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What tool to enlarge a hole in fiberglass?

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Forum topic by Toller posted 07-31-2019 05:16 PM 463 views 0 times favorited 14 replies Add to Favorites Watch
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Toller

37 posts in 2077 days


07-31-2019 05:16 PM

The inspection port on the floor of my sailboat cockpit broke. It has a 4” hole and inspection ports for 4” holes no longer exist; I have to make a 4.5” hole. The boat is moored 120’ from the nearest outlet.

I intended to cut it freehand with a battery rotary tool, but people told me getting a decent edge was impossible; it would move around too much. I cut a 6.5” hole in a piece of plywood; the rotary tool has a shoe on it that is 1” from the “blade”, so running the tool around inside the plywood should give a 4.5” hole. Seems foolproof. Right? I haven’t tried it yet, waiting for no wind or boat traffic; otherwise the swaying will make it interesting.

Dewalt has a 20v jigsaw on a sale with a really nice price. I don’t really need it (don’t often use a jigsaw, and have a good corded one) but would like it. The jigsaw would make less of a mess, but unless it is going to make a nice hole, I can’t justify buying it for this project.

Whatcha think?


14 replies so far

View DBDesigns's profile

DBDesigns

229 posts in 475 days


#1 posted 07-31-2019 05:21 PM

Buy a hole saw attachment for your battery drill. They are available in a variety of diameters from Home Depot. then cut the hole in a piece of scrap wood. Use it as an outside guide for the hole saw. Then drink a beer and celebrate your ingenuity.

-- I remember when Grateful wasn't Dead

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SMP

1333 posts in 383 days


#2 posted 07-31-2019 05:30 PM



Buy a hole saw attachment for your battery drill. They are available in a variety of diameters from Home Depot. then cut the hole in a piece of scrap wood. Use it as an outside guide for the hole saw. Then drink a beer and celebrate your ingenuity.

- DBDesigns

I agree on the hole saw. What I usually do in fiberglass is run it backwards the first couple seconds, then switch to forward, flip to other side halfway through(if possible)

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Toller

37 posts in 2077 days


#3 posted 07-31-2019 05:44 PM

Actually hole saw was my first thought. They are $15 on Amazon. I am sure they are junk, but I only need one hole. But I thought it would be impossible to hand hold it since the pilot drill would have nothing to go into; I didn’t think of cutting the template first.

Obviously a hole saw is a better idea than the jigsaw, how does it compare to the rotary tool which will be free?

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SMP

1333 posts in 383 days


#4 posted 07-31-2019 05:54 PM



Actually hole saw was my first thought. They are $15 on Amazon. I am sure they are junk, but I only need one hole. But I thought it would be impossible to hand hold it since the pilot drill would have nothing to go into; I didn t think of cutting the template first.

Obviously a hole saw is a better idea than the jigsaw, how does it compare to the rotary tool which will be free?

- Toller

Hard to say what you mean by “rotary tool” and how much torque. Like a basic dremel? For me personally, not enough torque and too much dust, but I have done it. You’ll get the hang of how much pressure and how fast you can move by hearing it bog down, letting off pressure til it speeds up and repeating this dance. Much prefer a pneumatic die grinder for speed. But if its more about not spending any money, then yeah that should work, just wear a good mask.

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bilyo

816 posts in 1580 days


#5 posted 07-31-2019 06:30 PM

If you think that a jig saw will do the job (not sure how neat the hole has to be), how about borrowing or renting a small generator to run your corded one? If the hole edge has to be smoother, cut it slightly under size and use a large drum sander on your drill to true it up

View jdh122's profile

jdh122

1093 posts in 3295 days


#6 posted 07-31-2019 06:34 PM

My choice would definitely be a hole saw, as long as you have a way to secure the template (double-stick tape would do it if you have no way to use clamps).

-- Jeremy, in the Acadian forests

View John Smith's profile

John Smith

1969 posts in 640 days


#7 posted 07-31-2019 06:44 PM

if you want to have that semi-perfect hole to fit the insert,
you would have to first fill in the hole so the pilot bit can be the guide.
without a solid surface for any kind of hole saw or fly cutter,
the results will definitely be less than desirable.

I would cut a 1/4” plywood oblong disk to fit inside the hole
with a string in the middle to pull it tight to seal the hole from underneath.
epoxy this into place with 5min epoxy. cut the string when cured.
[if you don’t want to use epoxy to hold the bottom disk in place, use double faced tape].
cut a 1/4” plywood disk and epoxy that firmly into the 4” opening.
it is critical that the 4” disk can not move after it is epoxied in place.
let it cure – then you have a solid foundation for the pilot bit for
the tool you choose to use. (just like patching a hole in drywall).
other than that, you will be doing it freehand and may achieve less
than satisfactory results.

inspection ports have a flange that is larger than the hole it goes in.
the flange will cover most irregular cuts and will never show.
just make sure you have enough “meat” to fasten the screws into.

.

-- I am a painter. That's what I do. I paint things --

View ibewjon's profile

ibewjon

894 posts in 3270 days


#8 posted 07-31-2019 06:47 PM

Hole saw is best, but run in reverse as long as it keeps cutting. Forward can be too aggressive with new, coarse teeth.

View RobHannon's profile

RobHannon

304 posts in 1008 days


#9 posted 07-31-2019 07:16 PM

If the plywood guide wont fit in your situation and you have a 4” and 4.5” hole saw you can stack them this way. They sell special arbors for stacking them, but I have always just used 2 arbors and a piece of 1/4 rod in place of the pilot drill bit.

View Lazyman's profile

Lazyman

3871 posts in 1865 days


#10 posted 07-31-2019 07:54 PM

John, have you ever tried using Rotozip or spiral cutter on a Dremel on fiberglass? It works pretty nicely on other materials. It seems like that would leave a nice clean edges as long as you have a circular guide or trammel setup to insure that its actually moving in a circle. The Rotozip bit listings include fiberglass in their descriptions.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Toller's profile

Toller

37 posts in 2077 days


#11 posted 07-31-2019 08:01 PM



If the plywood guide wont fit in your situation and you have a 4” and 4.5” hole saw you can stack them this way. They sell special arbors for stacking them, but I have always just used 2 arbors and a piece of 1/4 rod in place of the pilot drill bit.

- RobHannon

I can fit a plywood outside guide on, but I have an idea similar to the double saw. I have a 4.125” hole saw. Presumably that leaves a 4” slug. If I put it on the 4.5” hole saw pilot, that will be about the same, won’t it?

The inspection port flange is 5/8”, so it doesn’t have to be exactly perfect; but it should be pretty good.

Sadly, I suffer from OCD. If there were just one way to do it, I will just do it. But since there are several, I am perplexed.

View ibewjon's profile

ibewjon

894 posts in 3270 days


#12 posted 07-31-2019 08:39 PM

No reason to be perplexed. The hole saw and plywood guide is the easiest way to go. If the 4” slug fits snugly, you can try it. I would use the plywood template taped down.

View Craftsman on the lake's profile

Craftsman on the lake

2994 posts in 3915 days


#13 posted 07-31-2019 09:05 PM



If the plywood guide wont fit in your situation and you have a 4” and 4.5” hole saw you can stack them this way. They sell special arbors for stacking them, but I have always just used 2 arbors and a piece of 1/4 rod in place of the pilot drill bit.

- RobHannon

I’ve used this method and it works quite well. The existing 4” hole saw acts as a guide and the outer one does the cutting. With most hole saw arbors you can stack two blades one over the other. This is the best way to get a clean cut. With fiberglass, run it backwards to make a clean groove before you finish the cut in forward mode. Go lightly and the cut will be clean.

-- The smell of wood, coffee in the cup, the wife let's me do my thing, the lake is peaceful.

View shipwright's profile

shipwright

8355 posts in 3275 days


#14 posted 07-31-2019 09:32 PM

I had to cut a couple of holes in my sailboat this spring. I used a die grinder from HF and a composite cutting burr that I found online. It is 1/4” and would likely fit your tool and cost less than a good hole saw.

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese! http://thecanadianschooloffrenchmarquetry.com/

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