Avid CNC Basics and Setup Guide

A moving-gantry router keeps the table still and carries the spindle across the work. That layout gives a large envelope at a lower price than a mill, and it is the core of Avid CNC basics. The trade-off is stiffness: a gantry bolted from aluminum extrusion flexes more than a cast iron column.

The frame is usually 8020-style extrusion with linear rails and rack-and-pinion drive on the long axis. Ballscrews appear on shorter axes. Spindles run from a trim router up to a 3 kW water-cooled unit, and that choice limits what you can cut more than the frame does.

Every axis has a home switch and a stepper or servo. Control comes from Mach4 or a similar motion controller over Ethernet or USB. Understanding this chain matters because setup errors anywhere in it show up as dimensional drift, not as an alarm.

Build the frame on a flat surface and check diagonals before you tighten anything. A gantry that is 0.5 mm out of square over 1,200 mm will cut pockets that are visibly out of parallel. Measure corner to corner with a tape or a laser, then adjust until both diagonals match within 0.5 mm.

Level the machine feet so the frame does not twist under its own weight. Twist shows up as a table that is high in one corner. Shim the feet, then re-check with a dial indicator on a granite plate or a straight edge across the rails.

Tram the spindle once the gantry moves freely. Mount a dial indicator on a holder in the collet and sweep a 100 mm circle on the table. Adjust the spindle mount until the reading is within 0.02 mm across the sweep. Tramming affects surface finish on facing cuts more than it affects dimensions.

Set the home switches so the machine repeats to the same zero after every power cycle. Run a homing cycle ten times and record the position with an indicator. Repeatability under 0.05 mm is normal for a well-adjusted router.

Routers cut with a single-flute or two-flute cutter at high RPM. The rule is chip load: you want a chip thick enough to carry heat away. Too light a chip rubs the edge and dulls the tool in minutes. Too heavy a chip stalls a stepper or snaps a small end mill.

For aluminum on a router, start at 12,000 to 18,000 RPM with a feed of 60 IPM or higher and a depth of cut around 0.5 mm per pass. Use a rigid tool holder and flood coolant or a mist system to control chip evacuation. Climb milling gives a better finish on the side walls.

Wood and plastics tolerate much higher feed. A 6 mm two-flute cutter in MDF can run 200 IPM at 18,000 RPM with a 3 mm depth of cut. The limit is usually the acceleration of the gantry, not the cutter.

Test cuts belong on scrap of the same material. Cut a 50 mm square pocket and measure it. If the pocket is undersize, the tool is deflecting; reduce depth of cut or step over. If the corners are rounded, check tool diameter compensation in the CAM setup.

A router table with T-slots and clamps is flexible but slow. For production runs, a vacuum table or a fixture plate pays back quickly. The part must not move during the cut; a shift of 0.1 mm ruins a pocket that has to fit a bearing.

Use tabs or a sacrificial layer when cutting through the part. Cutting directly into the spoilboard is fine for rough work but it changes the effective depth of cut on the last pass. A 6 mm spoilboard surfacing cut every few months keeps the table flat.

Clamp low and close to the cut. Clamping high on a tall part lets it vibrate, and vibration shows as chatter marks on the side wall. For thin parts, support the underside with a machined pocket in the fixture.

Check that the part is parallel to the machine axes. A part that is 0.2 mm out of parallel over 300 mm will produce a pocket that is deeper at one end. Probe the corner or use a dial indicator on the vise jaw before you start.

A gantry router is not a milling machine. It cannot hold ±0.005 mm on a steel part or cut titanium at production rates. If a drawing calls for a tolerance tighter than ±0.05 mm on a metal part, the job belongs on a machining center.

Hardened steel, Inconel, and titanium wear out router tooling fast. The spindle speeds are right but the rigidity is not. Deep pockets in aluminum also push the gantry, because tool length grows and deflection grows with it.

GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers and 27 three-axis machines, with a maximum processing size of 4,000 mm. That capacity covers the parts a router cannot. We hold ±0.005 mm and Ra 0.8–1.6 μm on production work.

If your part is a prototype in plastic or a plywood fixture, a router is the right tool. If it is a structural aluminum bracket with a bearing bore, send it to a machining center. The decision is about tolerance and material, not about brand.