Eccentric Production

[John Bateson]

I am hoping to be able to tackle the production of a couple of eccentrics for some models on my own railway. The ones I need are for a pair of internal cylinders with a pair of Stephensons in the centre for the valves.
Every time I sit in front of the lathe to do some experimentation I always seem to find other things to do but, start I must. Especially since elsewhere in this forum is a long discussion on axle material which I have been carefully following and which may be very pertinent.
This sort of thing out to be feasible in 4mm scale, so if anybody would care to offer advise on materials and technique and assembly methods, it would be appreciated.
John

[grovenor-2685]

John,
Do you mean eccentrics or crank axles?
The former should be fairly simple as they are just discs with off centre holes in for the axle.
Crank axles are rather more difficult and if their location in the chassis makes them fairly obscure you can cheat and use eccentrics instead of the crank axles.
Have a look at the example Brassmasters have done for the 4f.
http://www.brassmasters.co.uk/images/4F/4F%20driving-cams.jpg
There are photos there as well.
Keith

[John Bateson]

Keith,

Do you mean eccentrics or crank axles?

A small company I know about already does eccentrics on their kits
Yes! eccentrics and crank axles on the same axle.

John

[grovenor-2685]

I've seen assembly of crank axles written up a time or two. IIRC the idea is to silver solder all the bits onto a complete axle, then, when it is all as desired cut the two surplus bits of the axle out. It may well have been illustrated in Roche and Templar but I sold my copy.
Regards
Keith

[Horsetan]

...It may well have been illustrated in Roche and Templar but I sold my copy.

It is certainly described to some extent in my copy.

[allanferguson]

I essayed the construction of a crank axle once, a while ago. I do remember that pushing the wheels on was alarming (I know about putting props in the gaps, but it's not so easy in practice). Also I had to put the connecting rods on before soldering the axle together, (as I couldn't work out how to split the big ends) and that was another heart-in-mouth moment. But it worked, and it was lovely to see something going up and down in that empty space under the boiler. I felt that eccentrics and their rods were a bit too far for me. Unfortunately the engine didn't work, for other reasons, and is now awaiting a round tuit.

Allan F

Crank Axle.JPG

[Terry Bendall]

The former should be fairly simple as they are just discs with off centre holes in for the axle.

Ideally this neds a simple jig to held the discs so that all the holes are the same distance off centre. A way of doing this is to get a piece of metal, I would use mild steel, but brass would do, and mount this in a four jaw chuck so that it is off centre to the correct amount. Create a hole in the steel so that the diameter is the same as the diameter as the eccentric. A simple way is just to drill a hole all the way through, but if you drill a slightly smaller hole and use a boring bar, (see Scalefour News 159,page 7), you can create a stepped hole with a recess that will ensure that the metal used to make the eccentric is held square to the centre line of the lathe. When the hole is created, take the metal out of the lathe and make a saw cut from the edge to the hole. What you now have in effect is a slipt collett. If this is held in a three jaw chuck, all the eccentics will have holes which gibe the same throw, and tightening the chuck jaws will cause the saw cut to close up sufficiently to clamp the disc.

A moderate amount of work, but once it is done you have the jig for evermore. You do of course need access to a four jaw chuck. A method of getting the jig off set to the correct amount is to mark the centre of the bar, then measure the amount of off set needed and centre punch a mark. When setting up in the four jaw chuck the centre punch mark can be lined up with the tailstock centre.

the idea is to silver solder all the bits onto a complete axle, then, when it is all as desired cut the two surplus bits of the axle out. [ /quote]

This method is commonly used in model engineering circles for crank axles on live steam locos. No reason why it sould not work in smaller scales.

(I know about putting props in the gaps, but it's not so easy in practice).

If the props are held in with soft solder, they won't fall out.

Terry Bendall

[John Bateson]

215 Eccentric Plate Front.PNG

215 Eccentric Plate Side.PNG

These are the eccentrics supplied, while I don't use the same method as Terry for producing them, they seem to work with a little care.
A pair of them, back to back, holds the rod to the steam chest. 16BA is a little small to hold them together but there is no side stress.

While I am au fait with the theory of turning the cranks, I have yet to do this with any certainty that the axle with both the cranks and eccentrics will end up square enough for P4 standards, especially with 6' 9" wheels being pushed on the ends. The Exactoscale method for example would put a lot of stress on the axle parts, silver solder or not and I have found that some ag wheels are a little tight on the axles.

When I can find the time I will have to do some serious experimenting - again!

John

[grovenor-2685]

There is nothing to stop you pressing the wheels on before cutting the axle.
Keith

[Terry Bendall]

There is nothing to stop you pressing the wheels on before cutting the axle.

How would you hold the axle in the lathe Keith? If the wheels are pressd on, the only way of holding it would be to grip one of the wheels in the lathe chuck. You could of course make the axle longer, but I think normally you would turn the cranks first and then turn the parts of the axle that take the wheels and go through the bearings. Or am I missing something?

Terry Bendall

[grovenor-2685]

Why would you want to hold it in the lathe to press the wheels on? Any of the usual press tolls such as the GW tool would do.
I am assuming that you assemble on the axle the crank webs, eccentrics and the big end journals and solder the lot up solid, then fit the wheels and as the last action saw through the axles between the crank webs and file flush so the connecting rods can be fitted.
The worry seemed to be that if the two sections of axle were removed prior to pressing on the wheels the axle may distort in the press.
If you are going to turn up a one piece crank axle from the solid none of this applies.
Regards
Keith

[Horsetan]

....If you are going to turn up a one piece crank axle from the solid none of this applies.

How about a cast crank axle like this

[grovenor-2685]

Ok with outside valve gear and rocking levers, no good if you need to fit eccentrics between the cranks.
Might give somebody ideas though.
Keith

[Horsetan]

.....Might give somebody ideas though.

Which was exactly the point.....