As many of you running a CNC router probably already know - work holding can be a royal pain! We have three differently-sized ShopBots at our TechShop location here in San Francisco, all three with their own unique limitations when it comes to work holding. Brass screws inevitably strip out, clamps somehow end up in the way of the tool path, and spray adhesive just doesn't seem to hack it. I just want to say one word to you. Just one word....Plastics. Seems Mr. McGuire in The Graduate was right! Modern polymer nails have most of what we need covered, though with a few notable exceptions worth exploring.
Step 1: Polymer Nails for CNC Workholding
Companies like Raptor Nails and Staples offer a variety of fasteners, all made from the same high-strength polymer and come in standard sized strips which fit most pneumatic nailers. The tensile (pull) strength of these nails is, in some cases, double that of their steel counterparts which makes for great work holding on your CNC table - as long as you're using a soft spoil board, like MDF.
Step 2: Polymer Nails for CNC Workholding
Penetration of even the 1" 18gauge brad nails is excellent, but this is where the little niggly issues start to come to light.
Step 3: Polymer Nails for CNC Workholding
Our standard Porter Cable Finish Nailer had no problem driving the nails through soft pine, regardless of thickness. Harder woods caused an increase in nail breakage and less overall penetration. Part of the problem we noticed was that our standard shop air - 125psi - was absolutely killing the nails as they were driven in. Running a regulator and lowering the pressure down to 75-90psi cured a bunch of the problems, but we still broke nails on those harder woods.
Step 4: Polymer Nails for CNC Workholding
So it sounds like these nails can be finicky, right?! Well, their shining glory is revealed when you need to remove your part. One quick tap with a soft mallet is all that's required to snap them off. While they're strong in one direction, they're basically mush in the other. Strong, but brittle, as it were. Ever see Samuel Jackson in Unbreakable? You get the idea.
Step 5: Polymer Nails for CNC Workholding
Flip the part over and you can see the results. Both the part and the table show clean breaks, darned near flush. Compare their impact to the screw holes nearby and you might find yourself jumping on their bandwagon too. Any stragglers rising above the surface can be leveled with a quick scrape of a putty knife or a straight edge. Don't bother with a chisel, you'll only damage your deck. Additionally, they can be machined flat or, unlike that broken brass screw you're loudly cursing, sanded smooth.
The pros: Polymer nails are easy to use, hold very well, and break off cleanly.
The cons: They require some tweaking to get the air pressure right, they don't work well through harder materials, and they only come from limited suppliers.
The conclusion: If you've got flat sheet stock of fairly soft material and are looking for a fast way of securing it, then use some of these, you won't regret it ;)
Lead DC - Facilities Manager