Isel CPM3020 cnc machine - electronics rebuild

[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]

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...

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 

Good practise dictate that you have separate limit and home switches

[philf]

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

Hi Abraham,

Certainly with Mach3 it's advantageous for safety and for the sake of the machine to drive to home or limit switches as a reference before you start machining. By setting the soft limits up correctly (and enabling "Use Soft Limits") you will then never run into the limit switches and before Mach3 will run any code it does a check that the soft limits won't be reached.

If the table or gantry is moving fast it may not be able to stop quickly enough before running into a mechanical limit which, as Andrew has pointed out, could lead to a ballscrew & nut being destroyed or worse.

I too always zero on a job but Mach3 works with two sets of co-ordinates akin to Absolute and Incremental on a DRO. I always home the machine on switch on and then I zero the edges of my part in the 'incremental' mode but the 'absolute' mode still knows the safe distance to the limits and thus can give you a warning.

Hope you're taking all this in Chris! If you get stuck you know where I am.

Cheers.

Phil.

[awemawson]

G54 Phil

[raynerd]

As always, appreciate your replies. Really helpful stuff and I think I now understand. Based on the info, I was fiddling last night I can see how you can set limit and home all on one pin - very smart.

I fully appreciate Andrew, based on what you said, that in industry or with big machines a limit is critical for safety of the operator but equally safety of breaking parts.

Thanks again for your time.

[raynerd]

The cnc machine is now up and running. I`m blown away with how fast and how accurate it is and how well it did with a 4mm cut depth in brass cutting some text.

[philf]

Excellent work Chris! 

Phil.

[awemawson]

Well done 

[raynerd]

Thanks Phil, Andrew and I appreciate how much help you have given me answering my questions through this thread. Its been a very nice build and it is great how much space there is in the back of the machine to fit the new drivers. I just can`t get over how accurate and quick it is - you can`t compare it to the TEP machine I converted some years ago.

Thanks again.

[DMIOM]

Well done Chris      

Can I repeat someone's earlier - the music really detracts from being able to hear you and appreciate the work you're showing - have it on the intro, and if you really really must have some quiet background at times, but not that loud please?

cheers / Dave

[raynerd]

No worries, cheers David - I presume no-one would really want to hear me blabbing on      - point noted. I think I have a poor ear for sound as to me, the music isn`t that loud at all after the first 10 seconds. I clearly need to drop it down to half of what I think. Cheers

Chris

[philf]

Chris,

Now you need a 4th axis connected in and try some wheel cutting on it and then crossing out the wheels!

Cheers.

Phil.

[raynerd]

Yes, now that certainly is on the agenda. Rather than build my own from scratch, I'd consider looking for a small 3" table and converting it just as I have my 4" vertex. It's too late now, but I've not actually sat my rotary table on the mill - expect it'll be a fraction too big.

Edit, RDG actually do a 2 3/4" table