Intro: Designing for Sheet Metal
What works on paper, doesn't always work in the physical world as planned, for a variety of reasons. What follows here are some of the various lessons I have learned while laying out sheet metal projects. Hopefully you can benefit from this guide and save yourself the time I spent learning things the hard way!
Step 1: Sheet Metal Has Thickness
When you're making something out of paper, the thickness of the sheet is negligible--everything will line up and work out most likely in the end. Sheet metal is a different creature in this respect. Bends have radius, sheet has thickness, and tools need space to access a bend. When laying out a simple box with tabs, the tabs will meet the perpendicular face of the next side when folded up to 90 degrees, so when you are folding your tabs, measure in from the edge of the material the length of the tab, plus the radius of the bend, plus the thickness of the material for a perfect corner.
When folding over a seamed edge, the folded-over portion is going to need to clear the adjacent faces on both sides. Slightly notching these helps resolve these issues. A file is sufficient for fine-tuning these bends.
Think of bending sheet metal as what it really is: stretching one face, while shrinking the opposite face in a controlled way.
Step 2: No Welder? Try Rivets or Brazing!
Welding sheet metal isn't always the ideal way to join sheet--especially thin or non-ferrous sheet. Heat causes metal to expand and warp if you're not careful. When spot welding, for instance, each spot weld causes the sheet to expand 360 degrees around the weld. If you start in the middle of the work and work your way out to edges, you'll have more uniform results than moving from corner to corner.
POP rivets are a good alternative to spot welds. Punch a dimple in the sheet metal before attempting to drill the pilot holes using an awl and mallet. Then, your bit has a depression to hollow out and won't stray and scratch the work. Put the rivet in the hole you've drilled, and draw the mandrel through the rivet shank with a riveting tool. You actually can dress these rivets as well. Doming them with a ball peen hammer helps a lot. Just be sure to use a steady hand, the appropriate size hammer, and a rivet set if possible. Solid rivets are another option, but doming them in tight spaces can be an issue, hence the choice of pop rivets for this box.
If rivets aren't your cup of tea, try brazing. Use a MAPP gas torch, brass brazing rods, and flux to heat the joint to braze and draw brass into the seam using capillary action.
There are also methods of joining metal without fasteners or brazing. Hemmed seams are a good way to join two edges. Make a v-shaped channel on each end on opposite sides of the sheet, nest the openings and flatten closed. Clean up the seam with a groover--a steel stamp with a rectangular cut-out. Or, overlap tabbed-seams with a seamed edge.
Step 3: Used Sheet Metal Is Still Sheet Metal!
If money is an issue, look no further than your closest alley. Cut the chassis off of a dryer, refrigerator, or filing cabinet. Cut down enough sheets that it's worth the trouble to negotiate a price with a sand blaster in your town. Pay a fair rate to have the sheets sand or bead blasted down to bare metal. If you're really strapped for cash, use a belt sander. If the dumpster gods aren't smiling upon you, try calling your city's trash collectors, inquire about large item pick-up day. Most towns have someone who breaks down appliances into recyclable materials, disposes of toxic things, etc. Ask for this contractor's contact info and negotiate a price for the un-processed sheet metal.
Sometimes your local steel yard will discount corroded material. Learn methods for removing rust and other types of oxidation. Ammonium works well on Aluminum Oxide. Sandpaper and stainless steel wire wheels work well on rust.
Step 4: Always Wear Gloves! No, Seriously!
Sheet metal is sharp. You think you'll be careful enough that gloves won't be necessary all the time, but you're wrong. I was wrong once and lost a week's worth of work time. All it takes is one slip and you're out a few hundred dollars for stitches or surgery.
Step 5: Rolling a Burrs and Flanges
Adding a perpendicular face to a curved surface sounds like a daunting proposition, but with the right tools, it's not such a big deal. The tool you want is called a Bead Roller. There are several companies that make them new now, Mittler Brothers Machine & Tool is the first that comes to mind, or several serviceable vintage machines are out there too, such as those made by Pexto and Roper Whitney. With the right combination of dies, you can roll that profile in 3-4 passes.
Step 6: Know Thy Shears
There are several different types of shears out there, and each of them have very specific uses. Trying to make straight cuts with a non-straight cut shear may lead you to believe that you're a crumby metalworker. Not true! Straight shears make straight cuts.
Types of Shears
I. Tinner's Snips: These are your grandpa's tin snips. They are a very sturdy and useful tool, deadly accurate, easy to sharpen and maintain, and damned near indestructible.
A. Straight Cut Tin Snips
- Have a long, straight jaw. Used for straight line cuts
B. Duck-bill Snips
- Useful for cutting short straight sections and curves
C. Circle Snips
- Used to cut circles of larger radius
D. Hawk-Bill Snips
- Used to cut circles with smaller radius. Allows for added clearance in tight cuts.
I. Compound Action Shears / Aviation Shears: used originally for making aircraft, these are easier to use because of their compound action, yielding a higher mechanical advantage. They are color-coded, though all you really have to do is look at the jaws and use your common sense. Yellow is straight, red is left, green is right. Long-cut aviation shears have the jaw angled 90 degrees from the handles and are good for making long, straight cuts where clearance of material is an issue.
A. Straight Cut
- Cut reasonably straight lines
B. Left Cut
- Make cuts that curve to the left
C. Right Cut
- Make cuts that curve to the right
Step 7: Never Trust a Factory Edge Without Sqaring
When you buy sheet metal it will have crisp, straight (on some axis) lines. Before you start laying out a project, take the time to check the edges for square. I use an aluminum T-Square for this. A carpenter's square, machinst's square, etc. all work as well. This will save you from tearing out your hair after 4 hours worth of work when your project is 1/8" off center!
Another option if you're buying your sheet metal from a specialized sheet metal supplier, they can cut the sheet on a squaring shear, a huge hydraulic monster machine that makes perfect cuts.
Step 8: Scribe, Don't Mark!
There's a reason why sheet metal workers scribe lines, rather than mark them with a sharpie. While fine-line marker might do a fine job, a scribed line has a pin-prick's diameter and can be very accurate. A scratch awl or a divider are traditional and effective. A nail even works in a pinch. Scribe along a straight edge, or use your divider to drag along your squared edge to mark off distances from the edge.
Step 9: Use Trig Only As a Last Resort
If you're laying out a part with angled faces, for instance, to determine the lengths of the sides, you would have to use trigonometry. Are we metalworkers or mathematicians though? What I do is draft it out exactly. I know the base measurement, I know the angles, and I can draw the intersections. From there, set the distances from point-to-point with your divider, scribe onto the metal, and go from there. It's much faster for non-math types like myself. If you need to have tabbed-edges on a piece with these kinds of angles, divide the internal angle measurement by 2, mark it out on your draft, and go from there. Much easier!
Step 10: Increasing the Strength of a Sheet Metal Part
Sheet metal in a flat sheet is not very sturdy. It can be bent, warped, and folded easily; that's why we love it! But when you're designing a sheet metal part, add a few strength-enhancing features to make sure your part lasts for generations.
Hems are created when you fold over the metal back onto itself. It saves time (less de-burring), gives a more finished look to your edges, and, most importantly, it adds tremendous amounts of strength to a face!
Beads are embossed lines that run across the surface of the sheet. These are made using a bead roller. These indentations add strength perpendicular to the bead. This is the idea behind corrugated tin roofing.
Grooved Seams are a great way to join two pieces of sheet metal, but they also create tremendous strength perpendicular to the joint. A properly grooved hem has 2 broken edges, and 4 layers of sheet metal in one spot. Very strong!
Step 11: Hand Seamers Can Break Edges
If you need to break small edges that are difficult to access, try using a hand seamer. These are designed for HVAC work, but work very well for making finger-break type work without the proper break. Fold the part over in the hand seamer's jaws, use a mallet to sharpen the edge, and release. These can be used to break long, continuous edges as well, but in 3-4 steps, working uniformly across the surface. I use them to finish hems.