Repairing worn lathe half nuts

[Holescreek]

I've had the Clausing 5914 sitting in the shop all dressed up with it's new DRO and splash guard taking up space and gathering dust for too long (just like the Atlas on the other side of the shop). I've used it for some minor turning work but nothing too involved because I don't have many tools set up for it (not enough BXA holders).

A few days ago I wanted to use the lathe for some barrel work and after trying to thread the muzzle I re-discovered that the half nuts were worn badly. My first clue was that when I engaged the half nuts the lines on the thread dial didn't quite meet up anymore. So after I finished my task I engaged the half nuts and looked down the length of the leadscrew while pushing and pulling on the carriage. The carriage moved quite a bit.

Clausing sells the half nuts for my 5914 for about $157/pair. Knowing that they were available (and expensive) made it easy to decide to repair them myself.

I pulled the apron and blueprinted the half nuts. They are cast iron and a bastard size acme thread, 7/8"-8.

There are several avenues of repair that I could think of to consider.
1) Make a tap, fill the threads with bronze filler rod, make a fixture to hold the halves in place and re-cut the threads.
2) Make a section of Acme rod that matches the leadscrew dimensions, buy a chunk of bronze or brass and replicate the nuts by single point threading it on the lathe then cut away the excess material to replicate the old nuts. (The matching acme rod is used to fit the internal acme threads.)
3) Make a brass insert to replace the threads and figure out a way to attach it to the half nuts.

 

[Holescreek]

Since I decided to make brass replacement threads I needed to make a dummy shaft to duplicate the lead screw. I started by locating my acme screw thread gauges.

I found another gauge used to align the tool bit to the bar, however I used it incorrectly in the photo below. I'll have a photo further down the page that shows the correct usage of the angle shown in the top right of this photo. For the rest of you, the "V" in the end is used for grinding the sides of your tool bit to the proper 29 degree angle and the notches on the sides are used to grind the flat on the end of the bit to the correct width.

Per the gauge, the depth of the thread is pitch/2 +.01". so 1"/8 tpi=.125" /2 +.01= .0725".

I set my compound to zero degrees(perpendicular to the cross slide) for acme threads and all infeed is done with the cross slide. Once depth was reached I used thread wires to measure the pitch diameter of the lathe's lead screw, then measured the PD of the stub I made. It was within .001" (meaning the tool bit was ground correctly or I was just incredibly lucky) so all it took was another cleanup pass to bring it in to size. If the space between the threads had been too small I'd have used the compound feed to widen it. Acme threads are the easiest to make in my opinion.

 

[Holescreek]

Next I made a fixture to hold the nuts in place for boring. There are two reamed holes that the dowels on the bottom of each half nut fit into and a gauge pin between the halves to keep them from rotating. I aligned the fixture on the cube and trammed the hole center in with an indicator.

I used an existing boring bar setup and got a little chatter from having the bar stick out too long but really don't care. The chatter will give extra holding power to the glue. I bored it to .950" ID then used a 1" end mill to counterbore each end of the bore.

 

[Holescreek]

On to the nut.

I set the side of the tool the correct way for acme thread this time:

The thread itself was simple, just keep pushing the cutter in at .005" per pass until the depth of .072" per side (to match the lead screw) was met then test the male acme plug I made. It wouldn't start so I put layout dye on the end and tested it again to see where it was hitting. I needed to widen the threads so I just advanced the compound .001"/pass and made a few more passes.

Once the threads were complete I switched chucks and grabbed my acme plug by the threads. I made a cleanup pass on the shank to true it to the threads then I flipped it around and used it as an arbor to hold the nut while I turned the OD down to fit the half nuts.

Then came the gluing. I used Loctite 620 retaining compound after cleaning everything up with acetone and slathered it on.

While the Loctite was wet I set everything back up in the fixture to keep it aligned while the glue dried.

 

[Holescreek]

24 hours later I band sawed them apart through the gap and sandpapered the excess brass flush with the cast iron.

I installed them this afternoon and then replaced the worn bushings in the lead screw support under the tailstock. A quick test after cleaning and oiling has everything working perfectly and the lines are back to matching on the thread dial.

 

[chips&more]

Very nice work! I’m wondering about the use of Loctite 620? It’s more of a retaining compound, like for making a bushing stay put in a hole. If the Loctite is just holding 1/2 a part, it’s not retaining, or is it? That job can be soft soldered. Cast iron can be soldered. Or maybe a good adhesive? Or maybe the Loctite 620 will work. Please keep it touch, I’m curious to find out. Again very nice work!

 

[hermetic]

Superb work! "yer a wizard Arry"

 

[Holescreek]

Very nice work! I’m wondering about the use of Loctite 620? It’s more of a retaining compound, like for making a bushing stay put in a hole. If the Loctite is just holding 1/2 a part, it’s not retaining, or is it? That job can be soft soldered. Cast iron can be soldered. Or maybe a good adhesive? Or maybe the Loctite 620 will work. Please keep it touch, I’m curious to find out. Again very nice work!

I was a little leery when I had to tap the dowels out from the bottom of the fixture with a punch, anticipating that the parts would crack apart once the dowels came free. Then there was the sawing and sanding. I don't know much about the Loctite I used other than it was the suggested type, but it worked very well. The brass nuts have a flange on each end so they can't shift longitudinally, and they only get used in a clamped position. According to the PDF it's resistant to common chemicals and has to be heated to 482 degrees to soften. I'll update the thread if an issue ever arises, but I doubt it will happen.

 

[Karl_T]

Good job, Holes.

I'll keep that adhesive trick in mind. If this does fail (unlikely) they make a low temp silver solder paste. Slather it on, assemble parts, heat to 600 F. Great for large areas like this. i've used it on weapons builds.

 

[Holescreek]

I have plenty of silver solder paste I could have used, I wanted to see how the Loctite 620 works out. The Loctite was about $21 shipped (tiny bottle in a basket ball sized box) from Zorro online. The $21 is my total expense in the repair.

I watched a video earlier in the week in the "how it's made" series on adhesives. They have come a long way during my lifetime.