A simple query about homing switches & milling practise

if your homing swithes move all you axis (axii?) back to 0,0,0 ...then that means you can't have your tool go past those points (I'm quick eh?!)

But when designing in CAD/CAM, often 0,0 is used as my extremeties in the X & Y axis...but if so, then obviously it would be impossible to cut anything along those lines (eg my cutter is 3mm in diamter so would need to be X (or Y) -3mm to cut a line along X0 or Y0.

So what do people do here in practise?

1. Make sure their CAD/CAM output never gets any closer to X0, Y0 than the width of their cutter (seems clumsy!) (ie offset in CAD/CAM?)

2. Some offset in the CNC software? (eg Mach3?)

3. Start making designs well away from the 0,0 reference?

4 None of the above?

I ask, because I'm just about to fit some homing switches to my Frankenstein CNC creation...but then while pondering I relaised that I'd hit this particular problem (as I tend to have my design extremeties touching X0 & Y0...just for simplicity)

Most commercial machines, Mach is included in this, use two co-ordinate positions, Machine Co-ordinates [ MC ], are set at the homing switches and are just used for this purpose.
Knowing where home is every time means that you can set soft limits accurately so you can't run off a table.

Once homed and MC set for the first time it can be forgotten.

You then switch to Work Co-ordinates [ WC] and this is where you set the corner of your work or vise etc.

A goto zero command will return the tool to this position, a goto home command will send it back to the homing switches.

Mach can use two switches per axis daisy chained together and connected to one input, either switch can be designated as the home switch so for example you can have the tool home at the far right or far left of the bed, your choice.

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Thanks for the quick reply! (Sat night too!)

But how for example, does Mach3 differentiate between the two?

When I import Gcode, the CNC moves the tool to act upon those coordinates in the G-Code - Mach3 doesn't ask "are these MCs or WCs?") ...therfore if, back to my example, I have a countour line running along X0, the tool will try & get 3mm past that (the diameter of the tool) , but the homing switch will inhibit that.

So how does CNC app differentitate between a home MC of 0,0 & then subsequently interpret / apply WC 0,0 to a different position?

I guess I'm trying to figure how you have two set of coordinates in practise!

Does that make sense?

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Originally Posted by HankMcSpank

Thanks for the quick reply! (Sat night too!)

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Yes I know sad bastitch aren't I ?

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But how for example, does Mach3 differentiate between the two?

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Button on screen just under the DRO's marked Machine Coords, second from right. toggle between the two states, lights up red when in MC.

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When I import Gcode, the CNC moves the tool to act upon those coordinates in the G-Code - Mach3 doesn't ask "are these MCs or WCs?") ...therfore if, back to my example, I have a countour line running along X0, the tool will try & get 3mm past that (the diameter of the tool) , but the homing switch will inhibit that.

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The homing switch will only inhibit this IF your part is that big it's shape is the same size at the bed travels.
If for example your bed travel in X is say 300mm then the max part you can cut in theory with a 3mm cutter is 300 - 1.5 - 1.5 so 297mm.
In practice it would have to be a tad less because you may get switch bounce.

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So how does CNC app differentiate between a home MC of 0,0 & then subsequently interpret / apply WC 0,0 to a different position?

I guess I'm trying to figure how you have two set of coordinates in practise!

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The two sets are always there but once set the MC can be switched back to WC and ignored, as stated before goto home or home all will travel to the home switches. Goto zero will go to your WC point.

This WC, MC is one of the stumbling blocks that catch beginners out the most. You can ignore the home switches and not set them if you don't want and just stay in WC and reset 0,0 to the corner of your work and keep changing this from job to job.
You can still have limit switches at both ends for safety, just no home switches allocated.


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Many thanks John,

I've been ignoring them until now...but I need replicability (in other words, I'm aiming towards a goal, where I can easily churn some simple identical parts out in short order).

Parking the replicability for a moment, I have a fairly imminent need to be able to mill a tiny pocket out of the end of some plastic threaded rod (I don't have a lathe) ..one way I've considered tackling this 'how to' problem is via a home switch. Basically, if I can start my spindle/tool from an exact known location everytime, then I could for example have the tool drill a whole in my cnc's base plate with the tool - self harming for CNCs! (actually probably some waste material clamped to the base plate!)....I can then tap thIS hole & insert/twiste my threaded rod into it...becaUse the cnc tool drilled the hole in the first place, it knows the exact centre...but the tool would need to start out from the same position each time (hence the home switch!).

It's got to the stage where I can see quite a use for starting the tool in a known position each & every time.

That can be done without home switches or anything.

Fasten your sacrificial block onto the bed, position the tool where you want the hole by guesswork / eyesight / ougie board etc Zero all the axis at this point in Mach by clicking the Zero x, Zero Y etc buttons at the side of the DRO's

Drill your hole with the cutter, move away, tap it, fit screw, press Goto Zero button, under the DRo's and it will return to where you drilled your hole every time.