Reverse Engineering a Part with an Inexpensive Microscope

Every once in a while you need to make a mating part or a holder for something that already exists. The part in the photo above is a flange from a hammer drill that we are converting into a custom tool, reusing the drill as a motor and gearbox donor. The mating part is the outer wall of the gearbox, so it needs a cover with holes positioned precisely over the gear axes.

Accurately measuring those hole centers the traditional way would take a set of machinist buttons and a good amount of patience. You fit a button to each hole, indicate it, write down the coordinate, and repeat. It works, and it has worked for a century, but it is slow and every transcription is a chance to introduce an error.

In TouchDRO Plus you can use a touch probe and the "Indicate Workpiece" function instead. Besides setting an origin, the indicated points can be added to the sub-datum list, so the coordinates are captured as you go rather than written on a scrap of paper. That is a large improvement over buttons, but finding the center of each hole still means a lot of cranking and careful touching off.

There is an easier way. An inexpensive "coin" microscope, the kind sold for inspecting coins and circuit boards, can locate part features very precisely, and it costs a fraction of a decent hobby-grade touch probe. The unit shown below came from Amazon for less than $40. A probe worth owning starts around six times that.

The Setup

Before using the microscope, you will need to find a way to mount it to your milling machine head. The mount doesn't need to be especially rigid, since nothing will be pushing against it. What matters more is that the optical axis ends up parallel to the machine's Z axis travel.

The reason is depth of field. A microscope at this magnification holds focus over a very short range, so on a part with features at different heights you will be refocusing constantly. If the optical axis is tilted, every change in height moves the crosshairs across the part, and the error goes straight into your coordinates.

To check it, find a sharp feature and align the microscope's crosshairs on it. Lower the microscope as close to the feature as you can and note where the crosshairs sit. Then raise it as high as the focus will allow and check the alignment again. If the feature has moved in relation to the crosshairs, adjust the mount and try again.

Creating the Probe Profile

TouchDRO needs to know where the microscope is looking relative to the spindle, so the microscope gets a profile in the "Probe Library" just like a touch probe would. The profile stores the offset between the crosshairs and the spindle axis, and the application applies it to every coordinate you capture.

Rather than measuring that offset and typing it in, you can have TouchDRO read it directly. Set up a known point, park the crosshairs on it, and let the profile capture the current position.

  1. Indicate a corner of a vise jaw, a 1-2-3 block, or anything else with straight sides. You can use the "Indicate Workpiece" function to set the origin at that corner
  2. Move the microscope over the same corner and align the crosshairs with the two sides
  3. Without moving the table, open the "Add new profile" dialog in the "Probe Library"
  4. Set the type to "Camera / microscope"
  5. Enter a name, or keep the default if it suits you
  6. Select your preferred units
  7. Tap the sync buttons next to "X Offset" and "Y Offset". Each one will take the current reading for that axis
  8. Tap "Save"

The new profile will be created and can be selected wherever a probe is used. The individual fields are described on the Probe Library page.

Digitizing the Part

With the microscope mounted and its profile saved, the part itself can be measured. The "Indicate Workpiece" function sets the absolute origin first, and that origin serves as the datum for every coordinate that follows.