Cutting round duct from the inside

[JohnD]

I have a duct built into a wall for a bathroom extractor. It is actually a length of UPVC soil pipe, around 100mm i.d.

Modern extractor fans have a slightly larger spigot than the ones it was built for, because they all seem to have a bulge for electrical connections. Therefore they will not fully insert into my duct.

I want to cut out a part of the duct. I can get my hand inside easily. A multicutter will not do the job because I need something that can cut round the diameter, not along. A Dremel might do but is intensely slow. The duct will not pull out and I can't hammer it from the outside because it is 9 metres above the ground.

Any suggestions?

 

[stem]

I needed to do something similar. A soldering iron fitted with an old tip bent to 90 degrees melted a slot through it quite nicely....."like a hot knife through butter"

 

[Tigercubrider]

you can get multi cutter blades with a 90 degree bend

 

[JohnD]

eh?

the spindle rotates in a back and forth direction.

How can it cut side to side?

 

[Tigercubrider]

https://www.amazon.co.uk/Imperial-B...ATFS02XN0DH&psc=1&refRID=A219KCZ1VATFS02XN0DH

 

[big-all]

get a 40mm dremil type wood saw blade will cut plastic in seconds
random link
https://www.ebay.co.uk/itm/7pcs-CIR...689418?hash=item2cfb3dc14a:g:5sYAAOSwT7NdwTCU

 

[JohnD]

thanks, I will give the circular saws a try.

 

[big-all]

top tip
use a normal drill get a 44mm hole saw and cut a disc from 12mm ply/mdf/wood
this will give you a disc around 39-40mm
chamfer/countersink the centre slightly to rebate the dremil shaft collar in flush double sided sticky tape this to the back off a 44mm blade this gives you a depth stop off around 2-2.5mm
if you need say 3mm cutting depth then oversize the centre hole by 0.5 mm then stick on concentric[off centre]