The Basic Difference: 16 mm vs 20 mm
The model numbers are fairly straightforward:
- SFU1605: 16 mm nominal screw diameter, 5 mm lead
- SFU2005: 20 mm nominal screw diameter, 5 mm lead
The important detail is that the lead is the same.
One revolution theoretically moves the nut 5 mm on either screw.
That means you shouldn't expect SFU2005 to automatically move the machine faster simply because it has a larger diameter.
The diameter mainly changes the mechanical side of the problem: stiffness, load capability, rotating mass, installation space, and the requirements placed on the rest of the axis.
This is where the selection starts getting interesting.
If the Lead Is the Same, Why Does Diameter Matter?
Think about the screw as part of the machine's force path.
The motor produces torque, the screw converts that torque into linear force, and the nut transfers that force to the moving table.
If the axis is lightly loaded, a 16 mm screw may have plenty of mechanical margin.
If the axis is carrying a heavy table or seeing significant external forces, the larger 20 mm screw gives you more mechanical section to work with.
That doesn't mean SFU2005 will automatically make a machine more accurate.
The guide rails, mounting plate, bearings, coupling, frame, and installation accuracy still matter.
But if the ball screw itself is the weak link, increasing the diameter is a much more useful fix than trying to solve a mechanical problem through servo tuning.
I've seen that mistake more than once.
The controls get adjusted, acceleration gets reduced, gains get changed—and the machine still isn't behaving properly.
Sometimes the mechanical system simply needs more stiffness.
Where SFU1605 Usually Makes Sense
SFU1605 is a good starting point for compact CNC and automation equipment where the mechanical loads are moderate.
Typical examples include:
- Small CNC routers
- Desktop or compact machining equipment
- Engraving machines
- Small linear stages
- Inspection mechanisms
- General positioning axes
- Light-duty automation
The advantage isn't just lower cost.
The smaller screw is also easier to fit into a compact assembly.
That can matter when the ball screw sits between two guide rails and there isn't much room around the motor coupling or bearing supports.
There is also no reason to use a 20 mm screw simply because it is larger.
If the machine only needs the capacity of the 16 mm design, the larger screw may add weight and cost without giving you a useful improvement in the finished machine.
When I Would Move Up to SFU2005
I start looking seriously at SFU2005 when the axis becomes more demanding.
For example:
- Heavier moving tables
- Larger fixtures
- Higher cutting forces
- More demanding machining operations
- Higher rigidity requirements
- Longer or more heavily loaded axes
- Industrial automation with substantial moving mass
The larger diameter gives the mechanical system more stiffness margin.
That can be particularly useful on a CNC axis where cutting forces are changing continuously.
A transfer axis that moves a fixture from point A to point B doesn't necessarily need the same mechanical stiffness as an axis supporting a cutting tool.
That's why application matters so much.
Don't Confuse Payload With Actual Axial Load
This is probably one of the easiest ways to oversimplify ball screw selection.
Suppose somebody tells me:
“The table is 40 kg.”
That isn't enough information.
I would want to know how that 40 kg is moving.
Is the axis horizontal?
Is it vertical?
How quickly does it accelerate?
Is there an external cutting force?
How long is the screw?
Where is the center of mass?
How is the screw supported?
A 40 kg horizontal table and a 40 kg vertical axis are very different engineering problems.
For a vertical axis, gravity is continuously acting on the system. During acceleration and deceleration, the screw and motor have to handle additional forces.
So before choosing between SFU1605 and SFU2005, calculate the actual operating condition instead of selecting purely from the payload number.
The 5 mm Lead Is Actually More Important Than Many Buyers Realize
Because both models use a 5 mm lead, the basic screw speed required for a given linear velocity is the same.
For example, at 1,000 rpm:
5 mm/rev × 1,000 rev/min = 5,000 mm/min
That's about 83 mm/s.
So changing from SFU1605 to SFU2005 doesn't suddenly give you a higher feed rate.
If your application requires more linear speed, changing the screw diameter while keeping the same lead doesn't solve the basic speed requirement.
You would need to look at the lead, motor speed, screw length, support arrangement, and allowable operating speed as a complete system.
This is one reason I don't recommend choosing a ball screw by model number alone.
Screw Length Can Change the Decision
Diameter gets most of the attention, but screw length is just as important.
A relatively short SFU1605 can be a very solid solution.
As the screw gets longer, however, factors such as:
- Screw deflection
- Critical speed
- Vibration
- Alignment
- Bearing support
- Overall axis stiffness
become increasingly important.
At that point, the question isn't simply:
“16 mm or 20 mm?”
It's:
“What diameter and support arrangement make sense for this screw length and operating speed?”
If you're designing a long-travel axis, I would make this calculation before deciding on the screw size.
Don't Forget the Guide Rails
A ball screw doesn't work by itself.
The linear guides carry the table and control its motion. The ball screw provides the drive.
If the guide system is too flexible, increasing the ball screw diameter won't magically make the entire axis rigid.
The same applies to the mounting plate.
You can install an SFU2005 and still have a poor-performing axis if the screw support is moving or the mounting surface is distorted.
When troubleshooting an axis, I therefore look at the whole force loop:
motor → coupling → ball screw → nut → table → linear guides → machine frame
The weakest part of that chain usually determines the practical result.
Installation Space Is an Easy Reason to Choose SFU1605
There are situations where SFU1605 wins simply because it fits better.
The 20 mm screw needs more physical clearance, and that can affect:
- Bearing housings
- Couplings
- Motor brackets
- Rail spacing
- Protective covers
- Lubrication access
- Table design
This matters especially when you're modifying an existing machine.
If the original axis was designed around a smaller screw, moving to SFU2005 may require changes elsewhere.
In a new machine, that's easier to accommodate.
In an existing machine, it may not be worth the redesign unless you actually need the additional capacity.
Motor Selection Should Come After the Load Calculation
I wouldn't choose the motor simply because the ball screw is SFU1605 or SFU2005.
The motor has to be sized around the actual operating requirements.
Look at:
- Axial load
- Acceleration
- Screw lead
- Required speed
- Transmission efficiency
- Axis orientation
- Duty cycle
The 5 mm lead gives both screws the same basic mechanical advantage from a lead standpoint.
But the actual motor requirement still depends on what the machine is doing.
For a vertical axis, also consider what happens during power loss or an emergency stop. A suspended load should have an appropriate holding or braking solution rather than relying on assumptions about the ball screw.
Alignment Can Matter More Than the Difference Between 16 and 20 mm
This is one of those things that becomes obvious after you've had to troubleshoot a machine.
A properly installed SFU1605 can run better than a poorly aligned SFU2005.
If the ball screw isn't properly aligned with the guide system, the nut can experience additional resistance as it travels.
Typical symptoms include:
- Higher motor current
- Uneven running resistance
- Heat buildup
- Abnormal noise
- Premature nut wear
- Positioning instability
If an axis feels smooth at one end of the travel and noticeably tighter at another, I wouldn't immediately blame the screw size.
I'd check alignment first.
SFU1605 vs SFU2005: How I Would Make the Decision
If I were doing the initial design, my selection would look something like this.
I'd start with SFU1605 if:
- The machine is compact
- The moving load is moderate
- Cutting forces are relatively low
- Space is limited
- Lower component mass is useful
- The screw length and operating speed are reasonable
I'd lean toward SFU2005 if:
- The moving assembly is heavier
- Cutting forces are significant
- Higher mechanical stiffness is required
- The machine is industrial rather than compact
- There is enough installation space
- The additional screw diameter provides useful mechanical margin
Neither choice should be made from diameter alone.
A Better Way to Ask Your Supplier
Instead of sending a supplier:
“SFU1605, 500 mm, quote please.”
Give them the actual application.
For example:
Horizontal CNC X-axis, 500 mm travel, 35 kg moving load, 5 m/min maximum speed, moderate cutting force, 10-hour daily duty cycle.
That gives the supplier something to work with.
The useful information includes:
- Travel
- Moving mass
- External or cutting force
- Axis orientation
- Required speed
- Acceleration
- Required accuracy
- Duty cycle
- Screw support arrangement
- Available installation space
A good supplier should be able to use that information to discuss the appropriate screw configuration rather than simply sending the biggest model available.
Don't Automatically Buy the Larger Screw
There is a natural tendency in mechanical design to think:
“If 16 mm works, 20 mm must work better.”
Sometimes it does.
But bigger isn't free.
You may also be adding:
- More rotating mass
- More inertia
- More material cost
- Larger bearing supports
- Larger coupling requirements
- More installation space
If those changes don't solve an actual problem, they're difficult to justify.
On the other hand, choosing SFU1605 simply because it is cheaper can also be a false economy if the machine really needs the additional stiffness of SFU2005.
The goal is not to select the largest screw.
The goal is to select the smallest screw that comfortably meets the real operating requirements.
Final Thoughts
SFU1605 and SFU2005 are not really competing products where one simply beats the other.
They are two different sizes of the same basic 5 mm lead concept.
For compact, moderate-load CNC and automation axes, SFU1605 can be a practical and economical choice.
For heavier loads, higher cutting forces, or applications where mechanical stiffness is more important, SFU2005 gives you more room to work with.
But before making the decision, check the things that actually determine ball screw performance:
load, force, screw length, speed, acceleration, support arrangement, alignment, and installation space.
And if you're replacing an existing screw, don't forget to look at the rest of the axis. A new ball screw won't fix a distorted mounting plate, worn linear guides, or poor alignment.
In the end, the right ball screw is not the one with the bigger number.
It's the one that gives the machine enough mechanical margin without making the rest of the system unnecessarily complicated.
+8615622924499
+8615622924499
