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« on: May 25, 2012, 09:18:18 AM »
Hi YNN,
Thank you for the kind words, it is much appreciated.
The contact ring, insulator and plate have all been lathe turned and the flat surfaces are as near parallel to one another as I can make them. These 3 items have then been fitted together using Locktite, under a small pressure until cured, making the assembly of the contact ring as plane with the top surface of the plate as I can. The 3 individual contacts have been lapped so that they each protrude from the rear surface of the PCB by exactly the same amount (as near as I can measure using feeler gauges on a surface plate).
The contact ring, plate and the 3 contacts are all made from A4 (austenitic) stainless steel.
The contact ring rests on the 3 contacts under spring pressure and the overall height of the completed probe measured with a micrometer – this dimension being entered into the Mach3 probing script as plate thickness. Any downward movement of the plate breaks one or more contacts for the G31 detection. Although the contacts are ‘normally closed’, the transistors invert this logic so the probe output is normally a weak High and only taken to a strong Low (GND) when contact is broken.
The amount of movement necessary to operate the probe is incredibly small (unfortunately I don’t have the necessary equipment to accurately measure this) but it is largely dependant on the flexibility in the top cover and operational movement is currently smaller than two of my Z axis micro-steps. It does not seem to make any difference if the plate movement is parallel (centre contact) or tilted (off-centre contact) there is never any contact bounce – it either makes contact or it doesn’t.
This type of contact break detection is used in some of the Baumer Electric limit switches – OK they have far more accurate manufacturing facilities than I have but they claim 1 micron repeatability. I don’t think I am anywhere near to that but, as I said, the triggering movement is incredibly small and beyond my micrometer measurement skills.
The over-travel (after contact break) is approx. 2mm but I have found that with Mach3 version R3.043.062 the G31 appears to trigger an immediate stop rather than a decelerated stop and plate movement is not even visible.
One new thing that I have noticed is that the work piece must not have any movement or flexibility when tool height setting – my vacuum table holds engraving laminate tightly but double sided sticky tape has a degree of springiness and this can cause a setting error. I am not quite certain just what is happening here but it needs further investigation.
You would really have to build a similar device for yourself to fully appreciate the accuracy of the operation but, as shown in a previous picture, it has enabled me to engrave 0.5mm high text to a depth of 0.05mm which I struggled to do before using this type of probe.
Hope I have answered all the questions.
Tweakie.
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