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Get a Grip on CNC Work

Six ways to hold work solidly

Computer numerically controlled routers do a lot of work quickly and precisely. But they don’t provide any clue that a workpiece will come loose from its worksurface until it happens. And then you have a minor disaster. For success—and safety—always firmly secure the workpiece; your grippy router mat simply won’t do the job. These six tested techniques ensure that workpieces stay put on a CNC machine. 

Commercial hold-downs

You’ll find many styles of hold-downs, including the one shown [Photo below, Sources], that engage the T-tracks in the tables of some CNC machines. These install and adjust easily, and provide plenty of pressure to hold your work. Go with aluminum hold-downs rather than steel, just in case the cutter hits one. 

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With the hold-down in place, do an “air cut” of the tool path before starting the machine to make sure the cutter will not collide with the hold-down or its attaching hardware.

Shop-made clamps

Hardwood scraps make great hold-downs, especially for machines without table T-tracks [Photo below]. In addition to wooden hold-downs being priced right, accidental contact between one of them and a router bit causes no damage to the bit. And these hold-downs have a low profile. To make one, rabbet one end of a piece of scrapwood 4–6" long. Cut the rabbet depth slightly less than the thickness of your workpiece. 

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Screw a shop-made wood hold-down to a wood auxiliary table or spoil board fastened to the machine bed. A coarse-thread pocket-hole screw works great.

Wedges

For machining multiple same-size parts, try wedges coupled with an anchor board and battens [Photo below], all made from MDF or plywood. They allow you to get a workpiece on and off quickly, and lock it into the same XY position every time. First, create the anchor board on the CNC machine, providing a hole at any inside corner for dust relief. Cut a 5° angle on the wedges. 

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Wedges hold a workpiece tightly against the sides but don’t provide downward pressure. Because of this, avoid using upcut spiral bits with this technique.

Screw the anchor board to your spoil board, with its edges parallel to the X and Y axes. (MDF spoil boards call for coarse-thread screws for good holding power.) Place the workpiece against the anchor board, slide a wedge against the workpiece, position the batten, and screw the batten in place. Repeat for the second batten. Secure your work by lightly tapping the wedges with a mallet.

Screws

You can screw the workpiece to a wooden table, if the design allows. Include the screw locations in your design and create a separate tool path for them [Photo below]. This guarantees they stay clear of the cutter while machining the design. 

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When laying out your design, include locations for hold-down screws. Here they are placed in a waste area that gets trimmed away later. If this were not waste, they could become mounting holes for the plaque.

Run only the tool path for the screw holes, using a V-bit with a 18 "-deep cut to mark the screw locations [Photos below]. Without moving the workpiece, drill the screw holes and screw the workpiece to the table.

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Secure your workpiece with painter’s tape to run the tool path for the screw locations. Mark the screw locations with plunge cuts straight into the work. Without any lateral force, painter’s tape provides plenty of holding power.

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Double-faced tape

Double-faced tape works fine for light-duty cuts, such as 3D carving and lettering [Photo below, Sources]. For maximum tape grip, make certain the spoil board and workpiece are flat and free of dust. You’ll probably have to pry your work from the spoil board after completing the cuts, so don’t try this with delicate pieces.

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Apply enough double-faced tape to the workpiece to hold it securely without making it difficult to remove. For the best hold, use new tape on each workpiece.

Hotmelt glue

This adhesive applied to all four edges has enough holding power for even the most aggressive cuts [Photo below]. But applying it and waiting for it to cool takes more time than other hold-down methods. 

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Apply hotmelt glue along the edge of the workpiece, not under it. Glue between the workpiece and spoil board prevents the workpiece from resting flat.

After completing the CNC cut, slice through the glue with a utility knife to release the workpiece. Before mounting your next workpiece, clean glue residue from the workpiece and spoil board with a sharp chisel . 

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 In addition to classes at Vondriska Woodworks in Hammond, Wisconsin, George Vondriska teaches at woodworking shows, guilds across the country, and Weekend With WOOD. Order his book, CNC Essentials: The Basics of Mastering the Coolest Machine in Your Workshop.

 

Sources:
Hold-down clamp

Double-faced tape

 

 

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