Simple and accurate home

.

I am genuinely bewidered, Nick

What do you mean by that recommendation ?

... A google search was't much help

MichaelG.

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http://lmgtfy.com/?q=shade+tree+engineering

Edited By Michael Gilligan on 31/12/2018 07:32:18

Indeed, but you don't need a home switch to do that, you can zero the machine coords manually, for example at a marked position at the tailstock end of the bed. The controller does need to know where the end of the material in the chuck is so it knows where to start machining, and generally this depends on exactly where you put it, so the Z offset needs to be set anew for each workpiece unless you can arrange a hard stop. For that I have a touch off system. This slowly moves the tool towards the bar end until it just touches, stops quickly, zeroes the Z work coordinate, then withdraws 20 mm, leaving the Z DRO reading +20. All the Mach 3 wizards, and my own Gcode, assume the bar end is at zero.

Maybe in our climate shade tree engineering is best done in an armchair by the fire?

I don't think Nick is casting aspersions on Butch Lambert and all the other good folk down at Shadetree Engineering and Accuracy!

This outfit is based in Terrell, Texas (pop 17239) which was home to the No. 1 British Flying Training School during WW2. However, Nick's Shade Tree is more likely to be found in the novel 'The No 1 Ladies Detective Agency', featuring Mme Precious Ramotswe and set in Botswana. The trope is that african gentlemen laze under the Shade Tree during the heat of of the day, wasting their time talking nonsense, while women do all the work. In the UK we use Armchairs for the same purpose...

Dave

I have a repositionable z axis home/limit switch, it clamps on the ways. Having collets, a 4 jaw chuck and two 3 jaw chucks that are all different distances from the headstock makes use of manual data input (mdi) risky if you make a mistake with z axis movements. Also using as a home switch before work comences allows everything to be switched off when I want to stop (always saving fixture settings on exiting Mach3) . Re-homing next time I power everything up means the z axis is set without touching off the workpiece. I also have x axis homing for the same cold start reasons. Since I sometimes have to move the tool post from one tee slot to another calibrated tool holders and tool tables are not much use to me.

Martin C

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Thanks Dave ... perhaps you could now tell me the relevance of Nick's closing comment:

... it's not funny, clever or useful.

MichaelG.

[ still bewildered ]

Why silver contacts? Silver is not suited to precision low current switching. It tarnishes for s start. It's not suited to any kind of reliable low level (below 100mA 10V) switching. Commercial index switches are available such as the the Omron D5A but they are not cheap.

I'm not an armchair or shade tree engineer, I'm a chartered engineer who has designed and built motion control systems with 1 micron resolution and 10 micron repeatability. This was confirmed using laser interferometry with nanometer scale resolution.

Robert G8RPI.

Edited By Robert Atkinson 2 on 31/12/2018 11:39:32

Kuka robots have relied on a mechanical reference to calibrate their robots for many years. The reference is simply a V groove and V shaped plunger, at its simplest a dial gauge is fitted and the centre of the V found by jogging the axis to find the null point. An electronic gauge is also available that interfaces to the control system and the robot will seek the null point automatically. This routine is only required after repair or replacement of any element of the axis drivetrain. Considering that a robot arm can be 3 to 4 meters from the point of reference and the arm is accurate to a fraction of a millimeter it has to be pretty effective.

Mike

Coming in late to this;

I think John Haine did a great job in making a simple, accurate home switch.

I understand the potential issues, but, I assume John does as well. What's wrong with making things rather than buying them?

Kudos to John Haine.

Here's one I did for a Unimat SL; since given on to another club member for his enjoyment:

(I expect to do the same for a little Sherline CNC lathe I've picked up)

Edited By John Alexander Stewart on 31/12/2018 14:40:02

Hi All

John Haine has used a simple contact limit switch method used by many DIY CNC router builders to sense the tool height by placing an insulated contact plate on the bed and lowering the tool until it contacts the plate that works well. If the contact can be kept clear of contamination it will work accurately.

Robert Atkinson mentioned that break contacts are more reliable. I agree and a while back made a small experiment to do research on wire alignment using direct contact (on break of contact). Wire alignment is still used to align large machines. Google Cern wire alignment.

**LINK**

Three images below:

Accuracy, repeatability and low cost were the main objectives. It also had to be able to apply a force sufficient move a small machine slide not just for contact testing.

The device comprises two 25mm sq BMS levers pivoting on short spring steel flexures, a flexure also drives the plunger located in the reamed round tube. the lever ratio is 10:1 The drive is a A 400 count per rev half stepped step motor driving a 1.25mm screw via a timing 4:1 ratio timing belt. Note the substantial spring that forces the levers apart and the way the motor mount is attached to the semicircular pivot. this maintains correct alignment as the levers move. yes the timing belt is too long but it was in my stock!

The levers provide 10:1 primary reduction closest to the measuring point, with negligible backlash. I used this mechanical system hand driven, no step motor or timing belt on a grinder head allowing very accurate wheel positioning to 0.0001” and it was repeatable.

The control system (Created by my mate Leo) is a baby Arduino, a stepper driver and a small touch panel. Many thanks due. The parts were remarkably inexpensive.

A number of tests were done; the test shown was a piece of stretched high tensile carbon steel piano wire that contacted a stainless steel pin. Not ideal, gold would no doubt be better but I wanted to see what simpler materials could do. The wire and contact pin was wiped clean. Apologies for the rough setup but it did the job and worked.

The control system was programmed to move the pin slowly into the stretched wire at various speeds then back off until contact was lost. The process was repeated thousands of times.

Overall it behaved as planned, the dial indicator needle constantly moving to the same position at each measurement. around 3 tenth's accuracy with a few outliers caused by mechanical sticking I plan to use this system in the future, a few improvements are planned .

Regards
John

Presumably, those working to micron accuracy, or better, are doing so in a an environment where temperature and humidity are closely controlled? This is what is required if we are to maintain, consistently, Calibration Room standards of accuracy.

Dare I suggest that for most of our hobby work, this is neither obtainable; nor necessary?

10 micron repeatability is pretty good, you are talking less than 0.0005" . Merely holding the part and/or the measuring instrument will probably introduce an inaccuracy of that size, within a short time. Your pocket rule may well be engraved "At 20'C" Human hair varies between 0.003" and 0.007" depending on colour and section.

Howard

[This posting has been removed]

The confusion between the US and the metric world in relation to what measure a Micron describes is confusing.

Being part of the metric world I have to teach myself to use the full metric "Micrometre" not the abbreviation Micron on documentation to make it clear what measurement system is being used.

Some parts of the US think a Micron is is a ten thousandth of an inch.

A Micrometre is actually about 0.4 approx of a ten thousandth of an inch. Less than half as big

Or 0.0000394"

Robert, I hesitate to cross swords with a practical and qualified expert, but in respect of silver contacts, the tarnish (Usually sulphide) is conductive, as is the oxide, which can be used to make batteries. In VHF/UHF circuitry, items are silver plated to aid conductivity through the 'skin effect'. Furthermore we are advised not to polish these surfaces because the conductivity gets worse.

Geoff, G8BMI

The computer world and high end radio controls use gold contacts.

I don't think there is a perfect contact material, you always have to consider what the switch is for.

Gold is reliable because it doesn't tarnish. Despite high cost, it's economic because it can be spread very thinly. But it's inferior to silver when low-contact resistance is essential, and it melts. In pure form it wears quickly and is sticky. You don't want gold contacts when switching power - sparks and arcs soon destroy it, though there are several gold based alloys with Copper, Silver, Tin, Cadmium, Nickel or Iron that improve performance.

Silver is good because it's tarnish resistant and because the tarnish is often conductive. It has the lowest possible resistance. Tends to be used for signal switching, quite often in combination with gold.

Platinum is a good all rounder (lowish resistance, good heat performance and doesn't tarnish) but rarely used because of the cost. Palladium is nearly as good as Platinum, and more common because it's cheaper.

Brass and other Copper Alloys offer good conductivity and are cheap. Corrosion is a problem but arranging the contacts so that they wipe when the switch is operated reduce the problem considerably - good for switching power. Power switches are likely to be spring loaded to minimise opening & closing times as a way of reducing arcing.

Morse keys are interesting because they straddle various requirements. Modern keys control the transmitter by switching low voltages and they favour contacts made to make that reliable - silver, gold etc. I have WW2 morse key that's designed to switch valve transmitters at rather high power levels - it's rated 400V at 13A. The contacts are made of tungsten. A WW1 era morse key working a spark transmitter might have been asked to switch even larger amounts of power - some of them have solid silver contacts about an inch across mounted on hefty finned blocks of copper to keep them cool.

I think John's choice of materials for his purpose is fine. Simple to make and adjust, easy to maintain, and reliable enough for what he needs.

Dave

Maybe I could bring this thread to a close? To sum up:

1. I wanted a HOME switch, not a limit switch. This is only used once per machining session, at the start, to set the X (cross-slide) axis machine coordinates to a known position that is defined as the zero. You then add a tool offset to that to get the actual turning radius which is the distance between the tool tip and the lathe centre line. All my tools are in QC holders that clamp back in the same position with pretty good repeatability. So once I zero the lathe at the start, I can turn pretty close directly to a set diameter - good enough for most purposes - once the right tool is dialled in. Actually having homed, at the next session the home is very close - usually within 0.02mm - which is a function of the small movements that steppers make when the drives are powered up/down.
   
2. When the Xslide is driving towards the switch, it's creeping at about 2mm/minute IIRC, and there is a good 5mm of overtravel, which is limited actually by the slide bottoming on the saddle not the switch. Yes the stepper would stall but so what? So far the contact closure has never failed to stop the movement.
   
3. The contact is between the domed end of a stainless steel screw and a small beryllium copper plate soldered to a piece of PCB. Both of these are reasonably corrosion resistant. It's an easy job to give the contacts a quick wipe to remove oil etc.
   
4. I have soft limits set at the extremes of travel and manually home Z with the carriage about in the middle of the bed. So far I haven't crashed the tool into a chuck, but if you do have Z limit switches it doesn't necessarily help because usually you want to be able to turn nearly to the chuck jaws anyway, and the poition of those changes depending on which chuck you fit. I have a controller macro with a touch off sensor that can auto-zero Z on the end of the stock, I don't rely on homing Z for longitudinal dimensions.
   
5. My guiding principle with the lathe CNC system has been to avoid as far as possible drilling any holes in Myford's ironwork. I looked at using microswitches and opto-couplers, either would have been quite tricky to fit and required some lathe dismantling. I already used a touch off system on my little mill and it works very reliably, so really I just did the same for the lathe.

It works for me - just thinking that other people might find the idea useful at least for its intended purpose.

Nice summary, John

... and your concluding statement seems 'spot on'

MichaelG.

Here's a practical observation.

Typical 3D printers use cheap, small size microswitches as limit switches typically with a lever that gives about 4: reduction in sensitivity.

Even with this unnecessary lever feature, the repeatability is surprising. The z-axis stops with the leadscrew within a few degrees of the same position every time, better than a thou with the 0.8mm pitch.

So.. consider using a small microswitch!

Neil

Barrie

You may well be matching one part to another to within a few microns, AT THAT TIME. In uncontrolled conditions the clearance may still be within 10 microns, but cannot be an absolute value. As you agree, heat transfer from the hand will change the physical size. We are arguing about the difference between Relative and Absolute values. if you want the latter, you have to have standardised conditions, otherwise Delusions of Accuracy take over.

When I took a part to the Standards Room to be measured, it was allowed to soak to the controlled ambient conditions for at least 24 hours before anyone started to measure it with the equipment kept in that environment..

Slip Gauges were checked in the Calibration Room, under even more closely controlled conditions of temperature and humidity, and then segregated into Workshop, Inspection, and Standards Room grade, differing by millionths of an inch.

In our hobby world, we cannot work to such precise dimensions, because our conditions are not controlled, nor are our measuring instruments that closely and accurately calibrated. The fits that we produce MAY remain the much the same, if the mating parts remain in the same ambient conditions, (We must accept that different materials have different coefficients of linear expansion) BUT the ACTUAL dimensions will change, as the ambients change, maybe almost imperceptibly, but they change.

Howard