How to enlarge exisiting hole from 50mm to 70mm?

[zillah]

Dear All

Looking to use two hole saws with a single arbor to enlarge existing ceiling gyprock holes from 50mm to 70mm something similar to this

https://bloximages.chicago2.vip.townnews.com/lakeexpo.com/content/tncms/assets/v3/editorial/6/0a/60a...

Reason to enlarging holes as I can't the downright 50mm with their brackets as I guess there are old style

I have got couple of hole saws set but problem thread on arbor wouldn't fit 2 hole saws 

 :

Thx

[Noyade]

Evening @zillah 

I just popped into the shed at 10 pm and looked through some old hole saw sets.

These are from Clints Warehouse from the early 2000's. Cheap and nasty I admit but would suffice for gyprock.

I can get the combination but the 50mm only protrudes 5mm above the 70mm, so certainly not as long as seen in the photo in your link above.

Another thought is a 50mm wood disc inserted into a PVC pipe section.

Just late night thoughts.

Cheers.

[zillah]

Thx Noyade for your help and appreciate your support

i am thinking about your suggestion 50mm wood disc

And this video exactly what I want to do from 50mm to 70mm

https://www.youtube.com/shorts/eAvr1ch-I6I

[EricL]

Hello @zillah 

It's great that you've received excellent advice from @Noyade. You can also achieve the same effect with the Craftright 16 Piece Holesaw Set. Please note that this is a light to medium duty holesaw kit for wood, plastic and plasterboard. 

Please remember to wear personal protection such as gloves, goggles and a mask when using your tools. 

If you need further assistance, please let us know.

Eric

[AlanM52]

Hi @zillah, CC @Noyade,

There are basically two levels of hole saw sets, entry level and pro.

The DIY one you have has a 'normal' cutting depth.

The PRO sets usually have 'extended' cutting depth, for example used by plumbers and cabinet makers.

I suggest the following workaround (which I have done) providing you can access that area and... it's quite laborious requiring some patience.

Cut to size a piece of plywood to fit between the joists and secure with brackets.

Back inside using a compass find the center of the 50mm plaster hole and boar your 70mm hole.

Cheers

[Noyade]

G'day @zillah 

"i am thinking about your suggestion 50mm wood disc"

It does work. I removed the 6 mm drill and replaced it with a 6 mm bolt. Cut the head off and 'notch' the other end for the grub screw.

But it's not pretty.

Cheers.

[MitchellMc]

Hi @zillah,

If it's just the one downlight, it likely would be easiest to just adjust the hole size with a jab saw.

Mitchell

[zillah]

Morning Noyade

Nice technique

I guess initially you used 50mm holesaw to boar in a thick timber to get 50mm cylindrical template

Then you attached that template to your arbor to be a pilot guide with 70mm holesaw

[zillah]

Hi Mitchel 

I have got few around 5 at the house