Boring and threading a back plate - confusion about matching the spindle threads

[WobblyHand]

Making a back plate out of a chunk of cast iron. I have turned it to a cylinder and faced the ends. Drilled to 1" and have started boring. I have also made a copy of the spindle to make it easier to do the final fit. When I measured the pitch diameter, it was near the minimum pitch diameter for its size.

So need to bore the hole to the right size, so I can single point the internal threads. Should I be boring to the Maximum Minor diameter of the thread 1-3/4 x 8? (1.6400") or the Minimum Minor of the thread (1.6150"). Everytime I think about it, I go around in circles. Can someone point me in the right direction? Thanks.

Obviously, Min Minor is safest. Just want to be in the ballpark, so I don't over or undershoot the threads.

 

[xr650rRider]

I'd shoot for 1.6275.

 

[pdentrem]

I also made a copy of the spindle and I did measure the thread minor diameter on both chucks. Neither were the same! So I just kept threading one pass at a time until the spindle copy fit as it did fit both chucks.
Pierre

 

[mmcmdl]

I'd shoot for 1.6275.

Yes sir , same here . Always shoot for nominal dimensions .

 

[benmychree]

Keeping the ID of the backplate as small as possible provides little strength to the finished thread, and invites jamming up from tiny chips and dirt, keep the ID consistent with the other backplates.

 

[WobblyHand]

Thanks everyone. Have a good idea what I need to do now.

Here's another question. Sorry about these simple questions, never machined a back plate before. Should I bore the register before or after threading?

 

[T Bredehoft]

If there's thought that the back-plate moved while threading, then I'd bore it last, or at least leave a cleanup for last op.

 

[benmychree]

I agree with Tom.

 

[SLK001]

Thanks everyone. Have a good idea what I need to do now.

Here's another question. Sorry about these simple questions, never machined a back plate before. Should I bore the register before or after threading?

The register is the vertical section that butts up again the spindle shoulder. What you are calling the "register" is just a clearance counterbore. It is usually bored to 0.010 to 0,020" over the size of the spindle clear area.

 

[WobblyHand]

The register is the vertical section that butts up again the spindle shoulder. What you are calling the "register" is just a clearance counterbore. It is usually bored to 0.010 to 0,020" over the size of the spindle clear area.

Ok, wasn't sure what that feature was called. What's your recommendation for boring the clearance counterbore. Before or after the single point threading?