Smart and Brown Lathe Crosslide Screw Renewal

[Darren]

Another new project for me,

I guess some of you remember me whittling on about my worn crosslide screw on my S&B lathe.

In case you forgot, it looks like this, a little past it's best   



Well I decided to do something about it and purchased a 1M length of 10mm x 2mm pitch Trapezoidal Spindle and a matching tap for cutting the nuts to go with it.
I did consider cutting my own screw thread on the mini lathe, but decided that a it might be a bit ambitious for me at the mo and a proper rolled thread would not only be finished better but last much longer as well.
The original thread on the lathe was 3/8 Acme at 8tpi. I decided to convert to metric for no other reason than I understand it better than imperial. My dials have 100 graduations so no probs there.



Commonly known around these parts as a "Crap-o-CAD"



This is a piece of 20mm 303 SS bar, soon to be the "handle" end of the screw.



Drilled and tapped 8mm 1.25 pitch



Now the screw thread, turned down and threaded 8mm 1.25 pitch to match



Viola.... they fit, not that difficult so far... 



A little bit more work and



As you can see I have made it 2" longer, the slide can take it and I hazard it may come in useful sometime. This lathe originally has a long crosslide but a short screw? not any more it doesn't !!

I still yet have to decide how to fix the threaded section into the "handle" end, locktight, cross pin or braze, maybe just solder will do? Any suggestions chaps?

Also the 3/8 thread needs doing for the slop adjusters, but that will have to wait for the mini lathe to return from the Doc's. Be the first job it'll do. 

Well, I hope someone found that interesting....

Darren

[Brass_Machine]

Nice start Darren.

Keep the pictures coming!

Eric

[John Stevenson]

Why didn't you saw the old end off the knackered screw and stub it into the end of the new thread ?

John S.

[Darren]

I did think of that,

But I needed the old one to make the new one, would have been a bit difficult without it....

Apart from that, the old one has some over sized clearances which I intend to tighten slightly.

Darren

PS, I could have done simply cos other the other tooling I have with this lathe, but, but....do I need an excuse, 

[bogstandard]

Darren,

Go the way of belt and braces, cross pin after assembly with a touch of loctite.

No need for anything spectacular.

Measure up a nail (of the hammer it home type), drill hole to same size, put nail thru with a dab of loctite, trim off a bit longer than needed, then wack nail until it swells in hole, clean off the hammered end bits, et voila, one solid fixed end, and if anything untowards happens, the soft pin will shear, without hopefully, any damage to the threads.

You have to read it all in one breath

John

[Divided he ad]

Carry on with this one Darren,

Sooner or later some of us will be doing similar to our machines.... It's always good to have a reference article to browse to help you on your way 



Ralph.

[Darren]

Thanks guys, I will continue later...

One thing I forgot to mention was the the "handle" end was machined to size after fitting it to the thread buy holding the threaded section in a lathe collet.

This should make it true to the thread.. .

Next job it to make the nuts, thinking of what material to try here....back later...

[bogstandard]

Darren,

The accepted material for the nuts is bronze (PB1), if you try brass, I think you will find it will wear very quickly. Steel is definitely a no-no, the nut and screw are liable to gall and friction weld together.

John

[Darren]

Opps, bit of underlining there...!!

I have some PB1 for one nut but not big enough dia for the other (I could get some of course), but I was thinking of trying Delrin or Teflon both of which I have.

I thought it might be interesting to give them a go?

Darren

[bogstandard]

Darren,

You have to remember, the lead screw and nut is not just for positioning, they take the full force of the cutting action. The metal is pushing against the two parts because it doesn't want to be cut, you are forcing the cutting tool into the metal.

That is why you have all your correct cutting angles and clearances, it helps to take the pressure off the cut, so the machine doesn't have to work too hard.

You can try it, and prove me wrong, but I suspect you will start to have trouble almost straight away with the finish, as the tool bounces backwards and forwards because of the flexibility of the nut. You could eliminate the pressure on the nut by tightening the jib lock before starting the cut, but that is not an ideal situation.

John

[Darren]

Good point John, Teflon deff out then, I read somewhere that it's neither a true solid or liquid, something in between, like glass.

But you could have a point with Delrin as well...umm...

[CrewCab]

Opps, bit of underlining there...!!

You can always nip back in and edit your post Darren

Phosphor bronze has been used for this sort of application almost since the stone age, and for good reason I suspect. ........... I used Delrin for the cylinder of a tiny model engine and whilst it ran well enough the thread for the securing bolt didn't last too long.

CC

[Darren]

Thanks, I was just thinking of experimenting just to see and learn really.
Anyway, for tonight I cut one of the nuts, from PB1 btw

Started with a piece of scrap that I'd been playing with recently



Turned it down to size and threaded it, thought I'd taken a pic but it seems not....

It needed a retaining slot to stop it spinning, I'm a little low on milling cutters and didn't have one small enough (some on the way in the post)
So I decided to give this a go in the lathe. It's a HHS parting tool on its side.



Not the neatest job I'll admit, Had to take very light cuts and take it real slow.....but it seemed to have worked.



It even fits the thread too, handy that...... Feels nice and smooth as well, the original screw was kinda rough.



Well that's it for tonight, a bit further on. Now I need to order some 35mm PB1 for the other nut....

Darren

[bogstandard]

Darren,

That is looking real well.

I see you are discovering methods all by yourself. Cutting the one off side groove on a lathe is almost exactly how it is normally done.

I see we have a natural machinist coming along here.

John

[Darren]

"almost" feel free to educate me 

I seem to remember seeing it somewhere cutting internally, broaching I think it was called? Possibly cutting a gear.

[John Stevenson]

What about using the phossie nut you have made and then using a Delrin nut spaced out from the phossie one with those dished washers.
That will provide zero backlash without relying solely on a soft Delrin nut.
You can make spare up at the same time for replacements down the line.

The dished washers will keep a preload on the whole shebang.

Just guessing here as i don't know the build up on a Smart and Brown lathe

John S.

[Darren]

Hi John, interesting idea, but not sure it will work in this case due to the design of the machine.

Thanks
Darren

[Darren]

Wow, this one had to have the cobwebs blown off.....

Well a little more progress today, first to make the backlash nuts shown in this picture from the SS bar end below the two nuts.



Roughed out with and old tip in the funny orientation holder to use the "other two" cutting edges.



Final shape with a new tip. I'm just putting a groove in for the friction spring that tensions the indicator dial.



Drilled,counter drilled part way and tapped 8mmx1.25.
The original was 3/8 with a 20tpi thread. 20tpi is almost 1.25mm metric pitch, so I chose an 8mm thread as the pitch was more important than the diameter...



I also threaded the leadscrew 8mm at the handle end by single pointing on the mini lathe as i don't have an 8mm die. Sorry forgot to take pictures of this 


Set up in a 5C collet fixture to drill a 3mm hole for the locking pin of these two parts.



Counter sunk with a 6mm spotting drill and then turned a 3mm brass pin. I chose brass in case I ever wanted to take it apart. Easier to drill a soft brass pin out than a hard steel one.
Also brass would be easier to beat into the countersunk areas to secure it.





Now beaten into place. The pin should also swell inside the hole to make a nice tight fitting.



This is how it all fits together. You can see the old two backlash adj nuts in the bottom of the picture. They have holes in them to insert a bar to open and close them for adjustment.
Firstly they can be rather tight, and with a small bar (drill bit) it can be very difficult to undo them. Secondly the drill bits usually bend 
Thirdly the holes become distorted in time making them useless....

So what to do? Mill two flats for a spanner fitting, drill similar holes but make a pair of proper bars, say a short 4mm diameter to fit the nuts and then expand to 6mm or more. Would be easier to use than drill bit 

Or something else, any suggestions welcome...

[John Stevenson]

Slots and a C spanner ?

John S.

[Darren]

Thanks John, I did consider that but the thought of making a c-spanner has put me off. I'll prob go for simple holes but make a proper fitting pair of keys/bars. Should cause less damage that way I think.

Indecently, I recently bought a Drill Doctor drill sharpener and this is the first real chance I have had to test the results. I'm happy to report that the sharpened bits drill better than some new ones I have, seem smoother. But the split points that this unit is capable of producing is a real eye opener. You can push a 10mm bit through a SS bar as if it was a 5mm bit with no smaller counter bore hole or center drill first.
I'm really impressed with the sharpening results.

I have re-sharpened about 200 drill bits so far so I consider it already paid for. Also I sharpened some snapped drill bits to make some stub drills to benefit from the extra rigidity.

A tool worth having IMHO....

[Darren]

The next part to this saga is one I have been agonising over for months. I just didn't have a piece of leaded bronze big enough....

For months I have been watching Ebay for a piece, but nothing has come up. New prices, well you just don't want to know....
I needed a 2" round and when they get to that size it just gets silly for making a simple nut.....!!


I have spent 6hrs today polishing my new concrete garage floor that was laid yesterday. That's on your hands and knees floating 24hr old cement. Floating, wetting, floating, wetting and polishing in an effort to get a smooth floor ready for painting when it has fully dried.

Anyway, some sanctuary in the workshop was called for, alone where it's quiet  

Then it struck me, I already had the material for this nut, I was just looking at it the wrong way.

I bet I've lost you all by now so better show a pic of what I'm on about....

This is what I want to make from this bar, but the dia is too small



But if we look at it this way on....



On the miller to machine a flat, I need somewhere to calculate from, a datum if you like



Turned it over to machine the other side flat, carefull not to take too much off



Center punched the position for the threaded spigot, and set up off center on the lathe in the four jaw chuck



Don't brass chippings just love to fly everywhere at great speed. Here's my patented brass chipping barrier   An old shop leaflet display re-shaped and help down with a microwave magnet....



Spigot now turned



Threaded with a trapezoidal tap, 10mmx2 pitch



Trimmed to length, note a tiny pip of the original bar curvature, couldn't be helped as there had to be compromises due to material size



Out of the four jaw, turned around and into a collet



I forgot to take the next pic, sorry, I simply rounded off the backplate a bit to size

Back on the miller in a collet fixture to machine two flats



And voila.....!!
The flat on the bottom is of no consequence, another compromise, but not an issue.





All that's now needed is two curved slots.....gulp....

That means the rotary table that I've yet to figure out how it works and how to mount this piece. I'm thinking MT2 blank end drilled and tapped to hold this nut?

[bogstandard]

Wonderful stuff Darren.

You are starting to look at pieces of metal how they should be looked at in our game. Forget what shape the original metal is, imagine the piece you want being in there. Then using your ingenuity to make it come out.

Now you have to take the plunge Darren, and get the RT set up. This just might be the push you need to get you working in another dimension. A rather different and sometimes difficult learning curve, but once you get the hang of it, you will wonder how you ever made things without it.

John

[Darren]

Thanks John,

I was quite pleased with myself today. Firstly I was just sitting there having a coffee and a smoke, as you do in your own little space, when it struck me.
Secondly cos it worked out quite well.

Next moment I get I'll set up the RT and have another "ponder" on how to proceed...

[Divided he ad]

Top job 

It'll certainly do it's intended job   



I think your RT set up idea sounds about right..... As I see it all you need is a slot drill of the right size (usually 2 flute) to make it a little easier!




You'll figure it out no worries Darren 





Ralph.

[sbwhart]

Nice Job Darren well done 

You'll have no problems with the RT, the M2 tapered mandrel is the way to go, I've used a similar technique myselve a number of times.
 


Have fun

Stew