Threaded backing plate for dividing head - how to machine shoulder without threaded lathe spindle?

[kevin.decelles]

I finally got around to putting together the chuck/backing plate for my dividing head. I had ordered the chuck/backing-plate from Shars last year, and figured it would be pretty straight forward as the backing plate only required the shoulder to be machined. That is....... until I though through the process and read the literature online. The issue is that I don't have a lathe with the same thread as the backing plate -- meaning that I can't simply thread this onto the lathe, indicate and machine it. My lathe is d1-5. My other two lathes are threaded, but different threads.

All the material I've read insists that you need to do it this way to ensure concentricity with the spindle -- which I understand. I bought a couple of the backing plates and I spent the day testing different approaches, non of which yielded a decent run-out. i've tried to center it in the 4-jaw, and indicate from the various surfaces. I can get close, but not close enough (would like < 0.003)

My next approach is to chuck a piece of free machining steel (say 12l14), indicate it true, then cut threads on it. Then, without removing it from the 4-jaw, thread on the plate, and machine the shoulder.

Looking for any ideas/suggestions.

 

[PeterT]

Can you show a pic, or it it like this? What size are the BP threads?

I'm not 100% but I think some of those threaded systems depend on both the machined OD recess surface and the machined square back surface to accomplishes the concentric fit. The threads are obviously just to retain it in place. So ya, if you don't have that thread on your lathe spindle nose then machining an arbor in-situ with the correct dimensions would probably be the way to go. A bit of work but it would be guaranteed to be true as long as you mount the BP to the machined fixture without removing it from the lathe. Screw it in place & then turn the face/OD/lip/whatever. And don't run in reverse for any reason LOL

I cant think of a male threaded arbor stub that that one could buy to chuck & indicate true with 4J chuck.

How does the chuck mount to the BP? Front mount bolts into threaded holes? Reason I ask is you may find that you machine a precision lip with 0.0005" clearance only to find that your 3J chuck jaws are out 0.003" & then you have no remedy. If so, it might be better to allow a bit of gap between the lip & chuck recess. That way you can dial it in perfectly with the hole clearance & bolt in that position. If its a 4J you are mounting, never mind, you have full control.

 

[trlvn]

My next approach is to chuck a piece of free machining steel (say 12l14), indicate it true, then cut threads on it. Then, without removing it from the 4-jaw, thread on the plate, and machine the shoulder.

I think the above is your best bet. Basically you have to copy the mounting system on the dividing head. You don't even have to worry about threaded arbor being indicated true before machining. By definition it will be concentric with the axis of rotation of the lathe after you turn the OD.

Craig

 

[kevin.decelles]

Some pics

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[PeterT]

That looks more finished than I was expecting. What exactly has to be machined? Just the rear & front face parallel to each other, or the collar area right adjacent to the threads?

Could you no mount the blank to your faceplate (your middle pic). Machine the small face (and really could turn the collar area at this point do if the thread axis was DTI concentric).
Then flip the part, grip the boss in a chuck with parallel spacer between chuck body & small face to machine the large front face parallel.

 

[kevin.decelles]

The part you ballooned in the picture is not quite wide enough. I need to widen it

The issue is that I have now touched all external facing surfaces and have no reference that I trust..... part of the learning curve and why I bought two backing plates

Even if I start with the new plate, how do I guarantee that the outside surfaces are in alignment with the threads?

This might me one of those things I need to see demonstrated! Too easy to get confused on the terminology




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[PeterT]

See sketch
- turn a short bar segment that is a snug slip fit in the thread ID to provide a temporary surface to reference the thread axis with DTI
- mount casting to lathe face plate, slightly loose so you can tap it around. Use DTI on bar OD to get assembly centered, secure the bolts, remove the bar
- now you can machine the ID collar surface, it will be concentric to the threads
- and machine the back face same setup
- flip the part & use a chuck to hold the boss
- have it back stop on an accurate spacer block referencing from the machined surface
- machine the large face (and OD if you like)

I'm assuming the large front face is reasonably flat & the threading bore is perpendicular.

 

[PeterT]

If your 4J chuck can hold the casting like in the first setup sketch (maybe jaws reversed), you would not need faceplate to mount as long as it was indicating planar.

I've never really tried but maybe a DTI can indicate across the ID thread flats directly without skipping in the thread troughs too bad & give an accurate reading of concentrically & that would mitigate the need for a short bar segment.

Now I'm curious, how did you set your up this far?

 

[kevin.decelles]

I tried a couple of variations with the 4-jaw, grabbing it either by the outer diameter or the inner boss. I indicated it level on the face and top, assuming the thread was perpendicular....... I faced the boss (need to remove some), then cut the shoulder (really happy with the fit btw) but the runout is too much. Thx for the updated pdf, that was very helpful and I'm going to go with that approach.

thx for the tips!