Cheap Tool Length Sensor Options

[AndyUK]

Why on the one hand do savvy builders on this forum appear to have no interest in a tool sensor that can save them so much time when it can be as simple as a £6 panel switch yet the market supports solutions at £50-100 and Shapeoko hobbyists go mad for the benefits and pay £120?

My spindle is grounded, which seems to also ground my bits, so there is nothing to clip on, and I have a block of cast aluminium which I use as a Z touch off puck on a coiled wire which can reach anywhere on the bed. I do my tool changes manually, and it's really no extra trouble to do a new touch off. Sometimes I even change touch off references during a job deliberately. The time saving (if any) of a fixed probe which would take up usable bed space is of zero interest, because I'm there anyway and it literally takes seconds. In three years I've broken a single £4 Vbit through accidentally driving it into my touch off puck (I got a cable stuck around my hand pendant which unexpectedly drove the Z down and made a whoopsie) so it's not a massive safety issue, other times the tool is strong enough and it just stalls the Z axis.

Soo, yeah, don't see the point. If I had ATC that would be a completely different story.

[Andrewg]

Soo, yeah, don't see the point. If I had ATC that would be a completely different story.

Thanks Andy. Having an earthed tool so no crock-clip needs attaching before probing is clearly a huge advantage (I did check, mine isn't). I can also see that the need to travers all the way to a fixed sensor and back is a waste of time that could out-weigh the need to position the probe. You make me reconsider the solution of leaving my touch plate permanently plugged in, change to a coil wire to reduce wire tangles and make a holder for the touch plate so it is safe on the machine when not in use and perhaps replace the tool crock-clip with a magnet so that connect/disconnect is easier. That is even easier and cheaper than wiring in a £6 switch!

[AndyUK]

Having an earthed tool so no crock-clip needs attaching before probing is clearly a huge advantage (I did check, mine isn't)

Damn. Did you measure where the continuity ends? Wondering if you could get lucky by connecting to the spindle mount? I'm using one of the Hyangyang 2.2kW spindles from fred, so everything is metallic and unpainted which probably helps.

[Andrewg]

Damn. Did you measure where the continuity ends?

Well I hadn't, and when I did, it was continuous from frame to tool, (or to acknowledge M_C's point, it was the six times I checked)
Previously I had just tried the touch plate without attaching the tool clip. So now I attached the tool clip to the frame.....and it works! So if I earth the -ve digital input for the probe my touch plate will (almost M_C?) always work without needing to use the tool clip

Not sure where that leaves me though. This might be worth while to streamline use of the movable touch plate. Still looking into suitable retracting or coiled wire to allow me to leave it permanently connected in a holder on teh machine (it is looking for it in the draw that adds the most time)

However it still leaves me weighing up the advantage of a fixed tool length sensor. Feedback here from people like Neale helps in that evaluation. It also helped looking at Ger21's screen set and the macros included for both approaches. If I have a fixed sensor, I certainly don't want to have to trigger it first to check it is working so a switch (With or without overtravel protection seems the way to go. I am now wondering if I can use my cheap panel switch to act as a touch plate by connecting the body to signal and use the switch as over travel protection (in case M_C is eventually proved right)

Thanks for your help

[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.