Workholding

Painter's Tape and CA Glue Workholding for CNC: How to Do It Right

Clamps get in the way of the bit, screws leave holes, and a vacuum table is a project of its own. The tape-and-glue method holds a flat part with nothing sticking up at all, and the whole kit costs less than one good end mill.

It's not magic, though. It holds well when the surfaces are clean, flat and large enough, and it lets go at the worst moment when they aren't. This guide covers the method step by step, the parts it suits, the ones it doesn't, and how to get parts off without cracking them.

How the method works

The idea is simple. CA glue (cyanoacrylate, or super glue) bonds very strongly but is hard to remove from wood and leaves a mess. Painter's tape peels off cleanly but has weak adhesive. Put them together and you get the best of both: the glue bonds tape to tape, and each layer of tape sticks to its own surface. When you're done you peel the tape off, and no glue ever touches the workpiece or the spoilboard.

The bond you rely on is the tape adhesive, spread over the whole contact area. That's why surface area matters so much. A large flat panel has a lot of tape holding it down, while a narrow strip has very little. The glue layer is rarely the weak link. The tape lifting off the wood is.

What you need

Some makers use special double-sided "CNC tape" or woodturner's tape instead. These are simpler (one layer, no glue), and they're a good alternative when you'd rather not handle CA. They usually hold less than a well-made tape-and-glue joint, and the residue can be harder to remove from some woods. The setup advice below applies to both.

Step-by-step setup

1. Clean both surfaces. Vacuum the spoilboard and wipe off dust. Sand or scrape any fuzzy, raised spots. Tape will grip a layer of dust instead of the board, and that's the most common reason it fails.
2. Tape the spoilboard. Lay strips side by side covering the footprint of the part, with edges butted together or slightly overlapped. Don't stretch the tape as you lay it, because stretched tape tries to pull back and lifts at the ends.
3. Tape the workpiece. Cover the back of the part the same way. Trim the excess flush with a knife so tape edges don't hang over where the bit will cut.
4. Burnish everything. Press both taped surfaces hard with a roller or scraper. Painter's tape is pressure-sensitive, so firm pressure noticeably improves the grip.
5. Apply glue to one side. Run beads or a zig-zag of medium CA across the tape on the part (or the board). Cover the area evenly, paying extra attention near edges and corners.
6. Mist activator on the other side. A light spray on the opposite tape surface. Too much can make the glue cure before you're in position, or leave it foamy and weak.
7. Place and press. Set the part down where you want it (you get about one chance), then press firmly or put a weight on it for several seconds. Then you're ready to zero and cut.

What it's good for (and what it isn't)

Good fits

• Signs and V-carving on flat boards, where clamps would interfere with the toolpath.
• Thin stock such as hardwood veneers, inlay plugs, thin plywood, or plastic sheet that would bow under clamps.
• Parts cut all the way through where you don't want tabs. As long as each finished piece keeps enough taped area under it, it stays put after the final pass.
• Small aluminum plates on a hobby router, when you take light passes with appropriate tooling. Many makers use this for brackets and panels.
• Second-side ops, when the first side is flat and you have a good way to locate the part.

Poor fits

• Tall, narrow parts. A small base under a tall part gives the cutter a long lever. The sideways force peels the tape up from one edge.
• Heavy roughing passes. Big stepdowns and aggressive feeds create cutting forces that tape adhesive isn't designed for. Lighten up or switch to mechanical clamping.
• Warped or cupped boards. If the part only touches the board at a few points, only those points are holding it.
• Oily woods and dusty, rough-sawn surfaces. Some oily species and porous surfaces make tape adhesion unreliable. Test on scrap first.
• Coolant and wet cutting. Liquids and painter's tape don't mix.
• Long jobs in hot or cold shops. Tape adhesive performs worse at temperature extremes, so a cold garage can make a reliable setup unreliable.

Getting the strongest hold

• Cover as much of the part's underside as you can, edge to edge.
• Keep the spoilboard surfaced flat. A wavy spoilboard means partial contact.
• Let tape sit a few minutes after burnishing if you can. Pressure-sensitive adhesive tends to grip better after a short dwell.
• Use climb or conventional milling with care. Whichever direction you choose, avoid passes that push sideways toward an edge that's already been cut free.
• Cut the inside features first and the outside profile last, so the part keeps its full contact area as long as possible.
• When cutting parts free, leave a thin skin (an "onion skin") and finish with a light last pass, so the final cut barely loads the part.
• Make test cuts on scrap of the same material whenever you change tape brand, material or toolpath strategy.

Releasing parts and cleaning up

Most parts that break do so during removal, not machining. A thin plywood sign or delicate inlay can snap if you twist it off.

1. Slide a wide, flexible putty knife under one edge between the two tape layers, or between tape and spoilboard.
2. Work along the edge with slow, steady pressure, letting the tape peel instead of forcing the part up at one corner.
3. For large panels, tap in two or three thin wedges along one side and let the peel happen gradually.
4. Peel the tape off the part afterward, pulling back at a low angle.

If tape adhesive stays on the part, a little mineral spirits or a citrus-based adhesive remover on a rag usually lifts it. Test on an inconspicuous spot first on finished surfaces. A brief, gentle warm-up with a heat gun can also soften stubborn tape adhesive, but don't use that on plastics that can warp.

How it compares with other workholding

For most hobby shops, tape and glue is a tool to keep in the drawer next to the clamps, not a replacement for them. Use it when clamps are in the way or the stock is too thin to clamp, and switch back to mechanical holding when the cut gets heavy.

Bottom line

Keep a roll of wide painter's tape and a bottle of CA glue with activator by the machine, and you'll find yourself reaching for them more often than your clamps.