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Ah, ok, thanks. Probably candle wax then. (Just looked at 37mm reamer prices - EEK! No!)

So anyway, here's the thing: The hole in perspex is reasonably clean. It's cutting a bit hot, but I guess I can put up with that. It was ½mm dia oversize at first, then a bit more filing and sanding brought it down to only fractionally over. Now to find a 37mm reamer, or more likely a round...

This thread has come to the end of its natural life, so thanks to all for your suggestions and advice. Though the pillar dtill is too weak, this works pretty well, and this (where I tried to reduce the cutting area and ground the tips down to "scrape" not cut) might have been ok if I'd been...

That sounds sensible, too! And there's nothing wrong with being a die-hard bodger - our day will come!

Haha, I've just discovered what I should have known anyway, that my 450W bench drill won't drive a large (36mm) forstner bit through plastic, or even wood... but as the hole has to go right through, it's a bit of a waste of energy cutting out the whole circular area anyway! I'm now thinking of a...

Lol, not quite as bad as 500mm square, but like this MkI: ...but a bit wider, and the 12x21mm bearing replaced with 30x37mm. (Not thinking ahead with enough clarity these days :-( ) So it needs to be a push fit, and alas it won't go on the lathe! Anyway, as I'll need a couple of dozen, it...

Thanks for the ideas - I'll probably try them all! I take it we think that somehow shrinking a 38mm forstner is a non-starter? (For anyone who has similar trouble with trousers... legs DON'T come only in even numbers. I've discovered that M&S realise that.)

Hi - I need to make neat and accurate 37mm holes in 5mm perspex. Forstners work excellently, but I can only find 36 or 38. Can anyone think of a way to accurately convert (grind down?) a 38mm one? I have a wood lathe with chuck, routers etc, but no milling machine. I haven't tried a flat bit...

Yes, I get that, but you'd think the long size would be the "special" rather than the default?!

Hi - I've been doing some drilling into steel with a 12mm hss bit, and of course it vibrated like anything - because it's 15cm long! Why? How many users are going to need to drill through 10cm of metal?! Very few, I bet. For the vast majority (correct me if I'm wrong), 99% of holes over about...

Ok, thanks all....!

Hi - I've wondered for some time about why, when sawing or jigsawing or drilling, wood plastic or even metal, it goes easier just before breaking through at the end? The sound becomes smoother and the cut seems to go faster. Is it the material somehow softening, or is it when the saw points...

Ok, thanks. I've gone off in a slightly different direction now, but I will bear that in mind, likewise all the other helpful/informed ideas.

@J-G: Ok, yes. Scratch "Mod 3" and insert "3mm tooth spacing". I'm sure there is a logic to the mod system but I haven't yet got my head to an intuitive understanding.

I'm thinking of slow-moving with potentially lots of strain, so could nylon cope? I did look (on ebay and in my memory!) at Meccano ones, but the teeth would be a bit small and the larger ones too thin.... It's for a robot knee joint, in case anyone's wondering. Mk's I to VI were with this...

I realise that precision hardened industrial steel gear wheels are going to be expensive, but I wonder why (apparently) there are no cheapo hobbyist-level ones available? I expected to find thousands from "£5 for 10" upwards when I looked for say 3mm thick 20-50mm dia mod 3 's, ideally with...

Good, thanks!!

I've ordered 100 of these from AliExpress. Perfect! Thanks.

Excellent! Good idea! The middle of the rod/bar wants to be about 20-something mm from the plate to get a 45° pivot, and they'd need to be strong, but it still sounds a good possibility. I'm looking!

Ok, from the side: The bits on either side are aluminium plates about 1cm thick (and 30cm square) that I want to swivel in a manner similar to those toy snakes you used to see, up to 45° either way. I put some effort into fabricating them before thinking that perhaps they, or something that...