By Martin Harris.

CNC Router Dust Collection and Dust Shoes

Good CNC dust collection starts with a dust shoe, a housing that surrounds the bit with a brush skirt that touches the material. Connect the shoe by hose to a shop vac with a cyclone separator on a small hobby machine, or to a dust collector on a larger machine with a 4 inch shoe port. Hang the hose from above so it never drags on the gantry, and use a grounded or antistatic hose so static does not build up.

I cover router tables and the basics of CNC capture in my router table and CNC dust collection guide. This page goes further into CNC router dust collection: shoe types, brush fit, hose routing through or beside a drag chain, which vacuum to use, static, and the extra care MDF dust needs.

Why a CNC is hard to collect from

A router bit spins fast and throws chips and fine dust outward in every direction. On a router table, the bit stays in one place, so a fixed hood can sit right next to it. On a CNC, the bit travels across the whole bed, so the pickup has to ride along with it. A fixed hood at the edge of the machine would be too far away from the cut most of the time.

CNC jobs also run long. A carving or pocketing job can go for an hour or more with nobody standing at the machine, and the dust keeps coming the whole time. WorkshopCalc's reference chart puts CNC routers at 500 to 800 CFM, and notes that hood geometry can matter more than the CFM number. Bill Pentz, who sets stricter targets aimed at fine dust, puts CNC machines at up to about 1,000 CFM. A tight dust shoe is how a hobby machine gets good results with far less air than that.

Dust shoe types

Sienci Labs, which makes the LongMill hobby CNC, sorts dust shoes into two main styles. The difference is what the shoe mounts to.

• Fixed style. The shoe mounts to the Z axis or the router body, so it moves up and down with the bit. It is simple and fits almost any machine. The catch is that as the bit plunges deeper, the shoe rises and falls with it, so the brush has to be long enough to stay on the material through the whole depth of cut.
• Adjustable style. The shoe mounts to the X axis carriage and stays at a set height while the Z axis moves the router up and down inside it. Sienci says this style is much better at collecting dust when you cut flat material like sheet goods. Many adjustable shoes use strong magnets so you can move or remove them quickly.

Carbide 3D's guide describes a third convenience feature that shows up on both styles: a brush section held on with magnets, so you can pull it off for a bit change and snap it back on. On a machine where you change bits often, that feature decides whether the shoe stays on or ends up sitting on a shelf.

Brush length and fit

The brush skirt is the seal. It closes the gap between the shoe and the material so the vacuum pulls air in right at the bit instead of from all around the shoe. I did not find a single published brush length that fits every machine, because it depends on the shoe style and how deep you cut. These are the fit rules I follow:

• The brush should touch the material with a slight bend. If you can see a gap under the bristles, the shoe is pulling room air. If the bristles are folded flat, they drag hard and can push small parts around.
• On a fixed shoe, check at full depth. Jog the bit down to the deepest cut in your job, with the spindle off, and make sure the brush still touches the surface at that height.
• On an adjustable shoe, set it to the material. Set the shoe height once for the stock thickness, and the brush stays put while the bit moves.
• Replace worn brushes. Bristles wear shorter and splay out. Once they no longer reach the material at your usual settings, capture drops quickly even though the vacuum is working just as hard.
• Watch the clamps. Plan toolpaths and clamp heights so the shoe clears them. A shoe that hits a clamp can knock the machine off position.

Shop vac vs dust collector for a hobby CNC

A shop vac and a dust collector solve different problems. A shop vac makes high suction (static pressure) but moves less air. A dust collector moves a lot of air at lower suction. Sienci's CNC dust collection page gives typical ranges:

Source: Sienci Labs, CNC dust collection fundamentals. Match the hose to the vacuum you have. A shop vac cannot push its airflow through a 4 inch hose at useful speed, and a dust collector choked down to a 2 inch hose loses most of its airflow.

For most hobby CNC machines with a shoe port of 2-1/2 inches or smaller, I would use a shop vac. The high suction pulls well through a small shoe opening and a long, thin hose. There are two catches:

• Run time. Carbide 3D's CNC dust collection guide notes that budget shop vacs are loud and not necessarily made to run continuously for hours. It points to Fein and Festool as better choices for long jobs. The Festool CT 26 HEPA and the Fein Turbo I HEPA are both in that class.
• Filter clogging. CNC chips fill a vac fast and fine dust loads the filter. A cyclone separator between the shoe and the vac, like the Oneida Dust Deputy Deluxe, drops most of the chips into a bucket before they reach the vac. My best dust separator roundup compares the options.

A dust collector makes sense on a larger machine with a 4 inch shoe port, or when the CNC is part of a shop that already has ducted collection. Carbide 3D notes that dust collectors are typically quieter than a shop vac and hold more chips, which matters on long jobs. See my dust collector vs shop vac comparison for the full tradeoff, and my best shop vac for woodworking roundup if you are buying a vac for the machine.

Hose routing and drag chains

A drag chain, also called a cable carrier, is the linked plastic track that guides the machine's wires as the gantry moves. It is tempting to put the dust hose in it too. I keep the hose out of the chain on most hobby machines, for three reasons. Most dust hoses are larger and stiffer than the chain was sized for. A hose rubbing inside the chain adds drag on the axis. And a plastic hose full of moving dust builds static right next to the signal and motor wires.

Sienci's advice is to hang the hose above the dust shoe so it does not press on the router or catch on the work, either from the ceiling or on an articulating arm. These routing methods work well:

• An overhead support above the center of the bed. Hang the hose from a hook, a spring, or a bungee above the middle of the machine. The hose then reaches every corner with about the same amount of slack.
• A cable or rail above the machine. Advice collected on WOODWEB's knowledge base describes hanging the hose from a steel cable stretched overhead, and running rigid duct to a point above the middle of the machine so only a short length of flex has to reach the corners.
• Alongside the drag chain. If you want the hose to follow the chain, tie it to the outside of the chain at a few points with enough slack at the bends so it never kinks or pulls.
• Inside a large chain. If you do run it inside, choose a chain with inside space larger than the hose's outside diameter and a bend radius the hose can follow without flattening.

Before the first real job, jog the machine to all four corners and to full Z height with the vacuum running. Watch the hose the whole time. It should never pull tight, kink, or touch the bit, the clamps, or the work.

Static buildup

Fine dust rushing through a plastic hose builds a static charge. At best that means small shocks when you touch the hose. On a CNC, the bigger worry is the electronics. Carbide 3D's guide says you should always use a grounded vacuum hose, and explains that grounding keeps static discharge from reaching the machine's control system. It describes wrapping copper wire around a standard hose as a less effective way to bleed off some of the charge.

• Buy an antistatic hose. Sienci points to purpose made antistatic hoses as the simplest fix. Some shop vacs and extractors sell them as an option.
• Run a ground wire through the hose. A common shop method, described on CNC forums, is a bare braided copper wire run inside the flex hose and connected to a ground at one end.
• End the ground at a proper ground. Plug the vacuum into a grounded outlet, and if you are unsure where a ground wire should connect, ask a licensed electrician. Do not tie ground wires into an outlet box yourself. My guide to grounding PVC dust collection covers the same questions for ducted systems.

Fine MDF dust

MDF is the most common sheet good on hobby CNC machines, and it makes the worst dust. Cutting it produces very little in the way of chips and a lot of fine powder, which escapes a loose shoe and hangs in the air. MDF is bonded with formaldehyde based resins. The International Agency for Research on Cancer (IARC) classifies wood dust as a Group 1 carcinogen, its top category, and it classifies formaldehyde as Group 1 too. Sienci's guide notes that MDF releases formaldehyde and other chemicals when cut or sanded and calls for better filtration and respiratory protection.

For MDF, I would stack several protections:

1. Tighten the shoe. Check the brush reaches the material everywhere, and use an adjustable shoe for flat sheet work.
2. Filter fine. Use a vac or collector with a HEPA filter, or a filter rated for fine dust, and put a separator in front so the filter does not clog halfway through a job.
3. Wear a respirator. Wear one while the machine runs and while you clean up. My woodworking respirator guide covers fit and filter ratings.
4. Clean the air. An air cleaner running during and after the job pulls down the dust the shoe missed. See my best air filtration for woodworking roundup.
5. Clean up with a vacuum. Sweeping or blowing off the bed with compressed air puts fine dust back in the air.

I go through the health data in more detail in is MDF dust dangerous, and the rest of the shop setup is in my complete guide to small shop dust collection.

Bit choice and chip direction

The bit changes where chips go. An upcut spiral bit lifts chips up out of the cut, toward the shoe, which makes collection easier. A downcut spiral pushes chips down into the kerf, which gives a cleaner top edge but packs the slot with dust the shoe cannot reach until the next pass. Compression bits do some of each. If collection matters more than edge quality on a job, an upcut bit helps the shoe do its work. When you run downcut bits, expect to vacuum the slots after the job.

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