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.
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.
- 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 always be used for stability because their shaft contact is thin.
C-Channels are like blocks except they lose their top, bottom, and 1 side face- but to compensate they are relatively lightweight while retaining moderate rigidity.
At this point, we have 6 design branches to go from: monoblocks, miniblocks, monoangles, miniangles, mono-C-channel, mini-C-channel. Mono and mini refer to the frame support for the axis shafts and stepper/leadscrew drives being one solid piece or 3 separate pieces (per drive).
Right away we can point out that the major difference between blocks and angle/c-channels is that blocks have top and bottom faces, a strong mounting-related advantage. However, the rigidity to mass ratio of angles and c-channels are quite high, and this proves useful for scalability purposes.
With mini-angles and mini-c-channels we notice that they require a supporting frame piece, leaving blocks as the only option, but if the column is a block, then we lose the rigidity-mass-ratio benefits of using mini-angles; if we use angle/c-channel support columns, then we get design redundancy (if mini-angles/c-channels and support angles/c-channels, more efficient to just go mono-angles or mono-c-channels. So mini-angles and mini-c-channels are out.
Mono-blocks are too massive to be scalable.
Mini-blocks require a supporting column, and it must not be blocks for efficiency. So flats, angles and c-channels must be used. But angles and c-channels need to be used in pairs for travel anyway.
We have concentrated our design branches into a combination of mini-blocks, and flats/angles/c-channels.
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