Mellor Cross slide nut replacement

[ZebethyalParticipant @zebethyal]

Hi there,

I will start off with a little bit of an introduction:

A while back I bought a Mellor lathe quite cheaply on a well known auction site, I also have a Sieg SX2 Mini Mill.

I am so far pretty happy with the lathe, I still need to obtain a suitable drive belt to go from the motor to the lay shaft, but that should be simple enough to obtain from my local auto factors.

The reason for this post is that I noticed a considerable level of "slop" or backlash in the cross slide, and after taking it apart, it would appear that the bronze nut is quite badly worn.

The screw itself has a left hand square thread form (not ACME) of 5/8" -5TPI, and whilst there is some wear, it is not that bad. An "as new" tooth at one end measures 0.104", the worst worn tooth is .0965", and most measure at around 0.01" so showing 0.004" to 0.009" of wear which I believe I can probably live with on a lathe of this age that rather than looking to replace it.

The Nut however when wound onto the screw can be moved by about 1mm (.04" ) , with the teeth looking visibly worn when viewed off the screw. On measuring an impression of the teeth, they measure at around 0.065" or .039" of wear, which when combined with the wear on the screw is over 1mm of slop.

Whilst this may be ok for general machining, I believe I will struggle if I need to cut any threads.

Any suggestions for a replacement nut, or any other alternatives, after all, it is not like I can simply nip out and buy a new nut )

I believe I could make the Nut itself, however I will no doubt struggle with the thread as from what I have read, cutting square thread forms is not exactly straight forward and can place big loads on the work piece.

Edited By JasonB on 23/05/2014 15:50:04

[ZebethyalParticipant @zebethyal]

[Clive FosterParticipant @clivefoster55965]

The acetal (delrin) nut replication process is said to work well and could well be worth a try if the fitting geometry and space available will accommodate a suitable sleeve to hold the plastic nut.

See here **LINK** for a description of the process and what looks to be an extended discussion.

Clive

[ZebethyalParticipant @zebethyal]

Thanks for the link Clive, I read that thread a few months back, before I realised I had this problem and thought it a great idea, but had since forgotten all about it.

I have been considering delrin as a possibility, but was thinking about machining the thread or some way of cutting it with the screw whilst the screw was hot, the method in the link is a much more elegant solution.

My cross slide nut is an odd shape, though not uncommon for lathes (sort of D shaped with a boss on the flat to hold it to the cross slide itself).

There is not much space in the "well" where the nut travels, so I will almost certainly have to go with an almost identical design if I make one from scratch.

The simplest solution may well be to drill out the existing thread in the nut as the unit pretty much is junk otherwise, and use the remainder of the nut as the sleeve to press the delrin insert into.

[BazyleParticipant @bazyle]

One interim soultion is to solder on some appropriate bits of brass on the outside of the nut and then cut it in half-ish such that the mounting is on the half nearest the handle.. Arrange two screws to push the halves apart and prevent rotation of the outer half. This can be used to take up the slack for a few years giving you time to perfect the skills to make a new nut.

In time you may also be able to make a new screw, or trim the existing one on the lathe itself but as you say it is plenty good enough for the moment.

Delrin nuts seem to be a good idea but then often present a mounting problem.

[ZebethyalParticipant @zebethyal]

Thanks for an alternative idea Bazyle, unfortunately I don't have the space under the cross slide to do what you are suggesting, the current nut at 20x27mm (0.8×1.07&quot only just fits with little extra space around it for anything else, which was also pretty much stumping most of my ideas that revolved around replacing the screw with an alternative, as the nut would pretty much still need to be a copy of the current one, albeit with a potentially different thread.

[BazyleParticipant @bazyle]

With no space how about turning the outer end of the 'D' circular and cut off part of this treated portion. Silver solder a thin brass tube onto the remaining stub. Wind the cutoff part onto the screw and into the tube with the assembly at the outer unworn part until it locks the thread as the two 'nuts' contact. Back off a tad and soft solder the nut to the tube. Being soft solder it can be easily redone until the effect is tight but not locked.

You can do the turning on your own lathe by locking the nutless cross slide and moving the topslide round to use crossways.

Edited By Bazyle on 23/05/2014 23:24:40

[TimSParticipant @tims]

What about casting a nut from epoxy? you might be able to use the existing void as a mold?

alternatively you could push the old nut into plasticine or clay? as a last resort lego or carved wood could do the same job

TimS

[Mauro MalvezziParticipant @mauromalvezzi63020]

Hi,

my name is Mauro and I am writing from Italy. I do not post much over here, but I make sure to read much of what's posted. I couldn't believe my eyes when I saw somebody talking of a Mellor lathe- I thought information about them was scarce, and the lathes themselves very rare.
Long story short, I purchased a Mellor lathe (nice bit of kit – with many accessories and a collet spindle), and I currently am in the process of restoring it to its former glory.
The lathe seems to be well worn, I am afraid, but I'll stick to my guns and rebuild it, it's a very nice small lathe (very large bedways compared to the swing), I want to do it justice.
I'd like to ask mr. Timothy how he removed the cross slide, since I have gotten to the point where almost everything is already off the carriage (auto-feed gears, top slide, dog clutch, cross slide handwheel), but I now am rather puzzled about how to proceed wit the disassembly of the slide.
Any help would greatly be appreciated.

Best regards from Italy,

Mauro

[ZebethyalParticipant @zebethyal]

Hi Mauro,

Always good to find another Mellor owner, every one I have seen seems to be slightly different, based on what has been lost or adapted over the years. Mine seems to be missing the thread counter as well as many of the covers and is V belt drive which in itself seems to be quite rare, I also have a 2MT tailstock which I believe only appeared on some of the later models, although the paint colour on the tailstock implies it was possibly from a different lathe originally.

I will have a look and double check, but I believe I dismantled it as follows (not all steps are absolutely necessary as I chose to remove the compound slide first):

Remove the 2 nuts securing the compound slide to the cross slide, these are the nuts securing the compound slide to the 2 T head bolts that allow the compound slide to swivel, this allows removal of the compound slide as a complete unit.

Remove the 4 fingered thumbscrew from the rear of the cross slide lead screw that secures the gear for the power cross feed, remove this gear as well.

Remove the lock nut securing the cross slide handle to the cross slide lead screw.

In my case the handle was held on pretty tight to the lead screw and I needed to jam the cross slide all the way to the front in order to allow me to unscrew the handle from the lead screw.

You should now be able to wind out the cross slide lead screw to the rear of the lathe.

Remove the 2 screws securing the gib to the cross slide and slide out the gib.

You should now be able to remove the cross slide from the lathe.

The cross slide lead screw nut is simply pressed into the underside of the cross slide and can be knocked out is required from the top, or pulled out by twisting the nut from below.

I think this has covered all the steps I followed, please let me know how you get on.

Regards

Tim Moores

[Mauro MalvezziParticipant @mauromalvezzi63020]

Dear Tim,

thank you very much for your explanation – it shows that I was very close to discovering the solution myself, given that I had come to the point of stripping everything off the cross slide, handwheel (which was rusted on in my case – I had to hold the leadscrew with a pair of rubber-jawed pliers in order to remove it) included.
So, I should now simply unwind the leadscrew from the rear of the lathe? I hadn't thought about that. I'll give a try and report as soon as possible.
With reference to Mellor lathes: yes, it's always nice to find somebody else who's got one! Mine was a bit of an adventure, I travelled all the way to Dorset in order to buy it from a very kind Englishman who helped me disassemble it into its major components and crate it. No v-belts, only a 3 speed flat belt cone, but I've got all the changegears and a full set of 5c collets. Main spindle taper seems to be 3 MT, while tailstock is 1MT.

As soon as I have it cleaned from almost 50 years' worth of debris, I'll start measuring wear on the bed and slides. I plan on rescraping everything, down to the last bit, to proper fit and alignment: the lathe, with its separate backshaft for feed (which could be converted to V-belts and driven on its own for key-and-spline cutting purposes) and wide bed, is worth the effort.
Stay tuned!

Mauro

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