Stepper Motors

STEPPER MOTORS
I am trying to select a simple CNC system and would like to ask your opinion on the following. I am building a lathe to my own design; it is about Myford ML7 size.
I intend to drive the leadscrew and cross slide by stepper motors. These stepper motors can be dis-engaged so that normal manual operation is also possible. I would like the stepper motors to be driven at a selectable fixed speed and engaged or disengaged by the operator in a similar manner to conventional lathes for fine 'X' feed (cross slide movement) and 'Y' feed (along the lathe bed). The lathe spindle will be belt driven from a 3-phase inverter controlled motor. The lathe spindle can also be separately driven by a further stepper motor ('A' drive), which can also be engaged and disengaged by the operator. I intend these stepper motors to be controlled by a cnc program, which will allow simple settings for the relative speeds of the motors. The 'A' drive motor will have two modes: 1) as a dividing system for indexing the lathe spindle. 2) As a slow speed drive which can be coordinated with the 'Y' feed for screw cutting. The cutting tool would be engaged/disengaged in a conventional manner whereby the carriage drive is coordinated via a screwcutting dial. I would like to ask your opinion on whether you think this will work? Will the leadscrew and spindle be coordinated enough by the cnc program without a further device to sense the spindle position? I also intend to use a fourth stepper motor to drive a small milling spindle mounted on the cross slide ('B' drive). Do you see any problem with using a stepper motor for this purpose? (Speeds approx 200 to 2000 rpm). It would be desirable to operate these stepper motors individually as well as in combination with each other. Therefore would it be better to use individual stepper motor drivers rather than one four way driver because this would allow stepper motors that are not being use to be switched off? I can give you further details of the stepper motor drive conditions if you require. I would be grateful for any advice.
Regards Alan Jackson
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Alan, This reminds me of Sir Frank Whittle when he was explaining to Ernest Hive's how simple the jet engine was...Hives was heard to mutter "Don't worry we'll soon design the bloody simplicity out of it ..." <bg>
Why not just do a full CNC conversion and put two electronic handwheels [ Manual Pulse Generators ] on X and Z for manual work. It surprising how soon you get a 'feel' for using these. I have seen people with no lathe experience soon be cutting metal on the MPG's.
By all means bring a second motor in for the spindle as an A axis for dividing but screwcutting can still be done on the AC motor. Screwcutting is usually done from 300 to 900 rpm on a CNC lathe and yes a simple single pulse spindle encoder will be able to keep up.
.
-- Regards,
John Stevenson Nottingham, England.
Visit the new Model Engineering adverts page at:- http://www.homeworkshop.org.uk /
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jackary wrote:

Sounds like a homework question.
Is this lathe on paper or is there any metal yet?
Yer gonna be all kacked up if you disconnect the steppers without having a system in place to track position. You will otherwise have to reset ypur zero points each time.
Steppers, compared to servos, come out looking rather a poor choice. If you are going to the trouble, build it with decent components that can work at decent speeds.
Cheers Trevor Jones
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Yes the lathe is well on the way. I have completed it for manual operation and I am now about to do my quirky method of electronic operation. I am trying to use the stepper motors purely as an electronic leadscrew. I did not want a cnc conversion, this is only a model makers lathe for one off etc. I plan to use the motors as speed controllers and do the tracking as you call it manually as you do on a manual lathe. I have been following the electronic lead screw group in Yahoo but they seem to be building a variation on cnc and are adding complexities to solve induced problems.(probably just as I am doing in my own way). I am (naivley?) trying to do a very simplified version so that I would minimise the computer control. I will post some pictures if I ever find out how. Regards Alan
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jackary wrote:

You post pictures by registering at one of the free hosting sites, such as photobucket, or putfile, you upload your pictures, and post a link to them.
Posting pictures directly to a usenet newsgroup makes the system administrators cranky, and can result in the loss of your internet access. Most newsgroup servers trim binaries off any message not posted to a "binaries" newsgroup, so posting them will generally not get them out to the people you want to see them.
Cheers Trevor Jones
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wrote:

For CNC lathes, I believe the axes are conventionally Z (along the bed) & X (cross slide/depth of cut). See 'Axis conventions' at http://www.dakeng.com/man/turbocnc.html I'll use these below.

8<
There is a slight issue with number 2 insofar as 'A' is usually in degrees and 'Z' is linear. The way you will be set up, you need something that tells the motors 'rotate the spindle 360 degrees and advance the carriage 2mm'. I don't know of a conventional G-code that will do a two-axis move (i.e. the 'coordination' you require) in this way.
This is not to say that what you want is not possible: have a look at Tony Jeffree's excellent pages on his CNC Taig mill, especially the bit about 'faking' a fourth (A) axis, but note that the 'resolution' of your fake axis depends on the diameter being cut.

Why? The only reason for using stepper motors is positional control. If you want a milling spindle, use a conventional motor with speed control.

If you use a driver board with four axes on it, and blow one up, the whole board might be scrap. If you use individual boards for each axis, you can swap out the broken one.
HTH
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wrote:

John Thank you for your comprehensive reply. As you see I am on a learning curve starting at the bottom. I did not know the convention for naming the axes and will take on board you comments. My reason for trying to use a stepper motor to drive a milling spindle was that the power supply and drive boards were available and could be used to power a stepper motor rather than source a DC motor and separate variable DC povver supply. It also occurred to me that to use a stepper motor to drive the lathe spindle would put the spindle under the control of the cnc program and enable the Z axis to be controlled in unison by the same program. I obviously need to learn a lot more about cnc programs to achieve my aims or adopt cnc conventional ways. However I do not want to be a cnc programmer for every small job. I just want the electronic advantages added to a lathe. I also thank you for your comment on individual versus four axis drive boards. Best Regards Alan
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On Tue, 12 Jun 2007 19:46:24 +0100, John Montrose

John -
Thanks for your kind words!
Actually, the "faking" a fourth axis technique can work very nicely for screwcutting if the spindle is stepper driven - if you look at the second pic up from the bottom of this page:
http://www.jeffree.co.uk/pages/divheadmk2.html
That setup is effectively what you would have in the lathe - the dividing head (actually a Taig lathe headstock anyway) is the lathe spindle, and the only difference between this and doing it in a "real" lathe is that I used a 60-degree V-tipped milling cutter instead of a 60-degree thread cutting lathe tool.
Given that you know how many steps per motor rev (400 1/2 steps in this case) and the reduction drive ratio of the drive to the spindle (72:1 in this case), then 28800 steps turns the spindle a full revolution. As I was using the Y axis signals to drive the rotary axis, and this axis was programmed for 0.000125" per step, programming a "linear" Y move of 3.6" gave a full rev.
So, to cut a thread, all you need to do is to create the necessary "G" code to move the Y axis (3.6" times the number of thread pitches needed) while the X axis moves by the distance needed for the desired number of thread pitches.
Regards, Tony
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wrote:

I should have added that a downside with this approach is that it is painfully slow. At a step rate of 10,000 1/2 steps/second, which would be a fair lick for the mill table (75"/minute) and probably faster than I could actually drive that dividing head, it would still only be doing about 20 RPM.
Hence, it is probably a more satisfactory answer to use a more conventional CNC lathe approach - stick an encoder on the spindle (and one pulse per rev works pretty well by all accounts) and drive it using one of the CNC software packages in common use (Turbo CNC, Mach 3 etc.)
Regards, Tony
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