spindle touch sensor

Dear bob:

Rubber is a notoriously bad choice. It wears rapidly, it changes size rapidly with temperature, and with vibration in the probe mount.

Measure current flow into/out of the shaft of the probe. As it nears the workpiece, the capacitance goes up, and the resistance (which will be quite high) will go down. A few hundred volts may be required, but microamps at most, so compromises to safety is less of an issue. "A continuous static charge creator".

David A. Smith

it's actually an air pressurised rubber bonnet with an abrasive pad bonded to it. it's compressed against the workpiece to produce an 'abrasive spot'.

are you saying that the bonnet would have an integral capacitor. current flow would change when in proximity to the workpiece? would this have the resolution to determine actual touch on? and would it be repeatable? it also needs to work with differing workpiece materials.

the other issue is that the bonnet and workpiece will have coolant on them, so any high voltage system will have to take this into account.

thanks.

Dear bob:

No. You would convert the bonnet into a sort of "capacitive proximity sensor" (which you can do a search for).

Yes.

Better if you didn't touch (or the abrasive pad were fully non-conductive), but stayed just out of contact.

As long as the bonnet geometry did not change, and the workpiece were fully conductive (metal or very conductive-and-grounded graphite composite)

Would absolutely not work then. Coolant usually means water, and water has a very high k value. This will bollix up the works.

Better look for another idea...

David A. Smith

Do you probe with the workpiece on the rubber fixture?

Can you rely on repeatable tool changes? If so, the general method is to use a commercial probe. Renishaw makes probes with deflection sensors that record the position where the tip is slightly deflected. If you can rely on the repeatability of tool change, then something like this might be your best bet.

If the tool must be in place, then why not measure the strain on the spindle (in a manner similar to what you use now)? Just use a (much) stiffer load cell or strain gages directly on the spindle without the compliance.

What do you think?

-Matt

The only thing comes immediately to mind is an insulating base for the machine vise and sensing resistivity when the (carbon-loaded?) rubber touches down on the work piece.

Brian Whatcott Altus OK.

One possibility is the use of precisely machined air jets. The basic idea is that a pair of orifices are placed in the proximity of the desired position, i.e. next to the rubber bonnet, in such a manner as to allow the approach of the piece to be machined. As the workpiece approaches, the backpressure of the airjets, which is a function of the air flow resistance and the workpiece position, is monitored until it reaches a predetermined critical threshold. At the threshold point, the motion of the workpiece is stopped, and the abrasive bonnet may be activated. Some information regarding this procedure is given here:

Dave