37mm hole?

[ChaiLatte]

So a forstner does not cut a perfect circle, then?

Forstner bits cut very good circles. The difficulty with the sanding idea is that the bit itself is not a continuous circle so it would be almost impossible to sand down and for it to remain round just by freehand pressing something onto it.

I did find one 37mm one, by Famag, but it appears to be discontinued, limited stock:

https://www.famagtool.co.uk/famag-b...yed-tool-steel-long-version-37mm-fam162303700
This image, from Tormek, shows a possibility. Replace the carving knife with the Forstner bit and leave the clamp only snug. Rotate the bit slowly as the wheel grinds it down.

You could rig up a holder for the bit and use a bench grinder or belt sander to reduce its diameter. You just need a sure-fire way of controlling its distance from the abrasive. That could be as simple as a stop block with multiple layers of masking tape totalling 1/2mm in the beginning. Push against stop, rotate and grind until sparks stop, remove one layer of tape and repeat until all tape is gone.

 

[Phil Pascoe]

I wasn't thinking of using any kind of bit, I was thinking of widening an existing hole. I think we got our wires crossed.

 

[loftyhermes]

Wouldn't it be easier/cheaper to change the bearing size? Or am I not understanding the problem.

 

[ChaiLatte]

Wouldn't it be easier/cheaper to change the bearing size?

From his post above, I think he has chosen that bearing because it has a 30mm bore.

If you search here:

https://simplybearings.co.uk/shop/advanced_search_result.php?search_in_description=1&keywords1=30&
30 ID x 37 OD is the only available option with 30mm bore and less than or equal to the 5mm thickness of his sheet.

 

[Inspector]

I would look at a Rotabroach hole saw. If the hole isn't a press fit you could epoxy the bearing into the hole.
https://www.engineeringsupplies.co....-carbide-tipped-tct-hole-saws-rotabroach.html
Pete

Adding they you should lubricate while drilling. Soap and water or cooking oil are not going to affect the plastic like some petroleum based oils can.

 

[Fergie 307]

1.5 inches is equivalent to 38.1 - so larger than a 38 I suspect.

Weird, if I ask Google what 37mm is in inches I get 1.456. But if I ask for 1.5 inches in mm then it says 38.1. Mitotoyo caliper set at 37.00mm then converted to inches says 1.4570. At a guess I'm thinking 0.03937 as the conversion factor for mm to inches may be more accurate than the 25.4 factor used generally for going the other way. Any mathematicians out there.

 

[Fergie 307]

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).

If you could get it on a lathe or mill then you could just cut the holes first to whatever size bit you have, then finish to size with a boring tool. Now we know the OP wants a few I wonder if it's not worth just going to a local machine shop. Especially now we know it's for a bearing where the hole will need to be precise. I doubt a 37 mm forstner but will cut a hole exactly 37mm, it is almost certain to be a gnats bigger.

 

[MikeJhn]

I did find one 37mm one, by Famag, but it appears to be discontinued, limited stock:

https://www.famagtool.co.uk/famag-b...yed-tool-steel-long-version-37mm-fam162303700
This image, from Tormek, shows a possibility. Replace the carving knife with the Forstner bit and leave the clamp only

It does say limited stock, but lets you put it in the basket and buy it.

 

[Fergie 307]

Lol, not quite as bad as 500mm square, but like this MkI:
View attachment 168899

...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...

If you have an adjustable boring tool that will fit the lathe headstock you can do it by mounting the piece on the slide, so the tool is turning in the chuck, and the work is fed into it. Only really doable on a metal lathe, where you can advance the saddle accurately. So not really a viable solution to your problem, but a tip that maybe useful to someone.

 

[ChaiLatte]

I doubt a 37 mm forstner but will cut a hole exactly 37mm, it is almost certain to be a gnats bigger.

Yes. I think if you took it to a machinist, you'd need to include a sample of the material and a bearing that is to fit in the hole. Then it could be tuned to do the job required rather than to any specific dimension.

If you have an adjustable boring tool that will fit the lathe headstock

A straight shank boring head in a drill press would do it, especially if you had a fixture for your stock. Rip through it with a 36mm bit, change to boring head and finish the last millimetre.

I wonder if an old fashioned tank cutter or lathe/mill flycutter would be OK for the final cut. More difficult to adjust than a boring head though.

 

[Fergie 307]

With you on the straight shank boring head. Very expensive solution unless you already have one. I did some holes in a hard acrylic a few years ago for someonr, probably quite similar. Used a four head boring tool works well but need a high speed on the tool for a neat cut, then slow feed and plenty of coolant so the stuff doesn't melt !

 

[Fergie 307]

I wonder, bearing in mind its quite soft stuff, could you use a 36mm forstner and then a hone as below. These are intended for cylinders so you would have to be careful to keep the pivot point of the shoes in line with the sheet to keep the sides parallel. But ought to work, and enable you to fine tune the size. Just Google cylinder hone.

 

[John Hall]

Use the smaller bit, then use a drum sander

 

[HamsterJam]

Have you tried looking for an imperial bit? 37 mm is 1.456 inch so a 1 1/2 inch bit would only be a gnats over, around 0.002 in. Depends how accurate you need it.

Actually 1.456 is an error of 0.044in which is just under 1.2mm.

A 38mm bit is closer and will make a hole 0.1mm too small. (1.5in = 38.1mm)

This all rather assumes the bits cut to this accuracy anyway. You might find a 38mm bit that cuts 38.1mm.

I’d be tempted to go 38mm and open the hole out 0.1mm with abrasive.

 

[Phil Pascoe]

But he needs a 37mm hole.

 

[Fergie 307]

Actually 1.456 is an error of 0.044in which is just under 1.2mm.

A 38mm bit is closer and will make a hole 0.1mm too small. (1.5in = 38.1mm)

This all rather assumes the bits cut to this accuracy anyway. You might find a 38mm bit that cuts 38.1mm.

I’d be tempted to go 38mm and open the hole out 0.1mm with abrasive.

You are quite right with the error, so used to working in metric I was taking the error as a mm figure, which of course it isnt. Only excuse is that I am sitting at home at the mo with a bad dose of flu, so the old grey matter not functioning as well as normal Still think slightly under then finish with a hone might be a good solution. I have used these hones with strong rubber bands on the either end of the shoes to keep them parallel whilst fine tuning holes in thin metal, don't see why it shouldn't work on perspex.

 

[Sideways]

Weird, if I ask Google what 37mm is in inches I get 1.456. But if I ask for 1.5 inches in mm then it says 38.1. Mitotoyo caliper set at 37.00mm then converted to inches says 1.4570. At a guess I'm thinking 0.03937 as the conversion factor for mm to inches may be more accurate than the 25.4 factor used generally for going the other way. Any mathematicians out there.

25.4 mm dead is the metric equivalent of one inch.
The inch has been defined from the metric using that factor since 1930
Any differences are either measurement error, truncation or rounding error, or some damn fool google algorithm deciding that just because 99 out of a 100 references to Pi on the internet are truncated to 3 digits or approximated as 22/7, then that's what Pi is....

(you get my point, crowdsourced opinions aren't a reliable reference for technical questions that have a genuine right answer.

 

[loftyhermes]

From his post above, I think he has chosen that bearing because it has a 30mm bore.

If you search here:

https://simplybearings.co.uk/shop/advanced_search_result.php?search_in_description=1&keywords1=30&
30 ID x 37 OD is the only available option with 30mm bore and less than or equal to the 5mm thickness of his sheet.

How about drilling a 38mm hole and wrapping the bearing with 0.5mm shim stock cut to 116mm long x 5mm wide? Would that not work? It'll certainly be cheaper than a 37mm drill bit. I'm saying all this but I'm no engineer, just a die hard bodger.

 

[HamsterJam]

But he needs a 37mm hole.

Doh!!!
I’ll write out 100 times “I must read the question before answering“.

 

[GrahamRounce]

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 bit of 38mm OD stainless steel pipe, mounted somehow, filed down to 37mm on the wood lathe, and with teeth roughly ground out with a dremel.
Either that, or two pieces of 3mm tool steel sharpened, epoxied 31mm apart, and mounted to make a sort of circle cutter.
Sigh. It would be nice to have a laser cutter, but they're still quite expensive for a decent wattage. (Or are they despised here? Quite right!)