Crankpins first. The clearance between the back of the crosshead assembly and the leading crankpin on the prototype was small, maybe half an inch. That's about 0.15mm at this scale, or, because of the perverse way the world works, nothing. Or minus nothing when my accumulated modelling errors are accounted for, plus more side play on the leading axle than is necessary. I didn't think about it earlier, and Mr Bradwell doesn't draw attention to the challenge. I'm thinking about it now though. I can reduce the height of the leading crankpins with some chicanery (probably next episode), but I certainly need to reduce the side play of the leading axle. Some of that side play is between the bearings and the horn guides, and there's not much I can do about that, but most of it is between the inside of the driving wheels and the face of the bearings. To address that it is necessary to take at least one wheel off the axle to add some washers. I contemplated this with a heavy heart, because I remember how much of a pain it was to get the original wheels to gauge, quartered and with an acceptably small wobble. In the non linear fashion of a Tarantino movie, we'll leave the crankpins here and flash back to an earlier time...
When I first joined E4um (Scalefour Society online forum back in the day), I joined in the middle of an energetic thread about the merits or otherwise of Gibson wheels. This was known as The War of Gibson's Wheel (after
The War of Jenkins' Ear - look it up, WT broadens your horizons doesn't it?). The warring factions were divided over whether the unavoidable wobbliness of an assembled Gibson wheel set was inherent in the design and manufacture of the wheel, or was down to the ineptitude of the assembler. The body count was kept to a modest level by the moderators, but I don't think a conclusion was ever reached. I guess it's continued as a low intensity conflict ever since. Fast forward again to Saturday 1st May...
With The War of Gibson's Wheel in mind, I thought that before I took a wheel off the leading axle, I'd better try pressing an axle into a new wheel using the
method described in an earlier post. That had worked really well on the tender wheels and I hoped it would work equally well on a 1/8" axle into a driving wheel. It didn't. The end of an axle was slightly radiused and polished to ease entry to the wheel, but as soon as the axle centered and started to press into the wheel it was apparent that the plane of the wheel wanted to be anything but perpendicular to the axle. Just by spinning the axle between my fingers made the wheel wobble shockingly. When I put the axle in a collet and measured the wheel tyre run out it was about 0.25mm radial (doesn't sound much but looks awful), and the axial wobble was...a lot... builders of my acquaintance work to the nearest half breeze block, so something of that order. How to proceed?
Straightforward engineering approach, but somewhat of an a*se at this scale. About 15,000 words worth of pictures coming up- and then a lot of words.
- A lump of something (an odd end of aluminium bar in this case) is held in a 3 jaw chuck in the lathe and a recess is made that is exactly the diameter of the driving wheels over the flange. It pays to measure all of the wheels you're planning to do, because chances are they'll all be slightly different. I measured ten P4 Gibson WD wheels of this type and they ranged from 18.95mm to 19.01 mm. Since I was only wanting to experiment with one pair I was able to choose two wheels exactly the same at 18.98mm. If I needed to do the lot I suppose I'd bore the recess to the smallest size first and then rebore it to accommodate the progressively larger wheels, finishing up with the largest. The depth of the recess was 1.5mm, which leaves 0.5mm of the tyre proud of the turning fixture to clamp the wheel. Of course now you have a means of holding the wheel tyre that is perfectly true to the axis of the lathe, you can't remove the fixture from the chuck without destroying the accuracy of the whole operation. This calls for some thinking ahead. You have to make any features for the wheel holding before the bar goes in the lathe. In this case I'd worked out exactly where to drill and tap M3 threaded holes to enable the head of an M3 cap screw to just pinch the edge of the tyre. It was easy enough to find the centre of the bar on the mill and make the holes in the right place. The bar won't centre perfectly in the chuck, but the holes don't have to be that accurate. There's a 5mm diameter hole drilled straight through to clear the drilling and boring tools used later.
- Another smaller recess is turned to clear both the crankpin boss when the wheel is held face to fixture, and the boss on the inside of the wheel when it is held back to fixture. In this case diameter 8mm x 1mm deep. If I was doing this again I'd calculate the depth of this recess carefully, for reasons that will become apparent around step 12 (don't skip ahead, you'll ruin the story).
- The first job is to hold the wheel face to fixture and machine off the moulded boss on the back. The cap screws don't need to be any tighter than snug. Hands go up at the front of the class and point out that if the tyre is not in the recess then the wheel can't be centred. True, but we don't care about concentricity at this point. Just centre the wheel by eye and it'll be good enough. We're working on the plastic insert now so we want cutting forces to be small. I use small tools with effectively no nose radius sharpened to an almost mirror finish on a diamond wheel. Light cuts and high speed get the job done without distressing the plastic centre. Just watch out for the tool contacting the whizzing and invisible cap screw heads. Already there are some clues to the root of our problem. The boss doesn't quite clean up to the back of the moulded wheel. It's flush on one side and still slightly proud on the other.. you might be able to see this if you zoom in on the photo. Not out by much, but I know from reading Colin Seymour's assembly notes on the wheels on the Alan Gibson Workshop website, the plastic centres are moulded separately and pressed into the steel tyres. That's an operation that has some margin for error, and I'm assuming that's why the plastic centres aren't quite in the same plane as the tyres. The axle hole is also moulded in and that therefore must be slightly off axis as well.
- Brutal stuff now. The wheel is turned round to locate the flange in the recess and reclamped. It's gratifying to see the tyre spinning perfectly with no run out. The axle hole is drilled out to diameter 4.0mm. Again, use a sharp drill and feed slowly. I have two sets of drills. One cheap set, all gold coated and shiny. Maybe from India or China and drilling approximately to size. They're used for rough drilling. And a set of genuine Presto HSS drills, that are kept for best. They cost more, but they're perfectly ground and very sharp when new, and for accurate work they're worth the money.
- The drilled hole is carefully opened out to diameter 4.5mm with a small carbide boring bar. Unlike a drill, which has some tendency to wander off centre and make holes slightly bigger than the drill, a boring bar cuts true to the spindle axis. At this point we're not too bothered about concentricity, but we need to size the hole accurately. The hole is opened out to be a slight press fit for the brass plugs described next.
- This is where I can claim some credit for thinking ahead. Doesn't happen very often. I'd made some brass plugs for the wheel centres before I made the turning fixture. The plugs are diameter 4.5mm to match the hole in the wheel, and they have a diameter 6.5mm x 0.5mm thick flange on the back. That flange replaces the moulded boss we scraped off back in step 3. They also provide some extra gluing area. Countersink the back of the hole very gently to ensure the flange sits against the back of the wheel. Remove the wheel from the fixture.
- Here you can see that the plug is proud of the front face of the wheel and the flange on the plug replaces the moulded boss. An virgin wheel is shown on the left for comparison. The parts were degreased with IPA and pressed together with Loctite high strength retainer. Titus the dog was walked while the retainer cured.
- Wheels back in the turning fixture, and the first job is to face off the brass plug flush to the crankpin boss. Actually I took a very shallow skim across the lot to level up. Bit of a funny tool set up to get in between the cap screw heads. It was easier to run the lathe in reverse and feed from back to centre. Again a very sharp tool and light cuts is best.
- Here's the facing op with motion blur for dramatic effect.
- That's the plug faced flush, and we're ready to bore the new axle hole.
- Tiny 0.5mm centre drill fed in gently.
- I started to make the axle hole with a new and very sharp 2.5mm drill. That found it's way through the plug without any drama. I had planned to open up the hole with a series of drills to around 3.0mm and bore to size, but this is the point where I discovered that the retainer hadn't fully cured. I'd wondered why the dog had wanted a longer walk. A 2.7mm drill started to push the brass plug back through the wheel. The wheel was removed and the plug cleaned up and re-fitted with low viscosity cyano. Rather than risk further movement of the plug, I elected to complete the work from this point by boring. Worth noting that If I'd faced the flange side of the plug to a known dimension , and calculated the depth of the small recess in the turning fixture accurately, I could have the back of the flange in contact with the fixture when I carried out the drilling and boring operation. Then it wouldn't be able to push through if the cutting forces were too high.
- That's the smallest boring bar that I possess. It's capable of starting in a hole of just 2.0mm diameter. The advantage of boring to size rather than drilling or reaming is that the cutting forces are tiny and don't risk dislodging the brass plug. I'm pretty sure a 1/8" reamer would have broken out the brass plug from the plastic wheel centre. Also the boring bar cuts true to the spindle axis and has no tendency to wander, unlike a drill. If the drill is a little off then mostly the reamer follows it. I used the shank of drills to gauge the size of the hole up to 3mm, there's no way you can accurately measure a hole of this size without some very expensive equipment. From 3mm to final size I used the axle to gauge the bore. Cuts were tiny towards the end, with only a trace of brass dust being removed. One nice thing with small, sharp tools like this is that the cutting forces are so small that there's almost nothing taken out of the bore on a spring pass. Patience, and creeping up on it with infinitesimal nudges of the cross slide, and eventually the axle can just be pushed in with a slight resistance and no discernible clearance.
- Both the wheels now have brass axle bushes dead to size and perfectly (well not really perfect, but to very close limits) concentric to the wheel tread (OK, actually concentric to the flange but it's turned in the same op as the tread, so practically the same). What's interesting is that the brass bush is clearly not central in the plastic wheel boss, which explains the radial run out measured on the original push fitted axle.
- The almost final product, with the wheels just slid onto the axle ends. I have to say I'm really pleased with the result. They roll across a flat surface with no discernible wobble or run out at all. They're every bit as good as your aristocratic Ultrascales, plus they cost less, you don't have to wait 6 months (or more), and there's a wider range to choose from. Finally The War of Gibson's Wheel has been won (by me at least). I'm sure this approach would also work on Slaters 7mm wheels.
Next it'll be front axle apart to measure up for side play and washers. It has to be easier to gauge and quarter the wheels when they're free to slide on the axle like this than when you're trying press them on, keep them straight and pay attention to the quartering at the same time.
, being set to alter the diameter of wheel bore by only about 1 thou over its short length. The lathe top slide is adjusted using a dial indicator traversed over a longer (calculated) length.
