Our Technique: Material Holddown

Material Holddown with Raptor Nails

On our PRS9648 ShopBot with a Porter Cable router, we haven’t setup a vacuum table. This is due to costs and the fact that our current workshop doesn’t have three-phase power. When we first assembled our Bot and cut a few parts, we got concerned about hitting the drywall screws we used to secure the material to the ShopBot table. After scanning the WWW, we found out about polymer nails from a company right here in Texas, RaptorNails.com. They are a blend of polymer and fiberglass, don’t rust, bond like glue with friction and are UV resistant. Most importantly to us, the router cuts right through them. Now any time we need to hold material we either use nails directly through it or in the case of foams or acrylic, we attach strips of MDF to the table that hold the material in place.

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Keep the Material on the Table

In every part file we create, “Step 0: Mark Holddown” is a peck drilling toolpath to mark the spots for holddown using whatever the first bit is at 0.05” depth. In the case of full sheets of 3/4” plywood, we’ve found that having a nail every 24” inches around the perimeter and through the middle provides enough holding power to the keep the plywood in place when cutting at 4 inches per second.

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Keep Parts in the Material

We have tried onion skinning, routing within 0.05” of the material thickness. It requires using a Laminate Trimmer with a flush cut bit and going back over the entire routed path to free the part from the material. Depending on the quality of the bit you use, you risk not getting a clean edge when removing the onion skin and spending more time per part. If you don’t use a bit with a bearing, you often burn the inside edge of the path with the bit.

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Using tabs, we have done various sizes and spacing with good results. You still use a trim router but only risk edge quality for the length of the tab. We have not yet worked out a formula for tab size but it does work better to place tabs in corners or shorter part sides than risk the clean edge on a long side.

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We have used a Porter Cable #7301 Laminate Trimmer for the last few months but are switching to a Festool MFK 700 Modular Trim Router Set for better dust control and accuracy.

Release the Material

In the case of plywood, we’ve found a quick way to free the material from the work table. Using a 5lbs dead blow hammer, if you smack the front edge of the plywood towards each corner, the polymer nails will shear and release the wood. Of course this might not always be appropriate if you have delicate parts or are using a fragile material.

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Before reloading the work table with new material, we sweep the surface with a dustpan or anything with a hard edge to find any nails sticking up. Nails can be cut with a utility knife or trimmed with a Flush Cutter like the Xcelite 170M.

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