# Forum Home Renovation Roofing  Cutting holes in gutter for nozzle

## paddyjoy

I need to cut a circular hole in a zl box gutter for a 90mm downpipe nozzle. What's the best tool to use for this, would it be aviation snips and if so do I need straight cut, left/right or offset? 
Thanks

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## justonething

A hole saw.

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## phild01

how about one of these: Hard Alloy CUP Type 90mm DIA Hole SAW W HEX Wrench KIT | eBay

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## paddyjoy

Thanks guys I reckon the hole saw will be easier than doing it with snips considering I only get one shot at it!

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## Bros

I see plumbers doing it with snips

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## toooldforthis

> I see plumbers doing it with snips

  theory is heat generated by fast moving (hole) saws causes cut to rust - so they snip.

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## phild01

> theory is heat generated by fast moving (hole) saws causes cut to rust - so they snip.

  Would lubrication prevent this?

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## Bros

> theory is heat generated by fast moving (hole) saws causes cut to rust - so they snip.

  Never thought there was any science in it just that it showed the skill of the plumber.

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## paddyjoy

> theory is heat generated by fast moving (hole) saws causes cut to rust - so they snip.

  Interesting, I assume that this would also be the case for trimming gutters, best to use a snips and not a grinder?

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## phild01

Cold cutting is always best.  Snips, nibbler or guillotine.

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## phild01

Nibblers | Metal Nibbler Tool At Bunnings Warehouse

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## paddyjoy

> Nibblers | Metal Nibbler Tool At Bunnings Warehouse

  Nibbler is a bit out of the price range so might have to get some practice in with the snips or just take the risk of corrosion with the hole saw.

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## RodEye

Red wiss snips would be my preferred option.Measure twice,mark once and cut accordingly.

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## r3nov8or

Mark your circle. Drill a 10mm hole just inside the line to take the tip of your snips. Use left turning/red snips (best for a right hander IMO in this case) and away you go.

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## RodEye

Another method is to  drill a series of 1/8" holes to form a cross in the centre of marked circle.Take your butcher's chisel(not your best chisel)slice through drilled holes and then trim out with what evert snips you are comfortable with,wether they be curved,left or right cut so long as they are sharp.I am left handed and always reach for the red pair first.
PS it is the first time I've heard a D/P pop referred to as a nozzle :Smilie:

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## toooldforthis

> Never thought there was any science in it just that it showed the skill of the plumber.

  theory or old wive's tale - dunno.
only ever see roof plumbers use snips or nibblers.  _"__BlueScope Steel recommend 'cold cutting' coated steel products. If this recommendation is followed, these products will 'self heal', that is, the zinc in the surrounding coating will protect the newly exposed edge."_  http://hia.com.au/bluescope/faq.php

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## paddyjoy

Picked up some red wiss snips the other day, cutting the holes is surprisingly easy once you have the right tool!

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## toooldforthis

> Picked up some red wiss snips the other day, cutting the holes is surprisingly easy once you have the right tool!

  funny that.  :2thumbsup:

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## FrodoOne

> theory is heat generated by fast moving (hole) saws causes cut to rust - so they snip.

  Please explain that theory! 
In addition, a "90mm down-pipe nozzle" will be inserted in the resulting hole and (presumably) sealed with silicone, to prevent leaks. 
Hence, the cut edge will be covered with silicone, under the flange of the "nozzle" and, hence, out of contact with water - and oxygen - except (perhaps) from below.

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## phild01

> Please explain that theory! 
> In addition, a "90mm down-pipe nozzle" will be inserted in the resulting hole and (presumably) sealed with silicone, to prevent leaks. 
> Hence, the cut edge will be covered with silicone, under the flange of the "nozzle" and, hence, out of contact with water - and oxygen - except (perhaps) from below.

  And if the nozzle is placed under the gutter for better water flow!  I think what was said is correct, cold snipping would result in a clean cut with less exposed steel to corrode and the zinc will remain better fixed surrounding the cut IMO.

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## OBBob

> Nibbler is a bit out of the price range so might have to get some practice in with the snips or just take the risk of corrosion with the hole saw.

  I have the $49 drill nibbler (Masters has them for $29! I think). Works great... I was cutting 2.7m long sheets of corrugated iron in half length ways no issue.  The only disadvantage is that it takes two hands to control,  whereas the purpose made electric or air versions could be used one handed.

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## intertd6

> theory or old wive's tale - dunno.
> only ever see roof plumbers use snips or nibblers.  _"__BlueScope Steel recommend 'cold cutting' coated steel products. If this recommendation is followed, these products will 'self heal', that is, the zinc in the surrounding coating will protect the newly exposed edge."_  http://hia.com.au/bluescope/faq.php

  I can't see much difference between power saw cutting with a metal blade & a hole saw. Tungsten carbide cutting steel, not grinding which is burning the steel & coatings.
regards inter

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## paddyjoy

Isn't the zincalume designed so that the zinc is sacrificial and corrodes first, so in theory it shouldn't matter how you cut it or how many edges are exposed, the steel shouldn't corrode until all the zinc is gone? Like how they bolt blocks of zinc onto pipes/ships, doesn't matter where the zinc is as long as it is in contact with the steel? 
Using the offset snips to cut a circle is like childsplays, it's one of things that looks like it would take some skill but in reality is a piece of cake.

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## phild01

> Isn't the zincalume designed so that the zinc is sacrificial and corrodes first, so in theory it shouldn't matter how you cut it or how many edges are exposed, the steel shouldn't corrode until all the zinc is gone?

  The coating needs to be close enough to the steel to be sacrificial, rough cutting (grinding) exposes more surface area of the steel in the cut whereas guillotining the cut leaves that exposed steel to a least possible level.  I have seen rough cuts with levels of rust that is greater than that of snipped steel, which is next to nothing.  But this would be dependent on the gauge, thinner steel would have exposed edges closer to the zinc coating and much less corrosion or none.

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## chalkyt

Might as well throw in my two bob's worth to add to (maybe!) the body of knowledge. Long ago I was told by a very experienced Sheetmetal Worker that shearing (i.e. snips, guiollo, etc) wipes the zinc across the cut so the exposed edge becomes protected just like the surface. Cutting or grinding (angle grinder, hole saw, jig saw, etc) generates heat which burns off the zinc. Don't know if it is correct, but that is the trade folklore and why they use snips in preference to saws.

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## r3nov8or

Link - BlueScope Steel Australia: Guidelines for using ZINCALUME® steel 
"_Use metal cutting discs or shears to cut sheets rather than angle grinder discs._"

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## intertd6

> Might as well throw in my two bob's worth to add to (maybe!) the body of knowledge. Long ago I was told by a very experienced Sheetmetal Worker that shearing (i.e. snips, guiollo, etc) wipes the zinc across the cut so the exposed edge becomes protected just like the surface. Cutting or grinding (angle grinder, hole saw, jig saw, etc) generates heat which burns off the zinc. Don't know if it is correct, but that is the trade folklore and why they use snips in preference to saws.

  almost correct, a sharp tungsten carbide hole saw cutter shouldn't burn, even when blunt they will turn the metal blue from friction but still not burn.
regards inter

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