Scanning a Quarter


This process was done with DeskCNC version on an IMService 121204 servo CNC router with .2 pitch lead screws in X and Y, .1 in Z.


One of the first things that a new probe user wants to do is to take a coin out of their pocket and scan in the face of the famous president.  This is not really a beginner’s task, but since it is something that most people want to do eventually, here is an example US Liberty quarter dollar coin, and the process used to scan it.


The scanning target is a well circulated, Washington head quarter.  It is attached to an aluminum plate with hot-melt glue.  This probe has a special 1mm ruby tip.  The scanned details of a quarter tend to wash out a bit when the standard 2mm ball tip is used.  This picture shows the scan nearly completed.



The TP-100 probe tip is initially set to a Z=0 at about .030 below the rim of the quarter.  A 1mm tip is .03937 inch in diameter and the shank is sized even smaller.  It is very important that care is used in setting up the probe to avoid running into immovable objects.  This size probe is very delicate and can be damaged beyond repair with a single fast movement into a clamp.


The scanning range is about 1 x 1 inch in X and Y directions.  Because we want to capture as much detail as possible, some special steps are to be taken for this scan.  The Max Steps per second for Z scanning is set to a very small value to permit the least oscillation in the Z-axis (with anti-backlash nuts installed) as the probe switch is opened and closed.  By moving this axis slowly in response to a contact detection, the mechanical delay of the switch re-seating will be accomplished in a very short distance.  The result of this setting is that the digital zigzag steps, from the step wise probing motion, will be significantly reduced and should be only just a small increment larger than the actual movement distance in the X direction.  This setting is on the Machine Setup – Tool Sensor/Probe tab.  Set the Max Z probing Velocity to 2500 SPS.  This is about 2.5 ipm on a 10-pitch screw with our Globe servomotors and belt reducer drive. 



If a test scan shows that the part Z seems to be drifting deeper into the surface of the target, and the resulting model is tilted at some angle above the actual position, it may be n3cessary to change the Invert Step Pulse setting to clean up the signal sent to the drivers.  This setting is on the Machine Velocities Tab of the Machine Setup dialogs.



Be sure to save the setting changes before proceeding.


Before we start the scan setup, we need to enter the probe tip diameter into the DeskCNC software.  Open the Settings-Options dialogs from the menu.  Navigate to the digitize tab and enter .03937 (1mm = 1/25.4 = .03937 inches)


Save your setting.


Now select the File-Digitize-3D probe Scan screen



Click the Reset button to release the controller from emstop.  Do NOT home the machine if requested when the reset button is clicked.


Use the job buttons just to the right of the green scan button to navigate to the smallest value in X-Y-and Z.  This will be at the front left in X and Y, and .030 below the rim of the quarter in Z.  Click the ALL button to zero all the coordinates.  Use the Jog drop down ( shown at .25) below to select jog speeds and distances to make probe setup easier.


After the coordinates are zeroed, click the Set X min and Set Y min buttons.  They should appear pressed in.


Jog the probe up about 1 inch in Z and about 1 inch over in X and Y.  Click the Set X-Y-Z max buttons.


Enter 15 ipm for the V (velocity) value.  Set the X-step over to 100 steps and Y to 275 steps.  These stepover values will calculate to .004 inches in X, which is the frequency at which the travel along the X-axis will be measured, and .010 stepover in Y, which is the Y distance between passes.  These values are using a 5 pitch lead screw on the X and Y- axis.  For a standard 10 pitch screw, use a value that yield about .004 and .010 ( this should be around 200 and 550 steps).


Once the settings are entered click the green Scan button and the probe will move back to the beginning of the scan a start to scan the part.  Press pause at any time to stop machine movement without interrupting the scan.


It is a good idea to test the probe early in the scan to make sure that it lifts up when the probe is contacted.  I usually test it twice during the first pass.  This can be seen in the screen shot below as two small zigs in the first line of the scan (the row nearest the front).


This scan collected 28764 points and took about 1˝ hours to complete.  This screen shot shows the scan nearly completed:



After the scan is completed, the probe will lift and you will be asked to enter a file name for the .stl file to be saved.


Here is a rendered view of the resulting .STL file in DeskCNC:



Here is the same file, opened and scaled in Vector Cad-Cam, with the background selected and turned blue.  The .010 stepover is quite visible, but the .004-inch distance along X is barely discernable.  If I wanted to create a replica medallion about 3 inches in diameter, I would next scan at a .004 x .004 pattern(or smaller) and see if that yielded a fine enough surface that when scaled up 3 times would still yield a smooth machinable surface detail.  This will of course increase the scan time and the file size.


Here is the fine-pitched scan with .004 x .004 steps, approx 95,000 points scanned.

The total time was about 3.5 hours.  The velocity was kicked up to about 35 ipm on the flat areas.

This is the original stl file, rendered in Vector Cad-Cam.