37mm hole?


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26 Jul 2007
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Bethnal Green
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 yet, but can't see it being all that neat, except for expanding a 36mm hole, *maybe*..

If you have a 37mm flat bit, could you drill your hole in a piece of wood. If you are happy then use this a s a template with a router and top bearing trim cutter to make a nice job of your perspex
Have you considered cutting a 37mm hole in a piece of MDF or similar and using this to guide a router with a trimming cutter-if you started with an undersize hole in the perspex?
Would an expansive bit be accurate enough? Probably better than a flat bit in perspex if taken steadily
A starrett or Bosch hole saw should do it , I use starrett but a trial cut to get the best speed is advisable . Place the Perspex on a sheet of scrap ply or mdf it’s best to drill the pilot hole 1st as the tip can get hot and it can melt the Perspex -then use the hole saw with the pilot drill in the pre- drilled hole in the perspex and off you go -
Templates for routing it out of the perspex sound good.

If you don't have a 37mm drill to make the template! 25mm forstner hole to start, 6mm rebate all the round the circumference.

Flip over template and flush trim from the other side - makes a 31 mm hole. 6mm rebate and flush trim again makes 37mm exactly

I'm sure there are videos that explain this router trick much better than me - but I can't find one right now!!

Good luck
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.)
Perspex can crack easily when being cut or drilled so use sharp tools and don't force it. Hole saw and router in template all good ideas. You can file a flat bit down quite easily , make sure you file equally from both sides. I think the type with spurs would be best as the cracking risk is less.
I've never used one, but I wonder whether this would work?
I think that would be an expensive way to create a chipped hole.I've drilled quite a number of holes in perspex and other acrylic materials and learned long ago that the factor that causes chipping is the rake angle of the cutting edge.By backing off the cutting edge (similar to a flat bit),there is almost no tendency for the bit to surge through and it tends to be this snatching action that pushes the body of the bit suddenly through the work.Those expanding bits would have all manner of uses and can be a tremendous help,in fact I have had an Irwin equivalent for use in a brace since the 1970's.I wouldn't risk that amount of money without some assurance that success is likely.I also have doubts that a flat bit would give a clean hole as it would be prudent to finish the hole from the other side and the tip of most flat bits would be likely to shake the piece and cause a ragged hole or mis-alignment.You could try a smaller hole and a bobbin sander,but they tend to melt the plastic and clog the bobbin.I stand by my earlier recommendation.

The lathe technique might be a possibility for a job that will fit the capacity of the lathe.If it is a finger hole in a 500mm square door,it could be a challenge.
What sort of finish do you want on the side of the hole?

Any drilling method will lead a cloudy, rough finish because the tool is moving in a helical path as it penetrates the material.

The routing method, where you are shearing chips from the full depth of the hole, while traversing a circular path will lead to a much better looking hole, if that is important to the application.
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 would be nice to have an actual bit, but I'm wondering about using a 36mm forstner, which I have, then making a 37mm flat bit to widen it, with its first half tapered slightly to hopefully get a smooth transition. I'll look for an Imperial forstner though, as that might just do...
It's half a mil on the circumference. Surely you could wipe that out with a fairly coarse wet and dry?

It is 0.5 mm on radius, 1mm on diameter and about 3.14mm on circumference, so you only a factor of six out.

The sandpaper strategy might work on a round object, slow and tedious, but you'd get there. However, a forstner bit has a very interrupted perimeter so every time you went over the gap (twice per revolution), the paper would fall into the gap. Even if you used a file, it would straightline between the start and end of the gap.

It would be possible on a metal lathe with a grinding spindle (e.g. Dremel, router with mounted abrasive point) held in the toolpost but you would need a Forstner bit _without_ the brazed carbide tips (e.g. the Bosch one).