View source for 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:

*[http://en.wikipedia.org/wiki/Accuracy_and_precision Precision]

*[http://en.wikipedia.org/wiki/Stiffness Rigidity]

*[http://en.wikipedia.org/wiki/Speed Speed]

*[http://en.wikipedia.org/wiki/Force 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

*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.

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 alone.

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 need to be used in pairs for travel anyway, and small blocks make a rigid connector.

We have concentrated our design branches into a combination of mono-blocks, mini-blocks, and flats/angles/c-channels.

Mono-blocks provide great rigidity, so should be used when the axis shaft holes are close to the leadscrew hole. Mini-blocks provide great rigidity as well, but require more parts to be manufactured, so use when the distance between the axis shaft holes are too large to effectively fit into a mono-block. Angle supports should be used to scale with both mono-blocks and mini-blocks. Flats and C-channels should be used for special cases where a specific reinforcement is required.

=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

http://www.rcgroups.com/forums/showthread.php?t=1118773

http://www.tcrconline.com/documents/electric%20flight.pdf