Cheap Tool Length Sensor Options

[JohnHaine]

For what it's worth here are some pics of one I made years back for my Novamill.

Basically laminated from a MS block with an inset magnet, then layers of 12mm epoxy fibreglass (could be e.g. Corian). There is a machined cavity inside which contains a stainless steel slug at the botton (insulated from the base), then a strong spring, then a 12mm square carbide insert as a hard anvil. Most of the layers are glued with epoxy but the top is bolted on to allow assembly. The slug has a hole drilled from the side for a 4mm banana plug to connect to the probe input. It fits to the table with the magnet, and the top face of the anvil is measured at 38.84mm above the table surface which is input as the setting plate thickness. As the Novamill table is small I generally just place this for setting the tool then take it away, with a bigger table it could be left in one place. The spring has a few mm free travel to reduce the consequences of a tool crash. Of course it depends on the tool being grounded. Total cost negligible as made from the scrap box.

On my lathe I have a rather different system to do an equivalent job which is to find the end of the stock in the chuck and zero the z axis. That's another story, it still relies on electrical contact between tool and work but senses that magnetically.