How to Choose Linear Rails & Ballscrews for Your CNC Build

The linear motion system is what turns a spinning motor into a precise cut. Pick rails and ballscrews that are too light and you get flex and chatter; pick them "stronger" without understanding the numbers and you usually just pay more. This guide explains the four decisions in order β€” rail type, screw size, lead, and support β€” and ends with a checklist of real stock.

Decision 1 β€” Square (profile) rail or round (shaft) rail?

Rail type What it is Best for
Square profile rail (HGR series) Hardened rail + preloaded block carriages (HGH/HGW) Router and mill axes that need stiffness and precision
Round shaft rail (SBR series) Supported round shaft + linear bearings Light duty, budget builds, low-speed axes

Square rails carry higher loads with less deflection and hold accuracy better over time β€” that is why most router conversions use HGR rails with HGH (wide) or HGW (square flange) blocks. Round SBR rails are cheaper and fine for light machines and slow axes. A common first build: HGR20 rails for the gantry, SBR16 only if you are strictly budget-constrained.

Example stock: the HGR15/HGR20/HGR25/HGR30 square guide rails with HGH20CA/HGW20CC carriages cover most router sizes from one listing.

Decision 2 β€” Ballscrew size: diameter sets stiffness, length sets travel

Ballscrew naming is direct: SFU1605 means 16 mm shaft diameter and 5 mm lead. Diameter determines stiffness and load β€” 16 mm (SFU16xx) is the mainstream for hobby and small commercial routers; 20 mm (SFU20xx) for longer spans and heavier gantries. Rule of thumb: keep the unsupported screw span short relative to its diameter, and order the screw longer than your travel by the distance needed for the nut, motor and supports.

Example stock: the SFU1605 / SFU1604 / SFU1610 ballscrews in 1700–2000 mm lengths let you pick diameter, lead and exact length from one listing.

Decision 3 β€” Lead: the resolution vs speed trade-off

Lead (mm per revolution) sets the trade between resolution and speed:

Lead Torque advantage Typical use
5 mm (SFU1605) High holding at the nut, finer resolution Milling, plasma Z, most router axes
10 mm (SFU1610) Faster traverse per RPM Long travel axes, laser/plasma XY

A 5 mm lead with a 1:1 motor gives roughly 0.0125 mm per full step at 200 steps/rev β€” fine for most routing. If your axis must move fast (laser work, long gantry traverse), a 10 mm lead reduces the RPM the motor must sustain. You can mix leads per axis; most builds run 5 mm on X/Y/Z for cutting machines.

Decision 4 β€” Support blocks: fix one end, support the rest

A ballscrew needs a bearing block at the motor end (fixed) and at the far end (supported). The naming: BK/BF are the fixed/supported pair with a bearing in the block; FK/FF and EK/EF are lighter variants. Match the block series to the screw diameter β€” BK12/BF12 for 12 mm screws, BK15/BF15 for 16 mm, BK17/BF17 and BK20/BF20 for 20 mm screws. Never mount a screw rigidly at both ends without allowing for thermal growth β€” that is a classic cause of binding on long axes.

Example stock: the HLTNC support unit range (BK/BF/BK12/BK15/BK17/BK20/BK25/BK30/BK35/BK40, C7/C5) matches every common screw size, and the BK10/BF10…EK/EF compact support set covers 12–16 mm screws when space is tight.

Skip the math: ready-made linear modules

If you want a tested assembly instead of matching parts yourself, linear slide modules pair the rail, screw and nut housing in one unit: the HGR20 + SFU1605 300 mm linear slide with HGH20CA carriage is a drop-in Z or small X/Y axis. For a full axis bundle, the HGR20 square rail + SFU1605 + BK12/BF12 kit includes rail, screw and supports in one set.

Ordering checklist

  1. Measure travel β†’ add ~30–50% for screw overhang, motor and nut β†’ that is the screw length.
  2. Pick diameter by span: 16 mm up to ~1 m spans, 20 mm for longer/heavier.
  3. Pick lead by work: 5 mm for cutting resolution, 10 mm for speed.
  4. Order the fixed/supported block pair that matches your screw (BK/BF + series).
  5. Choose square HGR rails for stiffness; order rail length β‰₯ travel + carriage length.
  6. Grab a matched kit if you are building your first axis β€” fewer mismatched parts.

Bottom line

Work through the decisions in order: rail type first (HGR for stiffness), then screw diameter and length by span, then lead by speed vs resolution, and finish with the matching support blocks. The links above are all current stock β€” mix single parts or take a ready-made module, depending on how much tuning you want to do yourself.