The Cerebral Forum

CNC Projects => CNC Questions and Answers => Topic started by: Davetech on July 15, 2009, 04:25:40 AM

Title: Setting up limit switches
Post by: Davetech on July 15, 2009, 04:25:40 AM
I'm making progress on "Tom's Easy Mill".  I'm to the point of setting up limit switches after building the Z-stage and then rebuilding it because it was oogly and I didn't like the first one.

The little instruction booklet for the driver board, which I have misplaced >:(, did not say whether the limit switches should be normally open or normally closed. But it did show separate limit switch inputs for each of the three stages.

I had charged ahead and wired them as normally open, but then I read a post where someone was talking about their system and their switches were N.C.

Is there a standard? I know I can find out with a little experimentation, but I didn't want to apply power to the board until I have everything right. I've written to the seller but no answer back yet. 

To show my noobness, I also just assumed that two switches are required for each stage... one at each end?  I wired them in parallel since they were N.O.

Another question I have is about placement of the Z-stage switches.

Placement of the switches for the x and y stages seems straight forward, but placement of the bottom z-stage limit switch seems critical, where you want the mill bit to be able to reach the maximum working depth without going too far and touching the stage platform. Do you normally have the work piece sitting on top of a scrap piece of wood or something and just set the limit to be at the surface of the scrap?  Should I make that switch height easily adjustable?

Yesterday, the mailman brought my parallel cable gender changer so I can now hook the board up to the computer. I'll be ready to apply power once I have these switches set up correctly. I had been wondering how the program knew where it was on the board. I found in the K-cam documentation that it figures out where it is when it is first started up by seeking out the three limit switches and bumping them three times to make sure it knew where the limit was. I don't know if other software does it this way or not... cause I'm a noob :P. But that does tell me I can't experiment until I have the switches in place.


Title: Re: Setting up limit switches
Post by: Gabriel on July 15, 2009, 06:19:08 AM
Both of the driver boards that I have used in the past were normally open, and I would assume that that is the standard because otherwise you would absolutely Have to have limit switches set up for the board to even work.

I had to learn how to program limit switches while writing my heliostat program, and I can tell you that it would be easy to make them either N.O. or N.C. with the code. I would guess, although I haven't actually looked, that CAM programs could give you the option to switch between the two.

I think the way you have it wired now is how it is typically done. Checking your CAM program's documentation might reveal some useful tips because the way it is wired is really up to the person who programed it.

I set the work piece on top of a piece of scrap like you said, and I don't actually use limit switches for homing. For me, I've always found it easier to jog the router bit to where I need it and then just reset X,Y, and Z to zero. Of course, I've never done anything that requires precise alignment before like what you're doing with PCBs.

For your Z axis, you could try what this guy did. ( You have to scroll down some to find what I'm talking about. He tells you how to set up a touch plate in Mach 3, but there might be a way to do it in K-Cam as well.
Title: Re: Setting up limit switches
Post by: BackyardWorkshop on August 18, 2009, 06:46:29 AM
I have my limit switches wired N.O. - and I use Mach3 and it allows you to change the Active Low or Active High for each switch - I too have wired them in parallel so if either switch closes it trips - I'm also using one switch on each axis as a home switch - and since their in parallel then you have to tell Mach3 that "home" is in the negative direction
Title: Re: Setting up limit switches
Post by: Davetech on August 18, 2009, 04:59:41 PM

Thanks for that.  When I first posted the question, I didn't have the hardware finished and was not yet familiar with the software. Once I had something to work with, I saw that my question was pretty much a non-issue since K-Cam and the other packages are very configurable.

So far as the home position, I'm with Gabe, in that it is easy to pick a point you want to be home, manually jog to that position and zero the axis'.

I've learned a lot, but I'm still a noob.  ;D