Homemade Eccentric sheaves

[Rob Pulham]

Although these parts are allied to the Princess build, I thought that I would add the making of them and the reasoning behind it as a separate thread just in case anyone has need to solve a similar problem in the future.


Readers of the David Andrews Princess thread will have noted that I had to make the brake gear removable to ensure that I could remove the wheels due to them being mounted on telescopic axles. This need stems from a similar issue but I will start at the beginning.


Although the gent that I am building it for doesn't want full inside motion. One thing that is quite visible through holes in the front frames, is movement of the crossheads. I was asked if I could add some element of movement using the spare outside motion bits from the kit ( a set of Premier rods etc. has been supplied for the build). Having started to think about it long before getting to the actual start of the build I realised that with the axles being hollow I couldn't use traditional cranks to make the crossheads move (or rather I wasn't comfortable that I could make it work without ruining axles wheels). So I thought that a pair of eccentrics would impart movement while being able to be attached to the hollow axles without having to cut them.





We agreed this as the way forward and I ultimately bought castings for the slide bars/crossheads and eccentric straps from Laurie Griffin. I was also going to buy some eccentric sheaves too until I recalled Nick Dunhill finding the new cast versions very time consuming to prepare. Further thought made me also realise that the LG cast ones wouldn't really work anyway. This is because in a 'normal' inside motion set up the eccentric sheaves are trapped between the cranks so the eccentric straps cannot slip off the edge of the sheaves. Of course, I planned to use them as a single eccentric so they would need to be restrained at either side.


Instead of ordering a set of cast eccentrics along with the other parts, I bought a length of 12mm round brass bar instead.


Initially I was going to offset in the lathe using a four jaw chuck to offset the bar but having done a bit of research online, most machinists were of the view that you could but if you had a mill it would be far easier to drill it in the mill. Not being as proficient with my mill as I hope to be in time I did use the lathe to put a small centre mark in the end of the bar before transferring it to the vice in the mill.


Then using a centre drill inserted into my centre mark to centre it quickly, it was easy to plot the offset (2.4mm, more on this later) and then drill it out to 4.5mm. I then took it to final size with a reamer.


Transferring it back to the lathe I took a skim off the outside and marked up for the first sheave. Some time later we had this.







I tried the eccentric strap for size and found it was just a little too wide for the strap to close completely around the sheave.





It was at this point that I noticed that the depth of cut of the groove to accommodate the strap was such that it was very close to breaking through into the offset hole. It seems that in setting my offset hole at 2.4mm from the centre to gain the maximum amount of movement was just too much.


You may be able to see the mark where it’s just about to break through in the image below. This was made worse by having to apply a second slightly larger reamer than 3/16 (4.74mm) to get the axle end into the offset hole








The plan is to start again from the other end with a 2mm offset hole and see how I get on.


To be continued…

 

[simond]

Rob,

you could speak to Nick Dunhill, he’s got some ready made

Atb
Simon

 

[Rob Pulham]

Rob,

you could speak to Nick Dunhill, he’s got some ready made

Atb
Simon

Hi Simon,

He has, and I had considered them but I need these to have a groove/slot to contain the straps and prevent them moving sideways. This is because I am only using one eccentric on each end of the axle to impart movement to the crosshead rather than the usual pairs or sets of four.

As I understand it, those that you did for Nick are plain on the outside with the cranks preventing any sideways movement of the straps once assembled between the cranks?

 

[simond]

Hi Rob,

yes, exactly. We can probably do some with a groove, if you want to shoot me over a dimensioned drawing, I’ll have a look and let you know.

Can’t promise on the timing, we would need to fit these into the work schedule, but the machine is damn clever…

Atb
Simon

 

[Rob Pulham]

Thanks for the offer Simon, it's appreciated, but this exercise is as much about me learning how to use my tools as the parts themselves (and I only actually need two for this job).

An additional bonus is that I had somewhat lost the mojo earlier in the week but thinking about this and machining them has rekindled my interest and to be honest that's the reason that I am doing them now when I don't actually need them for quite some time.

I can see how model engineering could easily become a hobby in it's own right rather than a means to an end...

 

[Rob Pulham]

I went through the process again this time drilling the offset at 2mm which proved to be perfect for this particular application.


I had been using a very thin cutting tool which someone had ground (not very well it turned out) which came in the box of bits with the lathe. After cutting the first one I thought that I would examine the tool to see if I could improve it or at least rub it on a stone to restore the cutting edge. When I looked closely at the cutting part I noticed that the bottom of the tool was wider than the top and although there was some rake away from the cutting edge the fact that it was getting thicker couldn’t be helping to make an efficient cut.

This is a sketch of what the tool looked like originally albeit the bottom of the wedge is somewhat exaggerated.




I have watched a few Youtube videos on sharpening lathe tools recently so I had a go at grinding the tool to take off some of the thickness towards the bottom making the two sides parallel.

This improved the cut and I successfully cut the groove for the first sheave. Then I parted it off but I was a little too close leaving a very thin edge. During the parting off, the parting tool moved slightly in its holder which pushed over the thin rim of the sheave slightly closing the top of the groove.

In the end it was usable but I decided to cut another pair to be on the safe side. It was while parting off the first one that I noted that my parting tool was in fact just the right thickness for cutting the groove without having to move the carriage as well as the cross slide to get the desired cut.

Having discovered this the next one progressed much faster and modifying the fixture that holds the parting tool so that it grips the parting tool more securely by squeezing it in the vice made the third one even faster still. The parting tool is one like this albeit mine only has one tool. You will note that it’s a pressed steel fitting that grips the part with the aid of a cap screw. Mine didn’t hold the cutting tool very closely to the holder but it does now.

https://images-na.ssl-images-amazon.com/images/I/41Uv-fAJmjL._AC_.jpg


So here we have the finished sheaves.





This is how they will fit on the axle once I work out their alignment with the crossheads further down the line.



It turns out that I wasn’t far out with my 16mm deep offset hole, I had a measure of the remaining stock while putting this post together, and there is just about enough material to cut a 4th sheave had I needed a full set for a conventional inside motion build.