CNC Circuit Mill/V2 Design Rationale: Difference between revisions
| Line 99: | Line 99: | ||
=Rotary-to-Linear= | =Rotary-to-Linear= | ||
*'''Selected''' - Leadscrew and Nut for high mechanical advantage | *'''Selected''' - Leadscrew and Nut for high mechanical advantage, lifetime operation, modularity | ||
*Belts stretch and have low mechanical advantage | *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 leaves room for ballscrews and corresponding nuts, which are extremely high precision and necessitate the frame-nut separation. | |||
=Axis Supports= | =Axis Supports= | ||
Revision as of 20:56, 16 May 2012
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.
- 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.
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 leaves room for ballscrews and corresponding nuts, which are extremely high precision and necessitate the frame-nut separation.
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