Tuning and Speeds


Adjusting the Machine's Axes for Maximum Speed


The speeds of the machine's axes are affected by the alignment and drag of the components, the capability of the steppers, the settings on the drives, and by the type of leadscrews and leadnuts.


Stepper Torque Curve
Stepper torque decreases with speed.

Stepper Speed

As shown in the chart, the torque that the steppers can deliver decreases as the steppers' speed increases.

The top useful speed of the steppers in these DIY machines usually ranges between 300 and 1000 rotations per minute, with 500 rpm being a rule of thumb.

The top speed of the stepper divided by the number of turns per inch of the leadscrew will give the highest speed that the axis can move.
 

Examples

A 10 turn per inch leadscrew will give a speed of 50 inches per minute with the stepper turning at 500 rpm.
500 rotations per minute ÷ 10 turns per inch = 50 inches per minute.

Likewise, a 2 turn per inch leadscrew will give 500 rpm / 2 tpi = 250 inches per minute.

Low cost hardware store 5/16-18 threaded rod will give a speed of 500/18 = ~28 ipm.

These values are reasonable speeds to target when adjusting the machine's axes during the set up and tuning process.
 

Axis Tuning, Background

Tuning the machine is a trial and error process.

First the axis’ speed is set in the controlling software at a rate that is well below the top speed of the stepper. The top speed is the value that was determined in the examples above.

A speed below half the top speed is a good starting place.
For example, a 10 turn per inch leadscrew would have a top speed of 50 inches per minute. The initial value could be set in the software at 20 inches per minute.
This slower speed will take advantage of the stepper’s more powerful low speed torque. With this high torque the stepper can power through some of the drag that is caused by the axis being out of alignment.

The stepper will stall at the points of most resistance first. Were the axis speed initially set at its maximum, the tuning process would be difficult because the stepper would stall at every point of drag. Specific causes of drag could not be found and adjusted.

Once the axis moves well at the lower speed, the software’s speed setting is incrementally increased and the process is repeated. The axis is jogged at the higher speed until it stalls, and the newly found problems are removed with adjustment.

This trial and error process is repeated until the axis can move no faster without stalling.
The speed in the controlling software is then backed down a little to give a safety margin so the axis will not stall while cutting.
The top speed of the axis can vary widely, either higher or lower, from the mathematically derived top speed.

The Z axis frequently cannot move as fast because the stepper is lifting the weight of the spindle rather than moving the load horizontally, as is the case on the other two axes.

Low quality leadscrews and leadnuts can also significantly lower the axis' top speed because of their increased drag.

Note that the values above are in inches per minute. The software may express the speeds in units per second. Therefore, the inch per minute values will have to be divided by 60 to be in inches per second. Speeds below 60 inches per minute will be less than one inch per second. For example, 30 ipm will be 0.5 inch per second.
 

Acceleration

The acceleration values in the controlling software are also set by trial and error. They can initially be set in the lower middle of the scale.

Extremely low acceleration will cause the axis to move sluggishly, and acceleration that is too high will cause the axis to snap around violently.

Higher acceleration will also cause the stepper to stall as it tries to rapidly bring the axis up to speed.

There is usually little to be gained by pushing the acceleration value to an aggressive setting. The slightly faster axis movement is not worth the higher risk of the axis stalling.
 

Finding Maximum Stepper Speed

A test to roughly determine the maximum speed of the axis can be undertaken after the leadscrew is fairly well aligned.
The axis is moved with as little drag as possible on the leadscrew. This test also helps to determine if the stepper, drive and power supply settings are correct.

The leadnut is loosened enough so it can move on its support, but it is still attached to its support. The axis is jogged with a powered stepper, and the speed settings in the software are increased to the fastest the axis can move without stalling.

This speed is usually in the range of the value that was determined by dividing turns per inch into 500 rpm.

The axis should move well, but with considerable play and backlash because of the loose leadnut. Should the axis not be able to perform at the expected speed, check that the electrical components are properly set. Incorrect jumper and current settings on the drives can make a huge difference in stepper performance.
 

Aligning and Tuning the Machine

The axis is assembled and adjusted for smooth movement without the leadscrew in place. After the axis moves well on its rails, the stepper, leadscrew, leadscrew bearings and leadnut are loosely installed.

The axis is moved to the stepper end of the leadscrew.

The leadscrew is held in place by eye so it is aligned with the stepper and the axis.

The stepper, the stepper end bearing, and the leadnut are secured into place.
This is not a final setting, so the hardware should be tight enough to hold position, but should not be permanently tightened.

The leadscrew is turned to move the axis to the other end of the leadscrew. This can be done by hand or with a powered stepper.
The leadnut will support the leadscrew and will align the leadscrew with the axis as the axis is moved to the far end.

The bearing, if there is one, on the far end of the leadscrew is tightened into place.

The leadnut is loosened and the stepper speed test described above is undertaken.

Once the axis moves well with the loose leadnut, the speed setting in the software can be lowered to prevent stalling as drag problems are found and addressed.

The leadnut and the other components are incrementally tightened into place. The leadnut may have to be shimmed on one edge of its flange so it does not cause excess drag on the leadscrew. The slightest leadnut misalignment can cause considerable drag.

All components are incrementally tightened as the stepper is jogged at increasingly higher speeds.

After the excess drag has been removed and the axis moves well at the final speed, the components are firmly tightened into place.

The last bit of tightening can introduce drag, so it is important to work methodically and incrementally.
 
As a note, tuning of some of the machines has taken just a few minutes, while others have taken a few hours. It is a trial and error process. A slight misalignment can cause significant drag that may be challenging to address.

The usual problems are incorrect drive settings, leadscrews that are not parallel with the axis' travel, steppers that are improperly mounted, and leadnuts that are out of alignment.
Note that new leadnuts can be extremely stiff in their movement. They can be run back and forth on their leadscrews so they 'break-in' to the leadscrews. Also note that slightly bent leadscrews can lower the top speeds.

Each axis can behave very differently. The speed, acceleration and micro-stepping values do not have to be the same for all axes.