Just curious have you tried feeding a sheet of paper through the driving gears, paper should print differently at any tight place. May just need a gear adjusted. I can't remember but I think you have a 12 x 28 which has to have a gear switched to change feed direction. Very well could be that simple, I can't take credit for the paper it seems that's whats used to set gear clearances on South Bend lathes. Hope it helps.
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Cut Depth Affected By Feed Rod Rotation?
- Thread starter barnbwt
- Start date
- Joined
- May 14, 2015
- Messages
- 179
Just got back in contact with Matt, so he's helping me figure out what voodoo got stuck in this thing. It looks like it'll be a major disassembly pain no matter what happens, since the feed rod and its gears are kind of at the bottom of the apron. At least parts should be available, since the parts diagram shows these lathes to be identical to SEIG C8 (and probably also C10) which have been out for a while.
TCB
TCB
- Joined
- May 14, 2015
- Messages
- 179
Ah-ha! Found it. The feed rod journals at each end were significantly off-axis when they were turned. The factory needs to fix their broken 3-jaw, or something. The rod is straight as an arrow, but the journals were a whopping .01" off the centerline. With the feed rod out, and just turning using the lead screw at slowest tpi (72), I get a very consistent surface, which is almost smooth if I use a large-radius tool. Matt is hooking me up with a new feed rod, and hopefully I can put this thread to bed once it gets here. A backup, I suppose, is to turn down the existing journals true and sleeve the machine. I'd rather not have to do that with just a leadscrew, lol (make temporary bushings & new journals, assemble, turn better bushings, disassemble, press better bushings onto rough thread-cut surface of journals, reassemble)
TCB
TCB
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- Feb 1, 2015
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- 9,621
Hopefully, you have finally found the problem. I have to admire your persistence and perseverance. Let us know what the final outcome is.
- Joined
- Jun 15, 2013
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- 1,199
Are you using a tailstock and center when turning those long stickouts? In the book for my Bison chucks it allows 1.2xD stick out before you must use tailstock support....glad you were able to find the problem and have it remedied, good on ya
- Joined
- Jan 2, 2014
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- 8,852
They do say that a lathe is the only machine that could make itself........
.However I agree, on a new machine get the new parts and enjoy the precision that comes with it!!!
I am glad you got to root-cause.
I bet you learned a lot more about your lathe in a shorter period that if you had no problems with it.
Thanks for keeping us informed.
-brino
- Joined
- May 14, 2015
- Messages
- 179
I tried a little experiment on a hypothesis I had; perhaps you all can tell me why it did not pan out;
If the eccentricity of the feed rod to the key profile was causing the carriage to wobble as it turned, would not turning down the support journals to be concentric (er) and letting them sit loose-ish inside their supports reduce the cut variance? Obviously not a long-term solution because loose moving parts like to wear and gall, but something to allow me to mess around while waiting for the replacement feed rod. I turned down both journals using my 3-Jaw and bunch of positioning/tweaking until it dialed within a couple thousandths of centered (the hex-rod itself is not very hexagonal, so it's a bit of an approximation), ultimately pulling off about .01" of diameter of both journals. The rod inserts into the driving socket more easily (shock), and rattles a bit when everything is put together. That should mean it isn't applying any forces besides the torque to drive the carriage, right? Cut still wobbles, though gib tension seems to flatten it out into a mere texture, now (well less than what I can feel, but just as visually apparent).
So now I'm wondering if there isn't still more going on, or if a loose feed-rod is still able to impart a wobble to the carriage somehow. perhaps the keyway in the driving end is binding on the key and pushes the rod to one side of the socket? Starting to get pretty good using the compound to turn stuff like these journals consistently, but I long for the glassy finish I'm getting from cheapo Harbor Freight bits run at 400rpm (only without the wobbles). I've at discounted the apron being the culprit, at least for now. Freed from the two journal supports, I can freely spin the feed rod and drive the helical gears in the apron without any periodic binding (I can feel a bum tooth in the main gear every three turns or so of the feed rod, but that's about it, and that's the wrong frequency for my wobble). If the feed rod isn't the villain, I have to assume it is the last gear in the headstock driving it that's out of round, which may also have something to do with the rattle I'm getting that's timed to the leadscrew (the leadscrew is also out of center about .007, btw).
I'd rather not have to dig around in that sensitive and fiddly area, but it may be the only way to determine whether or not I have egg-shaped gears binding on eachother. Come to think of it, an egg-shaped gear clashing with another in the headstock would push down on the end of the feed rod, causing the middle to bow upward (and due to the leverage, possibly a bigger affect than the journal offset)
TCB
If the eccentricity of the feed rod to the key profile was causing the carriage to wobble as it turned, would not turning down the support journals to be concentric (er) and letting them sit loose-ish inside their supports reduce the cut variance? Obviously not a long-term solution because loose moving parts like to wear and gall, but something to allow me to mess around while waiting for the replacement feed rod. I turned down both journals using my 3-Jaw and bunch of positioning/tweaking until it dialed within a couple thousandths of centered (the hex-rod itself is not very hexagonal, so it's a bit of an approximation), ultimately pulling off about .01" of diameter of both journals. The rod inserts into the driving socket more easily (shock), and rattles a bit when everything is put together. That should mean it isn't applying any forces besides the torque to drive the carriage, right? Cut still wobbles, though gib tension seems to flatten it out into a mere texture, now (well less than what I can feel, but just as visually apparent).
So now I'm wondering if there isn't still more going on, or if a loose feed-rod is still able to impart a wobble to the carriage somehow. perhaps the keyway in the driving end is binding on the key and pushes the rod to one side of the socket? Starting to get pretty good using the compound to turn stuff like these journals consistently, but I long for the glassy finish I'm getting from cheapo Harbor Freight bits run at 400rpm (only without the wobbles). I've at discounted the apron being the culprit, at least for now. Freed from the two journal supports, I can freely spin the feed rod and drive the helical gears in the apron without any periodic binding (I can feel a bum tooth in the main gear every three turns or so of the feed rod, but that's about it, and that's the wrong frequency for my wobble). If the feed rod isn't the villain, I have to assume it is the last gear in the headstock driving it that's out of round, which may also have something to do with the rattle I'm getting that's timed to the leadscrew (the leadscrew is also out of center about .007, btw).
I'd rather not have to dig around in that sensitive and fiddly area, but it may be the only way to determine whether or not I have egg-shaped gears binding on eachother. Come to think of it, an egg-shaped gear clashing with another in the headstock would push down on the end of the feed rod, causing the middle to bow upward (and due to the leverage, possibly a bigger affect than the journal offset)
TCB
- Joined
- Dec 27, 2014
- Messages
- 658
I have the SC8 and so far I have been unhappy with it. I have not had the time to really dig into all the problems yet.
One issue I did find is that the gear driven by the hand crank gear has about ~0.010" tir. Since this gear also turns when under power feed, it results in a rhythmic bind on carriage travel and a poor finish. The lathe is nearly unusable at the tight spots if you are trying to make a fine adjustment by hand.
I did not think to check the concentricity of the feed and lead screws. I will have to inspect that as well before I get back to the dealer I purchased it from.
BTW, Siegs customer service seriously impressed me. Mine had a broken part nearly out of the box and they airmailed it from China, whole process took less than two weeks. I was expecting it to take months for the part to get here.
-Josh
One issue I did find is that the gear driven by the hand crank gear has about ~0.010" tir. Since this gear also turns when under power feed, it results in a rhythmic bind on carriage travel and a poor finish. The lathe is nearly unusable at the tight spots if you are trying to make a fine adjustment by hand.
I did not think to check the concentricity of the feed and lead screws. I will have to inspect that as well before I get back to the dealer I purchased it from.
BTW, Siegs customer service seriously impressed me. Mine had a broken part nearly out of the box and they airmailed it from China, whole process took less than two weeks. I was expecting it to take months for the part to get here.
-Josh
- Joined
- May 14, 2015
- Messages
- 179
Sorta kinda half-assed fixed my cut problem. I figured that since the feed rod is not guiding anything, there is no reason it needs to be snuggly secure at each end, just so long as it transmits torque from the keyway. Not a long-term solution, but something to get me running for the time being. It 'works' in that I got no ridges and even got about half a barrel turned cylindrical (was tapered, which makes squaring it up a very tedious incremental process, especially when using a follow rest) with a decent finish.
The only problem was when I then went to work on truing up the backplate to be mated to my 4-jaw chuck. Crank the carriage all the way up to the head stock and the cross slide all the way out past the ways to work on it, and the ridges come right back. Seems that even loose in the journals, the shaft will bind to one side on the drive keyway, just not as forcefully as when the snug journals were camming the shaft around. Quick solution was to remove the feed rod, use the compound for the edge cuts and chamfers, then use the cross slide to manually face the plate. Seems to have worked nicely, a smooth, consistent 'brushed' gray surface is the result, which I assume is about as good as you can get with cast iron. Scribed a circle for the new bolt pattern (either I am getting tired, or the cross slide dial markings do not correspond to what the lathe claims; I couldn't hit the 4" diameter target until I rigged up a dial indicator and used it to set the cutter position).
I think having to crank the cutter out past the ways (to where it tries to tip down over the front of the machine) to cut the outside diameter of the plate was about 90% of the cause for trouble. It seemed pretty consistent when doing <2" diameter stuff, as far as trying to cut rings into the profile.
Since the feed rod is hexagonal, I have been 'truing' it in my 3-jaw, which means there is probably still about half the original runout no matter what I do (which may explain some of the lingering issues). If the new feed rod is also wonky, whenever it shows up, I'll have to buy a hex collet and borrow a buddy's collet chuck to get the journals dead nuts true, and make some bushings to fill the gap created. I'm really, really, really hoping that a square feed rod won't show these grooves, because that would mean the adjacent gear train for sure is the culprit.
TCB
The only problem was when I then went to work on truing up the backplate to be mated to my 4-jaw chuck. Crank the carriage all the way up to the head stock and the cross slide all the way out past the ways to work on it, and the ridges come right back. Seems that even loose in the journals, the shaft will bind to one side on the drive keyway, just not as forcefully as when the snug journals were camming the shaft around. Quick solution was to remove the feed rod, use the compound for the edge cuts and chamfers, then use the cross slide to manually face the plate. Seems to have worked nicely, a smooth, consistent 'brushed' gray surface is the result, which I assume is about as good as you can get with cast iron. Scribed a circle for the new bolt pattern (either I am getting tired, or the cross slide dial markings do not correspond to what the lathe claims; I couldn't hit the 4" diameter target until I rigged up a dial indicator and used it to set the cutter position).
I think having to crank the cutter out past the ways (to where it tries to tip down over the front of the machine) to cut the outside diameter of the plate was about 90% of the cause for trouble. It seemed pretty consistent when doing <2" diameter stuff, as far as trying to cut rings into the profile.
Since the feed rod is hexagonal, I have been 'truing' it in my 3-jaw, which means there is probably still about half the original runout no matter what I do (which may explain some of the lingering issues). If the new feed rod is also wonky, whenever it shows up, I'll have to buy a hex collet and borrow a buddy's collet chuck to get the journals dead nuts true, and make some bushings to fill the gap created. I'm really, really, really hoping that a square feed rod won't show these grooves, because that would mean the adjacent gear train for sure is the culprit.
TCB