Why I Don’t Just Pick the Bigger Ball Screw: SFU1605 vs SFU2005 in Real Machines

Start with the load path, not the diameter

A ball screw sits in the middle of a force chain. The motor turns the screw. The screw pushes or pulls the nut. The nut moves the table. The table rides on linear guides. The guides react the cutting forces. The frame holds everything together.

If any part of that chain is weak, the whole axis suffers.

A lot of builders think they can fix a flexing gantry or a loose bearing block by switching to a larger screw. But if the guides are bending or the mounting plate is twisting, the 20 mm screw won’t save you. The force still has to pass through the same weak structure.

That’s why I look at the entire loop before blaming the ball screw. A properly installed SFU1605 in a stiff frame will often outperform a poorly aligned SFU2005 in a flimsy machine.

The 5 mm lead is the real speed limit

One thing people often miss is that both screws have the same lead: 5 mm per revolution. That means for a given motor speed, the linear speed is identical.

If you have a stepper motor spinning at 600 rpm, the linear speed is:

5 mm/rev × 600 rev/min = 3,000 mm/min = 50 mm/s

It doesn’t matter whether the screw is 16 mm or 20 mm. You’re still moving at 50 mm/s.

So if someone is upgrading to SFU2005 expecting a faster machine, they’ll be disappointed. To go faster, you need a larger lead (like 10 mm), a faster motor, or both. The diameter alone won’t change the feed rate.

Where the 16 mm screw feels right

I’ve used SFU1605 on several small machines where the moving mass was under 30–40 kg and the cutting forces were modest. Engravers, small routers, pick-and-place units, camera stages—those sorts of systems.

The 16 mm screw is easy to package. It doesn’t require a huge bearing block or a bulky motor mount. It keeps the moving assembly light, which matters when you’re trying to accelerate quickly.

There’s also a cost difference. Not just the screw itself, but the bearings, coupling, machining, and everything around it. On a compact machine, those savings add up.

I remember a customer who insisted on SFU2005 for a small 300 mm travel engraving axis. The machine worked, but the larger screw forced him to widen the gantry uprights and move the linear guides further apart. In the end, he gained nothing in performance. The 16 mm screw would have been perfectly fine.

When I would step up to 20 mm without hesitation

The main reason I go to SFU2005 is stiffness and load margin. If the table is heavy, the cutting forces are significant, or the screw is long, the extra diameter starts to matter.

On a CNC router cutting aluminum, the tool can push back with a lot of force. The ball screw has to hold the table position without deflecting. A 16 mm screw might handle the load, but it will flex more under load than a 20 mm screw. That flex can show up as chatter, poor surface finish, or lost steps under acceleration.

For a vertical axis, the difference becomes even more important. A heavy Z-axis with a spindle hanging on it is constantly working against gravity. During rapid moves, the acceleration forces stack on top of the weight. The screw sees a peak load that can be double or triple the static load. In that situation, the stiffer 20 mm screw gives you more safety margin.

I also move to 20 mm when the screw gets long. A 1,000 mm SFU1605 is going to sag and whip at lower speeds than a 1,000 mm SFU2005. Longer screws need better support, and a larger diameter helps push the critical speed higher.

Don’t confuse static load with dynamic load

A mistake I see often is someone sizing a ball screw based on the weight of the table alone.

They say, “The table is 25 kg. SFU1605 is rated for way more than that, so I’m fine.”

That would be true if the table never accelerated and never saw cutting forces. But in a real machine, the load is constantly changing.

During acceleration, the force is:

F = m × a

If the table is 25 kg and you accelerate at 2 m/s², that’s 50 N of force. Add a cutting force of 200 N, a little friction, and some margin, and the screw is now seeing a few hundred newtons. Not huge, but not trivial.

The problem gets worse with a cantilevered load or a heavy spindle mounted off-center. That creates a moment on the carriage, which transfers uneven load to the screw and nut.

So before I pick a diameter, I try to estimate the worst-case axial load during acceleration and cutting. That number, not the table weight, is what matters.

The mounting support changes everything

SFU1605 and SFU2005 can both be ordered with different end machining. Usually you get a fixed end and a floating end, or sometimes both ends fixed.

How the screw is supported changes its stiffness and critical speed dramatically. A screw supported on both ends is much stiffer than one with a floating end, even if the diameter is the same.

I’ve seen machines where the designer used a heavy 20 mm screw but mounted it with cheap bearings and a loose floating end. The result was a screw that whipped at moderate speed because the support wasn’t adequate.

In contrast, a well-supported 16 mm screw can run smoothly at surprisingly high speeds for its size. The end machining and bearing selection often matter more than the 4 mm diameter difference.

Alignment will ruin a good screw choice

You can buy the perfect ball screw size and still end up with a bad axis if the alignment is off.

When the screw isn’t parallel to the guide rails, the nut gets pulled sideways as it travels. That causes uneven friction, higher motor current, heat, and premature wear.

I’ve had machines where the axis felt smooth at one end and tight at the other. The ball screw wasn’t the problem. The rails and screw weren’t parallel. Swapping to a bigger screw would have made it worse, not better.

So whenever someone complains that their SFU1605 is “binding” or “making noise,” I ask about alignment first. If the axis is properly aligned, the smaller screw often runs beautifully.

A quick rule of thumb I use

I don’t have a strict formula, but I do have a mental checklist:

• If the axis is under 500 mm long, the moving mass is under 30 kg, and the cutting forces are light, SFU1605 is usually more than enough.

• If the axis is 500–800 mm, the mass is 30–60 kg, or the machine cuts aluminum, I start thinking about SFU2005.

• If the axis is over 800 mm, the mass is over 60 kg, or the screw drives a vertical Z-axis with a heavy spindle, SFU2005 is a safer starting point.

But these are starting points, not rules. The actual calculation depends on acceleration, speed, cutting loads, and support configuration.

What I ask a supplier when ordering

Instead of sending a one-line request like “SFU1605, 800 mm, quote,” I give the supplier the full context:

• Travel length

• Moving mass

• Axis orientation (horizontal/vertical)

• Maximum speed

• Acceleration

• Cutting force or external load

• Duty cycle

• Screw support arrangement

That usually leads to a more useful conversation. A good supplier will tell you whether the 16 mm is sufficient or whether you should step up to 20 mm. They may also suggest a different lead if your speed requirement is high.

The point is to treat the ball screw as part of a system, not as a standalone part number.

Final thought: bigger is not automatically better

It’s tempting to choose SFU2005 just because it sounds safer. And in some machines, it absolutely is the right call.

But for a compact, light-duty CNC, the 16 mm screw is often the better engineering choice. It’s lighter, easier to install, cheaper, and completely sufficient for the job.

The key is to calculate the loads, check the screw length, think about the support bearings, and make sure the alignment will be good.

If you do that, the choice between SFU1605 and SFU2005 stops being a guess. It becomes a decision you can defend with numbers instead of a feeling.