Topics

1

General Mach Discussion / Motor Tuning Math

« on: December 13, 2015, 10:28:17 PM »

Hello, I have a problem with losing steps. Before I go and buy a new set of motor drivers, I wanted to explore the math behind the motor tuning (or I might end up right back where I am). I am cutting PCB copper to make circuit boards. I only need to penetrated 0.01 mm from the surface (0.05 mm is actually acceptable). Someone pointed out to me that is only 0.4 mil and is hard to control too. That made me think about the tuning parameters.

Okay, here's some math. I used a caliper to carefully measure the pitch of my lead screw. It seems to be very precisely 2.5 mm per turn (used several threads, then averaged). My system seems to have everything in metric, so that's why I chose it.

1 turn = 360 degrees, so 1/360 (turn/degrees) = 1  (unity)
2.5 (mm/turn) * 1/360 (turn/degrees) = 1/144 (mm/degrees)     This is my lead screw

My stepper motor clearly says "1.8 degrees per step", so
1/144 (mm/degrees) * 1.8 (degrees/step) = 0.0125 (mm/step), or 80 (step/mm)     This is my lead screw/motor combination

What's confusing me is that my CNC manufacturer has indicated "640" as the motor tuning parameter for "Steps per". In my case, that's Steps per millimeter.
Using that number: 640 (step) * 0.0125 (mm/step) = 8 mm

The answer should have been "1 mm", not 8. The funny thing is that this 640 value seems to work very well. I have no "scale" issues with X, Y and Z motions using this number. I just don't understand the math behind Mach 3. I think this might be due to the Tonsen controllers then. In the Mach 3 tutorial video on YouTube, the value they recommend is "2000". I've never seen a stepper motor with that kind of resolution.

HERE IS THE IMPORTANT PART:
Most importantly to me, I'm trying to get to "-0.01" mm (repeatedly), but I can see that my step resolution is actually 0.0125 mm/step from the math above. If I repeatedly go to 0, then -0.01, then to 2, then repeat, will I be loosing that 0.0025 mm every time it cycles through this sequence? These are the only values that I ever command my Z-axis to (except to park it at 4 mm when done).

Since my math (theory) shows my steps per mm as being 8 time greater than what practice says, does that mean my resolution is also 8 times better (0.0125 mm divided by  8 is 0.0015625 mm)? Somehow I think not.

I noticed my program (CopperCAM) can't generate values smaller than 0.01 mm. What precision of mm does Mach 3 support?

Sorry, I know I've asked a lot here. Thanks to anyone that helps. Tweakie and RICH have been great to me, but I don't want to look a gift horse in the mouth.

Sincerely, Richard V

2

General Mach Discussion / Recommended Encoders

« on: November 30, 2015, 02:13:06 PM »

Hello, I've read that one can buy a stepper motor driver with a built-in encoder to ensure that we can close loop on a target position. I've seen "Gecko" and "Romax" mentioned in these posts. They cost somewhere around $200, or so. I don't mind the cost, but I don't want to experiment and buy a controller that won't work with Mach3, or my system. I saw a 4-axis controller that is compatible with Mach3, but I only need 1 axis (the one giving me trouble).

Can someone please recommend some Mach3 compatible encoders/motor drivers? I'm using a 1.8 degree/step stepper motor (24 V, I think). My system is very small, so I don't want to buy one that is more expensive than necessary.

Thank you, Richard V

3

General Mach Discussion / Z-Axis Repeatability: Small Changes

« on: November 19, 2012, 05:26:42 PM »

Hello All, I am using MACH 3 to route out my Printed Circuit Boards (PCBs) designs. The main problem that I am encountering is that the Z-Axis looses track of position. My plunge depth is only 0.01 mm, so I cannot tolerate very much error.

For regular DIP or 1st gen surface mount parts, this is not much of a problem. The problem is that I cannot do the latest generation of surface mount footprints without wiping out traces. This is because my "V" bit gets wider as it goes deeper. It goes deeper (hence wider) unintentionally.

The problem occurs for even a small design with just 1 fine pitch part (0.65 mm spacing). Starting off, my vertical repeatability was very good. However, after a couple of hundred lines of code I can tell that the cuts get progressively deeper (can tell by sound). Looking at the Z Axis values, all should be well. However, when I stop the code, and recheck my vertical axis, I see that it has drifted (deeper). It appears that MACH 3 has lost track of a few steps. This problem is not as bad for X and Y axis, but for Z axis, my tolerances are a lot smaller.

I have noticed that the loss of tracking is always unidirectional (not random). Steps are always lost to make the machine cut deeper than intended, never in the opposite direction.

I have read as many post about "Z-Axis Repeatability" as I could find. I have kept my lead screws clean and properly lubricated. I have already decreased my velocity and acceleration to as far down as I could manage. I have also slowed down my feed rate to less than 60 mm/min, my plunge rate is 1 mm/S and my lift bit rate to 750 mm/min. I have also limited the amount of clearance to 1 mm (was 4mm) to decrease the amount of Z-Axis travel. None of the above affected the amount of error I detected (no improvement). I feel that I must be doing something wrong.

I'm not sure how to manage the "backlash", yet. I'm not sure if that is the problem (or one of), but it is something that I have not tried yet.

I would greatly appreciate any advice that anyone can give me.

Thank you, Richard V