Cut Depth Affected By Feed Rod Rotation?

Started roughing out (since that's all I can do for now) the backplate for my Skinner 4-jaw. Man is cast iron some messy stuff. Anyway, getting ridges every 1/16" across the facing cut as you'd expect for the 1:2 gear reduction in the cross slide. What's weird though, is the ridges did not appear until after the cutting edge passed over the near-side way, at which point they abruptly started. I'm not sure what to make of that, but it makes me think the .01" vertical play I see at front way (which cannot be adjusted through gibs as I can tell) may be a factor.

TCB
 
barnbwt said:
Started roughing out (since that's all I can do for now) the backplate for my Skinner 4-jaw. Man is cast iron some messy stuff. Anyway, getting ridges every 1/16" across the facing cut as you'd expect for the 1:2 gear reduction in the cross slide. What's weird though, is the ridges did not appear until after the cutting edge passed over the near-side way, at which point they abruptly started. I'm not sure what to make of that, but it makes me think the .01" vertical play I see at front way (which cannot be adjusted through gibs as I can tell) may be a factor.

TCB
Click to expand...
Repeat the last facing cut like a spring cut on a turned piece. The ridges will diminish or disappear.Also,do you lock the carriage before taking a facing cut? Not locking he carriage probably will only give you a convex face cut.
On the original problem,if you find the lead screw is bent I believe it can be straightened.Check with the tech from the manufacturer.A friend bought a used lathe that had a similar problem. He received a new lead screw from the dealer.The dealer straightened the original lead screw and is being sold as reconditioned.
mike
 
-Waves are consistent with rotation of the feed rod (not lead screw)
-A zero pass will ride the crests/troughs exactly if I repeat the cut with the same feed rod/carriage position, opposite alignment will cut off the crests and skip over the troughs
-Waves only appear when power feed is engaged (not when thread feed is engaged or when manually fed)
-Occurs whether the carriage or cross slide are locked (all my cuts are made locked in the respective fixed direction)
-Tightening rear carriage gibs helps, but did not eliminate the problem

I haven't found any mention online of bent feed rods, only leadscrews, which leads me to believe this is either a very uncommon scenario, or that bent feed rods do not display symptoms like lead screws do (makes sense as the feed rod fits loosely in the carriage and gear keyway). I believe one or (probably) both keyed worm gears driven by the feed rod have an anomaly that causes varying gear engagement thereafter. I need to open up the carriage to see what kind of adjustment I may have to work with, or if the gear imperfection is visible (I suspect it is causing periodic binding, and will appear shiny)

TCB
 

From a previous post by you, it looks like this is a new lathe. If so, what does the manufacturer and/or vendor have to say about the problem. A new lathe should have arrived reasonably well adjusted and not showing problems like you are experiencing. Certainly, wear is not an issue. It is possible that there is a manufacturing defect ( e.g. bad gear tooth,

I wasn't able to find a parts list for your lathe but did find one for a PM1127. Assuming a similar drive, the gearing in the carriage is fairly straightforward. Since you observe the pattern in both turning and facing, the problem would originate prior to the split for the carriage drive and cross feed drive. It appears that there are only a couple of gears between the worm and the split and the gearing is such that those gears would only make a partial revolution in a 1/8" carriage travel.

I would agree that a bent fed rod is unlikely to be the cause. Does your carriage advance 1/8" for each revolution of the feed rod? When you observe the feed rod turning as you drive the carriage/cross feed, do you see any hesitation in its rotation? It almost sounds like there is a problem in the gear box. I think if part of a tooth were missing on a gear, there could be a hesitation until the tooth was more fully engaged.

In any event, it sounds like a warranty issue.
 
To be honest, it doesn't really matter about the warranty. Unless Precision Machine happens to have a spare part lying around (they won't), it will be months/years until a replacement is available, I expect. Whining about the defect won't get the lathe fixed, and I'll be likely to be on my own, despite the best intentions of the distributor (doesn't absolve them of responsibility, but I prefer not to waste my time if there is a way around the problem). The lathe is incredibly similar to the SIEG SC10, so they could possibly be an alternate (although the SC10 is so new I'd be surprised if they have parts, either). Given the straight-gear quality elsewhere in the lathe, and that the gearbox seems to be consistent in effort to rotate manually (power feed disengaged from carriage, of course), my guess is the problem gear is the worm, simply because it is a more complex shape. My hope is that there is simply a piece of sprue or something fixable, so sourcing a replacement won't be needed (a friend has a 4 axis CNC, so a reverse-engineered replacement is potentially possible, though definitely not my preferred option)

TCB
 
Well, mystery solved. One, or probably both, of the spiral/worm gears that tap the rotation of the feed rod and direct it orthogonally into the apron gearbox are misshapen. They have a 1:1 ratio (keyed to the feed rod) so it is impossible to determine which/both are at fault without fully dismantling the apron, and that entails circlip pliers for starters. Not going there until I have a game plan.

I'm finding other serious issues with the lathe's ways themselves, so that game plan may entail getting my money back at this point.

TCB
 
Hi TCB,

Thanks for reporting back!

Can you provide pictures so others can see what you see for both problems?

Thanks,
-brino
 
Well, after lifting off the saddle (so the apron is 'floating' on the screw and rod while resting on some wood blocks), turning the spindle causes the apron to shimmy in several directions once per turn, consistent with the low/high parts of the cut I was experiencing.

The other issues I am keeping to myself for the time being, until I get a response from QMT (still nothing since last week, but I am aware they are busy; I'll give them until Friday)

TCB
 
TCB,

It sounds like your methodical approach to debugging the issues is paying off.
A meticulous investigation can take a lot of time, but it does lead to a full understanding.

I hope you get to a satisfactory conclusion on the issues!

-brino
 
Sorry to hear you are having issues with your new lathe, I hope that this is something that QMT can work out satisfactory for you.

Subscribing to keep up with the story.
 
You must log in or register to reply here.
Back
Top