A 4" 4-jaw will hold 2" stock easily, it leaves 1" of jaw engagement which is plenty at that size.

If you want to spend ~£100 try looking at HBM, Zither and Vertex.

Neil

Carl.

I have a small Myford M type Lathe with the small 7/8" 9 or 12 TPI thread,( not sure which ) I'm going through the same process as you, requiring a larger chuck, I would think twice if I were you about the 6" chuck, mainly because of the weight you would impose on the smaller mandrel.

A 4" lightweight one will be my choice

Michael G.

What sort of insert are you thinking of,a reducer from a myford thread to the 1" thread on Carl's lathe ?

James

The lighweight ones are also available for backplate mounting which may be easier than trying to convert a threaded one. I have a Bison one on my lathe.

.

Admittedly it's on an ML7 page, but I think this [from lathes.co.uk] is a fair summary:

[quote]

… The "threaded body" chucks were always the preferred choice, being designed to allow as close a fitting to the spindle bearing as possible, so reducing overhang and improving rigidity. Internally-threaded, and of a half-depth body design for lightness, the 6-inch diameter 4-jaw Burnerd chuck was designed to reduce the load on the front spindle bearing and had its mounting thread well recessed into the body. … … The 6-inch diameter Burnerd 4-jaw chuck is the perfect solution for those large and awkward jobs that would otherwise take too long to set up on a faceplate – and a good used one is well worth seeking out.

[/quote]

Now the 'M' lathe has a 1" thread and [correct me if I'm wrong] an MT1 taper; whilst the 'ML7' has a 1.125" thread [1.25" register] and an MT2 taper. … I have not done any math on this, but gut-feeling is that there is very little chance of the 'M' spindle failing under the weight of this chuck.

… Compare the rather magnificent faceplate with which it was supplied !!

MichaelG.

Edited By Michael Gilligan on 08/07/2016 06:58:40

Edited By Michael Gilligan on 08/07/2016 07:00:25

Other than Myford I don't think I have ever seen any direct screw fit chucks for other lathes. All use back plates. Other types of fitting too.

Chuck size is an odd area. Manufacturers do have a tendency to fit the smallest 3 jaw they can get away with and even offer a larger one as an alternative at times. The traditional 4 jaw for a particular lathe will generally be larger than the 3 jaw and slimmer too. I suspect that Myford chuck weight came more to the fore when the big bore arrived. I can't recollect any mention of it before that and my Myford came with the larger 3 jaw and I can't recollect ever seeing one with the smaller chuck.

As this is a smaller lathe I suspect a 5" 4 jaw would be the best answer but if the op can't turn up a back plate he's likely to have problems. He could probably just about get away with holding a back plate casting in his current 4 jaw with the jaws reversed. College Engineering offer a 5 3/8 dia one which is probably too big but the spigot could be held first to turn down the OD a bit. They also do a 4 1/2" back plate.

The other point is that 4 jaws usually last well as they aren't used that often. I can't help wondering if bell mouthed jaws are really loose headstock bearings but people have been know to machine jaws and if the lathe isn't perfect the result will be bell mouthed jaws.

If the OP chooses to buy a used chuck it's worth remembering that there are several old makers that produced chucks that are just as good as Bernard. There is a small Crown on ebay at the moment for instance. There are others that crop up now and again.

Just add that if anyone does make a back plate it really is a good idea to make a dummy spindle nose first. It can be checked for fit into one of the other chucks that are known to fit well or measured VERY carefully as it's made. The main aspect to get exactly correct is the register diameter but ideally the thread needs to be very close too.

John

–

Edited By Ajohnw on 08/07/2016 10:08:04

.

Carl,

I did not suggest threading an internal bore on a 6" chuck … I suggested making a suitably threaded insert, to fit a plain bore. … By doing things in the right order, and setting-up carefully as you proceed, it should be simple enough.

MichaelG.

Hi I have a 6" Bernerd light weight four jaw chuck that looks the same as the Pratt and Bernerd ones that have been linked to.

This one as Jason has said, is made for fitting a backplate. The backplate that is fitted to it is for a Myford ML7, code number on it is SK8052. The chuck is 1 5/8" thick and has a bore of 1 5/8" and weighs 4.1 Kg on it's own and 4.66 Kg with the backplate fitted. One thing about the backplate is that most of the threaded portion protrudes into the bore of the chuck, thus keeping the overhang to a minimum.

I don't have a lathe that this fits onto, although I do have a Myford M type with a 1" x 12 thread ( which incidentally I believe is UNF and not Whitworth) but I don't have a backplate to fit it, but the three jaw chuck that is on the M type is about the same weight, although I have never used it as it's just one lathe that I have waiting for an overhaul. Another old un-badged lathe that I have has a four jaw that is threaded into the body, but I don't think it is a Myford, I think it looks more like a RandA design, although it has a spindle nose the same size as the Drummond round bed lathe.

Regards Nick.

Hi Carl & Micheal G

Micheal i fully agree with what you are saying, and would have gone down the route you suggested, ie turn the thread out of P-B chuck and bushed it to suit the spindle to get the best of both worlds , largest capacity and minimal over hang i dont have a problem with that . Though one may need another lathe with the capacity to do so, though i suppose one could leave the old back plate on the spindle and then use the chuck jaws to hold the chuck to the back plate which would let one get the tool in place to turn the thread out.

Trouble is one then needs to screw cut the internal thread in the bush, as you said careful planning the operations and also the ability to work to a high degree of accuracy will be needed, this maybe a bit daunting and will depend on ones level of ability, which is an unknown factor ! The inverted back plate as shown in Nick`s photos looks to be a good idea. I would rather not get into another debate like the ML 7 post as it didnt go well at the end of the day ! so hopefully someone with a 1 inch spindled Drummond M type will advise on chuck size. so to answer the question as asked in the first post which make of chuck would one buy, for myself i would go for TOS, Zither or Vertex or a good condition S/h P/B ………….b