Making a couple of threaded spindle backing plates for my lathe

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When I sold my Atlas 10F24, I held on to my Bison 5” 3J, as it was fairly new. Naturally I needed to make a new backing plate to mount it onto the new lathe. The old plate was 1 1/2” X 8 while the new spindle is 2 1/4” X 8, so I needed new metal to do this. I recently purchased 2 backing plates from Busy Bee Tools. They are 8” semi raw castings. They are turned to clean off the rough cast surface and the center hole is 1” ID unthreaded.

First I started by reading up on what could be the rights and wrongs in making these backing plates. Recent posting here by various people, too many to list, have been of excellent help.

I make a copy of my spindle, so that I had a plug to try the threads as they neared finish size. I used the 3 wire method to make the plug match the spindle. Once the readings were the same between the two, I was finished. I did stamp the diameter and tpi for future reference.

I am making two backing plates. One for the above Bison chuck and the second for a 5C collet chuck. It was a local purchased, used but still excellent condition. CDCO supplied said the seller.

The first back plate, I chucked in my 4J, centered it and bored the spindle hole, threaded with the tool cutting on the back instead of the front side. This is so I could see what was going on. With a 2” hole this is easy to do. Faced the registration surface where the plate butts against the spindle shoulder, cut the plate back face and the shoulder OD to remove any run out. Mounted the new plate onto the spindle, cut the shoulder that fits into the back of the Bison chuck.

At this point I still had a 8” diameter plate. I needed to turn it down to slightly under 5”. I used a tool from work that is used to cut face o-ring grooves and can be used to part off the un-needed disk of material from a disk, as my plate had. Once this was done, I finished the OD and now needed to drill the mounting holes.

I did some measuring to find the center of the holes in the original plate from the Atlas and made a life size printout of the 6 holes arrayed around the shoulder that the chuck will mount to. With some careful cutting I fitted the paper to the new plate and transferred the hole positions and drill 3 holes for 1/4” X 20 thread and the alternating holes for unthreaded holes for the mounting bolts.

On the second plate I changed my method a bit. I mounted the plate first to my 4J chuck using the shoulder area and removed the outer disk first. Then I mounted it onto my 6” 3J and then cut the threads faced the registration surface and cleaned up the shoulder and back face. Next I have to check the collet chuck to see if it has any run out and then proceed with making the shoulder and mounting holes to match it. I think I will use the method mentioned on the write up at the cnccookbook.com website. I will let you know how that goes in the future.

100 spindle-plug.jpg 101 cutting-the-extra-off.jpg 102 disk of-iron-off.jpg 103 disk-of-iron-removed.jpg 104 setup-for-thread-cutting.jpg 105 finished-threads-and-recess.jpg 106 finished-plate-ready-to-drill-holes.jpg 107 mounting-face-and-shoulder.jpg 108 old-plate.jpg 109 need-to-make-holes.jpg 110 paper-pattern-for-holes.jpg 111 mounting-plate-with-holes-mounted-to-chuck.jpg 112 mounted.jpg
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Strange that I should stumble across this post at this time, I am making two backing plates for two new chucks. I took advantage of Enco's 20% and ordered a 4J independent Bison 6" and I also picked up a new Gibraltar 3J 6" for a good price. I bought the semi finished Bison back plate and am just about through machining it. For the Gibraltar I bought a piece of hot rolled 1018. Any thought on why lathe back plates are usually made from cast iron? I thought I would try making one from steel, cast iron is such a mess to machine.

Steel tends to ring more than iron. Iron has a better dampening effect on vibrations. When i was parting off the unneeded disks, the first one did not sing but the second one did. You bet that iron is dirty! I will be spending some time today with the vacuum cleaner!
Well I cleaned up the lathe and area and installed the chuck on the spindle to check the runout. I have a 3/4" ground shaft that is straight and the runout is less than .001" on my DI. So I am happy!

The best reason to use cast iron for lathe chuck back plates is that it does not tend to gall like steel; make one of steel, and sometime likely the threads will gall, and mess up the spindle. I have made many of these, mostly for larger lathes, and I always make a plug gage to try the threads, but I do not make it like a copy of the spindle with steps and recesses, I have made them all just like commercial thread plug gages, with the threaded portion longer than what I will be gaging, and with a knurled handle smaller than the threaded portion so that it may be easily threaded to full depth. For finding the centers of the mounting (tapped) holes, I use transfer screws, that when the plate is struck with a hammer in assembly with the chuck, the hole centers are marked on the backplate; these can be homemade, using an alloy steel heat treated bolt, cutting it into short sections and turning a sharp teat on one end; to install and remove them a saw cut slit can be made with a hacksaw just adjacent to the teat.
Do you remember the part number for the backing plates you got from Busy Bee?

The plates are B056. They were on clearance when I bought them a couple months ago. Likely have to call the main office as they were not listed even at the time I got them. Good luck.
You could use a steel plate, as long a you take precautions against it galling, A light smear of a good anti seize compound should ensure that it never galls
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