radius template

[marcros]

I have no idea where to post this.

I need to prepare a piece of acrylic to go into a blind routed slot in another piece of acrylic. I do not think that I can square off the slot, like I could in wood, so am looking at ways to bullnose the inset piece. The inset is a piece 40mm long x 5mm wide, probably 10mm deep.

My thoughts were to have a template of the radius and then sand the corners off a square cut piece. the inset needs to be accurate to a few thousandths of an inch in length. The thickness will be set by the piece of acrylic sheet. It will be glued with CA, but needs to seal when glued. Something like a washer would be ideal as the template, just clamped in place, but 5mm washers don't seem to exist. With this method, I could sneak up on the final size until it fits.

Any ideas what I could use as a template?

I could drill a hole in a sheet to use as a check gauge.

 

[Normancb]

You can buy 5mm diameter silver steel quite cheaply and it's ground to very close tolerances - could you use a couple of slices of that, clamped either side of the work? At that size very easily hardened too if you want to.

If you have access to a metal lathe you could make 5mm washers from silver steel - again easily hardened.

However, if you use Tensol cement instead of CA you'll have essentially a welded joint with enough gap filling to cope with quite a lot of error - and a lot stronger than CA.

 

[dickm]

+1 for the Tensol cement. Just having difficulty working out how you will hold the 5mm diameter template in place without whatever is holding it getting in the way of the router. Might it be better to do some careful shaping of the end of a strip of ali/paxolin/MDF, using the pieces of steel rod as a filing guide? They would need to be held against the strip in a vice, but this would still allow access for filing. The strip template could then be clamped on to the workpiece well out of the way of the router.
Or am I not visualising the situation correctly?

 

[marcros]

Dick,

I may not have explained it quite clearly, and probably have not thought about trying to clamp a washer in place.

The router is not for the shaping of this piece, it is only for slot cutting- I was going to use a sanding block of sorts. Mark it out, cut it to within a mm or two and use the template to shape it- steel being harder than acrylic.

This is not a great example, because it is laser cut into a tube, whereas my main part is to be solid https://www.pennstateind.com/store/PKSSB38.html. also my parts are straight rather than wavy.

Idea 2 is that I could use a needle file to square off the end of the slot. It may mean drilling the blank first because I wont be able to get to the very corner of a blind slot. My fear is then having to accurately re-drill a hole that is not evenly resistant on both sides of the drill bit. This may not be an issue, I have not tried it before. The large piece would be a 16mm(ish) tube, with an 4mm wall thickness- perhaps filing would be the easier thing to do.

 

[marcros]

and thank you both for the tip on the glue, I will look into that.

 

[dickm]

Sorry, Marcros, I should have read your post more carefully. But if you are "just" sanding/filing the end of the insert, then possibly make a jig to hold the bits of steel rod exactly colinearly (a sort of tuning fork with hole through the two legs into which you fit the rods?). This would keep everything lined up, and if the rods just project a mm or so when pressed against the acrylic, the whole thing could then be held accurately in a vice for filing?