Homebrew NC Mill

My office opens onto a machine shop. I was just now chatting up the machinist, and idly musing on the workings of the transport mech or
whatever it's called - it's got motors to drive the bed in X and Y. This is a Bridgeport end mill, with electric motors to slew the bed. And I was wondering about turning it with a stepper motor - with the gear ratio, it looks like you could get about .0005" per step. "Hmmm ..." I mused to myself, and the machinist (Jimmy) was talking about closed loop and open loop and the accuracy of servos because they use the sine and cosine, and that a stepper motor can miss a step, and so the feedback comes up. "How does the controller know if you've missed a step?" And I start thinking about position sensing, and asking about where's the best place to put a pot, and he says, "Well, you could just use the readout." I slaps meself in the haid - "Boy! Am I Stupid!" "What? For not thinking of that right away?" "No. For slapping myself in the head like that. That really hurt!" And I went, That's trivial. I've written stepper motor controllers in Z80. You can get a driver for a few bucks. And I have not only THREE computers (I inherited the one that they replaced in the front office), but an M68ICS05P in-circuit simulator/development system in my office.
I'm gonna _BEG_ the boss to let me do that. (slap together a 68HC705P6A and a stepper motor driver, hack into his Bridgeport, and have NC on a beer budget!)
Wish me luck!
If I get the project, I promise to make frequent progress reports, so everybody think good thoughts! :-)
Cheers! Rich
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Could be simple, could be tough. These mill retrofits vary. If you've got the motors and drivers lying around already, half the battle (and 75% of the expense) is over.
But then you need software to run the thing. TurboCNC (http://www.dakeng.com) will let you read G-Code files and generate all the pulses you need with only a Grandma-grade computer. No need to mess with HC705's or anything.
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In

Technically, this isn't "Homebrew NC", it's "Workbrew NC". :")
The nicest (cheap, fast, simple) driver board I've seen is: http://www.hobbycnc.com/driverboards.htm
One of these days I'm gonna get one or three of those, and build a zippy little PCB driller. And NC the lathe, and NC the mill... one of these days. Maybe. :)
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Mark J. wrote:

I've come across web sites describing DC pm motors used for this, rather than steppers, used. Steppers are more cost-effective for lower powers, if your interested in maximizing torque-speed-$$$

It is possible to sense back EMF on the stepper to figure out if your missing steps, but analog methods take tweaking and time.
And I start thinking about position

Yep, I wondered about hacking my own linear encoder from a laser-printed mylar strip, or magnetic tape. But for my (PCB drill) app, I didn't need to worry.
(I inherited the one

I love PICs. Far superior to the archaic Motorola chips.

Post this to rec.crafts.metalworking for some good advice! There's also a spam'n yahoo group mailing list for CNC too.

Don't know about that, I think I tried it & didn't like it. You need an RTOS for milling; Windows can't give you precise time-control for coordinated curves. I'd checked out EMC for Linux.

Checkout my web page for my PCB drill bot. Can hardly do it simpler, but perhaps cheaper if you use acme rather than leadscrew & make your own anti-backlash nuts.
--
Scott

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<snip>

join cad_cam_edm_dro on yahoo groups. they have done everything you want to do and will give you much advice.
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On Sun, 18 Jan 2004 08:11:59 GMT, the renowned "Rich Grise"

The other reason to use FB is to avoid backlash errors and screw inaccuracy.. it's also better if the mill uses ballscrews which get rid of the first, but retrofitting them costs $$.

When/if you get it, consider surplus servos as well. They have shaft encoders and you can even make them mimic steppers if you so desire.

Good luck!
Best regards, Spehro Pefhany
--
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<snip>

If you can get servos with quadrature output encoders (two channels of square waves 90 degrees out of phase) then you can use Gecko G320 drives which are about $100 and allow you to use step/dir from a micro or parallel port. Get an old PII running linux and download the free EMC software to drive your motors using industry standard G-code.
If you think the palm sized Gecko drives are too small, consider that I am using them right now to move the x,y, and z axis on a 7000 lb Shizuoka bedmill. I can do rapids of 180 IPM. I probably could go faster, but the mill starts dancing around on the floor!
Rick
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www.cnczone.com is another great resource for both the pre-made and home-brew.
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This magazine often has ads for home shop automation. http://www.homeshopmachinist.net /
If you haven't seen the magazine but are interested in mill/lathe work, get a copy. There are some impressive projects in it.
Back the the original question - Any serious machine uses some type of feedback. We have 5 CNC mills now and they all use AC servomotors with a variety of encoder devices. The most impressive is how a 500 pound machine head (whole machine is about 9 tons) about to tap a 2.5mm diameter threaded hole plunges at warp speed to within a mm of the workpiece, spins up and feeds while tapping. As much as I know about control systems, I am impressed every time I watch the machines.
Oppie
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On Mon, 19 Jan 2004 17:34:03 GMT, the renowned "Oppie"

Rigid tapping or with a tapping head?
Best regards, Spehro Pefhany
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wrote:

.
No tapping head. Just a tiny tap in the spindle of this huge machine.
Then there is also single point thread cutting where a single pointed cutting tool is spinning fast in the spindle. It decends slowly as the XY table moves in a circle. We use this for cutting larger diameter threads on occasion where it isn't practical to buy a tap.
Machines are all RS-232 but connect into an eternet to RS-232 converter so the files can be downloaded from the office server. parts are designed in AutoCad, edited with AutoCAM tooling path editor and downloaded to the CNC mill. Ain't technology wonderful?
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I think the cool way to do it now is threadmilling. Bore a hole out to the root diameter, insert a smaller tool with a single row of teeth at the right pitch, move tool to the edge of the hole and make a one helical rotation around the inside of the hole. Don't need to sync or even turn off the spindle, makes accurate threads and you can use one tool to replace a number of taps, and do non-standard diameters.
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NC Mill', on Mon, 19 Jan 2004:

That's 'thread chasing'. It's getting on for 200 years old.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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On Mon, 19 Jan 2004 20:06:53 +0000, the renowned John Woodgate

I note that turning metal on lathes is getting on for (at least) 1800 years old.
Best regards, Spehro Pefhany
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I read in sci.electronics.design that Spehro Pefhany <speffSNIP@interlog
about 'Homebrew NC Mill', on Mon, 19 Jan 2004:

Yes, but specifically making threads with the type of tool described is 'chasing'. You don't need a lead-screw, just a swift hand.
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On Mon, 19 Jan 2004 20:34:21 +0000, the renowned John Woodgate

I see what you mean. There's another meaning of "chasing threads", at least on this side of the pond, to refer to cleaning up existing threads with a die. The kind of tool you are talking about is a multi-point tool shaped like a slice taken radially from a tap.
(view from above) --------------- (rotating work)
- - - - - - - - - <- axis of rotation
---------------
/\/\/\/\ | | | | | | <--- fixed chasing tool, free to move <--->
Best regards, Spehro Pefhany
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On Mon, 19 Jan 2004 20:34:21 +0000, John Woodgate

As it happens, I can agree with John Woodgate.
Though the original writer TALKS about thread-milling (which surprise, surprise, is done with a rotating mill (looks like a tap) he actually described a tool with thead profiles cut down one side. This is a description of a thread-chase, which does not need to have a progessive cut - they often have threads to full depth all the way, and moves axially while the work rotates....
Take a look at this URL for a little more on milling threads (with a ROTATING mill!)
<http://www.sct-usa.com/millhelp.asp
Brian w
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wrote:

A common type of thread-milling tool uses a single carbide insert, which has a row of cutting teeth. A thread chase, as the name implies, is supposed to chase the thread for most of the length. A thread chase is most likely to be used in a lathe, replacing a single point threading tool for quicker material removal.
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Thread chasing is not the same as thread milling. Thread chasing is more suited for lathe operations, it's basically the same as single-point threading except it has multiple teeth with a tapered profile, meaning only one pass is necessary. The thread chasing tool is run the entire length of the threads. A thread milling tool has the same tooth profile along its entire length, and cuts all or most of the threads at the same time on the inside of the hole. Only one circular move around the hole is required, interpolated with one vertical move of one thread pitch. A thread milling tool can cut threads almost entirely to the bottom of the hole, while a thread chasing tool has several incomplete threads left at the bottom of the hole.
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One of the coolest useful things that you can do with an inexpensive NC mill is make 'lathe parts'. Just bolt down a plate, start the mill, and let it cut circles from the plate, without a lot of shearing, sawing, and slow-feed knocking the corners off.
I even thought it was moderately cool when I first saw it done 35 years ago, by a mill with two stepping motors on the table. We were going to make a fortune on that machine, until our customer noticed little steps in what were intended to be bearing diameters.
I'm not talking about the 'jaggies' you get at angles near an axis, from the discrete nature of the motion. I'm talking about a hard spot in the workpiece or a chip on a way or a cosmic ray in the wrong place causing the stepper to lose a step or two, and the remainder of the part cut after that being off in one direction by .001" or so. Bigger steppers didn't stop it from happening. Hydraulically amplified steppers didn't stop it from happening. Feedback stopped it from happening, a long time later.
Open loop is okay for pen plotters. For actual machining, especially where the material fights back, you need closed loop controls.
-Mike-
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