Is this any good

[Haydn CallowParticipant @haydncallow19480]

Hi, first off a big thank you to everyone who has helped me so far.

new to all this,

bought a Drummond M type in a basket……I now have it built and I know it has been attacked with a hacksaw in the past…..however it is mostly there and it will do what I want.

I have stripped most parts and cleaned and oiled reassembled.

I have just stripped the saddle and in doing discovered what a half nut is.

my question is having never having seen one before :- what do folks think of it ? Worn out, o.k, or good.

if worn out what can I do about it ?

[Haydn CallowParticipant @haydncallow19480]

Half nut

[Martin Johnson 1Participant @martinjohnson1]

Life expired. It will slip a thread at the most inopportune moment, so replace if you have any intention to do screwcutting.

Awkward to make, but you might be able to silver solder new nut halves into the old remains.

Sorry to bring bad news, don't shoot the messenger,

Martin

[Thor 🇳🇴Participant @thor]

Hi Haydn,

From your photos the half-nut looks worn, especially on the side where there is hardly any thread left.

Thor

[Haydn CallowParticipant @haydncallow19480]

So my next question :- can it be repaired ( beyond my ability ) or could I get a replacement ?

[Clive FosterParticipant @clivefoster55965]

The technical term is basically "well knackered".

As you aren't equipped to screwcut a new one and finding a decent used replacement is likely to be challenging an unorthodox repair will be needed.

Hot moulding two half circle bored pieces of delrin around an unworn part of the leadscrew is known to work well. The difficult bit generally being attaching the new delrin halfnuts to the mechanism. There are several write-ups of the technique on the internet. Most workers elect to bore out the halfnut casting and arrange flanges on the delrin half cylinders to keep them in place under screwcutting loads. Adhesive, pins or teeny screws being enough for retention when not in use.

In your position I'd be inclined to try a modern metal loaded filler. Put a couple of small holes through the thread remains to help the excess filler squidge out, coat the chosen part of the leadscrew with a release agent and squeeze the two half nuts together around the screw to form a thread. Soot was a common release agent when babbit bearings were poured, I imagine silicone spray, PTFE loaded lubricant spray or one of the thinner, retentive oils would also work. Hardest part is going to be making sure that both halves of the nut align. May have to be done on with the apron partially assembled leaving little room to work.

But many machines eg Colchester, Kerry et al get on just fine with only one side of a half nut. So doing only one side might well suffice. I imagine you have no plans for vast amounts of screwcutting so even an, objectively, insufficiently strong repair might last for several years.

Some experimentation on disposable things is usually advisable before committing to a unorthodox repair on hard to replace parts.

Clive

[not done it yetParticipant @notdoneityet]

As per MJ. You only need to look at offerings on an suction site to make a simple assessment of what you have.

Buy new, buy second hand, make yourself, or repair are the options. I would likely go the repair route, if possible, but ymmv.

A question might be re the lead screw condition, too.

Edited By not done it yet on 27/09/2022 13:37:53

[Ady1Participant @ady1]

Easiest route I can think of is a babbit pour, I will dig an article out asap

https://www.model-engineer.co.uk/albums/member_album.asp?a=58501

Just adapt your nut job to the situation

 

Edited By Ady1 on 27/09/2022 14:08:25

[old martParticipant @oldmart]

Could you measure the pitch and size of the leadscrew? It might be possible to get a nut made and cut in half, then attached to the arms.

[Ady1Participant @ady1]

You can get a lifetimes supply here

Edit: I did as old mart suggested a long time ago, its a very fiddly solution, I used aluminium

Now I'd just go for the pour

Edited By Ady1 on 27/09/2022 14:14:01

[old martParticipant @oldmart]

Casting babbit is only the start, how would you attach strong enough arms to the nuts?

[Ady1Participant @ady1]

You drill holes for the pour to grab, the pour goes into the hollowed nut. Did you read the article?

That's the best thing about the babbit route, you can fix your original LS nut forever

I burned through two perfect ones in 12 months when I first got my Drummond, Its the weakest technical part of owning those lathes

Edited By Ady1 on 27/09/2022 14:38:59

[Lee RogersParticipant @leerogers95060]

I think everyone is wide of the mark here . One of those is only a pressure pad and doesn't need a thread at all the other is not too bad and in my experience with 4 Drummond restorations it will probably be OK as it is. Try it , if it works then it's ok.

[BazyleParticipant @bazyle]

A 'get you home' trick that will last a few years:
Measure core diameter of leadscrew 'D' and find a bar offcut a little larger. Obtain brass or gunmetal if possible 1/8 X 1/4 X (pi x D /2) anneal (red heat for a few minutes and cool slowly). Bend it round your offcut in the difficult direction, ie ending up with a half washer 1/8 thick and 1/4 flange. Make two
Following the residual thread on the right hand end of your old nut file the end off the nut at this angle and at the right pitch spacing. You can hold it up to the leadscrew to check progress.
Twist your 'half washer' slightly to match the pitch of the leadscrew and offer it to the end of the old nut while fitting into the valley of the leadscrew. After a bit of tweaking you will be able to position it and drill for a couple of pins to hold it in place while you solder them together,

You now have a bit of screw thread at full depth. Repeat for the other half nut and possibly if you enjoy the process the left hand end of the nuts. The RH end is more effective as it takes the thrust under compression of the joint. Obviously not full strength and will wear faster than a full nut but will last long enough for you to learn enough screwcutting to make a complete new nut next year.

[Haydn CallowParticipant @haydncallow19480]

Hi,

thanks for all the reply’s…I have now discovered that the shiny top half should in fact not have any thread, what you see is where the lead screw over time has worn into it….it’s supposed to be a pressure pad….the lower half is the only part that has a ‘half’ thread. In view of this should I smooth out the top pressure pad and adjust it down a bit..

the lead screw looks to be pretty good.

thanks

[Haydn CallowParticipant @haydncallow19480]

Lee Roger’s….I think you are spot on once again….thanks. Should I get the pressure pad half back to smooth ?

[David George 1Participant @davidgeorge1]

Hi the top of the leadscrew nut should be smooth with no thread as it is just to support the thread and should be filed and or scraped this will allow the saddle to traverse with the handwheel. The top piece should be adjusted till it rests on top of the leadsrew and locked there.The bottom nut can be repaired by screw cutting a phosfor bronze full nut with a round outside. This should be cut in two go make two halves one which could be sold as a spare. The arm will need boring on a mill to take the replacement half nut and silver soldered in its place.

David

[SillyOldDufferModerator @sillyoldduffer]

Posted by Lee Rogers on 27/09/2022 16:37:45:

I think everyone is wide of the mark here . One of those is only a pressure pad and doesn't need a thread at all the other is not too bad and in my experience with 4 Drummond restorations it will probably be OK as it is. Try it , if it works then it's ok.

The picture looks awful to me but lathes.co.uk supports Lee's opinion:

The nut (being of the "half" type and likely to be pushed out of engagement) was assisted by an upper thrust pad – a plain, half-round bronze unit retained by a massive bronze nut against the inside face of the apron; this thrust pad often has, due to years of work, the appearance of a thread cut into it – but it should be plain and smooth.

So I'd put it back together and see how well it works. Hopefully it will be fine. Spares for a lathe that went out of production in 1952 aren't easy to find and making a half-nut isn't a job for a complete beginner. Best to get a feel for metalwork generally and build up the workshop before tackling a repair like this. A new nut can be made, but not in half an hour with a few basic tools and no experience.

Dave

[Ady1Participant @ady1]

Yes I saw what I wanted to see, a knackered nut, but it was the top half, which is an unthreaded pressure pad

Your current nut looks ok for the moment, try to use the rack as much as possible to save wear and tear as you position the saddle up and down the bed, only use the nut for useful work purposes as much as possible

I'm actually having a go at a couple of babbit leadscrew nuts in the near future so will "report back" on how it goes

[Haydn CallowParticipant @haydncallow19480]

Would there be any point in cleaning up the pressure pad half so it is smooth ?
also if I could cut the thread in the threaded lower half 1mm deeper would that be a good idea ?
I am a retired Dental Tec with 50 years experience and am quite confidant with small hand motors ( like dremel&nbsp. I think I could with the correct width of cutting disc deepen the thread a little…..of course I could end up with a nackered half nut……

thoughts please.

in the meantime I’ve cleaned everything up and lowered the pressure pad a bit and it does seem o.k.

[Ady1Participant @ady1]

It's meant to be smooth so that would be fine

Saves it "catching" by accident if the main half nut is disengaged

It does actually help slightly if it's threaded because the leadscrew bends and twists under load, so it grips more under load and actually helps

Which is why its worn those partial threads

I would actually fiddle about until it's happy under load instead of removing those threads, but that's just me

Edited By Ady1 on 27/09/2022 18:21:47

[Clive FosterParticipant @clivefoster55965]

Your current bottom nut is well worn but still most likely reasonably functional. But probably have seriously excessive play in the thread between nut and screw.

As Ady1 says babbitt metal makes a perfectly adequate cast leadscrew nut. I believe the half nuts on my P&W 12 x 30 model B are cast babbitt. P&W had a well deserved reputation for making machines of the highest quality so clearly an eminently satisfactory way of going about things.

However the strength properties of epoxy steel, epoxy ceramic et al industrial repair materials are of similar order to babbitt. Perhaps 1/2 to 3/4 as strong depending on parameter and what specific materials you are comparing. Objectively more than strong enough in absolute terms.

Hence, as I suggested in my earlier post, applying some to the nut and pressing the leadscrew in to form an accurately replicated thread should work well to give a new thread of minimal clearance. Fundamentally an easier way of going about things as you can work cold and there is no need to provide dams to prevent molten babbitt escaping. Now we know you only have to deal with a single half nut the jib becomes much simpler than I suggested earlier.

Here is a link to a basic overview covering the Devcon range of such products :- **LINK**

https://www.aimsindustrial.com.au/assets/files/Devcon-Product-Guide-Catalogue.pdf

Devcon are a respected supplier of such materials to industrial users with a pretty comprehensive range. That guide gives a good idea as to what sort of "trowel it on" repair materials can be found at the more sophistcated end of the market once you start looking beyond JB Weld and similar consumer varieties.

Clive

 

Edited By Clive Foster on 27/09/2022 20:07:28

[Lee RogersParticipant @leerogers95060]

Posted by Haydn Callow on 27/09/2022 17:17:08:

Lee Roger’s….I think you are spot on once again….thanks. Should I get the pressure pad half back to smooth ?

Just give it a go. Nothing to lose . My approach to these very old lathes is to go gently and only change what is not working. If you want a precision to the last thou your looking at the wrong lathe. Get it just so and you have a decent lathe. Enjoy it !

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