Resurrecting a failed little project, mini-lathe spider

[WobblyHand]

What seems like long ago, I had tried to make a mini-lathe spider. This unit replaces one of the jam nuts on the mini-lathe spindle. I tried making it in my mini-lathe, but being a new machinist, I made a whole lot of mistakes, both in design and machining. Drilling and boring went well, but the internal threading was a disaster. All kinds of chatter on the threads. Basically, I used a threading bar that was too skinny. Yesterday, I thought I might try to salvage this. I loaded the 1.5" stock onto my four jaw on my 10x22. I looked for the largest threading bar I could find which was a Widia 3/4" Top Notch style LH bar. Originally, I had attempted to use a 3/8" bar on my mini. This 3/4" bar just barely fits in the bore that I am threading. I'm reverse threading, using a LH bar, a RH thread.

I had the foresight to make a male thread pattern (long ago) so I could avoid pulling the spider from the chuck for test fitting. Yesterday, the pattern didn't fit in the spider nut. Had to make the threads deeper. After some fooling around and false starts, I had picked up the 1.5 mm thread. After what seemed like 10's of minutes, I managed to start cutting a better and deeper thread, slowly cleaning out the old chattered ones. Eventually, the spider accepted the male pattern. Unfortunately, the female threads are not pretty, but the male piece threads in pretty well and is snug. I patted myself on the back and decided to pull the part from the chuck.

As it turns out, that was a bad idea. The nut did not screw onto the mini-lathe spindle threads. I sat there scratching my head in disbelief. I finally pulled out a micrometer and found that my pattern was 0.005" larger OD than the threaded spindle! Spindle was 1.010" OD, the male pattern was 1.015". That means the female threads need to be deeper to accept the spindle. Groan. Ran out of time for the day.

So today I need to "start over" again, re-chuck it and have to pick up the thread, and take off about 0.005-0.007". I'm going to try again to salvage this piece. If I mess it up, I do have more stock to make another one.

Here's the ugliness as of the moment. They thread together very nicely, too bad that's insufficient. Yeah, make sure your pattern is the correct size!

The thread gutter is not well formed. Hope to save this, but hey, might just chalk this one up to a learning experience.

 

[VicHobbyGuy]

I also made a spider for my 7x mini-lathe. Mine just threads on to the projecting threads on the spindle - the jam nuts are left in place.
To re-mount a nut and pick up the existing thread is a challenge for me- good work!
I don't understand - if your pattern (male plug) is too large, won't the nut that matches it also be too large? I'd expect a bit loose fit on the spindle- which isn't a major problem as the spider registers against the jam nut. Perhaps there is just a typo in your post, or I'm not thinking this through. BTW, my lathe (with its two jammed ring nuts on the end of the spindle) can run for a short while with one jam nut, so I could remove one to use for checking my test plug 'pattern'.

 

[WobblyHand]

Well that wasn't too bad. Since I didn't have a good pattern, I had to dismount the 4 jaw chuck and bring over to the mini for fitting. Did that a couple of times. Also found out that while the chuck maintained concentricity, it didn't lock up exactly the same every time, so I had to partially re-sync the threads. They weren't off by much, but they had to be re-synced. The chuck is a little heavy to hold one handed while trying to screw it on to 1.5mm threads, but got it.

Now to see if I can find a drawing of this. Maybe I made one... If not, I'll make one. Like to machine from a print, one that I can scribble on.
Think all I need is to mill a couple slots for a pin spanner, drill some holes for the screws, and figure out how to make some brass tipped screws.

Threads are looking better, no chatter. Thread run out gutter is better as well. So far, this is a save.

 

[WobblyHand]

I also made a spider for my 7x mini-lathe. To re-mount a nut and pick up the existing thread is a challenge for me- good work!
I don't understand - if your pattern (male plug) is too large, won't the nut that matches it also be too large? I'd expect a bit loose fit on the spindle- which isn't a major problem as the spider registers against the jam nut. Perhaps there is just a typo in your post, or I'm not thinking this through. BTW, my lathe has two jammed rings on the end of the spindle, so I could remove one to use for checking my test plug 'pattern'.

The nut wouldn't screw on the spindle as the threads on the nut were not deep enough. All I know is that the deeper threads work. Maybe I posted the numbers backwards. Re-syncing metric threads on an imperial system is always a pain. In my case, the threaded spindle chuck on my 10x22 didn't quite return to exactly the same place. The chuck and spindle mating surfaces were clean, or at least they looked clean. Next time the chuck is off, I'll clean the surfaces.

What I should have done, (but at the time I didn't have a thread micrometer, or maybe even pitch wires,) was to measure the pitch diameter of the mini-lathe spindle and then make a pattern matching that. It's likely I used calipers, and cheesy ones at that. So I paid the price for that indiscretion. But the biggest mistake was using far too small tooling for the threading. That lead to chatter.

In the post above, you can see one of the jam nut rings on the mini, just like yours. The jam nuts spun on the pattern fine. The nut I made from the pattern was a tight fit to the pattern and did not fit on the spindle. Taking a couple spring passes on the nut fixed the problem. This is a case where a tap would have solved the problem in a minute, but I didn't have one. While the piece was still in the chuck and concentric to the bore, I faced the spider nut.

Since I have the male pattern (which still fits) I can use it as a mandrel to hold the nut to clean up the exterior jaw marks and the rear surface.

 

[WobblyHand]

It seems I hadn't made a drawing. Not that this is a complicated part. But here it is anyways. The LMS part https://littlemachineshop.com/products/product_view.php?ProductID=1136 uses cap screws, which I probably will use and the slots are not lined with the holes. I only did that because I was too lazy to figure out how to rotate them in CAD. It was so much easier just to put it on a principal axis...

Biggest issue might be interference of the mill head with the dividing head and chuck. Experienced something similar in the past. Will be finding out soon.

I will make brass tips for the M6 screws, or at least give it a try. Have some 1/4" 360 brass stock somewhere in the shop, should be able to turn some down and loctite the tips to the screws.

 

[wildcatter]

If you have an oxy-acetylene torch you can just add a drop of brass to the end of the screws. Then mill a flat on the brass end.

 

[WobblyHand]

Had to clean up a little down in the shop. It was a bit cluttered, especially on the mill. Got the dividing head setup and drilled and tapped the holes in the spider. I left one hole untapped so when I flip the piece in the chuck I can sort of line things up again. None of this is critical, but I'd like the slots offset 45 degrees from the holes. Took a break, had a snack and made this post. It's not as hot as outside, but it's 83F in the shop so a little hydration is recommended.

 

[WobblyHand]

If you have an oxy-acetylene torch you can just add a drop of brass to the end of the screws. Then mill a flat on the brass end.

That's an interesting idea. Might try it later. Too hot outside to be doing that right now. All my oxy-acetylene stuff is done outside, as my garage is antique and is internally sided with wood. Too risky to do open flame work in the space.

I haven't even looked for the brass yet, I was too busy drilling and tapping.

 

[wildcatter]

I completely agree. No point in risking your shop and tools! And slaving over a hot flame on a hot day is not always my favorite thing either.

 

[WobblyHand]

Save for the brass tipped screws and a little more de-burring, this is done. It could be better, but it's fit for purpose. As I anticipated, the jaws were in the way due to the short end mill length. It took some creative adjustments on the fly to sort it out. That means it's not as precise as it could have been. But hey, sometimes you just have to get it done.

Tomorrow will try to get the brass tips on the screws.

I don't have a spider for my 10 x 22 either, so this is a practice run for that lathe. Hope that one will be a little better. Right now the threads in the pictures above are kind of jagged and a bit rough. They catch on rags or paper towels, shredding or cutting them. Next one hope to do a little better on the internal threads.