Measuring Z-height on a digital (weight) scale

Hi Group,

I continually run into a problem with accurately measuring Z-height (vertical tool position) on my machine. Need >0.05mm depth precision at all times for decent engraving.

The unit has too little Z clearance to use a standard toolsetter - all I have seen are 50mm or more.

So I was thinking: These small, inexpensive digital scales - made for kitchen or drug-retailing use.

They rely on something called a "load cell" to measure weight - but in fact, I think they actually measure mechanical displacement vs. some spring resistance.

Readout seems to be an accurate function of vertical mechanical displacement. And there are hacks out there to interface to Arduino - so it's relatively easy to generate a metric look-up-table from raw sensor data.

Do you see where I'm aiming?

Any problems here that you can see - before I invest too much energy into a proof-of-concept?

Jakob E.

Are you sure that's the mechanism?  I haven't taken one of those cheap ones apart, but better scales use strain gauges as the sensors.

Could you just use feeler gauges or gauge blocks?

Could you just use feeler gauges or gauge blocks?

Click to expand...

This is what I use now, but I don't get the resolution I need - both because manual-steps in my software are at 0.1mm fixed, and because the 0.2mm routers I use breaks really easy. I'd like to have a soft-touch, absolute-unit-calibrated measurement.

And you're right about the strain gauges - these are the load cells i refer to. They measure bending or compression of the element, outputting a bridged resistance.

Jakob E.

Years ago I bought a kit digital scale (heathkit) that used a strain gauge to measure deflection of an aluminum beam. Modern stuff uses dedicated load cells. You can find application notes from the sellers of these load cells.

I don't quite follow what you are suggesting. but extra credit for thinking outside the box.

JR

Jakob,

Why wouldn't you just use a machinist indicator--something like that:

http://www.zoro.com/i/G1694314/?utm_source=google_shopping&utm_medium=cpc&utm_campaign=Google_Shopping_Feed&gclid=CN7x9tzy2sMCFY2TfgodkUkAlQ

Needs very little clearance, high precision, reversal (some are automatic, and some on its back have a switch, which changes the deflection direction), and besides... always good for setups like dialing parts, etc.

Just clamp it down, measure amount of deflection against some standard block, set your tool to the same hight (or just  calculate the difference) and you are good to go...

Best, M

I guess I'm not sure how the load cell will measure tool height - sounds interesting though, could you give more detail?

Are you worried a conductive touch-off plate will break the tools?  If you were getting really fancy, maybe some sort of optical system would work?

Marik said:

Just clamp it down, measure amount of deflection against some standard block, set your tool to the same hight (or just  calculate the difference) and you are good to go...

Click to expand...

Hi Marik,

Thanks - but I'm probably not machinist enough to figure out exactly how to do this: I tried, but it is quite hard to hit the very-small measurement-ball on the end of the indicator arm spot-on with a .2mm engraver. Hitting it off-centre decreases precision enough to be problematic. I've seen schemes where tool is lowered onto a parallel-moving plate mechanism, which's height is simultaneously touched and read by an indicator - but I haven't found any hardware resembling this..

Do you think the scale-route is wrong?

Dfuruta,

Yes, conductive touch-plate would not work, as I need an absolute-readout because of my 0.1mm manual stepping quantization.

Jakob E.

Jacob,

If I am understanding correctly you want to plunge the tool 0.05mm but you only have 0.1mm step resolution on your machine.

In which case just get a piece of 0.05mm shim ( feeler gauge).

Place it on your work piece.

Loosen the tool.

Drop the tool manually to rest on it fully.

Tighten the tool.

Pull the Z axis up by 0.1mm.

Remove the shim.

Drop the Z axis by 0.2mm.

Now the tool will plunge only by 0.05mm

For perfect engraving results you need a mechanical depth regulator on your CNC (in German: "Gravuranschlag" or "Tiefenregler") .

Like this one:
http://www.ebay.de/itm/BZT-Gravuranschlag-schwimmend-gelagert-Set-CNC-Frase-Frasmaschine-/390783608972?pt=Industriemaschinen&hash=item5afc84a88c

You get very precise results with that... even if your surface isn't absolutely flat.

...markus

Yes - this is usually the right way to do it.

Unfortunately, the router head I have (a W&H 160w high-speed spindle - http://reprap.org/mediawiki/images/6/6d/W%2BH_Tectorque_Inners.jpeg) has a chuck that slightly pulls the bit inwards when grabbing. Something like 0.2 to 0.3mm - but regrettably not an entirely predictable fixed amount...

Jakob E.

Marcus - writing as you posted:

Yes, I have such an engraving mount - and it's perfect for all painted and powder-coated surfaces. But when engraving anodized aluminum (which are 90% of the units I manufacture), small bits of metal and anodizing gets caught under the nozzle, is dragged around, and makes a "shadow" around engravings.

I've tried getting rid of debris with both vacuum, pressurized air, and a combination - but still get less than perfect engraving because of shadowing.

The best I've tried so far in this direction is a spindle from a LPKF pcb-router, which rests on an air-pressure nose - but it's still not enough to get rid of the shadowing problem.

Jakob E.

If you want to mark (black or coloured) anodized aluminium, look for a CO2 Laser engraver.
Looks way better than mechanical engraving in my opinion.
I got one and I love it

https://www.facebook.com/video.php?v=897743273599619

..markus

Beautiful indeed!

Want one, but probably can't afford..

How powerful a machine is needed for doing this? Estimated price? Running costs (like wear on tube etc.)?

Jakob E.

I imported mine directly from China.
Costs including shipping (by air) were about 3000.-  Euro.... which is not that expensive, I think ;-).


For marking anodized aluminium you don't need much power.
I do it with 10% of my overall 50Watts power.
But there's SO MUCH more you can do with a Co2 Laser Engraver. ;-)

The tube lasts (depending on the brand ) 1000-4000 hours.
Costs for replacement tube (50Watt) 200-400 Euro.

No additional running costs.

...markus

This is my machine:

http://www.triumphlaser.com/mini%20laser%20cutting%20machine%20TR-6040.html

...which is kind of "mid class" brand... there are lots of other chinese companies.

A european or american machine will cost minimum twice.


...markus

Sorry. I get it now. You want  to regulate the Z to compensate for the variation on the work piece surface.

How about a laser proximity sensor with digital  output?

I have seen engraving spindles in which the cutter is spring loaded. In the description it says this is for irregular surfaces.

gyraf said:

Thanks - but I'm probably not machinist enough to figure out exactly how to do this: I tried, but it is quite hard to hit the very-small measurement-ball on the end of the indicator arm spot-on with a .2mm engraver. Hitting it off-centre decreases precision enough to be problematic.

Click to expand...

Would a good magnifying glass or photo camera set in max zoom help? Just move the engraver tip over the ball a bit and note the highest indicator reading. I'd think this would be the easiest way of doing it...

Best, M

I was going to purpose a floating head but I see you already tried that.

I think you should find the way of getting rid of the play of the axis of the tool, probably not an easy thing to do since it's spinning so fast but maybe just using a heavier lube or changing the bearings may help to make it smaller. Ideally some preload would be used so you are sure the tool axis is always on top, so when you make force on that it doesn't have anywhere to go, that would be the only way of making it zero. Adding a new bearing with a spring loading it up could be the way to do so, but you need a way to mount the bearing both in the axis and on a slider on the machine, which would be spring loaded.

Then loosening the tool seems to be a nice, easy and precise way of doing that.

JS

A really fancy solution, like sahib mentioned, would be to set up a laser beam break sensor, adjust with the tool spinning until you find the tip of it, and then measure the height of your laser...