Ian_C's workbench - P4 and S7 allsorts

Discussion in 'Workbenches, including workshop techniques.' started by Ian_C, 21 May 2017.

  1. Ian_C

    Ian_C Active Member

    Taper pins. Yes, I've found it very variable. I ended up with some 1/16" pins from two different suppliers. One lot was very tidy, the other was a mixture of OK-ish and rough. The good stuff came from EKP Supplies www.ekpsupplies.com 1/16" x 1/2" Taper Pins Qty 10, about £3.50 I think. I only got the reamer from Tracy Tools, sorry if I didn't make that clear.

    Wider pony truck frames? Depends how itchy your hair shirt of authenticity is. You'd have to make quite a few new parts to do that. You could compromise and just put some extra profiled plates on the outside of the frames to space the spring castings out. Honestly, I don't think it's worth the effort.
     
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  2. Ian_C

    Ian_C Active Member

    Yeah, I like that approach. Thanks, I've learned something this evening! Funny how people find different solutions to the same problem isn't it? I wanted the freedom to choose an axle diameter that suited each job and wanted one tool to do them all, hence the V groove. Your design has the merit of being easier to get geometrically correct in the lathe than all the careful setting up in the milling machine to get the same result. Also provides more support for the wheel rim.

    I'd thought of quartering differently too. May post my solution here if it works out.
     
  3. Lyndhurstman

    Lyndhurstman Western Thunderer

    Good Grief. That's beautiful. Marvellous. Cap suitably doffed.

    Cheers

    Jan
     
  4. Threadmark: The quartering tool
    Ian_C

    Ian_C Active Member

    Here's the quartering tool. It follows the well known principle of having two surfaces set relative to the axle centre that contact the crankpins such that they end up 90 degrees apart. The wheel gauging and axle pinning block shown in a previous posting is used to hold the axle. The block can be used independently, as previously described, or installed in this quartering tool.
    quartering tool 1.jpg

    The quartering tool is designed around a 3/16" diameter axle and 3.0mm crankpins. Of course, once they're set 90 degrees apart you can use any diameter crankpin you like; they'll still be 90 degrees apart. I've made a pair of dummy crankpins specifically for this tool and they're stored in a couple of 10BA holes in the base plate when not in use. The horizontal reference block extends far enough each side of axle centre line to allow the crankpin to be set forwards or backwards for RH or LH lead.
    quartering tool 2.jpg

    If you use an axle diameter other than 3/16" then the axle centre height in the V groove changes, and that in turn alters the angular relationship of the crankpins dictated by the horizontal and vertical reference faces. Not by much, and theoretically you can get away with a couple of degrees either side as long as all axles are the same. The vertical reference block is fixed through slotted holes in the base plate so it can be adjusted by 1.5mm in either direction to restore the 90 degree relationship for different axle diameters or different crankpin diameters if you wish. The degree of movement and the resulting position on the base plate can quickly be plotted in CAD for any combination.
    quartering tool 3.jpg
    quartering tool 4.jpg

    The accumulated errors from making several parts on a milling machine that isn't to toolroom standard mean that I won't get exactly 90 degrees. But it'll be very close to 90 degrees and, more importantly, all axles will be the same. Once I've done all four axles and checked running with the coupling rods on then I'll find out whether all the theory is borne out in practice!

    If anybody wants to see the drawings then let me know and I'll post them.

    I'm getting bit ahead of things here because I haven't posted the approach to modifying the Slaters driving wheels and making new axles. I modified a pair, seen here unfinished, as a trial. It worked out mostly as planned. I'll do the rest of them when I get the time and post the learnings.
     
  5. Threadmark: Modifying the Slaters driving wheels
    Ian_C

    Ian_C Active Member

    The confident assertion I made in an earlier post about being able to remove and replace the lathe chuck and wheel holder without introducing any out of true error turned out to be slightly wrong. For a reason I don’t understand the wheel holding tool did not run quite true when the chuck was replaced on the spindle. The wheel holding tool had not been removed from the chuck meantime. It was only a little bit out of true but it should have been near perfect. Not a big problem because it was easy to reface the part of the tool where the wheel sits.

    The issue where, in S7, the Slaters wheel and crankpin boss projects far enough from the face of the wheel to cause clearance problems behind the slide bars was flagged up in a DavidinAus post some time ago. David opted to take 0.4mm off the face and it seemed to work for him. Taking a close look at the prototype drawings in the Wild Swan book revealed that…
    • the driving wheels measured 6-13/16” overall from front of axle and crankpin boss to inside face of rear boss
    • the rear boss on the prototype wheels measured 15/16” from rear face of tyres to inside face of boss (there's no rear boss on the Slaters wheels, we'd use a washer or two on the model)
    • the tyre was 5-1/2” in width
    …and converting these dimensions to 7mm and doing the arithmetic I conclude that the model wheels ought to measure 3.42mm from inside face of tyre to outside face of axle and crankpin boss. The Slaters wheels measure 4.23mm. So there’s the missing clearance of 0.81mm each side. The prototype ‘as drawn’ clearance between inside of slide bar and leading coupling rod pin was a mere 11/16” , or 0.4mm in 7mm world. If the MOK kit has the slide bars the correct position (and it should have because there are parts on the S7 conversion etch specifically for this) then half a revolution and CLANG is the guaranteed outcome.

    As it turns out you can’t sensibly face the axle and crankpin boss down by 0.81mm on the Slaters wheels. The inside ends of the spokes flare outwards to meet the moulded boss on the Slaters wheels to a greater extent than on the prototype. I suppose it makes the over thick Slaters wheel centres look in proportion. I’d never have noticed if I hadn’t checked, and in 0F it may not matter. It is possible to take 0.55mm off the thickness before you start to machine into the end of the spokes and the face of the boss starts to look like a starfish. If DavidinAus got by with 0.4mm and I can manage 0.55mm then I figure I’m in with a chance provided that I have appropriate coupling rod and pin projection on the leading axle.

    Mount a wheel in the holder and check that the tyre runs true. On some of the wheels you could see by eye that the axle insert was a little off true when the lathe was spinning. There's the root cause of the 'Slaters shimmy', as I've heard it called.
    driving wheel on chuck.jpg


    The first part of the job is to face off the crankpin and axle boss by 0.55mm using a sharp, tiny, pointy tool. This is the same tool described in the pony truck wheel thread somewhere above. The small point combined with a small depth of cut (0.15 - 0.15 - 0.15 - 0.1 as it happens) gives rise to small cutting forces which can be handled by the plastic spokes and which won’t upset the steel crankpin and axle inserts. A collateral benefit of this is that the moulding sink marks in the original surface of the moulding are removed and we have a nice flat boss like the prototype.
    facing wheel.jpg


    Next operation is to clean out the countersink and square axle hole with a sharp 4mm slot drill.
    wheel and slot drill.jpg

    The 4mm bore is opened out to about 5.0mm with the tiny boring bar. The rationale here is that a boring bar will cut true to centre where a 5.0mm drill might wander a bit depending on how accurately it’s been sharpened.
    wheel and boring bar.jpg

    The last operation differs slightly from the conversion of the pony truck wheels previously described. Scaling from the drawing, the prototype driving axles are about 9” diameter where they project from the driving wheel boss. That’s 5.25mm in 7mm scale, or 5.2mm for convenience. So the outer end of the axles must be 5.2mm in diameter and the bore in the wheel insert must be a gentle push fit onto the axle. For the pony truck wheels I’d made the axles to the required diameter and bored out the wheels to fit. That wasn’t straightforward as I couldn’t measure the bore accurately and had to take tiny cuts with the boring bar and keep trying the axle fit. I did it the other way round on the driving wheels. The wheel inserts are all reamed out to 5.2mm (I have a 5.2mm machine reamer) and the axle will be machined to fit the wheel. It is easier to measure the axle diameter accurately than the wheel bores and we'll get to the finished size faster and with more confidence, and since there are 8 axle ends to do so that’s a worthwhile time saving. The 5.2mm reamer works nicely through a 5.0mm bore.
    wheel and reamer.jpg

    With a bit of organisation it took me about 15 mins to do a wheel, so in about 2 hours one evening all the wheels were done.

    Before and after...
    wheel before and after.jpg

    ..and all done...
    all done.jpg

    Suppose I'd better make the axles next. This also makes me think about balance weights, which in turn means I need to decide which locomotive I'm actually modelling, which in turn might have consquences further downstream. I fancy a late inhabitant of the mighty 18A Toton, mid sixties, last knockings of steam on the Midland main line and I was a kid in short trousers growing up nearby. My brother reckons he remembers seeing 8Fs on the low level approach through the old Long Eaton station behind the Co-op car park. Must have been' 64 or '65 or at a push '66.
     
  6. adrian

    adrian Flying Squad Mick's Minions

    Thanks for posting that analysis. I admire what you have done with the wheels very impressive - although in some way I'm slightly relieved that I decided to go for Mark Wood wheel castings as I reckon I should be able to turn them to the appropriate dimensions, for which I'll use this post to refer to.

    However having looked at the Wild Swan drawings it ill behoves me to point out that it appears that the crank pin boss on the leading driver is recessed lower than not only the axle boss but the tyre as well.

    This is the leading driver and the coupling rod boss is offset inwards compared to the centre line of the coupling rod and the fluting of the coupling rod is deeper on the inside surface compared to the outside to clear the axle boss.

    IMG_8517.JPG

    Compare this against second driver

    IMG_0071.JPG

    If we apply the crankpin boss recess on the leading driver then it could give us a little more breathing space to the rear of the slide bars, although it will depend on how the coupling rods are built up in the kit.
     
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  7. richard carr

    richard carr Western Thunderer

    Ian

    I use soft jaws that have steps machined into them to hold the wheels in the chuck about 2mm deep. As the steps are cut in on the lathe itself they must be true to the centre of the lathe. Your little boring bar is a perfect tool to do that with.
    Your method relies on all the wheels being exactly the same size, which I doubt is the case, as the manufacturing tolerances are not that accurate.

    Having said that from my experience Slaters wheels run fine even with what appears to be a fairly large run out error.


    Richard
     
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