NOTE: See last week's Lab write-up to see the requirements for this week, which are the basis of your grade.

OBJECTIVES:

• Use a CNC mill to cut flat-faced and roller follower cams

BACKGROUND:
In the old days, before computers took over the world, machinists cut cams using plunge-cutting. As the name suggests, the machinist would plunge the milling cutter through the work piece, lift it up, move to the next points, plunge again--all away around the surface. Since you can only plunge at a finite number of points, "scallops" are left making the surface jagged.

Using a computer numerically controlled (CNC) mill, however, the mill cutter is in continuous contact with the work piece. The bit and/or table is then moved and controlled by an onboard computer. The cut path is still defined by a discrete number of points, but the continuous contact makes the cam smoother, not to mention reducing the manufacturing time. The computer reads x-y-z coordinates from a file and moves the cutter to that location. Because there are a finite number of points, the computer must interpolate between them to more smoothly move the cutter.

FILE FORMAT:
The CNC mill needs a specific format for the file it reads. The beginning of the file contains information about cutting speeds, feed rates, interpolation method and so on. The bulk of the file is the x-y-z coordinates describing the cutting path. You don't need to worry about the z-coordinate, however, since both of your cuts will only be 2-D (at a z-height of zero). Here is an example of a CNC cutting file:

The bold items are the items which you need to generate. Notice the format of the x-y coordinate specifications. There is an "x", followed by the x-coordinate value, followed by a "y", followed by the y-coordinate number--all without spaces between them.

The spindle on the CNC mill turns clockwise (looking from above at the x-y plane). For a better cam surface finish, it is better to cut the cam surface with "conventional milling" rather than "climb milling." Therefore, be sure to generate the cutter path to traverse around the origin in the counterclockwise direction, with the x-axis to the right and the y-axis up (looking from above). Note - Dr. Dave's lecture notes on cam design constraints and cutter path geometry provide equations that generate cam surface and cutter paths that travel clockwise around the cam. To change the direction to counterclockwise, just take the negative of either the x or y coordinates (e.g., remove the minus sign in the y_cam(θ) equation and change the minus sign to a plus sign in the y_cut(θ) equation).

GENERATING FORMAT:
Once you have parametric equations representing the x and y values of the cutting path (for example, x_cut(θ) and y_cut(θ)), making the CNC mill file is easy. The first step is to get 400 points in 360 degrees. To do this, specify a step size of 360/400 degrees and step θ from 0 to [360-360/400] degrees.

Eventually, you want an Excel file that looks something like this:

The first column is filled with the letter "x"; the second column contains the x-values; the third column is filled with the letter "y"; the last column contains the corresponding y-values. Once you have this, save it as a tab-delimited text file and reopen it in WordPad or Notepad. Replace (Edit | Replace) all of the tabs with nothing. In the "Find what:" field of the Replace dialog box, copy a tab from the file and paste it in this field. In the "Replace with:" don't put anything. Now you should have something like this:

Now open the CNC file template (as a text file in WordPad or Notepad). Copy your data and paste it below the line that says (what else?), ";Your data goes below this line." Also, copy your first line of data and paste it below the line that says, ";Your First Data Point." Don't change any of the other parameters! Save this as either "roller.nc" or "flat.nc", depending upon which cam cutter path you are storing.

Follow this procedure for both the roller follower and flat-faced follower cams. Bring your two NC files to class with you (as instructed by the TAs). Also bring plots showing the pitch curve and base circles for both cams, so the TA can review your designs and ensure they are within spec before attempted to cut them.

Be sure to sign up on the schedule posted by the TAs so the CNC mill will be available. You don't need to show up until your selected time slot.

After cutting your two cams, sand the surface to remove big burs (don't sand it so much it changes the shape--it doesn't have to be all that smooth). Bring your cams with you to class next week for testing.

NEXT WEEK:
We will be testing the cams with an actual roller follower and an actual flat-faced follower. You will measure the output of the followers and compare them to theoretical. See next week's lab for more information on testing your cams. Schedule your testing time with your TA (e.g., use the same schedule as for the cutting).

ADDITIONAL RESOURCES:

• procedure for manufacturing cam blanks (your TA should have this done ahead of time)
• CNC Mill setup and cutting instructions
• Chapter 8 of your Design of Machinery textbook

cam blank dimensions: