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

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:

• Precision

• Rigidity

• Speed

• 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.

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

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

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