14x40 lathe power feed improvement

Discussion in 'Machine Modifications' started by petertha, Apr 1, 2018.

  1. Apr 1, 2018 #1

    petertha

    petertha

    petertha

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    I posted this on another forum back in Dec-2017. Now after a few months of happy machining I think its safe to confirm I have the problem licked. In fact, it might be running better than it ever has. Thought some of you might be interested as this style of lathe is quite common among hobby machinists. Aside from the downtime headache factor, at least this has taught me a lot more about my lathe.

    The problem started under power feeding spring-2017. Nothing excessive, 0.025-.035” DOC in steel. My clutch started clicking, then complained louder, then finally disengaged altogether with the dramatic clatter. I removed the clutch springs & balls, stretched them a bit, no improvement. By hand I could feel the power feed bar was binding so something was amiss in the apron. I was very fortunate to meet another fellow 3 hours north of me who owns same lathe different name & he helped me through the initial disassembly which was a new experience for me. Thanks John!

    After teardown & apron removed I saw the worm gear had worn away at the cast iron C bracket that retains it. Rather disturbing because this machine has seen relatively light duty up until a few years ago. Fortunately the hardened steel worm gear was not damaged. FYI there is an excellent 12-part Keith Fenner YouTube series called Lets Look Under The Old Girls Apron where he rebuilds a distant Taiwan 14x40 cousin of my lathe & he had this same issue. Newer lathes look to have a different layout & improvement. I milled a nice flat in the bracket correcting the wear & John turned me a bronze spacer ring from a Princess Auto bronze bushing that acted as a glorified thrust washer.

    After reassembly it was slightly better but clutch still disengaged. Bummer! If I rotated the PF rod by hand it turned ¾ turn free then frictioned up. But this is also where the worm uses up its backlash & starts to engage & pull so the problem could be elsewhere. This began the journey of WTF 10 possible things that might cause friction anywhere in the system.

    I removed the carriage top & cleaned up the <cough> ‘scraping’ so it slid better on the ways. I checked all the carriage gears & shafts, they run smooth on the bench. I fiddled with the alignment of the C bracket because it’s a loosey-goosey bolt-on design that can rub on the shaft with the bolt pattern tolerance. I removed the helix gear & its shaft from the apron. Gear was worn but hard to know if sufficiently bad. Every time I made an attempted fix or adjustment I had to re-assemble the lathe to see if it worked.

    Finally I had the overdue thought to just check the PF rod runout itself by putting it back in the lathe supported by the clutch cup & tailstock bearing block. Then DTI mapping down the rod length. Sure enough it was bent, max 0.080” at about 1/3 length from headstock. Predominantly in the same plane as the keyway slot. I’ve heard that the shaft can stress relieve itself over time on the keyway, or maybe it got distorted when the bracket wore? Or maybe it was not true to begin with & the wear aggravated it?

    My options were straighten or replace, so I pursued both simultaneously. I could find a similar (19mm OD x 5mm keyway) from Chinese lathe which would need to be modified, about $200 & couple months delivery. I wasn’t too concerned about the tailstock end turndown mod. But the thing you come to realize about Asian iron is they do some bizarre things when it comes to drilling roll pins in these rotating parts. Like off center & at ‘hand drill’ angles. So matching that off-geometry to other parts like clutch cup wasn’t too appealing. Enter Modern Tool who went out of their way to source me parts from Taiwan including new helix gear (which engages worm) & some other spares. Unfortunately my shaft was no longer available. New shafts are very expensive. Milling an IMP (7/8”) shaft keyway is do-able, but I was told it would probably warp more than what I had.

    So I took my PF rod to a local driveline place where they do that kind of straightening press work. Unfortunately it just wasn’t getting done, so after 4 months of patient waiting I took it back. I then contacted Keith Fenner. He has a lot of talents but shaft straightening is his forte. Plus he is a super nice guy. I shipped it down & he had done within a week within 0.005”. I’ll attach a link where I’m now semi-famous (inside joke). BIG THANKS & LEGITIMATE PLUG TO KEITH!!!!!!

    Now with a perfectly straight shaft this baby is finally going together! Not so fast. Again, better but not still not right. WTF. More email exchanges with John. What could be different? I started to get the sense by elimination maybe it always was a bit out of alignment but basically ran despite itself. The driveline will overcome a certain amount of resistance by brute force. I noticed the bracket holes were oversize but shiny on one side (read bowing the bar into submission). There is also no datum, pins or contact registry of apron to carriage, just 2 bolts holding the apron on. That carries the bracket & worm along for the ride. Another potential misalignment?

    So I decided to replace the C-bracket & make 2 independent steel blocks, each with longer bronze bushings. This allowed me to float the blocks into position using the rod to dictate concentricity. I made them undersize so that I could shim the blocks to however they fit on apron & match the bolt pattern. The bronze gives me longer sleeve area & also have bushing boss material on either side of worm. This is desirable because when you power feed carriage in either direction or PF cross slide, the worm thrusts to opposite sides & contacts this.

    That led me to realize the brass helix gear that engages the worm & drives the carriage also has to be exactly centered to the worm gear. The gear is kind of a saddle profile so if it’s displaced in or out just a bit, the teeth engagement will bind up. So measured it out & made a shim washer to ensure it stayed aligned.

    Everyone still awake? :) Now it’s Dec & re-assembly again (I’m getting good at this). New clutch springs, new PT bushing blocks, new brass helix gear, and new alignment. After some careful power traversing & monitoring… It seems happy-happy now. After a couple hours rotating I could still see my rod bluing slowly rubbing off indicating the bushings are sliding nicely & doing their job. The clutch set screws are now set much more conservative than ever which indicates to me the feed power is using its energy traversing vs. overcoming binding & friction.
     
  2. Apr 1, 2018 #2

    petertha

    petertha

    petertha

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  3. Apr 1, 2018 #3

    petertha

    petertha

    petertha

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    Clutch related. Note the off center roll pin hole. No, this is not a visual sobriety test. Its actually quite common in these machines. Beware!

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    Last edited: Apr 1, 2018
  4. Apr 1, 2018 #4

    petertha

    petertha

    petertha

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    Keith Fenner's similar issue on a distant cousin Taiwan lathe. He brazed a ring on the predominant wear face.

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  5. Apr 1, 2018 #5

    petertha

    petertha

    petertha

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    Initial condition of my worm gear, brass helix gear & C bracket

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  6. Apr 1, 2018 #6

    petertha

    petertha

    petertha

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    After cleaning up the face of the C bracket, this was my original fix idea - a sacrificial bronze spacer washer just running free. PF rod run-out checking in the lathe supported in clutch cup & tail stock bearing without carriage influence

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  7. Apr 1, 2018 #7

    petertha

    petertha

    petertha

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    Sketches of original bracket/washer mod & my new block/bushing proposed fix.

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  8. Apr 1, 2018 #8

    petertha

    petertha

    petertha

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    New blocks with integral bronze bushings & shims Testing helix gear tracking & spacer shim washer to center.

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  9. Apr 1, 2018 #9

    petertha

    petertha

    petertha

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    Shimming the new bushing block & now also shimming the brass worm gear for proper engagement.

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  10. Apr 1, 2018 #10

    petertha

    petertha

    petertha

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    Here are some pics of top slide underbelly. Bit of a Hmmm moment actually. If this is what the 'better' Taiwan machines look like, I'd hate to see a rough lathe. The crisp, shiny, induction hardened & ground V rails & flats you see on the topside bed looks over-the-top inconsistent with what actually slides on top of it. Maybe that's the just the way they all are & I was expecting something nicer. I don't think I could use the word 'scraped' with a straight face. The pattern looked way to course for oil frosting.

    I blued the surface & carefully blocked off some high points with a fine stone & oil and de-burred the edges. I didn't want to get carried away here so it was 0.0000 type cleanup. I did a before & after pull test with a fishing scale & can say for sure I improved it. But its very subjective. It takes more starting force & drag force with something like WD-40 film on it just as a baseline reference. As soon as you put thicker way oil on, it still requires a bit of 'un-sticking force' but then glides like a curling stone. So that was never a significant hold up issue, but it made me feel better to look in there. I did have chips & sh*t in behind the V wipers. I'm going to come up with a better solution here using a thick felt wiper vs. just the plastic slider (Fenner style).

    The underside bed gib clamps (or whatever they are called) are just rough sticks of cast iron. They were ground reasonably flat but again, zero basic chamfering or deburring. I set them up gapped with feeler gauges. There is a fine line between sticking & sliding so required some set screw diddling.

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  11. Apr 1, 2018 #11

    petertha

    petertha

    petertha

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    This is showing the final adjustment process of the brass worm gear relative to the mating gear that it drives. I measured gear widths but also turned it while blued to confirm. The brass gear is keyed to a shaft and a washer / bolt to retain it. But I came to realize that simply 'tightening' the end cap could also displace the gear potentially off center. That is bad news. You can only feel this resistance by hand rotating, assuming everything else aligned. I have a feeling mine was always 'out' to some degree but being masked by power feeding. I'm not sure if its just a bad mechanical design or just loosey-goosey tolerances. Anyway, that's why I decided to make a simple bearing plate 'washer' once the gear centering was established & the end bolt gets some blue Loktite to retain the desirable end play position.

    Included a pic of the typical shaft lubrication hole. They are often not de-burred or chamferred, so a bit of Dremel grinding made the shaft run smoother. The ID bearing surface itself is not very pretty When you squirt oil into one of the top side nipples, it follows some galleys & down onto various shaft journals in the casting. In my particular case, the holes were plugged up with some waxy mung & probably never saw much lubrication ever. Now when I inject a (light weight way) oil, I can see a indication ring of lubrication.

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    Last edited: Apr 1, 2018

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