6 tpi leadscrew

[DavidA]

Ok, I've been out of the machining for a while,  but now that winter is coming I intend to crack on with my Fowler 4f and hopefully have something to show when next seasons running starts.
So I was thinking about screw cutting on my mystery lathe (no one seems to know what it is) and decided to check the lead screw.
It has  a pitch of six tpi.
Now, one of the things I have to make is a calibrated wheel for the end of the leadscrew as the wheel on the saddle goes the 'wrong way' and is also very high geared.
But six tpi gives me a saddle advance of 0.16666 inch per revolution.
I clamped a clock on the bed and checked this. It is so.

Then I wondered, 'is it metric' ?
That would give me 4.198 MM per revolution.  Still seems strange.

So,  any ideas ? Why has such an odd lead screw been fitted. And how do I set about cutting standard threads with it ?
Plus how do you suggest I calibrate the wheel ?

Regards

Dave.

[NormanV]

Instead of calibrating the wheel why not fit a digital read out, then you would have metric and imperial at the flick of a switch.
In "The Amateur's Lathe" by L.H. Sparey there is a screwcutting chart for a 6T.P.I leadscrew in the appendix p215 of my copy.
I could send you a copy if you wish.
Norman

[Pete W.]

Hi there, Dave,

Depending on the anatomy of your lathe at the RH end of the lead-screw, it might be possible to fit a couple of gears of suitable ratio, one to the lead-screw and the other to a hand-wheel.  You might be able to make one of them a sprung anti-backlash type (same principle as shown on one of the CNC mill conversions on this site).

Hand-wheels with scales do crop up on eBay from time-to-time (I know, I've just bought a couple for my little horizontal mill); you should procure the hand-wheel first - that will decide the gear ratio you need.

I've just received a message saying that someone else has posted while I've been typing this - sorry if I've doubled up on their suggestion.

Best regards,

Pete W.

[andyf]

As Pete says, it is possible to have a gear on the leadscrew, driving a dial on a separate shaft.  That's what I did with my 12tpi (0.0833" pitch) leadscrew, as shown here:
 http://andysmachines.weebly.com/leadscrew-handwheel-and-dial.html.
After a while, I have got used to the dial rotating in the opposite direction to the handwheel, but it does help to put a splash of paint by the zero on itl, to help count the number of full turns.

I often wonder why so many lathes have awkwardly pitched leadscrews. Even the often-encountered 8tpi (0.125" per turn) requires a bit of mental arithmetic if you want to turn something down over a length of (say) 0.60". It must be a hangover from woodworking, where 1/8" etc is used more often than decimal measure. 10tpi would be far more convenient.

Andy

[DavidA]

Norman,

Great minds think alike, and so it seems, do mine and yours.

I was just looking at the arc catalog for DROs. That would be the best way to go,  and the quickest.

But I still like the old calibrated scale.

I'm going to add Sparey's book to my library in the near future, But thanks for the offer.


Pete and Andy,

A geared scale would work.  A variation would be to use a scale on the end of the lead screw and let the readings finish at 0.166 Inch . Rather like the odd z axis scale on my X1L mill. It took me a while to realise what was going on there.

Let's see.  Dividing into 166 divisions.   Hmmm.

All in all,  I think I'll go for the DRO option.

Thanks for the help.

Dave.
But it would be awkward to make.

[DavidA]

Norman,

On second thoughts,  It would be a great help if you could send me a copy of the relevant page(s) .

I'd like to get a couple of boring tools,  and it would help if I leave the book purchase until later.

The Email address is in my profile.

Thanks

Dave

[DavidA]

Norman,

I know have a table of changewheels for the 6 TPI leadscrew pinned to my shed door. On the inside,  before anyone should ask.

Thanks again for your trouble.

Dave.