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| 6 tpi leadscrew |
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| 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. |
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