Thread: Steps per inch formula, What I am doing wrong?

1. Steps per inch formula, What I am doing wrong?

I am trying to determine my steps per inch for servos with a 1024 ppr encoder, but my calculation seems to be off. The screw has a 13/64" lead and I have the motors geared down using 32 and 20 tooth pulleys. I was using this...

1 inch /((1/ppr encoder count)*(Motor Pullet Tooth Count/ Ballscwew Pulley Count)*(Screw Lead)

Is this wrong? The software is Mach 1. One thing I am uncertain of. I am assuming G320's will count 1024 steps from the pulse source as one rotation of the motor, right?

2. Thread lead is 13/64 = .203125

1024 / .203125 = 5041.2307 pulse per inch at one to one ratio

But, at 20/32 ratio

5041.2307 / 20/32 = 8065.9689 p/inch

Question: are you sure about this lead? It wouldn't be a metric screw would it? 5mm = .19685 inch

BTW, if the controller reads the encoder as a quadrature encoder, you will know because the movements will be out by a factor of 4.

3. Ah, thats probably it. Quadrature encoder, forgot about that. I think I am off by a factor of four. I will have to check that. Screw lead is actually 13/64". Did not believe it myself till I took it off their website.

4. Thats the same number I got by the way so at least I know I don't have to go back 2nd grade.

5. Yup, it was the quadrature encoder. I will remember that.

6. 1024 count encoders may limit your max speed because you need 4096 steps per revolution. That is a lot. You may want to go down to 250 or 300 count encoders for gecko 320 drives. gecko also sells the 340 which is made to hook to high count encoders.

Eric

7. Yeah, I might do that later on. I am thoroughly enjoying this ultra high resolution though. The machine is certainly not unbearably slow, I would say 50-75 IPM rapids; Obviously I would prefer 200-300, but I can live with 75. I know Mach 2 is supposed to have a much higher maximum pulse rate. Do you know what that is? One other question. I bought a very fast PC with a nice 17" LCD as my CNC computer because I wanted to be able to surf the web, play games, listen to music, and watch movies while the machine ran. It seems though that Mach 1 is very picky about what programs it will let run in the background though. I can't seem to get WMP 9 running with Mach 1 and I am trying to locate a compatible DVD player with Mach 1. Any ideas? I have not tried Mach 2 yet. Is it any more compatible?

8. Mach2 and any other program that runs a CNC machine really needs the full attention of the PC. One job is run in realtime, the one that sends out step and direction pulses. But this job is dependent on the non-realtime task feeding it with data. If another realtime task (like playing music or video) is competing with Mach2, it is not easy to say which of these will win if both needs the CPU at the same time. This probably was never tested as it is not a common scenario. It is also not easy to tell if the Mach2 non-realtime task will get enough CPU time to calculate all the trajectory data in time when it's needed. Windows have no mechanism for setting up task priorities and preempting less important tasks.

So to be on the safe side, let Mach2 be the one and only job run by the PC when running the CNC machine. This is valid for any program doing a job like this, Mach2 is just mentioned because it seems that is what you are using. And if you run into trouble and need to call on Mach2 support, make sure you tell him what other tasks you were running at the same time.

I am in no way affiliated with Mach2 or Artsoft, I write this based on 20 years of experience working with realtime system support.

9. That is good advice, ESjaavik.

I still think that an "outdated PC" is plenty good enough for a cnc controller. Say a P500 or so is lots of computer for the task. Maybe not, if running a full software servo loop on High speed machining applications, but I don't see many guys trying to do that around here.

Keep the high powered PC for your CAD programs, and games, music, etc.

10. You have to remember Hu, that Mach2 is generating the step and direction pulses in software, not through a PCI or external hardware generator. And, it runs in Windows. I've heard that a 500 is about the minimum to run Mach2, and I'm not sure if a 500 is fast enough to use Mach2's 46khz step rate. You might be limited to around 25khz.

11. I think you guys hold mach 1 mill in a bit too high of respect. Windows is quite capable of dealing with this sort of thing. How is blasting 1.5 Mb/s through a firewire port, getting pics from a USB digital camera, watching a movie, and simultaneously surfing the web any different from pulsing 25 kHz to a parallel port? I am not familiar with just how mach 1 gets rights to the parallel port during these operations, but I don't think it is that different from a basic printing program. While I have had problems with music and DVD's, I have been able to play games and surf the web while my mill is cutting no with no problems.

12. Noah, the firewire/usb example can withstand a break in data transfer of a few tenths of a second I am sure, plus the pc can read what it has written to the camera/card to verify the integrity of the data. You don't that luxury with cnc. A break in data transfer can cause stuttering of the tool, which can lead to ruined work. This is why Mach2 excells, as it has a higher priority than almost all windows drivers/apps if it wants to send a pulse to the parrallel port, it is sent instantly, not queued. Hope this helps

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