Mechanicality
Leadscrews

Leadscrews

Lead screws are used in 3D-printers to convert the rotational movement of stepper motors into linear movement of eg heatbed or the elevator holding the extruder.

These motion converting parts are not just ‘screws’ – they are a vital part of the printer, and choosing appropriate design and quality is essential for getting good print quality.

Below, I’ve collected a few informations regarding the design that you should find enlightening, thus helping you choose wisely.

Performance

The issues here determine the conversion mechanism’s performance. In other words the linear-movement-per turn ratio.

Pitch

Pitch is the distance between screw grooves and is commonly used with inch sized products and specified as threads per inch.

Lead

Lead is the linear travel the nut makes per one screw revolution and is how ball screws are typically specified.

The pitch and lead are equal with single start screws. For multiple start screws the lead is the pitch divided by the number of starts. Note that loadholding ability is inversely proportional to lead value; high lead -> low loadholding ability. This may become relevant if you think of eg a heavy extruder unit driven at high speeds and accelerations. In this case the inertial force exerted by the weight of the extruder may exceed loadholding ability of the leadscrew+motor assembly and induce skips on both acceleration and deceleration. The motor assembly turns the screw as it decelerates and becomes the driver rather than the driven…

High lead, however, facilitates high linear speeds without hitting the leadscrew resonance frequency.

Low lead – given a tight fit between screw and nut – facilitates finer granularity in linear motion for a given stepper type.

Suitability

Thread

Three kinds:

Square thread

Square threads are named after their square geometry. They are the most efficient, having the least friction, so they are often used for screws that carry high power. But they are also the most difficult to machine, and are thus the most expensive.

Article: Square thread form

Acme thread

Acme threads have a 29° thread angle, which is easier to machine than square threads. They are not as efficient as square threads, due to the increased friction induced by the thread angle.[3] ACME Threads are generally also stronger than square threads due to their trapezoidal thread profile, which provides greater load-bearing capabilities. Additionally, the use of split nuts for wear compensation seems to work best with this type of thread.

Article: Acme thread form

 

Trapezoidal thread is a variation of the Acme, as it is of similar design. Here, the thread angle is 30° and all measurements are in millimeters. All metric threads (eg. M8) are trapezoidal.

Article: Trapezoidal thread forms

Buttress thread

Buttress threads are of a triangular shape. These are used where the load force on the screw is only applied in one direction. They are as efficient as square threads in these applications, but are easier to manufacture. It is also possible to use split nuts for wear compensation with this type of thread.

Article: Buttress thread

What to choose

X-axis

Belt drive – no lead screw. You get the precision defined by the steps/rotation of the motor in combination with pulley radius.

xmin=2*pi*rpulley/steps

Y-axis

Belt drive – no lead screw. You get the precision defined by the steps/rotation of the motor in combination with pulley radius.

Z-axis

The leadscrews are loadbearing. They must counter the gravitational force generated by the elevator assembly, which would indicate a Buttress thread design suiting the use. Given the nature of 3D-printing, I’m not sure the use of a wear-compensating split nut is needed. I mean – layer-to-layer-movement so abrupt that the whole X-axis-assembly jumps seems unlikely…

However, unless the screws are perfectly straight, a decoupling between the leadscrew nut and the X-axis-assembly would be nice, in order to prevent X-axis-wobble.

 

zmin=lead/steps

References

Wikipedia: Lead screw