Best Way to Cut 7 Inch Holes for HVAC Duct

[kach22i]

I need to cut four 6" or 7" holes for installing round metal ductwork.

What is the best way to do this?

I'll be cutting through thin fiberglass, ABS plastic and five inches of rigid foam (hovercraft).

I was not sure until just now if the openings were going to be square, round or odd shaped so I picked up a 7 amp reciprocating saw from Harbor Freight ($5 off coupon plus last day of sale = $32).

I'm going to make a cardboard template using drafting tools, and mark the cut holes with a magic marker. If there is a better way to cut holes, I'd like to know. Some of the working areas may be tight and I might end up on my back.

[bell]

i would say start with a dremel or jig for the fiberglass/abs....then use a drywall saw for the foam.....a tight circle is going to tricky with a sawzal imo.

can you get a hole saw that big?

if you're stuck with the sawzal then cut/grind the blade so it's thin (the back side....not the thin side), it'll make getting an accurate curve easier (thick blades don't turn tightly easily) i think that made sense

[kach22i]

Originally posted by bell
i would say start with a dremel or jig for the fiberglass/abs....then use a drywall saw for the foam.....a tight circle is going to tricky with a sawzal imo.

can you get a hole saw that big?

if you're stuck with the sawzal then cut/grind the blade so it's thin (the back side....not the thin side), it'll make getting an accurate curve easier (thick blades don't turn tightly easily) i think that made sense

I've used a Roto-Zip cutting piece in my H.F. dremel knock off. It will cut the fiberglass but a hand held hole saw is a lot quicker.

That's it, two of the holes can be done by hand. The other two cuts through the thick foam and ABS can be done with the sawzal.

I have a carbide blade pack by "boa", the little one (thin) looks like it can do tight curves.

EDIT: My Roto-Zip is still in several dozen pieces, POS.

[rcecale]

If you can get a sawzall on the area you need to cut, why not try a RotoZip instead? Much smaller piece of equipment to work with and the rotary action of the cutting bit will allow you to follow the curve of the hole much easier.



Randy

[rcecale]

Originally posted by kach22i
I've used a Roto-Zip cutting piece in my H.F. dremel knock off. It will cut the fiberglass but a hand held hole saw is a lot quicker.

That's it, two of the holes can be done by hand. The other two cuts through the thick foam and ABS can be done with the sawzal.

I have a carbide blade pack by "boa", the little one (thin) looks like it can do tight curves.

EDIT: My Roto-Zip is still in several dozen pieces, POS.

LMAO!

Well, I guess you've already considered this then.

Randy

[Porsche-O-Phile]

Isn't that a means & methods issue?

[dad911]

Yep, roto-zip=POS. Mine failed twice, this time terminal. If it is flat, use a router with a circle cutting attachment. If not, sawzall, and a Dremel for cleanup.

[kach22i]

Originally posted by dad911
Yep, roto-zip=POS. Mine failed twice, this time terminal. If it is flat, use a router with a circle cutting attachment. If not, sawzall, and a Dremel for cleanup.

Two of the four holes will not be flat - slight arc to the plane.

My buddy also had a Roto-zip, his also kept locking up on him. The bit gets locked on and will not come off. I had my parts soaking in PB Blaster for a year before they came apart, now I can't figure out how to put it back together. I guess I'm no Henry Ford, his deal was watches as a boy.

[azasadny]

I use a Porter Cable reciprocating saw with Bosch "fine material" blades to cut this sort of material. I'm getting pretty good with this saw...

[rcecale]

Interesting that you've had problems with your RotoZip. I've had mine since early 2000 and have used it quite a bit without issue.

I've cut everything from drywall and pegboard to rebar and ceramic tile, using the appropriate blades. Never an issue with this thing at all.

What stage if disassembly is yours in? Post some pics and surely we can help you work out the re-assembly procedures.

Randy

[rcecale]

You might also wanna try cutting the foam with an electric carving knife. The same kind you use on a Thanksgiving turkey.



My wife refinished a sofa and used one of these to cut the foam for the cushions. Worked like a champ. Not sure how it will do on the ridgid foam you're talking about, but worth a try.

Randy

[WaA420]

The sawzall is what I would use to cut the hole also. I cut holes all the time to run duct work through all sorts of places. Just be really careful were you cut any exposed holes or ones lack of skill shows big time.

Have you figured out the patterns for the holes cut on the non flat surfaces?

[kach22i]

Originally posted by rcecale
You might also wanna try cutting the foam with an electric carving knife. The same kind you use on a Thanksgiving turkey.



My wife refinished a sofa and used one of these to cut the foam for the cushions. Worked like a champ. Not sure how it will do on the ridgid foam you're talking about, but worth a try.

Randy

A new idea, I like it - not right for this project but will have to keep it in mind.

Simple works too, a old picture below, hand held hole saws really work well, a classic.

[kach22i]

Originally posted by rcecale
Interesting that you've had problems with your RotoZip. I've had mine since early 2000 and have used it quite a bit without issue.

You need to buy some lottery tickets man, you are lucky.

[Jims5543]

I just wait until 1st thing in the morning then punch the hole out with this "tool".

Edited. -Z-man.

[928ram]

Bosch 1590/1591 jigsaw for the glass/abs ($149 at Lowe's); uber-accurate. Cut the foam with Sawzall or keyhole saw and dress up with 40 grit sandpaper.

[kach22i]

Originally posted by 928ram
Bosch 1590/1591 jigsaw for the glass/abs ($149 at Lowe's); uber-accurate. Cut the foam with Sawzall or keyhole saw and dress up with 40 grit sandpaper.

I don't think there will be enough room for a Jigsaw around the thin fiberglass area, but if there is it will be my first choice.

I really just bought the reciprocating saw for the ABS and foam sandwich.

Thanks for the input everyone.