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Improved Aluminum Knife Plans

By Larry Zepp, EAA 81346, for Experimenter

Knife

When set with the task of cutting sheet aluminum without the use of a shear, the options become rather limited. Some crafty homebuilders have discovered that the Olfa P-800 heavy-duty plastic laminate (Formica) cutter does a nice job of also cutting aluminum. The term “cutter” is perhaps a bit misleading, as it “cuts” in a manner similar to a glass cutter, in that it merely scores a line in the material that when bent, the material will break along the line. This article tells how one builder built his own heavy-duty version of this knife, but using a carbide blade which is nearly indestructible.

Even though the Olfa scoring knife gets the job done when cutting sheet aluminum, I kept thinking there must be a better way to cleanly cut aluminum sheet stock. Knife blades are designed to cut soft and flexible materials. When we use an Olfa or utility knife blade to cut T6 tempered aluminum, the aluminum resists spreading at the cut line and requires many repeated scores. Although the aluminum is thin, the blade must cold-flow the material before it is scored enough to snap. Also, hardened aluminum seems to dull the steel blade quickly.

In industry, if hardened aluminum can’t be sheared, a cutter or saw tooth removes aluminum chips in order to cut through. I’ve designed an improved aluminum knife that uses a long-lasting carbide “THINBIT®” lathe grooving insert to cut aluminum sheets much faster than the Olfa.

Knife

A carbide cutter is mounted in a modified handle of an inexpensive “donor” 8-inch crescent wrench with a little work. This uses a carbide “THINBIT®” cutter insert in a handle to make a better aluminum scoring knife. This carbide lathe bit has a cutting edge that’s only 0.019 inch, making a clean-cut chip of 0.005 to 0.010 inch thick per stroke—much faster than the Olfa knife. The carbide cuts aluminum like butter, and the insert should stay sharp for an entire aluminum airplane!

Since the insert (blade) is double ended, if the first cutting edge gets dull or chipped, remove the screw and rotate the other cutter into position. Because carbide is brittle, be careful not to twist or side-load the cutter. To make a cushioned grip, cut a 4½-inch-long piece of 3/4-inch ID rubber hose and slip onto the handle. This tool costs $28 to $37 to make, and (if you have to buy everything new) it saves a lot of building time.

Making Your Own

1. Purchase an inexpensive 8-inch crescent wrench, or retire one from your toolbox. Cut off at the narrowest point close to the head at a 110-degree angle, as measured from the bottom of the wrench.

Knife

2. Order the carbide grooving insert (blade) from MSC Industrial Supply (www.MSCdirect.com). For right-handers, order one item number 78677408, 0.019-inch-wide carbide grooving insert, right hand. For left-handers, order one item number 78677507, 0.019-inch-wide carbide grooving insert, left hand. These cost $18.42 each.

If you have access to a 4-40 US tap, you can make your own hold-down screw using a 4-40 x 1/4-inch or 3/8-inch-long socket head cap screw (SHCS). If not, order one metric hold-down screw (item number 78685997, $3.37 each) and one M3 x 0.5-millimeter tap (item number 04991204, $5.91 each).

3. Follow the drawing shown below for a right-hand aluminum knife with the carbide cutter blade mounted on the right side of the handle. For the left-hand version, make the groove holding the blade on the other side and use the left-hand insert.

Knife
Larger view

4. Locate the hole position with a center punch. If you’re using a 4-40 SHCS, use a 3/32-inch or number 43 tap drill. You can use a 40 tap drill for the M3 metric screw. Drill and tap, then chamfer the thread 0.06/0.07 inch deep for clearance and deburr. Locate the blade at 110 degrees from the bottom of the handle with the screw, then hand tight. Mark the clearance groove for the 0.278-inch-wide carbide blade with a fine-point Sharpie or measure 0.145 inch on either side of the hole’s centerline and scribe.

Knife

5. Mill, grind, or file the groove in the handle to support the width of the blade, approximately 0.070 inch deep or deep enough to slot the handle’s ribs and clean up (flatten) the surface between them. Test-fit the blade using the screw. If the carbide blade doesn’t have a flat shoulder to support it, then file, grind, or remachine the groove to fit.

Knife

6. If you’re making your hold-down screw using a 4-40 SHCS, use a short piece of 3/16-inch rod to make a holder for filing or grinding the chamfer. Tap the rod center with a 4-40 thread. Thread the 4-40 SHCS into the rod and chuck the rod in a drill press or cordless drill. Use a file or grinding stone to make a 90-degree chamfer in the head of the screw to match the blade chamfer.

7. If you’re using the metric hold-down screw, it’s tightened with a T8 Torx bit.

8. Mount the blade in the handle and tighten the screw. If the blade isn’t clamped tight, the thread chamfer isn’t deep enough or the groove surface isn’t flat. With the blade held tight, this improved aluminum knife will make a clean-cut chip of 0.005 to 0.010 inch thick per stroke. If the first cutting edge gets dull or chipped, loosen the screw and rotate the other cutter into position. Because carbide is brittle, be careful not to twist or side-load the cutter. To make a cushioned grip, cut a 4½-inch-long piece of 3/4-inch ID rubber hose and slip onto the handle. The tool costs $28 to $37 to make, and it saves a lot of time building your plane.   

Best regards and happy building!

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