Half the emails we open start with the same sentence: "We are replacing the Z axis on our mill, and we are not sure whether to stay with lead screws or switch to ball screws." Usually the machine is 15 years old, the OEM no longer lists the parts, and somebody on the shop floor read that ball screws are "better." Better is doing a lot of work in that sentence.
The honest answer depends on the axis, the load, and one thing most people do not think about until it matters: what happens when the power goes off. Here is how we actually walk customers through it.
The efficiency gap is not a small difference
A ball screw converts rotation to linear motion through recirculating steel balls rolling in a gothic-arch raceway. A lead screw slides a nut along a threaded shaft - brass, plastic, or steel on steel. That difference sounds mechanical, but it shows up in numbers you can quote.
Our ball screw range runs at 90 to 98 percent efficiency. A comparable Acme or T-lead screw sits around 20 to 25 percent. Put the same motor on both and the ball screw moves roughly three to four times the load, or moves the same load at three to four times the speed. That is why every CNC router we build uses a ball screw on the X and Y axes - at those feed rates a lead screw would need a motor so large it would not fit under the table.
The flip side of that efficiency: ball screws back-drive. Push the nut hard enough and the screw turns on its own. Lead screws, with their sliding friction and steeper thread angle, mostly do not. We will come back to that because it decides the Z axis question.
Self-locking changes what happens when the power goes off
Here is the thing we tell every customer who asks about vertical axes: a ball screw will not hold a load when the motor stops. The balls roll, the nut slides down, and the spindle drops. On a machine with a brake or a counterweight, that is fine. Without one, it is a crash waiting for a careless moment.
A lead screw with the right lead angle self-locks. The load cannot drive the screw backwards because the friction angle exceeds the helix angle. You get a free brake that works even with no power in the building. That is why vertical adjustment columns on manual machines, and a lot of Z axes on lighter routers, still run T lead screws after all these years. It is not nostalgia. It is physics.
The two families do not take the same lubricant, and lead screw vs ball screw lubrication explains why a grease that suits a trapezoidal screw can be wrong for a recirculating one.
The compromise some builders use: a ball screw on Z plus an electromagnetic brake on the motor. It works, it adds cost and wiring, and it is the answer we usually recommend when the machine needs high speed on the vertical axis. But if speed is not the goal, the lead screw is the simpler machine.
Where the precision budget goes
Precision is where ball screws earn their reputation, but it is worth understanding what you are paying for. Rolled ball screws like our SFU series are cold-formed, which keeps the price down while still delivering C7 to C5 positioning. Ground screws - the SFE series - are ground after hardening, hit C3 and better, and cost accordingly. A C3 screw on a machining center repeats to a few microns.
A lead screw of the same diameter will not hold those numbers. The nut has clearance, the thread form wears, and thermal growth moves the position. None of that matters on a manual feed handle or a hobby machine moving at 200 mm/min. It matters a great deal on a machining center cutting at 6,000 mm/min. The question is not "which is more precise" - it is whether your process needs that precision.
Preload is the other half of the precision story. Ball screws can be preloaded to remove backlash - light preload for positioning, heavier for cutting loads. We covered the trade-offs in our guide on ball screw preload and backlash. Lead screws have no practical preload mechanism; you adjust by replacing the nut, which is cheap precisely because it is not precision hardware.
Ball screw vs lead screw at a glance
| Factor | Ball screw | Lead screw |
|---|---|---|
| Efficiency | 90–98% | 20–25% |
| Self-locking | No - back-drives under load | Yes, with suitable lead angle |
| Precision grades | C3 / C5 / C7 | No standard precision grade |
| Backlash control | Preload (light to heavy) | Nut replacement |
| Noise | Louder - recirculating balls | Quieter |
| Lubrication | Every ~200 operating hours | Occasional, more tolerant |
| Typical cost | Higher | Lower |
Noise, rigidity, and the things nobody puts in the datasheet
Ball screws are louder. The balls recirculating through the nut make a constant rattle that surprises people the first time they run one on a bench. Lead screws are quieter, which is why so many 3D printers use them - that, and the fact that a 3D printer Z axis moves a few hundred grams a few times a day. Putting a ground ball screw there is like fitting a racing clutch to a bicycle. We sell plenty of DFU ball screws to people who genuinely need double-nut rigidity for heavy reversing loads. We also tell the ones who do not.
Rigidity goes the other way in one respect: a lead screw is stiffer in bending because the shaft is typically larger for the same load class, and sliding contact damps vibration. A long, thin ball screw whips at speed unless you account for critical speed and support it properly. Length, diameter and mounting decide how fast you can actually run a screw - we have a separate guide on ball screw critical speed with the formula, because this is the mistake that bites people with 1.5-metre axes.
Maintenance habits differ too. Ball screws need clean, regular lubrication - every 200 operating hours with a lithium grease like Shell Alvania or SKF LGMT is our standard advice, and a dry screw is the most common cause of early failure we see on returned units. A properly made lead screw tolerates neglect far better. Grease it occasionally and it will outlast the machine it is fitted to.
When a lead screw is the right answer
We make both, so we have no reason to push one. In our experience the lead screw wins when:
- the axis is vertical and the load is moderate - self-locking removes the brake problem entirely
- speed is not the requirement and positioning accuracy beyond C7 is not needed
- the budget is tight and every component counts
- the environment is dirty - sliding threads shed contamination better than recirculating balls
- the machine runs a few hours a week, not three shifts
And the ball screw wins when you need speed, efficiency, long travel life, or repeatable precision. If you can put numbers on it - axis length, load, feed rate, and the accuracy you actually hold on the finished part - the choice stops being an opinion.
Send us your stroke, load, speed and accuracy target and we will come back with a recommendation the same day - usually with a drawing mark-up showing the critical speed and column load check for your length. That check is free, and it saves the most expensive mistake we see: ordering a beautiful screw that whips at the speed it needs to run. Standard sizes ship in 3 to 15 days, and we can machine SFU, DFU and SFE screws to custom length with end journals turned to your drawing. Samples are available before mass production. We have been making both types since 2010, and we will tell you honestly which one your machine needs.
