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View Full Version : Limits or homing switchs???



Ross77
25-03-2010, 08:11 PM
I was planning to fit the limit switches to my machine this weekend but after reading on PMINMO's site that the best soulution is to use home switches instead and use the soft limits instead. (as you can then use the full travel of the bed as it wont have to back off the limit switch.)

Is this a good solution or is it best to still use limits as well?

Robin Hewitt
25-03-2010, 11:32 PM
I've never needed limit switches, if the cut looks like it will go anywhere near 'max travel' I check for 'enough travel' before starting the cut.

A home switch is handy if you are worried about losing position. Whether you want to set it near the job or in a remote corner probably depends on how long the round trip is going to take :smile:

Ross77
26-03-2010, 01:13 AM
cheers, so no limits or homing then just good planning before the job, makes sense i suppose.

Not sure I'm that confident yet, E-stop is a must tho?

audioandy
26-03-2010, 07:51 AM
Hi Ross

I have both home and limits on my machine, but I must say after I set up the soft limits I have never hit a limit switch. If you are running servo's then I would still install limits as it would not be the first time a servo has gone AWOL which could do some serious damage.

Home switch is nice for repatability.

Andy

ptjw7uk
26-03-2010, 05:05 PM
What is the accuracy of homing switches as it would appear to me that if some form of micro switch is used then the position it switched at would vary depending on the direction you were homing from!
Can the homing be controlled so as to only happen in one direction, as micro switches have a thickness to the control arm.

Peter

Robin Hewitt
26-03-2010, 05:10 PM
One solution is an opto slot phototransistor and a comparator. That has no hysteresis and doesn't impede the movement.

Problem is keeping it clean and dry :beer:

Ross77
26-03-2010, 06:58 PM
Cheers everyone



I have both home and limits on my machine, but I must say after I set up the soft limits I have never hit a limit switch. If you are running servo's then I would still install limits as it would not be the first time a servo has gone AWOL which could do some serious damage.


I'm using steppers on this one but will bear that in mind for the next one. switches and soft limits! dosnt that eat up more of the travel?


Can the homing be controlled so as to only happen in one direction, as micro switches have a thickness to the control arm.

I think thats the reason mach backs of the switch.




One solution is an opto slot phototransistor and a comparator. That has no hysteresis and doesn't impede the movement.



I was tempted use a similar type from the old machine but maybe I wont know. :smile:

tribbles
27-03-2010, 11:50 AM
I've got some magnetic sensors for mine, but I haven't got around to fitting them yet.

However, I am contemplating using a laser with a cross-hair generator on it, and visually aligning to a known point (which would be more than accurate for my needs).

Ross77
27-03-2010, 04:05 PM
I've got some magnetic sensors for mine, but I haven't got around to fitting them yet.


That was another option I looked at but the Mach 3 guide didnt recommend them as the get covered in swarf etc.



However, I am contemplating using a laser with a cross-hair generator on it, and visually aligning to a known point (which would be more than accurate for my needs).


That sounds serious........I think youve been watching too much TV lol


Think I'll start with the home switches and soft limits, and work from there.

irving2008
27-03-2010, 05:02 PM
theres a good thread on CNCZone (http://www.cnczone.com/forums/showthread.php?t=101878)about magentic hall effect devices...

i think if you used weak magnets (see the thread for reasons why) and mounted them at the limits and put the sensor on the moving part (yes i know it complicates things but means the 'sticky swarfy' bit is generally away from the cutting area...) then they could work well... and its cheap...

tribbles
27-03-2010, 10:05 PM
Since I'm cutting foam, I'd be more concerned about the foam dust blocking up the optical sensors.

I'm not sure how aluminium swarf would impact magnets (sure, steel swarf would)...

I've got reed switches rather than hall effect devices (which can easily replace microswitches). However, I'm not sure about their accuracy for home switches. Since the frame of my machine is steel, I can easily move the limits since the magnets stick to it :)

(Seriously, I'd glue them down when I was satisfied they're in a good location).

Ross77
28-03-2010, 04:17 AM
Cheers Guys. I started with the question of limits or homing, but the actual choice of switch seems to be the crucial factor. Theres more to this CNC malarky than meets the eye......

I've been using small microswitches because of the size constrant (and because i had them lying around) but the lever arm is really flexable and caused no end of problems as homing swithes. So Ive fitted them as negative limits (@ 0,0) but still called them homing switches and set the soft limits as -0.75 and max travel. seems to work ok.

One point that might be worth mentioning with regard to switch accuracy is the actual settings of Mach 3 (or what ever you are using). Initially I had the velocity set quite high, so it didnt take so long to setup the limits, but the acceleration was low. This meant that there was excesive over shoot when joging and the switches and soft limits didnt work resulting in lost steps. uping the acceleration improved the response and switch performance.