Additional G3 probing cycle - finding the centre of a cylinder


This posting is likely to be edited for clarity and provide supporting information to MASSO to justify this feature request.


The original probing functionality was implemented Aug 2019 (prior to the release of the G3 controller). The previous G2 controller had limited storage capabilities and therefore the "absolute essentials" was implemented.

Intended Users

Mills used in a non-production environment (prototype development / hobbyists). Milling scenarios that do not use work holders / jigs.

How does this feature deliver value?

The traditional method of finding the center of a cylinder involves mounting a dial indicator in the spindle and checking the indicated position in the X+, X-, Y+ & Y- directions. The X/Y position is manually adjusted and the surface of the cylinder is swept with the dial indicator again to determine if the position correction results in the same indicated value in the 4 axis directions. Once the indicated position is identical then the X/Y axis are set to zero.

The traditional method has the following challenges:
  • Tedious and repetitive adjustments
  • May not have enough room to mount a dial indicator
    • Near the spindle of a Sherline mill there is only 45mm between the spindle center and the Z axis dovetail

  • The dial indicator often faces the mill Z axis dovetail for the Y+ direction, this location is almost impossible to view directly and using a mirror is a challenge

By comparison having the touch probe touch off three points and calculate the center of the cylinder (zero the X & Y axis) is far more preferred.


The attached ZIP file contains several PNG files showing how a cylinder probing cycle could be implemented and the necessary formulas for taking 3 points on the circumference of a circle to locate the center of the circle. Also included is an Excel file with the formulas implemented using "named cells" in lieu of variable names, this is intended to be a verification tool for the software developer to reduce the effort during implementation.


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The following is further evidence why probing for the center of a cylinder is preferred over using traditional indicator methods for a Sherline mill due to space constraints.

My Sherline mill is a 2010 model with a 2" column riser added to increase vertical clearance. I made a tool to allow using a test indicator to zero the X/Y axis against the cylindrical work piece. Due to the length of a test indicator the Z axis is only 5 mm to 10 mm from the maximum Z axis position. By comparison the Drewtronics touch probe when mounted in the Sherline 3/8" Endmill Holder is about 30 mm shorter. Potentially the back plate of the Drewtronics touch probe could be replaced with an aluminium plate with a 3/4" 16 TPI thread mouting for the Sherline spindle nose, this would provide an additional 40 mm clearance. To put this in perspective the total mill Z axis travel is 140 mm (30 mm is 21% of the Z axis travel)

The mounting tool for the test indicator was made using two 3D printed parts, a M6 x 40 mm hand bolt from Bunnings, a 3/8" x 1-3/4" x 0.035 compression spring (Bunnings), a 100 mm length of 6mm harden ground linear shaft rail (eBay) that was cut by hacksaw into 2 x 40 mm + 1 x 20 mm lengths. I found cutting the hardened ground shaft was an excellent way to remove the teeth from the hacksaw blade ( *grumble* ).

Using a test indicator was challenging as the range of the indicator was 0.4 mm, this means that the position of the axis needs to be within 0.4 mm of the true center before the probe remains in contact with the cylinder for a complete revolution. Once the touch probe was properly in contact with cylinder making the final correction to center the spindle over the cylinder was fairly quick. Total time from mounting the indicator in the spindle to zeroing the work was about 5 minutes.


Using a touch probe has advantages of both clearance (21% additional Z axis clearance) and speed (30 seconds vs 5 minutes).


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A quick bit of design work for an alternative "lid" to the Drewtronics touch probe so it can be screwed on to the spindle nose threads.

3D printed in ABS with the threads as part of the 3D model, needed to use an appropriate tap to "chase out" the oozed plastic. Nice and snug fit.

This touch probe arrangement has an addition 65 mm clearance over the test indicator.


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Just received a CNC milled & anodised "lid" to mount the Drewtronics probe on the nose thread of the Sherline spindle (ordered on-line, took 3 weeks to process / ship). The anodised colour match was more a fluke (ordered "red" and came out pretty much spot on). Run-out on the tip was adjusted to within 0.01 mm, which is repeatable after removing and reattaching the probe to the mill.

Next job is to prod Masso to give us more probing options... (bad flashbacks of cncnutz probing video, "alien abductees").

ZIP file of the files required to place the order are attached if any Sherline mill owner wishes to repeat the exercise.



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