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

Frame Material

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

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: Difficult to do planar measurements, difficult to drill along non-centerlines, low contact rigidity when mounting on its curved surface.

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

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