Limit/position switches

I want to fit some position switches to my mill. I'd like the switching action to be as accurately repeatable as possible and/or reasonable. Any
ideas?
thx
Peter Fairbrother
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On Sat, 23 May 2020 22:16:36 +0100, Peter Fairbrother

Back in the 80s I built my first CNC positioning table. It was a simple X Y table that I bolted on to my Bridgeport table. I used stepper motors on it and snap action Micro switches for homing. I used the Micro brand but I think any high quality snap action switch would just as well. They repeated reliably within .0005" total. They were so reliable that I stopped worrying about checking position all the time and just made parts. The biggest problem I had was the steppers themselves because they would lose position if I made them move too fast. Now I just use servos. In any case, try a snap action switch, I'm sure you will be pleased. Eric
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On 23/05/2020 23:55, snipped-for-privacy@whidbey.com wrote:

I was wondering about the switches used to locate objects to be machined - do you know anything about them?
0.0005 is good, but I'd like a micron or less if possible. This is a small envelope mill (converted/beefed-up BCA) used to make small parts.
Peter Fairbrother
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On Sun, 24 May 2020 18:50:12 +0100, Peter Fairbrother

It sounds like you need some sort of digital encoder. You can buy linear encoders with .0002" resolution, which is half a micron, for pretty cheap on ebay. Then use the encoder as the switch. The encoders are pretty standard and are easy ti interface to. For info try https://www.usdigital.com/products I use their products. They sell chips for pretty cheap that interface with digital encoders and these chips can be used for getting a signal that is very easy to use. For example, I have an encoder that is driven by a shaft. I use the signal from that encoder, conditioned by a chip, to provide the signals for a servo amp that in turn drives a servo motor. If you are looking for a mechanical switch that has 1 micron or less resolution then I can't help. If you use linear encoders then you can also use the encoder for a position display. You could even hack a typical digital readout to get your position signal using one of the chips from US Digital. And I'm sure these chips are available from other vendors closer to your neck of the woods. I use US Digital because they are close to me and shipping is fast and cheap. Eric Eric
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On 24/05/2020 19:42, snipped-for-privacy@whidbey.com wrote:

Eric,
Maybe you left out a 0 but 5 micron is 0.0002", 0.5 micron is 0.00002" .

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wrote:

Yup, I was absolutely wrong about the Micron. Dunno what I was thinking. A micron is almost exactly 40 millionths of an inch. Thanks, Eric
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On 25/05/2020 16:38, snipped-for-privacy@whidbey.com wrote:

Linear encoders are available reasonably cheaply with 1 micron readout - but usually they are not absolute encoders, so they would still need a zero/position switch.
Peter Fairbrother
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On Tue, 26 May 2020 00:41:23 +0100, Peter Fairbrother

Digital encoders often have a "Z" line that is used for homing. All of my rotary encoders do and at least one of my linear scales does. If you were to look at the lines on a linear scale you would see a whole bunch of lines in a row. Every so often will either be longer lines or separate lines above the row. These lines are used for a homing or marker pulse. So what you do is use a mechanical switch that is close to the marker line and when the switch is triggered you move slowly to the marker or Z line. The read head will have 3 LED and photodetector pairs. These are A, B and Z. The A and B pairs are staggered, so either A will lead B or B will lead A, depending on the direction of travel. This allows the scale to be read in quadtrature, so you get a resolution that is 4 times the native resolution of the scale. The read head will have A, B and Z wires along with ground and V+, usually 5 volts. So if a read head that comes with the linear scale has 5 wires it is a pretty good bet the scale has the Z lines on it. Eric
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On 26/05/2020 00:41, Peter Fairbrother wrote:




I wonder how accurate they actually are. I have a Sylvac digital DTI which indicates to 1 micron but the calibration chart shows the actual deviation along the 25mm travel and that varies from 0 micron up to 2 micron and with a quoted sensor error of 2 micron it says maximum error maybe 4 micron (sensor + scale error).
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wrote:




Well, there is accuracy and there is resolution. But mostly it doesn't matter in machine tools as the error is over a large distance and you just care about the resolution over a small amount of travel. So a part is made, measured, and then the machine cuts are adjusted to get the desired part dimensions. Over time the machine warms up, cutters get dull, tool pressure goes up, and the machine cuts are adjusted again. Even when it comes to inspection indicators are rarely used over their full travel to make absolute measurements but are instead used as comparators. Even excellent screw micrometers can have very high resolution but a relatively high error over the full travel distance. Same thing with machine tool ballscrews. To get better accuracy with machine tool positioning the errors in the leadscrews are mapped using standards, gage blocks for example, and the errors are programmed into the machine tool parameters. Then the machine tool takes these errors into account when positioning. Sometimes it is just the backlash of the leadscrews that is measured and sometimes it is the actual positioning error at several places or even over the complete travel. Eric
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On 26/05/2020 23:51, snipped-for-privacy@whidbey.com wrote:




Eric,
I understand those items and the calibration chart gives the +- error along the range and is mainly 0 or +-1 micron, it's only 2 micron at 1 point, so if I want to do some really accurate stuff I can chose my range and have workshop quality (grade 2? UK made) slip gauges to check if I want that level of accuracy. Temperature can be critical even in less accurate items as the heat resulting from machining can throw out the readings by some 0.001", so if important I go away and leave things to stabilise before taking final cuts. One of my neighbours has a home made CNC router with a 3' x 3' bed and he quoted some accuracy and I had to correct him as the machine is in a shed and not temperature controlled so a mainly aluminium machine will move but he realises this and for what he does it isn't important, others he knows don't and assume the machine is as accurate as the ball screws. Ball screws can vary, a company I used to work for made force and torque testing kit and the ball screws were made in Japan IIRC but later moved to a new plant in Europe and they found the original offerings from that new plant weren't to the same standard of accuracy even though they were supposed to be.
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On 24/05/2020 18:50, Peter Fairbrother wrote:

Yes I do, for example the probes produced by Renishaw.
You do not rely on the movement of a switch toggle because that is unpredictable as to when it will flip over because temperature effects affect the springiness of the spring and there can be variation in the distance you have to move the carriage to make the switch flip over.
So, that is what you DON'T do.
What you have are two contacts, possibly brass, that are in contact with each other, one fixed, and one movable.
The movable contact is held under very light pressure just enough to maintain contact.
What you arrange is for your carriage to come along and just lift the movable contact OFF the fixed contact, and that point of disconnect, no matter how diminutive the movement, is your signal.
This is something that you can make for yourself.
Just to summarise, you are looking for the break in electrical contact to indicate that your carriage has arrived at your fiducial point, and the point of that breakage is reproducible to a gnat's cock of distance.
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On 24/05/2020 21:27, Gareth Evans wrote:

Actually Peter, a chap I know, not a hundred miles away from you in The People's Republic Of Trowbridge, used this very scheme for the homing positions when he developed his own multi-colour 3D printer with an 18" cube building capability, although he used 4 (NSEW) fixed contacts and one movable contact with the ball end being the actual contact so a slight movement of the probe would result in at least one of the contacts breaking. That does mean 4 sets of electrickery because you can't just wire them all together because just one contact breaking would be masked by the continuing contact of the other three.
With these sorts of detection, the movement required at the sensor is just the distance of one molecule to achieve a break.
Remember this? :-) ...
Two little atoms Walking home from school Bumped into each other And made a molecule.
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On Sun, 24 May 2020 18:50:12 +0100, Peter Fairbrother

Greetings Peter, I was way off in my dimensions, as David pointed out. I don't know what I was thinking. Anyway, I can measure accurately to 1/2 micron with my 20 millionths of an inch per division indicators. I have had to make parts that were round within 30 millionths of an inch so that's why I have the inspection equipment to check this kind of stuff. And temperature REALLY changes dimensions at that resolution. Even touching a part with your fingers can make the part measure out of round because of uneven heating. Is your machine really accurate enough to want sub micron resoultion? In any case a digital encoder would still probably be the cheapest way for you to get sub micron positioning. The way CNC machines determine position using digital encoders is to use a switch for coarse position feedback and then the machine moves slowly until it sees the index on the encoder. You will need to do the same if you don't want to constantly overshoot your position marker. Just how small are the parts you are making? And how accurate do they need to be made? Cheers, Eric
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On 25/05/2020 18:19, snipped-for-privacy@whidbey.com wrote: [...]

Us metric folks use "hundredth's" almost exclusively as the measurement division while hand machining - that is 1/100th of a millimeter or 10 microns. This division is approximately equivalent to the imperial "few tenths".
We are taught to estimate the dial reading to a tenth of a division, ie one micron, though we would only do that on the most accurate parts.
My analogue micrometers read to a hundredth and can be estimated to a micron or two, my digital mikes read to a micron. My (small, A4 size) granite surface table is accurate to 1/4 micron. Wanting a part accuracy of a few microns, requiring a basic mill reference accurate to +/- 1 micron or better is not really a big deal.
(I don't do submicron work - as yet anyway)
For this mill a large part would be 80mm long and 50mm diameter. 10mm to 30mm part dimensions would be more typical.
Mill is a converted BCA with hella-expensive 2mm pitch ball screws and chunky overpowered 200 step motors using 16 microsteps/step to give 0.625 micron resolution, and a 30,000 rpm 600W ER8 fluid damped spindle to reduce cutting forces.
XY repeatability (without zeroing) is within 2 microns using LinuxCNC motion control software. With present zeroing (standard microswitches) repeatability gets considerably worse.
Peter

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