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What exactly is probing?

Discussion in 'CNC Mills/Routers' started by nportelli, Jan 23, 2018.

  1. nportelli

    nportelli Well-Known
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    I know I've seen the metal plate with wire attached and an alligator clip....but I'm not exactly sure what, or why you'd use it. I'm guessing to find the top of your work surface? Are there any links describing how to set one up and use it?
     
  2. Sprags

    Sprags Veteran
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    i use a Renishaw Probe on my 5 axis machine to establish work offsets. The tool probe actually touches off on the tool to measure its diameter and length.

    OK...more for OpenBuilds routers...I imagine you can set it up to establish the top of the part. I think if you actually attache the wire to a tool and you know the diameter you can use it to establish X0 and Y0 also.

    I just started building my machine but have been a manufacturing engineer for years machining parts so other member will have to go into detail on how to do it with the software you are running.
     
  3. Jonathon Duerig

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    The basic idea of a probe is to have two pieces of metal, one that is grounded (usually the bit), the other that is on a logic pin (like a copper plate) and detect whether they are touching by seeing if a circuit is made. In theory you can do all kinds of measurement this way, as @Sprags mentioned in a professional machine context.

    In a DIY context, they are almost exclusively used to find Z-position. X and Y position can be easily found (and re-found) using homing switches. The Z-position of the router itself can likewise be found and re-found using a homing switch. But the distance between the router and the tip of the bit varies. Every time you change a bit you are potentially changing that relationship. And you want your Z position to reflect the end of the bit, not the position of the motor.

    So you use a plate of a known thickness and run a GRBL command to slowly move the bit down until it just detects that it is touching the plate. Now you know that the bit is that thickness above whatever you have the plate on and you set your work position accordingly.

    Probing is often a slow and manual process in the DIY world (professional machines can often do this kind of thing automatically). So you want to do it rarely. Ideally you will have homing switches on X, Y, and Z axes. Then when you probe, use the plate to set the work coordinates relative to the bed of the machine. Z=0 would be the surface of your bed, and all cutting would be done above that plane. Make sure your CAM program knows this (so it doesn't cut into the bed). If you do this, you only have to probe when you change bits.

    And your efficiency can be improved further if you buy bits with plastic rings. The rings are precisely placed on the shaft of the bit at the same distance from the tip. So if you use a bit with a ring, just make sure the ring is flush against the collet and you know that the geometric relationship between the tip of the bit and the router hasn't changed.

    One other thing is that the electric contact probes offer a useful shortcut when putting bits into your machine and taking measurements. But this is something people do in other ways as well. Maybe they lower the bit gradually and eyeball it. Or use a piece of paper and stop when the paper tears. Or do one of the above methods and then tweak things if they cut and find they are cutting too shallowly or deeply. So don't feel like you have to figure this out right away. Keep the idea of adding a probe in your back pocket and when you get frustrated, pull it out and see if it will help. No need to have all the bells and whistles on a machine for it to be useful.

    -D
     
  4. Giarc

    Giarc Master
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    I am a visual learner so, here are a couple videos that made it easy for me. As mentioned, probing is a quick and accurate way to zero the z using a touch plate of a known thickness.


    It can also be used to find the edges of your work,



    and to define the surface you want to engrave.



    My new favorite way is with aluminum tape like in the last video. I had a roll left over from some duct repairs and it is sticky, easy to remove, and sticky enough to use again and again. I just pull it off the material I am cutting and stick it to my Y axis gantry plate until I need it again. I used the same piece multiple times while cutting all the plates for my laser cutter. The thickness is 0.06 mm.
     
    #4 Giarc, Jan 23, 2018
    Last edited: Jan 23, 2018
  5. GrayUK

    GrayUK Openbuilds Team Elder
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    Probing is also using a tool to measure and store 3d points in space, and then replicate them again and again, as is the use of a CNC.
    The small tool is put into the chuck of your router/spindle. It is usually made up of a sprung probe, and a micro switch, at it's very basic.
    The machine is set to move down, left or right etc, until the probe touches an object, when it does, like the other probes mentioned, it will retract, having given a reference point to the program. For a very detailed image the program will do many hundreds of probes, in a small area, building up a picture to replicate.
    I have used this method in the past to create a very detailed copy of a very very old, and deteriorating picture frame for a client in the restoration business. I only needed to probe about 300mm because the pattern was repeated. I made him 600mm of frame in oak, and he was able to cut and splice in the pieces he needed.
    It took a long time, and a many, many hundreds of probe readings, but these people expect to pay top money for the right result! I know I did when I used to restore buildings. In this case about £1 per mm of probing. Where else could he get what he needed? Plus the responsibilty, if my Z had plunged into the frame, my insurance would have been seriously justfied! Apparently, as it happens, I was quite cheap.
    And he happily paid my invoice.
    Gray
     
  6. T4Concepts

    T4Concepts Veteran
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    That's interesting stuff guys ! ................. I had no idea a cnc machine could do that ( the last video that Giarc posted ). I was looking at 'PhotoGrammetry' for duplicating one of my prototype moulds, but now it seems that I could 'copy' just about any shape ! Does this procedure generate a G-Code file then ?

    I had a look around and found this > Home | Software Redefined GRBL Controller software made easy. SoftwareRedefined.com
    Or does any type of G-Code generator have the ability of doing this ?

    I have haven't received my WorkBee cnc yet ( later today hopefully :D ) and already I'm finding new ways of using it !



    TURK
     
  7. nportelli

    nportelli Well-Known
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    Awesome. Has anyone thought of doing this with a laser range finder or ultrasonic? Or something similar? I suppose the location would have to be at the same as the bottom of the bit.
     
  8. GrayUK

    GrayUK Openbuilds Team Elder
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    It is always set from the Zero points to start, so a laser would probably work, probably even faster. :rolleyes::thumbsup:
    I'm just a mechanical guy, so mechanical probes seem more comfortable. :)
     

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