TinyG Tuning

Alden Hart edited this page Jun 9, 2014 · 29 revisions
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This page covers things you may want to do once the wiring and physical setup is complete.

Setting Maximum Velocities and Feed Rates and Motor Currents

Background

The velocity maximum - aka traverse rate - is the top speed of a machine axis under no cutting load. Traverses (G0's) move the machine at the maximum velocity and generally don't change from job to job. A good maximum velocity will drive the motor reliably at high speed and allow for a little headroom where the motor is still running well. Attempting to set this rate above this speed may cause the motor to operate erratically, drop steps, or stall.

Bear in mind that with traverses (G0) the actual speed of movement may well be above any of the traverse rates of the individual axes as it's the cartesian sum. For example, if xvm and yvm are set to 10,000 mm/min a G0 from (0,0) to (100,100) will actually run at 14,142 mm/min (assuming it has room to accelerate to the target velocity).

The feed rate is the maximum cutting speed the axis can sustain for a given tooling, material and type of cut and may change considerably from job to job. The max feed rates set here are just an upper limit that a Gcode file cannot exceed. The actual control of feed rate should be done from the F words in the Gcode file itself. The max feed rates should be set lower than the maximum velocity and generally set after these have been set.

Tuning Procedure

The following procedure can be used to set the max velocities ($xvm, $yvm...) and feed rates. Some notes:

  • Values shown are in inches. Millimeter values are also provided in square brackets for comparison [mm]. MM values may be approximate but accurate enough for these purposes.
  • Setting the machine is inches mode is done by issuing a G20 command at the command line (or in a file). Issue a G21 for mm mode.
  • Settings strings in this example, such as $xvm show the x axis. Other axes are similar, such as $yvm, $zvm, $avm, $bvm, $cvm.
  • The example shows motor 1 mapped to the X axis. Other motors mapped to other axes are similar.

Steps
Do these steps for each axis in turn.

  1. Ensure the step angle ($1sa), microsteps ($1mi), and polarity ($1po) settings are correct for the motor and wiring. In general, positive X moves to the right, positive Y moves away from you (towards the rear of the machine), and positive Z moves upwards. Typical values are $1sa = 1.8 degrees per step, $1mi = 8 microsteps, $1po = 0 (not inverted).
  2. Make sure the travel per revolution ($1tr) is set correctly. Example: $1tr = 0.100 is a value for a 10 thread-per-inch lead screw [or 2.54 in mm]. Some machines have different travel per revolution for different axes.
  3. Start with the current potentiometer at the middle position - 12:00. Never exceed the potentiometers' range of motion, which is 270 degrees, or from about 8:00 to about 4:00.
  4. Set a maximum jerk value ($xjm) where you can audibly hear the motor come up to speed. A value of 20,000,000 [50,000,000] is a good starting point. Note: commas are accepted by the configuration routines in text mode, but not in JSON mode.
  5. Test traverse rate with a G0, such as G0 X5 [G0 X100]. The motor should accelerate, cruise at speed, then decelerate to a stop. The motor should not stall or fail to start. Lower the velocity if this is the case.
  6. If the motor hums but doesn't start it's probably not getting enough current. Turn it up to 1:00 or 2:00. Alternately, if the motor stops and starts; or stutters; and the driver chips are excessively hot the motor is getting too much current. Turn the pot down a bit.
  7. If the the motor more or less works but seems to be dropping steps it could be any of the mechanical system (too much friction), the current setting, or the velocity max being too high; or possibly some combination of all three. Experimentation is required. It's best to try to fix them in that order. Start with the mechanical system, then the current, then the setting.
  8. Once you have the axis working you can see if raising the jerk ($xjm) and providing more current can increase the top speed. This is a case of experimenting. Try to avoid excessive current, however, as after a point the current setting provides diminishing benefits and only heats up the motors and risks thermal shutdown.

It's worth noting that the mechanics of the axes may not be identical, and the achieveable traverse rates may differ for each axis. You can set them optimally for each axis and moves in more than one dimension will takes the individual settings into account. 

It's also worth noting that on some machines mechanical resistance is greater in some parts of the travel than others (e.g. more resistance at the ends of travel due to shaft coupler runout or other mechanical factors). Be sure to test the entire travel for each axis before finalizing the settings.

Set feed rates ($xfr) similarly. These often require adjustment for a given job or material as the cutting loads may vary. The traverse rates should not require job-by-job adjustment.