Coupling Rods

[AndrewP]

Ok chaps and chappesses, I'm machining the coupling rods for my Rob Roy and have a question regarding the centre joint which is the type where the pivot of the joint is the crankpin iteslf.

I can't tell from the drawings whether the rectangular section that sticks up above the boss and carries an oil point is thinned down to 1/8" with the tongue of the joint or left at 1/4" which is the full thickness of the rods.

The drawings look as if it is thinned but Martin Evans refers to the use of a 9/16" counterbore to reduce the thickness of the tongue part of the joint which would leave the oilway boss at full thickness.

Any ideas anyone?

[Shawki Shlemon]

Hi
I am not sure about the details but just an advice , normalize the material before machining to prevent distortion . May be someone else can answer the detail question . Good luck .

[baggo]

Hi Andrew,

I machined up a new set for my brother's Rob Roy and I left the oil box at the full thickness of the rod which I think looks better. I think I used an endmill to machine the recess rather than make a special pin drill.
Martin describes the same joint for Simplex but the write up does imply that the oilbox is reduced to the same thickness as the tongue as he suggests that the tongue could be filed if necessary. I don't suppose it matters really-just your choice.
Personally I don't see that having the joint on the crankpin makes the job any easier than having the conventional knuckle joint behind the pin but that's just my opinion!

John

[hat]

I am also in the process of building Rob Roy, I have gone through the book and the drawings and it certainly isnt clear. I will leave the part full thickness and have a look how its looks.- Its easier to take it off later than put it back. However the question has raised the issue of normalisation - how do you advise this is done, I understand why, to destress the material before machining but in a small workshop what is the process - please

[ron]

Andrew
The Simplex one is thinned but with it being a larger scale it still leaves plenty of metal to drill an oiler, it might make it a bit small on the RobRoy? but as John says I don't think it really matters, do what you prefer the look of.
This might offend the purists, but I normalise with a hammer and a vice after machining, although I can't say I've had to do it very often!
Ron

[mackintosh]

Hello Andrew
Nomalising is achieved by heating to cherry red (1500 F) and allowed to cool in air.
Bob

[AndrewP]

Thanks everybody.
Full width it is, for starters at least, I had in mind an end mill since I don't have any silver steel that big and realise that it does leave a slightly convex surface - hoping to traverse the mill off once at correct depth.
I usually normalise all my material when it arrives, since I have a big kiln it just goes in on a cycle up to 880 degC and then cool to room temp which takes about 24 hours.
These rods are actually Malcolm High's laser cut parts, cut from HROP steel, whatever that is, and I didn't normalise because I was too impatient - might come back to bite me I suppose, oh well there's always the hammer ;D
The skin on the laser cut parts is like glass but once through that they are a joy

[mutley]

HROP is 'hot rolled oil pickeld' so they shouldnt need normalising. Laser cutters like this steel as it tends to be quite soft and easy to cut.