Worm gear bracket fix on 14x40 lathe

petertha

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So what I initially assumed was a clutch problem in this post
http://www.hobby-machinist.com/threads/14x40-lathe-clutch-issue.56867/#post-469823

..is now pointing to a worn worm gear bracket assembly. After some not-so-taxing 0.015-0.020” DOC power feeding on 1144 steel, the clutch started clicking & complaining as per the post. But now I think it was just coincidence that pushed it into the ‘fix me now’ zone.

With the carriage now disassembled I can see obvious wear on the inside of the cast iron U bracket that contains the worm gear. It seems to me when the lathe is new, the internal sides of the bracket are nice & square. The gear fits the outer bar with just a small gap. Now it appears the worm has worn a ~0.1” depth divot into the cast iron. Amazingly the worm gear is in great shape despite this & damage confined to the bracket from what I can tell. I will polish off some minor jaggy’s on the one gear end with the Dremel stone. Same goes for the brass gear it engages, teeth look fine.

So now I’m going over fix scenario pros & cons in my mind.

1) Buying a new bracket is possible, but it’s a late 1997 Taiwan 14x40 (King brand). There is a possibility to locate one from far away lands or maybe a current cousin in N-Am, could take a while.

2) I could mill a brand new bracket from chunk of steel & incorporate some improvements.

3) Utilize / modify the existing bracket, mill away the offending material kind of like the sketch & make up the gap with a bronze bushing that would take the lateral load of power feeding. This is somewhat equivalent to what same issue Keith Fenner remedied in his Youtube series ‘Lets look under the old girls skirt’. But he dressed the lip & OA torch & silver soldered a bronze ring. I dont have that equipment so do you think I could just insert what would amount to a glorified bronze flat washer of that thickness? I almost prefer that option as its less fuss & replaceable. The downside is I lose some rigidity in the arm from lost cast iron, but its not like I'm hogging on the machine.

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Here are some screen grabs from Keith Fenner's Youtube series fix. (I highly recommend watching btw).

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And this is the part we are talking about relative to the carriage assembly

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Actually thinking about this some more dimensions wise, a bronze 'washer' spacer would probably have to be milled off on one side to align the shaft hole which occurs quite close to the edge of the casting. This flat would prevent spacer spin. I wonder if a part like this were made similar to this shape (but adjacent to the worm gear inside the gap obviously) without being permanently attached to the casting by soldering or whatever, would that be a good workable solution? I guess one advantage is when it wears, another could just replace it.

Just curious if anyone has example of how other lathes accomplish this? I would think a thrust bearing might not appreciate this kind of duty or environment?

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I think every lathe that I have ever worked on had bushings inside the bracket. They wear out, but then it's just a matter of replacing the bushings. If it were mine I think I would go with the spacers or bore the bracket and mount bushings.
 
Just looking at the drawing, it might be that a slippage of the key (part #71) is gouging the casting (part #73). If that could occur/reoccur, a bronze bushing inset in the casting won't last long. Could be that a thrust bearing is
a better bet, because the thrust washer plate blocks key motion. Don't know sizes, but something like
this <https://www.vxb.com/TC1220-Thrust-Needle-Roller-3-4-x1-1-4-x5-64-inch-p/kit12483.htm>
with a couple of hardened washers might be made to fit. You'd want to machine clearance on
either side of the worm, just a little over the thrust bearing OD size, so that it aligns on
the center hole, against the shaft.

Another approach, with less lubrication maintenance, would be to remanufacture part #73 with
some permanent-lube sealed bearings (moderate speed, moderate thrust, so maybe ball
bearings can do it).

Minimally, counterbore inside for bronze 0r oilite thrust bushings, and use
hardened thrust washers against that. It'll take some fiddling with loose parts at install time, though.

An extra thrust washer would make a space for fitting in shims in future years, to
reduce the worm backlash.
 
Thanks. I examined the key & its set very securely in the worm so I don't believe it was doing the gouging. The wear was just a bit on the worm facing the casting, but substantial divot on the CO bracket face itself. The other side looks pristine, both worm & bracket. Which kind of makes sense, I rarely power feed under load from left to right.

I've had a chance to draw up the components to see what I can & cant do. I haven't looked at thrust bearing dimensions yet but guessing with a 0.75" nominal ID for shaft by whatever minimum thickness they come in, I will probably be hogging out a lot of bracket material to the point it loses strength or the notch/counter-bore interferes with the mounting holes. I would really prefer to preserve that pattern in the carriage. It would be nice if I could thread in a bushing so when it wears, just turn it in to compensate wear. But the shaft hole is very close to the edge of the bracket so virtually no allowance to accommodate a thicker bushing. Here are some views to give a better sense.

Another thought is to make 2 blocks rather than a more complex U bracket, kind of like pillow blocks. Then I could monkey with the thickness & shim accordingly. But I suspect they design it this way for a reason, stronger bracket & 4 bolts is more secure.

So far the simple bearing ring is winning out me thinks.

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