This is a very topical thread for me too, as although I've already posted elsewhere on the forum about something on these lines, I thought I'd found the solution, by filing down a brass strip, which would form the base slot carrier, and possibly adapt an Incra V127 mitre guide to fit onto this. The only trouble being swapping between two different machines, with different size guide channels. Both of my machines are old machines (though excellent working order and condition).
If you're right, and someone on the forum did have his cast bed machined out (I wondered about having the same thing done, if I could find a company willing to do it, and whom wouldn't charge an arm and a leg) and it was £90 some years back, then that's quite a steep price in my view. Okay, so if there is enough material to retain the bed's integrity and strength, then I could I suppose hammer them a bit on price for doing two, but it's still not going to be good value really.
I'm very tempted by the Incra V127 mitre guide, as it has laser-cut teeth accurately cut at each and every degree position. So setting up specific angles would be both quick and precise. However, removing the swivel from the slide might, might prove easier said than done; or rather, making a replacement to which I could either screw or rivet the Incra to, would be difficult to do with complete accuracy, and especially in a way to possibly design it to also fit another machine tool too? Maybe some rivets, which would be tapped end on, to swell their diameter to that of the hole in the channel guide slide bar. I could then make up two slide bars (sleds?) both with the same size holes, and a threaded thumb-tightening, knurled knob which would help aid a secure angle-hold (to back up the 1-degree laser cut grooves).
This would then facilitate swapping from either my bandsaw to my circular saw, or vice versa. I don't really want to have to buy 2 mitre guides, if I can manage well with one, that's set up to do both, or either machine, and it can simply be removed and affixed to the other channel guide.
I'm not sure if I've explained things well here, but I've tried my best.
I also want to extend, lengthways, not widthways, both of my saw table beds, so I can attach this when needed. I don't really want a solution that is permanent, or semi-permanent, for it'll take too much valuable space up. But this is a subject for another thread. I'll explain in more detail in that thread, for I've not heard or read anything on what I'm trying to achieve, nor how I can envisage achieving it.
Anyway, interesting read everyone.
