Centroid CNC4

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23 Jun 2011 10:23 #10811 by andypugh
Replied by andypugh on topic Centroid CNC4
forhire wrote:

If your looking for an off the shelf system you might look at Mach Motion. I spent some time on the phone with them this week. It's not emc2 based but rather it runs Windows and Mach3. Looks very clean and priced well. It's actually very tempting. machmotion.com/cnc-products/cnc-kits/mil...achine-controls.html

Naturally you won't find a lot of love for Mach here, as we have all made the choice of EMC2 and are human enough to want to defend that choice.

I am not sure that Mach can use the encoders. To be fair, I am not sure that EMC2 can use the encoders to any great effect. The reason comes down to the question "what should the machine do if there is a following error". If the problem is a lack of motor torque for the cut being taken, then trying to make up the difference by making extra steps, running the motor faster, will give you even less torque, making the issue worse.
I have considered the possibility of running a stepper/encoder machine in velocity mode and connecting following error to the adaptive feed pin. That might work but is untried.

To roll your own using emc2, as I understand it, and assuming you'll be reusing your steppers, you'll need three stepper drives, a power supply, stepper control, and a computer with emc2.

Depending on the amount of auxiliary IO you might be able to omit the stepper controller. A parallel port has just about enough pins for a 4-axis mill with PWM for spindle control. It is tight for limit switches and there is no way to wire encoders with only one p-port.

Of the EMC2-compatible drive boards the cheapest (after a second parport) is the Mesa 7i43 at $80. It communicates via the p-port and offers 48 IO pins which can be GPIO or allocated to a range of functions such as encoder counters (4MHz), step generators (also MHz) PWM, SPI, UART...
There are compatible stepper drivers from Mesa which plug straight in, or the card can drive Gecko or other stepper drives directly. My machine uses
www.motioncontrolproducts.com/drives/msd...ping-drive.php?cat=2 wired direct to the 7i43.

You can drive the stepper controllers directly from the parellel port but if you want spindle on/off, coolant on/off, indexer, and etc, you well need something that gives you some relay outputs like the Universal Stepper Controller.

The ULN2003 chip can be useful for this, if funds are tight.

If I recycled my PC, power supply, and steppers from the cnc4 I suspect I could build it for around $700.

I think sounds about right.
The controller I am building now is:
Intel Atom D510 mini-ITX £60
8GB SSD drive £20
1GB ram £20
Pico-PSU £25
12V PSU £30
17" touch-screen £120
Mesa 5i23 (PCI card, 72 IO) £150

Add on 3 x Gecko G203 and that comes to £705, except with a new PC and touchscreen included to offset against the exchange rate.
Cheaper stepper drives are available.

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23 Jun 2011 10:31 #10812 by BigJohnT
Replied by BigJohnT on topic Centroid CNC4
All you should need providing you can harvest the steppers and power supply and pc is the G203v's and a breakout board like the C1G from CNC4PC so about $500 for 3 axis.

www.cnc4pc.com/Store/osc/product_info.ph...th=33&products_id=49

Take the LiveCD and boot it on the PC you plan to use and run the latency test.

John

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24 Jun 2011 01:19 #10821 by forhire
Replied by forhire on topic Centroid CNC4
andypugh wrote:

You might not even lose speed. It will take less torque to move at the same speed, so depending on the motor speed/torque curve you might end up in the same place or not too far away.

Handy pulley-spacing and belt-length calculator:
www.hpcgears.com/calc.htm


Thanks for the calculator link. The existing pulleys are 30 tooth... I assumed 60 tooth would give me the desired 2:1 ratio. I ordered a couple 60 tooth pulleys and some 720mm belts. I went cheap and had them shipped ground... so it might be a few days before I can try them out. :)

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24 Jun 2011 02:36 #10822 by forhire
Replied by forhire on topic Centroid CNC4
BigJohnT wrote:

Take the LiveCD and boot it on the PC you plan to use and run the latency test.


I downloaded the live cd and ran the latency test for an hour. Max jitter was 38955 and 42165. Looking at the wiki it appears to be acceptable but not ideal. Looking at the wiki there were numbers all over the board. My machine is an old P4 (Intel BF86510A). with 1 gig of ram and runs Ubuntu well enough. I've been using it for several years with my existing controller. I'm not really expecting to increase my feeds too much because my spindle can't do the rpms required. Should I expect the machine to do ok? Or should I look for something with less jitter? Any recommendations? I know several people have mentioned the Intel Atom D510 mini-ITX, how do these compare? Computers are cheap these days.

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24 Jun 2011 08:43 #10827 by andypugh
Replied by andypugh on topic Centroid CNC4
forhire wrote:

I downloaded the live cd and ran the latency test for an hour. Max jitter was 38955 and 42165

Do you know what your current peak step rate is? (You can work it out from the current rapid rate, gear ratios and leadscrew pitch.
You can then see how close you will end up to the limit with a 50uS base-thread rate.
Example calculation: 30ipm rapid rate, 4tpi screws, 1:1 gears and x4 microstepping:
30ipm = 0.5 in/sec. = 2 rev/sec = 2 x 200 x 4 steps/sec = 1600 Hz = 625uS/step. In this case a 50uS base thread is fine.
You want "a few" base threads per step ideally, otherwise you end up trying to make huge instantaneous speed jumps from 3 to 2 periods per step, then from 2 to 1.

Should I expect the machine to do ok? Or should I look for something with less jitter?

I would suck it and see. I can't remember if you were planning to use an external step generator (Pico or Mesa). It will be perfectly OK if you are.

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24 Jun 2011 11:51 #10829 by BigJohnT
Replied by BigJohnT on topic Centroid CNC4
Andy, if he uses the G203v drives they are fixed at 10 microsteps.

In the Stepconf Wizard if I put 42,000 for max jitter it shows 50,000ns base period and a max step rate of 20,000Hz.

On an axis page I put
200 steps per revolution
10 microstepping
1:2 pulley
4 leadscrew pitch
1.25 in/s maximum velocity

that gave me 20,000 pulse rate at max speed of 75 IPM.

John

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24 Jun 2011 12:40 #10830 by andypugh
Replied by andypugh on topic Centroid CNC4
BigJohnT wrote:

that gave me 20,000 pulse rate at max speed of 75 IPM.


But that would imply an instantaneous jump from 37.5ipm to 75ipm, and I doubt that is mechanically feasible.

However, it depends on what the machine can manage. If the motors run out of steam at 20ipm then it will be fine, otherwise I would be looking at external hardware.

Actually, when a 7i43 is $80 I can't imagine ever setting up a direct parallel port machine for anything other than experimenting.

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24 Jun 2011 13:47 #10836 by BigJohnT
Replied by BigJohnT on topic Centroid CNC4
andypugh wrote:

BigJohnT wrote:

that gave me 20,000 pulse rate at max speed of 75 IPM.


But that would imply an instantaneous jump from 37.5ipm to 75ipm, and I doubt that is mechanically feasible.


I'm real sure I don't understand what your talking about, I just followed the bouncing ball in the Stepconf Wizard and that is what it told me.

John

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24 Jun 2011 13:57 #10838 by andypugh
Replied by andypugh on topic Centroid CNC4
BigJohnT wrote:

andypugh wrote:

BigJohnT wrote:I'm real sure I don't understand what your talking about,


If 75ipm is one step every base period, then 37.5ipm is one step every 2 base periods. You can't make a step every 1,5 base periods. As the speed increases the increments between the possible speeds get bigger.

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24 Jun 2011 14:03 #10840 by BigJohnT
Replied by BigJohnT on topic Centroid CNC4
I see... I wonder why Stepconf would tell you that then?

John

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