Backplates

Hi all,

can someone explain to me why some chucks need backplates and some just screw straight on the spindle as is.

thanks

chris m.,.,

Hi Chris,

Some chucks are directly mounted onto the lathe, placing the chuck closer to the spindle bearings. So the chuck will only fit a lathe with the correct type and size of mount. Something like this (from Ajax):

Many chucks needs an adapter plate (backplate). As long as you can find a backplate that fits your lathe you should be able to mount the chuck on your lathe. Discussed here.

Thor

Edited By Thor 🇳🇴 on 09/10/2022 11:06:03

Various ways of fixing chucks to lathes, each with pros and cons.

A single screw on spindle is easy to make, accurate, and reasonably quick-change but the chucks tend to get stuck and risk coming off if the lathe is run in reverse. So best for lathes that run in one direction only. Ask the forum if you ever need to remove a stuck screw-on chuck! Broken teeth on Myfords is a common fault because ill-advised owners use the back-gear to hold a stuck chuck so they can apply excessive force.

Bolting the chuck to a flanged spindle is easy to make, accurate, never sticks, and can run both forward or reverse. But changing the chuck is slower and fiddly, especially on a small lathe if you have big fingers.

Best of all are the various forms of cam-lock. These are accurate, never stick, do forward and reverse, and are quick-change. Not easy to make and hence expensive. Typically made for workshops where time is money.

Although chucks can be made to fit one specific lathe, it's usual to attach a standard chuck via a backplate that adapts it to whatever spindle fixing the lathe comes with. Makes it much easier to buy and attach chucks.

Of the cheap and cheerful systems, screw-on seems to have been most common in the past, whilst most modern machines favour bolt-on. I think this is because most modern lathes are designed to run in reverse which it's very convenient for some operations such as metric screw-cutting.

Dave

Using a backplate allows one chuck to be used on many different designs of lathe. Only the backplate needs to change. If the chuck body is threaded then it severely limits the number lathes that the chuck will fit.

Andrew

A directly screwed on chuck will normally save a bit of overhang adding slightly to the length of the bed, useful on smaller machines.

As Andrew mentions the drawback of directly threaded, I bought a NOS Toolmex 6" four jaw independent lightweight chuck threaded for Myford. The centre got bored out to max size and a Smart & Brown 1 3/4" threaded backplate fitted. Myford lovers will be issuing a fatwa against me now.

Edited By old mart on 09/10/2022 15:51:49

One of the other advantages of a backplate mount is that the backplate can be trued in-situe which should eliminate any issues with the screw thread or spindle nose and give the chuck a true face to be fixed to. If you have issues with the spindle nose screwing/bolting the best chuck in the world to it won't cure those issues.

Also means the manufacturers don't need to make and stock every combination of mounting for all the chucks in their range.

SOD:

The American L series long taper is still in use, of course, and avoids the unscrewing risk but does increase overhang. The backplates are not cheap to buy and rather expensive to machine from solid.

It is only when the manufacturer knows a large volume of chucks will be ordered that it becomes viable to make them specific to just one lathe nose thread. Myford as mentioned above and Boxford also did a threaded low profile 4 jaw for the schools market.

It's quite possible Myford used a chuck with an internal thread and no backplate as a cost-saving measure. No backplate means one less precision part to manufacture. Just screw the chuck straight on.

The added advantage of less overhang is a nice bonus.

Edited By Hopper on 11/10/2022 04:00:40

As Jason says, a backplate can be trued on the machine on which it going to be used, improving accuracy of location.

For an independent manufacturer of chucks, not "building in" a mounting means that the chuck can be produced in greater volume (Reducing costs, and hopefully, price ) so that a separate backplate allows it to be fitted to a larger variety of machines.

To me, those look like good arguments in favour of backplates

Howard

I think the only chuck that Myford regularly supplied that did not need a backplate was the 6" 4 jaw independent which needs to be both slim and have minimal overhang to fit in the bed gap. As far as I am aware all the Myford supplied chucks were made by Burnerd in its various guises.

Rod

[Rod]

MichaelG.

Exactly. So if Myford ordered a thousand plain, unthreaded three-jaw chucks from Burnerd, Myford would then have to either pay Burnerd to cast a thousand backplates, machine the stepped faces for the chuck to mount to and then screwcut the thousand threads. Or Myford would have to get the thousand castings made at a foundry and machine the backplates themselves. Cheaper to pay Burnerd to cut out the casting and facing steps and simply screw cut the thread straight into the chuck. Burnerd would have already had the chuck mounted in a capstan lathe to machine the bore and back faces. A flick of the capstan lever and bingo, the thread is cut. Quick and easy.

Quite a cost saving. That is the way Myford did things. It is written all over their machines and was how they could sell their lathes cheaper than their competition eg Raglan, Boxford and even the old M-Type. The ML7 is a masterpiece of production engineering design in many similar ways. Eg extensive use of Mazak castings, lightweight bed, gap bed, etc etc. They never missed a chance to save a few pennies if it could be done without damaging the machine's usefulness.

I think Myford must have inspired Nevil Shute who said something like "An engineer is a bloke who can do for two quid what any fool could do for ten." That was the prevailing philosophy in mid-20th century Brit industry.

Edited By Hopper on 11/10/2022 11:05:38

It is written all over their machines and was how they could sell their lathes cheaper than their competition eg Raglan, Boxford

Agreed, it is written all over the machine, but both the Raglan and Boxford were in a different league to the myfords of the time.

Greater centre height, a separate feed shaft (avoiding wear on the lead screw and half nuts) were just a couple of refinements simply not even made for their cheaper cousin.