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Enlarging An Existing Hole
04-Nov-2006

Enlarging An Existing Hole

In order to complete this article I used a scrap piece of end grain balsa deck structure. I wanted to use a scrap piece so that I could make cut-a-ways to show the inner workings and what actually happens on the "inside" of the deck.


Here I have drilled three holes; one for the #115 bit, one for the #654 and one that will remain as 1/4" with no epoxy fill.


The center hole, as you can see, has been counter sunk. This was originally a 1/4" hole that I over sized to 5/16" to make the #115 bit fit into it. Over sizing holes in fiberglass can often result in an "oops" and some serious deck chipping.


So here's the trick to over sizing holes:


#1 Pre-countersink/chamfer the hole to just beyond the width of the drill you will use for the over sizing, in this case 5/16". If your countersink bit is new and sharp get in the habit of using reverse, not forward, when countersinking fiberglass. Forward will remove too much material too fast if you are not careful.


#2 Place your brad point drill in the chamfered hole and use reverse. You will cut through the top skin only and have a nice clean hole. You do however need a brad point drill set. You won't find them at Home Depot and will find them a Rockler or on line. This is the quickest and cleanest way to bore the top skin without the drill wandering or causing serious gelcoat chipping!

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John 02-Apr-2010 16:02
I have had good success avoiding gel coat chipping with drill bits made plexiglass - smooth bevel to a point. I just centerpunch and drill. (package with manufacturer name is on the boat, but I can get it if anyone wants the info).

Also, I used a 1/2 inch aircraft boring tool (requires ~1/8" pilot hole) recommended by NFM to install their ports. It worked very well making a nice smooth cut with no chipping.