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Starting from Zero, need advice
« on: June 13, 2008, 12:19:06 PM »
First, I'm VERY new to CNC. This is the first time I've even had access to one.

I just finished my Sherline Mill conversion to CNC, and now I'm wondering if I should have asked around first (No plans, nothing from a kit, I did it all from scratch, doing what I THOUGHT was right). I assumed that limit switches were most important, so I used up my Parallel Port inputs on X, Y, and Z limit switches. Now it seems home switches are more important. Is there a way to simulate a home position from a limit switch? Or should I just add home switches? If so, what's the best place to put them? Can or should I put all my limit switches on one pin? My home switches? I'm using Linistepper drivers with stepper motors. Forgive me if this was answered before, but there is a LOT of information on here, and after much looking, I haven't found it. Thanks!
« Last Edit: June 13, 2008, 12:21:07 PM by RChadwick »

Offline Hood

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Re: Starting from Zero, need advice
« Reply #1 on: June 13, 2008, 01:38:14 PM »
If you have normally closed switches you can connect them all in series and use just one pin in Mach. When homing Mach treats the limit switch as a home switch and automatically goes back to treating it as a limit after the homing has completed.

Offline jimpinder

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Re: Starting from Zero, need advice
« Reply #2 on: June 13, 2008, 01:44:33 PM »
Don't worry  ;D

You are certainly getting very advanced before you even start.

You don't need home switches, or limit switches to enjoy CNC. If you have your axis working from Mach 3 then that is all there is to it.

You're main job is to get the axis up and running reliably and accurately, which means setting up your motor tuning (see Config/Motor Tuning). Setting your "Steps per Unit" and your sped and accerleration for each axis is the first thing to do. Do backlash next - and you are ready.

You seem to have a grasp of the Ports and Pins, if you are talking about Limit and Home switches, so I will move on quickly.

CNC depends on your machine knowing where it is. There are two ways of doing this, and the easiest is to set up your work on the table, move your cutter to a fixed point, and tell the machine that is where it starts from. This is usually point 0.0.0. which is the bottom left hand corner of the work, with the cutter resting on top of the work. (This is where most G Code programs are written from) This is known as program co-ordinates. The machines says "right" and carries on the program. You can do that many times, as long as you are satisfied that the machine is starting in the same place each time (and therefore knows where it is)

The second way is more professional, and involves "home switches". These are fitted to the machine in a specific place (one on each axis) and if you press the "Ref All Home" strip, then Mach 3 will automatically move the table and cutter in turn, until all switches have been contacted, and it will remain there. (Any axis without a switch is just zeroed, with no further movement).  The table therefore has zeroed itself and knows where it is and this position is called "machine co-ordinates" 0.0.0

It is doubtful whether these switches would be in a place that is any use at all for machining anything, and certainly would not be at a place from where a G Code program would be written (unless you were a mathematical genius). To make use of this is the job of the Offset table (see Config/Fixtures) and here can be entered offsets to move the table to a convenient position to start work. Offsets start at G54 to G58 and G59P* (where * is 7 to 255). If you enter an offset, then tell the table to move to 0.0.0, the table will move to iits new starting position, which can be anywhere for your convenience. The new position is shown in Program Co-ordinates.

That's enough of that, but it gives you the idea.

Fortunately Mach 3 can double up on limit and home switches, so you can designate the same switch to be both a home switch and a limit switch. Mach 3 will change the function of the switch as it needs to. You need to make sure that they are designated on Config/Ports and Pins/Input Signals.
Not me driving the engine - I'm better looking.
Re: Starting from Zero, need advice
« Reply #3 on: June 14, 2008, 11:37:20 PM »
I want to thank both of you for the replies! Just what I was looking for. I've got movement in the X, Y, and Z, just floundering a bit from here. I feel like a teenager who just got a brand new Mustang, but doesn't know how to drive yet. I'm trying to tread carefully so I can avoid doing something really stupid :) The Mach3 Interface is a bit intimidating to me. My extent of usage has been to jog the axis's around, playing with my power supply voltage to get enough tourque without baking the motors. I figured out the steps per unit to 8000 (20 turns for a 1" movement on the mill, 200 steps per rev for the motor, and half-stepping) I think I'll soon add home switches, but good to know I can get by without them for now. I'm curious... Let's say my motor skipped a step or two. While machining, it happens to go over a home switch. Would it correct itself? Also, I'm assuming the home switch has a concept of direction, like backlash compensation, so it will only read it going one way?

One more quick question... Is there a way to test the machine? For instance, traveling from end to end on all 3 axis's, making sure steps don't get missed. In addition to getting to know the machine better, I'd like to be able to do something with it before I figure out how to make something in CAD, and then LazyCAM.

One more quick question... I haven't found where to adjust the direction of an axis. I think it was my X axis, and when I clikced on the button to go one way, it went in the opposite direction. Is there a way that I can reverse this without having to rewire the stepper motor?

Thanks again!

Offline Chip

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Re: Starting from Zero, need advice
« Reply #4 on: June 15, 2008, 12:19:37 AM »
Hi, RChadwick

Axis direction can be changed in Config, Home & Limits "Reversed".

Mach has no formal feed-back, It relies on proper use of G00 moves (Velocity and Acceleration settings) and G01,02,03 moves with F speeds set correct to not miss steps.

Hears a link to Mach3 manual it covers allot programing info. http://www.machsupport.com/docs/Mach3Mill_1.84.pdf for a start.

Hope this Helps, Chip
« Last Edit: June 15, 2008, 12:31:18 AM by Chip »