Machining for the Brave!

[HopperParticipant @hopper]

Posted by Nigel Graham 2 on 25/01/2023 14:24:10:

Ah! Thank you for that tip on cutting multiple threads, Hopper!

Among the ways I am investigating for my steam-lorry's steering is a screw and nut but it needs be at least 1/4" lead, so a 2-start 1/8" pitch, 1/4" lead screw would be best, and within my larger lathe's abilities as its lead-screw is of 4TPI.

(As long as it's within mine too, we'll be right!).

You're welcome. Yes advancing the topslide one pitch is easier sometimes than marking change gears and rotating the job 180 degrees etc. Multi-start threads are fun to do. As with any thread, I do the nut first to the theoretical specs, then do the male thread and machine it to fit the nut using bearing blue to read where it is binding at the final stages. You do need a lot of clearance angle the leading flank of the tool bit for that coarse helix too.

[Nigel Graham 2Participant @nigelgraham2]

Thankyou for that!

I envisage using a tool ground from round material to simplify the clearance problem, and having sculpted the basic form by off-hand grinder, using the tool-&-cutter grinder to finish accurately.

I've some simple round-bit holders I'd made originally for the little EW lathe, but if they give enough clamping area and clearance there's no reason they can't be used on hefty lumps like the Harrison L5.

The grinder is a 'Worden' design, Hemingway kit.

I'd thought cast-iron or leaded bronze for the nut, mild-steel for the screw, but make an aluminium lap to finish the screw to a decent surface. Or possibly silver-steel screw hardened and tempered (molten-lead soaking is is about blue-purple temperature) then lap to polish. I used an aluminium lap held in a big die-holder, and valve-grinding paste mixed with oil, to finish the crankshaft ends.

[HopperParticipant @hopper]

You probably don't need to harden it for the amount of use it is likely to get. Not going to be commuting on it daily, I presume! Make life easy for yourself and use some nice free machining leaded steel. Silver steel can be a bit awkward to screwcut nicely. You should be able to get a good enough off-tool finish with freecutting steel to do the job, with the usual running of a file along the thread tips to knock the burr off. You get the final smooth finish on the flanks by using the topslide to take tiny tiny cuts off each flank (allowing for backlash when setting to clean up the trailing flank). Will it be an Acme thread or square? Either way , be sure to leave clearance space at top and bottom of threads so it bears on the flanks only. The when you get to the specified thread depth, if nut does not fit, skim the flanks until it does.

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