Steel selection for lathe backplate

[Craig_J]

I have a new Grizzly G0602 (10x22) lathe and bought an ER-40 collet precision machined onto a 5.2" flange, all of which needs a 1 3/4 -8UN threaded backplate with a turned boss for registration. Long story short, I bought a cast iron backplate, pre-threaded, then basically screwed it up in the first machining operation, boring and facing the surfaces that join the spindle. I didn't figure this out until I had put in 3 days of work facing the other side to joint the colllet chuck. When threaded onto the spindle, there is a gap in the facing surfaces (ranging from 0 to .010), and the chuck side has a runout of .011. Intolerable! (LOL)

I want to try again and am thinking about making one out of a steel rod (rough dia. 3") turned down into a press fit into a 5.2" x 1" disk made from plate, with tack welds to guarantee it stays together. I know I can fixture the rod accurately, and final facing after screwing it onto the spindle should also be very accurate. Turning the cast iron was slow and ugly! I can buy another cast iron plate, but I really didn't like the quality of its threads or finish--I think it was a very porous casting. BTW, I am using indexable tools.

Question is: what material to use? I know why cast iron is used for backplates, but this should not be a shock-loaded piece. Is 1018 good enough? Should I use a hardenable steel? 1045 is also available to me; 4xxx steels would be much more expensive. Any advice appreciated!

Craig

 

[swatson144]

My 2¢ Is to buy the semi machined plate and try again, or see if grizzly sells machined fitted back plates for the machine. I too hate cutting cast iron and the mess.

If you do decide to cut your own from scratch you will want to take the time to make a duplicate of your spindle nose ~-.0000 +.0005. It's a lot of work but sure worth it in the long run. Then you have something to test the fit of the internal threads vice taking them out and hope it fits, chase threads and repeat.

You can make the nose protector as practice then you'll have it when you use a collet/drawbar. Tolerance isn't as tight but if you get it perfect on the 1st shot you'd be well on the way with the project you mentioned.

Steve