CNC Circuit Mill/V2 Design Rationale

Basic Concept

• A milling bit rotates. The milling bit moves into and cuts the workpiece.

Features

• Computer Numerical Control (to improve precision and minimize operator involvement)

• Stepper Motor Direct Drive with Leadscrews, Anti-backlash Nuts, and Thrust Bearings (for precise open-loop rotary-to-linear motion and transmission simplicity)

• Dual Shaft Per Drive with Anti-friction Sleeve Bearings (for structural rigidity with minimal motion resistance)

Uniform Part Dimensions

• Plate Dimensions: 6.35mm x 609.6mm x 609.6mm

• Block Dimensions: 38.1mm x 76.2mm

• Flat Bar Dimensions: 25.4mm x 101.6mm

• Round Bar: 25mm DIA

• Round Bar: 12mm DIA

• Round Bar: 5mm DIA

• Setscrew: M5x0.8 THREAD

• Screws: M5x0.8 THREAD

Uniform Mounting Dimensions

• Center-to-Center Distance between Block Mounting Screws: 55mm

• Center-to-Center Distance between Support Shaft and Leadscrew: 45mm

• Stepper Motor Mounting Square Side Length: 47.14 +/- 0.20mm

• Sleeve Bearing Mounting Circle Diameter: 30mm

• Leadscrew Nut Mounting Circle Diameter: 19.05mm

Stepper Motor Notes

• Higher inductance steppers provide greater torque at low speeds and lower torque at high speeds.

• Steppers run hot (50 to 90 degrees Celsius), but excessive current will cause overheating.

• Greater current provides more torque but increases rigidity.

• Wiring a stepper's coils in series provides full inductance of the stepper (hence greater torque at low speeds than parallel wiring); in parallel (half-coil) provides half inductance of the stepper (hence greater torque at higher speeds than series wiring).

• Steppers should be run at a voltage higher than their rated voltage for better performance (torque and speed).

Stepper Driver Notes

• Stepper motors overshoot each step and vibrate at their resonant frequency. If the resonant frequency equals the stepping frequency, the motor loses synchronism and suffers positional and directional problems. This problem is worst with full stepping. Microstepping drivers mitigate this problem and allows higher speeds without causing mechanical resonance.

Platform Parameter Overview

• Dimension 1: 500mm (390mm support shaft length until end of X Block + 45mm block-stepper spacing + 55mm stepper length + 10mm clearance = 500mm; maximum X travel range = 400 support shaft - 10 shaft collar - 40 block - 40 block - 10 shaft collar = 300mm)

• Dimension 2: 530mm (5 clearance + 37.5 (count as 40) half of X Block + 20 half of Y block + 360 support shaft until end of Y Block + 45 block-stepper spacing + 55 stepper length + 5 clearance = 530mm; maximum Y travel range = 400 support shaft - 10 shaft collar - 40 block - 40 block - 10 shaft collar = 300mm)

• Dimension 3: 5mm

• Mount Hole Diameter: 5mm

• Mount Hole Spacing along Dimension 1: 35mm, 375mm (5 clearance + 10 shaft collar + 20 into block = 35mm; 500 - 5 clearance - 55 stepper length - 45 stepper-block spacing - 20 into block = 375mm)

• Mount Hole Spacing along Dimension 2: 15mm, 70mm, 355mm, 410mm (5 clearance + 10 into block; that plus 55mm in-block spacing; 5 clearance + 37.5 half X Block + 20 half Y Block + 320 until start of Y Block - 27.5 (half in-block spacing) = 355mm; that plus 55mm in-block spacing)

Block Parameter Overview

• Block Dimension 1: 40mm (to mount leadscrew nuts and sleeve bearings)

• Block Dimension 2: 75mm (to mount stepper motors)

• Support Shaft Hole Diameter: 12mm (to handle radial loads during operation)

• Bearing Lip Thickness: 10mm (to handle axial loads during operation)

• Bearing Lip Inner Hole Diameter: 6.35mm (to stop thrust bearing)

• Bearing Lip Outer Hole Diameter: Xmm (to fit thrust bearing)

• Stepper Motor Mount Hole Spacing: Square, 47.14mm Center-to-Center

• StepperMotor-Block Inbetween Spacing: 45mm (2 stepper centerbulge + 22 stepper shaft + 6 clearance + 15 leadscrew stickout = 45mm)

• Leadscrew Nut Mount Hole Depth: 20mm

• Leadscrew Nut Fit Hole Diameter: 12.7mm

• Sleeve Bearing Mount Hole Depth: 20mm

• Sleeve Bearing Fit Hole Diameter: 18mm

• All holes unless otherwise stated: M5x0.8

• All holes are centered unless otherwise stated or evident

X Axis Parameter Overview

• Bottom-Shaft Spacing: 30mm

• Shaft-Leadscrew-Shaft Spacing: 90mm

• Shaft-Top Spacing: 15mm

• Block Dimension 3: 30 + 90 + 15 = 135mm

• Mount Hole Depth: 30mm

• Mount Hole Spacing from Block Dimension 1: 10mm

• Mount Hole Spacing from Block Dimension 2: 20mm

Y Axis Parameter Overview

• Narrow Side Bottom-Shaft Spacing: 20mm (9mm half-dia of sleeve bearing, leaving 11mm support)

• Narrow Side Shaft-LeadscrewNut-Shaft Spacing: 90mm

• Wide Side Bottom-Shaft Spacing: 130mm

• Wide Side Shaft-StepperMotor-Shaft Spacing: 90mm

• Wide Side Shaft-Top Spacing: 15mm

• Block Dimension 3: 130 + 90 + 15 = 235mm

Z Mount Parameter Overview

• Narrow Side Bottom-Shaft Spacing: 20mm (sets maximum Z working height: 140 - 20 = 120mm)

• Narrow Side Shaft-Leadscrew-Shaft Spacing: 90mm

• Narrow Side Shaft-Top Spacing: 90mm

• Block Dimension 3: 20 + 90 + 90 = 200mm (is maximum Z travel range; 200mm)

• Z Axis Mount Hole Spacing Along Dimension 1: 6mm, 34mm

• Z Axis Mount Hole Spacing Along Dimension 2: 25mm, 50mm

Z Axis Parameter Overview

• Side-Shaft Spacing: 15mm (after 6mm support shaft radius, 9mm support thickness left)

• Shaft-StepperMotor-Shaft Spacing: 90mm

• Shaft-Side Spacing: 15mm

• Block Dimension 3: 15 + 90 + 15 = 120mm

• Special Block Dimension 2: 20mm (10 lip thickness + 10 to fit thrust bearing)

• Block Mount and Stepper Mount Hole-line Spacing: 10mm, 57.14mm, 67mm, 95mm

• Special Block Dimension 1: 101mm (95mm + 6mm clearance)

End Mount Parameter Overview

• Side-Shaft Spacing: 20mm

• Shaft-LeadscrewNut-Shaft Spacing: 90mm

• Side-Shaft Spacing: 20mm

• Block Dimension 3: 130mm

• Special Block Dimension 2: 75mm (maximum Z travel range minus this value gives the approximate actual Z travel range; ideally the actual Z travel range should equal maximum Z working height; 200 - 75 = 125mm)

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 and support shaft radial loads effectively, 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 Mounting

• Comments: The first split of design branch is either if we want to mount the stepper motor with 1-way fasteners (screwing screws only in one way along an axis) OR with multi-way fasteners (screwing screws in more than 1 axis-along ways).

Multi-way fasteners practically have 6 screw-ways, these ways being perpendicular to the faces of a cube for visual reference. 1 of the ways is practically necessary due to the stepper motor already having a set of tapped holes along one screw-way. Given this screw-way, the 4 screw-ways perpendicular to the stepper screw-way are ineffective due to manufacturing inaccuracy issues as well as relatively low rigidity anyway. Therefore the multi-way design branch is practically forced into 2 screw-ways, one way being into the stepper motor tapped holes and the other way along the same axis but in the opposite direction.

Source stepper motors with open hole mounting such that the you avoid the disadvantage of relatively long screws. For example, for stepper motors with tapped holes: the mounting screws goes through the block, through the spacer, then screws into the stepper motor; for stepper motors with open holes: the mounting screw goes through the stepper motor open hole, through the spacer, then screws into the block. Because the stepper motor open hole plate is much thinner than the block length, significant screw length is saved with open hole mounting.

Axis 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 Transmission

• Comments The shaft coupling between the stepper motor and the leadscrew should not incur large radial or axial loads. The optimal way to achieve this is to have the stepper-leadscrew junction occur behind the supporting block. This way, the supporting block's thrust bearing takes the axial (and some radial) load, putting minimal such loads on the stepper shaft and shaft coupling.

Axis Axial Load Supports

• Comments Use a thrust bearing on both ends of all leadscrews and set the thrust bearing inside the block within a lip.

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