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Old 11-10-2012, 06:21 PM   #1
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have sheet metal, need hole.

How to cut large interior shapes?

I'm working with kinda think steel.
I need one 3 3/8" circle in one piece,
and ~ 4"x2" rectangle in another.

My metal working tools consist of a dremel, angle grinder and hacksaw.
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Old 11-10-2012, 06:26 PM   #2
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Grinder with a cut-off wheel might work for the rectangle. You probably want a hole saw for the circle, if thats not possible, draw a circle with a Sharpie and use a cutoff wheel on the dremel.
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Old 11-10-2012, 06:26 PM   #3
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"think" steel? Is it thick or thin?
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Old 11-10-2012, 06:33 PM   #4
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A circle is nothing more than a series of really short straight lines.

So a small diameter cutoff wheel on the dremel may work.

A holesaw and a 1/2" drill motor would be better, drill the piolt hole first, then use the holesaw and arbor.
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Old 11-10-2012, 06:32 PM   #5
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sheet metal and "kinda think steel" are different beasts.

How close tolerances are we talking about? Does it need to look awesome or is garage good enough?

assuming you really are dealing with thick steel, I'd try and talk a local waterjet shop into burning it for you. Shouldn't take more than a minute especially if you have the cad file already done. I have a local guy I have do these types of jobs for me. And I have a lot more tools that you do. It just comes down to a time and money thing. I could easily do the rectangle with just a jig saw and some lube and a file. But if it's thick metal it might take awhile. The waterjet cuts perfectly and it just looks WAY better than I can cut.

I know my limitations when it comes to cutting like a machine tool can and I have gotten much more used to farming that stuff out and concentrate on things that require my time and thinking.

But if you do want to do it yourself, I would do the rectangle with the angle grinder being careful to cut inside the line. A jig saw would make this a lot easier if you can spring for one of those. The circle might be easier to buy a hole saw and use that
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Old 11-10-2012, 06:38 PM   #6
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Got a torch and a circle guide?
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Old 11-10-2012, 07:33 PM   #7
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I once went to a local steel supplier to see about getting some sheet metal. I was thinking 18ga or so, something I could bend over a wood form with a mallet. I found out that what they call sheet metal starts at 1/4" and goes up.
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Old 11-10-2012, 08:45 PM   #8
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Quote:
Originally Posted by troidus View Post
I once went to a local steel supplier to see about getting some sheet metal. I was thinking 18ga or so, something I could bend over a wood form with a mallet. I found out that what they call sheet metal starts at 1/4" and goes up.
That isn't sheet metal. That is plate. You measure sheet metal by gauge. You measure plate by inch or mm.
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Old 11-10-2012, 11:07 PM   #9
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fly cutter is one way to make a big hole,

if you have time and a drill.... draw the circle, drill holes along the circle that just barely touch & file smooth.

as for the rectangle... drill a hole in each corner & use the cutoff wheel to run between the holes. clamp a guide strip if need be, and don't try to cut all the way through in the first pass. the thicker the metal, the more passes
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Old 11-11-2012, 01:58 AM   #10
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Quote:
Originally Posted by Beezer View Post
fly cutter is one way to make a big hole,

if you have time and a drill.... draw the circle, drill holes along the circle that just barely touch & file smooth.

as for the rectangle... drill a hole in each corner & use the cutoff wheel to run between the holes. clamp a guide strip if need be, and don't try to cut all the way through in the first pass. the thicker the metal, the more passes
Fly cutter on sheet, but on plate you can drill the periphery and file, you could speed that up a bit by drilling say 8 holes, connecting the dots with a dremel, or even better, a jig saw and metal cutting blade (or 6). Slow and steady with the jig, but it would move through the metal a bit faster than a dremel.
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Old 11-11-2012, 04:48 AM   #11
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The sheets I'm using are about 1/16" I guess. (Does that make it 16ga?)
Just had it lying around so I'm not sure.
I say kinda thick, because cutting it with a hacksaw isn't fun.

I'm shooting for garage good.
Close tolerances need not apply.
The rectangle should be easy enough.
But the circle does need to be a circle.

Flycutter or hole saw?
What can be used with just a handheld drill and get decent results?
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Old 11-11-2012, 05:21 AM   #12
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16 gauge is 1/16" but that's a coincidence, gauge and thickness numbers don't correlate.
Forget about a flycutter if you only have a hand drill.

With your tools I'd try the hole saw and cutting oil. If you cheap out on the hole saw it may not last the first hole.
If you do try this, make sure the work is securely fastened because it will grab hard and spin if it gets a chance.
Also, use a radial handle on the drill or it'll try to break your wrist when it grabs.

Sheet metal is dangerous to work with so you need to be prepared.
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Old 11-12-2012, 04:54 AM   #13
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You need one hole right? Ever plan on drilling a hole that size again?

Go to a machine shop. By the time you mess around with getting it 'close' you should be able to get it done for a few dollars.

Otherwise I'd probably chain drill it or use a jigsaw. I'd probably make a practice cut inside the hole with a jigsaw to see how it looks then go from there.
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Old 11-12-2012, 05:28 AM   #14
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For round holes in sheet metal, use a chassis punch of the desired size. See here: http://www.mcmaster.com/#chassis-punches/=k4qaw6
(shipping is fast from this company)

Taking it to a machine shop is very likely to have you end up paying a minimum one hour charge which will be at least $60, unless the shop is small and has nothing else to do or you know somebody who works there. This is especially true if you ask them to do both the hole and the rectangle cutout. A waterjet shop is almost certain to charge you a minimum one hour fee. The abrasives in the water are not cheap and they are generally not reclaimable.
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Old 11-13-2012, 02:28 PM   #15
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Quote:
Originally Posted by LuciferMutt View Post
For round holes in sheet metal, use a chassis punch of the desired size. See here: http://www.mcmaster.com/#chassis-punches/=k4qaw6
(shipping is fast from this company)
They want $320 for a 3" Greenlee punch.....

If there's only one hole, I'd buy a Harbor Freight jigsaw and cut it and then have a jigsaw for other projects.
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