183 lines
8.1 KiB
Markdown
183 lines
8.1 KiB
Markdown
# Rotation distance
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Stepper motor drivers on Klipper require a `rotation_distance`
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parameter in each
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[stepper config section](Config_Reference.md#stepper). The
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`rotation_distance` is the amount of distance that the axis moves with
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one full revolution of the stepper motor. This document describes how
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one can configure this value.
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## Obtaining rotation_distance from steps_per_mm (or step_distance)
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The designers of your 3d printer originally calculated `steps_per_mm`
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from a rotation distance. If you know the steps_per_mm then it is
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possible to use this general formula to obtain that original rotation
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distance:
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```
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rotation_distance = <full_steps_per_rotation> * <microsteps> / <steps_per_mm>
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```
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Or, if you have an older Klipper configuration and know the
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`step_distance` parameter you can use this formula:
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```
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rotation_distance = <full_steps_per_rotation> * <microsteps> * <step_distance>
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```
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The `<full_steps_per_rotation>` setting is determined from the type of
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stepper motor. Most stepper motors are "1.8 degree steppers" and
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therefore have 200 full steps per rotation (360 divided by 1.8 is
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200). Some stepper motors are "0.9 degree steppers" and thus have 400
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full steps per rotation. Other stepper motors are rare. If unsure, do
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not set full_steps_per_rotation in the config file and use 200 in the
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formula above.
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The `<microsteps>` setting is determined by the stepper motor driver.
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Most drivers use 16 microsteps. If unsure, set `microsteps: 16` in the
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config and use 16 in the formula above.
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Almost all printers should have a whole number for `rotation_distance`
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on x, y, and z type axes. If the above formula results in a
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rotation_distance that is within .01 of a whole number then round the
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final value to that whole_number.
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## Calibrating rotation_distance on extruders
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On an extruder, the `rotation_distance` is the amount of distance the
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filament travels for one full rotation of the stepper motor. The best
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way to get an accurate value for this setting is to use a "measure and
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trim" procedure.
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First start with an initial guess for the rotation distance. This may
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be obtained from
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[steps_per_mm](#obtaining-rotation_distance-from-steps_per_mm-or-step_distance)
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or by [inspecting the hardware](#extruder).
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Then use the following procedure to "measure and trim":
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1. Make sure the extruder has filament in it, the hotend is heated to
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an appropriate temperature, and the printer is ready to extrude.
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2. Use a marker to place a mark on the filament around 70mm from the
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intake of the extruder body. Then use a digital calipers to measure
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the actual distance of that mark as precisely as one can. Note this
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as `<initial_mark_distance>`.
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3. Extrude 50mm of filament with the following command sequence: `G91`
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followed by `G1 E50 F60`. Note 50mm as
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`<requested_extrude_distance>`. Wait for the extruder to finish the
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move (it will take about 50 seconds). It is important to use the
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slow extrusion rate for this test as a faster rate can cause high
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pressure in the extruder which will skew the results. (Do not use
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the "extrude button" on graphical front-ends for this test as they
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extrude at a fast rate.)
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4. Use the digital calipers to measure the new distance between the
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extruder body and the mark on the filament. Note this as
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`<subsequent_mark_distance>`. Then calculate:
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`actual_extrude_distance = <initial_mark_distance> - <subsequent_mark_distance>`
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5. Calculate rotation_distance as:
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`rotation_distance = <previous_rotation_distance> * <actual_extrude_distance> / <requested_extrude_distance>`
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Round the new rotation_distance to three decimal places.
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If the actual_extrude_distance differs from requested_extrude_distance
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by more than about 2mm then it is a good idea to perform the steps
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above a second time.
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Note: Do *not* use a "measure and trim" type of method to calibrate x,
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y, or z type axes. The "measure and trim" method is not accurate
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enough for those axes and will likely lead to a worse configuration.
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Instead, if needed, those axes can be determined by
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[measuring the belts, pulleys, and lead screw hardware](#obtaining-rotation_distance-by-inspecting-the-hardware).
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## Obtaining rotation_distance by inspecting the hardware
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It's possible to calculate rotation_distance with knowledge of the
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stepper motors and printer kinematics. This may be useful if the
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steps_per_mm is not known or if designing a new printer.
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### Belt driven axes
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It is easy to calculate rotation_distance for a linear axis that uses
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a belt and pulley.
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First determine the type of belt. Most printers use a 2mm belt pitch
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(that is, each tooth on the belt is 2mm apart). Then count the number
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of teeth on the stepper motor pulley. The rotation_distance is then
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calculated as:
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```
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rotation_distance = <belt_pitch> * <number_of_teeth_on_pulley>
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```
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For example, if a printer has a 2mm belt and uses a pulley with 20
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teeth, then the rotation distance is 40.
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### Axes with a lead screw
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It is easy to calculate the rotation_distance for common lead screws
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using the following formula:
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```
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rotation_distance = <screw_pitch> * <number_of_separate_threads>
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```
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For example, the common "T8 leadscrew" has a rotation distance of 8
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(it has a pitch of 2mm and has 4 separate threads).
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Older printers with "threaded rods" have only one "thread" on the lead
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screw and thus the rotation distance is the pitch of the screw. (The
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screw pitch is the distance between each groove on the screw.) So, for
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example, an M6 metric rod has a rotation distance of 1 and an M8 rod
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has a rotation distance of 1.25.
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### Extruder
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It's possible to obtain an initial rotation distance for extruders by
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measuring the diameter of the "hobbed bolt" that pushes the filament
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and using the following formula: `rotation_distance = <diameter> * 3.14`
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If the extruder uses gears then it will also be necessary to
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[determine and set the gear_ratio](#using-a-gear_ratio) for the
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extruder.
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The actual rotation distance on an extruder will vary from printer to
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printer, because the grip of the "hobbed bolt" that engages the
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filament can vary. It can even vary between filament spools. After
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obtaining an initial rotation_distance, use the
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[measure and trim procedure](#calibrating-rotation_distance-on-extruders)
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to obtain a more accurate setting.
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## Using a gear_ratio
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Setting a `gear_ratio` can make it easier to configure the
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`rotation_distance` on steppers that have a gear box (or similar)
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attached to it. Most steppers do not have a gear box - if unsure then
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do not set `gear_ratio` in the config.
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When `gear_ratio` is set, the `rotation_distance` represents the
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distance the axis moves with one full rotation of the final gear on
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the gear box. If, for example, one is using a gearbox with a "5:1"
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ratio, then one could calculate the rotation_distance with
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[knowledge of the hardware](#obtaining-rotation_distance-by-inspecting-the-hardware)
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and then add `gear_ratio: 5:1` to the config.
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For gearing implemented with belts and pulleys, it is possible to
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determine the gear_ratio by counting the teeth on the pulleys. For
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example, if a stepper with a 16 toothed pulley drives the next pulley
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with 80 teeth then one would use `gear_ratio: 80:16`. Indeed, one
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could open a common off the shelf "gear box" and count the teeth in it
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to confirm its gear ratio.
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Note that sometimes a gearbox will have a slightly different gear
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ratio than what it is advertised as. The common BMG extruder motor
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gears are an example of this - they are advertised as "3:1" but
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actually use "50:17" gearing. (Using teeth numbers without a common
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denominator may improve overall gear wear as the teeth don't always
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mesh the same way with each revolution.) The common "5.18:1 planetary
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gearbox", is more accurately configured with `gear_ratio: 57:11`.
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If several gears are used on an axis then it is possible to provide a
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comma separated list to gear_ratio. For example, a "5:1" gear box
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driving a 16 toothed to 80 toothed pulley could use
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`gear_ratio: 5:1, 80:16`.
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In most cases, gear_ratio should be defined with whole numbers as
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common gears and pulleys have a whole number of teeth on them.
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However, in cases where a belt drives a pulley using friction instead
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of teeth, it may make sense to use a floating point number in the gear
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ratio (eg, `gear_ratio: 107.237:16`).
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