enlarging holes in fiberglass or acrylic

[Tonyz]

I have a set of the std hole cutters from Bunnings.
I have seen various adjustable diameter hole cutters but they appear to be about 6mm increments. what do I do when Id need to get about 2 mm larger than hole cut.

Is sanding/filing the only way...sigh

[China]

I have used a home made flap sander to do this in fiber Cement sheet

[BobL]

What hole size are you after, how deep is the hole, and what is the "thing" you are drilling into?

[Skew ChiDAMN!!]

A couple of times in the past I've wedged a block of wood into the hole to give the drill something to centre/bite into while re-drilling with the correct size cutter.

I do not recommend it, there are both safer methods. The only reason I've gone that path was time:cost:urgency issues and insurance was in place.

I 2nd China's suggestion of a flap sander... only I wouldn't bother making my own; they're cheap enough to buy, come in a variety of grits & sizes and last a surprisingly long time.

I've used the same 2" 60grit on melamine, fibreglass, AC sheet and similar when fitting taps for about... 10 years(?) now. (Admittedly, I only use it a couple of times a year. But still.)

[Tonyz]

hole size 94mm

2mm fiberglass and acrylic sheets,

flap sander sounds good, would be best in drill press rather than cordless drill?

[BobL]

Flap sander will definitely get you there.
If you have many to do and want a more accurate hole then a small router and template would be the way to go.

[Skew ChiDAMN!!]

I'd use a flap-sander for a couple... but for more than that BobL's advice is on the money.

It'd be quicker and give better results.

[BobL]

Either way wear a mask.

[rrich]

If possible, drill the correct size hole in a piece of plywood. Clamp the plywood to the piece where the hole needs to be enlarged. Carefully align the plywood and use the plywood as a guide to drill the larger hole.

You could even back up the material with an additional piece of plywood to insure a smooth hold.

OR

With the same plywood and hole drilled, use it as a template for a bearing guided flush trim bit in a router. Probably the best solution.

[Tonyz]

err rrich, to be rude here mate, cobber, bro, buddy, fraulein but if I could drill the correct size hole in plywood then I could do it on the other stuff.

oh....I see where your coming from I said about enlarging a hole. hole cutter I have is 89mm and I want a 91/92mm hole.. yeah ok. but again if I could drill 92 in ply then I could drill it in anything.

dang think another red is overdue.

[Arron]

Would it work to clamp a piece of timber behind somehow, then redrill with one of these?

Hole Saw Drill Bits - The Ultimate Guide - scroll down to adjustable hole cutter.

[Tonyz]

this is what Iam looking at but still has limitations

51mm – 178mm
6.4mm increments

6mm increments is still a sizeable gap. hence why I am asking for widening ideas.

[KBs PensNmore]

Any chance of a pic showing what you're trying to do, please? How thick is the fibreglass that you're trying to open up?
Sometimes a picture can save a thousand words. What do you have available to use? Telling us what tools etc, helps everyone, no point in suggesting the use of a router, if you don't have one. If you do have a router available, you could use a piece of masonite, MDF or ply, draw the size hole required and use the circle attachment to cut the hole, then stick with GOOD quality double sided tape, or screw it to the fibreglass. Depending what side you have access to, use a copy collet or a trim bit on the router, and cut away.
Hope this helps.
Kryn

[malb]

I am not a big fan of the cutter type you are showing in the pic above, they were originally intended for cutting rubber and leather washers with a hand drill, rather than hard materials with a power drill. The particular unit illustrated appears to work in steps because the screw head at the inner end of each arm locks into the notches along the opposite arm.

I suspect that if you remove the arms, and take the inner screws out and fit lock nuts on the screws, then replace the screw and arms, you should be able to set the unit a size smaller than you need, micro adjust the system to required size by winding the screws out a maximum of 3 or 4 turns, and tightening the lock nuts on the screws against the end of the arms. Should be plausible if you already have the tool, but if you don't, I think I would visit an industrial supply place and buy a quality hole saw cutter at the correct size, they used to come in about 1.5mm increments if you are getting reputable brand ones. If you go that way, drill slowly so you don't melt the acrylic or chip the fg sheet.

[ian]

err rrich, to be rude here mate, cobber, bro, buddy, fraulein but if I could drill the correct size hole in plywood then I could do it on the other stuff.

oh....I see where your coming from I said about enlarging a hole. hole cutter I have is 89mm and I want a 91/92mm hole.. yeah ok. but again if I could drill 92 in ply then I could drill it in anything.

the most accurate way to "drill" or cut a 92 mm hole in ply is to use a router and trammel to cut the initial 92 mm template, and then use the router and a template following bit to make the "real" hole.
If you don't have a suitable template following bit, the same technique will work with a guide bush -- just that the guiding hole needs to be a little larger, by half the difference in diameter between the outside diameter of the guide bush and the diameter of your router bit.

The above is easiest with a trim sized router