While I’m messing around with other things I wanted to leave a follow up / observations / tips if you are thinking of making one:
(caution - long boring slog)
The plastic square/s are legit for 90’s, they are as accurate as you want them to be, but it’s pleasing & preferable to view them as ‘consumables’. I suggest making a couple at a time.
I don’t mean that they are not durable, they are, I mean that since they are inexpensive, can be cut into without dulling blade etc, you don’t have to baby them. So they are well suited for certain on site work situations where you might prefer to keep your machined rail square reserved for the shop. I.e. dropping one of these isn’t much of a consideration nor is it’s disappearance.
The ability to check accuracy on site by flipping them on rail, without precision squares in tow, also lends itself to on site work environments. No tools or accessories needed for 45ºs keeps things simple too. Hypothetically if there was a way for it to go out of square enough to matter, a sheet of sandpaper and any reasonably flat surface quickly dials it in. But this would be a high abuse / high neglect scenario.
Not sure if I would notch them for clamps for 90º cuts, I think those two long empty spaces have taken enough material out but you can try it. However I don’t see any problem with trimming about 20mm off the 45º corner to create clamp space that works for both regular & reverse 45º cuts.
I think for the trouble of setting clamp/s, requiring that you go to other side to install and tighten a clamp, you might as well be measuring and setting to a line or second measured mark anyways. An on-rail square is about avoiding that extra work, when applicable.
But if clamps are a must you can always glue in media to reinforce the gaps (recommend only Loctite
20min Plastic Bonder for this Polycast material and a patient 24 hour cure. Or just melt in some metal or more Polycast with a soldering iron).
The 45º edge can flex in the middle with enough pressure because of the long empty slot but the ends, that count for registration, don’t flex. So if you know you won’t be registering the full length of 45 edge then glue in a stiffener. This will also help if you plan on plunging the 10~12 mm into material for reverse 45º’s.
Incidentally the thick edge of the square, that drops into the rail clamp slot to register, is almost exactly 48º in relation to bottom of splinter guard edge maybe missing blade by a hair on max bevel cuts.
I discovered that some Kreg t-bolts have off centered heads biasing bolt to one side after I drilled my holes tight so just be aware to check/select them or drill holes slightly larger to prevent bowing / binding as knobs tighten.
Recently saw a bright orange Polycast square made by another company and it had more empty spaces as well as the perception of being a different formulation, slicker material. All were warped. So choice of square is going to be important. The blue one in images is Empire.
I’ve kept both of mine stock but you could trim down thick edge so square sits flush on surface. Though I think the square benefits from the mass so I just use extra washers.
When I did my accuracy tests I used two washers in one location and one in another because the square has different relief depths molded in and I didn’t want to flex the square when tightening. Again you can just fill this space in, glue on a stand off, etc if you don't want to mess with washers. If you punch or drill the rubber washers to the right size they stay put gripping the t-bolt threads.
To mount square - flip rail over, insert Kreg’s with washers on, drop square onto bolts and position FIRST,
then push the square’s thick edge into clamp slot, then tighten knobs. Sliding square while edge is engaged in slot doesn’t work (if it does you sanded way too much off). To remove - loosen knobs (a lot) and with finger tips pull up on square edge to pull out of clamp slot. With edge disengaged and knobs loose you can reposition/slide anywhere you want.
I didn’t like the idea of clamping atop that raised molded text so you just take a razor blade and slice them off flush around knob - but it’s not necessary.
Obviously this is a hang-off-the-edge rail square. So since you have the space it’s nice to use oversized knobs.