Allchin 1 1/2” scale Traction Engine Shaft Manufacture

I would value the opinions of fellow modellers, and particularly from anyone who has built the Allchin Royal Chester Traction Engine. My problem is how to manufacture the external splines on the second shaft in the gear train, the shaft is approx 5” in length and it has splines for about a 2” section. These splines are 1/8” wide and there are four spaced equally about a 17/32” diameter shaft, their purpose is to locate the sliding hub with spur gears mounted on it to effect changing the fwd speed of the machine. Unfortunately I do not have access to a broach, nor do I have the means to manufacture and use one. My first thought was to slot drill the shaft and use 1/8” key steel silver soldered in to the slots, then my thoughts progressed to a vision of the shaft ending up like a banana with all that heat applied, so I thought the key steel could be soft soldered into the slots or even use the correct engineering adhesive to secure the key steel / splines, once assembled with the sliding hub there is no way the key steel could part company with the shaft, I realise that the splines would need to be a good tight fit in the slots, or has anyone got any thoughts on this or of any other method of achieving the objective. Dave W

Hopefully Andrew will post his pictures of the proper way to do it. In the mean time the drawings for my 2" fowler show the method you describe with separate keys soft soldered into slots which is the way I did it.

Hi Jason, many thanks for your reply it looks the way to go, your shaft looks very smart. Dave W

I made mine over 50 years ago,so without loctite or other adhesives, I milled 4 equi spaced slots and fitted mild steel keys ,they were secured by long rivets,nowadays I would use high strength Loctite, or machine the keys out of solid,which if I remember correctly W hughes suggested machine from solid in his write up in ME.

W Hughes, ME vol139, wrote that some builders silver soldered the splines into milled grooves, though he didn't seem to really approve, possibly non-original.

His preferred method was making a "horizontal" milling cutter from case-hardened mild steel to cut the splines on the shaft and also a matching broach, also case-hardened, with photos to prove that it worked. I'm surprised he didn't use silver steel

IIRC, I used a combination of end-milling and planing in my long-suffering old Grayson lathe, with dividing via a change-wheel.

Whisper it not, my splines only fit in in one position, but otherwise OK and WJH himself admits to the same, so I'm in good company.

Nigel & Clive thanks for your replies,I will go with Jason’s suggestion of milling slots using key steel soft soldered in place, I know that W Hughes would probably disapprove but as long as it all fits together I shall be happy, even if it’s only in one position. Dave W

Edited By Samsaranda on 01/08/2021 09:24:21

When it came to machining the 6-sided splines on my Burrell SCC crankshafts I considered the slot 'n' key method, or using an offset endmill to form the sides of the spines and then nibbling out the space in between the splines. Neither seemed satisfactory, or prototypical. It is possible to buy commercial spline cutters, but none were available in the size needed. So I designed and made my own, from gauge plate which was hardened and tempered:

The splines were cut on a horizontal mill plus dividing head, the same setup as for gear cutting. Finished splines:

The internal splines were cut using a HSS toolbit and a slotting head with rotary table:

A length of HSS was ground on a cylindrical grinder to get the correct end radius (by measuring the diameter) and then cut in half and clearances added by hand:

The internal HSS toolbit is a couple of thou under width so final fitting of the gears to the splines was done with files. I made a test piece on scrap bar, and a messed up gear blank, to check out the process beforehand:

These are the finished splines, and outer gear:

Note that there is clearance on the inner and outer diameters, so the splines drive purely on the sides. The gears fit in all six orientations, although the shake varies from nothing to a little in the different orientations.

I made a few mistakes along the way. I didn't support the middle of the crankshaft when cutting the splines. Consequently the splines vary in width by a thou or two along their length. Each gear only works on half the spline length, so they are hand fitted to that length. The inner gear goes all the way along the splines, while the outer gear will only go half way down the spline. Originally i made a set of gears with a lot of inner clearance and with a straight ended slotting tool. But they didn't look right, so I binned them and made another set. In addition I hadn't got to grips with machining EN8 at the time, so the finish on the first set of gears was poor - another reason to recycle them.

Andrew

I made the cutter as indicated by Hughes, it worked perfectly, the gear hub slid on without the sleigh test bit of binding. Still got the cutter somewhere in my specials box!

Andrew, as always I am impressed by your attention to detail, your methods have given me much food for thought, however I lack a horizontal mill so manufacturing options are limited, having received information on the various methods it looks like I will be going down the route of the slot-n-key method, here’s hoping that the splines index correctly. Dave W

As the Allchin is less than half the size of Andrews it should be possible to do it with a rotary table or dividing head provided tailstock support is added and then use the form tool in a vertical mill, you could even rough mill most of the waste away and just use the form tool to finish.