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Author Topic: Zeros For Different Tools With No Home/End Switches  (Read 2213 times)

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Zeros For Different Tools With No Home/End Switches
« on: April 11, 2013, 04:54:32 AM »
Hello everyone,

Since I got my first mill a few months back I've:

- Successfully switched my machine over from some DOS control software to Mach3 (which was the hardest step by far)
- Gained some basic knowledge about tools
- Established a few supplier relationships (for tools (bits) mainly). I now have a supplier for the cheaper HSS tools and have a company that customs makes carbide tools for me. Interestingly enough it's both faster and cheaper for me to have them made instead of ordering them to be delivered (they make them for me in 2-3 days).
- Started basic usage of the mill (mostly for cutting plexiglass/acrylic and forex/pvc foam sheets)

Things are pretty great so far and I've come a long way but there is one part in my workflow that is particularly annoying and I'm sure there is some other way to go about it.

My machine doesn't have home switches so more or less every time I add a new workpiece onto the machine I have to zero the home positions to match new workpiece. I know a lot of people will keep telling me that I don't actually have to zero all to each workpiece and that I could instead just do that with the workpiece coordinates. Here's the thing though, I like having maximum usability of the material I'm milling and usually that's within 10mm of the edge of the workpiece and since I don't have end/home switches my soft limits are based around the ref all home as well.
I am guessing however that there isn't a way around this one without installing home switches, if there is please let me know.

Now to the main issue. I don't have materials or tools setup in my CAM software (CamBam) so when I'm milling a new piece I do the following: measure the thickness of the material I am about to mill, select the tool and setup everything in CamBam. Now instead of selecting a material or a pre-defined tool I just enter the depth I want to cut, so for example if I'm cutting 3mm acrylic I just set the depth in cambam to -3.5mm and export it to g-code. Then when I import that into Mach3 I just select the x,y zeros for the workpiece, activate the spindle and jog the z so it touches the material a bit, then I just set that as the z and start milling. Mach3 will lift the tool to 3.0 with G3, go to the starting point of the cut and lower it to -3.5mm as defined in CamBam.

The problem here is, it's pretty annoying having to do this every time but that's not even the main issue. The main issue here is the all important z soft limit. I defined the z limit (soft) with one tool and set it at -57 for example, then if I change the tool then I have to change the z limit again. This quickly becomes annoying but that's not even the worse of it. I usually know right away what the issue is but my employees, who I'm also teaching to operate the mill, probably won't know the solution and will just turn of soft limits and continue machining. As you can imagine that can have serious consequences if someone made a mistake in the CAM part and it's inevitable that someone will make a mistake and that something will happen.

I think that this part can be solved with tool offsets but I have no idea how to setup these because as said, I'm pretty new to the world of CNC machining.

Looking forward to your replies and thanks in advance!
Re: Zeros For Different Tools With No Home/End Switches
« Reply #1 on: April 11, 2013, 08:27:03 AM »
I don't think that Home switches will help you here anyway, since their position will be fixed but the control point will be at different places depending on the tool length.  I think you need to zero the axes to the workpiece (not the home position).  The way I do this is to use a probe in the spindle to pick up the edges of the workpiece to set X and Y; and a tool setter on the table to set Z for the particular tool in use. 

My tool setter is a plate at an accurately known height, which is isolated from ground so that I can connect it to the probing input.  There are several tool height setting macros around on this site including a good one from Tweaky.  This way I know the height of the tool above the table surface; and I tell my CAM what the height of the material surface is above the table; so it can calculate Z to cut the right depth.  I can also do a quick search on "Z-" using a text editor on my gcode to check that the machine isn't going to try to gouge out the table.
Re: Zeros For Different Tools With No Home/End Switches
« Reply #2 on: April 11, 2013, 12:27:55 PM »
i have had issues with sticky, imprecise homing switches on my mill at times.  by clamping a tooling ball in my rear table t slot far to the x+ side as possible (this puts the ball in the general direction where my x-y home on my machine) and leaving it there, i can always go to the ball, indicate it, and set x-y zero.  then by using a tool holder with a flat ended rod i can go to x-y zero and set a z zero using a flat block or a shim top of the ball to the flat rod face.  it should be simple from there to find a pseudo xyz home for your machine near the limits of travel. if you set the tooling ball xyz zero each time you start up, travel to the pseudo home and re-zero, then when using your work offsets g54 thru g58 and g59 p1 thru g59 p253 as your work coordinates, you will always have a known reference point on your machine to re-check to, and your work coords should always be valid after using this "rube goldberg" homing scheme.  do learn to use your work and tool offsets, they are what make cnc machining a precise repeatable process with minimum headache.  hope this helps.

i have also considered adding a bored hole to my table to fit a tooling ball with a ground stem and flange, so the tooling ball can be inserted, picked up, then removed.  have not done it yet.