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Don@2480
14th May 2018, 06:18 PM
G'Day All,
After putting up with my lathes (AL300) original 4 jaw chuck backing plate for far too long I decided to do something about it.
The problem being that the fit between the backing plate thread & the lathe spindle nose thread (Ø2.250" x 8 tpi) was to no standard ever used, except by the manufacturer. This resulted in uncontrollable chatter for any reasonable depth of cut & was impossible to part off any size steel workpiece.
As I rarely use a Ø200 4 jaw, I have kept putting this job on the back burner. Now as I need it for a fair bit of turning I have had to do something about it. After acquiring a couple of Ø210 x 45 pieces of mild steel plate I had no more excuses.
Fortunately, the oxy cut surface had been removed by the "supplier" making things a little easier.

373298
Rough material setup to start.

373299
Plate after rear face turned to size & profile.

373300
Plate after bore & register diameter turned to finished size.

373301
After screw cutting, the plate being turned to require thickness & the chuck location spigot turned to size.

373302
The finished plate after milling ops. for the securing screws.

The chatter caused by the old plate can be seen on the thread finish. The last cut had to chatter didn't it, but the fit is good. The plate has to be screwed onto the spindle nose with a spanner on a chuck jaw for the last 10mm now.
So now, decent cuts & parting off can be achieved.
Regards,
Don.

KBs PensNmore
14th May 2018, 08:37 PM
Thanks for the pics and how to, I need to do a couple for my Hercus, when it gets here.
Nice work, even with the chatter. if you wanted to be pedantic, make another to mount the chuck on with the new plate, then remachine the earlier new one.
Kryn

malb
14th May 2018, 09:54 PM
If you need to drive the backplate onto the spindle for the last 3 to 4 turns, you will be locating the chuck on the spindle threads rather than the spindle registers. The concept for a threaded spindle nose was that the chuck/faceplate/backplate threaded onto the spindle freely and with a small clearance in the threads, and that alignment was controlled by the accessory mating with the register and having enough thread clearance to allow the register to true the accessory.
In your case, if the threads mate that tightly that you need to use tools for the last few turns, there will not be enough clearance in the threads for the registration surfaces to do their job.

Don@2480
15th May 2018, 04:52 PM
Kryn,
I can put up with the chatter on the thread as I am not making another.

Malb,
The threads are not binding prior to the register faces meeting. I made a master thread gauge years ago when making backing plates for 4 other smaller chucks. The thread tolerance is approx. H7-g6 fit, with the register diameter turned to H7-h6 fit. The use of the spanner is to bring the two diameters together & only a small effort is required.
Regards,
Don.

Nrcollar
30th Jul 2018, 12:24 PM
most of the time you can solve the chatter with slower speeds, the unfortunate thing is they will only go so slow.
Nelson