Modifying chuck for front mount.

A year or two ago, I bought a 4" Pratt Burnerd three jaw scroll chuck on ebay. It was not a size that particularly interested me, but the price was remarkably low, especially as it was NOS sold as used. It has both sets of jaws, and during lockdown, I chanced upon a 6" backplate with the Smart & Brown 1 3/4" X 8 W thread. Having a small chuck has its advantages at times, so I have married up the pair. I will not be reducing the backplate diameter in case a four jaw independent happens to catch my eye, there are holes to mount three different chucks already in the backplate. I wanted to fit the spindle thread lock to the backplate, and as the chuck is small, the mounting of both became a problem. The chuck had the common rear mount M6 threads, but rear mounting would interfere with the lock, so a modification to front mounting was the answer. I like to have a loose fitting register to enable fine adjustment of three jaw scroll chucks, but I also like to play safe with six bolts rather than three. There is plenty of room in a chuck to have six mounts, I already have Pratt chucks with both front and rear mounts from new. The procedure starts by mounting the chuck on a rotary table and lining up the existing holes and drilling them right through the body. Care has to be taken to set up the chuck so as not to risk drilling the jaws of the chuck holding the one being drilled. It pays to check carefully where the holes will go to miss any mechanism. I had a scrap 4" Pratt with front and rear mounts in it to compare first, and the hole spacing is 60 degrees. After drilling the holes 6.6mm, the chuck was turned round and realigned to Counterbore for the SHCS, which will be cut down 60mm length. The chuck had to be dismantled and washed after modifying

Edited By old mart on 01/08/2020 20:02:22

What I like about front mounted chucks is that they can be easily adjusted for that particular sized part being held, apart from the ease of taking it from one platform to another. Years ago , I made a square adaptor plate for a 3 jaw chuck that located onto a modified face plate. It allowed a turned part to be transferred to the mill for its other operations when only making a few parts. They are still using it 10 years later.

An adaptor plate to fit this chuck to the mill bed or rotary table is in my list of things to do. The lightweight 6" four jaw independent Toolmex which was also NOS had the Myford thread originally, but soon was modified to front mount to fit directly in place of the very poor worn out Pratt on its backplate. As the 6" rotary table has four tee slots, that chuck can have a dual purpose.