making a chuck backplate out of CRS

[awander]

I recently couldn't resist the sale price on an import 5" 5C collet chuck, and so I need to make an adapter/backplate to mount it to my lathe spindle.

The spindle itself has a 5" diameter flang, with a raised register in the center. Chucks are held to the spindle by 3 M8 bolts which pass through the spindle flange, and thread into the chuck(or it's backing plate).

I had a length of 5" diameter CRS, so I figured I would use that. I cut off a 1" thick slice, mounted it in my 4-jaw, and faced it off-then I bored a recess that fit nicely over the spindle flange register, marked, drilled and tapped the M8 holes, and mounted the backplate to the spindle. I marked the orientation so I could get it back on the same way, in case that mattered.

Then I faced off the side that was newly facing out, and turned it to have a register that fit snugly inside the back of the 5C chuck.

The 5C chuck mounts using 3 M8 bolts that pass through it and thread into the backplate. I marked, drilled, and tapped the corresponding holes in the backplate, centering the pattern between the 3 holes that are used to mount the backplate to the spindle.

Deburred, and bolted it all up. Tested for runout on the 5C chuck and got .005 TIR on the inside nose taper, or on a piece of 1/2" stock held in a 1/2" 5C collet!

Initially, I figured the chuck must be bad, since the backing plate was turned in place on the spindle so it had to be accurate, right? I took the chuck and backplate off the spindle, and measuring the runout of the spindle itself, I get less than .001" TIR.(closer to .0004)

I remounted just the backplate to the spindle, and my nice new register was giving me .005" TIR.

SO-I am thinking maybe CRS isn't a good choice for this backplate. Or maybe I need to start with a larger diameter piece? Since the diameter of the steel I had is 5", and the lathe spindle and chuck are also 5", diameter I couldn't turn the outside diameter of the plate-the holes are pretty close to the edge. Is the CRS moving around on me?

I have an old barbell weight that I may be able to use, though I have heard less than good things about the quality of the cast iron used in them.

 

[Ray C]

I'm not sure if I have the right mental vision of what your setup looks like but, I would be inclined to just mount the plate to the spindle, face it then, flip it over, face the other side and if you prefer to have a shoulder/register, cut-in a little shoulder. I personally don't care for shoulders as often times, the corresponding area of the chuck is not true. In that case, I lightly bolt the chuck to the plate, insert a collet and piece of known true stock and put a DI on it; then, rotate the chuck and tap it lightly until the DI zeroes. Finally, tighten the chuck bolts.

If you could post a picture, that would be helpful as I might have the wrong image of your setup.

In any event, I doubt highly the piece of mild steel is not conforming/cutting properly. A piece that thick is not going to flex any measurable amount under normal cutting pressure.


Ray