to load the output bearings enough to make it bind. Could be thats all that ails yours. The screw should turn smoothly but without a lot of slop if you have it set right.
The worm gear bushings shouldn't really play into this much unless its got just an extreme amount of wear. As long as its running in the same direction it'll take the slop up. If the surfaces are overly screwed up, you might have to give them some TLC to make the nut and the bushing fit up better. If they aren't worn horribly, you should be able to just face them off and then adjust the nuts so that the worm sits nice and centered without gross amounts of slop. Off the top of my head I forget what I was told to set mine to, but somewhere in the .005 range comes to mind.
01-30-2017, 09:05 AM #36
Well, I had some time yesterday to look at things. I decided to do some quick visual tests first.
Had the lead screw turning, with a flashlight on the headstock side of the apron pointing towards the tailstock side. While looking down the lead screw from the tailstock end, I could see a rotating gap between the worm drive and the leadscrew. The gap didn't change if the clutch was engaged or not.
I also loosened up the 2 screws holding the lead screw support bracket so it had some play in it. While the machine was running, the bracket would pivot from front to back by over an 1/8". My thinking behind this was, if the lead screw is straight and the gearbox is inline with the apron, there should be very little movement...especially if the clutch is not engaged.
I loosened and shimmed the gearbox hoping to see some changes, but didn't see any improvement. I also took test cuts, confirming that there was no improvement.
Unfortunately, I'm traveling all week for work and won't get a chance to work on anything until next weekend. I think my next step is to remove the leadscrew and check for straightness....if there is a bend in the leadscrew, I don't think it matters how everything else is aligned.
01-30-2017, 04:49 PM #37
Any significant bend in the lead screw should be noticed by driving the lead screw and look for wobble.
Another thing to try is to position the cutting away from the center of the saddle as possible, then position it over the center of the saddle, then to the other end of the saddle. The check here is to see of the end of wings are worn more than the center, allowing the saddle to twist.
01-30-2017, 06:18 PM #38
again, if the fault is in the leadscrew and the carriage is stable, then the surface finish problem is a velocity ripple causing surface finish problems, nothing else.
If there is significant slop in the hole that the floating worm screw rides in, then when the worm screw moves towards and away from the worm gear, the angular velocity of the worm gear varies periodically.
because there isn't an 8TPI problem when cutting with the half nuts, that leads me to believe the carriage is stable.
It doesn't matter how badly worn the rack and pinion is, that problem if there is one, generates a velocity ripple at 4.45 tpi.
if i recall correctly, you said that by assisting the hand wheel makes the problem go away or less worse, and by resisting the handwheel it makes it worse. this leads me to believe it is a velocity ripple problem and not a carriage rocking issue. but, there is a test for that:
you can verify if the carriage is stable by putting a shim between the carriage and the tailstock flat way, causing the carriage to ride on the flat way and the rear inverted V way. You will need to hang about a 10 pound weight off the far end of the carriage to keep weight on the rear V way. When you do this the carriage will be supported kinematically and it should be completely stable. Obviously, the force exerted on the cutter needs to point towards the points of contact, or it must be less than that needed to tip the carriage to the left or right. The handwheel pinion is to the left the shim, so once you exceed too much force the carriage will tip to the left. more weight on the carriage will solve that problem.
01-30-2017, 07:05 PM #39
I have a question... What motor is on the lathe? Single phase, or 3-phase?
01-31-2017, 11:37 AM #40