7 1/2" hole saw

[drboom]

This feels like a stupid question but I can't find a hole saw in the appropriate size. I need to make a 7 1/2" hole in my house for an ERV vent (see the instructions) and can find no source for a hole saw of the appropriate size. Where can I source an appropriate hole saw - hopefully not crazy expensive given I need to make only 1 hole in the house (standard sheathing + siding or the wood sill, no masonry)?
I see some adjustable diameter drywall hole saws but it seems questionable if they will survive wood and most of them are aimed at a material thickness no more than an inch or so. I also see a few conventional hole saws in 7 1/2" but they are all only for depths of around 1". The sill is certainly more than that and I'm unclear on how thick the sheathing + wrap + siding is.
Thanks

 

[Mr onetwo]

drill a bunch of holes on the sill, ect. and then use a scroll blade in a sawzall

 

[PCustoms]

1 off large diameter cut?

Draw it out (string, stick etc.) and cut with a jig or reciprocating saw.

If you had a series to do something more consistent would be worth investing in.

 

[PCustoms]

Also where did you even search?

Hole Saw 7-1/2" (190mm) for Wood, 7.5 inch HSS Bi-Metal Hole Cutter with Pilot Drill Bit for Woodworking, PVC Boards, Drywall and Plastic, 3/8" Triangle Shank https://www.amazon.com/dp/B0BM4KLQBQ?tag=atomicindus08-20

I bought a vikiton branded one a few months ago in 6" and it worked great for 6 holes I needed

 

[engineer2]

Amazon has them. Close to 190 mm.

 

[Beerhippie]

The chances that you come away from boring a 7 1/2" hole with a hole saw in a hand-held drill while standing on a ladder without being laid up for a few weeks? Low.

Scribe, start-drill and use a jigsaw. Use a long "bell hanger" bit to establish the holes on both sides of the wall. I leave the bit in the hole and use it as a hub for a "string trammel" to mark the diameter.

 

[Rc_Guy]

Adjustable hole saw

 

[Beerhippie]

Adjustable hole saw

FFS! The guy needs to cut two holes!

 

[Jim greengo]

This feels like a stupid question but I can't find a hole saw in the appropriate size. I need to make a 7 1/2" hole in my house for an ERV vent (see the instructions) and can find no source for a hole saw of the appropriate size. Where can I source an appropriate hole saw - hopefully not crazy expensive given I need to make only 1 hole in the house (standard sheathing + siding or the wood sill, no masonry)?
I see some adjustable diameter drywall hole saws but it seems questionable if they will survive wood and most of them are aimed at a material thickness no more than an inch or so. I also see a few conventional hole saws in 7 1/2" but they are all only for depths of around 1". The sill is certainly more than that and I'm unclear on how thick the sheathing + wrap + siding is.
Thanks

Try multiple cuts with the 1 that goes 1".
I'd definitely recommend a Super Hawg and a long handle,I've drilled 6" holes before that made me seriously consider drinking heavily after the fact!
That's a monster you're dealing with.

 

[finn]

That’s why they invented wood chisels.

bore as deep as your new Amazon hole saw will cut, withdraw, and have at it with a hammer and wood chisel. Break it out to the depth of the first cut that way and start again with the not deep enough hole saw.

Rinse and repeat.

 

[engineer2]

I did a 6-1/4" hole with my cordless drill for my range vent. You need to use your drill's side handle and take it easy. Once the pilot hole is all the way through, you can start from the outside going in. If your rim joist is 1-1/2, even the cheap holes saw will go deep enough by attacking from both sides.

 

[Mandres]

Jig saw would be my choice. Or even a sawzall if the hole will be covered and neatness doesn't count

 

[Hakeem]

The chances that you come away from boring a 7 1/2" hole with a hole saw in a hand-held drill while standing on a ladder without being laid up for a few weeks? Low.

Scribe, start-drill and use a jigsaw. Use a long "bell hanger" bit to establish the holes on both sides of the wall. I leave the bit in the hole and use it as a hub for a "string trammel" to mark the diameter.

This was my first thought but it’s just going through 1/2” sheathing and siding, at a height of 6’8”. Admittedly the biggest hole saw I’ve used was only 4” but I don’t think it should be too much of a challenge.

Drill a pilot hole, go at a high speed and use a light touch. Maybe try running it backwards first to score the outline of the hole, before running it forwards to cut the hole out. The potential for kickback is real so I’d probably set up a little scaffold of a couple 2x8 propped on 5gal buckets, over using a stepladder.

E: whoops I did the math wrong, 90” is 7’6”. That’s a bit dicier. I’d still do it but maybe have someone to hold the ladder

 

[Muckin_Slusher]

Make one (or modify one).

Also, if you do use a hole saw, drill the pilot hole first with a regular twist bit. In the hole saw arbor, use a good grade 1/4 inch bolt as the pilot. (use the smooth part of the bolt, not the threaded part).

The advantage of predrilling the pilot hole without the hole saw is that you won't dig in with the saw when the pilot breaks through.

The advantage of using a bolt instead of a bit for the pilot is it's much, much more resistant to bending, and it also doesn't waller out the hole sideways while you drill.

 

[PCustoms]

Make one (or modify one).

Alright, I'll bite.

Got an example where you've made a hole saw?

 

[Muckin_Slusher]

Alright, I'll bite.

Got an example where you've made a hole saw?

Sure.

I don't recall ever specifically building or modifying a standard hole saw, although I've sharpened quite a few.

I built these. Silver one is an ice fishing hole enlargener (takes a 10 inch hole and makes it 14 inches).

Black thing "funnels" out the bottom of the hole so the pickeral rig hooks don't get stuck on the ice.

Little thing is an old timey brace bit that I modified to drill holes in ice to put trail markers.

I rolled the steel for the black and silver one, and if you look close the black one has a single tooth to chip and clear ice, just like those lenox single tooth hole saw.

 

[Muckin_Slusher]

Also, to the OP, if you DO decide to swing a hole saw that big, hang on and be careful, because it's gonna be a handful.

I'd use a hole hawg type drill because they're designed for larger rotating mass, don't have automatic brakes and also usually have overtorque clutch to help save you.

 

[PCustoms]

Sure.

I don't recall ever specifically building or modifying a standard hole saw, although I've sharpened quite a few.

I built these. Silver one is an ice fishing hole enlargener (takes a 10 inch hole and makes it 14 inches).

Black thing "funnels" out the bottom of the hole so the pickeral rig hooks don't get stuck on the ice.

Little thing is an old timey brace bit that I modified to drill holes in ice to put trail markers.

I rolled the steel for the black and silver one, and if you look close the black one has a single tooth to chip and clear ice, just like those lenox single tooth hole saw.

Nice build but that's a little different then a hole saw for siding/sheathing IMHO.

 

[The Cobbler]

In the hole saw arbor, use a good grade 1/4 inch bolt as the pilot.......... and it also doesn't waller out the hole sideways while you drill.

the center drill is redundant once the hole saw has started to cut.

 

[lardy1]

Sabre saw sure seems a lot easier. I recently did something similar on the rim joist and sheathing to install a range vent in the basement. That's a lot of hole saw to try to hold onto.

 

[danski0224]

Even if you buy one, odds are that you will break your wrist using it. Even a 4" hole saw can inflict serious bodily harm if the drill is not braced properly.

There are jigsaw style blades for a reciprocating saw (Sawzall). Very easy to cut curves and a Sawzall might fit where a jigsaw won't.

 

[RPH]

All this for a hole saw. Lots of doom and gloom. They’re some good suggestions though.

 

[Old tool guy]

the center drill is redundant once the hole saw has started to cut.

Except that once the pilot bit pops through the other side, it marks the hole so you can drill from there.

 

[speed bump]

Use a saw (jig or reciprocating), it has a cover so the hole doesn't have to be pretty.

 

[Beerhippie]

This was my first thought but it’s just going through 1/2” sheathing and siding, at a height of 6’8”. Admittedly the biggest hole saw I’ve used was only 4” but I don’t think it should be too much of a challenge.

Drill a pilot hole, go at a high speed and use a light touch. Maybe try running it backwards first to score the outline of the hole, before running it forwards to cut the hole out. The potential for kickback is real so I’d probably set up a little scaffold of a couple 2x8 propped on 5gal buckets, over using a stepladder.

E: whoops I did the math wrong, 90” is 7’6”. That’s a bit dicier. I’d still do it but maybe have someone to hold the ladder

I was making the assumption that the OP was going to use a gear-reduction drill, like a Hole Hawg, as God intended. Get the hole saw the least bit sideways and your arm is wrapped around the drill. I dislocated a shoulder this way once--the fall from the 28' ladder didn't help any. Gear reduction drills don't stop immediately when you release the trigger--and if you use one left-handed (Hole Hawg, I'm looking a t you), it's easy to hit the trigger lock without realizing it.

 

[drboom]

Also where did you even search?

Hole Saw 7-1/2" (190mm) for Wood, 7.5 inch HSS Bi-Metal Hole Cutter with Pilot Drill Bit for Woodworking, PVC Boards, Drywall and Plastic, 3/8" Triangle Shank https://www.amazon.com/dp/B0BM4KLQBQ?tag=atomicindus08-20

I bought a vikiton branded one a few months ago in 6" and it worked great for 6 holes I needed

the depth is the challenge with that hole saw. I'm not sure the ~1" depth is adequate. I do see some creative suggestions in this thread for chipping out a partial cut and going again. Or, the obvious, with cutting from the other side as well using the pilot hole as the template. That may be adequate if I don't go through the sill.

 

[The Cobbler]

I do see some creative suggestions in this thread for chipping out a partial cut and going again.

I wouldn't call that creative, it's done all the time. even when coring concrete . I cored hundreds of 4" holes thru concrete block for a job installing range hood fans. core bit was maybe 4" deep, had to use the same process to get thru it as there was a kitchen cabinet on the other side and no access for the core drill .

 

[drboom]

Thanks for pointing out the torque problem, I may have not thought through that. It may be a good excuse to buy a hole hawg - will have to check the red thread for deals.
Based on the feedback above, I'm inclined to try and cut this carefully with my rotozip type tool. That seems more practical than a jigsaw. I was hoping to make a nice, clean cut. Or, buy the hole hawg and hope that those 1" depth hole saws are adequate, even if done from both sides.
I appreciate the guidance.

 

[The Cobbler]

I would not use a drill motor with tons of torque, I would use a drill that would stall if it jams. a hole hawg will tear your arm off.

 

[PCustoms]

the depth is the challenge with that hole saw. I'm not sure the ~1" depth is adequate. I do see some creative suggestions in this thread for chipping out a partial cut and going again. Or, the obvious, with cutting from the other side as well using the pilot hole as the template. That may be adequate if I don't go through the sill.

Depending on specific construction, a normal hole saw isn't deep enough to go through either.


There's probably 15 different ways you could do this. Don't overthink it, go at it with the tools you have.

 

[Beerhippie]

I would not use a drill motor with tons of torque, I would use a drill that would stall if it jams. a hole hawg will tear your arm off.

My experience with low-torque drills--especially battery drills--is that they begin to let the smoke out after running constantly for too long. Too long usually being as long as it takes to get a larger hole saw through siding and sheathing. Hence the Hole Hawg and the risk of breaking an arm. If you use ladder stand-offs so you can get right behind the drill, it's far more controllable.

Up here in the north, we have walls with 2X6" (or equivalent) or greater stud depth. Using a hole saw from both sides it the only way you're using a hole saw to go through an outside wall. I have 8" and 12" long 1/4" drill bits in my hole saw toolbox for just this reason.

 

[danski0224]

Thanks for pointing out the torque problem, I may have not thought through that. It may be a good excuse to buy a hole hawg - will have to check the red thread for deals.
Based on the feedback above, I'm inclined to try and cut this carefully with my rotozip type tool. That seems more practical than a jigsaw. I was hoping to make a nice, clean cut. Or, buy the hole hawg and hope that those 1" depth hole saws are adequate, even if done from both sides.
I appreciate the guidance.

All you need are a couple of jigsaw style blades for a Sawzall and a drill with a 3/8 bit for the pilot hole.

The 1" deep hole saws are primarily for cutting holes for can lights in drywall.

A regular drill will not spin slow enough for a 4.25" hole saw... much less a bigger one.

Someone at my employer has issues providing proper tools, and I was given a Milwaukee 1/2" Hole Shooter to run a 6" hole saw through 10 gauge steel. They would not provide the jigsaw and blades I asked for.

The drill was smoking hot, and smoking. The hole saw was ruined despite using cutting compound. It was something that had to be done, and the electrician that happened to be there lent his jigsaw to finish.

Closest cordless update for the corded Hole Hawg is the M18 Super Hawg. A 4.25" hole saw will engage the clutch on that drill.

A corded Hole Hawg does not have a clutch. Your wrist is the clutch instead.

 

[Hakeem]

As luck would have it, at work I watched some grizzled old HVAC guy cut a 7” hole for a vent with a 18v Makita sawzall. Took him like 15 seconds, I tried to snap a photo of him doing it but he was too fast.

 

[PCustoms]

As luck would have it, at work I watched some grizzled old HVAC guy cut a 7” hole for a vent with a 18v Makita sawzall. Took him like 15 seconds, I tried to snap a photo of him doing it but he was too fast.

Looks a little oval

 

[Beerhippie]

Looks a little oval

HVAC guy. I'm surprised he didn't just cut through a stud.

 

[danielbuck]

Adjustable hole saw

hah! handheld, I don't think I'd try that. I use one in my drill press though more often than I though I would

 

[danski0224]

HVAC guy. I'm surprised he didn't just cut through a stud.

Better than a joist cutter (aka plumber).

 

[no704]

Do they make a drill attachment for a bobcat?

 

[danski0224]

Do they make a drill attachment for a bobcat?

Yes.

https://www.bobcat.com/na/en/attachments/auger

 

[drboom]

Thanks to all for your advice. As many of you suggested, I was overthinking this. While the 7 1/2" whole saw from amazon was not awesome quality, it did it's job with an M18 drill. Moving gently, I was able to avoid excessive torque braking and injury. I suspect had I tried to go through the sill instead of the sheathing, that may have been different. To my surprise, the whole saw had it's worst trouble with the vinyl siding and I had to finish that with a blade.