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Isel CPM3020 cnc machine - electronics rebuild

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raynerd:
That's my exact understanding but if the limit switches are hit then what's the difference between hitting those and a home switch ? Why can't they be used as a home as they are identical switches?

I know I'm missing something...

geoff_s:
On my CNC routers, the home switch is the limit switch for minimum direction. A separate switch is used for the maximum direction. When using soft limits, none of these switches is ever activated - except the home switch when initially homing the system.

Running Linuxcnc

philf:

--- Quote from: raynerd on January 21, 2016, 02:30:40 AM ---That's my exact understanding but if the limit switches are hit then what's the difference between hitting those and a home switch ? Why can't they be used as a home as they are identical switches?

I know I'm missing something...

--- End quote ---

Hi Chris,

Home switches aren't wired in series with the limit switches so triggering a home switch in the middle of running some g-code won't affect it. The home switch goes to a separate input.

I only have one home switch which is on my z-axis. Waiting for the z axis to go to its limit switch takes far too long so I added a home position (using a hall effect sensor and a magnet) so the table drops to the home position which is just enough to clear any tools in the chuck before the table homes in X & Y.

For those who aren't familiar with Mach3 you only need to use 1 input for all 6 limit switches. When you do a 'home all' it will drive the axes one at a time to a limit switch (or home switch if fitted). It knows which axis it's driving and the direction so it knows which limit switch it has hit. It drives until a switch changes state and then backs off until the switch switches back before setting the axis coordinate to zero.

Phil.

modeldozer:
Hi Chris,

When I built my CNC router i fitted 6 limit swithes in series and 3 separate home swithes.  The limits tiggers an E- stop and the3 homes go to three different inputs.  To be honest I never use the home switches as I normally set machine zero (X, Y, Z) to a datum on the part.

Cheers
Abraham

awemawson:
On a full size 'proper' cnc machine the limit switches will lock out the controller until someone with nouce can determine if it is safe to re-set them.

Remember you are playing with tiddly little stuff, but a full size machine can have several kilowatts of axis drive power that can shift things pretty fast and do very much damage.

On my Beaver Partsmaster the previous owner screwed up the Y limit switch setting, and it had managed to destroy a VERY expensive 40 mm ball screw and nut - if only those students hadn't fiddled ! (ex Bristol University)

On my Traub lathe the mass of the tool turret and slide mechanism is huge with servo drives of commensurate size  - you don't mess with the limits on those unless you have very deep pockets  :bugeye:

Good practise dictate that you have separate limit and home switches

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