Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: Mueller on November 08, 2008, 06:39:54 PM
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Going to use the factory switches on my Bridgeport series I with my new computer and Mach3 (I bought Mach3 2 years ago and never once used it)
I was wondering about the accuracy or repeatabilty of the homeing switches.
Does anyone verify that they are tripping at the same place?
Am I worried over nothing??
If machine is completly turned off, I take it Mach3 will not remember where the mill was before shutdown?
Thanks,
Mike
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It will depend on the type of switches you use I suppose. On my Bridgeport I have optical switches and I have confirmed they are accurate by doing 100 homing moves and confirming the actual position by using a glass scale and DRO, out of the 100 moves one or two (cant remember exactly as it was at least 4 yrs ago) were out but the amount was only 0.005mm, the rest all had the DRO reading zeros.
I have heard prox switches also can achieve good accuracy but I have no experience with them so cant comment further.
On my lathe I use switches but they are just to trigger my servo drives to look for the index pulse on my encoder and the mill I am retrofitting at the moment will be homed in exactly the same manner as well.
For your other part of the question, yes Mach will remember where it last was if you have persistent DRO option chosen. However it can not confirm that it is in that same position as it is open loop.
Hood
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I can confirm Hood's remarks. I also use an optical system - mine are lasers shining on detectors, but if you test them by zeroing without the automatic zero facility, it shows then innacuracy between the stops and on mine is is between 0.005 and 0.001 thou.
It all depends on your switches, and I think optical switches are more repeatable, particularly those fitted with a Schmit trigger.
The DRO's will retain their information between sessions, as, indeed all the other information in the system is kept between sessions - but what you cannot be sure of is that someone (or something) has not moved the machine (even by a fraction) since you last used it.
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Question on switches. Does the dro indicate the small movement created when the stepper is powered after being shutoff?
Maybe a dumb question, and no quick answer needed, as i don't use switches yet. But curious.
RICH
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Question on switches. Does the dro indicate the small movement created when the stepper is powered after being shutoff?
If you're steppers move when powered up, Mach has no way of knowing this. So no, the DRO's won't reflect the movement. The DRO's only show movement commanded by Mach.
Not sure how that's related to switches?
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I'm sort of new to this so maybe I'm missing something in the question. Sounds to me like you are using the home switches 0,0 as a position to machine by.
The home switches are used on startup to set the MACH machine co-ordinates (for whatever it's worth) to 0,0,0 is but those positions are never used to reference a part when machining.
i.e You probably never set a part on the table and use the home switches as the botom corner of the part or anything like that.
The table is usually moved from there to somewhere in the middle of it's travel and the part placed in the middle of the table. Both a long ways away from machine home 0,0.
Then a corner (or whatever) of the part is indexed and then the working co-ordinate DRO's are zeroed. Those are what you work by starting from there.
The machine DRO co-ordinates at that point might be at X8 and Y4 but it makes no difference.
Bottom line - I don't see what difference it makes if the home switches are off 1,10 or 100 thou. You shouldn't ever be working near the home or using them for anything. I rarely even look at them.
(there are reasons to take them into consideration like whether you have enough room to set a fixture offsets or whether soft limits are going to be exceeded but for the most part I never even look at the machine co-ordinates.).
Maybe I'm missing something.
Sage
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Bottom line - I don't see what difference it makes if the home switches are off 1,10 or 100 thou. You shouldn't ever be working near the limits or using them for anything. I rarely eve loo at them.
(there are reasons to take them into consideration like whether you have enough room to set a fixture offsets or whether soft limits are going to be exceeded but for the most part I never even look at the machine co-ordinates.).
Maybe I'm missing something.
Sage
Machine coords and accurate homing are very important. One instance is you set up a job that you need to run a lot of, you have a fixture of some sort so that you can place the parts in for accurate placement. Ok so you have 100 done and its getting late but tomorrow you will finish the rest so you shut down Mach and go home. In the morning you come in, reference your machine and away you go, if you didnt have accurate home switches you would need to find the zero position of your fixture again.
Another instance may be you are doing a one off, its a long job and you need to go home, its either leave it running, leave Mach on all night or if switches are accurate you can shut down and next day home and go straight to where you left off with confidence.
Another instance would be on a lathe, you set the tools up for zero to be spindle centre on the X axis, if you have no way of homing accurately you need to take a cut each time and set the dia for your master tool by measuring the stock dia. I have accurate homing so I know once I have homed I can tell Mach to cut at 30mm dia and it will be 30mm dia and I then have no need to set the tool coord every time I start Mach.
Hood
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Another example. On our woodworking machining center (not mach3 though), there are 4 different zones with pop up pins to locate your parts. You place your part in the correct zone, step on a foot pedal to activate vacuum, and push a start button to run your parts. The machine is homed once a day when turned on, and that's it. We run several hundred unique parts a day, and it only takes seconds to load the program (with barcode scanner), place the part and start running.
I'll be using a similar system on the router I'm currently designing. Home to switches, and have 0,0 offset form the switches (X,Y) by 2-3 inches. Most parts will be placed at 0,0 (via some type of fence or stop system, not designed yet) and run. No need to zero the tool to run parts.
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Valid reasons to have them accurate I guess.
Being that I don't do production work I don't find it a big deal to just zero over something on the part. I also turn the machine off in that part referenced position. Nobody tampers wih my home shop machine.
My trick for getting accurate machine co-ordinates after power up is to use the set of DRO's (legacy) also on the machine. I zero them on the part at power up. Then turn on Mach and the Stepper drivers (just in case something jumps the DRO's will read it)
I home the machine with Mach (which moves the table and sets the machine co-ordinates to 0,0) and then I send it back to the part using the MDI and the numbers registered on my DRO's.
I guess we all work out tricks.
The home sensors would have to be something better than a simple micro switch (like I have). I don't think a uswitch can be expected to be repeatable especially considering the conditions. 've never actually tested their accuracy or repeatability.
Apparently others have worked out some more accurate methods like lasers to do the homing. I don't really require it. (I don't think).
Sage
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The switches I have on the lathe are accurate but they are expensive Honeywells but I do the homing via the encoder anyway so I could probably get away with cheaper ones. The optical switches however are cheap and easy enough to make a coolant/dirt proof housing for them and they are deadly accurate.
Hood
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What are the optical switches you mention. I've done a lot of work with optical interrupters which are an infrared LED, and receiver placed across a gap from each other, but they require some electronics to condition them. We used to use them in ticket transports to detect very fast moving tickets. I suppose you might be using a commercially packaged version of those giving an electrical "contact closure" or circuit path.
Sage
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Use Optek OPB917B, only thing you need is a current limiting resistor. Heres how I enclosed the one for the Z axis on the Bridgeport, there are springs to go either side of the flag to keep it centered and 'o'rings go in the housing where the rod protrudes. I could have made it in two parts but didnt have thick enough Alu on hand at the time so just made it up out of thinner plate. The ones on the X and Y were ready made from Industrial Hobbies but I dont think he does them any more.
Hood
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OK. Those are (except for manufacturer) exactly what I'm familiar with. And you are correct, you would only need a resistor for the LED half. I forgot about the BOB having a pullup to 5v on it's input so that would take care of the photo transistor collector side.
Hmm.... now you have me thinking.
I like your hardware to make it swarf proof too.
[edited - removed my comments about using optical switches for limits. The discussion was using them for home switches]
So you say they are "deadly accurate". I know they seemed to be in our old ticket transports but then our firmware used to jocky the ticket edge back and forth a bit in front of the sensor to get it just right.
How fast do you approach the sensors?
Thanks
Sage
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I think I just homed at the normal 40% at that time, it was on my first conversion that I did when I tested them with the glass scale and DRO. On that machine I think it was 4M/min on the X axis so suppose that would be about 1.6M/min homing.
Hood
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thanks, I have a bunch of different opto switches from other projects, never thought about using them in this application....I'll have to whip something up now !!!
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Automation Direct has many types of environmentally sealed limit switches for about $38 each.
They are quality made in Italy units and similar to the original Bridgeport units.
May be a lot easier to go this route than mess with making up optical ones that are sealed.
Scott
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Mueller I've been using the original limit switches on my Bridgeport for around a year now, I've found them to be quite accurate but it does seam to depend on the speed that you home at.
Give them a try!
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I've managed to score several Industrial Hobbies optical limit switches at a great price. I can copy them for the rest I'll need.
Regards,
Ray L.
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They are nice switches Ray but just be carefull with the wires, they are easy to pull off. I put a cable tie (zip tie to you guys I think) round the wire inside so that there was slack inside even when the cable was taught.
Hood
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They are nice switches Ray but just be carefull with the wires, they are easy to pull off. I put a cable tie (zip tie to you guys I think) round the wire inside so that there was slack inside even when the cable was taught.
Hood
Hood,
Will do. Thanks!
Regards,
Ray L.