Best method to get 6" hole

[Jeremy Gibson]

I'm working on some blast gates for 6" PVC duct work. Based on the link on Bill Pentz's site I've been trying to cut a 6" hole in 3/4" MDF, Mellamine, or Plywood. I have a circle cutter and I'm using my new Delta floor drill press on the lowest speed (215 RPM). This works, but it is very slow and the cutter is nice and blue after just one hole.:mad: I used very little pressure on the cutter.

So, what is recommended? I could use a jig saw, router with a circle jig, buy premade gates from LV or ClearVue. What else is out there I haven't thought of? Oh, I have coupler ends on the PVC now so I was going to make the gates with short pieces of standard pipe.

[Jim Hager]

Make a trammel point base for your router and cut them out with that. It is the best way I know of getting something repeatable and well done on your kind of task other than buying a 6" hole saw. Those are expensive and if you have a router and a 1/4" bit your in business.

[Doug Shepard]

If you've already got a hole through one pc of MDF you could keep using that piece as a template and switch to the router with a bearing guided bit. Just one thought.

[pat warner]

This plate (http://patwarner.com/6182_offset.html)is pre-machined to cut a 6.00" hole with a 1/4" cutter.
Design can be adjusted for most plungers.

[Bill Pentz]

I'm working on some blast gates for 6" PVC duct work. Based on the link on Bill Pentz's site I've been trying to cut a 6" hole in 3/4" MDF, Mellamine, or Plywood. I have a circle cutter and I'm using my new Delta floor drill press on the lowest speed (215 RPM). This works, but it is very slow and the cutter is nice and blue after just one hole.:mad: I used very little pressure on the cutter.

So, what is recommended? I could use a jig saw, router with a circle jig, buy premade gates from LV or ClearVue. What else is out there I haven't thought of? Oh, I have coupler ends on the PVC now so I was going to make the gates with short pieces of standard pipe.

Jeremy,

I've cut so many of these I now cheat and use a big Starrett circle cutter. Unfortunately, it only works on the metal as most PVC is not an even 6" hole size, but some fraction more. Metal is mostly fine right at 6", but double. Check anyway.

These big hole cutters that can handle the high heat of cutting this big of a circle are pricey, plus don't work for the odd sized holes we need for PVC, so I then use a different technique using a lower mounted bearing 1/2" router straight bit in my router table or in a board setup as a router table.

To make the template I trace out an actual pipe onto a 1/4" Masonite to be used as the routing template. Put large vertical and horizontal centering lines through the drawn circle. These marks make it easy to align properly on the parts to be cut later. Use a jig saw to cut the template out about 1/8" shy then carefully with a drum sander sand right to but not through the line. In my template I drilled and countersunk two holes to let me just screw it tightly to the piece. That makes both easier to clamp for routing. I actually make my template the finished size of the outside parts of my blast gates so I can cut both the hole and part at once.

I found trying to just route directly does not work well at all. Instead, rough cut the holes and parts with your jig saw leaving only about 1/8" for the router to remove. That makes it easy to work from a router table. After doing the rough cut, then screw the template to the final with flat headed screws so all remains flush. Set the bearing height to ride on the template which rides on top of the work, then route away. You could use an upper mounted bearing bit, but I think having the bearing up top when working on the table is easier and safer.

bill

[Roger Meeker]

Jeremy,
I just finished making some of the blast gates from Bill's website. Since Gorilla glue is recommended you're OK if you're within 1/16" or so. I traced the PVC onto the melamine, roughed it with a jigsaw and finished with a router bit. Fitted it to the PVC, and now use it as a template for the rest with a bearing guided bit. Close enough is good enough with the Gorilla Glue. It's also pretty fast...since you'll be doing plenty of them.

RSM

[glenn bradley]

Bill's template idea is the way to go (who'd-a guessed he'd have it down cold?).

[Larry Reimer]

Jeremy, I think the suggestion of make a template and cut with a router will be the fastest way to get a bunch of these holes cut. I would like to say something about the original circle cutter you used. Sounds like a "fly cutter" or at least that's what I was taught to call it. That original cutting tool on the cutter is ground to cut thin material and not 3/4" thick. If you regrind the cutting edges it will work. The bottom of the cutter needs to be ground flat with a bevel back from the cutting edge. The sides of the cutter both need to be sharpened with a bevel back from the cutting edge. Originally the cutting tool is 1/4" thick and after sharpening mine it cuts about a 5/32" grrove. Sharpened in that manner it will cut 3/4" material very quickly, cleanly, and without the burning you're seeing.

[Jeremy Gibson]

Thanks everyone for the advice. It's never lonely in the creek! :D

I'm going to take this advice and make a template with 1/4" masonite I have and then use the router mounted in a table to cut the final pieces. Most of the material will be removed with the jig saw and then cleaned up with the router.

[Matt Meiser]

I use one of those adjustable beam hole cutters. Works great and was inexpensive.