Cutting dogs on starting handles
|martyn nutland||20/03/2021 18:25:12|
|133 forum posts|
I'm wondering if anyone who has done this successfully could give me a steer; my efforts have not been entirely successful? I'm trying to cut dogs on the starting handle shafts of Austin Sevens. This is the type that have a solid male on the shaft and a matching female screwed into the crankshaft nose - not the later type that has a large hollow dog on the crankshaft side to mate with a peg on the handle's shaft.
All along, by various 'sophisticated mathematical means' - eyeballing, protractor, paper and Sharpie, I've 'calculated' the slope of the two teeth is 20°. Though accuracy is impaired by nearly 100 years' burring, etcetera, on the pattern.
First off, I shaped the two teeth by sawing out opposing segments at the prescribed angle across half the shaft diameter then finessing with a file. But I found the bite on the crankshaft was tenuous and intermittent and certainly not nearly sufficient to 'swing' the engine - or even pull it briskly over for starting.
Since then, I've set-up the shaft at 20° in a swivelling vice on a milling machine
and cleaned up and sharpened both teeth with an end mill. It has definitely improved the situation and the handle will pull the engine over part way (possibly until it encounters a compression!). Yet, there's no way the starting handle would bite sufficiently to start the engine.
Any thoughts, please.
|Howard Lewis||20/03/2021 18:38:33|
|6005 forum posts|
The faces that drive the engine should be square to the shaft. (i e Axial )
The angled faces allow the engine to overrun the handle when it fires and runs up, like a ratchet and pawl.. So the angle is probably not critical, possibly nearer to 45 degrees would be suitable..
As the engine fires, the "square" faces separate, and the angle faces push the handle away from the dog, so that the handle does not rotate and injure the person acting as "self starter"
What breaks your thumb, if it is not on the same side of the handle, as the fingers, is when the engine kicks back and drives the handle in reverse, via the square faces. With the thumb alongside the fingers, the worst that you get is a rap on the knuckles, when the handle is kicked out of your grip.
Howard. How can I make SO many typos?
Edited By Howard Lewis on 20/03/2021 18:41:00
Edited By Howard Lewis on 20/03/2021 18:42:04
|john halfpenny||20/03/2021 18:58:53|
|232 forum posts|
Could the crankshaft side no longer be 'square' ?, so that the handle cannot apply sufficient torque without being forced out of engagement. IIRC, this is a problem for which an upgraded male/female is sold by A7 specialists.
|Dave Halford||20/03/2021 19:13:45|
|2004 forum posts|
One suspects the crank end may be damaged.
If you get the ramp too shallow it wont engage far enough, if the ramp it too steep it wont matter.
If the crank end is eaten away you get the same effect.
I'm afraid my only cranking experience was a low comp Range Rover V8, if you found the sweet spot it was easy
|Speedy Builder5||20/03/2021 19:44:30|
|2590 forum posts|
This worked for me.
|noel shelley||20/03/2021 21:51:57|
|1278 forum posts|
The Austin Champ (RR B40) had a 3 dog version that got me home many a night ! my fellow workers made the dog and I made the crank handle. The RR B60 had the same set up but longer. Noel.
|bernard towers||20/03/2021 22:17:10|
|568 forum posts|
Try Machinery Handbook. I have done it a couple of times for internal handlebar controls for vintage American bikes, it’s a bit of a faff but involves gearing your workpiece to handwheel turns which in turn ( sorry) moves the x direction giving you you’re spiral which self ejects saving your pinkies
6188 forum posts
Cant see you'd need a spiral to make one like in Speedy Builder's picture. Appears a flat plane he has cut rather than a spiral. That should do the job. It is only clearance for when the motor starts, and does not affect the "grip" of the dog's driving face.
What is critical is the flat driving face of the dog. When making similar ratchet drive mechanisms for old motorbike kickstarter drives, we undercut that face on both halves by a few degrees, maybe two or three. This ensure positive engagement and is pretty standard on drive dogs in motorcycle gearboxes that use the same principle for main power transmission.
Two flat faces bearing squarely together can tend to half engage and then slip out of mesh, Two slightly undercut faces will tend to slide into full engagement under pressure and motion. Highly critical when your entire body weight is perched on the kickstarter lever and your knee will turn inside out if the drive ratchet lets go mid kick. Guys have ended up in hospital from it with what the doctors call hyper-extended knee. Usually on older model Harley Sportsters which are notorious for it.
If you have already machined your part, and have the matching part on the end of the crankshaft already, some careful work with a thin disc in a Dremel can put the slight undercut on each surface. A check with bearing blue to make sure both faces on both pieces are engaging evenly will make an even better job.
The pic shows the principle where the driving face is cut a few degrees past square so it kind of "hooks in" under drive load, yet lets go when the engine starts, or when you pull the clutch in to shift gear on a transmission gear.. Google "gear dog undercutting" for more info.
Edited By Hopper on 21/03/2021 06:45:00
Edited By Hopper on 21/03/2021 06:59:28
Edited By Hopper on 21/03/2021 07:01:40
Edited By Hopper on 21/03/2021 07:04:11
|martyn nutland||23/03/2021 09:11:49|
|133 forum posts|
Very many thanks everybody.
The maths is now clearer.
I think wear on the crankshaft-side dog (not surprisingly) is an issue, apart from the chewing (sic) the tips (points) are worn round unlike the nice sharp ones in the picture.
I also think the initial cutting on a milling machine, rather than sawing them, as I did, is acceptable. But I like the idea of finessing the shape carefully with a Dremel.
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