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:
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. Along an axes, 2 sets of angles must be used for stability because their shaft contact is thin; this is a severe manufacturing and design burden; this low-contact characteristic of angles lowers their design value relative to blocks.
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
- Selected 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
Axis Support Positioning
- Selected Double Support Per Drive for stability
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