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Comment by colechristensen | Hacker News Reader

I've been acquiring tools to fix/measure/improve cheap CNC mill and 3d printer, the $100 dial gague I picked has 10 µm as its smallest increment, you can just barely see a 2µm variation. The force the measurement tool imparts on the part and its support structure is more than that.

The next steps are either carelessly blinding myself trying to build a laser interferometer or spending a ridiculous amount of money on more precise gagues.

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Sam’s lasers sells some refurbished HP laser interferometers on eBay (HP5517 head+the receivers). He has also worked with someone to supply open source software/counting electronics. My notes on the kit are here:

http://41j.com/blog/2017/02/hp-interferometer-notes/

You can often pick up the laser/receivers/optics quite cheap (few hundred USD). And then just use the software supplied by Sam. The Sam kit is tested which is nice (but it’s ~1000USD IIRC).

Interferometers as used on high end CNCs often.

There is a cool effect where a laser reflecting back into itself will cause interference inside the laser diode and affect the power required to drive it in a measurable way and with it you can extract a surprising amount of information (distance, rotation, angle, speed, etc.)

I was ... hasty ... with my first laser diode and burned it out in about 30 seconds. I like to tell myself the money wasted was spent on learning lessons. The second is coming in the mail.

The upshot with this method is that it can be extremely low cost and give you access to write code that interfaces directly with physics instead of somebody else's device.

The idea for hacking on these things comes from Ben Krasnow https://www.youtube.com/watch?v=MUdro-6u2Zg and several research papers.

This is known as a self-mixing interferometer (as is explained in the video)

https://en.m.wikipedia.org/wiki/Self-mixing_interferometry

The black matrix glass in an LCD is typically accurate to a few microns since it's made in a photolithography process. If you couple that with a microscope/camera (e.g. mounted where your tool usually goes) you can use that to calibrate your CNC mill across its work area with some machine vision software. This is probably going to be a lot cheaper and simpler than using interferometers and for any practical purpose should have better accuracy than what you can expect to ever get out of a regular CNC mill. I'm not sure if you can just buy those somewhere or you'd take a display apart or something along those lines...

Glass scales for DRO are relatively inexpensive. The Bridgeport EZ-Trak I bought at work uses them in the servo system. So getting static accuracy is probably not too difficult, but the dynamic accuracy is a whole different can of worms.

I can’t remember who wrote the white paper, but it described “rounding a square”, and measuring how much tangential material was left over at the flats, as a test of dynamic accuracy.

Even static accuracy is not so easy.

The scales will measure the axis position. But when you move the table over to the other end of travel, the shifting weight tilts the knee, and now your cuts that were supposed to be parallel are not. None of the scales measure this rotation.

Then, as the machine warms up, differential thermal expansion starts to twist things further...

My interests are more in general purpose tools for increasing accuracy, and perhaps using them to increase the accuracy of inaccurate tools through feedback systems.

That is, throw out chasing perfection of rigidity, dimension, and flatness of a mill (etc.) Instead keep adding measurement and feedback until the precision of the machine parts doesn't matter. (or, continue to amuse myself and annoy my partner by making messes and spending my disposable income trying)

It is, in my opinion, no coincidence that backlash and battiness both start with a B. Keeping up with (semi) predictable flexture would be... well, not a doddle as such, but... if you didn't also have to deal with backlash. Sure, there's a point where everything is, to a first approximation, made of rubber, but it's a lot easier to move that point by modifying or improving the tool than trying to trim the end of a diving board with a laser-assisted pole-mounted pruning saw.

My thinking goes, the end of the road is always "everything is rubber" when it comes to precision, so I might as well start off with that reality and see where it can take me.

On the other hand: wear. Even the best machines all experience wear all around and there is a lot of work keeping up with it or recovering from it. Keeping precision despite drift from wear and being able to trivially quantify it are really valuable.

The very best machines don't really ever wear out. The surfaces mate over the full area so even sliding bearings experience hydrodynamic contact. Or, they're built with fluid bearings to begin with.

(Assuming they're handled properly and kept clean, of course).

Having tried this, with some fancy measurement tools and a pretty custom and high-performing control systems, I've concluded that it's a route to madness. Fancy software and feedback turn out to be a very hard way to emulate large, straight pieces of cast iron, and there's no substitute for rigidity once you actually start cutting - vibration is a huge issue, and very tricky to actively damp. Essentially every machine tool that does have crazy accurate realtime feedback (diamond turning for optics, ultra precision milling, etc) starts with a rigid and long-term stable frame and adds the $$$ controls.

Granted, my switch over to "more rigid more better" may have gone a bit too far - I now own a 10ee (3200lbs), and am looking for a good jig borer (2400lbs+).

You're right that one can't compensate for lack of rigidity with software but you can have an inaccurate but rigid structure and compensate on top of that. It's pretty common practice to use some sort of software compensation to improve accuracy (at least in certain applications where it matters).

Metal milling machine need to be super rigid because of the cutting forces... You can build super accurate machines (e.g. with granite) that can't be used for milling but can place a tool with a (sub-)micron accuracy.

Even with metal you can take lighter cuts and trade off some rigidity for accuracy, but then it's gonna take much longer to get anything done...

An inaccurate but rigid structure, unfortunately, won't hold its calibration.

Go for the jig borer, and get them while you still can. It's what Professional Instruments does, and it's what I would do if I had space for one.

It can. Basically a repeatable machine can be made accurate by calibration. It doesn't have to be accurate to start with. If it's not repeatable (i.e. won't hold its calibration) then yeah, it won't ;) But accuracy != repeatability. I've worked on precision machinery (1um accuracy) and we used calibration techniques.

This is also my field. The issue is, the machine will be repeatible on the short term but not on the long term. It will look like it's behaving well during calibration and the next few months of operation.

But if the ways and mating surfaces aren't almost-perfectly straight and flat, they'll experience accelerated wear. (If you're using hydrostatic bearings, they won't work to begin with unless the surface is accurate). Then the calibration is gone. And that's just in the static case.

If your ballscrew has uneven pitch, is eccentric, or any number of other issues, you can calibrate it out. But now to move at a constant speed, your servo controller has to drive that inertia at a wobble, and everything shakes.

The machines I worked on had air bearings. No wear. Basically we're talking about something like an error of 10um over 2 meters. It's practically impossible, or cost prohibitive, to remove that error without calibration. The granite surface the air bearings ride on is locally flat but not perfectly accurate over its entire length.

I agree this setup is almost perfectly straight and flat. But it's still not accurate without the calibration.

Just to be clear, there's weren't milling machines...

What kinds of control systems did you use?

My ideas are in the realm of developing detailed physical models of machines and developing control systems out of them using precise measurement tools to both tune the model and act as feedback mechanisms. (I'm not talking about machine learning)

I don't think my living room floor would support three tons of machine tools unfortunately.

As msds was saying, if you're e.g. looking to cut metal you need to have a rigid structure. Otherwise the tool just bounces around given the cutting forces which really can't be compensated for. You entire structure deflects and vibrates under those forces. Accuracy is a different story, if you have a repeatable but not accurate system you can certainly fix the accuracy via calibration or feedback (feedback can be what gets you the repeatability).

Not only cutting forces; the machine will also warp and deflect as the axes move around and shift its weight. Even when all the sensors read that the tool position is correct, there's 6 degrees of freedom and a lot of coupled error from the stack up of the errors that each sensor doesn't measure. You can't really have accuracy if the machine isn't rigid, or if it has any play. And even if you do a calibration, if the ways aren't accurate to begin with then they'll wear and change.

You can certainly correct for all these errors if you are able to measure them. E.g. if your tool moves 1um down in position (x,y,z) due to the machine deflecting all you have to do is adjust z by 1um at that point. As long as there's no other issues and the only issue is the accuracy of placing that tool you're good. One example is precision ball screws, those typically come from the factory with data for compensating the screw. The screw isn't perfectly accurate on its own but if you apply the correction data you get better accuracy. The screw isn't less repeatable because of this, it doesn't wear down faster, it's just that it's absolute accuracy isn't perfect out of the factory so they just measure that and give you the corrections (let's say on the order of 5-10um over 1m). In usage the accuracy is also a function of temperature, so you may want to measure that and correct for it... or you may use a linear encoder or an interferometer ... point is, the screw does not have to be perfectly accurate, it just has to be repeatable and the remaining error can be corrected...

If you look, there's an excellent paper on an academic team that did a low cost construction project on a small mill for micromachining. They achieved very high accuracy with fairly ordinary components.

Here's one link to the project:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&c...

What is your goal with improving the cnc machine?

I’ve used a capacitive distance sensor to measure vibrations in the micrometers and nanometers. At that scale things get unintuitive pretty quickly. I doubt your motors and linear tables will be able to keep up with much precision beyond what you can measure with a cheapish digital caliper.

Eh.

When I'm following my own drives I tend to build something up from basics and improve it's ability to do X until it reaches a certain level of usefulness and I get bored and move on because my goal was building the thing not ever actually doing anything in particular with it. That section of the journey appeals to me.

Ultra-precise laser range finders scratch a particular itch. I so often have found myself wanting one and being unable to find something suitable to buy. Let's say, 1mm to 50µm precision at 1cm to 10m distances, in that ballpark.

What could I do with it?

* Characterize and quantify improvements in machine precision, most of what I do seems to be wild shots in the dark without some better measurement tools.

* Add feedback mechanisms to machines – work around imprecise parts and close the loop with cheap and super-precise measurement tools.

* Lasers.

The key features are programability and range+precision.

Cool!

Here’s a couple thoughts I have, forgive me if you already have these in mind.

Feedback and Controls: I’ve found that feedback suddenly made a lot more sense and seemed less mysterious once I understood the math behind it.

This is the textbook I used: Control System Engineering - Nise

And these videos were particularly helpful: https://www.youtube.com/user/ControlLectures

Regarding Precise Measurement:

It’s hard, something that might not look moving to the naked eye might be vibrating back and forth several uM. I’ve heard that lasers are subject to error from the atmospheres but I’ve never used them myself.

Good luck!

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