A tale of two springs

[phil-barrett]

Two springs are key elements of the design.

The inner spring, Spider Spring, forces the Spider Pins against the sensing balls. When the pins are in place, the current flows making it essentially a normally closed switch. When a lateral or vertical force on the spider moves the pins, the switch opens.

The outer spring is designed to hold the spider cage against the leveling screws in the barrel bottom. This allows the Spider Core to be adjusted to ensure that the probe is perfectly vertical.

The sizes of the springs in the current model are approximate and require testing/experimentation. In addition, the force of the springs needs to be determined. I believe that the outer spring needs to be stronger than the Spider Spring, to prevent the Spider Core from moving when the Spider moves.

I am interested in any thoughts or ideas people have on this.

[Overlights]

Hi Phil,
It's an interesting project and I'd love to help you.
But it is difficult to give advice at such an advanced stage of development.
Regarding the electronic part, I take it for granted that you did a very good job, also considering multiple revisions of the PCB.
But a 3D probe is primarily a precision mechanical device and should be handled as such.
So in your place, I would prefer to fine-tune the mechanical aspects first, and only then the electronic functions.

In this regard, I would like you to clarify me the usefulness of some of the features provided.
For example, the need to use working voltages up to 24V.
I thought that voltages between 5 and 12 Volts already covered 99% of possible use cases.
Or even the possibility of NO (Normally Open) operation, in addition to the basic NC (Normally Closed) operation.
Adding functionality that is not strictly necessary complicates the circuit and the space required by the components.
The risk is to overshadow the mechanical aspects, those most directly involved in the precision of the probe and last but not least, in the ease of construction.

Directly answering your question about springs, a key aspect of your design, my opinion is that two springs should not be necessary.
And if they were not indispensable in the original Renishaw design from 1973, perhaps they are not even necessary in a DIY design, which, for the same performance, should favor simplicity and ease of creation.

At the moment, the current probe model is composed of various quite complex parts, which must be coupled together and which increase the possibility of inaccuracies and errors.
Finally, I think it would be better to modify the Design by increasing the diameter (and perhaps length of the lever arm and therefore precision) and reducing the height of the probe, which on the practical side takes away travel from the Z axis (and the ability to probe taller objects).

[phil-barrett]

Thank you for your comments.

On 24V. Many machines use 24V for their signaling, limit switches and such. It adds no cost to support it.

The outer spring is used to allow alignment of the pcb/ball assembly so that the plane defined by the measurement balls is perfectly parallel to the measurement plane and the spider's probe is normal to that plane. Otherwise, I do not see how to make it adjustable to deal with the imperfections of balls.

I hear you about the size. Once we have a working prototype, we will look at improvements. Size is certainly on the table. For what it is worth, it is similar to other probes out there.

[whiskeyblacksmith]

The spider spring is definitely needed. The outer spring with leveling screws doesn't matter as much and can be done away with.

Instead of using an outer spring to hold it down, you could just use two opposing screws. One on the top and one on the bottom to sandwich the board in place and still allow leveling.

Another thought I had is that a perfectly vertical probe does not matter unless you are using something like a star stylus. All that matters is that the ball on the probe rotates concentric to the spindle. One of the things that's missing from your current design is any way to indicate the probe tip into the center.

If you look up the manuals from renishaw on how to replace a probe stylus, you will see that there are three set screws between the shank going into the spindle and the main probe body. To indicate the probe and you have to put the probe into the spindle and put a dial test indicator on the probe tip. You then rotate the spindle and adjust those three set screws until your dial indicator reads zero all the way around.