Broken Half-Nut Woes

[Robo_Pi]

I've just recently been getting quite excited about getting back into machining. In fact, I just bought a bunch of new accessories for my Lathe/Mill. And then the darn thing BROKE!!!! What a bummer!

Before I begin let me first say that I used to own a really nice South Bend lathe, complete with tons of accessories. It had 3-jaw chuck, 4-jaw chuck, full set of 5-C collets with drawbar, Taper Attachment, Tool post Grinder, Center Rest, etc, etc, etc. It was great. Unfortunately when I moved I sold it because I didn't have a place to set it up. I would give anything to have it back today. That was about 40 years ago that I sold it. In the interim, I had an really nice Atlas lathe, again with tons of accessories, and even a full turret tailstock. Again. Sold the thing and would give anything to have it back today.

Now I own an Enco HQ400 Chinese 3-in-1 Lathe/Mill/Drill. It's a piece of junk I know. But it was working up until about an hour ago.

Here's the machine:



It was pretty nice until it broke. I mean, for being a cheap machine.

Anyway, I just bought a brand new set of 3MT collets for it and I was in the process of making a drawbar for them using an old Sewing machine handwheel. (see photo below)



On the right is the handwheel I'm hoping to use. Next is the piece I'm currently working on to mount the handwheel to the threaded drawbar rod. It was while turning down this piece of steel when the lathe half-nut broke.



You can see the half-nut lever just to the right of the table handwheel. Note that this half-nut serves for both thread cutting and to drive the table during normal turning feed. I was hogging off about 40 thousands of material when something banged and the lathe quit cutting. I knew something broke but wasn't sure what it was. My first thought was the carbide tool bit broke off. I was using a cheap brazed-on carbide tool at the time. But nope it wasn't the tool. It was the half-nut lever, broke clear off and was laying on the bench beneath the table.



Apparently only 2 small screws hold it on. One screw had apparently been missing entirely for who knows how long. The other screw was most likely working it's way out and simply broke off a piece of the lathe casting with the last couple threads it was hanging on with. I feel guilty for not having realized the lathe was falling apart. Had I not let it get this loose it most likely would have never broken.



Above is a picture of the half-nut still in the table. If you look close you can see that a very small part of the table casting actually broke off just underneath the half-nut. That was the screw that was still hanging on by a few threads. The screw itself didn't break. And fortunately there appears to be plenty of threads still left in the table part so that I might be able to put this back together again by just getting a longer bolt.

Below is a picture with the half-nut removed. You can better see where the casting was broke off on the bottom screw hole. I'm guessing this was some kind of casting since it broke like cast iron.



Below you can see the half-nut. The threads appear to be fine. No damage to the actual half-nut itself. I'm a bit surprised though. This truly is just one-half of a nut. It seems to me that both my South Bend lathe and the Atlas actually had two-halves that clamped together to serve as a "half-nut"? I can't be sure about that. But this is truly just 1/2 of a nut. That's all that drives this table.




I'll need to find some metric bolts to see if I can put this back together again. I did notice that in a worse-case scenario the threaded holes could be drilled clear through the piece mounted on the table and this could be put back together using bolts and nuts. There's enough room on the far side to do this if necessary. But for now it looks like it might be repairable by just replacing the screws with some a tad bit longer.



Above is the piece of steel I was turning down when this broke. I think I was hogging off quite a bit of material at the time. But nothing this lathe shouldn't handle. The lathe itself wasn't struggling to cut this. I don't think the half-nut mechanism would have broken at all had both mounting bolts been property seated and tightened.

In any case, I need to fix this now before moving on.

I have a second job to do for this lathe. I just bought a brand new 6" 4-jaw chuck for it. It came with a 4" 3-jaw. The lathe has a 17" swing so I decided to get the 6" 4-jaw just for the larger capacity. Plus I wanted the independent jaws of the 4-jaw too. This lathe should handle this chuck easily as long as I don't put anything too monstrous in it.

Here's the new chuck:



And I bought a blank piece of 1/2" thick by 7" steel plate to make into a backing plate.



Lot's to do, but no half-nut to do it with.

Oh, yeah, one more thing,....

I'm anxious to try cutting threads on this lathe too. I cut the threads shown below on the collet drawbar using a standard thread die by hand. But I need to thread the other side even longer, so I'm hoping to cut those threads on the lathe.



I have lots of work for the mill too. But I can't use that with this broken half-nut either. I use this same table for cutting on the mill.

I suppose I'm supposed to ask a question in this beginner's forum?

Has anyone else experienced a similar breakage on a cheap lathe?

Hopefully I can fix with without any major loss of performance.

 

[Mark Needham]

You've answered your own question. Suck it up., and get into it.
Oh, and have fun.

 

[benmychree]

Also, you could drill And slot the lever cast part and add two more holes for two more bolts above and below the existing holes, that is, a symmetrical bolt pattern. The handwheel setup for the drawbar may not be a real good idea, think of the amount of inertia it has, when started on a high speed, it may loosen and the cutter fall out; a drawbar with a nut pinned on one end for a wrench is the usual arrangement.

 

[markba633csi]

These machines usually have some weak points which need attention- you just found one
John's fix sounds like a good one
mark
Suggestion: try to feed by hand as much as possible - save the half-nut for threading operations mainly

 

[Robo_Pi]

The handwheel setup for the drawbar may not be a real good idea, think of the amount of inertia it has, when started on a high speed, it may loosen and the cutter fall out; a drawbar with a nut pinned on one end for a wrench is the usual arrangement.

Thanks for the heads-up. This sewing machine handwheel is pretty heavy. I never thought of it spinning loose. That's certainly something to keep an eye on. I was originally going to go for the nut arrangement, but then I was looking up MT3 collet drawbars on ebay and I saw some of these handwheel designs. I immediately thought that would be far nicer than having to grab a wrench every time. I think my South Bend had a handwheel tightener for the 5C collets if I remember correctly. My Atlas lathe also had 5C collets, but it had the level that you push side-to-side to tighten them. I wish I had that Atlas back. These MT3 collets won't allow for long rods to be pushed all the way through the spindle. But I don't think this lathe can use the 5C collets. The spindle bore isn't big enough. So I'm stuck with the MT3 collets. It's better than no collets at all.

I'm going to give this handwheel a try. But thanks for bringing this up. I'll be sure to keep an eye out for that potential problem. It it becomes a problem I might try going with a light weight alluminum handwheel before giving up on the handwheel idea entirely. This lathe take a while to start up and stop. So it might not be a problem for this lathe. But I will keep that in mind. I might be able to rig up some kind of lock on it too. Something to lock it to the spindle after it's been tightened.

Also, you could drill And slot the lever cast part and add two more holes for two more bolts above and below the existing holes, that is, a symmetrical bolt pattern..

I just came back from Tractor Supply. Thankfully they had the exact metric bolts I needed. These aren't real big. They are only M5 X 0.8. But yeah I'm going to be adding more holes. I just took a bunch of pictures I'll try to post them soon. This turned out to become a much bigger job that I had originally hoped.

 

[Robo_Pi]

UPDATE:

I'm not feeling as guilty anymore. This thing could never have had the second top screw in it. There simply isn't enough clearance for it to fit. I just came back from the store with two new screws and there's no way the top screw could ever fit. There is ZERO clearance between the Screw and the table when the screw is screwed into the hole. The cap screw touches the table when screwed into the hole. See photo below:



In the next photo I'm holding up the half-nut lever, you can see there is a good 1/8" of meat that needs to fit between the cap screw head and the bottom of the table. This screw could have never been installed from the get go. This screw had to be missing when I bought this lathe.



So this relieves me from any responsibility for being an idiot and allowing my screws to fall out. I'm only an idiot for having bought a piece of junk lathe. That's a shame they screwed this up at the factory. It's not a bad lathe when put together properly.

So I ended up having to take the whole thing apart. I had to remove the lead screw:



Then I was able to take the half-nut assembly off the bottom of the table. That was nice. Better than having it part of the main table entirely.

As you can see in the next photo that screw would never go in that hole when the half-nut lever is flush with the table bottom.


So then I had to disassemble the half-nut lever, and a whole bunch if tiny parts came out. Ball, spring, set screw, etc. I gotta put this all back together and make it work. I'm getting in deeper and deeper.



Finally I'm down to the nuts and bolts, project.

I'm going to reuse the original bottom hole, and try to mill out the top hole to "Zero Clearance", so the old top bolt might still be usable to some degree. And then finally I'll put two brand new holes to fill out the square. There doesn't appear t be anything in the way so this should be doable. They had slots in their original design, but I think I'll just line everything up perfectly and just make the two new screw holes straight in. I don't see the need for the slots. They were probably put in originally to allow for sloppy manufacturing.

Below is a photo marked up with the newly planned screw holes.



I think there's enough room to do this without a problem. I can still use the mill part of this machine. I'll just lock the table down. I have compound X-Y mill vice that I can use to drill and mill these new holes.

When I'm done it will be far better than it was originally. At least that will nice. It's not just a fix, it's an improvement.

 

[WarrenP]

Do you think the slots were there for adjusting the half nut clearance, etc ?

 

[Robo_Pi]

Do you think the slots were there for adjusting the half nut clearance, etc ?

Probably so. The problem is that they wouldn't work too well anyway because this plate won't move any further in than it already is. You probably can't see in these photos but there's a thin piece of sheet metal riveted onto the plate and that sheet metal is flush up against the half-nut assembly. So the bulk of the slot is unusable anyway. There may be a very tiny bit of adjustment where it could be moved out a bit. I'll check the clearance before I drill the holes. I could slot them slightly just to give a little bit of adjustment. This isn't a very good design.

I'm actually thinking about re-manufacturing the entire half-nut assembly. Although that would probably be over-kill. The best way to have this adjustable would be to have the actual half-nut lever screw in or out for more or less tension. That way you could set it live as you're working.

But yeah, I'll check the clearance before I drill the actual holes. This lathe isn't going to be high precision no matter what I do to it. This half-nut is probably tight in places and loose in places along the length of the lead-screw anyway. It's not a super precise machine. I'll have quite a bit of play in the screw clearance holes. I'm not planning on making them real tight. So they'll be some play there to work with anyway. I think there's also a bit of play in the ball-detent of the half-nut lever. In other words the half-nut lever isn't going to hold this half-nut as solid as a rock. It's actually going to be "floating" on the ball-detent spring of the half-nut lever.

This is one of those lathes that makes things, "Almost Round". When you have a lathe like this you save a lot of money on micrometers and dial indicators because there's no point in buying them. If things look good to the naked eye, that's close enough.

 

[markba633csi]

Actually, a large proportion of lathe duties involve making "just good enough" parts, at least in my garage
And I use my caliper much more than my micrometer
mark

 

[Robo_Pi]

Actually, a large proportion of lathe duties involve making "just good enough" parts, at least in my garage
And I use my caliper much more than my micrometer
mark

It's true that I bought this crappy lathe knowing full-well at the time that I wasn't going to expect high precision out of it. I almost bought a better lathe at the time, but there were two trade-off. The first trade-off was the swing. The better lathe only has something like a 6 or 8 inch swing, while this one has a 17" swing. And that's nice for doing brake drums and rotors etc. The second trade-off was that the better lathe was just a lathe only and didn't have them milling head. I realize the milling head on this machine isn't all that great, but it's better than none at all.

I used to have a really nice South Bend Lathe and Bridgeport Mill. I'd love to have those pieces back. But they were also quite large. And I don't really have a shop large enough to deal with machines that big anymore. This little 3-in-1 machine is a featherweight in comparison,. It doesn't take up much room and only weighs 400 lbs so it's somewhat easier to move if necessary too. But yeah, in terms of precision I'd much rather have my old South Bend lathe and Bridgeport mill back. And my old larger garage too!

My life has been going downhill instead of uphill. I guess I need to wake up to the fact that I'm just a failure. What a bummer.

But yeah, WarrenP is right. I'll need to take a look at potential clearance adjustments before I drill any holes. I probably will end up milling out a little bit of play for adjustment. I'll go out to the shop and take a close look at the situation. I can put the lead screw in the bearing and reassemble the half-nut and get an idea where it needs to be before drilling any holes. It would be nice to have some room for future adjustments as the half-nut wears down.