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Saturday, June 16, 2012

Holding material down without a vacuum table.

Using a CNC has got to be the easiest form of woodworking. I once heard another CNC user say "I can't find a hammer that fits my hand." At the time I didn't get the joke but there's allot of truth there. I got a splinter the other day for the first time in what seems like years. I barely handle the materials i work with, and when i go to the local Woodworkers Club meetings, I make it a point to let the real woodworkers know that i have none of their skills. I know I have a hammer, I used it the other day to  break apart some trash before it went in the dumpster.

Unlike other forms of woodworking, there is very little "holding" the
wood. When your using a hand tool, or even a power tool, there is allot of handling, gripping, and otherwise touching whats being worked on. This make a lot of sense from a hand-eye coordination point of view, but when there is machinery involved, cold, unfeeling, one tends to think of how hard it might be to eat pizza with only one hand worth the fingers. But that doesn't mean the material can't be held in place, just that it needs to be done without your hands being involved.

With automated machines, there are several methods for accomplishing what is called "hold-down." Soe methods include Vacuum, Clamps and T-Track, Adhesives, and Screws.

Really fancy machines often have an option for Vacuum clamping. This is essentially a Machine with a tabletop that literally sucks the material tight to the surface. It's not always great with small parts, but by and large its pretty dang effective. First time I saw one in use, the operator tossed a piece of scrap wood on the cutting surface and i couldn't for the life of me get it off until he turned off the suction. Here is a demonstration of just how powerful vacuum clamping can be using a setup on a work area.

Clamps and T-Track is pretty standard on a CNC machine, if it has anything for hold down, it often has this aswell. This setup work by having metal slots in the table which allow bolt head's to be inserted down, and a nut is threaded to the bold pressing down a "clamp" (typically a small block with a hole or slot in it's center.)

Some projects require adhesives, possible because the object is very small, or because there isn't a place to actually "hold" the material except underneath. I most often use this when working with styrofoam.

There there are screws. This is the poor man's route. Well, let's be honest, I don't have t-tracks, and sure as heck don't have a vacuum system. While this system probably takes the longest to setup, its probably the strongest method for holding down materials. There is a trick.

Screws are metal, and if the cutter hits one, your likely going to have trouble. Broken bit, chipped cutters, or worse, the bit survives, but the machine is thrown out of alignment and cuts all over the place.

When I first started working with entire sheets of material, i would literally crawl onto my machine while it was running, and start screwing the material down while the machine was going. I always had the material held down from the edges, but it it bowed and needs to be held down in the center aswell. So while the machine was running i'd stand onto, and when the machine had cut stuff in one region, and i knew it was safe to place a screw, i would insert a screw while the machine was out of the way. The one day i had a better idea.

Why not pick safe places for screws before cutting the parts? Well, there is a trick.

Once you have your designs all programmed and your ready to get cutting, do one last step. I take my circle tool and place circles with a diameter of .3" (That's about how big the head of a typical wood screw) between parts where its out of the way of the cutter. How many? Well, that depends on the shape and size of parts on the board, where they are positioned, etc. However, when i am doing it, i try to get atleast 9 or 12 screws into my material in addition to the screws a the edge.

 Selecting all the circle place, I create a drilling tool path and use a .125 (1/8") bit to pre-drill all the screw holes. Run this operation first, screw it in place and then run the cutting file. This has saved me tons of time scrambling over my machine while its running to place screws, and it still doesn't look like I am getting vacuum hold down any time soon.

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