MasterCAM/CNC Machining Project
Original Project Outline Handouts
outline in PDF (adobe acrobat) format
What Your Section Gets in the Parts Kit
2 Pens - Grand & Toy "RoundEdge" #99011
hex portion is approx. 4" long
distance across the flats on the hex portion is 0.305"
pens can be turned down to 0.290" dia.
2 Paper Clips - Grand & Toy #99526
3-3/4" long when stretched out
wire is 0.035" dia.
5 Rubber Bands - size #64 Grand & Toy #99011
about 3" when not stretched
will safely stretch to 12"
K= 1/2 lbf/inch or K= 0.8 N/cm
coefficient of friction on tile floor about u=0.9
1 Wax birthday cake candle
approx. 2-1/4" long
Medium Density Fiber (MDF) board
(MDF is a wood pulp product)
3 boards - 7"x4"x3/16" thick (nominal)
6 - #8-1/2" long sheet metal pan head screws, Canadian Tire #61-6705-2
1 - sheet of 8.5 x 11" white paper
1 - small DC electric motor
motor dia. = 1.105"
motor body length (not including shaft) = 1.175"
shaft dia. = 0.078"
shaft length from motor = 0.400"
specs: 2500rpm with no load
0.3W with no load
3.57W at motor stall
(you must make your own MDF pinion gear)
1 - motor shaft bushing
This presses on to the DC motor shaft, allows you to mount your MDF gear
with a 0.160" dia. hole in the middle
overall length = 0.275"
length of bushing = 0.25"
dia. of bushing = 0.165"
NOTE: MACHINE A 0.160" HOLE INTO YOUR PINION GEAR
FOR PRESS FIT WITH THE BUSHING
1 - twin AA battery case with switch
Fitting Parts Together
The milling machines you will be using are accurate to within a few
thousanths of an inch.
Generally, if you need a 0.250" dia. shaft to rotate in a 0.250" hole, draw a 0.250" hole in AutoCAD,
bring the drawing into MasterCAM, generate G-code and the machine will cut an exactly 0.250" hole.
Remember, small holes can be filed (or sanded) larger. You can't make a big hole smaller!
If you are using the pens for shafts the following fits may be
These are measured across the flats of the hex.
Again, the pens can be turned down the a 0.250" dia. if you need.
Mounting the Motor
The motor can be press fit into the mono-rail car body.
Machine a 1.100" dia. hole in your car body for a press fit.
Close-up of motor mount.
Designing Gear Trains
For the purposes of this project, we will be using a cycloid
gear tooth profile,
instead of the standard involute profile used in industry.
Since you have yet to take learn about gear trains in your course
a few definitions are in order.
The big gear is called a gear, the small gear is called a pinion.
Two gears should be sparated my the half of the sum of the two pitch circle diameters.
We have found that gears of a diametral pitch of 10 is about the
size possible with the MDF board.
(diametral pitch is the number of teeth on a gear per inch of its diameter)
All gears drawn to the same diametral pitch will mesh properly with each other.
This makes calculations easy, if you want a 36 tooth gear, its pitch circle diameter is 36/10=3.6".
Thus for the above figure, with a 36 tooth gear and a 11 tooth pinion, they should be separated by:
Gear pitch dia. 36 tooth /10 diametral pitch = 3.6"
Pinion pitch dia. 11 tooth /10 diametral pitch = 1.1"
Shaft to shaft distance = (3.6"/2) + (1.1"/2) = 2.35"
For the above gear train, each revolution of the 11 tooth pinion,
of course, move the 36 tooth gear through 11 teeth.
11/36 = .31 of a revolution
.31 x 360 degrees = 110 degrees
Drawing Gears Simplified
For this simplified cycloid gear profile;
the distance from the pitch circle to the top of the gear tooth is the addendum,
the distance from the pitch circle to the bottom of the gear tooth is the dedendum.
Begin by drawing the pitch circle.
(number of teeth/10 = circle diameter)
Now draw the the top tooth.
Notice that the center of the tooth radius is at the intersection between the tooth thickness and the pitch circle.
For simplicity, a radius of 0.0625" is used at the root of the tooth.
If you are drawing in AutoCAD, draw one tooth at the 12 o'clock position as shown.
Make sure that all the lines end and start at the same point.
Use the "PEDIT" command to transform all the individual lines into one long "Polyline".
Then, use the "ARRAY" command, in Polar mode to copy your teeth around the gear.
Finally, join all the gear teeth together with the "PEDIT" command.
The tables below list the appropriate values for gears and pinions
10 diametral pitch.
|Dedendum 6 tooth||0.151"|
|> 10 teeth||0.179"|
Devin's Rubber Band Car