Maslow accuracy - simplification proposal

Technical Details
3

Hi @TomD,
Your proposal covers quite some ground and contains several interesting assumptions.

Now I have worked thoroughly the maths and firmware experiments around chain length calculation and I can say:

  • it is easy to compute the sled position to achieve less that 1.5mm over the workspace.
  • the residual error varies slowly and closer points will show smaller errors
  • positioning the sled is one thing, keeping it in the right spot when routing is a different task. (for the later, read on later in the post)

One important assumption:

We could place the bit “near” each “known” location on the workspace, then “measure chain length”. Placing the sled in the right spot requires a known good target to something in the order of 0.1 mm accuracy, and a tool to get a close view of the bit over the workspace to get it aligned with a target. Then finding chain length involves chain pitch variations and chain stretch. (Counting links will not yield chain length).

A second assumption:

Positioning the bit at the right place would allow cutting accurate parts. When the bit touches the material, side forces will be applied to the bit and to the sled. To keep the bit in the right position, mechanics must maintain forces equilibrium. And there you need something like:

  • a triangular link
  • 11.5 feet or more top beam to reduce dangling sled cases in lower corners.
  • designing routing paths to allow some deflection then accurate finishing passes

Now it would be great to ease even more those assumptions. :slight_smile: (ideas welcome)

“Home Made” is a value.

Each Maslow frame is unique. And that is good because Maslownians makers can do the best with what they have at hand. And it is possible to tune a Maslow with minimal work and to make it cut usefull straight parts.

One achievement of the MaslowCNC is to provide a solution that works with simple measurement tools. (Yet if you want to CNC very accurate parts, you’ll likely need a 6 inch caliper. But that is a general precision CNC machining requirement)

I say “achievement”, yes, because we are at the stage of firmware improvements integrating the latest findings in a beta test. Then finalizing integration with simple working instructions.

So why the oval circles?

Up to recently, something was misunderstood., and something was missing:

A) Maslow realy has only a few important parameters, and among that there is only one needing help: Chain tolerances. This parameter describes chains link real average pitch. Misadjusted, it can warp the workspace in a unique way.

B) Also, there was one parameter missing into the MaslowCNC control to provide Chain stretch compensation. That is because steel is elastic and it contributes to center top vertical scale error. But it is easy to find and generalize for a chain type like the RS25 used into MalowCNC. (Despite it was missing, the easy fix was to exagerate left-right motor distance to pull up the center top workspace and reduce somewhat the vertical distortion.)

For both of these items, the Holey Calibration topic resulted into a beta version to address both issues and improve the workspace scale precision and uniformity.

I personnaly went through a parallel firmware development effort and came to the same conclusions, achieving the Zipper Tree Challenge.

So my position is the following:

  • With the Holey Calibration, the MaslowCNC control is progressing quite well. And trying it would be a baseline to estimate what will soon be the current user experience. (Considering GroundControl can also automate some steps).

  • Each mechanical phenomenon involved in chain length calculation is simple to model. I also found that the chain sag compensation factor is a sufficiently precise estimation of the catenary equation, but requires some (easy) tuning.

  • Overall, mathematics into the MaslowCNC are actually well encapsulated and the specific chain length calculations fit in less than 100 lines of code (for the triangularInverse model) into the file Kinematics.cpp.

  • With that the latest parameters set, you can repeatedly reach below 1.5 mm error over the range.

  • For those who want to get near 0.5mm error over the range, I could share my approach in a cooperative way in a new “High Precision Cutting” Maslow forum category.

Regards :slight_smile:

5 Likes