Newbie- Home Switch (Mechanical, Photo or Proximity)

[mkenney]

I've read some info on the different types of home switches but am honestly a little overloaded. My goal is for something that has a high level of accuracy and repeatability so that when I pair it with a tooling plate I can reduce/eliminate some of the set-up time when switching between different fixtures.

Mechanical switches seems to be the least expensive and most widely used. On the downside it seems to be the least accurate, please correct me if I'm wrong.

There is a photo kit from cnc4pc that is interesting but I don't see an accuracy listed with it:
http://www.cnc4pc.com/Store/osc/prod...roducts_id=183

I know I saw a proximity system listed somewhere in the forum a while ago. I can not seem to find it again no matter the amount of searching I have done???

Any advice and recommendations on products would be greatly appreciated.

Thanks again!

Mark [mkenney]

Editted to say HOME switched instead of the Limit switch I intially listed it as inaccurately. Thanks Crevice!!!

[Crevice Reamer]

I think you mean HOME switches. Limit switches do not worry about accuracy/repeatability--They just need to stop motion when limit of travel is reached. Actually, with a stepper system and mach3 soft limit settings, you could forego limit switches altogether.

Mechanical home switches need to be roller type and are only as accurate and repeatable as their mounts and trigger blocks are solid. Good roller switches solidly mounted and protected from chips/coolant are very accurate/repeatable.

CR.

[mkenney]

Originally Posted by Crevice Reamer I think you mean HOME switches. Limit switches do not worry about accuracy/repeatability--They just need to stop motion when limit of travel is reached. Actually, with a stepper system and mach3 soft limit settings, you could forego limit switches altogether. Mechanical home switches need to be roller type and are only as accurate and repeatable as their mounts and trigger blocks are solid. Good roller switches solidly mounted and protected from chips/coolant are very accurate/repeatable. CR.

Thanks Crevice, I updated it to say home switches as it should of, I miss-speak on this often, going to have to watch myself better!!

Are these roller type?


Are we talking .001 in repeatability or better then that?

Thanks again for the help!!!

Mark [mkenney]

[Crevice Reamer]

Yes, and they are very repeatable, but only if they or their trigger ramp don't move.

CR.

[Crevice Reamer]

Repeatability also depends on the accuracy of your steps/inch settings. If Mach3 says your axis moved three inches, but the axis actually moves 2.95 inches, then repeatability will be non-achievable.

CR.

[Crevice Reamer]

The usual method of wiring home switches is Normally Open. It will close and complete the circuit when actuated. This requires three inputs and one ground connection on the Break Out Board.

Limit switches are usually wired Normally Closed and in series. If a switch opens (Or a wire breaks) all axis motion is stopped. These require only ONE input and a ground connection to the BOB.

The switch on the left of your pic can be wired either NO or NC. The one on the right will only be one or the other.

CR.

[mkenney]

Originally Posted by Crevice Reamer Repeatability also depends on the accuracy of your steps/inch settings. If Mach3 says your axis moved three inches, but the axis actually moves 2.95 inches, then repeatability will be non-achievable. CR.

Been meanning to check this. I intially set it based on the screw rate and stepper specs. From what I'm gatherring, this doesn't mean it is set correctly.

On the switches, my current ones are definately not mounted as rigid as it sounds like they should be! Also used the standard arm microswitches without the roller as that is what I had at the time and was waiting for the other's to arrive.

And the learnning curve goes on Thanks for the continued assitance!

Mark [mkenney]

[Crevice Reamer]

Your SPI may or may not be correct, depending on the mechanical accuracy of the axis. Make a fixed distance cut or line in each axis, measure it, and compare the actual travel to the Mach3 DRO travel to be sure.

CR.

[BobWarfield]

I was really impressed with the accuracy, low cost, and durability of Vince's proximity sensor setup:

http://www.cnczone.com/forums/showth...846#post541846

Cheers,

BW

[HimyKabibble]

I currently have roller micro-switches for home/limits on my knee mill, and they work OK. Repeatability seems to be at least 0.001". However, the little roller arms are delicate, and easily damaged, and, of course, they are not coolant/oil/chip friendly. I am switching to fully enclosed optical limits, which I expect to give much greater accuracy, and, being completely sealed, will be oblivious to all the nasty stuff flying around a mill. This also allows me to have separate home and limit switches in a small space, by putting two optos next to each other, both operated by the same flag.

You normally want to connect home switches as normally closed, and simply connect them all in series. There's no advantage whatsoever to having separately wired switches. Also, using normally closed, you'll know immediately if one fails. If wired normally open, you won't know if one has failed until you try to home, and the axis sails right past the switch and hits the hard stop. Similarly, all my optical switches are normally conducting, and turn off only when the home/limit is hit.

Regards,
Ray L.

[LaserImage]

You may want to look into Hall Effect switches. You would be amazed at how many places they are used and their accuracy. They have three connectors - +v, -v, output. For my G540 I connected them directly to my 24v psu and the output directly to the G540 - works like a charm! Accuracy is a smaller number than I can measure with the equipment I have - a .001 dial indicator. The hardest part is getting your magnets aligned to where you want them. The other nice thing about them is the cost - $2.30 from DigiKey - p/n US5881LUA-ND. The magnets you can get just about anywhere, I salvaged mine from some cheap, $1.00, wooden boxes from a Michaels craft store - 2 magnets per box!

Gary

[escott76]

Originally Posted by HimyKabibble You normally want to connect home switches as normally closed, and simply connect them all in series.

I would think you want independent homes, and tied together limits no?