Linear Scale (Grating Ruler): Full Closed-Loop Positioning Explained

Nearly every machining center sold today positions semi-closed loop: the control counts motor turns, multiplies by the ball screw pitch, and assumes the table moved accordingly. That assumption holds until heat, backlash or screw elasticity creep in. A linear scale, sometimes called a grating ruler, removes the assumption by measuring the table position directly, and it is offered as optional equipment on VITOP machining centers for exactly this reason.

Semi-Closed Loop vs Full Closed Loop

In a semi-closed system, the feedback point is the motor encoder. Everything between the motor and the table, including the coupling, the ball screw and its thermal growth, is unmeasured. A 1-metre steel ball screw grows by roughly 0.01 mm for every 1 degree C rise, so a machine that runs warm for hours slowly drifts even though the encoder says it has not moved.

With a linear scale fitted to each of the X, Y and Z axes, the scale reads the actual distance between the saddle and the guideway. The control then corrects for whatever the screw, the nut and the temperature have done. This is what engineers mean by full closed-loop positioning: the measured element and the moving element are the same body.

When You Should Specify One

You do not need a linear scale for every job. Bracket work at plus or minus 0.05 mm tolerance will never notice the difference. You should consider it when:

• Tolerances sit at or below about 0.01 mm on long moves, such as precision mould cavities or mating plate sets.
• Cycle times are long. An eight-hour aluminium mould roughing and finishing run gives the screws plenty of time to heat up and drift a semi-closed machine.
• Matching parts are machined on different days or different machines and must assemble without selection or shimming.
• Thermal environment is tough. A shop in the tropics with 35 degree C mornings and coolant-heated enclosures sees more screw growth than an air-conditioned plant.

How It Is Fitted and Used

The scale is a sealed extrusion mounted along each axis, parallel to the guideway, with a reading head carried by the moving element. As the head passes the fine graduations, it produces position signals that the CNC uses in place of, or together with, the motor encoder. The closed scale design protects the glass strip from chips, coolant and dust, which matters in a machine that also does heavy roughing.

For the retrofit or factory-fit decision, factory-fit is the safer route. The axis cables, the control configuration and the compensation tables all need to be set up together, and on a VITOP machine the scale is specified when the machine is ordered so it is commissioned and laser-verified as part of the build. Every VITOP machine is checked with laser interferometry and ballbar testing before shipment, so the actual positioning result you get is documented, not just promised.

Practical Advice Before Ordering

Match the scale to the travel. The scale length must cover the full axis stroke plus the head length, so state your machine model and travels when you enquire. Keep the scale protection in good condition; a dented cover bellows is the most common cause of linear scale failure, not the scale itself. And ask for the machine's accuracy report with the scales fitted, because a full closed-loop machine that is never verified gains you nothing.

If your work is precision moulds, gauge-quality plates, or any part where thermal drift eats your tolerance budget, a linear scale is one of the cheapest accuracy upgrades available on a new machine. Send your typical part drawings to VITOP and ask whether closed-loop feedback is justified for the accuracy you need.

Talk to VITOP Machinery

Planning a machine purchase or need help matching a model to your parts? Send your drawings or questions to vitopmachinery@163.com, or message us directly on WhatsApp: +86 183 1908 0212. You can also browse all machines at www.vitopmachinery.com. We reply within one business day.