Like most hackers, I’ve run into a part that looks like it might do what I want, but the only documentation came from a company so thoroughly defunct their corporate office is now a nail salon and a Subway.
So, as any hacker who’s wandered through a discount store with a spare twenty, at one point I bought a Chinese caliper. Sure it measures wrong when the battery is low, the temperature has changed, if I’ve held it in my hand too long, the moon is out, etc. but it was only twenty dollars. Either way, how do I get accurate measurements out of it? Well, half-wizardry and telling yourself educated lies.
There are two golden rules to getting accurate measurements by telling lies. It may be obvious to some, but it took me quite a bit of suffering to arrive at them.
- Engineers are lazy. So lazy. Most things are going to be even numbers, common fractions, and if possible standard sizes. If sheets and screws come in 2 and 3mm then you bet you’re going to see a lot of 2mm and 3mm features. Also, even though the metric world is supposedly pure, you’re still going to see more 0.25 (1/4) mm measurements than you are .333333 (1/3) mm measurements. Because some small fractions are easier to think about than decimals.
- Your eyes lie. If it matters, measure it to be sure.
Stupid Caliper Tricks
Using a caliper should be straightforward. After all, it’s just two parts that slide against each other and a means of measuring how far it’s travelled. Nonetheless, between three measurement surfaces and a few tricks to use each one, it’s worth looking into.
The Flat Bit and the Wedgey Bit
The caliper has two types of surfaces on its jaws — a flat portion near the readout, and a wedgey region that gets thin toward the tips. Use the flat bit when you want an average measurement, and the wedgy bit when you want a point measurement.
For example, if you want to know how big to make a hole for a screw to fit through, use the flat bit. (But if you want to measure the minor diameter of the screw — the diameter inside the threads — use the wedges.) Usually, you’ll measure diameters with the flat bit. Even after measuring, though, you’ll want to use your head. Screws are usually a bit smaller than the size written on it, so an M3 screw will read 2.95 mm on the caliper. The extra play will make it pass easily through a 3 mm hole.
If you want to measure a delicate or fragile material, you also use the flat bit. The caliper should be made out of fairly hard steel. (It gets an extra point on the hardness scale for every ten dollars you spent on it.) So if you try to measure something delicate it will damage or indent the surface. This is especially true for plastic parts. For plastic parts the wedge applies a point load and deforms the surface, throwing off your measurement.
Don’t drag your calipers across surfaces when you’re done measuring. This is a pet peeve of mine. I had a coworker who would use his 250 dollar calipers to measure the width of a circuit board, and then draaaag it off the edge. The wedge is a thin, precision, hardened surface. You’re either damaging the calipers or the thing you’re measuring. Open your calipers, then remove them.
The Caliper Has Math Functions Built In
Using the zero button you can do simple addition and subtraction. Imagine that you wanted to measure the distance between the center of two holes. You could measure the distance between the outsides of the holes and then subtract off each hole’s radius to get the center distance. Or you can use the zero function when the two holes are the same size. Measure the diameter of a hole, then click zero. Now measure the outside to outside points of the holes. That will be your center to center distance — the diameter (two radii) is automatically subtracted away. Neat!
The zero function is also useful when trying to decide if something will fit into something else. What if you want to know if a shaft will fit into a hole? You could measure one, then measure another and subtract the difference to get the clearance. With calipers? Measure the shaft, click zero, then measure the hole. The value on the screen is the clearance.
The same trick is used to check the difference between two similar measurements on different parts. Measure one, press zero, and then measure the other. I also use this feature to measure how much printer filament has been extruded with a tool I made.
A Example Application
With these basics and a few tricks, we can reverse engineer a thing accurately. Our victim is the new cold-end from E3D. The cold-end on my printer broke, and I needed a new one. Since I was thoroughly in an egg-chicken situation, I hoped to get someone to print me a new one at MRRF. I told them about my plight at MRRF, and they agreed to give me one of their prototypes if I designed a sled for the Prusa i2.
I started off by figuring out just exactly which measurements I needed to take: where the case and the nozzle are in relation to the stepper motor. This positions the nozzle and gives me rough outlines of the cold-end so nothing collides into anything. The rest of the features of the cold-end and their locations are not needed. You don’t need to model the whole part — just the bits that impact the part you’re building.
It looks like we have a mount for a stepper motor on the back. Since all steppers are built to a standard, we can apply Rule 1 right away. A quick search shows us the pattern for a NEMA 17 motor is a square hole spacing of 3mm holes at 31mm with a 27mm hole in the middle. We haven’t even taken the calipers out yet.
Okay, so let’s get the outside dimensions of the cold end. I’m getting 43.97 and 46.44. I’m going to translate that to 44mm and 46.5 via Rule 1. Likewise, I’m getting 24.67 for the depth, so 25mm it is.
Now, some of the more experienced of you will say, but Gerrit, what about draft angles? The bottom of the part is a whole 0.75mm smaller than the top. Well, again, Rule 1, but on myself this time. It really doesn’t matter. I’m only interested in this measurement to know when the part will hit something, so the biggest dimension with do.
Now the next thing is an application of Rule 2. It looks like the shaft is sitting right in the middle of the box, which means that the stepper pattern should be right in the middle of the box. However, with a second glance it becomes apparent that the centricity is an optical illusion caused by that offset screw in the bottom right corner. It’s an important dimension, so I’m going to measure it to be sure that it’s centered.
To start, I picked the easiest corner to measure. I can assume via Rule 1 that nothing is rotated and that the stepper pattern is going to be perfectly square with everything. Even if there was some unknown advantage to rotating a stepper hole pattern, CAD software really hates that sort of thing. If I find the offset of one hole, the rest can be determined. So I simply measure the distance from the edge to the inside of a hole, and then add one half of the hole diameter to it. If the math were harder, I could have done this using the zero-offset trick.
Last, we have to get the offset from the stepper shaft to the center of the nozzle assembly that slides into the cold end. There’s no good way to get this measurement, but by combining all the skills up till now it’s fairly easy to get a good guess. Though if forced to be honest, I would throw an error of +/-1 mm on this.
So, I’ll measure the widest point of the slot. I’ll cut that measurement in half and set my caliper to that. Then I’ll set it against the side of the slot closest to the shaft. Now I’ll mentally (or with a crayon) mark the point where the other edge of the caliper sits. That’s the center of the slot. Then I’ll use a piece of paper to draw a line from the middle of the shaft to the edge I’m measuring. Now I have one measurement! 11mm.
To get the offset from the back of the cold-end to the center of the nozzle, I can apply Rule 2. I know by the documentation for the nozzle that fits into this that it’s http://wiki.e3d-online.com/wiki/E3D-v6_Documentation going to be a hole of 16mm or bigger for it to work. So, on a hunch, I look and see if the arc is 8mm tall, making it a 16mm circle. If that’s the case then I can measure from the lowest point of the arc to the back and then just add 8mm to get the offset. It ends up being 13mm from the back. Which is, just slightly forward of the center of the assembly. Thanks Rule 2!
While we’re at it, let’s get the depth of the hole that the nozzle fits into. I didn’t mention the stick thing that comes out the back of the caliper because it usually doesn’t work, but that’s what it’s there for. A quick measurement to the mating surface gives us 12.1, or 12 mm. And there we have it.
After a bit of CAD, we have a model that for all practical purposes lets us design this part into an assembly. You can see it/download it here. After a bit more work, I used it to design the promised sled. I had some other issues with the first iteration of the design, but as far as the clearances for and locations determined by the cold-end, I had no problems at all. A few iterations later, I had a final design that works pretty well!
When I got started with this sort of stuff I would always agonize over getting the model exactly right. I would make a lot of paper drawings and keep tables of measurements. As I went on I realized that a bit of cleverness and familiarity with the tools are just as good as a 3D scanner for practical purposes, and certainly a lot quicker. I wouldn’t recommend using these tricks to do a quality inspection at your job, but to get a good model with the least agony it works pretty well. Do you all have measurement tricks to share?