Is this normal in backing plates

I was turning brass washers today 3/4" inside dia and my chuck went floppy . I stopped the lathe and it was sitting lopsided.

I took it off and all the threads in the back plate are broken . Is this a cheap back plate or has anyone else had problems like this?

Thanks for looking

Edited By Martin Newbold on 30/04/2018 18:20:47

To be clear - you mean the threads that hold the chuck to the backplate? And the threads have crumbled away?

If so, this does rather look as though either: the attachment bolts were done up much too tight, or, the backplate casting material is faulty (porous, not the right specification, etc).

If on the other hand, it is the main attachment thread - on to the lathe spindle itself, then I think you can rule out overtightening, so the casting is the likely problem>

Whichever, I suggest you take this up with the supplier, and until you hear from them, do not destroy the evidence.

If overtightening was to blame, ask the supplier for details of the instructions which he (she) issued, - which in my experience would be a rare occurrence. And if there were torque settings, you know what to do next time ...

Regards, Tim

The photos are of the backplate to spindle mounting thread.

Edited By Mark Rand on 30/04/2018 19:17:03

To answer the original question, no it's anything but normal.

And has presumably developed (deteriorated) as time goes by. How old is the backplate?

Visually the back plate looks like it is cast iron. Which will crumble if overloaded, but I can't for the life of me see how you could have "overloaded" the thread in any normal service - or any un-normal service either - to such an extent that it explains what you have described.

So what other possible causes are there:

Faulty mating thread, e.g. insufficient depth of engagement (or oversize female thread), thread angles cut wrongly so remnant thread in female part is over-slim

Swarf or dirt jammed in the threads causing premature wear

Major screw up eg tool dig in (it would have to be a "good" one!!!)

Impact damage - not sure how

New replacement called for whatever is the cause.

HTH Simon

Edited By Simon Williams 3 on 30/04/2018 19:41:16

Thanks everyone for answers its the spindle thread. The three bolts were good and solid. It was not wrenched on and was cutting brass pretty soft stuff.

Have no clue why it failed . I have only just put it together bought it on here and its had really little use Is there any reason why they are made from cast iron and not steel . Can i buy a steel one its a ml7 thread on a drummond lathe spindle it was not over tightened . The original 5" I have replaced it with is steel backplate. can i get hold of one please? I am worried if i buy a new myford one it will do same thing?

Can anyone help thanks for looking

Thanks M

I'm sorry if my early response was over-complicated. I was relying on the term 'all the threads' to mean, well, all the threads.

And now the later questions:

Cast iron is used because it is very stable - it does not tend to distort after machining, as long as it is treated properly in its early stages, which is a range of well-known procedures (aging, etc). It is also easy for the final user to modify to fit his (her) chuck in position on the lathe, as this is the best way to achieve accuracy. And it is plenty strong enough - usually. The only time it is not preferred is when very high rpm is in use - some CNC stuff, but not home workshops.

I am sure that you could find a steel version - or have one made - but I see no advantage. The most important checks before you do anything about replacement is to have your lathe spindle checked for distortion, wear, etc, as that could have influenced the failure. Now we know that your spindle is 'non-standard' it does point more in that direction.

Cheers, Tim

A final thought. Now we know it was brass being cut is another factor - it does tend to grab, especially if the cutter is not made or modified specifically for brass, and if the machine is a bit worn. The tool can dig in to the work, and it will move inwards to take up all the slack in the system. This can bring a lathe to a dead stop in a very short time (much less than one revolution) and has been known to break tools, holders, slides, and yes, mounting threads. In these circumstances a cast-iron backplate is a big advantage - a backplate thread failure is much easier and cheaper to correct than a new spindle or some other no-longer made lathe fitting.

This has already been mentioned by Simon Williams 3 as 'Major tool dig-in' and he didn't know that brass was involved, either.

Regards, Tim

How does that work? The Drummond spindle thread is smaller diameter than the ML7's.

There is no way a tool dig in would tear the chuck off its threaded spindle if everything was correctly made. Something was not right in the set-up in the first place.

It may be you only had very partial thread engagement between the 1" diameter Drummond spindle and the 1-1/8" diameter ML7 chuck backplate thread.

Edited By Hopper on 01/05/2018 01:40:22

Steady on, hopper! ‘May be’ is a gross understatement if there is a difference in OD of 1/8th of a whole inch! That would be a simple explanation for an expected failure due to to a dangerous installation! Nowt to do with metallurgy failure here if there is such a mismatch of threads.

A 8 tpi Whit form thread has a depth of 0.080", so running a 1 1/8 x 8 tpi female thread onto a 1" male thread would give only 0.035" engagement (0.0175" a side). IF this is what the OP has done, I would not be surprised that the thread crumbled, and "the chuck went sloppy". The top of every thread would be torn off! My surprise would be that the chuck went on, and the thread survived tightening, at all.

Hope that this is not what has happened.

Only hope of recovery would seem to be boring out the backplate and bushing it with a thread that matches the spindle.

Just 'cos "it has a thread on it" as some Iranians used to say to me, takes no account of thread pitch, angle or size, and does not mean that it will work satisfactorily!

Howard

LOL. Well, I was not sure if that what Martin was saying in his original post or not, it was a bit unclear. Perhaps he can clarify?

.

Regular readers might recall that Martin's lathe is much-modified and, following his unfortunate experiences with one motor installation, was recently fitted with this: **LINK**

http://www.model-engineer.co.uk/albums/member_photo.asp?a=45842&p=759888

The 'available torque' may [or may not] also be pertinent to the backplate thread failure.

MichaelG.

No I havn't been following the whole saga closely enough to remember every detail. Do the modificationss include a larger spindle thread to match an ML7 backplate?

I would expect the belts to slip or the tool bit to snap off long before a lathe tore the chuck off the spindle if the threads were properly made and engaged.

If Martin could clarify, and post some in-focus pics of the damage in good light we might be able to offer some more informed opinions from the panel.

Either way, it looks like this backplate is toast. If the spindle thread is 1" x 12tpi as per M-type Drummond, Tony at lathes.co.uk sells backplates for the M type at a very reasonable price, or did a couple years back when I enquired. He may have others available if thread is the earlier 3/4" diameter etc?

Edited By Hopper on 02/05/2018 09:49:53

.

No modified spindle thread, so far as I am aware

MichaelG.

Only Martin can confirm but a combination of wrong threads on an over-powered lathe looks like a good explanation of the damage to me. All my reading on lathes suggests it's a bad idea to put an excessively big motor on a machine. Apart from the risk to working parts during a dig-in, it's all too easy to get into the habit of using the extra power. The motor won't complain but bearings, gears, frame, belts, spindle, chucks, tool-post etc. are all stressed. In Martin's case, the thread mismatch is the likely weak point.

You can put a big souped-up engine in a little car, but don't be surprised to wreck the clutch and find you need better brakes, fat tyres, and a roll-cage!

This may also shed some light on the issue, have a look at Martin's post from October 2017, seems it's been wobbling for a while!

Interesting link, Jason

MichaelG.

I'm not sure what the Myford to Drummond apaptor actually is, searching Myford's site for "Drummond" gives no result?

There is also a pic in his albums that suggests a 10tpi thread.

Probably best not to waste more time until Martin can explain what he has.

SOD - his best shot may be to run one of the belts a bit slack as a safeguard in future.

Michael - Yes one of his pics shows a vernier on the spindle showing 1" (nom) spindle thread so I would say that is the problem confirmed: 1" spindle in a 1-1/8" BSW threaded backplate hole.

Jason - Thanks for the link. Not sure where Martin is now compared with then. As you say, over to Martin now to clarify. No sign of a Drummond adaptor on Myfords site as pointed out.