Machining a chuck backplate

I would like to have an ER collet chuck for my lathe. The machine has a D1-4 spindle nose and to save money on buying a new backplate I'm making an adaptor to convert the one from the four jaw (which I too rarely use) to fit the new chuck. Four jaw and backplate:

The register is 125mm diameter and about 4.5mm deep on the backplate - the recess in the chuck is about 6mm deep, so I'm making the adaptor in the same style:

So to get to to my question - given that the 'rim' on the adapter is only 12.5 mm ( I had a bit of 150mm CI to hand) , is this the way to do it? My gut feeling is 'yes, it'll be fine', but I read somewhere on t'internet that I should make the rim short so the faces internal to the rim mate.

Not done this before, so any advice/opinions would be welcome.

Robin

Edited By Robin Graham on 02/05/2019 23:52:37

It's only a register so just a few mm would be fine Robin. 12.5mm is more than ample for the job.

Seeing as the face of the boss on the existing backplate is not machined you have to use the rim of your adaptor as the registration surface.

Even if both surfaces were machined I'd still make the central recess deeper to give clearance. Less chance of having a bit of swarf or dirt trapped and causing misalignment. The rim is the larger diameter so will be the most accurate register.

Robert G8RPI.

You could probably have saved some work by machining a double register on the existing back plate simply by reducing half the height of the existing one down to the size you need for the collet chuck.

Hi, I've always understood that four jaw chucks should seat on the inside and three jaw chucks on the rim. Logic tells me that whatever position the bolts are in, that is where the contact between the backplate and the chuck should be. Bolts should always be used in tension so that the two mating surfaces they are holding have friction between them and that is where the force from one component to the other is transferred, the bolts should never have shear forces applied to them, which if you have even the slightest gap, can lead to a shear force. If you so wish the inner part of the face can be undercut inside the bolt hole circle, leaving a rim like contact face and reducing any chance of rocking over the largest portion of the face on a four jaw backplate.

Regards Nick.

I wouldn't assume - I would check what the most accurate face on the chuck is and use that.

Neil

Thanks for replies. In retrospect JasonB's suggestion of a double register might have been the way to go. However I am of a nervous disposition and have an irrational fear of modifying 'official' parts - daft really, they're just lumps of metal. I was pleased that the adaptor I made fitted the backplate with a wriggle, a push and a satisfying click when it went home, so maybe my skills have advanced enough to cross the line.

Nick - I read the same about 3/4 jaws somewhere and didn't understand it. I still don't. Surely if the bolts are tight enough the frictional forces between the mating surfaces are dominant and the bolts don't see any significant  shear forces?

My original plan was to buy a chuck from Arc, but having slept on it I'm going to have a go at making one from scratch. I'm after ER40 and have enough 3" EN1A or CI in stock to make the chuck body. Is their any advantage in using one or the other? I like CI because it machines so sweetly, but maybe steel would be better?

Robin.

Edited By Robin Graham on 03/05/2019 23:56:29

Edited By Robin Graham on 04/05/2019 00:17:18

Steel will take the fine thread for the nut better and not break away over time

I would try to find some EN24T or similar as much better quality than EN1A

Ta Jason and Colin - steel it'll be then. I'll ask at the local steel place if they can do me a part bar of EN24T, but I want to crack on with this so shall make a MK1 with the EN1A I have in stock.

Next problem is how to measure the PCD of the blind M10 threaded holes in the backplate so I can transfer to my adaptor. ARC do the very thing for the job in the form of threaded transfer punches, but I don't want to wait. Preliminary attempts to do this gave a difference of almost 1mm between measuring from the holes in the 4-jaw to which the plate was attached and from the holes in the backplate itself. Maybe that's why I had to take a hammer to to an Allen key to get the screws out in the first place, or maybe I'm not measuring right.

Robin.

Could someone tell me if it is better to make a chuck backplate out of stainless steel or ductile cast iron, ignoring the difference in cost of raw material ?

Edited By Adam Harris on 16/09/2019 20:00:23

Ductile cast iron, unless you already have the SS. There are some grades of Stainless that machine easily, if you do your homework. We have some beautiful pieces of stainless laying about at the museum, but I wouldn't want to machine any more of it.

The machining of ductile cast iron is so much cleaner than the common stuff, I have a 160mm unmachined cast iron chuck backplate on order from Chronos which I will fit to the Atlas, which has a 1 1/2" X 8? thread like a Boxford, I am not looking forward to the mess.

As for differences between three and four jaw chucks, the physical differences are obvious, so the placement of the fixings and registers varies. My 6 3/4" Pratt with the serrated jaws has three fixing studs directly in line with the centre of the scroll. A four jaw independent is easier to make if the register is inboard of the screw bearing blocks. I notice that the OP's four jaw is the other way round.

 The bolts will only have shear forces on them if you forget to tighten them.

Edited By old mart on 16/09/2019 20:25:53

Edited By old mart on 16/09/2019 20:29:39

Adam, I think it will be hard for forum members to answer your question until you explain what you mean by 'better'. Since starting this thread I have made two backplates from cast iron (so I'm an expert!) - they are accurate and work well. I would have found it harder to make them from stainless steel because the stuff is (in my experience, on my machines) more difficult to machine to tight tolerances.

I can't think what advantage stainless would have except that it'll probably stay shiny for longer.

Robin.

If I was making an ER collet chuck to fit on to my lathe I wouldn't have a register as that would be a set back to getting the collet set to run true. I would have a a flat flange with at least 6 screws radially and be able to put a standard bar in the collet and clock the bar true then tighten the screws to stop the set moving. There is little chance of the collet set moving and if you have a major bang you can always recover by resetting the set. The register on a large diamiter chuck has at times more side load as when you have intermittent cutting but on a collet set the face friction is plenty to hold a collet chuck.

David