CNC Circuit Mill/V2 Design Rationale
Design Overview
- Platform Baseplate (For Simplicity and Ease of Mounting)
- Blocks Scaled With Angles (For Frame)
- Double Stepper Motor X-Axis (To Clear Middle Area)
- Single Stepper Motor Y-Axis (For Simplicity)
- Single Stepper Motor Z-Axis (For Simplicity)
- Leadscrew and Leadscrew Nut (Per Stepper Motor; consider ballscrew and ballscrew nut for higher precision)
- 2 Support Shafts (On Axis Per Stepper Motor)
- 4 Sleeve Bearings (On Axis Per Stepper Motor)
- Shaft Coupling (On Leadscrew Per Stepper Motor)
- 2 Thrust Bearings (On Leadscrew Per Stepper Motor; use angular contact bearings for larger loads if necessary)
- Screws (Into Tapped Holes Per Fastener Connection)
Block Parameter Overview
- Block Dimension 1: 40mm (to mount leadscrew nuts and sleeve bearings)
- Block Dimension 2: 75mm (to mount stepper motors)
X Axis Parameter Overview
- Bottom-Shaft Spacing: 30mm
- Shaft-Leadscrew-Shaft Spacing: 90mm
- Shaft-Top Spacing: 20mm
- Block Dimension 3: 30 + 90 + 20 = 140mm
Y Axis Parameter Overview
- Narrow Side Bottom-Shaft Spacing: 60mm
- Narrow Side
Z Axis Parameter Overview
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.
Axis Frame Part Shape
- Block Advantage: High approx-uniform rigidity. 6-flat face mounting versatility. Drawback: Massive.
- C-channel Advantage: Moderate rigidity. Drawback: 3-flat face mounting options.
- Tube Advantage: High rigidity. Drawback: 4-flat face mounting options. Low thickness material for tapping.
- 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.
Blocks are the only remaining part shape that accommodate bearings, so singular and multiple blocks are the 2 design branches to go from here.
Single blocks should be used for a given shaft-stepper-shaft drive when the shafts are close together.
Multiple blocks held together by angles (ideally at each corner) should be used for a given shaft-stepper-shaft drive when the shafts are far apart.
Rest Base
The frame should be able to stay still as well as properly mount its workpieces. 2 main options: large plate with holes to mount workpieces or special mounting platforms, OR rectangle made with angles at the bottom of the 4 columns of the frame. Potentially, the platform could be cut to a U-shape and the machine put on lockable castor wheels to produce a portable high-volume CNC mill!
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-bottom area and retain movement stability. Single drive for Y and Z-axis for simplicity.
- Comments:
Clearing the middle-bottom area is important for 3 major reasons: the working volume can potentially go below the frame, workpieces need not be placed onto the frame for machine operation, and machine installation requires a much lower area to be cleared. These are significant general usability scenarios; for instance- below-ground operations, heavy material transportation, site-to-site portability. Because double-drive x-axis still leaves 2 side faces clear, we retain all the workpiece mounting advantages of single drive x-axis placed low on the frame.
The X-axis could use single-drive for simplicity and place the drive high up so that the middle bottom area is clear, but then we run into 3 inter-related issues: the x-axis supporting frame needs more material to be placed high up (plus top-heavy = less stability), the high-up x-axis acts as a ceiling that limits z-axis travel range, and the high x-axis ceiling is furthest from the workpiece material at the bottom (relative to the rest of the frame) resulting in significant torsion.
Rotary-to-Linear
- Selected - Leadscrew and Nut for high mechanical advantage, lifetime operation, modularity
- Belts stretch and have low mechanical advantage
- Comments: Having a leadscrew and nut as opposed to directly screwing the leadscrew into the frame material strikes high on the modularity scale. Especially where precision is involved, machining a small part to be mounted on another piece is easier than without. Plus this separation prepares for ballscrews and corresponding nuts, which are extremely high precision and necessitate the frame-nut separation.
Note that the leadscrew must only be used to convert torque into linear motion; the leadscrew is not structural support, and in any case forcing that function would reduce precision and decrease durability anyway.
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