A slide table specification usually leads with one figure — "±0.005 mm" — and leaves the reader to assume that everything else follows. It does not. Repeatability, positioning accuracy, straightness and flatness are four different measurements, and a slide that excels at one can be mediocre at another. Buying tolerance means knowing which one your process actually consumes.
The four numbers, in plain terms
| Specification | Typical for this class | What it means on the machine |
|---|---|---|
| Repeatability | ±0.005 mm | Returning to the same position, same direction |
| Positioning accuracy | ±0.01 mm | Commanded vs actual, including lead error |
| Straightness | 0.005/300 mm | Deviation from a straight line along travel |
| Table flatness | 0.01 mm | Deviation of the mounting face from a plane |
For most machining operations, repeatability is the figure that matters. A boring cycle returns to the same position twenty times; if that position is repeatable, the bore is consistent even when the absolute position is 0.01 mm off. Accuracy matters when you move between features cut in different set-ups, or when the axis must agree with a second machine.
Screw grade: the biggest single lever
Lead error dominates positioning accuracy over long travel. C7 rolled screws, at ±0.05 mm per 300 mm, are adequate for transfer and general automation but will not hold a precision boring tolerance across a metre. C5 ground screws at ±0.023 mm per 300 mm are the standard choice for machine tool axes and cost perhaps 30 % more than C7 at the same size.
C3 ground screws at ±0.008 mm per 300 mm are available and appropriate for grinding and metrology. On a normal shop floor, without temperature control and without a scale, the C3 advantage is consumed by thermal growth before the first part is finished.
Preload: cheap tolerance
Reversal error is the cheapest tolerance to buy and the most commonly skipped. A preloaded double nut removes 0.02–0.03 mm of backlash for a modest price increment. Skipping it and compensating in the control works for positioning but not for contouring: the control cannot predict which way the axis will be pushed by cutting force.
Cost order for tolerance
Spend in this sequence; later items only pay off once the earlier ones are in place.
- 1. Nut preload
- Low cost, removes reversal error
- 2. Machined datum
- Included in a good assembly
- 3. C5 screw
- +30 % vs C7
- 4. Linear scale
- +15–20 % of axis cost
- 5. C3 screw
- 2–3× screw cost
Rail class and block count
Profile rails are graded by running parallelism and height tolerance. Normal grade suits general automation; high grade is what we fit as standard on the precision tables. Beyond grade, block count matters more: four preloaded blocks on two rails double pitch and yaw stiffness compared with two blocks, which is what keeps an extended axis from chattering at the end of travel.
When to add scale feedback
A linear scale measures the table, not the motor, so it removes lead error, thermal growth and most of the frame compliance from the positioning loop. It costs 15–20 % of axis cost and requires a control that accepts an external encoder.
Add a scale when the process tolerance is below 0.02 mm over more than 500 mm, when the machine runs long shifts without thermal stabilisation, or when two axes must agree with each other. Do not add a scale to compensate for a flexible frame — it will report the deflection accurately without removing it.
Matching tolerance to the operation
- Rough and semi-finish milling, ±0.05 mm — C7 screw, standard preload, no scale. A standard LX160 in its base configuration is more than adequate.
- Finish milling and boring, ±0.02 mm — C5 screw, preloaded double nut, four-block carriage.
- Grinding and precision boring, ±0.01 mm — C5 screw plus scale, cast iron frame, temperature-stable mounting.
- Drilling and EDM positioning, ±0.03 mm — dovetail slide with a graduated handwheel or a C7 screw if motorised.
Verify on arrival, not after the first reject
Every TEBAK assembly ships with a dimensional report: straightness at three positions, backlash in both directions, table flatness and the stroke achieved. Check one unit against the report when a shipment arrives. If the numbers agree, the packing and transport are sound and the rest of the lot can be trusted; if they do not, the problem is found before it reaches a machine.
Tolerance is not a single grade you buy at the top of the range. It is a budget spent in the right order — and for most machining axes, preload and a machined datum deliver more than an expensive screw grade ever will.
Common questions
It corrects lead error, thermal growth and loop compliance, but it cannot remove structural deflection under cutting load. A flexible frame with a scale still cuts oversize.