Cutting thin sheet metal

Sheet Metal Fabrication techniques, questions and help. "Tricks of the Trade"

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Mr Ron
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Cutting thin sheet metal

Post by Mr Ron » Sat Sep 19, 2015 1:37 pm

I want to use .025" or .031" thick aluminum sheet as sides for a large scale locomotive I'm building (3/4" = 1' scale). I have to make cuts for windows that measure around 2" square. What tool is recommended to make these cuts? I would like for the cuts to be clean and not leave ragged edges.
Mr.Ron from South Mississippi

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Re: Cutting thin sheet metal

Post by drmico60 » Sat Sep 19, 2015 3:46 pm

I have used a nibbler for cutting aluminium sheet up to 0.08" with no problem. Nibblers give nice clean edges but they can be difficult to control. I modified my nibbler to make control easier and also built a nibbled table with a guide to facilitate straight cuts. See:

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Re: Cutting thin sheet metal

Post by Harold_V » Sat Sep 19, 2015 3:50 pm

Nothing wrong with using an end mill, assuming you can get to the location once the piece is firmly clamped to the table, set on a piece of wood (think plywood). Key to success, not leaving ragged edges, steps, excessive burrs, is to understand proper roughing technique, followed by proper finishing, which would include climb milling the final pass. Use a sharp end mill and you won't have burrs. Avoid some of the soft aluminum grades if you can, as they tend to be more difficult to machine, and readily chip weld. 6061-T6 or T651 would be a good choice. Softer grades will work, they're just a little more finicky to machine.

I'd suggest opening the hole with a larger end mill, one that is robust enough to resist breaking. Run the end mill as fast as your machine permits, using kerosene or WD-40 for lubricant. Either can be brush applied if necessary. Don't attempt to open the window(s) dry, as chip welding is a problem, regardless of the speed you choose to run.

Rough the window(s) within no less than .03", which you should leave for final orientation and finish passes. Switch to a smaller end mill for the finish passes, to limit the size of radius in the corners. Keep in mind, small end mills must be applied with correspondingly smaller cuts, so take light passes for finishing.

To locate the windows where desired, if you choose to trust your dials, measure each side, noting the dial setting. In this case, you'll read the dials directly, as you'll have compensated for backlash (when the opposite side is machined) because you'll have measured, knowing how much must be removed to establish finished size. I like to make a + on the machine table (wax pencil), or on a piece of paper. At the tips of each end of the cross, I write the dial setting where the part is on size. That way I don't get confused, or forget the dial setting I'm looking for.

A little tip. Coming in to corners tends to leave chatter. The next axis to be moved will have the backlash in the wrong direction, so there's a delayed pickup of movement. To get around that problem, on the final pass, I unlock the slide and reverse the handle just as the cutter approaches final depth. The moment it does, I start feeding the other axis. Takes a little repetition to get the hang of this method, but it yields seamless corners, without chatter and/or undercut.

With thin material, it may be required that you clamp near the cutting points. I like to use parallels on top, although not real close to the cuts. Clamp the parallels with finger clamps, which will prevent the helix of the end mill from picking up the thin material. If the location of the windows is far from the edges, use a long piece of material that reaches the edge on each side, where you hope to clamp, then place a piece of paper under the clamp near the window, so the bowing of the clamp will still provide pressure where needed. Use more than one thickness of paper if you must.

Hope this helps.

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Re: Cutting thin sheet metal

Post by Mr Ron » Sun Sep 20, 2015 12:06 pm

Thank you for your replies. Your description is a bit more lengthy than I expected. Think of it as the side of a passenger car with many windows close together. It could be quite tedious cutting them with a mill. Other than a punch and die set-up, I can't see any easy and quick way. I won't even entertain a CNC, laser or water jet as they are not practical solutions if they are not available. I tried scribing, but it didn't work well; fine for straight lines (not windows). I can scribe, then bend and it will snap apart. This doesn't work on internal shapes. I was thinking about an "L" shaped punch that I could punch out the four corners, leaving a small bridge to remove.

Another thought: They used to sell chassis punches in round and square patterns. A hole was drilled and a 2 piece punch and screw was used to punch out the shape. It may not be the required size, but it would give me room to trim the opening to the required size with snips or a nibbler. What do you think?
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Re: Cutting thin sheet metal

Post by steamin10 » Sun Sep 20, 2015 12:33 pm

Electronic chassis punches are a good thought, with a backup block of oak or similar wood (mdf?) Horror Fright has rather cheap corner chisels that may be worth a try. They should work fine on thin material with a backup to cut into.

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Re: Cutting thin sheet metal

Post by Mr Ron » Wed Sep 23, 2015 11:32 am

Thank you DRMICO60 and everyone else who responded to my my question. I'm still tossing around ideas. Being retired, I can afford to spend the time experimenting. I had been using thin plywood and even illustration board, and would like to try using metal for more realism. Building a model is sometimes more difficult than building in 1:1 scale.
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Re: Cutting thin sheet metal

Post by Metalman » Wed Sep 23, 2015 1:50 pm

Only other method I can think of in your situation (no CNC or waterjet), would be to use a router with a center collar guide inside a pattern cut oversize to adjust for the collar size. Pattern could be plastic or whatever material easy to cut, finish but be hard enough to hold it's edge. With some care it could be moved and repeated. Do clamp everything down and back up the thin aluminum.
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Re: Cutting thin sheet metal

Post by Glenn Brooks » Mon Oct 23, 2017 7:23 pm

Mr Ron,

Did you ever come up with a method to cut your window openings? Iam contemplating cutting similar window openings in slightly larger sheet metal, and just wondering what eventually you decided to do.

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Re: Cutting thin sheet metal

Post by NP317 » Tue Oct 24, 2017 10:42 am

You have access to local (relatively) laser and water jet cutting businesses. They might be you best solution.
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Re: Cutting thin sheet metal

Post by johnfreese » Tue Oct 24, 2017 4:05 pm

I think I would try a wood chisel with suitable support under the part. I think UHMW or Nylon would be fine. A kitchen cutting board would work if not too deeply textured.

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