Monday, July 11, 2011

CNC touch probes review

Looking for a simple inexpensive touch probe for parts measuring and hexapod calibration, during last two months I became a kind of market expert in touch probes. So, here's a summary of my investigations.

There are many top-quality industrial probes with 1um and better precision, based on kinematic, optical or strain gauge technologies. The most popular industrial probes are Renishaw.

Renishaw RMP600 machine tool probe
Renishaw PH1+TP20 CMM probe
Renishaw TP1S manual CMM probe
Their list prices are unacceptable for home use ($2000 and more). Used probes might be purchased on ebay for $300-$500. Renishaw probes usually need special shanks, connectors, interfaces, receivers etc.

Another option is simple industrial probes with ruby tips and near 25um precision like CNC SharkDeskCNC touch probe ($249-$299).

or KE industrial ($199 on ebay).

or DamenCNC (€ 199.99), derived from successful homemade design

These probes are compact (20-30 mm dia.) and nice looking.

If we go cheaper, there's a kind of semi-homemade probes: larger (50 mm dia.) and less accurate (50-75 um precision), with metal  tip:

Wildhorse Innovations Econo-Probe ($120 or $96 as a DIY kit). There are useful and interesting assembling videos on their site.

Hubbard CNC Inc. probe ($129.99 on ebay)

There's also the cheapest probe from Hubbard CNC Inc. Not so good looking and only 0.15 mm accuracy.

All these simple probes based on kinematic technology (U.S. Pat. No. 5146691, Renishaw, 1991, now expired).
The kinematic location consists of a pivotal plate 1 that is spring-loaded against three bearing points 2 by a helical compression spring 3. These bearing points are formed by a combination of rollers and ball bearings.
Bearing points act as electrical contacts such that when the pivotal plate is deflected, the electrical circuit changes its characteristics and causes the probe interface to send a trigger to the CMM controller.
Following this trigger event, the stylus ball must be removed from contact with the surface to enable the probe to return the stylus ball to its repeatable position. (c)

There's newer patent illustrating the same principle ( U.S. Pat. No. 5146691, 2007)

There's a lot of homemade DIY probe designs using similar principle:

arie kabaalstra's probe, which improved design he later sold to DamenCNC

Probe from RoboCNC project

And other homemade probes found on youtube

Such homemade probes are intended to work with CNC like Mach3 or EMC2. They connect to input pins of parallel port or breakout board. Each probe is usually equipped with LED indicating its state.

Main problems of homemade kinematic touch probes:
  • The resistance of contact between small ball and rod might be not so low, and the probe circuit includes 6 such contacts connected in series. For better contact golden-plated balls and rods can be used, or electronic circuit to increase the input sensitivity and stabilize the current through contacts.
  • If the probe parts made inaccurately, its rods might not contact the balls at all 6 points simultaneously in initial position.
  • If balls and rods diameters and distance between balls are not chosen properly, the probe might not always go back to initial position when released, or stick between balls.
  • The probe tip center usually does not coincide with probe shank axis, therefore some device needed to adjust the stylus tip position.

1 comment:


    This is exactly what I was looking for. I am in the market for a decent, yet affordable, touch probe and your post has helped me immensely!