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L0 Integrated backplate for a 9_1/2” Pratt Power chuck - Homemade tools

Howie Dines

Sep 17, 2020
Germany (Formally UK)
Rather than buying 10” diameter semi-finished back plate, which are quite expensive to buy in Europe. I though It would be nice to make one myself. This way I would not have to compromise with the amount of overhang from the headstock.

The rear of the chuck showing the 6 outer and 3 Inner allen screws. Due to the threaded rear, the inner 3x screws would need to be fitted from the front. I had a spare piece of steel (Not cast iron) that I planned to use. I wasn’t sure what grade it was, but It did come with a lot of tool steel, and I’m assuming it was actually something like O2 / 1.2842 as it turned out being tough to work with.

The blank was 230mm Diameter x 110mm weighing in at about 32Kg / 16lb

The fun starts on the Colchester master lathe, firstly facing the chuck side and the outer diameter. Flipping in the chuck and then forming the thread blank and internal taper. For this indicating the top slide to the taper on the lathe spindle.
The thread was cut with a HSS hand ground tool. I don’t have a thread pin set so used 3x drill blanks. Machining to match the thread size of the dog plate seen on an earlier picture.


The finished and the blued up taper. Not the best of fits, so I’ll come back to this later after the keyway has been cut.

To be continued ......

The keyway is to be a blind one as per a standard 8“ chuck. This required an undercut using a 1_1/4” x 5x16” woodruff cutter on the Holke F-10-V Milling machine.

Then onto the fly-press to stamp my mark 😊 (F.J.Edwards 6DB press)


Back to the mill, with the Bridgeport Type-E slotting attachment. (Video file link to follow)


One of the planetary gears was a big problem to remove. A quick light grind soon took that problem away.


As mentioned earlier, I wasn’t happy with the fit of turned taper. I re-clocked in the compound slide and using a Duplex tool post grinder, re-ground the taper. This time it now “grips” on the spindle when you try to remove it.

Next is what will be the inner machining of the backplate.


This is where I had a glitch. The 6x outer fixing holes I machined using a rotary table, but in hindsight it would have been easier to just use the PCD hole patten function on the DRO. Oh well too late now. But I also had a problem with the holes in the chuck. It appears that two were not equally spaced on the PCD. I therefore re-drilled and tapped another set of holes in the chuck body rotating the backplate 5-10 degrees.

To be continued ......

An arbour was used to hold the backplate in the dividing head. Despite clamping it also to an angle plate to prevent rotation, It didn’t ! It only happened once and I was more careful after. The second operation was the undercut with a T-Slot cutter.


Each of the three planetary gear holes were clocked, bored (Criterion DBL-76MD boring head) , drilled and reamed for the gear nose, then spot faced to depth.

Video of boring operation to follow.

To be continued .....

The final four pictures show the inside of the original backplate with my new one. Also this picture shows the poor condition of the L0 spindle on my lathe. Perhaps another project for grinding later.

Then we have the final chuck in place and the final shot showing the inside of the taper, the blind keyway slot.

I hope someone finds this useful. Ask any questions as needed.

Regards Howie