Advice sought: Need to cut a 4 3/4" hole in sheet metal dash

[VinceAtReal4x4s]

I've cut a few larger holes in sheet metal before with hole saws but I'm trying to figure out a way to cheaply cut a clean hole of exactly 4-3/4" in a metal dash for a speaker. All the metal hole saws I looked at, or have that are close, are 4" then jump to 5" or maybe 4-1/8th" or something like that. The speaker mounting holes that I am dealing with are just under 5" apart so 4 3/4" would work but 4" won't because of the slope rise of the speaker's housing wont allow a flush mounting.

I saw something once that looked like a simple device where you drill a hole in the middle of your circle and a little bolt/nut/bushing fits into the hole, then a flat bar that can rotate on that bushing goes on and it has a metal-cutting router type bit at the end of it that you put on your drill and it cuts the circle to any size you want by adjusting the router bit up and down the bar. I have no idea what it was called.?

Any advice on a cheap solution or on what that thing might be called?

EDIT: Well I just found a few hole-saws on ebay that fit exactly. Now to figure out what kind of pilot bit is needed for a saw that doesn't come with one. (why are these things so complicated?!) With shipping this is going to be an expensive freaking hole!

http://www.ebay.com/itm/4-3-4-Hole-Saw- ... 35a020885c

[Pinzgauer Pete]

Vince the hole saw you posted from ebay will take a standard 1/4 inch drill bit and goes right into the arbor as is.

[CentAr712]

I suggest possibly drilling a hole and then using a fine-tooth hacksaw blade in a jig-saw. This method works best if you can hold the workpiece rigidly. After you cut the hole, finish it up with a file. I cut sheet-metal like this when I have the same problem as you are having.

[krick3tt]

I think the one on eBay is a deal as I paid much more for my hole saw used to cut two holes for new tail lights
and only used it twice. But at least it was the correct size.

http://s400.photobucket.com/albums/pp88 ... MG3348.jpg

[cowetadirtfarmer]

I have cut many holes in sheet metal by drawing your circle, use a small drill bit and drilling holes as close to your line as possible. knock out the center and file to a perfect hole. Works just fine if you do not want to buy a hole saw.

[mopar]

I saw something once that looked like a simple device where you drill a hole in the middle of your circle and a little bolt/nut/bushing fits into the hole, then a flat bar that can rotate on that bushing goes on and it has a metal-cutting router type bit at the end of it that you put on your drill and it cuts the circle to any size you want by adjusting the router bit up and down the bar. I have no idea what it was called.?

Vince
That tool would be a rotozip and craftsman makes their own version in a big kit that includes the circle cutter atatchment:

http://www.rotozip.com/en-us/Pages/Cate ... x?pid=18_8

[Stekay]

http://www.mcmaster.com/#adjustable-hol ... rs/=g822a1

I drill the center hole, then flip the bit in the circle cutter so its like a dowel pin in the hole to avoid any drift of the cutter from the center bill acting like a router bit.

clamp the panel, use a drill press, go slow

[krick3tt]

Wow...I have one of those. Inherited it from my Dad, no idea it was for metal..thought it was for wood.

[Jimm391730]

The tool that has the flat bar and a cutting edge on the end of it is called a "fly cutter". My suggestion is to get a sheet metal nibbler (manual or pneumatic), with these you can cut any shape you want simply, cleanly, and while they may be a little slower the frustration factor and chance of really messing up the project is greatly reduced.

[VinceAtReal4x4s]

Vince the hole saw you posted from ebay will take a standard 1/4 inch drill bit and goes right into the arbor as is.

Thanks everyone for the advice. For price, precision, and considering what I have available in resources, it looks like that hole saw will work best for this particular job.

Pete, so how does this hold a standard 1/4" bit, or did you mean a bit with a hexed shaft like used for quick-change adapters? If not is there a set screw or something that i cant see? Thanks

[David Dunn]

With all the suggestions, they're all more expensive than to buy the hole cutter for a few holes. Rather than using only a pilot bit to center the hole, drill the 4 3/4" hole through a piece of plywood and securely clamp it to the piece of metal you want to drill. With hole saws on metal, they always "wobble" using just the centering bit.

Also Vince, I don't have a hole saw that big, but do have all the other options mention above, plus a plasma cutter. If you want, you can bring the pieces to my shop and use whatever tools you like.
Also, what type of material and it's thickness? I have a few other options available also.

[David Dunn]

Vince,
There's a set srcew to hold the bit

[cowetadirtfarmer]

I just ordered on of these circle cutting guides for my plasma cutter with all of the talk of cutting holes.

http://swagoffroad.com/Plasma_Circle_Cutter.html