Once upon a time, the movements of master machinists were measured, recorded, and stored on cards and paper tape. The stored numbers were used to control motors that moved mills, lathes, and other machines exactly the way the machinists had. This was referred to as numeric control (NC). After World War II, computers found their way into manufacturing and were used to control the machines, which is called computer numeric control (CNC). Broadly, computers controlling motors that move tools includes modern 3D printers, laser engravers, stencil cutters, and the target of this skill builder: CNC devices using routers (Figure A) or motor spindles.
In general, we use the term CNC to refer to subtractive manufacturing techniques. Subtractive manufacturing removes material with cutting bits, end mills, or other tooling — the opposite approach is additive techniques like 3D printing, where the part is constructed by building up material. 3D printers rely on plastic adhesion to keep the print from moving during production. CNC machines that grind, cut, and drill, however, are much more aggressive — unless the piece is firmly held in place during these operations, the part is ruined and expensive bits are broken.
CNC workholding is a topic that can, and has, filled books, but we’ll take a look at the basics of how to hold parts in place. Which method you choose depends to some degree on the table or work surface on which the part is mounted, and the material being held, but ultimately a few techniques cover the important ideas.
On a CNC machine, the work surface can be unbroken, or have built-in methods to hold items, such as T-slots. A T-slot is a T-shaped cutout (Figure B), where the crossbar of the T is on the underside of the table or inside the table itself. A nut fits into the slot to provide a movable location for inserting a bolt or clamp for workholding.
There are many types of clamps that can be used. The different designs offer various advantages and disadvantages. It’s surprising how much holding power a small clamping area can provide. But misaligned clamps (Figure C) can cause trouble when they fail to hold. Clamps that have a flat surface need to be placed level to the workpiece. If they hold at an angle, especially against the edge of the work, the holding force is less than intended, and may even contribute to the piece moving when under pressure. A bolt or other support at the back of the clamp is adjusted so that the clamp is horizontal under load.
Many work surfaces that don’t have T-slots will have thread inserts mounted at regular locations (Figure D). Bolts can be threaded into the inserts to hold clamps of various designs.
When cutting parts, your workholding strategy must take into account whether you’re cutting all the way through a piece or just cutting on the surface. If you’re cutting all the way through, it’s common to use a “spoilboard” under the workpiece so that the bit doesn’t cut into the table. It’s also important to consider if the part being cut out might move once there is no more support and get thrashed by the bit. Many CAD programs will provide a feature to add tabs to the design (Figure E). Tabs are small, uncut regions left at the base of the part, just large enough to keep it in place during the remaining operations.
Some clamps are designed to hold with an edge, rather than flat (Figure F). These are usable at an angle and are supported on the back by the worktable.
For many operations, a vise clamped to the table holds the workpiece (Figure G). This allows quick swapping of workpieces but only registers the piece in one direction.
When Clamps Won’t Cut it
When absolute positioning is necessary, the concept is taken to another level with the creation of a jig. A jig is a structure that correctly positions the workpiece in all dimensions. The piece is slid or dropped into the jig and held in place with one or two quick clamps. Cam clamps (Figure H) that rotate to lock a piece into place with horizontal force are common in jigs, as are toggle clamps (Figure I) that clamp things in place vertically.
Clamps can have the disadvantage of getting in the way while cutting. Running a $30 bit into a metal clamp is a distressing (but not uncommon) experience. Many of my clamps bear the mark of shame (Figure J) that motivates me to take extra care.
Many operations need the workpiece to be fully exposed without clamps sticking up. In these cases there are three basic approaches. The first is to screw or nail the piece directly to the table. This requires careful planning so that the cutting bit doesn’t run into a screw. It also doesn’t serve to hold the entire piece down; If sections without tabs are cut away from the screws, the piece can move.
The second approach is common on many large CNC routers used in cabinet and woodworking shops. The worktable will have small holes in the surface and channels underneath through which a vacuum is drawn, like an air-hockey table in reverse. This pins the workpiece in place for the duration of the cut and quickly releases it afterwards. Smaller vacuum clamps are available and can be used like vises (Figure K).
The third approach is to mount the workpiece in place with double-sided tape. This is easy with small pieces, but difficult with larger ones. Be careful when positioning tape so that it doesn’t leave some areas without support. Even a hundredth of an inch difference in height between supported and unsupported areas can be a problem with many materials. Unfortunately, double-sided tape in quantity is expensive and frustrating to work with. Worse, many varieties have very weak holding power.
Tenacious, Tailer-Made Tape
Ben Crowe, a master luthier at Crimson Guitars (crimsonguitars.com) has come up with a popular variation on the double-sided tape approach. He uses regular masking tape (generally the very wide variety) on both the table and the workpiece, then applies thin squiggles of cyanoacrylate (super glue) to one side of the tape and, optionally, cyanoacrylate accelerator to the other side. When placed together, this creates a wonderfully strong, homemade double-sided tape that’s actually affordable. When the work is completed, the workpiece peels off the tape and the tape peels off the table.
I’ve been using my own variation on this method for even larger pieces. Rather than using masking tape, I’ve been using 12″- or 24″-wide adhesive shelf lining paper (sometimes called contact paper) and adhesive spray (Figure L).
This has allowed me to adhere large pieces of leather to my worktable very solidly and very cheaply without having to carefully apply multiple strips of tape. This is much easier than aligning tape strips and hoping I didn’t overlap them, creating bumps, leaving gaps, or getting glue on the table and workpiece. I use newspaper to cover any areas that I don’t want to get adhesive spray on (like the motors or rails of my CNC). The spray gives me even coverage of adhesive and the shelf paper easily peels off the surfaces without problems when done, but holds firmly while in place (Figure M).
This article only scratches the surface of a gripping topic, so I hope you’ll experiment and research more ideas for holding your work successfully!