A practical guide for machine builders and buyers on where ball screws are used in CNC machinesâX, Y, Z axes, spindle head, and tool changersâand how to choose between rolled C7-class screws and ground higher-precision classes. Includes support-end configuration decisions and a supplier checklist.
If you are specifying a CNC machine or replacing a worn screw, the ball screw is one of the few components that directly limits positioning repeatability. This article explains where ball screws are actually used inside a CNC machine, when a rolled C7-class screw is enough, and when a ground, higher-precision class is warranted. It also covers the support-end decisions you will be asked to make as a buyer, and closes with a supplier checklist.
Key takeaways
- Ball screws convert rotary motor motion into precise linear motion on the X, Y, and Z axes, and often on spindle heads and tool changers.
- Rolled C7 screws are the common choice for general positioning; ground screws are chosen when accumulated error, smoothness, or thermal stability matter more than cost.
- Support-end configuration (fixed, supported, or free) is a purchasing decision you should settle before ordering, because it affects bearing selection and mounting.
- Always confirm accuracy class, preload, end machining drawing, and warranty terms with the supplier before placing a PO.
What a ball screw does in a CNC machine
A ball screw converts the rotary output of a servo or stepper motor into linear motion. Recirculating balls run between the screw shaft and the nut, so friction is much lower than in a traditional lead screw. That lower friction and the preload that can be applied to the nut are what allow a CNC machine to move an axis quickly, hold position, and reverse direction without excessive backlash.
For a machine builder, the practical question is not what a ball screw is, but which axes or assemblies need one, and what accuracy class each of those positions actually requires. Over-specifying raises cost and lead time; under-specifying shows up later as poor surface finish, position drift, or premature wear.
Where ball screws are used in a CNC machine
Ball screws are used in several distinct places, and the requirements differ from one to the next.
X and Y axes (table or gantry motion)
On a typical vertical machining center or router, the X and Y axes carry the table or the gantry. These are the most common ball screw positions. They usually see the highest travel and the most frequent reversals, so nut preload, screw straightness, and support-end rigidity all matter. On a gantry machine, the two sides must be matched well enough that the gantry does not skew.
Z axis (vertical motion)
The Z axis often carries the spindle head, so the screw may need to hold position against gravity when the brake is off. This is where support-end design becomes important: a vertical screw with a poorly supported free end can sag or vibrate, and the load direction is constant rather than reversing like the X and Y axes. Buyers should expect the Z-axis screw to be specified with more attention to support bearings and, in some cases, a larger diameter for the same travel.
Spindle head and quill motion
On some machine designs, a ball screw drives the spindle head or quill rather than the whole column. This is common where the head travels and the workpiece stays fixed. The screw here is short but must be stiff, because any lost motion shows up directly in the cut. When this position is driven by a ball screw, the support end is usually fixed on the motor side with a preloaded bearing pair.
Tool changers and auxiliary axes
Automatic tool changers, pallet changers, and rotary or tilting auxiliary axes also use ball screws or similar linear actuators. These are not cutting axes, so the accuracy requirement is usually lower, but repeatability and cycle life still matter. It is common to use a rolled screw here to control cost.
The table below summarizes where ball screws are typically found and how the duty differs.
| Position | Typical duty | What matters most |
|---|---|---|
| X / Y axes | High travel, frequent reversal | Preload, straightness, matched pairs on gantries |
| Z axis | Vertical load, holding position | Support-end rigidity, bearing selection |
| Spindle head / quill | Short stroke, cutting load | Stiffness, minimal lost motion |
| Tool / pallet changer | Intermittent, non-cutting | Repeatability, cycle life, cost |
Rolled C7 vs. ground higher-precision screws
The two broad categories you will be quoted are rolled screws and ground screws. Rolled screws are formed by rolling the thread, then typically heat-treated and sometimes polished. Ground screws are ground after heat treatment to remove more of the lead error and surface irregularity.
The difference buyers actually feel is this: rolled C7-class screws are adequate for general positioning, while ground higher-precision classes are chosen when accumulated axis error, motion smoothness, or thermal behavior becomes the limiting factor. C7 is a commonly used class for general CNC positioning, and it is the class most cost-sensitive machine builders start with. Moving to a ground, higher-precision class costs more and usually lengthens lead time, so the decision should be tied to the machine's accuracy requirement, not to a preference for "better."
Use this as a starting point for the conversation with your supplier:
| Consideration | Rolled C7-class | Ground higher-precision |
|---|---|---|
| Typical use | General positioning, routers, standard VMCs, auxiliary axes | High-accuracy machining, grinding, measuring, precision spindles |
| Relative cost | Lower | Higher |
| Lead time | Usually shorter | Often longer |
| When it is enough | Positioning requirement can tolerate the class's accumulated error | Not enough when the machine's accuracy target depends on the screw |
| What to confirm | Class, preload, end machining, straightness | Class, preload, end machining, inspection report |
One practical rule: if the axis is closed-loop with a linear scale, the screw's own accuracy class matters less for final position, because the scale measures the table directly. In that case a rolled C7 screw can be the right engineering choice even on a machine with a tight positioning target, because the scale handles the correction. If the axis relies on the motor encoder alone, the screw's accuracy contributes directly to position error, and a ground class becomes more justifiable. This is a design decision, so confirm it with your control and mechanical engineers before locking the purchase specification.
Support-end configuration: the decision buyers often miss
The support end is how the screw is held at each end. There are three common arrangements, and they are not interchangeable.
- Fixedâsupported: one end fixed with a preloaded bearing pair, the other end supported by a simpler bearing. This is the most common arrangement for machine tool axes and is a reasonable default for X, Y, and Z.
- Fixedâfixed: both ends fixed. Used where stiffness and thermal stability are critical, for example long screws on high-accuracy machines. It demands good alignment during assembly.
- Fixedâfree: one end fixed, the other free. Suited to short screws and lighter loads. On a long or vertical screw, the free end can sag or whip, so this is rarely the right choice for a Z axis.
As a buyer, you will usually be asked to confirm: which end is driven, the support arrangement for each end, the bearing type or support unit you want, and the end machining drawing. If you do not have an end machining drawing, the supplier cannot finish the screw to fit your machine, so this is usually the first document exchanged.
Other selection factors worth settling early
- Preload: determines backlash and stiffness. Higher preload reduces lost motion but increases drag torque and heat. Match it to the axis duty, not to a maximum value.
- Nut type: flange, cylindrical, or other mounting styles. This must match the housing on your machine.
- Screw diameter and lead: driven by load, speed, and the required resolution. These are design outputs, so confirm them with the machine designer.
- Environment: dust, chips, and coolant affect whether you need a wiper, bellows, or additional sealing.
- Lubrication: grease or oil, and the service interval, should be defined before installation.
Supplier checklist before you place an order
Once the engineering side is settled, the purchasing side comes down to a short list of confirmations. Ask your supplier for these in writing:
- The accuracy class and preload you are being quoted, stated explicitly on the quotation.
- The end machining drawing or confirmation that your drawing has been reviewed.
- The support-end arrangement and whether the support unit is included or supplied separately.
- Certification documents relevant to the product, if your market requires them.
- Warranty terms and what they cover.
- Lead time and packing method, especially for long screws that are easily bent in transit.
For reference, Xiamen Dongfeng Bearing is a manufacturer of linear motion bearings, guideways, and ball screws, founded in 2014. The company serves automation and machine tool builders, offers OEM/ODM capability, states that its products carry CE and SGS certifications, and provides a 24-month warranty on manufacturing defects. Buyers should still confirm the specific class, preload, and end machining for their own machine, because those are order-specific.
Frequently asked questions
Is a ball screw the same as a lead screw?
No. A lead screw uses sliding contact between the nut and thread, while a ball screw uses recirculating balls. The ball screw has lower friction and can be preloaded to reduce backlash, which is why it is used on CNC positioning axes.
Do all three axes of a CNC machine need the same ball screw class?
Not necessarily. The X, Y, and Z axes have different duty, load direction, and travel. It is common to specify a rolled C7-class screw for general axes and reserve a ground, higher-precision screw for an axis where the machine's accuracy target depends on the screw. Confirm this with the machine designer.
Can I replace a ball screw without an end machining drawing?
The end machining must match your machine's bearing housing and coupling. Without a drawing, the supplier has no defined geometry to machine to, so measure the existing screw or obtain the original drawing first.
What does the support unit include?
Support units commonly include the bearing housing and bearings for one end of the screw. Whether both ends are supplied, and whether the unit is pre-assembled with the screw, varies by order. Confirm this on the quotation.
Next step
The fastest way to get an accurate quotation is to send the supplier three things: the axis the screw goes on (X, Y, Z, or auxiliary), the required accuracy class and preload, and the end machining drawing or the dimensions of the existing screw. With those, the supplier can confirm class, support-end arrangement, and lead time without guessing. If your machine has a closed-loop axis with a linear scale, mention itâit can change which class is the right choice.
This article is general technical guidance for machine builders and buyers. Final specifications depend on your machine design and should be confirmed with your supplier and engineering team before ordering.