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Index backplate on which chuck?

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Chris V27/06/2020 10:22:55
262 forum posts
41 photos

I have an ER32, 4 jaw independent & 3 jaw s/c chucks all needing 100mm backplates.

(I also have Square & Hex ER32 collet blocks)

I have three ready threaded backplates one of which has index holes around the rim.

Question is which chuck would be most useful going forward to have the index backplate please?



Nigel Graham 227/06/2020 10:31:40
792 forum posts
16 photos

I think only you can decide because you need establish the nature of the majority of the parts you are going to need to divide, therefore the chuck best suited to the largest range of them.

However, you could add index holes to the other two back-plates to allow dividing with any of the chucks .

Nick Clarke 327/06/2020 10:36:12
935 forum posts
33 photos

I would say it could be useful to have the facility on all of your chucks, but if restricted to one it has to be the 4 jaw as that can hold both circular and irregular material and can be set to run truly, if nor more truly than the collet. It can also hold larger material than the collet.

Howard Lewis27/06/2020 10:55:27
3757 forum posts
3 photos

+1 for Nick Clarke 3.

Might it be possible, having fitted it to the 4 jaw, put another backplate into the 4 jaw, centre it and index that as well, for fitting to the 3 jaw? the you can have the best of both worlds, for square, irregular and hexagonal or round material.


ega27/06/2020 11:12:34
1850 forum posts
157 photos

Chris V:

As a matter of interest, how many divisions does your indexed backplate give you and what are you using to pick up the indexing?

I find twelve divisions is the most convenient number.

Edited By ega on 27/06/2020 11:12:49

Chris V27/06/2020 13:30:38
262 forum posts
41 photos

Thank you all, I was leaning towards the 4" 4-jaw but have trouble making decisions especially as I'm inexperienced.

Typically the 4 jaw chuck fixing screw centres will mean the screw holes in the back plate will come through the back plate half in the main large flat area and half in the outer portion of the threaded section, which would look really messy. The 4" backplate is new and thick, is it acceptable to have the bolts come through the chuck and thread 1/2" deep into the backplate, ie in stopped holes?

ega, There are 24 holes in the backplate, no idea as yet what to pick up with, I figured I'd cross that bridge when I come to it. I was just pleasantly surprised to find a ready made index backplate to fit the Myford M type thread, so went ahead and bought it.


old mart27/06/2020 16:24:47
2193 forum posts
164 photos

The four jaw is the most versatile, so I would vote for it to have the indexed backplate.

Nigel Graham 227/06/2020 23:21:05
792 forum posts
16 photos

I take Ega's point - 12 holes will cover most indexing, but 24 has that useful extra integer of 8 - octagons and 8-hole pitch-circles are not very common but thinking a little laterally, this also allows setting-off at 45º. It's one of those rare things, an "extra" you might need only very rarely but are glad to have when you do, yet takes up no extra space!

I'd agree too with Old Mart if you elect to use only one indexed back-plate, use it for the 4-jaw.

No reason you can't have stopped holes for screws provided they engage fully. Holes that break out in a corner as you describe do look messy but worse, the breaking-through risks broken drills or taps. I take it the chuck is the pattern with long cap-screws running right though the chuck from its outer face.


A tip when indexing: clean the rim with a cloth moistened with meths, then mark the needed holes with a fibre-tipped pen or marking-out fluid. These marks can be cleaned off afterwards in the same way.

{I used similar for circumferentially and axially dividing the 120-off 3mm dia. holes in a strainer made from a bit of standard PVC sink waste-pipe. Having no formal method at the time, the dividing-engine was my EW 2.5" lathe simply standing on a table. The 60-tooth change-wheel on the spindle, with the relevant teeth marked by two differently-coloured felt-tips and aligned by eye with a convenient edge, gave the staggered bands of holes. A second, inked change-wheel acted as lead-screw "dial" for spacing the bands. I used a hand-held battery-drill, with a brass guide-block gripped by the tool-clamp. }

ega28/06/2020 00:07:57
1850 forum posts
157 photos


I would have opted for more than twelve divisions but for the problem of dodging the existing bolt holes (my divisions were put in with a centre drill).

A spring-loaded plunger with hold-out is a straightforward job as long as there is somewhere to mount it. Top tip thereafter; disconnect the power when using!

ega28/06/2020 08:34:18
1850 forum posts
157 photos


Overnight, I remembered the idea of using a removable "dividing ring" temporarily fastened to the chuck/backplate via grub screws. One such with 48 divisions was available from Hemingway for use with the Quick Step Mill but could readily be shop-made.

The answer to the original question could then be that dividing could be available on all chucks with the same size backplate and, in principle, with a range of divisions.

There was also a design of spring-loaded detent for use with the division ring.

Chris V28/06/2020 09:16:36
262 forum posts
41 photos

Ok this is all great, thank you all.

24 holes was the only option available ready made & I'm glad to hear its a generally approved of number if divisions.

Yes Nigel, long cap screws through the front and yes I had also thought of the taps & drill breaking. In due course I will fit it to the 4 jaw and as has been suggested might use it to modify other backplates if necessary.

I'm also glad to learn I'm not the only one who uses marker pens to help me, that and masking tape!

One downside with this backplate is it has a register already turned, and guess what its too small a diameter to suit my chucks. But when I bought it i did not know it was there, so my intention is to turn the existing away, turn a larger diameter recess & bolt a new register in place, then trim to size to get it running true. Its more work but still a lot less than starting from scratch with an iron casting.

ega, when the time comes I will look at detents, and thats an interesting thought about removable dividing rings!,

Plenty of food for thought.

Ok off to play with my nuts now, brass dome heads, I think I have to start again )-:



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