One of the new functions in TouchDRO Plus is the Reference Image overlay in the Graphical View display. In essence, it allows you to set an arbitrary image as the background for a workspace and calibrate it to the machine coordinates. As a result, as you move the spindle/cursor, the features in the image will translate 1:1 to the physical part. There are a few different uses for this functionality. In this post, I will go over the three ways I've used the Image Overlay [so far], and then explain how to set it up on your tablet.
UPDATE Jul 7, 2024 - This post has been updated with new screenshots and some corrections in the text to avoid confusion, since the final version of the feature ended up a bit different than what I captured in the original post.
Use Cases
Side note:at the time of this writing TouchDRO can use BMP, PNG, and JPEG image files as a workspace background. I am working on adding support for vector formats, but it won't be ready for a while (parsing vector files is pretty complicated).
Visual Reference
The simplest use case for the reference image is to set it as a visual reference. If you design your parts in some sort of CAD software, you can export the drawing [with or without dimensions] to a file and import it into TouchDRO. Once you anchor the image to the physical workpiece coordinates, you can use TouchDRO as you normally would (i.e. enter sub-datum coordinates, etc.). This way you get all the accuracy you would get from the DRO, but Having the CAD drawing in the background would help you avoid making silly mistakes, like drilling a wrong hole size, etc.
Virtual Layout
A more interesting approach is to use a CAD drawing loaded into TouchDRO as a virtual layout directly. In other words, instead of marking the workpiece with a layout fluid and scribed lines, you can use the TouchDRO display to position the spindle. This can save you some time from not having to scribe the layout on the part. More importantly, you can get better positioning accuracy, since the screen can be zoomed in, and the lines won't get scratched off or obscured by the swarf. Not to mention that you won't need to stick your head into the spindle while trying to watch the cut.
Recreating an Existing Part
There are times when you need to machine something that mates with an existing part. If the part is irregular, or there is no easy way to measure the distance between the holes, it might be hard to drill the holes on the mating part accurately. Fortunately, the tablet that you are using to run TouchDRO likely has a camera that you can use to take a photo of the part.
The photo will likely be skewed, so you can't use it as is, but once you set the coordinates of four points in the photo, TouchDRO will automatically correct the distortion. You can use the corrected image to position the spindle over the relevant holes, and make an accurate mating component.
Side note: there is one important nuance that you need to be aware of - almost all general purpose camera lenses introduce some amount of pincushion and barrel distortion. You can read more about those on this Wikipedia article. The good news is that the cameras that are used in name brand tablets are very well corrected in the software, and these distortions are completely eliminated.
Pocket, Slot, or Contour Milling
Finally, since the Graphical Display in TouchDRO can show the actual cutter diameter and position in relation to the reference image, you can use this function to machine pockets, slots, or complex contours with a reasonable degree of accuracy. You can use a CAD drawing, or a photo of an existing part for the background. To show the cutter radius, you can use the Cutter Radius Compensation function with no offset. The display in TouchDRO will look similar to the screenshot below.
Setting Reference Image Background
Now let's walk through the process of setting a reference background. For the sake of this demonstration, let's say that I need to enlarge some holes in the machinists block shown in the photo below. I admit that this is a rather contrived example, but it will be easier to explain things using a simple part.
There is a way to open the camera app directly from TouchDRO using the camera icon in the Graphical View display. I almost never use it, though. Instead, I'm going to exit TouchDRO and open the camera app manually. Normally, I like to give myself the best chance to get an accurate layout, and try to get the part to be as square as possible in the photo. For the sake of this example, I'm going to pretend that I've had a few cold ones already, so my photo looks like this:
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| This image is not ideal. It has too much perspective distortion |
Worry not, dear reader, we can fix this image. To do so, I will need to locate four points on the part, and link them to their positions in the photo. Since this part is rectangular, I can simply measure it and use the size to set the corner coordinates. This is not always possible, especially if the part is irregular, so I'm going to do this the hard way. This tooling block has a grid of small dots. I suspect the maker marked the holes locations, and then changed his/her mind. Using a wiggler center finder, I will locate four of these dots in the opposite corners and set them as my references.
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| Using a center finder to indicate the first dot on the part |
With the part held in the vise, I will reopen TouchDRO, and switch to the Graphical View display. Although it's not strictly necessary, I like to set the workspace's datum to the lower-left corner of the part. In this case, it will be the lower-left-most dot. To do so, I will locate it as precisely as I can using the center finder, and then press and hold the Set Zero button in the right panel.
Now that I set the zero point, I can tap the "Open File" button (second button in the upper left corner), navigate to the photos directory, and open the photo I took a few minutes ago. After I confirm the file selection, TouchDRO will open the "Setup Image" dialog that we will use to calibrate the image.
First, I like to tweak the photo. It's a matter of personal preference, but I usually convert the photo to grayscale, and invert the colors. With the dark image, it's easier to see the crosshairs and other on-screen graphics.
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| The first point will be set as zero |
We can now start setting the reference points. I haven't moved the spindle since setting the workspace datum, so I'm just going to tap the X and Y sync buttons in the "Reference 1" section. Next, we need to move the image around (using a stylus or a finger) until the dot we located is under the crosshairs. Accuracy matters here, so I like to zoom in (using the pinch-zoom gesture) first, drag the image as close as I can, and then use the arrow buttons to position it as precisely as possible. When done, I'll tap the checkbox in the "Reference 1" section to latch in the coordinate.
Next, we need to repeat this for the lower-right, upper-right, and upper left corners. That is:
- Locate the physical location of a dot that is closest to the relevant corner
- Tap the X and Y "Sync" buttons
- Move the photo until the dot is under the crosshairs
- Tap the checkbox button to latch the value
With the four corner references set, we can check what the image will look like by tapping the "Preview" button in the lower-left corner of the dialog. The screenshot below shows the result I got.
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| Image distortion is corrected after four reference points are located |
The image looks right to me, so I'm going to tap "Apply" to set it as the background and go back to the Graphical View display.
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| Zooming in on the first point improves accuracy |
You'll notice that the "Camera" and "Open Image" icons are gone, and there is now a new icon in the upper right corner. Tapping this icon button will cycle between hiding the image and showing it at 50% and 100% opacity. To completely clear the image, long-press the button.
This is all there is to it. Now we can center the crosshairs over any of the holes [by moving the spindle] and perform whatever modification we wish.
So, What About Accuracy?
Using a CAD drawing or a photo as a virtual layout is nice, but you might wonder what sort of accuracy you can achieve. To answer this question, we need to consider the things that affect the accuracy, especially when using an ad-hock photo.
First of all, as I briefly mentioned earlier, almost every lens will introduce some amount of optical distortion. There are very robust [and well understood] software algorithms that can completely remove those distortions, either in the camera module, or in post-processing. Tablets made by the mainstream manufacturers, such as Samsung, Lenovo, Amazon, etc. are usually very well corrected. Non-name Chinese tablets are hit-or-miss. Some are good, and some have pretty horrendous distortions. An easy way to check how your tablet's camera behaves is to take a photo of a chess board (you can print a standard calibration grid from the internet as well) and check how straight the lines are.
Second concern is the pixel resolution of a photo. A 12 megapixel image is usually 3000x4000 pixels. If you take a photo of a 3x4" part (and fill most of the frame with it), after calibration you will have a bit less than 1000 pixels per inch. In other words, each pixel will represent a bit over 0.001" of spindle travel. If the part is 6"x8", each pixel will map to a bit over 0.002", and so on. In practice, you can often split the image pixels on screen by positioning the cross hairs in the middle of the line, but whether or not this is acceptable depends on the required tolerances and your comfort level.
Finally, you need to keep in mind that a reference photo works well for flat parts, since all of the features are on the same plane. If you take a photo of a 3-dimensional part with multiple planes, there is no way for TouchDRO to correct perspective distortion.
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