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Main => Woodworking => Topic started by: sol on August 03, 2008, 03:16:29 am

Title: 3 inch trackball hole
Post by: sol on August 03, 2008, 03:16:29 am

I was wondering which is better to use to achieve a perfect 3 inch hole in mdf or plywood?

A hole saw

or

a 3 inch forstner bit

Title: Re: 3 inch trackball hole
Post by: Boz on August 03, 2008, 03:18:53 am

MDF, Hole Saw.

Title: Re: 3 inch trackball hole
Post by: Blanka on August 03, 2008, 01:40:36 pm

With a router and a 6-8mm diameter long shaft bit. Make a plate that acts as a spirograph. You can cut any size you like, even if there are no exact hole saw matches.

Title: Re: 3 inch trackball hole
Post by: shmokes on August 03, 2008, 01:59:34 pm

Quote from: Blanka on August 03, 2008, 01:40:36 pm

With a router and a 6-8mm diameter long shaft bit. Make a plate that acts as a spirograph. You can cut any size you like, even if there are no exact hole saw matches.

More info please.  This sounds super useful, but I don't quite get how it works.

Title: Re: 3 inch trackball hole
Post by: Blanka on August 03, 2008, 04:59:34 pm

Pick 2 spare pieces of mdf/plywood. In the first of about 20x20 cm you drill a hole of around 20mm in the middle. Thats where the router bit goes through and will be on top of the sheet that needs to be cut. Next you put the second piece under the sheet where the hole comes (it will hold the loose circle after you went round, and makes sure you don't route into the table. If the table may get little damage, just use the table as second piece!
Then screw a screw on a certain distance from the hole in the first piece, through the circle center in the actual sheet that needs to be cut, and through the help sheet underneath (or into the table). The distance can be calculated, but it is better to make a test circle about the final size in spare wood, and then measure the difference in diameters to adjust the distance of the screw to the routerbit-hole.
Then you screw the router on the 20x20 cm help sheet, centering the routerbit with the hole in the middle. Finally fasten the actual sheet and the bottom help sheets with clamps onto the table.
Now you can start routing with the around-the-screw rotating helpsheet. Go 3mm deeper every turn until you get through. Because the screw holds the cutaway part, it won't fly away or kill the routerbit by locking it up.
If i was not on holiday, i would have photoshopped a sketch.

Title: Re: 3 inch trackball hole
Post by: shmokes on August 04, 2008, 12:05:30 am

Quote from: Blanka on August 03, 2008, 04:59:34 pm

i would have photoshopped a sketch.

Heh . . . that would be helpful.  If you remember, and are feeling generous, put that sketch together when you get home.  A picture says 1000 words . . .    ;D

Title: Re: 3 inch trackball hole
Post by: sol on August 09, 2008, 09:02:49 am

Thanks for the info  :)

I ended up getting a expanding bit it did the job well.  :cheers: