Mounting a new chuck

Gents,

I recently bought a new P&B 125mm chuck without a backplate. By chance I found a 125mm backplate that fits like a glove. Almost as of it was made for my new chuck, but here's the dilemma: the backplate holes are not tapped. It is mounted to the chuck with 3 bolts that go from the backplate into the back of the chuck. I can mount it this way to the new chuck without a problem.

Now the P&B chuck came with 3 mounting bolts (long ones) that go from the chucks face, through the chuck and screw into what I assume must be tapped holes in the backplate. So that would be going the other way.

Does it matter which method is used? The one disadvantage I can think of is the empty holes on the chuck face but other than that?

Any advice is appreciated, thanks.

Mark

I'm slightly confused. If mounting through backplate to chuck then is there a set of tapped holes in the chuck? If mounting from chuck to backplate bolts then options must be a new set of holes or perhaps partly overdrill/mill flat bottom the holes there and fit tapped plugs?

pgk

Hopefully. the Backjplate O D runs true when on the Mandrel? Is it screwed onto the mandrel, or

If the Chuck has the facility to be retained by long bolts from the front, surely the easy way is to fit the Chuck to the backplate, and spot through to drill and tap for each hole.

Other tappings in the Backplate would be needed for the setscrews, or studs that retain the Chuck onto the Flange on the Mandrel, unless it is a screwed nose on the Mandrel.

Howard

@pgk pgk,

yes thats the strange part. The chuck also has its holes threaded about 8mm deep (measured from the chucks back face).

@ Howard,

The backplate mounts directly onto the Myford mandrel. You propose to rotate the backplate slightly and drill/tap new holes 120 deg apart and use the P&B long bolts?

Hi Mark I have seen similar chuck fixings and i have used both fixings with socket cap screws from front and bolts from rear to give a solid fixing.

David

If the Backplate OD runs true when on the Mandrel, and is a snug fit in the Chuck, I would spot through and use the P & B long bolts to hold the two together.

With the best will in the world, a three jaw is never going to hold work absolutely concentric. The only way to get two or more diasmeters concentric is to turn them without disturbing the work in the Chuck, before parting off. Teaching Granny to suck eggs!

Howard

Thanks to all for the wisdom and advice. As Howard says, that chances of the old backplate being absolutely true with a different chuck are small so I will buy a new backplate from Myford and turn it to fit this chuck and then use the long P&B bolts front to back to fasten it.

Thanks again,

Mark

You can try the chuck on the backplate by putting a stub of bar in its jaws and pushing it up against the backplate with the tailstock. Then clock the body to see if it runs true. Also unless the backplate is paper thin you can turn off the current register and cut a new one.

That's not really what Howard said. If the register on the backplate is true to the Myford spindle (which it should be if it was originally turned in location) .... and .... if that register is a snug fit in the new chuck, then what are you going to achieve by getting a new backplate and turning it?

The way I interpret what Howard said was that any 3 jaw chuck will never run absolutely concentric not simply your new chuck fitted to the old backplate.

Thank You Bandersnatch! You have taken my meaning exactly

I did not want Mark, or anyone else, to think that fitting a 3 jaw Chuck to a Backplate would lead to absolute concentricity, of a piece of round bar clamped in a 3 Jaw. Which is why, if two, or more, diameters have be concentric, they should be turned in succession, without disturbing the Chuck in any way. My technique, for right or wrong, would be largest / larger diameter first, than the next decreasing diameter(s)

The best that can be achieved, in my view, is as you say, turn the Backplate, on the Mandrel, to be a snug fit in the Chuck, fix the Chuck to the Backplate, and then start making swarf

Howard

It doesn't matter but if you use the long bolts that mount from the front you have the advantage of the socket caps which are generally better quality (and stronger) fasteners, plus you plug the holes which prevents the inevitable accumulation of swarf or dirt to be thrown out when you least expect it. If the bolts are long enough you could even put nuts on them but the simplest way would be to fit the plate to the chuck and spot through the holes with a close-sized drill, then drill and tap the spotted marks to accept the capscrews.

Added to that there's no real requirement to have a register on the backplate. The chuck will work perfectly well bolting the flat faces together, the register only saves you the bother of clocking the chuck true before tightening the bolts.

In any case, you should take a thin facing cut on the backplate before fitting your new chuck to it (but AFTER drilling and tapping the mounting holes) just to make sure it runs true on the face.

I would disagree with the idea of not needing a tight fitting spigott on the back plate. It is that which resists any axial movement under excessive forces. I don’t consider it just as a ‘register’ although it will serve that duty as well, of course.

Same here for same reasons, make it a good fit and tighten both together.

You may be able to cut the existing face plate back and re register it.

May be harder to align, drill and tap from the front plus any downwards pressure on the protruding threaded stub will have to be sorted.

Other way from the rear is remove the jaws then you have a wide flat spreadable surface.
Pick up any hole by movng the chuck around with desired drill and centre up.
Place back plate on without moving the chuck and drill that hole through.
Open that hole up if pukka job counterbore and bolt together before next hole.
Then same again twice, job done.
Only need a bench drill.

Arn't the chucks as Mark describes which have holes through them used to mount chuck on a rotary table using tee bolts the threaded holes in back of chuck for mounting on lathe ?

Mick

It's you're right to have your own opinion, but what do you call excessive forces? Three m8 bolts clamping a chuck to a backplate will give a phenomenal clamping force, something that only a heavy crash would overwhelm .

My 8" TOS chuck is fitted to the backplate with no register, has been since I got it new. I've slipped the belt and stalled the 3hp motor but it's never bothered the chuck.

Correct, but clearly I am not alone (thanks, Jon). I would (personally, again) prefer a belt and braces approach where safety is concerned. For instance, I would not trust any chuck, fixed only with three bolts and no spigot - rotating at perhaps in excess of 2000rpm - if I purchased a second hand item. Would you?

That is the ‘test’ I would apply to considerations such as this.

Question: Are chucks, fitted like yours, supplied by OEMs? I, and likely a lots of time served machinists, look forward to your response.

The last (3-jaw) chuck I fitted has a "loose" register - a few thou over - and was tapped to run true holding a ground 20mm ER16 extension chuck (which I've only used once in this way). That was some time ago and I did check it again recently and there was no apparent movement. I used the same loose arrangement when fitting my ER32 chuck. Neither have moved in use and whilst I guess a really bad dig-in might shift them, I don't think safety is compromised by doing this. It's not a new idea BTW - although I forget which of the "Gurus" originally suggested it. I also have an old 'Grip-True' but have never fitted/used it but the general idea is the same.

Perhaps a little perversely, I did make my 4-jaw backplates a snug fit on their registers as there's no need for any adjustment. However, some of this just comes down to personal preference and expected use. In industry it's not possible to predict what abuse/stupidity will occur on a machine but in my workshop the only abuse that happens is through my own ineptitude/carelessness and fortunately these events seem to be a bit rarer these days (a really good 'fright' is an excellent learning experience I've found and one not willingly repeated).

Regards,

IanT

If you do manage to generate enough force to shear the bolts it's going to come off the register anyway.

The advantage of the 'loose reister' that IanT talks about is that if you want a chucked part to run true you can chuck it, loosen the bolts slightly and clock true with a few taps. Tighten the bolts again and you are good to go. It's essentially a poor mans GripTru.

regards Martin

Yes of course. That's not to say I wouldn't check over any chuck I bought - they all get stripped down but the register does nothing to add to the connection between the backplate and the chuck except ease the process of centering the chuck - all the load is carried by the friction between the faces created by the clamping bolts.

The loads on a lathe chuck are a lot less than people imagine. Two of my lathes have the chuck fitted to the spindle with no bolts at all.

Howard, now I get it thanks. The old chuck came with a backplate already machined for it. So not machined on my lathe.