I’m at the point in my OX build where I’m thinking about limit switches, but it seems like they might be optional, because you can use soft limits? Questions:
I was initially thinking of induction sensors, but a bag of 10 roller lever arm switches is only $5, versus something like $30 for the sensors (I know, not a big difference on a $1500 machine, but still.)
+Peter van der Walt I know I’ve asked about this before, but it looks like the SN-04 uses 10-30 volts, while the above link mentions (emphatically) that it cannot handle more than 3.3v. Have you used these with a TinyG board before? Here is the SN-04 (NPN normally open) switch I’m looking at (if this will work, I’m going to pull the trigger on 6 of them right now):
From their page:
"The inputs are 3.3v logic inputs and must not have 5v applied to them or you will burn out the inputs. The inputs are de-glitch filtered with a resistor-capacitor circuit (RC circuit), and pulled up to 3.3v on the board via 2.7K ohm resistors (strong pullup).
Two 3.3v outputs are made available for opto-coupled and other powered switch options. if using active switches you will need to be careful not to exceed 3.3v on the inputs or you may damage the inputs. If you draw the 3.3v do not pull more than 30 ma."
This makes me think I would need to power them directly from the board power supply (to avoid drawing too much current), and then drop the output voltage?
Hmmm…more info apparently recommending against SN-04:
“To connect a switch to an input pin simply wire the switch across the ground and the input. This applies to both normally open (NO) and normally closed (NC) switches. Either NO or NC switches may be used, but all switches must be of the same type. We recommend using NC switches for better noise immunity to prevent false firings.”
Think of the limit switch as an active circuit, powered by 10-30V, with an open collector NPN transistor as the output stage; that open collector switches the load voltage (here, tinyG port pulled high to 3.3v by 2.6kohms).
If you are using primarily for homing, NC would probably work OK. The primary noise source is Spindle noise, most folks don’t home with spindle energized.
Use shielded cable for the hookup, ground shields at tinyG end(only).
I guess most importantly, the need to understand homing, machine zero, and Z axis zero would determine if u want to use switches at all.
If you are a production shop, homing switches save time to get set up. All the homing does is help define machine zero. This makes it quicker to touch off for Z part zero. Also home is the base line for offsets and that allows one to set up all sort of differnt fixture locations.
With all that said, if you just want to set up run some parts its super easy, and no need at all for any switches. The control gui makes setting 000 one click away at any time.
Drag the head to the center of the work table, touch the tool to the material (use a rolling paper between the tool and materail as a spacer) u want to cut in the center. Hit zero all, and run the gcode… if you use the return to zero code after the end of each part run, the head goes back to 0,0,0.
Just secure a new piece of material on the bed, and press play again. It really is that easy.
I have now. Other videos I’ve seen place the home position at the front left, not the back right. Not sure yet which is right, or if this is just a matter of taste. But my gut tells me to put it at the front left.
You see it matters not where Home is… It’s just HOME, it’s the reference from which all of the moves take place. It’s X0, Y0,Z0 also known as Program Zero .
Google CNC Coordinate System, and you will soon see as long as you have a fixed place to work from “home”. Home is where you want it to be. There is a system called Machine Coordinates, its the baked parameters that the controller works from, it’s the framework.
You can set all kinds of offsets, but what is most important is the Work Coordinates. This is the Zero based on the part you are working on. It the Touch Off location for Z, as well as the point at which the Ggode will start to run the routine.
It may seem confusing, but in the end its not. We chose that Home location because when the machine is homed it opens the work bed the most. That is to say the head and is out of the way the most relative to the place where he loads and unloads the machine with work.
Some people work from a vise, they may wish to make home the top left of the jaws. In a hammer vise only the back section moves, so the top left is a fixed position that never moves. That home would make for very fast part changes. However if you had multiple vises on your work table, you could set up multiple offsets to home on the different vises.
I get that; I’m not completely unfamiliar with Cartesian coordinate systems. But there is a tool chain involved here that all needs to be “on the same page” or else you may end up with cuts that are mirrored and/or flipped. But I guess this is probably easily transposed in CAM before generating the g-code. As I said above, it may just be a matter of taste. If so, then unless I hear a compelling argument otherwise, I will probably keep it the same as my 3D printers, which has 0,0 at the front left.
I think I’m going to pull the trigger on 6 of these SN-04-N2 (normally closed): $4.96/each, and free Amazon Prime shipping means I get them soon, too. 
