3D Edge Finder, Touch probe, Digitizing Probe

Once you start milling you soon realize all work has to be referenced, easily done with a round bar with known diameter, but a lot of calculating with manual dials.

Mechanical devices like wiggler are easy to build but only work in 2D and can not interface to the DRO.

My mill came with a Heidenhain DRO VRS 760b.

Not sure when they where manufactured, but it looks massive, stuffed with electronic components dating back to the late 1970's. Old but quite advanced for the its age, it supports an edge finder (kanten taster) KT120, terminating in a connector resembling a game port DA-15.

This seems like a basic tool to have , but oh my word selling on ebay for EUR 450 on a trade in of defect unit. Way too much for my hobby budget. So I have to build my own....

The idea was to start with what else than a google search on "3d touch probe diy" ...

https://www.google.com/patents/US5657549

https://www.google.com/patents/US5365673

https://www.google.com/patents/US5509211

http://www.homemetalshopclub.org/news/12/presentation_1202.pdf

http://fadedbits.com/2011/02/touchprobe/

many reads further I discovered most design's are based on a lapsed patent with three ball contacts connected in series. Soon I discovered this is quite inaccurate, and my precision OCD kicked in, there has to be a better way in using electronics to accurately measure this. The problem with the three contact points is depending on the angle of attack, the displacement until contact breaks is different.

My thinking is along the lines of amplifying the movement/acceleration with mechanical advantage, and measure with MEMS, optical, capacities or inductive.

Requirements:

1.  Repeatability is more important than Accuracy of measurement.
2. approach angle should not matter
3. battery powered
4. wireless
5. LCD/led indicator, indicate power on and edge detected, if more accurate measuring is possible indicate measurement.

Evaluation:

1. 3-Axis Gyro/Accelerometer IC
   Accuracy of less than 0.5 deg possible, https://electronics.stackexchange.com/questions/33374/choosing-an-accelerometer
   
   1. tan 0,5 deg = 0.00872686779 , this is a minute displacement, worth investigating  further. how will noise influence repeat ability?
      
2. Opto reflective sensor
   1. displacement of 1mm easy to measure
   2. further testing required to establish repeatability
    
   

My Design

The body is fixed to the spindle, is houses a moveable part, which pivots on a spherical ball in the center, which locates in a tight fitting sleeve, this facilitates the measurement of small variances in horizontal and vertical plane.