Wood moves, of course, and CNC operators must deal with that phenomenon when machining with a spoilboard.
So, what causes stock to move on a CNC bed? The most obvious force is shear, when the tool moves sideways against the material as it feeds into the cut. A less obvious stress causes stock to move up or down along the axis of a bit. This is most significant with a spiral upcut bit which creates a force that pulls up on the stock. This type of stress may also cause vibration in thinner stock, and that can show up as a poor surface finish. There may also be reaction wood, where movement is the result of tension or compression stresses being relieved during the CNC processing.
The preferred method for securing large flat parts to a spoilboard is vacuum. But what if the shop needs to secure smaller parts and solid wood?
Nails and screws
One safe way to secure parts is to use plastic composite nails and staples such as the Raptor line. Of course, this requires investing in a special nail gun, but the advantage is that a tool won’t be damaged if it passes through the fastener.
In small shops with occasional needs the handiest solution may be screws, but there are several caveats here. First and foremost is that the screws can’t be placed in the path of a cutting tool. More often than I care to admit, I’ve secured material with those ubiquitous gold deck screws only to see a tool nick a screw and chip the cutting edge. So, I prefer to use brass screws that are less likely to damage a bit if there is inadvertent contact.
If a screw penetrates all the way through the spoilboard, it will create a volcano on the underside where it’s not visible. Even worse, it can create a volcano on the phenolic bed of the machine. Volcanos on the top surface of the spoilboard are easily removed with a sanding block, but invisible ones on the underside will compromise the flat surface and will not be discovered until the spoilboard is removed. A carefully prepared counterbore of appropriate depth will ensure that the screw penetrates adequately into the spoilboard, but not so far as to create a volcano on the bottom. Be sure that the tip of a screw comes no closer than 1/4” from the bottom of he spoilboard to prevent these hidden problems.
Sometimes parts are large or thin enough that they must be secured in the interior (in addition to around the edge). If a vacuum isn’t available, screws can be used if applied with care. They should be in an area that will be waste in the final part, or in a hidden spot that is counterbored below the machined surface where they can be filled if needed.
Not so simple clamps
If the bed of a CNC router has T-track, clamps can be used to bear down on the top surface of the workpiece. The most immediate risk with clamps is that they stick up above the workpiece and create a risk of collision with the router bit. There are several approaches to mitigate these risks. The most obvious is to use clamps and bolts that don’t stick up more than necessary. The second is to use clamping materials that are less likely to damage a router bit, such as hardwood bodies with nylon bolts. Nylon serves an additional role in a school workshop because the bolts will snap when a student chooses to torque them too much.
Software programming and operational practices can also reduce risk. The height for rapid Z clearance above the workpiece can be set greater than the thickness of the clamps. When manually locating the spindle prior to beginning the job, the spindle can be located over the cutting area so that the initial moves are less likely to plow into a clamp. One can be thoughtful of the sequence of toolpaths and machine interior features first before perimeter cuts that are intended to release the part from surrounding waste stock.
Another approach is to use a 3D roughing cut before a 3D finishing cut. I prefer to save machining time by using only a 3D finish toolpath but in some situations, this is a false economy. As the machining progresses, the material can move as residual stresses are relieved. A roughing toolpath that removes the bulk of the material means that most of the movement will have occurred prior to the finish toolpath, so the final part would be truer to the intended shape.
There are best practices for applying clamps, too. To maximize the useful clamping force, the bolt should be located close to the edge of the workpiece. The clamp should be close to horizontal, with a small amount of slope toward the material to avoid applying the clamping force on the corner of the material. Spacer blocks under the back end can help here.
A trick I use from time to time is to make a groove in the edge of the stock with a table saw, and then secure the stock with a clamp that fits into the groove. This allows the clamp to have a very low profile and avoid collisions. It’s also useful on taller stock where I would have had to use longer bolts. The trade-off is that there needs to be enough material for the groove, and the groove will need to be removed or incorporated in the finished part.
Clamps sold for use on CNC routers are often fabricated in metal, and they use steel bolts. However, there are some clever versions that can be useful in specific situations. One is a cam type that presses sideways against the edge of the stock. It can be low profile to reduce the risk of collisions. Just be sure the stock is also being pressed down against that CNC router bed with another device.
In-line clamps, such as those designed for use in the dog holes of workbenches, tend to tip back a small amount when force is applied, and they may lift the stock off the table.
With some workpieces, the horizontal force required to securely clamp a part may be sufficient to cause the workpiece to bow. That can be dangerous.
Double-sided tape can sometimes be used to secure stock, and you may need to plan for the tape’s thickness in your cut. It’s also necessary to apply sufficient pressure to activate the adhesive surface on the tape. For thin materials, stock can be pressed into the tape with a roller or even pounded into the tape with a rubber mallet. For thicker stock, it may not be possible to apply that kind of pressure, so CNC users have developed the blue tape and CA glue technique. Blue painters’ tape is applied to each surface, and then super glue is applied to one of the tapes before the parts meet.
Generally speaking, adhesives don’t do well if they come into contact with a router bit – they can stick to the bit and cause problems. And keep in mind that it will be necessary to detach the stock from the bed once the machining is complete, and then remove the adhesive from the part. If the stock is thin or delicate, it could be damaged during the removal process.
There are also times when the best solution may be to complete production of a part on a different machine after machining on the CNC router. One approach I use on routers that don’t have automatic tool changers is to machine a V-groove around the perimeter of the part to guide removal of the waste with a band saw or even a table saw on straight parts, and then touch up the edges on a horizontal belt sander. The part spends less time on the CNC router. It also eliminates the time and risk involved with a manual tool change.
For one of our class projects that involves the machining of a 3/16”-thick small part, it has always been a challenge to secure the thin stock so it’s flat. An alternative is to machine the part in 3/4”-thick material and then cut a 1/4”-deep groove around the perimeter. Once it’s off the CNC router, the machined piece is brought to a band saw where it is released from the thicker blank using a resaw technique – the machined side of the blank is facing away from the fence, so the part simply drops out on the table once the blade passes through.
Experience can be a strict tutor, and most of us have learned that wood moves. It doesn’t have to.
This article was originally published in the May 2022 issue.