DIY CNC Machine How Hard Can It Be?

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I haven't look yet..but I'm imagining some smart motor control IC's might make things easier. Also..I'm certain I'll end up programming a uC too. D from BC

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I had to go on wikipedia to look up G code. I'm really a newbie at this.. :(

About XY positioning.. Yeah...It's probably a crazy idea to transfer every micron XY position from the PC to the motor controllers just to get smooth movement. It'll be alot of data and it'll have to move fast. USB comes to mind. It's to dodge making a motor controller that has to interpret G code.

D from BC

Well, suppose you're doing a G01 X1. Y1.2 from part zero, you're going to have to be telling it an awful lot of stuff to get that movement smooth.. then consider a G03! (CCW circular interpolation). That stuff is standard on pretty much the oldest

Best regards, Spehro Pefhany

PLX9054?

I used that on a design a few years back too. Worked like a champ. The documentation was a little confusing in a few places but not too bad.

Mindshare has some very good books. I made good use of their PCI book, even though my employer was a member of the "secret society", so I knew the secret handshake to download the spec directly. The Mindshare book was far easier to understand.

But it's *easier* to make incompatible products! ;-)

Agreed. May it stay dead!

Not hard. ISA simply sucks.

I've had great success using the brushed DC drivers from geckodrive.com. Cheap, compact, covering large range of motors, and are virtually indestructible.

Not associated with them at all, just a satisfied customer.

robert

I use

I use a dspic33 to control everything, it also direcly bitmap drives a

You will probably have several coordinates that you will want to move at the same time, so each point will probably need all the coordinates you have available, and a time to get there in or a velocity as well as spindle speed etc, or laser on/off period.

the micro will have to work out all the points in between that coresponds to each change of state on all the stepper motors.

I started making something years ago wich relied on dual lead screws to do the drive and also guide each stage. im not sure if this would of worked very well as I never finished it, i decided i was making it too big, after I had got into SM devices.

one idea I had was to use a disc drive with the work mounted on the top platter, with a magnetorestrictive sensor to sense the angle from a bitfield impressed onto one of the platters, or opto slots etc, and the drill mounted on the head actuator.

I have an old 8" disc drive knocking about wich would probably gave me the idea.

each bit is quite simple, but the overal design is quite a lot of bits. ive seen many cnc projects on the internet, its surprising how they are all quite different. the commercial ones all seem to be more the same.

Colin =^.^=

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You could go to some middle ground. The micro can be trusted to step off the positions following a rule but the G code can be translated in the PC to a simpler form.

It doesn't take much of a micro to be able to do this:

The interface from the PC sends 7 numbers for each motion that the micro has to implement. Numbers are all 32 bit values. The position numbers have the stepper / encoder value at the MSB end. ie: there are bits below what would actually cause a motion.

If I was doing it, I would send the numbers in base 16. Perhaps not as hex but instead as the letters "A" through "P" since that makes the decoding easier.

The input would look like this

LEAD STUFF X0 X1 X2 Y0 Y1 Y2 COUNT

The micro does this:

variables: StepperX, StepperY, StepChange

while COUNT > 0 COUNT = COUNT -1 StepChanged = False X1 = X1 + X2 X0 = X0 + X1 if MSB(X0) StepperX then StepperX = MSB(X0) Xport = StepperX StepChanged = True end if

Y1 = Y1 + Y2 Y0 = Y0 + Y1 if MSB(Y0) StepperY then SteperY = MSB(Y0) Yport = StepperY StepChanged = True end if

if StepChanged then Delay end if end while

This allows the micro to do second order curves and sections of circles etc but doesn't require much in the way of smarts in it.

Probably one of the things that makes DIY CNC's differ ant than commercial units is that "whatever-I-can-get" parts are used for DIY. Whereas, commercial CNC's machines probably get lots of nice custom machined parts. Also, for commercial machines...the electronics engineer may have wanted to justify working longer (for more bucks) and so made the design in the most bloated way possible :P Example: Using any off-the-shelf computer supply would be fine but a roll-your-own was done.

D from BC

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Now there's something that's keep me thinking for awhile :) D from BC

Yep, PLX, not PXI -- my mistake. (We have this National Instruments rack in a screen room that proudly proclaims "PXI" in a very large letters... must be rotting my mind...)

Agreed, it's a very nice chip. The chaining scatter-gather DMA was really nice!

Yep, it sure is!

---Joel

This is one of the reasons conversions of manual mills to CNC are so popular, I think -- you already have the means to make custom parts, even if it is slower initially!

You get a lot of that in bigger companies... there's a "DSP guy" and a "microcontroller guy" and a "PC interface" guy and the end results uses a lot more hardware than what's really necessary.

Actually, a "computer" supply typically wouldn't have the higher voltages outputs (e.g., 24/28VDC) that many motors would like. Still, there are tons of off-the-shelf switchers available in pretty much any voltage you want...

---Joel

Happy birthday to you. Happy birthday to you ..

-- "Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get into it." (Stephen Leacock)

You don't want steppers for CNC. They put steps in the work. Slopes and curves come out all rough.

Nobody "in the business" uses steppers.

DC is easiest and probably cheapest. 3-phase AC permanent magnet synchronous is mostly current practice.

You need to tell them how fast, too, unless all you want to do is drill holes, when only the Z feedrate *really* matters.

If you want to mill slopes and curves, velocity is all-important.

Depends on what you want. In my case, all my holes are precisely spaced and lines are straight. I don't need or want curves, not for my current project.

I guess many precision tool makers are not "in the business". That's why I can't buy the right tool. They keep telling me what I want is not what I want.

You can safely extend that to all motors.

And if you want the stepper to be able to go at more than a tiny fraction of its speed capability, it will get up to a speed where a momentary (well say millisecond) interruption to the stepping signals will result in it losing step. Basically, stepper motors cannot stop suddenly without first ramping down to a low speed. Either you need to always be running at a painfully low speed, or you need a control computer that does not go AWOL for a millisecond here and there.

I remember seeing a PC-controlled circuit board milling thing that used stepper motors. Because it used some M$ operating system or other (can't remember the exact vintage), sometimes the OS would find something more important to do ... Step X Step Y Step X Step Y.... See if the network card is doing anything interesting.... ooh let's try the screen saver... let's page some virtual memory to disk...where was I ....Step X Step Y.... The motors would lose step and half of the tracks and holes on the PCB would be in the right place, and the other half would be shifted off to one side by a few millimetres, sometimes cutting through the first tracks. Completely useless. You really need to be in full control of whatever software is controlling the motors, even if you're only using stepper motors. I haven't tried that "EMC" software, (supposedly running some real-time linux variant), but the only other alternative I can see would be to use a microcontroller for the time-critical stuff.

Chris

Whilst I expect that you are right that the USB standard if implemented correctly, would work well, I think that many PC don't implement the standard very well and peripherals are generally even worse. I guess that this has probably improved with time as the other interfaces are disappearing and the PC with broken USB becomes more and more useless, but some of my first USB experiences have left a very negative impression on me.

Thank you for drawing my attention to that IC, and the book. I didn't know about them.

Well you are welcome to that opinion. I think that it *is* good that PCI and USB and WiFi exist, but I think that it was a BAD THING when the PCI club decided to take the specification off their website, and started sending take-down notices to the ISPs of anyone who continued to make available copies of the standard. To me that says pretty clearly "We don't want you to use this standard for free", and since I want to do things that can be done for free, then that makes me not want to use their standard. I know it won't hurt them measurably but it gives me no pleasure to work on equipment of which the technical details have been deliberately suppressed and kept from public knowledge. I'll work on closed systems for money but then it's just a job and nothing more.

About 10 years ago, with a piece of edge connector hacksawed off a broken videocard, a bunch of HCT logic chips, a copy of Horowitz and Hill and a spare weekend, I made a 32-input data acquisition card that enabled me to gather some data that I needed to gather. I think PCI would have involved rather more stringent layout and construction requirements, USB would have involved ordering and waiting for a fancy IC and then would have required a lot of messing around to get the fast latency that I needed, and VME was not available on my ex-dumpster PCs. Certainly ISA is not elegant but it got the job done.

I'll have a look at that PCI chip that you mentioned. A cheap (

Hmmmm that points out a problem wit the method I suggested elsewhere. The rate of motion can't be controled by the PC and it varies by about

The PC needs to send the delay in a form that the micro can handle without too much trouble.

When either stepper actually moves, we should delay by a PC determined amount. When both steppers step we want to delay by 1.414 times the normal amount.

To improve on this, when both steppers change, the delay could be calulated like this:

Temp = Delay + (Delay SHR 2) Temp = Temp + (Temp SHR 3) Delay(Temp)

This would be good to 1/2%.