Enlarging a round hole

[Stevep]

Hi all -- for years I have read about a technique to enlarge a hole by mounting two hole saws on a single mandrel. One hole saw, the smaller one that is the size of the existing hole, is used as the "pilot bit" for the larger hole saw. However, I have never met/talked with anyone who has actually done this. I have a great need now to enlarge an existing round hole and this technique looks like it would work best. I have several different kinds of mandrels and hole saws but none seem to be suitable for this double hole saw process. So, have any of you used this technique? If so, would you please let me know the brand/type of mandrel and hole saws that you used? All hints and suggestions would be very much appreciated. Thanks.
Steve

 

[Nereussailor]

Hole saw

I've used this technique before when I had access to the back side of the hole.
Fasten a board to the backside of the hole that covers the hole completely. Then take the hole saw that cut the original hole and dill the pilot bit into the board. Then take the hole saw for the size of the hole you want now and put the pilot bit in the hole in the board. Drill your new hole.
Hope this works for you.

 

[Ross]

Steve rather than spend more money for a tool may use only

once, try this: Cut a hole of the size you want in a scrap of 1/2 inch plywood, center that on the hole you wish to enlarge and fasten with hot melt glue, bore your new hole and pop the plywood loose. This method also allows you to move the center of the hole.

 

[Brian D]

I would agree with that technique

But I would make the new hole a silly mm smaller that what you need. Then use a curved rasp or file to gently increase the diameter. This will prevent you from expanding the hole larger than you originally wanted. Measure twice, cut once!

 

[jviss]

Brian, ...

So, I must ask, where would one buy a hole saw that's "a silly millimeter" smaller than what you need?

I've installed tons of instruments on boats (professionallY), and believe me, no one who knows what they are doing does this.

 

[Timo42]

The tool you are looking for is

available at Rockler, works well, I have used mine twice, well worth the 8 bucks. I even like the name, though I have never mismeasured a hole *D Tim

 

[John]

A simple alternative method

Cut yourself a peg that you can drive into the existing hole. It should be tight, with then end more or less flush with the material you are trying to drill through. Then use this peg to hold the bit steady for the new, larger hole. Simple as that.

 

[Brian D]

The point...

And it appeared to have been missed, is cut smaller than measured and gently expand.

 

[Ross]

Obviously there are several ways this job can be

accomplished and they will all work. The next thing to do is the pick the most convenient method for you and go with that. You can also scribe the new diameter and cut it with a jigsaw.

 

[Ross]

John, I must ask. What method do you use to cut

a 1 7/8 inch diameter peg to make a tight fit in a 1 7/8 diameter hole?

 

[jviss]

The point...

... was not missed, Brian. As a product designer myself, I can all but assure you that the instrument designers designed the housings of their instruments to fit properly into holes produced with commonly available tools. Doing otherwise would be foolish, since a difficult-to-fit instrument would meet with an inhibition to sell on the part of dealers. Companies selling internationally will design instruments with enough fitting tolerance that English or the close metric equivalent will work (although I wonder about Raymarine, who specify 90mm with the English equivalent of 3.54"; a 3.5" hole saw might be over by .040, but maybe not. Simrad's unit dimension is 88mm, although in their manual they give the English equivalent of 34.6"! Whoops! Makes you wonder about any calculations the instruments might be doing, eh? I'm guessing a 3.5" hole saw will work in either case).

 

[Les Blackwell]

Another way....

No one has mentioned a way that I learn to cut a slightly larger hole. I wanted to install a new faucet in the galley. After removing the old faucet, I mixed and inserted some putty stuff that hardens in a half hour. You can get it at West Marine and most hardware stores--a tube goes along way. It sticks to wood, metal, iron and even hardens under water. Once it became hard, I drilled a pilot hole and then use a circular saw on my drill. Got a perfect new hole.

Les

 

[NautiDuck]

I have done it

Just as you described. I had 1 1/2" thru-hulls and needed them to be 2" so I nested a 1 1/2" hole saw in a 2" hole saw. The 1 1/2" stood out just a bit which made it perfect for drilling the new hole.

 

[Stevep]

thanks

thanks everyone for the greatly appreciated tips and advice. With these ideas, I know that I will be able to successfully enlarge the hole.
Steve

 

[BART]

oops arbor for hole saw

The part you are looking for is called an"oops" pilot arber from Starrett part #66078 A19. list for 2008 is $8.20. It allows you to use 2 hole saws on the same arbor, one smaller that the other. Contact Starrett at www.starrett.com.
Or call them 888-674-7443. You may be able to buy online with a credit card.
The other choice is to find a good industrial supply house in your area that handles Starrett.

 

[Pete]

Hoof beats and zebras....

Keep it simple if you can. If you have to enlarge an existing hole only a "bit," you may be able to do that with a Dremel (either cutting or sanding disks)or a sanding stone on an ordinary drill. Or perhaps use a wall board hole saw or a hand coping saw, all of which are found in most tool boxes. If the new hole is going to be covered by a bezel around the new instrument, that will give you a lot of "wiggle room" to enlarge the hole and not really care what it looks like as it'll be covered up. What I always try to avoid is having to buy a new tool that will only be used once.

 

[sandpiper]

alternative tool

Never tried this intriguing idea but why not draw out the desired hole then cut with a jigsaw?

 

[tedsbeds]

HOLE IN A HOLE

When I have to enlarge a hole I use a drum sander mounted in my drill. You can get them in many different diameters and grits. I just run it around the old hole till I get the size I need. I also use this attachment on an angle to take the rough edges that could chafe a hose passing through.