So I'm having to restock/refit parts of my workshop as I get back into model engineering, and in the process I've left myself wondering about BA threads. Back before my dad sold off a large chunk of the tools, we had been in the process of switching to an entirely (or mostly) metric workshop; metal stocks, drills, taps, dies, etc. Of course it never would have been entirely metric, as most (older at least) designs are drawn in imperial, and most fittings are ME threads; but new machine tools are metric, as are most metal sizes. Thus things like axles are 20mm instead of 3/4", use a M5 tap instead of 3/16" and so on, with imperial dimensions used only where necessary. This brings me to where I am now, and particularly in replacing smaller taps and dies. The smallest tap/die I currently have is 3mm, below that I need to buy new as the old sets got sold off (or were claimed by my brother, and thus either lost or broken by now). So I'm wondering if I should get new BA taps/dies, or switch to the metric equivalents. Going all metric would simplify some things such as identifying a thread, and the idea of a "standard" thread type feels somewhat more organised to me. Anytime a BA (or other) thread is specified on a drawing, I'd just use the nearest metric equivalent, as there's metric thread at 0.2mm (or less) intervals in the smallest sizes, down to 1mm; equivalents to 0-14BA, plus some in between. But, smaller metric fasteners are not necessarily easy to get hold of, so I'd likely end up having to make a lot of nuts and bolts myself. On the other hand if I stick with BA for small threads then fasteners are easier to get hold of, compared to the smaller metric sizes. Plus there's no need to deviate from the design. But there'd also likely be a crossover point, either in the case of a project having larger threads in metric, and smaller in BA; or in having both metric and BA tools and fasteners on hand, and choosing one or the other depending on the project. I've been dithering on what to do for a while, but I'm going to need to buy one or the other (or both) soon, so a decision needs to be made. Basically this is my chance to go all metric, if I want to, which I'm not sure if I do.
Currently drilling and tapping the joiner holes for the rod bosses, I need to get a M6 plug tap to finish this off. Also gradually cutting the centre sections for the rods, this is 4 x 9" cuts in 6mm plate, so is taking a while. Just doing a bit each day so as to not wear my arm out or get bored and rush it. Also contemplating building a 16mm scale loco as a side project, this would be built to sell to help fund a bigger lathe. The little Taig isn't big enough to machine the blowfly cylinders, and the crowdfunding campaign I have going is not going very far.
Dad came into the workshop tonight as I was boring out another coupling rod boss. Even though it was late (about 11:30PM), and he was a bit sleepy from getting out of bed, we still were able to have a good chat for about 30 minutes about what I was doing, and what's next, and so on. Considering his dementia has progressed to the point that he often can't remember where he lives, even when standing in his bedroom, this was really great to see. It's been a couple of years since I was able to regularly just chat with him like that.
Yey, 2 hours work and I have a hole. Just bored out the crankpin hole in one of the front coupling rod bushes, this includes a 2mm deep 20mm counterbore for the retaining collar to fit in.
That was quicker than I expected; all the boses are now machined to size, so I can get on with boring the crankpin and joiner holes next. Will have to sharpen up a boring tool first.
That's one boss machined to size, and now all nice and square and shiney. Five more to go. Hey look! I cleaned the lathe benche (didn't realise how messy it was till I say that last photo). Sitting on the bed is the finished-to-size boss, with another mounted in the 4-jaw chuck ready to be machined to width. Lathe belt is off, as I took the pic' just as I was done for the night.
Just to show I am actually doing stuff, here's the coupling/connecting rod bosses (as per earlier drawing) getting machined to size in the lathe; to give some idea of scale, that chuck is 4" diameter. The other 5 bosses are sitting on the cross slide for the photo. All have been machined to length, currently machining to height, next machine to width. Then I'll bore the holes for the crankpins and bushes, and then drill the holes for connecting them together into a finished rod. This all looks messier in close up.
Well that's one dead (or at least blunt) hacksaw blade, now waiting for some new ones to arrive, which should be a few days. I could get a pack of cheap blades froma local store, but those cheap carbon steel things are utterly useless; akin to cutting a steak with a stick of butter. So I've ordered some good high speed steel blades. But as I've got a couple of the coupling rod bosses cut to (rough) length, I've started machining those to square them up; the combination of rough drawn black steel and my battling on with a blunt hacksaw blade on the last one means they're far from square now. As I was setting the job up I gave the 4 jaw chuck a good clean and some fresh oil; apparently went overboard on the oil as I got a face full of it when I first started the lathe, oops. But none-the-less the little Taig lathe is good for this kind of small job, and even with mild steel it cuts without fuss. Still took an hour to face the ends of one boss down to size, 6 to do total (4 for the coupling rods, plus 2 for the connecting rod), so longer task than one might think for something so simple. Plus that's only 2 faces done, 4 more faces per boss to do, then bore out the pin holes, and make the bushes. Hopefully I can find plenty of time to work on it though.
My arm is threatening to go on strike, still cutting metal. Still at least when I've got it all cut the rest of the work on these rods will just be machining; which I can pretty much sit down and do. On the plus side it's still less than half the cutting work needed for the original design, so I shouldn't complain too much.
Giving the coupling rods some thought, I'm considering deviating from the plans; partially so they're easier to make, but also to give them a bit more shape, and centre the bearing surfaces on the rods' forces. The standard Blowfly design calls for 25x7mm steel, shaped into a dogbone, but I'm thinking of using 2 pieces of 25x10mm for the rod ends, joined by a length of 12x6mm which would be threaded and silver soldered into the ends. This would give what to me is a more pleasing shape, while resulting in considerably less hacksaw work. Also, unlike the original design which has the bushes overhanging the rods by several millimetres on one side, this would centre the forces of the rods on the bearing surfaces, which could reduce wear (would also centre the oil hole in the bush, thus better lubrication). Here's a quick workshop drawing I made tonight of the intended coupling rod design.