Author Topic: Precisely tune the Z probe  (Read 3522 times)

Offline Mark81

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Precisely tune the Z probe
« on: December 23, 2019, 10:28:17 am »
I have a simple Z probe made with a precision micro-switch. I put this device under the spindle and using a G38.2 I retrieve the depth of the switching point. To get the actual "zero" (the surface below the probe device) I need to know exactly the offset between this point and the switch.

Before go ahead I made tons of measurements with G38.2 to check the repeatabilty of the swtich. It seems below 0.008 mm. With small bits there's a problem with the position: if the tool touches the plate slightly off the center, it will tilt a bit and the measure will be affected. Unfortunately I need the maximum precision with tools of 0.6 mm.

Anyway, how to precisely measure the offset between the surface and the switching point of the probe? I did the following:

1. make a dummy tool with insulated shaft
2. place a metal sheet on the bed to make a contact probe with the tip of the dummy tool
3. with G38.2 get the Z value of the surface
4. put the probe device on the surface and with G38.2 get the Z value of the switch
5. the offset is (should be) the difference between the two measurements

But still I have an error more than 0.1 mm >:( too much.

Questions:

1. is my procedure wrong? Or perhaps it was only badly executed?
2. my ER20 collets are not insulated from the ground, so I cannot use a contact probe. Is there any other way to precisely find the zero?

Thanks in advance!
« Last Edit: December 23, 2019, 10:29:51 am by Mark81 »

Offline Bob La Londe

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Re: Precisely tune the Z probe
« Reply #1 on: December 23, 2019, 15:21:51 pm »
You are going to struggle with a single point of contact probe.  Either as a touch plate or as a spindle probe.  

There are a couple ways to make a probe that does work.  Even a single point of contact one.  Interestingly I use a mechanical tool height setter to set Z-height, and I do have a relatively expensive electrical height setter that plugs in on one of my machines.  

To tackle your specific problem I might consider a height gage.  Lower the height gage scribe onto your tool height setter until it triggers.  Read the height gage.  This is best done on a surface plate, but I expect if you had a surface plate and a height gage you would have already done this.  It should get you within 0.0005 inches.

The method used for mechanical height setters is to build them to a specific height.  2 inches is common because you can use a set of 123 blocks to calibrate the dial.  Some come with hard points on the top that are supposedly ground to exactly 2 inches.  In theory you can set the edge of a good parallel across those points and then adjust the dial to zero.  

There are generally three types of electrical height setters that work.  

1. The simplest is a precision flat plate. You hook a wire to the tool and a wire to the plate.  When they touch you have the height of the tool above the surface below.   Just mic the plate.  Of course this is subject to wear and the tool to crashing and chipping depending on the hardness of the plate.  It does not work if there is an electrical path between the work piece and the tool.  You could insulate it with a piece of plastic.  

2.  Next is a simple spring loaded plate.  You make a box that the plate fits inside of.  You put a spring or multiple springs inside the box to consistently hold the plate up against an opening in the top of the box.  With the plate held up against the opening machine (or grind) flat relative to the base.  Works the same as 1. above except the plate moves when touched so it doesn't break tools or wear the plate as easily.  Electrically it has the same issue as method 1.  You could make the box out of plastic, or mount a plastic plate on the bottom.  

3.  A decent tool height setter will be made somewhat similarly to number two, except the plate will have 3 contact surfaces (usually gold plated round rods) that rest between contact points (usually gold plated round balls).  If the plate moves downward in any angle from contact point on the surface of the plate it breaks the circuit.  Depending on the MFG its either made very precisely or the circuit board that holds (soldered) the round balls is adjustable to adjust the plate flat.  Its usually made of all metal, because both wires hook to the circuit board.  Not to the plate or the body of the setter.  

(alternate)  An electronic probe (or mechanical 3D Taster) is place in the spindle, but only if the tools can be taken out of the spindle and put back in the spindle consistently every time.  Then all the tool and the tool probe height are saved in the tool table of the machine.  

As I scan over this I see I over simplified a lot of it, but my main goal was to steer you away from the single micro switch tool height setter.  

Typically I am used to seeing a "probe" as something mounted in the spindle, and a tool height setter as resting on the stock.  I don't know if this is a standard or a norm, but its the way I usually see them referred to.  

The picture below is the height setter I have resting on 4 of my 5 CNC mills, and similar to the one I have on the 5th.  I rough adjust them on the surface plate with 123 block and a parallel.  Then I final adjust on the machine it will be used on by zeroing a tool, and then lower the spinning tool to a piece of milled flat and lightly sanded aluminum stock until it leaves a tool mark, but does not leave a mark I can catch a finger nail on.  Usually there is only 1-3 ten thousandths difference.  After I have the tool height setter adjusted and dialed in I leave it on that machine until it dies or I crash it.  Coolant and chips can gum them up, but as long as you don't get any inside the indicator you can clean and readjust them fairly easily.  If you drive a tool down into them it damages the plate, but you might still be able to save it.  You just have to avoid the tool mark in the future.  Unfortunately with a Mach 3 driven machine running programs with a million lines or more it can be cludgy and slow to respond to manual controls causing that sort of crash.  

« Last Edit: December 23, 2019, 15:24:52 pm by Bob La Londe »
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Offline Mark81

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Re: Precisely tune the Z probe
« Reply #2 on: December 23, 2019, 17:09:29 pm »
First of all, thanks a lot for the time spent to write this answer!


To tackle your specific problem I might consider a height gage.  Lower the height gage scribe onto your tool height setter until it triggers.  Read the height gage.  This is best done on a surface plate, but I expect if you had a surface plate and a height gage you would have already done this.  It should get you within 0.0005 inches.

Right now I have a height setter and this "caliper" that I can put into the spindle to check the flatness of the table:

https://www.amazon.it/gp/product/B07CMLYXCG/

What's its English name?

Quote
The method used for mechanical height setters is to build them to a specific height.

Yep! But this was made by the manufacturer of my machine and it's not "built to a specific height". He calibrated it for me when he sold the machine, but it always had some errors.

Quote
1. The simplest is a precision flat plate. You hook a wire to the tool and a wire to the plate.  When they touch you have the height of the tool above the surface below.   Just mic the plate.  Of course this is subject to wear and the tool to crashing and chipping depending on the hardness of the plate.  It does not work if there is an electrical path between the work piece and the tool.  You could insulate it with a piece of plastic. 

This is what I've done, but the problem here is the collet that is already connected to gnd. So I must isolate the tool, not the plate and it won't work this way.


Quote
3.  A decent tool height setter will be made somewhat similarly to number two, except the plate will have 3 contact surfaces (usually gold plated round rods) that rest between contact points (usually gold plated round balls). 

This remember me the "3D touch probe" mechanism.
I guess this is the best method because it will trigger in any condition even with a very small tool. I'm going to search something like this.


Quote
(alternate)  An electronic probe (or mechanical 3D Taster) is place in the spindle, but only if the tools can be taken out of the spindle and put back in the spindle consistently every time.  Then all the tool and the tool probe height are saved in the tool table of the machine. 

I use this method for low precision tasks only (i.e. to cut plywood). But I cannot take out and put back a tool in the spindle with an accuracy lower than 0.05 mm.


Quote
I have the tool height setter adjusted and dialed in I leave it on that machine until it dies or I crash it.

That's why one needs a big machine ;D
Unfortunately I need to use every cm of my working area. But perhaps I can place it outside the front edge. Most machines have a Y travel longer than the T-slot table so you can move the spindle over its edge (I guess to easily access the spindle).

« Last Edit: December 23, 2019, 17:11:03 pm by Mark81 »

Offline Bob La Londe

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Re: Precisely tune the Z probe
« Reply #3 on: December 23, 2019, 17:15:36 pm »
Dial indicator.  In this case digital dial indicator. 

On THAT machine.  Not in the work envelope of the machine.  I specified this because i have multiple machines and each one has its own height setter.

I typically zero to the work piece surface unless I am making multiple identical work pieces or I have to hit a specific target thickness. 
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Offline lloydsp

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Re: Precisely tune the Z probe
« Reply #4 on: December 23, 2019, 17:18:18 pm »
"This is what I've done, but the problem here is the collet that is already connected to gnd. So I must isolate the tool, not the plate and it won't work this way."

-------------------------

No.  Being an electronics engineer by past careers, I can say that it would be simple to make the tool ground, and make the tool-setter touch plate 'hot' through a resistor, so that contact can be sensed, but there would be no dangerous current flowing if something shorted 'dead out' to the plate.

It's a simple change of relationships, and easy to do.

Lloyd
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Offline Mark81

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Re: Precisely tune the Z probe
« Reply #5 on: December 23, 2019, 19:02:51 pm »
I can say that it would be simple to make the tool ground, and make the tool-setter touch plate 'hot' through a resistor, so that contact can be sensed, but there would be no dangerous current flowing if something shorted 'dead out' to the plate.

I'll try it, it would be the simplest and most accurate solution.

Thanks

Offline Garyhlucas

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Re: Precisely tune the Z probe
« Reply #6 on: December 24, 2019, 03:15:08 am »
You are making too much of this I think. Touch off any tool on your microswitch. Zero the Z axis DRO. Remove the tool setter and touch off that tool using a known feeler gauge on the table. The DRO indication + the feeler gauge is the the height of your microswitch tool setter.
 
Just as a matter of interest I am trying an experiment on the CNC router I am building for the FIRST robotics team. I insulated the entire T slot table from rest of the machine on fiberglass blocks. So I can apply a small voltage to the table and sense the tool with the table, vise, or metal part.  Trying to automate the process for the students.
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Offline Mark81

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Re: Precisely tune the Z probe
« Reply #7 on: December 24, 2019, 10:08:18 am »
You are making too much of this I think. Touch off any tool on your microswitch. Zero the Z axis DRO. Remove the tool setter and touch off that tool using a known feeler gauge on the table. The DRO indication + the feeler gauge is the the height of your microswitch tool setter.

It sounds great :o
I will try as soon as possible and report back the results!

Offline Mark81

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Re: Precisely tune the Z probe
« Reply #8 on: December 26, 2019, 12:13:40 pm »
You are making too much of this I think. Touch off any tool on your microswitch. Zero the Z axis DRO. Remove the tool setter and touch off that tool using a known feeler gauge on the table. The DRO indication + the feeler gauge is the the height of your microswitch tool setter.

Well, reading it again carefully it's exactly the same thing I've done in the five points described in the first post.

I mean, the hard part is when you say "touch off that tool using a known feeler gauge on the table". You cannot just see when it touches. You need something to get the exact point... I used my simple insulated contact probe tool. Then it makes no differences if you use a feeler gauge of the table itself as reference plane.

Am I wrong?

Offline lloydsp

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Re: Precisely tune the Z probe
« Reply #9 on: December 26, 2019, 12:37:54 pm »
Mark,
It makes a little difference.  It doesn't change your measurements, but 'feeler gauges' are cheap and readily-obtainable.  A marred table, not so cheap!

Lloyd
"Pyro for Fun and Profit for More Than Fifty Years"

Offline Mark81

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Re: Precisely tune the Z probe
« Reply #10 on: December 26, 2019, 15:08:30 pm »
Mark,
It makes a little difference.  It doesn't change your measurements, but 'feeler gauges' are cheap and readily-obtainable. A marred table, not so cheap!

I apologize but I don't understand what you're saying  :'(
Perhaps we're using the same words to describe something different.

"Garyhlucas" said:
Quote
Touch off any tool on your microswitch. Zero the Z axis DRO. Remove the tool setter and touch off that tool using a known feeler gauge on the table. The DRO indication + the feeler gauge is the the height of your microswitch tool setter.

If I touch the feeler gauge the height of the microswitch is calculated as DRO indication + feeler gauge thickness.
But because the feeler gauge has a known thickness it's actually the same of touching the surface directly: it doesn't add or remove any information, it's just a constant.

The problem is how to "touch" either the feeler gauge or the table surface precisely, without using any type of probe (like the contact one discussed before).

Am I wrong?


Offline lloydsp

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Re: Precisely tune the Z probe
« Reply #11 on: December 26, 2019, 17:19:19 pm »
Mark,
Your point is true, and will work fine.

My point about the cost of feeler gauges is that they are very inexpensive, and if you make a mistake 'touching down' (I assure you, it will happen), it would be far-better to damage a feeler gauge than to damage your bed platform.

Lloyd
"Pyro for Fun and Profit for More Than Fifty Years"

Offline Mark81

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Re: Precisely tune the Z probe
« Reply #12 on: December 26, 2019, 19:03:43 pm »
My point about the cost of feeler gauges is that they are very inexpensive, and if you make a mistake 'touching down' (I assure you, it will happen), it would be far-better to damage a feeler gauge than to damage your bed platform.

Got it, thanks.
Anyway, this should be done only once.


Offline lloydsp

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Re: Precisely tune the Z probe
« Reply #13 on: December 26, 2019, 19:21:47 pm »
Once?  I don't believe so.  If you're wise as to how machines wear and change over 'usage' time, you'll set a schedule to do it regularly, just to check things -- say, every three months of operation.

It's not a really big deal to do, and ensures you keep the accuracy you started with.

Lloyd
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Offline Bob La Londe

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Re: Precisely tune the Z probe
« Reply #14 on: December 27, 2019, 00:47:04 am »
Ultimately the last definitive test is always the smoke test.  Turn it on and see if it smokes.  

In this case the smoke test is a tool touch off.  

There are things you can use to minimize the chance of damaged.  A feeler gage is suggested as something that can go between your tool and your table to prevent marring the table when you do a touch off.  

I used to use a sheet of printer paper, and drag it back and forth as I lowered the tool until it start to drag without tearing.  This would tell me the tool was the thickness of the paper off the touch off surface.  Then I would mic the paper and do the math.  I always felt this was better than a feeler gage, because if you over travel a few ten thousandths the tool does not contact the surface.  It compresses the paper.  Before I had other better tools I did this for every single tool change.  It was slow.  It was tedious.  It got me within a thousandth of an inch or better consistently.  At one time I used to have a stack of little strips of printer paper next to my machine for this.  I still use this method for locating stock when nothing else will work.  

Anyway, you have to measure the height of the electronic height setter when activated one way or another.  No matter what metrology you use you won't know 100% for certain that you have the best number for that until you do a touch off to the surface its resting on.  

I can come up with all kinds of methods to get pretty darn close, but the definitive test is the one that could damage something.  The "smoke test."

If you know the backlash of your axis you could even do what I call a reverse touch off with a precision ground block.  (On some machines Z-axis backlash is not an issue because gravity overcomes it.)  Set your height setter on something like a 123 or 246 block.  Lower the tool until it trips the height setter.  Zero the Z height.   Remove the height setter and slide the block out of the way.  Lower the tool until its just a tiny bit lower than the top of the block.  Raise the tool a ten thousand at a time until the block just barely slides under tool.  Or just a thousandth at a time if that's good enough for you.  The DRO now shows the height of your tool height setter minus the backlash of the axis.  Add the backlash to the number and you have the height of the height setter.  

I use the reverse touch off 123 block method just to find zero on my CNC wood router because it doesn't have enough Z axis clearance to use any of my other height setters once I get a stack up of backer, material, etc mounted on the table and a tool hanging down from the spindle.  

There are a BUNCH of different ways to get there.  Now quit screwing around and pick one.  



« Last Edit: December 27, 2019, 00:51:50 am by Bob La Londe »
Getting started on CNC?  In or passing through my area?
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