A new prototype paradigm How to make things 5/25/2010

5/25/2010
A new prototype paradigm
How to make things
Robin Coope
Engineering Group Leader
BC Cancer Agency
Genome Sciences Centre
With help and examples from Dan Gelbart, Bernhard
Zender and Jon Nakane
The New Approach
• When the designer can build his/her own prototypes quickly, the pace of R&D is
greatly accelerated,
• No formal documentation is needed.
• Change design on the fly to for materials on hand or fabrication errors.
• No need to communicate design details to others
• Traditional prototyping via CNC machining of aluminum blocks is
• Too slow (requires careful CNC programming) and expensive in time and
materials.
• Student engineers can not be taught to become a machinist in a short
time.
• Turnaround for even simple prototypes is at least days, even in well run
places. Any changes require a new iteration and delays.
• The great majority of parts can be made in a few hours by the designers,
without formal drawings or CNC programming, of formed sheet metal or
minimally machined plate.
•The more innovative the project is, the more likely it is that many rounds of
prototyping will be needed. Cutting each cycle from days to hours has a major
impact.
Source: Dan Gelbart
Start with a sketch
Example Project: Improving the Multiplate Vortexer
Three deep well plates per holder, four holders total
Solidworks
Solidworks
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Solidworks
To OMAX Layout….
…And the Water Jet Cutter
2’ x 4’x 1.25mm mild steel
Can cut metal, plastic, rubber stone, glass, wood
Cut Parts
Bend and Spot Weld
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Quick Corrections
Quick Corrections
Sand Blast and Powder Coat
Fabrication Time: Two Hours
Other Advantages
An $800 Part, or, why engineers
need shop time.
• The designer/engineer/researcher will gain more and
more experience from this process so his/her designs will
actually get more efficient over time.
• Parts made this way can be easily scaled to production
through simple machining or production waterjet or laser
cutting
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Hierarchy of Design
1.
2.
3.
4.
Figure out what you need.
See if you can buy it somewhere..
Try to design it with no post-waterjet machining
Try to make it from sheet, with only bends, spot welds
and weld nuts/studs
5. Design it with added screw threads only – use more
than one part if need be.
6. Design a machined part with as much of the difficult
machining done by waterjet as possible.
A Gelbart Box
Design Examples
A Gelbart Box
Step 4: Nut/ Stud Welding and Spot Welding
Note bent electrode for access under a lip.
A Gelbart Box
Simple Part Stack
Electrode Holder from the GSC’s Size Selection Robot.
Stacked Simple Parts are used here rather than any
machining. Only threads are tapped
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A Prototype Medical Tool
All parts except for the turned parts were done by waterjet cutting,
bending and spot welding. Total fabrication time (not including design)
is about 8 hours. (Dan Gelbart)
Precision Parts
Cut from 12.7mm aluminum plate, painted, then machined at the
critical areas. Total fabrication time 2-3 hours per part. (Dan
Gelbart)
Complex Shapes
Flexure
High Density Polyethylene
Or Delrin
A QC device for prostate
brachytherapy
A mounting system for two wiring boards which
allow the boards to be removed without tools
Project Mango!
Design Informed by Physics
or How to Have Structural
I t iti
Intuition
You can never be too
young, too rich or too
thin
- Wallis Simpson,
Famous divorceé
You can never be too
light, too stiff or have
too much torque
- Autonomous Actuator
Truism
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Making Things Stiff
Bends 27mm @ 5 Newtons
A 100x200mm piece of 24ga (0.6mm) mild steel
Fixed Here
Force = 5N
0.34mm @ 5N
(0.39mm with 15mm sides)
Closed Box: 0.013mm under 5N!
Fixed Here
Fixed Here
Force = 5N
25mm sides
Force = 5N
Deflection
Deflection y(x)
y max
y max 
3
4WL
Ebd 3
Stiffness  thickness3
WL3

3EI
BD 3  HK 3
I
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The strength-to-weight ratio
depends on getting material farther
from the neutral axis, without
introducing other bending modes
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Closed Structures
Good Robot Design
• Optimized forces
• Tidy wiring, good
connectors and easy
to remove boards.
• Low force
mechanisms
• Torque is the enemy.
Stiff!
Not so stiff!
Closed structures prevent twisting into less stiff configurations
Good Robot Design

 F
a
m
Drive Train Optimization
Minimize mass to maximize
acceleration and minimize
rolling resistance
, T
Acceleration:
I   mi r 2
i
Place masses close
to the center of
gravity to minimize
yaw moment
a = F/m
F = T/r  a ~ 1/r
r
Speed:
vmax = max r
Important: Driving forces >> friction/load
Gear Ratios
N2 teeth
N1 teeth
, T2
Torque:

Slower = more torque.
, T1
Parts for the 2010 Race Car
Bot
Angular velocity:

Faster = less torque
There are practical limits to the torque you can transmit
through gears. Meccano gears will split under excessive
torque
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The Importance of Solidworks
Lazy Susans, Gears and Racks
The best approach now is to learn Solidworks really well and
design the robot as completely as possible. Include all offthe-shelf parts w/ masses. Add wires if possible.
Steering: Ackerman Angle
Bumpers
King pin
1.
Tie rod
Good approximation to ideal Ackerman angle
2.
Source:
http://en.wikipedia.org/wiki/Ackermann_steering_geometry
Fasteners
Fasteners
Machine Screws
2-56/4-40/8-32
Rivets
POP i t
POP-rivets
Loctite thread
fastener.
adhesives etc
Epoxy
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Adhesives
Key Holes and Standoffs
Adhesives in the 253 lab:
• Epoxy
• Cyanoacrylate (superglue)
• Hot glue
• Adhesive tape (double sided)
• Loctite
Remember to clean and
degrease surfaces to be glued.
Images from MIT 2.007: http://pergatory.mit.edu/2.007/lectures/lectures.html
Shafts, Wheels, Transmissions
•Shafts should only be supported
at 2 points – use flexible
couplings as needed – eg servo
potentiometer
•Supporting either side of gears
is better.
•Bearing choice is determined by
speed, load, & cost.
Clamping to Shafts II
Set Screws
Positives: Easy to make, can use on
any shaft.
Negatives: Can mar the shaft, not
much torque resistance, easy to strip
4-40 set screws, need to use shaft
flats for better grip larger shafts.
Best used with small shafts – eg
Physics 253 robots!
Best used for bushings, not
torque loads.
Clamping to Shafts I
Splines. Great torque resistance. Can be endbolted if the spline is tapered or split bolted (e.g.
Futaba servo output). A loose fit will “nag” the
splines and wreck the shaft. Can also be used for
longditudinal sliding torque transfer.
Woodruf keys: A single spline for a steel shaft
and hub. Material of the key (e.g. brass) is
chosen so it can break under a specified load,
as a safety mechanism.
Clamping to Shafts III
The best method for torque resistance without
spines is a split clamp.
MUST have an accurate fit to the shaft before the
screw is tightened.
Actuator
Machine Screw
No threads
Cut out
Threads
Rigid Link
Shaft
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Wheels and Hubs
Tire
Wheels
• The key to good hubs for gears
and wheels is concentricity and
good grip.
• Shaft fits should be tight to
prevent vibration.
•Make hubs long enough to make
set screws accessible
• Good hub/shaft overlap (eg
~20mm for our motors), gives
good stiffness which helps keep
wheels tight.
Wheel
Hub
Motor
Hub Set Screw
Wheel Screws
For two-part wheels and gears, the mechanism for tightening onto the
shaft must be separate from the mechanism for holding the wheel and
hub together!
While the “non optimal” solution at
f=F*L/d left is, yes, not optimal, in practice
mounting wheel directly on motor
shafts has always worked for 253
robots.
robots
Images from MIT 2.007:
http://pergatory.mit.edu/2.00
7/lectures/lectures.html
Bearings
Thin Bushings – easy, quick but
beware of binding!
More about Bending and Spot
Welding
g
Nylon Bushings – cheap, can make
custom shapes, higher friction
Ball Bearings – expensive! heavy
very good alignment required.
Guides from CNC back guage
Top die
Bottom Die
Our 12 ton press brakes can bend > 120cm @
1mm and ~ 30cm @ 2.5mm for steel
The guage fingers can get in close enough to bend ~
8mm flanges with the small die, 14mm with the large.
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Dies are cut into different lengths to allow
complex bends
12 kVA spot welder can weld up to 2mm + 2mm steel.
Power can be adjusted from “1” to “7”
Rule: Always keep the timer on minimum: 0.1s
Stiffener plates get welded either side of mitre
An 8-32 weld nut going being spot welded by flat dies.
The shoulder locates it in the waterjetted hole.
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Sheet Metal Rules
Source: Dan Gelbart
Material
Sheet Metal Rules
Source: Dan Gelbart
Final Thoughts
• Steel
– Cheap, spot weldable, (Was) hard to machine, needs
finishing. ~ $3/kg ($12/kg for stainless)
– Density = 7.9 g/cm3
– Young
Young’s
s Modulus = 2000MPa
• Aluminum
– Forgiving to machine, low density. $12/kg
– Density = 2.7 g/cm3, Young’s Modulus = 600MPa
• Plastic (Polyethylene, Delrin, Teflon)
– Insulating! Makes Bearings! Transparent to X-rays!
– Otherwise, soft, weak and yucky
• Do as much Solidworks design as you can
without slowing down the team too much.
• Keep
p yyour design
g jjust simple
p enough.
g
• Avoid high driving forces/torques
• Keep your wiring organized
• Be prepared to change course if things
aren’t working.
• Work as a team!
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