Home-shop CNC guys: Home & Limit switches, again?

Home-shop CNC guys: Home & Limit switches, again?

Joined: January 1st, 1970, 12:00 am

November 8th, 2017, 2:00 am #1

I know I asked this before, but I'm not sure we really answerd the question- at least, not to the point I could grasp the concept.

I'm in the middle of converting the CNC lathe over to my new Centroid Acorn board (at least, as an when I have time) and to do it up right, it needs some home and/or limit switches.

I know a limit switch can be a fairly conventional- and not necessarily terribly precise- microswitch. That part's easy (other than trying to mount them away from chips, swarf and possibly even coolant.)

The "home" switches, however, need to be considerably more precise, as the computer takes that "home" position and bases distances moved off of it. If the home switch is only good for plus-or-minus five thou, the size of your part is only going to be good for plus-or-minus five thou, especially on multiple parts.

I also know that the home and limits can be the same switch, my question is, what kind of switch?

Lots of people recommend an optical switch- apparently a shutter in front of an emitter-detector pair is or can be sub-.001" accurate.

Most I've read about, however, are homebrew, and at this stage- that is, where I really still don't quite know what I'm doing - I kind of need an off-the-shelf solution.

Any suggestions?

Doc.
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Joined: November 23rd, 2014, 9:43 pm

November 8th, 2017, 6:13 am #2

Your limit switches are coarse switches which stop the travel before the machine hits the hard stops like you said.

Your home switch should only need to be used for each initial power up. A .005" difference between power ups on a machine you are hoping to run production (I'm assuming here) and the associated diameter compensation isn't something I would be particularly worried about. Depending on your tool holding it might be as simple as adjusting the X value on your G54 to compensate all the tools for the new home position. This would be no more difficult than setting a tool on a conventional DRO.

This is all assuming you're using something more advanced than open loop steppers.
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Joined: November 17th, 2014, 11:09 am

November 8th, 2017, 12:32 pm #3

Search for Optical Switch Reprap on eBay to find the stuff used on home 3D printers.

Fix some dust shields for them, and buy plenty of spares...

Home switches are an absolute necessity if you're swapping tools during the job.
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Joined: October 10th, 2014, 3:09 pm

November 8th, 2017, 2:44 pm #4

I know I asked this before, but I'm not sure we really answerd the question- at least, not to the point I could grasp the concept.

I'm in the middle of converting the CNC lathe over to my new Centroid Acorn board (at least, as an when I have time) and to do it up right, it needs some home and/or limit switches.

I know a limit switch can be a fairly conventional- and not necessarily terribly precise- microswitch. That part's easy (other than trying to mount them away from chips, swarf and possibly even coolant.)

The "home" switches, however, need to be considerably more precise, as the computer takes that "home" position and bases distances moved off of it. If the home switch is only good for plus-or-minus five thou, the size of your part is only going to be good for plus-or-minus five thou, especially on multiple parts.

I also know that the home and limits can be the same switch, my question is, what kind of switch?

Lots of people recommend an optical switch- apparently a shutter in front of an emitter-detector pair is or can be sub-.001" accurate.

Most I've read about, however, are homebrew, and at this stage- that is, where I really still don't quite know what I'm doing - I kind of need an off-the-shelf solution.

Any suggestions?

Doc.
They're just ordinary microswitches on my Denford Orac, but there's no actual need to home.

Z-zero can be found by touching off to the end of the stock and zeroing the coordinate system (G54)

X-zero can be set my taking a skim cut, measuring the diameter and entering that number into the coordinate system.
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Joined: November 2nd, 2015, 3:29 am

November 8th, 2017, 3:01 pm #5

I know I asked this before, but I'm not sure we really answerd the question- at least, not to the point I could grasp the concept.

I'm in the middle of converting the CNC lathe over to my new Centroid Acorn board (at least, as an when I have time) and to do it up right, it needs some home and/or limit switches.

I know a limit switch can be a fairly conventional- and not necessarily terribly precise- microswitch. That part's easy (other than trying to mount them away from chips, swarf and possibly even coolant.)

The "home" switches, however, need to be considerably more precise, as the computer takes that "home" position and bases distances moved off of it. If the home switch is only good for plus-or-minus five thou, the size of your part is only going to be good for plus-or-minus five thou, especially on multiple parts.

I also know that the home and limits can be the same switch, my question is, what kind of switch?

Lots of people recommend an optical switch- apparently a shutter in front of an emitter-detector pair is or can be sub-.001" accurate.

Most I've read about, however, are homebrew, and at this stage- that is, where I really still don't quite know what I'm doing - I kind of need an off-the-shelf solution.

Any suggestions?

Doc.
Maybe there is something I don't understand, but for your application with a manual lathe, I don't see the use of a home switch.

The use of the home switch is to have a repeatable position of your G54 when you restart your machine so you don't have to teach the origin of your work piece every morning. As long as the machine is powered up, the CNC doesn't care for your home switch. It will go with your encoder or stepper pulse count.

So if it doesn't disturb you to teach the position of your work piece after every start up, I think you don't need them. But I could be horribly wrong also.

Le_KiD_Qc
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Joined: January 1st, 1970, 12:00 am

November 9th, 2017, 5:59 am #6

Your limit switches are coarse switches which stop the travel before the machine hits the hard stops like you said.

Your home switch should only need to be used for each initial power up. A .005" difference between power ups on a machine you are hoping to run production (I'm assuming here) and the associated diameter compensation isn't something I would be particularly worried about. Depending on your tool holding it might be as simple as adjusting the X value on your G54 to compensate all the tools for the new home position. This would be no more difficult than setting a tool on a conventional DRO.

This is all assuming you're using something more advanced than open loop steppers.
Full build details can be found here. It's a homebrew CNC conversion of the very common DIY sort, using common 1.8 degree steppers and eBay ballscrews.

I started out with Mach 3 on an old XP PC, but that's not a particularly friendly setup for those of us that have no real prior experience with the software side of CNC, and the system was badly limited anyway by its inability to use more than single-pulse-per-rev spindle encoders.

I just picked up a recently-released Centroid Acorn and will thus be running the latest CNC12, which, for doofi like myself, includes (supposedly) easy to use conversational programming.

The eventual plan is to use gang tooling (likely just four or fewer tools) and do short-run production. Being able to properly "home" the machine initially, and for it to be able to repeatedly home itself at the end of each cycle, helps with part-to-part consistency and overall accuracy.

Doc.
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Joined: April 15th, 2003, 12:28 pm

November 9th, 2017, 5:50 pm #7

I know I asked this before, but I'm not sure we really answerd the question- at least, not to the point I could grasp the concept.

I'm in the middle of converting the CNC lathe over to my new Centroid Acorn board (at least, as an when I have time) and to do it up right, it needs some home and/or limit switches.

I know a limit switch can be a fairly conventional- and not necessarily terribly precise- microswitch. That part's easy (other than trying to mount them away from chips, swarf and possibly even coolant.)

The "home" switches, however, need to be considerably more precise, as the computer takes that "home" position and bases distances moved off of it. If the home switch is only good for plus-or-minus five thou, the size of your part is only going to be good for plus-or-minus five thou, especially on multiple parts.

I also know that the home and limits can be the same switch, my question is, what kind of switch?

Lots of people recommend an optical switch- apparently a shutter in front of an emitter-detector pair is or can be sub-.001" accurate.

Most I've read about, however, are homebrew, and at this stage- that is, where I really still don't quite know what I'm doing - I kind of need an off-the-shelf solution.

Any suggestions?

Doc.
I've got 2 lasers in the shop, both fairly accurate. The small one with strippers uses a magnetic switch of some sort, that just has to have a sliver if steel interred to the right point. The big laser uses standard lever switches, with rollers on them, to zero x and y. It's pretty good, good enough that if the power goes out mid job, It can boot up, zero, and continue. But it also uses servos.

I would be worried about optics in a shop embodiment. Most cnc machines h have some sort of magnetic switching to do limits and such, so that might be your best bet.
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Joined: January 1st, 1970, 12:00 am

November 9th, 2017, 7:44 pm #8

Servos virtually always have encoders built in, for a closed-loop system. They can, therefore, "self home" using a regular limit swtich.

The process is that the carriage travels over to the home switch, and trips it. Now that it "knows" where it is in relation to the limit, it starts "looking" for wht's known as an index line or index count in the encoder. So it sneaks past the limit switch until it spots that index line and then stops. It now knows, to whatever the resolution of the encoder is, where it is.

As I understand it, theoretically I could add encoders to my steppers, and the Acorn board, with some help, could read them and do that "self homing" routine.


However, at the moment I am NOT anywhere near experienced enough with the necessary software in general or the Centroid stuff in particular, to be able to pull that one off. So I'm gonna try and stick with mechanical limit switches. At least those I can understand.

Doc.
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Joined: October 28th, 2014, 1:06 am

November 9th, 2017, 10:02 pm #9

"...So it sneaks past the limit switch until it spots that index line and then stops..."

I don't think they would sneak past the limit switch. Often the limit switches aren't just sensors, sometimes they physically interrupt the power to move in that direction. So I think most systems would move to the limit, then move back to the home position.

There are some switches that stop the rapid travel, but still allow slow travel, to prevent a fast crash, but I (personally at least) don't think of those as the limit switches.
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Joined: November 2nd, 2015, 3:29 am

November 10th, 2017, 2:03 am #10

I worked with a AXYZ cnc router table for a project at my old job.

The way the homing worked on this machine is that it will go at a certain speed to activate a proximity sensor. After the first trigger, it will back up and reactivate the proximity sensor at a slower speed to increase the accuracy of the homing. I don't remember the exact model of the prox sensor, but it was something like a E2A from Omron in M8. The AXYZ table was using stepper motor and we did the homing routine only at the start up or after a big crash where the stepper might skip a step. We didn't have any problem with precision

Also like someone already said, in a shop environment, you don't want optical system. Yes it's a precise solution, put it's not the sturdiest.

Le_KiD_Qc
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