Sienci LongMill MK2 Complete Setup Guide: Assembly, gSender, and Your First Cut

The Sienci LongMill MK2 has become one of the most popular entry points into desktop CNC routing, largely because Sienci Labs ships it as an open, well-documented kit rather than a locked-down appliance. It comes in three bed sizes — 12x30, 30x30, and 48x30 (inches) — built around the same C-beam gantry, NEMA 23 steppers, and GRBL-based control electronics, so the assembly and setup process below applies to all three with only the frame dimensions changing. This guide walks through unboxing, mechanical assembly, wiring, homing, and the first cut, plus the calibration steps that separate a LongMill that cuts accurately from one that fights you on every job.

What's in the Box

A LongMill MK2 kit ships as extruded aluminum rails, printed and machined brackets, linear guide rails with wheels, NEMA 23 stepper motors, GT2 belts, an MX3 or MX-FT (fixed table) controller board, a trim router or spindle mount (router/spindle sold separately unless bundled), and a bag of fasteners organized by assembly step. Sienci's assembly instructions are genuinely good — a full-color, step-numbered guide — so this article focuses on the parts of setup that trip people up rather than repeating every bolt-by-bolt step.

• Frame rails and gantry — pre-cut aluminum extrusion, drilled and tapped
• MX Controller — GRBL 1.1-based board with onboard stepper drivers, limit switch headers, and a spindle relay/PWM output
• NEMA 23 steppers (3x on standard config, 4x if you add a second Y motor on the larger beds)
• V-wheels and eccentric nuts for the linear rail system
• GT2 belts and pulleys
• Limit switches (homing switches on X, Y, and Z)

Assembly Order and Common Mistakes

Build the base frame first and get it dead square before anything else goes on — measure diagonals corner to corner and shim or adjust until they match within a millimeter. Everything downstream (rail parallelism, gantry racking, cut accuracy) traces back to this step, and it's the one people skip because it feels like it's "close enough" out of the box.

StepWhat to Watch For Base frame squaringDiagonal measurements must match; don't trust the extrusion edges alone V-wheel tensioningEccentric nuts should let the wheel spin freely but with zero perceptible play — too tight and you'll cook the wheels, too loose and you'll get chatter Gantry squaring (racking)Use a dial indicator or feeler gauge against the rail to confirm the gantry is perpendicular to the Y rails before locking down the coupling bolts Belt tensionShould have a firm "twang" like a guitar string tuned low — loose belts cause backlash and skipped steps under load Limit switch mountingConfirm the switch trips before the gantry hits a hard mechanical stop, with a few millimeters of margin

Wiring and First Power-On

The MX controller uses keyed connectors for the steppers and limit switches, so miswiring a motor is hard to do, but it's still worth confirming polarity on the limit switches before you rely on homing. Connect the controller to your computer over USB — Sienci's firmware ships GRBL-configured for the LongMill's steps-per-mm and travel limits already, so you generally don't need to touch $$ settings unless you've upgraded ballscrews or changed pulleys.

Install gSender, Sienci's own GRBL sender, which is purpose-built around the LongMill and handles homing, work coordinate zeroing, and probing cleanly. It isn't the only option — UGS and CNCjs both work fine over the same GRBL connection — but gSender's job recovery and surfacing tools are worth using if you're new to the machine.

Homing and Squaring the Gantry Electronically

After the mechanical build is solid, home the machine ($H in gSender or the Home button) and watch that both Y-axis motors (on 30x30 and 48x30 beds with dual Y steppers) arrive at their limit switches at the same time. If one side homes noticeably before the other, the gantry is racked and needs a mechanical adjustment — loosen the offending coupling, square the gantry against the rail by hand, and re-tighten. Running the machine racked will produce cuts that drift out of square over the length of a job, and it's the single most common accuracy complaint on the LongMill forums that traces back to skipped squaring.

First Cut: Setting Zero and Running a Test File

Before cutting real material, run a simple test file — a 50mm square pocket in scrap MDF is enough to validate the whole chain. Set your work zero (X0, Y0, Z0) at a corner of your stock using either the manual jog-and-touch method or a probing puck if you've added one; the LongMill doesn't include a touch probe standard, though Sienci sells one as an add-on and it pays for itself quickly in setup time. Load your G-code (generated in Vectric, Fusion 360, Carbide Create, or Easel with a LongMill post-processor selected), verify the toolpath preview looks correct, and run the job at a conservative feed override — 50% — for the first pass so you can hit the hold button if something looks wrong.

Feeds and Speeds to Start With

The LongMill's NEMA 23 steppers and C-beam construction are rigid enough for wood and plastics at reasonable rates, but it's not a metal-cutting machine without significant modification. As a starting point for a 1/8" (3.175mm) single-flute upcut bit in plywood: 60-80 in/min feed rate, 0.75-1.0mm depth per pass, spindle speed around 18,000-24,000 RPM if you're running a trim router like the Makita RT0701C. Dial these in with a test cut and adjust based on chip formation — you want clean chips, not dust (too slow/high RPM) or chatter marks (too fast/low RPM).

MaterialBitFeed RateDepth/Pass Plywood / MDF1/8" single flute upcut60-100 in/min0.75-1.5mm Hardwood1/8" or 1/4" upcut40-70 in/min0.5-1.0mm AcrylicSingle-flute O-flute30-50 in/min0.5-1.0mm, finish pass at full depth Soft aluminum (with mods)2-flute carbide, coated15-25 in/min0.1-0.2mm — only attempt with upgraded rigidity/spindle

Safety and Workholding

Wear eye protection any time the spindle or router is running, and keep hands and loose clothing clear of the moving gantry — the belt-driven motion is fast and doesn't stop instantly on impact with a hand. Use hold-down clamps, double-sided tape, or a spoilboard with screws to secure stock; a workpiece that shifts mid-cut turns into a projectile risk and will also wreck your bit. If you're cutting anything that produces fine dust — MDF especially — run dust collection and wear a respirator rated for wood dust, not just a paper mask.

The LongMill rewards the time spent squaring and tensioning it properly during assembly — most of the frustration people report online traces back to a racked gantry, loose V-wheels, or belts that were never tensioned past "snug." Get those right once, keep the rails clean and lightly lubricated, and it's a genuinely capable machine for signs, furniture parts, and small-batch production well past what its price point suggests.