CNC Circuit Mill/V2 Design Rationale

Frame Objectives

The function of the frame is to move a part of the frame in 3 dimensions (relative to its base).

For performance, a good frame has:

• Precision

• Rigidity

• Speed

• Force

For design, fabrication, assembly, and usage, a good frame also has:

• Uniform Dimensions and Other Parameter Values

• Maximum Simplicity (without sacrificing performance)

• Ease of fabrication, assembly, disassembly, and usage

• Modularity

• Durability

• Scalability

• Safe Operation

For the CNC Circuit Mill, a good frame also has:

• Workpiece mounting platform

• Large working volume of moving part relative to the mounting platform.

Choosing an Axis System

• Selected - 3 linear axes: all linked. Advantage: scalability

• 3 linear axes: 2 linked, 1 separate. Advantage: rigidity

• 2 linear 1 circular axes: all linked. Drawback: manufacturing and control complexity.

Frame Material

• Selected - 6061 Aluminum Alloy for rigidity, ease of machinability, and accessibility

Frame Overall Shape

• Selected Rectangular wireframe provides simplicity, rigidity, and flat base for resting stability. Also accords well with the 3-linear axes design parameter. Additionally can mount wallplates for improved rigidity if necessary.

• Spherical and similar wire/solid frames are similarly rigid but much more complex, non-stable while resting, and does not accord with the 3-linear axes design parameter. Additionally, mounting wallplates is much more difficult due to the vastly increased number of faces for such structures.

• Triangular and pyramidal wireframes provide simplicity, rigidity, and flat base for resting stability, but does not accord well with the 3-linear axes design parameter and has marginally higher faces to mount wallplates to.

Axis Frame Part Shape

• Block Advantage: High approx-uniform rigidity. 6-flat face mounting versatility. Drawback: Massive.

• Angle: Advantage: Lightweight. More rigid than flat bar. Perpendicular 2-plane rigidity. Drawback: 2-flat face mounting options.

• Flat Bar. Advantage: Most Lightweight. Drawback: 1-flat face mounting option. Rigid only along 1 plane.

• Round Bar. Advantage: High uniform rigidity on curved surface. Lathe-machinable. 2-flat and circular face mounting versatility. Drawback: Massive. Difficult to do planar measurements, difficult to drill along non-centerlines, low contact rigidity when mounting on its curved surface.

• Comments:

Round bar is out immediately; the marginal rigidity uniformity compared to blocks is negligible; lathe machinability is unnecessary because the precision of frame parts are already uniform through cutting of stock metal; circular mounting is unnecessary because the simplicity and rigidity of rectangular wireframe was chosen. Blocks > Round Bars in all cases.

Flat bar alone is out immediately; 1-flat face mounting and 1-plane rigidity cannot practically achieve 3D rigidity in a wireframe structure. Flat bars can only be used to complement other part shapes, so is restricted to combinations only.

Angles alone are limited to 3 design options within a rectangular wireframe: tri-angle junction corners, multi-hole mounting, and mini-angles.

Axis Frame Part Volumetric Allocation

• Multiple

• Singular

Axis Drive

• Selected - Stepper motors provide high resolution in a simple open-loop system

• AC or DC motors with encoders necessitate complex closed-loop control systems

Axis Drive Positioning

• Double drive for X-axis to clear the middle area and retain movement stability

• Single drive for Y and Z-axis for simplicity.

Rotary-to-Linear

• Selected - Leadscrew and Nut for high mechanical advantage and lifetime operation

• Belts stretch and have low mechanical advantage

Axis Supports

• Selected - Precision Shafts for ease of manufacturing and direct mounting

• Precision V-Rails

• Precision Extrusions

Spindle Drive

• Selected - Outrunner Brushless DC Motor has maintenance-free operation, over 90% efficiency, commutator-free long lifetime, precision speed control possible, quiet operation, more torque than inrunners

• Brushed DC Motors require regular maintenance, short lifetimes, noisy operation

http://www.youtube.com/watch?v=fPLdHeRQp_w

http://www.rcgroups.com/forums/showthread.php?t=1118773

http://www.tcrconline.com/documents/electric%20flight.pdf