cnc & manual touch probe.

by:kingtool aluminium machinery     2020-04-13
This type of touch probe described here was invented by Sir David mcwetter
The founder of Renishaw, responsible for checking the components of the Concorde engine.
They are now widely used in CNC milling machines and CMMs (
Coordinate Measuring instrument)
Unfortunately, the products of companies like Renishaw are priced in the range of thousands of dollars.
The probe is used for machining settings and component inspection, and also for touch scanning of 3D surfaces in reverse engineering.
The principle and main components are very simple, and the network is full of videos and photos of the home build version.
Many DIY versions are fairly rough, but other versions are of good quality, although the accuracy is not high (
(Micron)
At the top of the range probe, but within the range of normal machining requirements.
Many DIY probes are made by people doing 3D scans with CNC routers to copy the contours in the wood, in which case it is neither possible nor requires high precision.
In my example, I have two intended uses, first of all, I often use my milling machine to reverse engineer objects such as motorcycle turning boxes to determine the center and diameter of bearing holes, etc.
In other words, treat my factory as a CMM for the poor.
Secondly, I plan to use it on the milling machine for part positioning and alignment.
In the past, I have used traditional edge finder and dial indicator for these tasks, but I expect the probe to be faster.
My milling machine has high quality DROs with glass scale, but the minimum resolution is 0.
01mm, I hope the resolution and repeatability of my probe is at least better than that.
The basic principle of this type of probe is that when the probe touches the features in the XY plane at a given Z position or touches the surface in Z, the continuity of the circuit is broken by the direction at a given X & Y.
Many CNC programs can be scanned automatically on preset grids, but can usually be done manually as well.
Currently, I expect to use it manually, that is, position the mill coordinates by hand.
There are usually six balls (I used 6.
Remove 35mm balls from ball bearings)
Three Pins (
I used 6mm hardened pins.
Form a conductive path.
The ball is arranged in the insulation material (I used Delrin)
A circular pattern of three pairs and electrically connected in series.
The main photo above shows the principle in a chart.
The pin is installed in another piece of insulation with a spacing of 120 degrees to match the spacing of the ball.
This piece has a stylus for touching the object under test.
The photos in the other steps show this better than my text.
When the top part with a pin is loaded onto the lower part with a ball spring, the circuit is off.
Even the tiny movement of the stylus raises at least one pin a little and disconnects, indicating that the stylus has come into contact with the workpiece.
This requires a small amount of stylus for displacement, called \"pre-
Travel \", which represents an error in the measurement, but as long as this pre-
The stroke is repeatable and can be allowed on each of the three axes in the measurement software or in manual calculations.
Resolution and repeatability ratiotravel.
The ball spacing of the pin is an important consideration.
The second photo shows three possibilities.
If the gap between the balls is the same as the pin diameter, then the position of the pin, so the stylus will be very good, but the pin needs to be significantly lifted to disconnect the circuit, in other words, pre-
Travel will be excessive.
The other extreme is that the gap between the center of the ball is zero, in which case the pins are placed together at the top of the two balls.
This will provide us with the lowest pre-
Travel but no fixed position stability.
Obviously, we need to compromise between these two extremes.
I don\'t know what quality commercial probes the contacts use, but I walked half way and chose 45 degrees as my best stylus position and minimum pre-travel.
The electrical output is like a switch, which is basically on/off. many DIY probes are directly fed into CNC controllers such as Mach3, but in order to obtain good repeatable results, it is better to use a little intermediate electronic device, such as a comparator, to determine the switch point more accurately.
There is an intermediate range between pure on/off, where the contact resistance changes between close to zero and close to infinity.
The use of the comparator can detect minor changes in the resistance, so that the stylus touch can be detected earlier, thus reducing the pre-
Travel and improve repeatability.
Analog multimeter is the simplest method (
But most inconvenient)
Monitor the switch status using the resistor function.
In fact, this is what I used for the first \"working\" test.
As always, different manufacturers will have different tools and materials, so the detail design points can be different to suit anyone who wants to make it on their own.
Here is a list of the tools I have available and the materials in the scrap box, and although I ordered Renishaw ruby tip stylus, I have nothing to buy.
This may not be the case for everyone, but none of the materials is expensive.
The bottom of the tool material closes the lower part of the probe assembly and provides installation for 6 balls.
This involves the turning and milling of some lathes.
Milling requires a precise index of the holes in the ball, which can be done by writing some simple G code to use the CNC function, but I think in order to demonstrate the use of the turntable with the Chuck, this is fast and good.
If there is no lathe, the lathe work can also be done on the rolling mill with a turntable.
First, rotate a socket to fit inside the aluminum outer tube, and then make the hole using a flat head 6. 35 mm (1/4\")milling cutter.
The holes are precisely made into the same depth, and a flat bottom is used to ensure a good vertical position of the ball.
A separate test shows that hitting a hole in a milling machine can create a very slight interference fit on the ball, which makes their accessories without glue.
Grind the radial groove to get the gap of the pin and make the cutting edge groove for the connection line between the balls.
A tilted hole is drilled on the drill press of the external cable.
First of all, I grind the small plane on the ball as a place to weld the wires.
This means that at least most of the welds are left inside the sphere of the ball, preventing them from fitting in with the mounting holes on the base.
The first photo shows the plane on the foreground ball and a pair of connected balls.
I connect with very thin copper wire because it is easier to weld the ball.
The ball is supported on a brass rod with holes.
To facilitate welding, the rod is first heated as a heat source to maintain the temperature of the ball.
A drop.
The resin-based flux is placed in a flat part, touched with a soldering iron, quickly loaded the ball into the tank, ready to take out the pre-
Tin-plated copper wire.
First, I lowered part of the Delrin rod to 34mm diameter, a loose fit of the 38mm ID aluminum tube body to allow free movement.
I left part of the bar at the original 50mm in order to fit safely in the chuck.
Then I put it on the turntable and drilled 3 holes 120 degrees apart for the pin.
Turn back to the lathe down, facing off, revealing nearly half the diameter of the pin.
This is what I do because it is easier and more accurate to drill a complete round hole than to drill a half hole.
On the big end left a tits as the position of spring.
The pin is cut into length and mounted on the stylus holder.
This is the main structural part of connecting the whole thing together.
The tubes are separated and faced on the lathe.
The end cap was drilled and reworked a little under size for the mounting shaft that extended to the interior as a spring guide.
Then press it into the tube.
In order to obtain a strong interference fit, no additional processing is required for both the 38mm rod and the tube.
The photo above clearly shows how the top and bottom Delrin blocks are aligned by Ball and pin movement.
When the pin is in contact with each pair of balls, the geometry allows only one stylus position.
Therefore, whenever the stylus is moved by touch, the spring applies the pin force to the ball to ensure a repeated and precise adjustment of the stylus assembly.
I have ordered a Renishaw ruby tip stylus, but due to impatiently trying the probe, I made a temp by soft soldering the hardened ball to the M6 Bolt with reduced diameter
Frankly, for the relatively small amount of use I will give the probe, I was wondering if the professional stylus would do a better job.
What is shown here is the finished assembly.
Please note the three M3 screws that secure the Delrin part on the main body.
These are deliberately not drilled at a 120 degree interval to ensure that it must always be done in the same direction.
A small dent was drilled on the main housing to indicate the cable position as a quick guide to correct hole alignment.
Maybe a little obsessive,. . . . . . .
Main drill 2.
5mm for M3 screws.
I mentioned in another step that it would be better to add some electronics to get more accurate and repeatable touch sensing.
Initially I just used the resistance range on the multimeter to check if the probe actually works.
Upon completion, I assembled a simple breadboard prototype for the comparison circuit to control the LED as a touch indicator.
The first schematic shows that the LED connection of the LED is turned off when the touch is perceived, and the other schematic shows the simple changes needed to light the LED on touch.
It is possible to simply connect the LED and current limiting resistors in series with the probe contacts, but I am afraid that over time, the contact area of the ball and pin will become pit-to-LED due to the required current.
On the other hand, continuity must be ensured by a sufficiently high current.
Very small currents often lead to unreliable contact continuity.
I chose to use 5 mA, which I think is enough to get reliable contact without reducing the contact area.
For some time, I have been thinking about using Arduino to read the position of the slide on the mill so that it can start quickly when using the mill in manual mode.
Currently, I have to start the PC and CNC system in order to read the slides.
When I talk about this, it would be simple to enter the probe status and automatically record the touch position.
If this happens, I will make another note.
To accommodate simple electronics, I made a simple box using a 3D printer designed to be bolted to the aluminum housing.
It could have done less, but the extra space provided some proof of the future.
There are two LEDs on the front of the box, one green to indicate the power supply and the other red as a touch indicator.
There are two connections on the back of the box, one for 5 v power supply and the other for connecting to the CNC controller (optional).
I don\'t take surface value into account when measuring things, so I measured resolution, repeatability and accuracy before believing that the probe is a useful tool.
My milling machine has DROs with a resolution of 0 on each axis. 01 mm.
I hope to be a little less, but this is what I have and is OK for most of my work.
So my first concern is that the resolution and repeatability of the probe is no worse than the DROs.
To test these parameters, I clamped a 123 block to corner plate on the mill table.
I could have used the corner plate directly, but the surface finish is not as good as the ground finish on the 123 block.
Then I moved the mill until the touch signal was sent out by my LED indicator.
I repeated 20 times on each axis and in all cases the DROs showed the same reading indicating that the probe was at least as good as the mill\'s measuring capacity.
It also confirmed the pre-
The itinerary on any axis is very consistent.
The next series of tests involved measuring the accuracy of the hole and the features of the bump.
To do this, I moved the grinding disc so that the probe approached 123 pieces from the opposite side and recorded the DRO reading.
The difference in values on both sides of the block should be equal to the actual width of the block plus the stylus ball diameter, which are 25 dimensions respectively. 40 mm and 4.
76mm give 30 respectively.
16mm as the value of the check probe.
The probe gives 100% consistent values of 30.
Measuring 13mm along the x-axis and 30.
11mm along the y axis.
This means there is an error (pre-travel)of 0.
X 015mm, 0. On Y 025mm
However, since this is consistent in more than 20 Tests, it is possible
Travel the measured difference to eliminate the error and obtain accurate results.
Most controllers allow the input of this data when using CNC probes.
Accurate Results depend to a certain extent on the force required to move the stylus.
If the spring is too hard, then the structural deformation can increase the pre-
Travel, if too soft, then the pin may not be properly fixed on the ball after each touch, leaving the contact on.
Some commercial probes have the function of adjusting the spring force.
For the sake of simplicity, I have avoided adjustments that favor trial/error/success.
My initial spring needs about 8n on the Z axis to represent the touch, about 1 to 1.
5 N on the x and y axis.
The force required on the X-axis and y-axis depends on whether the touch is on one side of the probe ball or on the other, due to the geometric effect of the triangle layout of the contact point (balls and pins).
This first spring worked fine, but it felt a little too strong, and then I tried it with a spring close to half the assembly force.
This is also good, reducing the pre-Travel a little.
I decided to use that spring. I didn\'t try any other spring.
Here we can see an example of the typical usage of the probe I made for it.
This photo shows my inspection of the motorcycle starter box processed from a solid block.
The probe saves a lot of time compared to the methods I used in the past.
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