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motoxy
27-08-2012, 10:48 PM
I noticed that on the x axis when it hits the homing switch it 'bounces' back a few steps. On the z and y axis it hits the switch and just stops. Is this a setting or does it just happen. using mach 3 btw.

Bruce

i2i
27-08-2012, 11:44 PM
it should come back on all three axis

JAZZCNC
28-08-2012, 12:02 AM
I could be off the switch but you haven't seen it move.? . . . easy way to tell is to go into diagnostics screen and check the " input signals current state" the M1,M2,M3 home led should be off after homing.!

Just watch the diagnostics screen while it's homing and you'll see the pin state change then go back to off.

motoxy
28-08-2012, 12:45 AM
Thanks i2i and dean. I know its working just suprised that the distance was differant. I have a small problem that when the home switch is also a limit switch sometimes the z when it homes does not clear to allow the x and y to home. Have to check that out on wed as I actually have work for tomorrow.

Bruce

motoxy
28-08-2012, 06:49 PM
Okay got home early and fiddled. When the z homes although the diagnostic says the switch is off the y will not home or it will do a couple of steps and stop. If I move the z down and then home the y its fine. I was curious as to why the x backs off maybe 1/2 a turn but the z and y almost appear to not back off at all although the switch does clear on the diagnostics???

Bruce

russell
28-08-2012, 10:10 PM
Could it be the hysteresis of the switch (the difference between the on and the off threshold positions)? Or even backlash on the x axis?

Russell.

motoxy
28-08-2012, 10:24 PM
Could it be the hysteresis of the switch (the difference between the on and the off threshold positions)? Or even backlash on the x axis?

Russell.

Do not think its backlash as the motor does not try to back it off. I was looking on the mach forum and realised that I have not connected the shielding to earth yet. Naughty me. So I will try that tomorrow and see if it helps.

Bruce

russell
29-08-2012, 02:46 PM
Do not think its backlash as the motor does not try to back it off. I was looking on the mach forum and realised that I have not connected the shielding to earth yet. Naughty me. So I will try that tomorrow and see if it helps.

Bruce
Sorry, I'm not understanding. I thought you said that the x motor backs off by about 1/2 turn???

Russell.

motoxy
29-08-2012, 06:48 PM
okay I think I have got to the bottom of the problem.....crappy switches.
When a homing switch is triggered then the motor will back off until the switch clears, as was said earlier. You can test this by homing an axis and the pressing the switch manually. If you hold it in the motor will keep backing off until you let go. Because these switches are, cannot think of the right word, flicking off then the motor winds back until it flicks back. Trouble with these switches is that they start to lift off the connection before they fully flick off. Therefore the circuit is broken and the motor only goes back a very small amount to make the switch. This means that the switch is resting on the gantry and any vibration at all will lift it off and so the next homing is reset early. Thats why when I backed off the offending axis the rest homed perfectly. This also explains why one switch, which must flick as it breaks, allows the motors to rewind a small amount before it flicks back.

Hope this makes sense to anyone else and serves as a warning on cheap switches.

All of this is what I think Russell indicated when mentioning hysteresis of the switch. I had to look that up:whistle:

Bruce

Now I have to investigate why when I home the x axis the b axis which is slaved to it does not operate. So near and yet so far....

motoxy
29-08-2012, 08:15 PM
Industrial Limit Switch TZ Series | eBay (http://www.ebay.co.uk/itm/Industrial-Limit-Switch-TZ-Series-/270745711183?pt=UK_BOI_Electrical_Components_Suppl ies_ET&var=&hash=item3f09b3b24f#ht_1937wt_1111)

Anyone know if these are any good? I was looking at the tz8104
Bruce

C_Bubba
29-08-2012, 08:28 PM
They look exactly like the ones I purchased about 10 years ago for my machine. I only had problems with one of three that I got and it was my fault for not designing a proper mount and interface to the moving parts. I sealed the electrical connections and use flood coolant with no problems.

Jonathan
29-08-2012, 08:57 PM
I got a heap of these recently:

SN04-N DC 10-30V NPN NO 3-wire 4mm Inductive Proximity Sensor Switch Detector | eBay (http://www.ebay.com.au/itm/SN04-N-DC-10-30V-NPN-NO-3-wire-4mm-Inductive-Proximity-Sensor-Switch-Detector-/280950446097?_trksid=p4340.m263&_trkparms=algo%3DSI%26its%3DI%26itu%3DUCI%26otn%3D 18%26pmod%3D390447284717%26ps%3D63%26clkid%3D16741 77735205510498#ht_1953wt_1139)

There's good reviews about them on CNCzone. I'll post when I've tested them, but that may be some time.

Robin Hewitt
29-08-2012, 10:13 PM
Big chunky limit switches do not not necessarily give you good positioning, they are more concerned with reliability than a fine switching tolerance.

The best positioning switch is probably an opto slot with a built in Schmitt trigger. But if the switching point chances to align with a completed step you can still go +- one step.

Perfection is probably a side paddle arm on the screw that winds into an opto slot. That would allow you to adjust the switching point to mid step.
I'm going to give it a go, but that's only because I am totally anal :beer:

Jonathan
29-08-2012, 10:26 PM
Perfection is probably a side paddle arm on the screw that winds into an opto slot. That would allow you to adjust the switching point to mid step.

Perfect perfection would be to put the opto slot on the stepper motor or ballscrew - the rear shaft on the motor would be pretty convinient. If you put the sensor/slot at say 30mm radius then one step is amplified to just under 1mm movement instead of 0.025mm (or whatever). Trivial to detect 1mm, but you would need two sensors. One standard switch on the axis as normal to get it close, then look for the signal from the 'rotary' sensor to get it spot on.
That's what I'm doing on the next machine anyway - inductive to get it close, then optical on 30mm radius disc for perfection.

boldford
30-08-2012, 10:44 AM
As I've no need to machine metals I'm thinking of Hall Effect devices. I'm concerned mechanical switches or slotted opto devices might be affected by dust. No firm decision made yet.

irving2008
30-08-2012, 11:27 AM
As I've no need to machine metals I'm thinking of Hall Effect devices. I'm concerned mechanical switches or slotted opto devices might be affected by dust. No firm decision made yet.

No need to use hall effect... simple magnetic reed switches work well although need debouncing just as a microswitch would. A hall effect device might just be too sensitive. With modern high-strength mini-magnets I found for one application I needed the hall effect device over 5cm away from the magnet before it stopped recognising it. Also the reed switches in burglar alarm sensors are ideal and not expensive 3.99 from maplin (http://www.maplin.co.uk/miniature-contact-switch-12565) or 1.45 on ebay (http://www.ebay.co.uk/itm/MAGNETIC-REED-SWITCH-DOOR-WINDOW-CONTACTS-ALARM-SWITCH-SECURITY-/110943803378?pt=LH_DefaultDomain_3&hash=item19d4c417f2)

Jonathan
30-08-2012, 12:10 PM
I think the inductive switches I linked to in post 12 sound a lot easier than using reed switches, unless there's some advantage to using reed switches over them? SN04-N are the same ones DMM-tech use with their servo systems.

boldford
30-08-2012, 12:40 PM
With so many ideas to choose from it will probably come down to cost v repeatability.

Hummm. . . . . So many design decisions. . . . . .

m_c
30-08-2012, 01:26 PM
I'd vote for Robin's suggestion of opto slot sensors.
I used Buy Photoelectric Sensors U-shaped thru-beam sensor,PMR44P Sunx PMR44P online from RS for next day delivery. (http://uk.rs-online.com/web/p/photoelectric-sensors/4805231/) for the X-axis on my lathe, and repeatability is good. They come in various mounting styles, so you can normally find one that will fit with minimal effort.


From experience, inductive sensors generally have quite a wide tolerance, both in range and switching time. I've tried using a few different ones to control hydraulics in the past, and I could never get them consistently detecting motion. Good old fashioned mechanical heavy duty limit switches worked far better, even though the occasional one would get wiped out when things moved.