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- Jul 28, 2017
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- 2,379
The rotary shear I ordered online arrived today, so I mounted it on a scrap piece of wood and did a few tests. Like the metal snips I've used in the past, it appears to take some attention to get relatively straight cuts .... but it isn't quite so difficult to "keep on track" compared the snips because the sheet halves don't have to spread apart so much when making longer cuts. It also doesn't warp the pieces quite so much, for the same reason. The metal on the right side gets a little more bent up, but that was for cuts that weren't particularly wide. Wider cuts might result in less warpage on the "waste" side of the shear.
The serrations on the top cutting wheel do a good job of grabbing the sheet metal for easy feeding, but the downside is that they leave a distinctive serrated edge. My current projects don't depend on cosmetics all that much so it's not a problem at the moment. One way to get around that, at least when making enclosures, is to include "hems" that are bent toward the inside of the box, so those ugly looking edges are out of sight.
Now, on to some dimensional tests I did. I cut a couple of 2" long "coupons" out of some .025" thick aluminum sheet, scribed a centerline, then bent them around that centerline on my el-cheapo HF 18" brake. To minimize the radius on the inside of the bend I went as far past 90 degrees as I could, given the top clamping piece, then bent the flap back to 90 degrees. The clamping piece is ground at a slight angle to permit bending a little past 90. BTW I have the clamping piece offset from the bend point by the thickness of the sheet metal stock, .025". This might be a factor in the results I got, see below.
Then I measured the distance from the top edge to the inside surface of the opposing bend, and did the same from the bottom edge. In this case the "top" edge is the free, or unconstrained side of the bend and the "bottom" is the clamped-down side.
I measured some noticeable offset -- the clamped-down side loses about .026" (compared to the exact center of the unbent coupon) and the free side gains something between .016-020"
Are these results typical for bending up sheet metal, or am I doing something wrong? The main reason I'm asking is that I want to make matching lids for the boxes so they need to reasonably match the box outside dimensions. I notice that the clamped-down side's offset is pretty close to the thickness of the sheet metal, so I'm thinking that the offset I've got for the clamping piece is playing a role here. As long as the offsets are consistent I can allow for that when I lay out the bend lines.
The serrations on the top cutting wheel do a good job of grabbing the sheet metal for easy feeding, but the downside is that they leave a distinctive serrated edge. My current projects don't depend on cosmetics all that much so it's not a problem at the moment. One way to get around that, at least when making enclosures, is to include "hems" that are bent toward the inside of the box, so those ugly looking edges are out of sight.
Now, on to some dimensional tests I did. I cut a couple of 2" long "coupons" out of some .025" thick aluminum sheet, scribed a centerline, then bent them around that centerline on my el-cheapo HF 18" brake. To minimize the radius on the inside of the bend I went as far past 90 degrees as I could, given the top clamping piece, then bent the flap back to 90 degrees. The clamping piece is ground at a slight angle to permit bending a little past 90. BTW I have the clamping piece offset from the bend point by the thickness of the sheet metal stock, .025". This might be a factor in the results I got, see below.
Then I measured the distance from the top edge to the inside surface of the opposing bend, and did the same from the bottom edge. In this case the "top" edge is the free, or unconstrained side of the bend and the "bottom" is the clamped-down side.
I measured some noticeable offset -- the clamped-down side loses about .026" (compared to the exact center of the unbent coupon) and the free side gains something between .016-020"
Are these results typical for bending up sheet metal, or am I doing something wrong? The main reason I'm asking is that I want to make matching lids for the boxes so they need to reasonably match the box outside dimensions. I notice that the clamped-down side's offset is pretty close to the thickness of the sheet metal, so I'm thinking that the offset I've got for the clamping piece is playing a role here. As long as the offsets are consistent I can allow for that when I lay out the bend lines.
