One thing I wanted to avoid when designing the chassis for the locomotive was too much space for the axle and layshafts to move around as that would seriously affect the running. To try and ensure a sensible fit I designed all the holes to be the exact same size as the part that would fit through them. In fact all the holes are designed to be 1.5mm in diameter as both the layshaft and axles are 1.5mm. My plan was to then ream out the holes until I got a nice sliding fit. In fact my first attempt failed probably because I didn't ream out the holes far enough. Part of the problem was that I was opening out the holes using tapered reamers.
I own a cheap set of tapered reamers which allow me to easily open up a hole, but they don't give any control over the size of the resultant hole. Also it is often difficult to ream out a perfect cylinder given that the reamer is tapered and you can't always access both sides of a hole. I'd always assumed this was something we just had to live with until Jeff (in a comment on NGRM) introduced me to the concept of a straight chucking reamer which are apparently the tool for reaming out bearings. Given the wonderful chassis Jeff has produced I wasn't going to argue with him.
As you can see this reamer isn't tapered (it has a very slight taper at the point to help start the hole) and looks more like a drill bit. This allows you to ream out a perfectly sized hole, which as Jeff pointed out, is exactly what you want for a bearing. As both the axle and the layshaft are 1.5mm in diameter the suggestion was that I should buy a 1.51mm reamer. Unfortunately it turns out that while the wheels I was using are advertised as having a 1.5mm axle they actually measure out at just under 1.54mm so I was rather annoyed to find that they wouldn't fit through my perfectly reamed out hole. The layshaft fitted very nicely though. My annoyance was exacerbated by the cost of the reamer.
I have a set of 20 mini drill bits that cost me around £10 and a set of 5 tapered reamers that I think were about the same price so I was rather shocked to discover that the cheapest 1.51mm straight chucking reamer I could find (part number A01510BAM from Toolex) would set me back £25! Now you can see why I was annoyed to find the axles were larger than advertised. My initial plan was to use the reamer for the layshaft holes and then open up the other holes in the second prototype using the tapered, but less exact, reamer. Since then, however, I've tweaked the chassis design and it would now be very difficult to ream sensible bearings using a tapered reamer due to the size of the bearing holes and access to both sides of the holes, so I've now invested in a 1.55mm reamer as well (part number A01550BAM) and on a quick test with some scrap this produces a perfect hole for the axles.
So two new tiny tools for almost the cost of a small locomotive kit, but hopefully they'll allow me to produce a number of my own models, which might in turn become kits and cover the cost. Not sure how successful this will be, but I can guarantee you'll get to read about the journey.
There is such a tool as an expanding reamer but I doubt they do them in such small diameters.
ReplyDeleteI did see mention of those when hunting around but didn't find one that was small enough. I'll certainly keep my eye open for a slightly larger one though if I ever need bigger bearing holes, as one of those has to be cheaper than a whole bunch of these in 0.01mm increments.
DeleteThe price of scarcity and small runs is high to say the least.
ReplyDelete