Allchin Steering Gears

The worm and wheel steering gears on my 1.5" Allchin are commercial items, bought, years ago, from Reeves. They are 4-start. This makes for quick steering action but don't look very prototypical with a worm of rather small diameter and this has always irritated me, so I'm considering making new ones which seems to be an interesting mini-project.

The Hughes' design calls for a 2-start worm of 5/16" lead, of which he writes "screw-cut in the ordinary way". Same for the hob. Even single start at 5/32" lead seems daunting to me. Has any-one done this on a typical model-makers lathe? Seems a recipe for possibly chewing up the change gear train.

Edited By Clive Brown 1 on 27/09/2022 15:38:36

If you drive the leadscrew by hand it won't overload the drivetrain

In general single start is prototypical; anything else will back drive. On my engines I threw the drawings away and designed my own single start worm and worm wheel. On my 4" engine the worm is 1.5" OD, so 1/2" seems a little small for 1.5" scale. Can't help with the screwcutting as I CNC milled my worm. These are finished gears:

Andrew

Prof. Chaddock suggested driving through the leadscrew for the Quorn column thread, but he used a milling spindle for the square thread form rather than a single point tool, which would still require a lot of torque through the gear-train to turn the chuck. My Boxford has Tufnol tumbler gears and I don't want to break them.

Nice work Andrew. Hughes suggests 2-start as a compromise between steering and appearance. I'm inclined to go for single start to make things easier, but the job's still at the "thinking about it" stage.

Jumping off from your observation above, could you use a milling machine to thread mill the worm?

Let us assume it has a 5 (or 25/5) tpi leadscrew: the required tpi of 5/16" lead is 16/5 tpi. So you gear between the leadscrew and a simple spindle holding the blank (e.g. spindexer, vertical rotary table with worm out of mesh) in a 16:25 ratio (32/50, 48/75 etc.). Any old idler gear(s) to make up the height between leadscrew axis and workholding spindle axis.

It is like a very basic version of spiral milling using a dividing head.

You could rough out the blank using a normal mill no bigger in diameter than the root of the thread. Then make a D-bit or buy a tapered endmill with 30 degree included angle. As your gear ratio has an even number in it, it is easy to index for the second start by disconnecting the gears and rotating 180 degrees.

The drawings for my engines called for a 2-start 1/2" pitch, 1" lead worm and a mating spur gear but with teeth on a slant. However, the dimensions didn't add up and information was lacking, so I re-designed, sticking with a 2-start worm. My lathe will cut a 2tpi thread, but not 1tpi. So I decided to use the CNC mill to make the worm. On another forum it was pointed out that a 2-start worm will back drive. So I changed the design to single start and also a single enveloping worm wheel to match full size. I stayed with using the CNC mill for the worm, as I was concerned about using the lathe to cut an Acme full form 2tpi thread; the lathe leadscrew is 4tpi.

The worm was roughed out using 6mm and 4mm end mills and finished with a 15 degree (half angle) tapered commercial endmill:

A similar process was used to make a hob from silver steel:

The worm wheel was gashed and then free hobbed:

The dividing head is used to gash the worm wheel but plays no part in the free hobbing. The worm wheel is free to spin on the arbor.

Of course the above is all rather long winded, but I was interested in the design process, and to see if I could make a matching worm and worm wheel.

Andrew

I suppose the big question is do you intend to use the Alchin to pull you around a lot or will it be more of a display model when finished. At 1.5" it's not going to cope well on grass so you would be limited to events on flat tarmac and also be happy bent double trying to run it. In this case the 2 start would mean less handle twiddling so be the better option

If just for display then a single start would be easier to cut and you are not worried about the amount of handle twiddling to steer it

My 2" fowler is a 2 start 2tpi but I bought the worm & wheel

Edited By JasonB on 28/09/2022 11:51:33

I have just cut some 6tpi square threads in freecut steel on 20 mm bar. Not quite the same as yours, but close if you go for the single start option. It wasn't a problem, but grinding the tool is most of the job. Make sure the tool is tilted over to the mean helix angle and generous side clearance.

You will also have to make a new wheel to match the new worm.

Martin

It's worth consulting a book like Ivan Law's Gears and Gear Cutting, which goes into detail on correct tool shape and setting, etc..

An alternative method for cutting the second of a two-start thread, avoiding rotating the work-piece (e.g. by re-setting the change-wheels half a turn round) is to advance the top-slide by the pitch.

This is a problem I am facing, twice in fact.

1 - to make a 4TPI single-start worm as replacing the missing power-feed on a Denbigh H4 horizontal mill, keeping the exiting worm-wheel.

2 - For my steam-wagon. This has Ackermann steering but with no surviving works drawings or wagons, I've to "design" it as I go. The worm and wheel on there at the moment is of rather light duty so a temporary rig. Investigating, suggests a 1/4" lead worm and wheel but that would give at least 4 steering-wheel turns lock-to-lock, which seems excessive. Or a screw and nut for which I have a worm from I-don't know-what (washing-machine I think), of an odd 0.45" lead 2-start so giving about 2 turns on the steering-wheel.

So 1) Cutting a new worm, probably from free-cutting mild steel (I'm never going to wear it out!),

2) Cutting a hefty, internal, 2-start ACME-like thread in phosphor-bronze or cast-iron.

The Harrison L5 lathe will cope, according to the Operating Handbook that also says, with great care. The change-wheels for that 2.22 r. TPI thread work out at an exact 100 / 45 teeth.

I've investigated the model-engineering suppliers but none list steering worms and wheels!