LOCO BUILDS – TURNING ECCENTRIC SHEAVES
Alex describes his method for easier measuring for these essential loco components.
Iam fully aware that anyone who has ever built a steam engine other that the simplest oscillator will have made eccentric sheaves before and that the standard method for doing so has been well documented, not least by LBSC in his ‘words and music’. My aim here is to present a slightly different method for determining the position of the eccentric bore which I find easier than the more usual technique.
Once the outside of the eccentric has been turned in the three-jaw chuck to fit the eccentric strap, the standard technique is to remove it from the chuck and, having scribed a centreline across it and using the turning marks on the faced end to show where the centre is, to measure the eccentric throw from this position, mark it and then centre punch the new centre.
With the eccentric moved to the four-jaw chuck, it is then re-centred using the centre-punch mark and possibly a wobbler before the bore is drilled, bored and reamed.
Measuring issues
My issues with this technique are that I find it relatively difficult to measure and mark the position of the bore accurately and, when centring it in the four-jaw, you have to centre it in both directions to get the centre-punch position to run true.
To illustrate my approach, I am making the four valve gear eccentrics for Don Young’s 3½-inch gauge GWR 43XX mogul, the drawings for which I have reproduced here. I don’t know about you but I find having a drawing presented makes the understanding of a textual process description so much easier to understand.
These eccentrics are 1 ¼ -inch diameter by ½-inch long so I start by hacksawing off four ¾-inch long blanks from mild-steel bar of the appropriate diameter. Then, using the three-jaw chuck, I turn down a 3⁄16-inch length by about 1⁄16-inch to form a shoulder that provides repeatability for the next two stages. The diameter here is unimportant but it makes sense that all four are turned down to the same diameter as it will make the four-jaw work later on more repeatable.
This done, each blank is gripped with this shoulder against the jaws of the three-jaw chuck and, using a suitable parting tool of about 3⁄32-inch width and my old 1946 Myford M-type lathe running in back gear, I form the seat for the eccentric strap, using the already machined strap as a gauge.
When forming this seat care is taken to allow an extra 1⁄16-inch between the end of the blank and the edge of the seat; once the seat is formed, this extra 1⁄16-inch is turned down to a diameter twice the throw of the eccentric, in this case 7⁄16-inch as measured with a micrometer. This is shown in Photo 1.
Once all four eccentric blanks have been so machined, the three-jaw chuck is replaced with the four-jaw and, with no additional marking out, the first of them is chucked such that the very edge of the short 7⁄16-inch diameter stub is on the lathe centreline – I use a centre in the tailstock to make the initial setting. It doesn’t matter where, circumferentially, on the edge of the stub you end up as the thing is axisymetric at this stage.
Turning marks
Once it is set about right by eye, the lathe is started (no back gear needed this time) and a small 5 thou cut taken across the face of the stub. The turning marks this leaves will quickly indicate whether the chuck is adjusted correctly, if not, a small adjustment on one pair of jaws only is made to bring the edge of the stub closer to the lathe centreline (Photo 2).
After each adjustment, another 5 thou cut is made; you have space for
12 such adjustments before the 1⁄16-inch long stub is completely machined away. With the lighting in my workshop, I find that, if I stop the cut a couple of thou radially before I get to the centreline, the light reflecting off the small pimple so produced can make it easier to see if it (the pimple!) is accurately on the edge of the stub.
Once you are happy with the adjustment, the rest of the stub is
“The turning marks this leaves will quickly indicate whether the chuck is adjusted correctly...”
completely machined away and the eccentric is drilled, bored and reamed in the usual way. Although I always slacken and tighten the same pair of jaws when mounting each blank so the centring should be accurate, I still take a cut across the face just to check.
Machining of the 13⁄16-inch diameter boss on a stub-mandrel and drilling and tapping of the grub screw hole are then all that are required to finish the job.
I find this method gives me four eccentrics (Photo 3) with accurate uniform throws (as near as I can judge!) but without as much fiddly marking out.
■ Have you come up with a technique to make your workshop life easier?
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