Chuck backplate

[dell]

I have to replace the backplate on my Pratt four jaw independent chuck because the one on there is made of aluminium and not very good anyway. I purchased a Myford backplate when I was at the model engineering show last week but I am Unsure about mounting, as can be seen from the picture there is a register approximately 1.5mm deep X 70mm so do I turn the whole backplate down or just enough to allow it to sit in register, the aluminium one is just the size of the register, if just enough turned down to fit in register do I need to leave a gap between rest of chuck or should it sit on outer part of chuck , my other chucks the backplate is same size of chuck.
The Myford manual suggests that only scroll chucks should have backplate same size as chuck body, 4 jaw independent backplate should just fit the recess in back of chuck is this correct?
thanks Dell

[dell]

For some reason I have no pictures

[jo479]

The backplate should be the same size as the chuck, the register should just fit into the chuck but not bottom. The chuck body sits against the backplate

[ettingtonliam]

OK, normally backplates for 4 jaws are smaller than the chuck, and just fit snuggly into the recess in the back of the chuck. The fastening screws/bolts should be within the recess. You can leave the backplate larger in diameter if you don't want to turn it down, but you should leave clearance between the backplate outside the recess and the chuck body

If for some reason the bolts are outside the recess, as with most 3 jaw chucks, then reverse the above advice and make the backplate outside the recess touch the body, but leave clearance between the backplate and the face of the chuck body inside the recess.

[SteveW]

Oct 16, 2023 17:18:22 GMT dell said:

(Snipped)
The Myford manual suggests that only scroll chucks should have backplate same size as chuck body, 4 jaw independent backplate should just fit the recess in back of chuck is this correct?
thanks Dell

Just a thought ... maybe a three jaw self centering back plate is simply just a chuck back plate and not part of the registration system. Maybe any retaining screws can't be guaranteed to be flush so a gap is essential.

In essence the chuck's back just a bit of steel keeping the works in place. As such the interface plate to the mandrel should remain clear of it only referencing to the back surface and big detente where the securing bolts go.

[uuu]

Here's some chuck fitting instructions from Lathes.co.uk

Wilf

[dell]

The four bolt holes are in the recess.

[ettingtonliam]

Oct 16, 2023 18:09:58 GMT uuu said:

Here's some chuck fitting instructions from Lathes.co.uk

Wilf

Yes, can't argue with that.

[ettingtonliam]

Oct 16, 2023 17:47:14 GMT SteveW said:

Oct 16, 2023 17:18:22 GMT dell said:

(Snipped)
The Myford manual suggests that only scroll chucks should have backplate same size as chuck body, 4 jaw independent backplate should just fit the recess in back of chuck is this correct?
thanks Dell

Just a thought ... maybe a three jaw self centering back plate is simply just a chuck back plate and not part of the registration system. Maybe any retaining screws can't be guaranteed to be flush so a gap is essential.

In essence the chuck's back just a bit of steel keeping the works in place. As such the interface plate to the mandrel should remain clear of it only referencing to the back surface and big detente where the securing bolts go.

I know its a bit contentious with the 'poor mans Griptru' crowd, but in my book a 3 jaw's backplate is part of the registration system. With any chuck, the part of the face that the bolts go through is touching the backplate, any other aree having clearance. Backplates should be a snug fit in the recess in the back of the chuck.

[SteveW]

Oct 16, 2023 19:29:47 GMT ettingtonliam said:

Oct 16, 2023 17:47:14 GMT SteveW said:

Just a thought ... maybe a three jaw self centering back plate is simply just a chuck back plate and not part of the registration system. Maybe any retaining screws can't be guaranteed to be flush so a gap is essential.

In essence the chuck's back just a bit of steel keeping the works in place. As such the interface plate to the mandrel should remain clear of it only referencing to the back surface and big detente where the securing bolts go.

I know its a bit contentious with the 'poor mans Griptru' crowd, but in my book a 3 jaw's backplate is part of the registration system. With any chuck, the part of the face that the bolts go through is touching the backplate, any other aree having clearance. Backplates should be a snug fit in the recess in the back of the chuck.

Perhaps I was a little clumsy with the my point.

From memory, the chucks I've fitted to back plates have had a recess which provide radial location on to the 'spogot' machined on to the front of the embryo back plate. The complexities hidden in the back of a three jaw chuck are protected by a plate which isn't part of the chucks registration system. As such any matching 'spogot' should not be long enough to bottom against this plate.

The rear of an independent four jaw is a much simpler, likely machined with the rest of the chuck, so can be part of the referencing system. However, there is little point in matching the 'spogot' exactly to rear profile of either chuck type. Better just have enough 'spogot' to provide a reliable radial registration only.

thought it is normal practice to match the diameter of the chuck with the diameter of the back plate rather than having a back plate just big enough to provide radial registration and metal to fully support the securing bolts.

Perhaps I just misunderstood the question.

(109)

[dell]

Well I turned down the chuck backplate today, what some messy stuff, small chips everywhere, had to turn over 1” per side off but am very happy with the fit in recess, just holes and threads to do now , is there anything special to do when threading cast iron as I have never touched it before.
Dell
IMG_2003.jpeg

[ettingtonliam]

Just drill it and tap it dry.