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3 Jaw Chuck Backing Plate for Rotary Table

Janger

(John)
Administrator
Premium Member
#1
I wanted to mount a chuck to my rotary table so I could use it more easily and do some dividing activities too. What I have on hand though won't mount directly. It is a 6" back mount chuck. The RT has a Morse taper 3. So I decided to make a backing plate with a Jacobs 3 taper hole and holes to bolt to the table and the chuck. The idea was to have a self centering plate using a MT3 to Jacobs 3 adapter. This was a tricky project for me but it turned out better than I hoped.

The 8" RT and morse taper to Jacobs 3 adapter.
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First I cut down a slice of 4140 from the big log purchase and turned it round and flat.

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Roughed out clearance and a 1/2" starting hole.

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Setting the compound angle using the crazy BXA dial indicator holder as discussed elsewhere. I tried using the lathe taper attachment and it was far too fussy to set the necessary exact angle (1.5251degrees). This method worked fine. Keep turning the compound handle and adjusting angle until the needle does not move. Then used a boring bar from china with carbide inserts. Shallow cuts and lots of measuring. A very small cut makes a BIG change in the depth of the adapter.

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Testing - it fits and does not wiggle! wow. Lots of careful measuring to ensure the depth matches the plate to RT distance. I managed to get it close enough that a few mallet blows makes it fit right and surface to surface plate to RT mate flat.

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Then I had to flip the plate over and remount in the 4 jaw. To get it centered I used a dial indicator in the jacobs taper hole. I also ensured it was mounted flat with the dial indicator too. With some back and forth that worked with patience.

To center the chuck on the plate and hence the RT I made this boss on the plate. This chuck came with a R8 backing plate so I just reused their concept.

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I bought this at KMS


Here I reproduced the boss on my plate. The boss is 3.940 +0.005". I was able to turn it to 3.955 and it slips on ok.
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Chuck fits and stays on with friction! runout measured 0.001"+. I was very happy with that.

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Seems to work! Yes I know the stickout is ridiculous. Now I need a tailstock for this.

In retorspect using the MT3/J3 taper adapter was convenient and self centering but there is no through hole of course. That is a draw back. The RT only has about 1/2" diameter through though... any better ideas to have it self centering but keep the through hole?

Fun project.
 
#2
I just learned on saturday that when you're supporting your "stickout" from a dividing head it is called a footstock. (Not tailstock)

I know thats more annoying then helpful but figured I would share anyhow.
 

John Conroy

Well-Known Member
Premium Member
#4
John, just a thought. Could you machine a backing plate large enough to register on the OD of the rotary table on its bottom side and register on the ID of the back of the chuck on its top side? It could still be bolted down using the T nuts. That would leave the center hole open.
 

Janger

(John)
Administrator
Premium Member
#6
John, just a thought. Could you machine a backing plate large enough to register on the OD of the rotary table on its bottom side and register on the ID of the back of the chuck on its top side? It could still be bolted down using the T nuts. That would leave the center hole open.
Brilliant idea John! I just need some 8.5" round now... :)

And I remember your post on the tail stock you made. It was great. foot stock? I think somebody is pulling my leg.
 
Last edited:

PeterT

Ultra Member
Premium Member
#7
In retorspect using the MT3/J3 taper adapter was convenient and self centering but there is no through hole of course. That is a draw back. The RT only has about 1/2" diameter through though... any better ideas to have it self centering but keep the through hole?
I made a similar self centering plate + chuck on my old RT. Even though the components were pretty good, the jaws were rarely within a couple thou TIR and yes, the hole was plugged which could be a nuisance on longer parts. So I took a different approach. I made the center hole in the plate oversize clearance. I found an MT3 / straight shank test bar 6" shorty. (Geez I should be a saleman for these guys, just finished talking about that on the lathe alignment post HaHa).
Anyhoo:
- loosely position the plate + chuck assembly on RT. Chuck is bolted firm to plate
- drop the MT arbor in the RT hole
- lightly clamp the chuck jaws to the extended cylindrical portion of the test bar. Chuck is now coaxial to RT and with a bit of pre-load
- now tighten the plate to the T-slots
- open the jaws & withdraw the test arbor
- Presto done

A 4J chuck paired to a RT allows you to get it bang on & less critical of how the plate is mounted. But it takes longer to set up, isnt as repeatable with a series of parts & you need some kind of reference on the part to indicate. But you can also hold irregular pieces in a 4J for RT work which a 3J cannot.
 

Janger

(John)
Administrator
Premium Member
#8
I made a similar self centering plate + chuck on my old RT. Even though the components were pretty good, the jaws were rarely within a couple thou TIR and yes, the hole was plugged which could be a nuisance on longer parts. So I took a different approach. I made the center hole in the plate oversize clearance. I found an MT3 / straight shank test bar 6" shorty. .
Love it Peter. I just finished this project now I have to do it again. Twice. I think your idea and John's post both merit making a plate.
 
#9
It was great. foot stock? I think somebody is pulling my leg.
Dictionary.com
footstock

noun
  1. another name for tailstock
So at min they are interchangeable words.

I do like pulling on peoples legs though. Makes me feel like a wrestler....
But in this case I am just repeating what I heard from someone else. So I may be wrong, I maybe right.
 

Alexander

Super User
Administrator
#10
Yes on a mill it is called a foot stock. John you need to avoid stick out and your maching will improve. Even the quill should be up if possible. Same stuff on the lathe 3x diameter is max and shorter is better :)