Shopbot CNC Basic Guide

• 1
  How a Shopbot actually moves
• 2
  Holding the sheet: vacuum, screws, or tape
• 3
  Feeds, speeds, and chipload on a router
• 4
  Toolpaths, stepover, and what CAM hides
• 5
  Wood, plastic, and light metal on one table
• 6
  What accuracy a router can and cannot hold
• 7
  Gantry router vs machining center: which fits the part

A Shopbot CNC basic setup is a gantry router. The spindle or router sits on a bridge that spans the table, and that bridge travels along the long axis. The cutting tool moves in three directions: X along the table length, Y across the width, Z up and down into the material.

Most benchtop and mid-size units drive the gantry on rack-and-pinion gears for X and Y, and a ball screw or lead screw for Z. Rack and pinion is cheap to build in long lengths and tolerates dust, which is why it dominates woodworking machines. It also has backlash, usually 0.05–0.15 mm, and that number sets the floor on your accuracy.

The table does not move. You clamp or vacuum-hold the sheet down, set a zero point, and the tool comes to the work. That is the opposite of a lathe or a machining center with a moving table, and it is why a full 2,440 × 1,220 mm sheet can sit flat while the head does all the traveling.

Motion comes from stepper motors on most Shopbot-class machines. A stepper has no feedback: the controller sends pulses and assumes the motor arrived. If the tool hits a knot or a clamp hard enough, it loses steps, and every cut after that point is shifted. Re-zero after any crash. This single habit prevents more scrap than any parameter change.

• 1
  Rack and pinion on X and YLong travel, tolerant of dust, 0.05–0.15 mm backlash
• 2
  Screw on ZShorter travel, better resolution, less backlash
• 3
  Open-loop steppersNo position feedback; lost steps shift everything after the crash

The most common failure on a router table is not a bad toolpath. It is a part that moved. Sheet goods are thin and springy, and a climb cut pulls the workpiece toward the tool. If the hold-down is weak, the part lifts a few tenths of a millimeter and the edge finish goes with it.

Vacuum holding is the standard for production. A bleeder board under a spoilboard spreads suction across the whole sheet. The catch is small parts: once you cut a profile through the sheet, that piece loses its vacuum and can shift on the last pass. Tabs, also called bridges, keep the part attached to the surrounding material until you cut it free.

For one-off parts, screws through the waste area are faster than setting up vacuum. Place them outside the cut path and well clear of the tool diameter. Double-sided tape works for thin plates and soft metals, but heat from the cut softens the adhesive, so keep passes light.

Clamps are the worst option on a router. They sit above the sheet, and a Z move into a clamp breaks the tool and often bends the gantry. If you must clamp, put the clamps outside the sheet perimeter and set a safe Z height above them.

• 1
  Vacuum plus tabsBest for nested production on sheet goods
• 2
  Screws in wasteFast for one-offs, keep clear of the toolpath
• 3
  Never clamp above the sheetA Z plunge into a clamp bends the gantry

Feeds and speeds on a router are set by chipload, the thickness of material each cutting edge removes per revolution. Chipload equals feed rate divided by (RPM × number of flutes). Get it too low and the edge rubs instead of cutting, which burns the wood and dulls the tool fast. Too high and the tool deflects or snaps.

For a 6 mm two-flute carbide end mill in MDF or hardwood at 16,000 rpm, a chipload of 0.10–0.15 mm per tooth gives a feed of roughly 3,200–4,800 mm/min. That is a starting point, not a rule. Plywood with voids, particleboard, and plastic all want their own numbers, and the machine's rigidity sets the ceiling.

Depth of cut matters as much as feed. In softwood and MDF you can often take a full tool diameter in one pass. In hardwood, start at half the diameter and listen. A router that changes pitch mid-cut is telling you the chipload just dropped, usually because the tool entered a denser region.

Spindle speed on a router is often the easiest variable to change and the least useful. Lowering RPM without raising feed reduces chipload and makes burning worse. If the cut is burning, raise the feed first. If the tool is chattering, lower the feed or reduce depth.

• 1
  Chipload firstFeed ÷ (RPM × flutes); target 0.10–0.15 mm per tooth in wood
• 2
  Burn means raise feedLowering RPM alone makes rubbing worse
• 3
  Depth of cutOne diameter in MDF, half a diameter in hardwood

CAM software converts a 3D model or a 2D drawing into toolpaths. The choices that matter are tool diameter, stepover, stepdown, and whether the cut is climb or conventional. Everything else is bookkeeping, and most of it is defaulted to something safe but slow.

Stepover is the side-to-side distance between passes on a pocket or a 3D surface. A common starting value is 40–50 percent of the tool diameter for roughing, and 8–12 percent for a finishing pass. Smaller stepover means a better surface and a much longer cycle, roughly in inverse proportion.

Climb milling, where the tool rotation pushes the workpiece into the cut, gives a better edge on most materials but pulls the part upward. On a router with modest hold-down, conventional milling is often more stable, even though the finish is slightly rougher. This is a machine-rigidity decision, not a material one.

A profile cut needs a lead-in and lead-out. Plunging straight down into the finished edge leaves a witness mark that shows on the part. A ramp or a helical entry spreads the load and keeps the edge clean. If your CAM defaults to a straight plunge, change it.

• 1
  Stepover 40–50% for roughing8–12% of tool diameter for finishing
• 2
  Ramp or helix entryStraight plunges mark the finished edge
• 3
  Climb vs conventionalClimb for finish, conventional for weak hold-down

Router tables cut sheet goods and solid wood first. MDF and plywood machine cleanly and hold vacuum well. Solid hardwood cuts well but moves as internal stress releases, so a part cut from a wide board can bow after the last pass. Rough cut oversize, let it rest, then finish.

Plastics behave differently. Acrylic chips and can crack at the edge if the chipload is too high or the tool is dull. Polycarbonate wants a sharp, polished flute and a lower feed, because it grabs the tool and welds chips to the edge. HDPE cuts easily but flexes during clamping, so support it across the full face.

Aluminum is where a router starts to struggle. A wood router spins at 16,000–24,000 rpm, far too fast for aluminum, which wants 6,000–10,000 rpm with a single-flute tool and air blast for chip evacuation. You can cut thin 6061 plate, but the spindle speed, the lack of coolant, and the light frame all work against you.

Composites and carbon fiber plate are harder still. The dust is abrasive and harmful, and it wears guides and screws quickly. If you cut carbon on a wood router, expect to replace wear parts. For structural parts in 6061, 7075, 304 stainless, or titanium, a router is the wrong machine, and no parameter change fixes that.

• 1
  MDF and plywoodClean cuts, good vacuum hold
• 2
  Acrylic and polycarbonateSharp polished flutes, lower feed, watch edge cracking
• 3
  Aluminum and compositesPossible but marginal; abrasive dust wears the machine

A well-tuned gantry router holds roughly ±0.1–0.3 mm on sheet goods, and that is fine for cabinets, signs, and enclosures. The limits come from three places: backlash in the rack and pinion, deflection of the gantry under cutting load, and thermal movement of a long aluminum frame during a long cycle.

Backlash shows up as a step where the tool changes direction. If you cut a square and one corner overshoots, the axis reversed but the gear had slack to take up. Some controllers offer backlash compensation, which helps but does not remove the error entirely.

Gantry deflection shows up as a taper on deep cuts. The tool pushes away from the work, so the top of a deep pocket is wider than the bottom. Taking lighter passes reduces it. A stiffer machine removes it.

On metal parts that need ±0.005 mm and Ra 0.8–1.6 μm, a router cannot get there. Those numbers need a rigid frame, closed-loop control, and a temperature-stable environment. A 5-axis machining center with a Ø400 mm rotary table and travels up to 4,000 × 400 × 150 mm is built for that class of work, and a gantry router is not.

• 1
  ±0.1–0.3 mm is realisticFine for panels, signs, and enclosures
• 2
  Backlash shows as a stepAt every axis reversal
• 3
  ±0.005 mm needs a different machineRigid frame, closed-loop control, stable temperature