Movement Control in the Linear CNC Industry: 4 Checks Before You Release a Drawing

• 1
  What matters before you cut metal
• 2
  Separate motion error from cutting error
• 3
  Backlash and lost motion on the drive train
• 4
  Thermal growth over a long axis
• 5
  What to put on the drawing for a linear CNC industry part
• 6
  How we machine linear motion hardware
• 7
  Which error source to chase first

When a linear CNC industry part comes back out of tolerance, the first instinct is to blame the machine. Sometimes that is right. Often it is not. A linear axis has three error sources stacked on top of each other: the servo loop, the mechanical drive train, and the cutting process itself. If you cannot separate them, you tune the wrong thing.

Start with a no-load test. Command a slow traverse along the full travel and read the position feedback with a laser interferometer or a granite square and dial indicator. On a 1,000 mm axis, most machines in good condition show 5–15 μm of positioning error. If the no-load number is already at 30 μm, the problem is in the machine. If it is clean, the error is being introduced by the cut.

Load-induced error shows up in a different signature. The part is fine at the start of the pass and drifts as the tool wears, or it is fine on the finish pass and out on the rough. Check the depth of cut, the tool runout, and the workpiece clamping before you touch the servo gain.

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  No-load firstMeasure the axis alone. Do not cut anything yet.
• 2
  Then loadCut a test bar and measure the same features.
• 3
  Compare the two numbersThe gap tells you which side of the loop is wrong.

Backlash is the movement you get when you reverse direction before the tool actually moves. On a ball screw axis it comes from the nut, the bearing preload, and the coupling. On a rack and pinion it comes from the mesh. It rarely shows up on a straight line cut. It shows up on a circle that is slightly oval, or on a corner that has a small flat spot where the axis reversed.

Measure it the same way you measure a thread: approach a dial indicator from one direction, zero it, back off 0.1 mm, then come back and read the difference. New ball screws with a preloaded nut typically measure under 5 μm. A worn nut can be 20 μm or more, and no amount of gain tuning will remove it.

For parts where direction changes are frequent, such as a cam profile or a helical slot, backlash is the dominant error. For a long straight rail, it barely matters. Decide which one you are making before you spend money on the drive train.

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  Circle testA 100 mm circle reveals reversal error fast.
• 2
  Acceptable rangeUnder 5 μm for a preloaded nut, 5–10 μm for a standard nut.

A steel ball screw grows about 12 μm per meter per degree Celsius. An aluminium part grows about 23 μm per meter per degree. On a 4,000 mm axis, a 3 °C shop temperature swing is enough to move the part 140 μm, which is 28 times the ±0.005 mm tolerance we hold on small features. This is why long parts are usually machined in a temperature-controlled area, or measured after they cool.

The practical fix is not to chase the temperature. It is to let the machine and the part reach equilibrium before the finish pass, and to keep the coolant temperature stable. If a shop runs a 4,000 mm travel machine without temperature control, long parts should be expected to have a looser tolerance than short ones.

For parts with a tight tolerance over a long span, we cut the datum features first, let the part settle, then cut the critical features in a second pass. It costs one extra setup. It saves a rejected part.

• 1
  Steel grows 12 μm/m/°CAluminium grows about 23 μm/m/°C.
• 2
  Equilibrium firstLet the machine soak before the finish pass.

A drawing that says ±0.005 mm everywhere is not a tight drawing. It is an unbuildable one. The tolerance should match the function. A mounting face needs flatness and hole position. A rail seat needs straightness and parallelism. A bearing bore needs roundness and size. If every dimension carries the same tight tolerance, the shop will either quote high or ask for a deviation.

Call out the datum. On a linear axis part, the datum is usually the mounting face or the rail seat, not the outside edge. If the datum is ambiguous, two inspectors will measure two different parts from the same piece. This is the single most common cause of a passed part being rejected at the customer.

Give the material and the finish. Anodizing adds 5–25 μm per surface and can close a bore. Hardcoat is thicker than clear anodize and can change a press fit into an interference fit. If a bore is anodized after machining, the pre-plate dimension has to be smaller by twice the coating thickness.

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  Match tolerance to functionNot every surface is a bearing seat.
• 2
  Name the datumAmbiguous datums cause rejections.
• 3
  Account for coatingAnodize closes bores by 5–25 μm per surface.

We run 127 high-precision CNC machines across three wholly-owned plants, including 16 simultaneous 5-axis machining centers and 16 mill-turn centers. For linear motion parts, the useful spec is the travel: 4,000 × 400 × 150 mm on the large machines, and 750 × 1,150 × 550 mm or 600 × 600 × 600 mm on the medium ones. A rail, a gantry beam, or a long linear stage housing can be cut in one setup instead of two, which removes the joint line that a second setup always leaves.

The tolerance we hold is ±0.005 mm on critical features, with surface finish from Ra 0.2–0.8 μm on bearing bores up to Ra 1.6–3.2 μm as-machined. We inspect 100% of parts before shipment, and reports are available on request. The shop is ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022 certified, which matters if your motion hardware goes into a vehicle, a medical device, or a customer's confidential program.

Materials for linear hardware are usually 6061-T6 or 7075 aluminium, 303 or 17-4PH stainless, and 1045 or 4140 steel. We cut all of them, plus titanium and Inconel when the application calls for it. Quotation and a free DFM analysis come back within 12 hours, and production can start within 24 hours of a released order.

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  Long travel4,000 mm in one setup for rails and beams.
• 2
  Tight tolerance±0.005 mm on critical features.
• 3
  No MOQOne prototype to 10,000+ part runs.