Max speed for delta tower axes

justus2342

Hi,
I have a problem with configuring the maximum feedrate on my big delta printer (Modified Tevo little Monster with Duet2Ethernet, running RRF3.4.4). Maybe it is just my lack of understanding.

I would like to tell RRF how fast it may drive the tower axes at maximum. But as far as I see, this is not possible.
Instead the only configurable feedrate limit is for cartesian axes (aka X,Y and Z) via M203. Since the kinematic of a delta printer implies a non-linear relation between tower-axes moves and cartesian XY-moves, configuring only limits for cartesian moves might not suffice.

The only thing I could find in the docs was a rule of thumb:

As a general guide, your maximum XYZ speeds in mm/minute multiplied by your tower steps/mm should be no more than 3.6 million when running firmware 1.09i-dc42 and later

Considering the named firmware version, this might be outdated. Moreover it does not answer my question.

So I am a little puzzled. The EMF Calculator tells me, that I should stick to a maximum of 245 mm/s on the tower axes, but how do I configure RRF to keep this limit?

dc42

@justus2342 said in Max speed for delta tower axes:

So I am a little puzzled. The EMF Calculator tells me, that I should stick to a maximum of 245 mm/s on the tower axes, but how do I configure RRF to keep this limit?

If you specified the kinematics as Delta in the EMF calculator then the results it gives are already converted to feed rate. So just multiply 245 by 60 to convert to mm/min for the M203 command.

justus2342

@dc42 Thank you. That answers my question and solves my problem.

But I am curious. It seems, the EMF calculator just takes the delta-geometry into account with a factor of 1.7. But why 1.7? Where does it come from? (Follow-up question: Why 1.414 (≈ √2) for CoreXY?)

achrn

@justus2342 1.7 because it's tan(60). It assumes the 'rule of thumb' that the delta arms will be at a maximum of 60 degrees from vertical, and with that configuration the motor runs tan(60) times faster than the head is moving directly towards that tower, by geometry.

You can see this for yourself (roughly) knowing nothing more than pythagoras and sin/cos:

Suppose you have 400mm long arms, at 60 degrees from horizontal, extending directly away from a tower. Therefore, the tip of the arm will be offset 400 x sin(60) = 346.41mm horizontally from the tower end, and 400 x cos(60) = 200.00mm vertically.

Suppose the carriage now moves 1mm towards the tower, so the horizontal offset is now 345.41. The arm length obviously doesn't change so is still 400. By pythagoras therefore the vertical offset must now be sqrt(400^2 - 345.41^2) = 201.72mm That is, the head moved 1mm horizontally and the carriage moved 1.72mm.

If your delta has different geometry at extreme reach, you could/should use a different factor, but 60 degrees is the norm.