Hall Effect Home / Limit Switches

[dcartier]

Now that I have my SO2 assembled I am looking forward to adding some home / limit switches to help with repeatability. I can hardly wait to know where "here" is between machining sessions.

For a few years I have been following a thread on cnczone from Roman Black (Romanlini) for some DIY open source hall effect limit switches. So before I even ordered my SO2 kit, I was sure to order in the sensors I would need to use this idea on a SO2.

You can see Roman's original thread here: http://www.cnczone.com/forums/open-sour ... -easy.html

... and a the post on wiring here: http://www.cnczone.com/forums/open-sour ... post758009

... and a post on magnet tests here: http://www.cnczone.com/forums/open-sour ... post847057

So for the So2 (or SO1), I started thinking about how to mount these. Looking at the rails, it dawned on me that a T-Nut slot is readily available for every axis. I just needed a way to be able to mount the sensors using the slots, hopefully with some room for adjustment.

Here is what I have come up with. This will be machined out of 12.5 mm acrylic and should allow the sensors to use the existing TSlots. This photo shows the magnet sitting above the hall effect sensor body. It also hows an LED included in the body, but I expect that I will have to forego that feature as it will be quite tight just to get the sensor connected. The sensor mount is 32 mm x 10 mm.




Here you can easily see the magnet floating above the sensor and the yellow sensor in the slot below. The sensor is 4 mm x 3 mm x 1.5 mm. Soldering that in a way so the connections remain inside the sensor body (and get encased in epoxy) will be a challenge. Of course the soldering will be done first and then the soldered unit installed in the body.


Unlike Roman's version, the tight space will require the resistors to be mounted at the far end of the cable. I can add LEDs for each axis there as well.

While I am waiting for some 1/16" endmills to arrive, I have been trying to workout how to cut the needed reliefs in the backside. I think I can do it by cutting bolt holes in the stock, bolting the plexi to table , cutting the front pockets/reliefs, and then flipping the stock on the same bolt pattern. I am just worried that the machine position will not be in sync with the bolt spacing, but I do know how far apart the bolts are as I cut them using the machine. Guess I will have to try what I "think" will work and see.

Any advice appreciated.

Dennis

[dcartier]

I finished up my home/limit switches today. However the original design that I posted above proved problematic. I was cutting it from the side of the body, so it required flipping the material to cut the undercuts. It was my first attempt at cutting acrylic, and the results were less than spectacular. Each of the bodies came out very brittle and chalky. I thought it was cast acrylic (it had paper covering), but it may have been extruded. Also I was using the 2 flute straight bit and I am pretty sure I over heated the acrylic. When I went to drill the hole on my drillpress for the TNut, all but 1 of them promptly shattered. Meh, I decided that a redesign was in order.

This time I bought some thinner acrylic and made sure that I could cut the units all from the same side. I also switched to using a single flute up cut bit. This time the acrylic cut like butter.

Here are the bodies that I designed out of yellow opaque cast acrylic.


I cut everything with a 1/8 bit, except for the slots in body for the sensors and the slot on the end for the wires. For those I used a 1/16 4 flute endmill.

Squeezing everything into the small pocket on the bottom of the units was quite a challenge. To hold them firm, I used lots of CA glue. Making sure the legs did not short was always a concern.


Here are the X,Z and Y magnet bodies. This shows a single magnet installed, but for these small magnets (1/8x1/16) I had to use 3 or 4 of them stacked to get a good result. For that reason I drilled the later units all the way through and used CA glue to hold the stack of 3 magnets in place.


Here they are on the machine.



I am quite pleased with how they turned out.

Dennis

[edwardrford]

Dennis,

Really nice work on this project! Your switches turned out fantastic by the looks of everything.

Did you end up doing a max limit on the Z axis, or are these 'just' on the X and Y axis?

-Edward

[dcartier]

Thanks Edward.

I put a home switch on X,Y and Z, and limit switches on X & Y. For the Z home sowitch I used one of the existing holes about mid way down the Z plate with a standoff and a rectangular magnet body that forms a T and runs along the side Tslot. I thought the Z might be more complicated than the X and Y, but it turned out to be quite easy.

A Z limit would also be also be possible by adding a sensor lower in the same T slot, and adding another set of magnets on the lower arm of the T body.

Dennis

[dcartier]

As they say, a picture is worth a thousand words, so here is a photo of what I was trying to describe.


My next project tomorrow is a dust shoe because that single flute bit chucked acrylic everywhere!

Dennis