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Lathe Chuck Backplate

My lathe, as I may well have mentioned elsewhere, is a somewhat elderly and obscure Faircut Senior, made in Sheffield by the same family that went on to make Henderson's Relish. It is rather favourably described in great detail by Tony on, and since being made a present of it by a chap whose barn I helped clear out, I have grown rather partial to the old hunk of iron.

The only downside to running an obscure lathe is that, unlike users of the more common lathes like Myford and Colchester, I cannot simply go to a website and order a new part or attachment. Generally I can get by with what I have, or modify something else to suit, but one item I have a pressing need for is a four -jaw in dependant chuck. As luck would have it, I do in fact own such a thing, but in order to mount it onto my lathe I need what is known as a Backplate. This is effectively a disc that bolts concentrically to the back of the chuck and then screws onto the spindle thread of the lathe. Easy, you say? Well, yes, it is, if you can identify the thread on the spindle and cut a matching one accurately. It took me what seemed like an age to identify it. BUT, identify it I did, and here are the steps I went through to make my Backplate...

Step 1

Take your chunk of cast iron, and make sure it is thick enough to take the whole of your spindle thread and a bit more on top, and ensure its diameter is about equal to that of the chuck body. Then, mark the centre as accurately as you can, mount it onto the face plate of your lathe, and bore a hole as large as you can in the middle. You can also do this with a pillar drill, but in either case it is wise to start with a spot drill, then run a small drill bit all the way through, then work up a couple of drill sizes at a time until you have a hole large enough to get a boring bar into on the lathe.:

Step 2

With the disc mounted firmly on the face plate still, use a boring bar to enlarge the hole to the inside diameter of the spindle thread you are aiming for. This is the diameter at the base of the threads, and if you have identified your thread you can usually find the recommended drill size listed for tapping it...this is the figure you need. In my case, my Faircut Senior has a spindle thread of 7/8 BSF, and the drill size for that  25/32". So I bored my hole out to exactly that diameter:


Step 3

This bit is fairly simple, but contains one important point in it. Here is where we thread the hole. If you are happy doing it on the lathe with change wheels and a thread cutting tool, then by all means go for it, but I wanted to be safe with this one and I had bought for a small amount off eBay the correct tap for the spindle thread. So, using this I tapped the hole in the usual manner, but here is the important bit I mentioned: Start tapping while the disc is still mounted on the lathe. That way, you can use the tail stock centre as a guide for keeping the tap perfectly true going into the hole. After a good few turns, when the tap is solidly cutting its way in, you can remove the whole affair and put it in a vice where you can get better grip, but start in the lathe. You really, really want this to be a straight and true hole, believe me.

Step 4

Do a test fit! At this point, the thread is the most important thing, so check it on the lathe spindle itself. It should fit perfectly and screw all the way down to the unthreaded part of the spindle nose, which in most cases is known as the spindle register:

Assuming that is all hunky dory, you then need to turn the entry to your threaded hole down to sit over the spindle register. From now on though, you won't need to mount the disc on the face plate to work on can simply screw it onto the spindle directly, and all your work will be truly concentric! Useful, eh? So, measure the diameter of your spindle register, and how far back it goes before the next flat surface that the Backplate will rest against. Screw your iron disc onto the spindle with whichever side is going to face the tailstock in normal use facing the head stock instead (so, backwards). Use your boring bar to turn out the threaded hole to that diameter, as far in as you measured. Your disc should now, when turned around again, sit nicely onto the spindle register. The hole in mine looks a bit like this at this stage:

...And for reference, this is the spindle with its register that it fits onto:

Step 5

Now for a bit of truing up. Screw your iron disc onto the spindle the right way round, so that the register is engaged nicely. Turn on your lathe at a medium speed, and observe how the disc wobbles slightly. This effect will vary in degree, depending on how accurate you were at finding the centre during drilling and boring of the hole, but it will certainly be present! An eccentric Backplate means an eccentric chuck, and we do not want that. So, turn the outside diameter of the disc down in small stages until the edge of the disc is perfectly straight and true, and the disc does not appear to oscillate from side to side as it spins. Make sure your gib strips are nice and snug, this is another stage where accuracy is a Very Good Thing:

When you've got the outside edge running concentrically, with a nice smooth finish, you will then need to tidy up and true the front and back faces as well. Face them both flat, removing only as much material as is absolutely necessary to get the surface consistent:

Step 6

Here is yet another bit where you have to be spot on accurate. If you look in the back of your chuck body, you will see a depressed circle concentric with the body of the chuck. This is like a big, shallow socket and it is called the chuck register. The front of your backplate needs to have a corresponding register like a big, shallow plug that fits into that socket perfectly. It is this more than anything else that ensures your chuck body is running concentrically with the backplate, which is itself running concentrically with the lathe spindle because you turned it while it was mounted on it. I hope you're following this...

So, measure that register on the chuck as accurately as you possibly can, and turn down the corresponding section to a tight fit on the backplate. Test the chuck body on it repeatedly as you get closer to the final dimension, you really do want this as accurate as possible! When you are happy, remove the backplate from the spindle and place it on a flat surface, face up. Place your chuck onto its register, and use a transfer punch through the bolt holes on your chuck to mark where the tapped holes in the backplate need to be:

It is a good idea, in case you ever wish to separate backplate and chuck in the future, to mark which hole corresponds to which numbered jaw of the chuck on the backplate, to help you align it when it comes to reuniting them. I've used a set of number punches, as you can just about see in the picture above.

Step 7

Decide what fastenings you are going to use for attaching your chuck to its new backplate. You want a bolt whose head fits snugly into its hole, without standing proud of the face. This rules out hex bolts, as there will be no way of gripping it with a spanner to tighten. I would recommend cap screws, or countersunk machine screws, but whatever fits your particular chuck will do fine. One word of caution though: make them as large as will fit in the holes available. These bolts are what will be resisting the twisting forces of your lathe tooling, so you want them to be man enough for the task. Then, spot drill the punch marks you made on the back plates face:

Then follow that up with the correct drill for the size of the tapped hole you want...I went with some countersunk M6 bolts I had to hand, so I drilled a set of 5mm holes:

Finally, grab an appropriate tap (as I say, in my case that is an M6 tap) and thread the holes. If your chosen bolts are only short, you may wish to only drill part way through the backplate and in this case you will need a second and a bottoming tap as well, so the threads can be cut right to the bottom of the hole. In my case, my bolts were exactly the right length for the ends to be flush with the back face of the backplate, so I just tapped the holes all the way through.

Step 8

Assembly time! Fit your chuck onto the backplates register, line up the jaws with the numbered marks you hopefully made earlier, and bolt the little tinker down securely. Mount the whole assembly on your lathe spindle, and voila! One nice new chuck, all ready to roll! To get the full appreciation of how accurate your work was, stick a Dial Test Indicator on the rim of the backplate, and then on the body of the chuck itself. Both should read negligable runout, and you can allow yourself a very smug grin!