January 2014 Issue
Home Shop Machinist
Cutting aluminum on the table saw.
The first time I watched someone cut a one-inch thick slab of 7075 aluminum with a table saw, I thought, "You must be crazy!"
It turns out they were not crazy at all. The table saw, when equipped with the right blade, is an awesome tool for cutting aluminum. Add a sled with some basic hold-downs, and you can create parts that look like they were made on a milling machine.
My friend Richard Cunningham did just that. His design, "Lucky Dog," features an aluminum tube fuselage with abundant gusseting. With some clever fixturing and a basic sled system, he made all the gussets, attachment plates, and brackets using only a drill press, table saw, and disc sander. Since most of Richard's plane required several of each type bracket, he batch processed them by stacking the blanks for common drilling, sawing, and finally sanding the corners round.
Table saws come in many sizes and shapes. Portable benchtop saws like the kind sold at home improvement centers run from $200 to $600. Low-end portables are very noisy, not very sturdy, and they are not accurate enough for anything other than basic carpentry. At the high end of portables the saws are more solidly built and capable of reasonable accuracy. Portables don't pack the power of their stationary brothers, so that limits their usefulness for aluminum work. But if your project consists of brackets, plates, or gussets 3/8-inch thick or less, a good portable saw will do just fine.
Stationary saws start around $600 and go up (way up!). At the low end are so-called "contractor's" saws and at the high end you'll find classic "cabinet" saws like the Delta Unisaw. Even further up the scale are enormous saws with sliding tables for handling 4x8 plywood sheets; but for the purpose of this article and the type of parts typical for kit or plansbuilt aircraft, the cabinet saw would be tops.
Clamping the fence square for screwing and gluing. The front block is a spacer for clamping and is not part of the sled.
Table saws come with 8- or 10-inch blades for woodworking. You must install a carbide-tipped blade designed for non-ferrous materials (aluminum, brass, etc.). You might have to go to a specialty tool store or an aluminum supplier to find a blade. If you're using a portable saw or a contractor's saw, get an 8-inch blade. Just like propellers, the smaller diameter helps compensate for lower power. A cabinet saw typically has three horsepower or more, so a 10-inch blade works fine. My personal favorite blade is the Alumi-Cut by Tenryu. Their 8-inch Alumi-Cut has 60 teeth and the 10-inch is available with 60, 80 or 100. The 100 tooth blade is more expensive (about $80 versus $70 and $50 respectively), but will give better cuts in thinner stock and still work fine for thicker material.
One thing about cutting aluminum on a table saw is it makes a lot of everything: noise, chips, and heat, in that order! So wear earplugs, safety goggles and, before you start, clean out the dust collection system of wood dust and chips. I use my regular dust collection system when cutting aluminum. It doesn't suck up all the chips, but it gets most of them and without it the shop would look like it was droned with a glitter bomb. Although aluminum chips coming off the blade probably aren't hot enough to ignite sawdust, why take the chance?
Material, Layout, and Prep
Good parts start with good material and layout. If your plans don't call out the type of aluminum, be sure to research your options. There are big differences in strength, corrosion resistance, and machinability across the spectrum of alloys that make up the aluminum family. The better machining aluminum alloys include 2011, 2024, 6061-T6/T651, and 7075. With a sharp blade and steady feed, any of those alloys cut cleanly. Avoid un-tempered alloys like 6061 T0 (basically anything that is not T6 or T651), as it will gum up and ruin your saw blade. 2024 and 7075 are the strongest. The best machining aluminum is 2011 and it is comparable to 6061 T6 in strength, but it gets a "D" for weldability and is three times the price. Why would anyone use it? There are many production settings where the faster machining time outweighs the higher price.
The key to this project is a sliding table saw accessory called a sled. Woodworkers will recognize the configuration as a cross-cutting sled: a plywood base, hardwood runners to fit the miter slots, and a fence. The fence serves two purposes: as a reference surface for squaring cuts and a convenient handle for advancing the sled into the blade. The base is made from ¾-inch Baltic birch plywood, the runners are sized to fit the table slots (usually 5/8 x ¾), and the fence is 1¾-inch hardwood that is square and straight. The fence must be positioned two or three inches from the trailing edge of the sled. When you make the initial cut into the sled, position the blade only as high as is necessary for the thickness of material you plan to cut, and then just enter the fence partially.
After ripping a strip of 1/8-inch thick 6061 T6 to width on the table saw, I used the fence to prepare three blanks of equal length. A simple plywood scrap was clamped to the fence to create a repeatable stop. With one blank marked to locate the holes and center-punched, I stacked and clamped them into the drill vise for "gang" drilling. I like to start by spot drilling. Spot drills are stubby rigid drills designed to provide a more precision "center" than you get with a center-punch. Twist drills may or may not be straight; so even if lined up on a center-punch mark, the hole you drill may not be square to the mark. Spot drilling or "spotting" the hole helps that problem. Another way to make precision holes is to "pre-drill" the hole slightly undersize and then finish it to size with an end mill (works only if the hole size is a standard end-mill size). It only works to spot, pre-drill, and then plunge-mill the hole to size if your blanks are secured in an anchored vise. You'll never hit the hole on center with an end mill unless you've clamped the part securely.
If your blanks are not equal or your vise less than great, one or more of the blanks might slip out of alignment as you drill. Combat this by adding a clamp, or bolt them together after drilling the first hole. It's more important the holes be perfectly aligned than the raw edges (which we'll saw off anyway).
Now comes the fun part: bolting the blanks to the sled and sawing. Last month I introduced the Vix bit for centering pilot holes. The parts need to be positioned so the cut line is right at the kerf edge on the sled. You will notice in the photos the parts are located on the sled some distance from the fence. Since none of the cuts are square to the fence, it plays no part in locating or holding. You can fixture the parts anywhere along the kerf. In fact, for small parts, the sled doesn't need to be any bigger than the width of the miter slots. But once you have a sled you will be using it for different jobs, so it's a good idea to make it a reasonable size to survive repeated uses. After a few years, if and when it has so many holes it looks like Swiss cheese, you can make another one. The hold-down bolts can be flat head or pan head screws or whatever works. You will have to countersink or counterbore the backside according to the hardware you have. This is the reason to use ¾-inch thick plywood: to have enough material to counterbore for the hardware.
With a second setup and the parts secure, the second cut is made. Check out the companion video below.
Two bolts are required to hold the parts down. If one is very close to the saw, be sure the nut or bolt head clears the blade. Use a spacer if needed to move the hardware clear of the spinning blade. The Alumi-Cut blade works great on aluminum, but if you collide with a steel screw or nut, it could damage the blade. Best to avoid that!
Rounding over corner number one: The center pin is pegged to the plywood sled, which has a runner attached to index it to the miter slot in the table. Pegs are inserted into the open holes to keep the three parts aligned and identical.
CLICK IMAGE TO SEE MORE
Check the alignment and cinch the parts up tight. Fire up the saw and, with even and steady pressure, feed the sled into the blade and make the cut. You can't go too slow at first. With a new blade, you probably won't feel any resistance so watch carefully for the parts to sever and back the sled back out. The process is the same for successive cuts: line up the cut line, position the anchors, tighten and cut. The demonstration part required two fixtured cuts. The third side was established with the sawing of the initial blanks, and the hole locations were referenced from that side.
With some simple tools and basic skills, we have three professionally made, but even more important, identical parts.
Check out this video on the Kitplanes® YouTube channel that demonstrates the set-up and feed rate. It will give you an idea of what to expect.
With a second setup and the parts secure, the second cut is made. You can see this in the companion video, too.
You might ask, "Is it really necessary to do it this way?" Of course not! If you only need one of a particular bracket or gusset, a band saw and handwork on the disk sander will get you there. But if you have batches of identical parts to make, in the long run it's always better to set up a fixture.
Got a question for or job that needs assistance from the Home Shop Machinist? Send us a description of the part and where it's used. We'll consider it for a potential topic for a future HSM column. If we pick your request, be prepared to supply the raw material and, if it's part of a published plan, access to the original design drawings. Send inquiries via e-mail to: firstname.lastname@example.org and put HMS in the subject line. Due to liability, certain items may be limited to the construction of a demonstration part for instructional use and display only.
Bob Hadley is the R&D manager for a California-based consumer products company. He holds a Sport Pilot certificate and owns the VW-powered Victory Stanley Fun-Kist.
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