Making a cross slide nut - Tips for a Newbie

[Pete H]

Hello,

I'm new to the world of Model Engineering but have just finished putting an old Myford 4" Precision back together and am itching to start making my first stationary engine.

The only problem I haven't yet solved on the lathe is the wear in the cross slide nut. It is very worn and there is a half turn of backlash. The screw looks good enough to get me going.

I have tried contacting Myford without success for a nut I could moify or a tap so have decided to have a go at making one. I have a lump of suitable phos bronze but thought I would ask for any tips or advice before I start. It is a 3/8 x 12 tpi square thread and I have attempted to make a tool but the tip is only 1mm wide and was very hard to form on a bench grinder. Also because of the reach required, about 25mm (1"), I'm worried it will snap or chatter.

Any help gratefully received.

Regards,

Pete

[SteveW]

Pete,

I'm surprised Myford can't help. Their support for old stuff is sort-of legendary.

I've seen articles describing a route out of your problem. This involves using the existing lead screw as the inside of a mold with some form of outside support. Then it's just a case of pouring some molten white metal into the gap and waiting. Quite how successful this is I don't know, maybe it just stems from desperation.

If you really what it make it and you don't fancy cutting the long internal thread maybe do it by first making a tap of the correct form in silver steel, tempered etc.. I've seen that outfit at Dartmouth do various sizes of ACME taps. These might be a slicker bet if you have money.

[steam4ian]

G'day Pete.
Have you tried working over the existing nut? Cheaper lathes use dodges like tilting the nut or slitting the nut to control its tightness on the screw.
The Chinese mini lathe tilts the nut to take up the slack. This works after a fashion. Some owners have cut across the nut cutting it in half and arranged adjustment by spreading the nut apart.
The Taig lathe has a longitudinal cut along the nut and "adjustment" is made by first squeezing the nut in a vise, crude but apparently it works.

Regards,
Ian

[Pete H]

Thanks for your quick repies guys:

SteveW
I haven't managed to contact Myford. Their phone is either directed to fax or just rings out and emails get bounced back as their mailbox is full. I don't know how they manage to sell anything. I will keep trying now though after you say they may be able to help. Not sure about the casting method but I might try making a tap......I hadn't thought of that. Do you think I would need to anneal the bronze before tapping it??

Ian
Thanx for the tip but the nut is so worn I'm worried that the little thread that is left might shear off during heavy work.

[ace]

Hi Pete

I had the same problem on mine. So to lose the backlash try this. -- you need a new feed nut from Myford part No70/1302 - metric A1648 ( i got mine at the Harrogate show last year from Myford Stand). when you have it cut off the barrel part leaving the triangular piece with the two holes, file the end smooth, then remove the two feed nut screws from the saddle and offer the new nut up to the face of the old one. replace the two screws and tighten up loosely at first. Replace the cross slide feed screw, this should now feel tighter and then place the end plate screws in the cross slide and tighten them up. if this has done the job tighten the feed screws up. as the wear begins over the next few years just file a bit of the new feed nut replace it as above and it will be tight again. Mine has a very smooth feed to it since I did this.
Hope it helps you out mate.

ACE

[abby]

Steve's suggestion is a good way to go , I have made new half-nuts and cross-slide nuts by this method and can testify that they give very good service , many small lathes had original nuts made from white metal.

[ace]

Hi Pete


On the note you say the feed screw nut is already worn then you will have to buy 2 feed nuts to carry the method I described before. I did but two at the time as I realised this was the part that will wear first not the feed screw its self. I have checked Myfords web page and the parts are listed @ £11.08 inc vat. My feed screw had very little wear on it so you should only need the nuts.

ACE

[Pete H]

Thanks Ace,

Hopefully I'll see Myford at the Guildford show at the end of this month so I'll have a look at that part number you gave me. I'd like to try and use it all if I can but thanks for the 'split nut' idea. I might be able to make use of it.

Pete

[Pete H]

Abby, Thanks for your reply.

Can you tell me where I can find or buy 'White metal' and how do I melt it???

Would a few old car engine main bearing shells do??


Pete

[ace]

Pete

Contact Myford on 0115-925-4222 or email them on [e-mail] and order the parts for you collection at the show. They do a great little book for the ML7 'operation installation and maintenance' gives you pictorial parts list as well, very useful and detailed.

ACE

[chris vine]

Hi Pete,

Some years ago I acquire a box of "junk". In it are some acme (square) thread taps.

Tomorrow I will have a look and see if there is 3/8 by 12 tpi amongst them. You would be welcome to borrow it if it exists......

Chris

[Pete H]

Thank you Chris,

That would be a great solution for me!!!

Pete

[abby]

If you had enough bearing shells you might get sufficient for your needs but the bearing metal is a very thin coating on a steel backing shell - unlike your lathe mandrel bearings.
The alloy I used was tin/antimony/lead in the ratio 10/15/75 , you could substitute type metal although the exact alloy will be unknown , but the parts are a piece of p*** to make so you can replace them easy if they wear too fast, The stuff melts easily , like lead , and after 2 or 3 tries you will get a good un ! there are suppliers of these metals on the web but if you get electrical solder you have 60:40 tin:lead then you need some more lead to lower the tin and some antimony , I have some type metal if you are stuck.

[SteveW]

Pete,

It occurs to add, if you do go down the casting route, to give what is in effect a die (as in die-cast) a good heat to above 100C before getting any runny metal in there.

The aim is to dry out any moisture before the runny metal does it for you and causes a blow-back with attitude.

[Pete H]

Thanks Steve,

I found a local shop with some obsolete white metal figures so I might get those, melt them down and see what happens. I might try making a tap first although I don't have any milling gear so will have to rely on a file (or maybe an angle grinder) to make the flutes. If this fails to cut the bronze then I'll start casting.

Can phos bronze be annealed?

[AndrewP]

Pete
Most copper based alloys can be annealed by heating to red and water quenching once the red has gone, jeweller's bronze certainly can. I don't see why phos bronze should be any different but be aware that some alloys go straight from red to puddle on floor very quickly
Angle grinder sounds the ideal tool for making the flutes to me!

[chris vine]

Hi Pete,

Sorry for the delay in replying, and even more sorry that my box does not contain a 3/8 x 12 acme tap. Lots of other sizes but not that one. Some are even left handed which might be useful oneday......

Chris.

[Pete H]

Thanks for looking Chris, I appreciate it.

I have spent most of the day trying to make a tap from a bit of 3/8 silver as Steve suggested and considering it's my first ever project I'm quite pleased with the results so far.

The big test will be tomorrow when I try and cut a thread with it.

Thanks for all your help and suggestions guys!!

I'll let you know how I get on.

[steam4ian]

G'day Pete.

Well Done! Have you hardened it yet? Please let me know how you did it. Did you use the angle grinder to cut the flute/s?

Regards,
Ian

[Pete H]

TA DA !!!!!!!!

Three home-made taps later and I now have a lump of bronze on my cross slide screw, nice and free but with no play.

It was a real trial though as the taps kept binding and twisting. I ruduced the amount of 'thread' on the last tap with the angle grinder and it seemed to free up a bit.

Ian, I bought a piece of 3/8" silver steel, the correct OD for the tap, held it in the chuck and then supported the other end with a tailstock centre. I turned the reduced shank 0.4mm under the tapping drill size to allow for clearance and act as a register for the thread depth. I then turned the thread, removed the tailstock and turned a taper on the end. I started with 7 degs but ended up with about 2.5 degs to give more of a lead in when cutting the thread. I then parted it off and ground a flat on the top to hold in the tap wrench. I used an angle grinder with a worn metal cutting disc, i.e. only about 2.5" dia so the cutting speed would be lower. The angle was quite agressive but it was all I had. I identified which edges would be the cutting ones and tried to 'under-cut' them slightly to get a good cutting angle. I then heated them to cherry red or just passed and quenched them in water to harden although I don't think this seemed to work too well as they kept twisting.

Any way job done now. Thanks again for all your help and ideas.

Pete