How to cut a Larger Hole where there is already a smaller Hole!

[Habebty]

I am fitting new instruments. I need to cut/drill a 92mm hole where there is already a 60mm hole in the GRP hatch garage. Bit tricky to centre a conventional hole cutter with drill bit, is a jigsaw my only option?

 

[Sticky Fingers]

Make a jig out of small piece of scrap half inch ply or mdf by drilling it through with the hole cutter size you want, tape the jig centred over the existing hole, then position the hole cutter bit in the jig before you start the machine….

 

[Daydream believer]

Drill a hole with a hole cutter of the new size in a bit of 6mm ply. Clamp this over the position where you want it. You could screw it in place if you can find a position for 2 screw holes Then stick the new hole cutter in the ply hole & it acts as a guide..
Alternatively, You could screw a piece of wood or ply across the hole on the inside, screwing it through an area that is going to be cut away. Then using the drill bit of the new hole cutter start drilling the new hole. Through that bit of wood & the area you want the instrument.
As you get deep enough into the instrument wood the hole saw will sit in the groove it has cut so you can carry on & eventually the guide piece & the instrument piece will fall away complete with the 2 screws that are holding the starter piece of wood.

 

[Martin_J]

I would probably do the same as the two previous posters...

If you were doing this every day, or just liked buying tools, perhaps look up an Oops Arbor.

Something like this one from Starrett..

 

[Habebty]

Funnily enough the above ideas popped into my head as soon as hit "post" !! despite thinking about it all day

 

[johnphilip]

Really easy, I was given the tip when I had the same job. Put the 92mm cutter on the arbor and a 60 mm cutter screwed inside it. There is just enough thread on the arbor to take the inner cutter and the 60 mm one will protrude just enough to engage the smaller hole and pilot the bigger cutter.

 

[Refueler]

I am fitting new instruments. I need to cut/drill a 92mm hole where there is already a 60mm hole in the GRP hatch garage. Bit tricky to centre a conventional hole cutter with drill bit, is a jigsaw my only option?

OK - two ways :

If you can get back of the existing hole ... glue a thin piece of ply or material to cover the existing hole. But size is less than the new. Mark centre and then use the Hole Cutter ... the centre drill uses the ply as its centre.

If you cannot glue piece behind - then first you need to draw the new circumference onto the console with old hole centred. Now take piece of ply or material and glue over the hole ... making sure you can still see the marked new ... set your drill to centre so your hole cutter cuts the marked hole.

 

[seeSimon]

I'm facing at least two similar hole enlargement conundrums this winter...beginning to wonder just how much boat will be left after say another two instrument generational size increases and upgrades?

A common problem it seems?
...also currently trying to find a moderately sized/smaller estate car (golf/focus/astra or similar) for narrow lanes and difficult parking spaces here in SW UK.
However, Plenty of pointless 2WD fat SUVs, Passats or Audi A4s estates to be had!

 

[Boathook]

Purchased this and actually used it to enlarge a hole.
https://www.screwfix.com/p/starrett-a19-hole-enlargement-arbor-58-6mm/386rf

 

[Boathook]

Really easy, I was given the tip when I had the same job. Put the 92mm cutter on the arbor and a 60 mm cutter screwed inside it. There is just enough thread on the arbor to take the inner cutter and the 60 mm one will protrude just enough to engage the smaller hole and pilot the bigger cutter.

I tried that and it didn't work for the sizes I was using.

 

[Habebty]

Purchased this and actually used it to enlarge a hole.
https://www.screwfix.com/p/starrett-a19-hole-enlargement-arbor-58-6mm/386rf


Brilliant, thanks.

 

[alan_d]

Google "oops arbour".

 

[Ammonite]

If #6 doesn't work for you or you want the larger hole slightly off centre.

Screw a scrap strip of plywood over the existing hole with the screws 5mm outside of the edge. This will centre the larger holesaw. Use the holesaw in reverse when it first touches the gelcoat so it doesn't chip /grab.

 

[PCUK]

Screw the 92mm saw on the arbour, then screw a 60mm saw inside the 92mm saw. Job done.

 

[fredrussell]

Really easy, I was given the tip when I had the same job. Put the 92mm cutter on the arbor and a 60 mm cutter screwed inside it. There is just enough thread on the arbor to take the inner cutter and the 60 mm one will protrude just enough to engage the smaller hole and pilot the bigger cutter.

This^^^. Job done.