Proper way to fix slack in the compound

MyLilMule · Mar 14, 2021

In the process of rebuilding the lathe, I noticed this slack in the compound. It's not in the lead screw or the nut. It's as if the lead screw shaft is too long.

This doesn't seem right. There's .026" of play. What is the proper way to fix this? I'm just getting the lathe back together, so I don't quite trust the precision of it yet. And I don't have any other machines to make a new bushing, or a spacer. Options would be most appreciative.

Erich · Mar 14, 2021

Look very closely at the face of the shoulder on the lead screw, also the end of the threaded piece. You will see they are worn, not square to the axis anymore. Chuck them up and clean them square, measure the gap and fit an oilite washer to fill the gap.

MyLilMule · Mar 14, 2021

I’ll take a closer look at it when I go back out to the shop.

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MyLilMule · Mar 14, 2021

Erich said:
Look very closely at the face of the shoulder on the lead screw, also the end of the threaded piece. You will see they are worn, not square to the axis anymore. Chuck them up and clean them square, measure the gap and fit an oilite washer to fill the gap.

I’m not seeing any noticeable wear.

However, assuming you are indeed correct, there’s a flaw in your plan. [emoji6] Without this being in the lathe, I have no compound and no way to mount a tool post. Not being a machinist, and not having any machinists in my local circle of friends, and the pandemic shutting down access to the lathes at the maker space, I have no way to square these up.

But, I think if I get a right sized bushing in the same place, .025” thick, it would still do the trick.

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1yesca · Mar 14, 2021

lock the gibb with the set screw so it won't move and use the cross feed to machine with the compound locked up by the gibb

MyLilMule · Mar 14, 2021

1yesca said:
lock the gibb with the set screw so it won't move and use the cross feed to machine with the compound locked up by the gibb

Thanks for the tip. I still don’t believe they are worn, but I have some brass ring shims on order to use as a bushing. I’ll take an even closer look once I have them.

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michiganbuck · Mar 14, 2021

You most always have some slack, .002, or more in a screw-type crossfeed. The cutting forces push against the screw so the slack has little effect.
I can understand fixing and likely would do the same, but that won't change performance very much.

You could use the lathe with having that gap to counter face the mating surfaces to flat if you find they need that.

I would deliberately leave .002 slack.

MyLilMule · Mar 14, 2021

michiganbuck said:
You most always have some slack, .oo2, or more in a screw-type crossfeed. The cutting forces push against the screw so the slack has little effect.
I can understand fixing and likely would do the same, but that won't change performance very much.

This is NOT backlash. Backlash is normal and expected.

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michiganbuck · Mar 14, 2021

It is not backlash at the lead screw nut but I would still call it backlash to fit due to wear at the two shoulders and the compound faces. It has the same effect as a backlash at the nut and screw fit. Agree a washer would be a good/Ok fix.

Another fix might be the relocation of the tapered dowel if held with a tapered dowel, bumped to a .002 shim.

mattthemuppet · Mar 14, 2021

Mine had a lot of wear as mentioned above. I measured the gap, then turned down the screw holder enough for a needle thrust bearing to fit and made a sleeve to go over the thrust bearing (the screw holder was really chewed up). That took the gap down to a few thou and a small wrist twitch of rotational movement.

MyLilMule · Mar 14, 2021

michiganbuck said:
It is not backlash at the lead screw nut but I would still call it backlash to fit due to wear at the two shoulders and the compound faces. It has the same effect as a backlash at the nut and screw fit. Agree a washer would be a good/Ok fix.

Another fix might be the relocation of the tapered dowel if held with a tapered dowel, bumped to a .002 shim.

It's not held in that way. The bushing screws into the casting.

Screen Shot 2021-03-14 at 11.09.42 PM.jpg

Homebrewblob · Mar 14, 2021

My SB16 does the same thing. I would put a brass washer in between the nut and shaft

Could the screw have worn down the face of that nut?

Kevin T · Mar 14, 2021

Great video in your opener! I really don't like slop in my compound either. I thought I fixed mine with some arbor shims in between the handle and the dial but it's still not great. Please update this thread with how you attack the issue.

jim rozen · Mar 15, 2021

Homebrewblob said:
My SB16 does the same thing. I would put a brass washer in between the nut and shaft

Could the screw have worn down the face of that nut?

That's where the wear happens - between the underside (small, inner diameter) of the bushing, and the top (or outside) of the collar formed on the leadscrew.

Typically there's not that much wear on the top (outer, larger) portion of the bushing which mates with the underside of the graduated dial - although there will be some there, as well.

My approach is to remove the wear rings between the bushing and the dial, by facing them, so they are dead flat. Then if there's a lot of grooving between the collar and the inside of the bushing,
face those as well. This of course *increases* the lash in the assembly. The next step is to see if there's too much lash to make it up by shorting the stub on the end of the lead screw. Shorting that means possibly running out of threads for the funny slotted nut to hold the entire thing together.

If need be the next two approaches are, in order, make a up a steel washer to put between the leadscrew collar and the inside of the bushing, or if need be, cut the old threaded end of the leadscrew off and sliver solder in a blank, and re-thread for 12-24, allowing the seating location for the handle, to be moved inward as much as needed to make the lash near zero.

Bottom line is, wear makes the lash increase everywhere in the assembly overall. You either need to add new material in (washer) or shorten the distance between the critical surfaces (between the leadscrew collar and the end of leadscrew where the handle seats) to make it all work the way it was intended.

michiganbuck · Mar 15, 2021

I agree with Jim Rozen, if the mating place is not flat but dished it will make a spongy attitude to the cutting forces. The push side most important to turning and the pull side most important to boring.

Spongy as in bending the spacer washer so it acts like a spring with bending.

likely a hone skim would tell if that area is dished.... a flat spacer mating to a wore in dished mating of two surfaces.

Simple grooving not so important if the surfaces are still flat.

jim rozen · Mar 15, 2021

The grooving does impact the function of the crossfeed dials to some degree, and it's so easy to fix. So I do that when I can. It means five thou off each part typically. Which of course when you are done can add put to 20 thou overall. Which means going farther down the rabbit-hole. But I figure, if I care about the machine and I'm in there anyway, I take the extra time.

MyLilMule · Mar 15, 2021

Homebrewblob said:
Could the screw have worn down the face of that nut?

It's a brand new (old stock) nut. The screw looks to be in great shape. There's tiny, tiny perceptible backlash in the screw/nut (as expected).

MyLilMule · Mar 15, 2021

Some great advice here. I have the brass shims coming in a couple days. I bought several different thicknesses.

But I have some other things I want to do with the lathe before I would feel comfortable trying to repair it with it.

I need to adjust the shims under the headstock bearing caps. It appears the front bearing has about .002" much play in it. I understand that the excessive play will contribute to chatter. The shims are laminated, so I should be able to peel off a layer or two and have that dialed in.

I have a 4 jaw chuck coming that would help me get things dialed in concentrically (like the screw itself) - but I have never done that before, so it will be a learning process for me. But I would also like to face the bushing and the back of the dial, as suggested, and might be able to do that in the 3 jaw with some copper shims. The back of the dial is heavily grooved.

jim rozen · Mar 15, 2021

Yep that's a little grooved.

=)

You don't need to buy shims really. Just calculate what the thickness of the washer you will need, and make one of those out of piece of 1018 steel.

michiganbuck · Mar 15, 2021

I think it is good to check headstock clearance with a little warm-up, not dead cold... perhaps 15 minutes to a half-hour.

Be very sure to micrometer check current shims with a map of front and back, right and left. Notice how tight the hold-downs were at tear-down and hold dows are put back the same place and same way.

A cold check .002 may be in spec.

Qt: {I have a 4 jaw chuck coming that would help me get things dialed in concentrically (like the screw itself)}

Might be as good/better done between centers...if it has centers.

Proper way to fix slack in the compound

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