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01-21-2017, 03:06 PM #1
10L - Poor finish with power feed
I'm hoping someone can help me diagnose an issue with my newly restored heavy 10. When using the power feed to turn, my finish is really bad....it's like you can see pattern of the lead screw on the part. Based on some other threads I found I thought it might be leadscrew / clutch play.
I made a brass spacer to take up any side to side play in the leadscrew and took the apron back apart and tightened up the threaded collars that capture the clutch drive screw. (I'm sure that's not the correct name for the part) I also made sure there was no chips in the apron gears.
In an effort to troubleshoot things, I tried turning a test part by using the half nuts instead of the clutch and the finish was smooth as glass. Make a second pass with the power feed and it's back to the wavy pattern.
Since the finish is fine with the half nuts, I believe I've eliminated the drive train / lead screw or slop in any gibs out of consideration.
Has anyone else had this issue? I'm hoping someone can point me in the right direction.
01-21-2017, 03:41 PM #2
01-21-2017, 05:14 PM #3
Just a dumb question but are you changing the gearbox position for power feed vs half nuts? The power feed is slower than it would be with the half nuts engaged. Forget the ratio, but its printed on the gearbox chart. Could be you don't have the right speed, or if you are changing it you may have some trash in the gearbox causing uneven feeding.
01-21-2017, 06:39 PM #4
Thanks for the ideas, my assumption is that if there was an issue in the gearbox or leadscrew it would show the same results with the power feed or the half nuts....correct?
Yes, I adjusted the speeds between using the power feed and the half nuts to the same approximate feed rate.
When using the power feed, the pattern is identical at almost every feed rate.
01-21-2017, 09:00 PM #5
( my assumption is that if there was an issue in the gearbox or leadscrew it would show the same results with the power feed or the half nuts....correct? )
Not necessarily true the half nuts would hold the leadscrew much stiffer it's going to be in the gear train some where and I would start with checking out the gear box or leadscrew it's self.
01-21-2017, 09:13 PM #6
01-22-2017, 06:15 AM #7
01-22-2017, 09:06 AM #8
Not saying that I couldn't have anything in the gearbox or apron, but I disassembled and cleaned / painted everything a couple of months ago. It hasn't been used that much. I've had this finish issue from the beginning.
Is it possible that wear in the key, or in the ID of the apron worm gear?
Forgot to mention that I checked the leadscrew alignment with a dial indicator while moving the apron back and forth, and made some adjustment on the bolt connecting the side of the gearbox to the bed casting. Everything seems good there as well.
I will try and take a couple pictures today that will hopefully help.
01-22-2017, 09:55 AM #9
I ran out to the shop and took a couple cuts on a piece of 6160.
You can see the pattern on the left which was turned with the power feed, and the smooth portion on the right turned using the half nuts, with the gear box changed to be roughly the same speed per rev. Both were turned with the same HSS tool.
The rough looking lines on the left are actually high spots, if I go back and turn the carriage by hand very slowly, it will remove them.
Last edited by Jsnfox; 01-22-2017 at 01:40 PM.
01-22-2017, 02:21 PM #10
Since you're there and we're here you are gonna have to be a lot more observant of what exactly is happening when the carriage is feeding. Is there a stop/start pattern to the carriage movement? Put a mag base on the bed with an indicator on the front edge of the carriage and watch it advance. Is it smooth or irregular?
If that looks good, is the carriage lifting? Move the indicator contact to a machined surface on top side of carriage(top of compound maybe, qctp,???)
Figure out what's moving that shouldn't be, or, what's not moving smoothly.
01-22-2017, 05:41 PM #11
Using the indicators is a good idea...thank you.
I have tried putting pressure down with my hand on the carriage, with no change, so I don't believe it's lifting up. An indicator would obviously be more accurate than my hand though.
01-22-2017, 05:48 PM #12
Do you have the same issue with feeding in the opposite direction?
01-22-2017, 06:21 PM #13
From the photo, it looks like the cross slide is moving in and out - while you're using your indicators you might want to check that also
01-22-2017, 07:49 PM #14
Great question on feeding in reverse, I wouldn't have thought of that. I had a couple minutes and ran out to the garage and took a quick cut in reverse, I hadn't changed the setup since I took the cuts and picture this morning.
It still had the same pattern running in reverse, but not as pronounced...not sure what that could mean.
As far as the cross slide goes, it is an older well used lathe with plenty of backlash in the cross slide and compound. It's on my list to replace both down the road. In the meantime I always make sure to take up any backlash before taking a cut. Again, I would assume if the issue was in the cross slide it would show up using the half nuts as well??
I hope to have a chance to try checking things out with the indicators tomorrow evening.
01-23-2017, 08:58 AM #15
IMO, after eliminating the above suggestions, you have a few issues, all wear related.
1- saddle drop- this results in...
2- wear to the tips of gear/rack teeth and affects the involute form of the rack/pinion in such a way that they no longer "roll" and promotes a jerky,inconsistant motion.
3- likley also wear at the ends of the saddle leaving a high "pivot point" in the middle
it doesn't show up as bad in reverse because the gear teeth are considerably less worn in that diriection, and the saddle is being pulled rather than pushed.
Suggest a couple test-
Fix an indicator to the left side from saddle wing and put the indicator point on the tailstock flat way- crank the saddle from one end to the other...that will give you a general idea of drop due to bed wear but not saddle wear.
Whatever result you get you can reasonable assume there is at least that much wear to the saddle ways too.
Confirm your finding by fixing the indicator to the bed and place the point on the saddle wing directly over the front v way/rack- lift up on saddle to see how much lift you get before the pinion makes contact with the rack.
When new and properly meshing the lift you would get would be something around .010-.015" before the pinion "bottomed" on the rack.
If my suspicions are correct there are a couple simple things you can do to VASTLY improve the situation.
1- scrape some clearance in the middle third of the front saddle v way so it does not make contact with the he bed- doesn't have to be pretty, you just want to make it sit firmly at the ends.
2- shim the rack down to improve the mesh of the rack and pinion, you may also do the same to the gearbox and leadscrew bracket if necessary
none of the above is at all a proper fix but can still make a world of difference.
01-23-2017, 12:05 PM #16
Thanks for the well thought-out response and ideas....I hope to have some time this evening to check some things out with the indicators and will add your tests to the list to check.
Without performing any tests I can tell you that the fit between the rack and pinion have some wear / slop in them. Why would any wear in the suggested areas you mentioned, not show up while using the half nuts as well? I'm not questioning your ideas, just trying to get my head wrapped around the issue.
Is it possible that it's due to the drive position? Meaning the power feed clutch is roughly in the middle of the apron (possibly the high spot of the saddle) and the half nuts are on the far end.
Considering how many well used and worn South Bends that are still being used today, I assumed this would have been a more common issue.
01-23-2017, 05:11 PM #17
The power feed turns the handwheel for you, to move the carriage. The half nuts push the apron directly, and drag the handwheel along for the ride. What you are seeing looks like uneven feed rate to me- whenever the rate slows for bit, the cutter gets to make a longer pass in the same spot. Most likely cause for that is something between the worm and the rack pinion. If you don't feel any kind of uneven drag when hand feeding, the worm and clutch in the apron are most likely.
01-23-2017, 09:24 PM #18
First, I have no discernible rocking in the saddle, like it's riding on high spots in the center, I will still double check it with an indicator though.
I for sure have drop in the saddle and a fairly large amount of gap in rack and pinion teeth, by eye I would estimate .060 plus. I did some quick test cuts with the same setup as my picture above and tried the following:
1) 0.010 cut while firmly pressing down the saddle with both hands - No change
2) 0.010 cut while firmly pressing in on the qctp (testings slop in the compound / cross slide - no change
3) 0.010 cut while "helping" the hand wheel....While using the power feed, I applied a slight amount of pressure to the hand wheel, helping it to turn. - Improvement! See the picture below.
I believe my issue is in the rack / pinion fit. When I get the time I'm going to try the suggestion on shimming the rack down to give a better fit. You can tell that the pattern is still present, but I would guess it's maybe 75% better.
It feels good to be making some progress in tracking this down....everyones help is much appreciated!
01-23-2017, 09:43 PM #19
What is the pitch of the pattern? It looks to be 8 tpi.
01-24-2017, 01:08 AM #20
Is there a significant difference if you move the tool far to the right of the center of the carriage. Usually its to the left of center, can you put the cutting tool say, an inch from the right side of the carriage. If the problem goes away or gets worse then I think you have an unstable carriage due to wear in the ways.
It doesn't take much, you need a tenths indicator to prove that the carriage instability is causing this problem. it is hard to notice .001" of movement by eye, if you take a rubber mallet and strike the 4 corners of the carriage you should be able to figure out which two corners its resting on.
the hand wheel (pinion) applies pressure to the (edit: gear rack) at a 14 or 20 degree angle (i forget which) this effectively pulls the left most corner of the carriage down, while pulling it towards the headstock. This is also the corner of the carriage that likely has the most wear. as the teeth wear the force will be periodic and variable, but its still at the same general angle. usually gears wear into larger or higher pressure angles far as i know so the contact angle will vary periodically, at 4.45 tpi for 14 dimetrical pitch, which is what i believe that lathe is. so the force will be varying in direction, and magnitude too because the teeth are worn.
the sliding worm screw that floats on the leadscrew may be worn offcenter and even if the leadscrew appears to be in the correct postion, you could still have a periodic force that will be exerted once per leadscrew rotation (this will not be 8 tpi while power feeding, but rather 2.93 times that, or about 24 tpi)
or you could have the sliding worm screw rubbing on the leadscrew which causes varying amounts of friction that repeats at 8 tpi. as you appear to be taking light cuts that friction is enough to vary the velocity of the carriage which causes the variable surface finish.
Last edited by johansen; 01-24-2017 at 01:13 PM.