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Thread: convert lathe to cnc

  1. #1
    Join Date
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    Question convert lathe to cnc

    Hi
    I though I saw a little while back a post on converting a manual lathe to cnc.Tried a search with no luck.So here goes:
    I have a small 11" lathe that has no trail stock (long story)anyway I use it to make small plastic and teflon parts ~100 per month.
    I was wondering if anyone has any ideas on how to convert this machine to CNC.
    I do not need to thread on this machine or hold close tolerances. +/- 0.005" is ok
    Plans,books etc that may help would be greatly appreciated.
    It's going to be a winter project during the slow days in the shop
    eddie

    [This message has been edited by motorworks (edited 11-17-2002).]
    please visit my webpage:
    http://motorworks88.webs.com/

  2. #2
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    OK, lets go. I want to do the same thing with a 10" Rockwell. Seems to me we need a lathe with ball screws, stepping motors and a count wheel on the spindle to tell the position of the spindle. Next a computer to send the infomation to the lathe from software of your choice (from comments posted here, there is a wide choice on software)and finally "THE BLACK BOX" that connects it all and makes it work. If this correct, the lathe conversion is straight machine work. The computer and software are computer work so the only problem to me is "the black box" or controller.
    Roland Friestad, who writes for the magazine on mill/drill conversion to CNC, is working on a news letter and has told me that he is also working on the controller and hopes to have a circuit for home building and at a reasonable cost. As is, his company sells the controller for $4,000. I do not know if that included stepper or not.
    So, if this is correct we need a source for controllers and stepping motors. What do we need. How many inch ounces of torque are needed on the motors, what brand, voltage, etc. Have any of you done this? More important, is it something that can be done by the home shop machinist? John

  3. #3
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    Post

    I thought HSM recently did an article on this? Have you looked at old copies?? Good luck, I'm sure it can be done.

  4. #4
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    Eddy and John,
    Yes this can be done reasonably easy and not at a high cost, especially if you raid the scrap bin.
    Grab a chair as this will kinda get long.

    Fist off lets divide this into modules and take them one at a time. We need in no apparent order the following:-
    [1] Lathe [ obviously ]
    [2] Ballscrews
    [3] Stepper motors and couplings
    [4] Motor power supply
    [5] Motor drivers
    [6] Controller
    [7] Spindle encoder [ for threading ]
    [8] Sundries [ wire switches, coffee etc ]

    OK now we'll work thru the list.
    We'll miss [1] out as being that obvious, and move to [2] Ballscrews. You will need two of these, one to relace the leadscrew and one to replace the cross slide. The top slide is removed completly as it's not needed.
    [3] Stepper motors, these have to be matched to the size of the machine. Something like a Sherline needs about 150 oz in. A 10" or 11" lathe will need about 600 oz in. You need 4,6 or 8 wire motors. Steer clear of surplus 5 wire motors as these need a special driver.
    Couplings are of the shelf non backlash type. You also need to fabricate a bearing housing for mounting the end of the ball screw, coupling and motor mount so there is no backlash.
    [4] Motor power supply is just a transformer that takes shop power, reduces it to a voltage that is useable by the motor, turns it into DC and smooths it. Sounds complicated but all it is is a transformer, bridge rectifier and a capacitor. Can be bought off the shelf but easily made.
    [5] Motor Drivers: For something in the 10" to 11" range needing 600 oz in there isn't a lot of choice.
    The Gecko drive is well though of and is cost effective. See http://www.geckodrive.com and look at the 210 model. You will need two of these.
    [6] Controller, This is were it gets complicated and can get expensive. You can go from a black box costing $1,000's to a cheap software approach driven by a PC's parallel port.
    For a lathe you need [ although Eddy has said he doesn't ] a threading capability. This is an input signal from the spindle to keep the leadscrew in time with the work.
    Most of the cheaper PC options are mill based software and don't have this threading capability. There is a cheap one out there called TurboCNC that can do lathe threading. It costs $20 to licence it and although crude looking it does work. link at http://www.dakeng.com
    [7] Spindle encoder. This is either a bought item putting out 500 to a 1,000 counts per rev or in the case of the cheap TurboCNC it's just a one count unit that can be made from a slotted disk and a IR sensor.
    [8] Sundries I won't go into except to say get decent coffee.

    It's obvious from this list above which is very brief that you have a multitude choice of options and that many choices are determined by other items. No good buying a 2.5 amp driver if your motors expect 6 amp.
    The choice of most parts once made can be kept, ballscrews, motors etc
    The controller software is changing all the while. If you go the PC based software route you can easliy change later to a better product with low costs being incured. If you go the hardware route you could be stuck with an expensive box that you have no option but to keep.
    I feel at this state of the game it's better to keep an open mind. Things are changing fast. There are up and coming options both in software and hardware that can radically change to way these things will work.

    My personal take on this would be for two ball screws, 2 600 oz steppers, 2 Gecko 210's, roll your own power supply, use TurboCNC as a start for $20 and make your own shaft encoder. Coffee will have to be personal choice.

    John S.
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.




  5. #5

    Smile


    I recently built a 4 axis mill using a milldrill to start with.
    I used ball screws and a kit I got from
    MaxNC....they advertise in HSM.
    The system runs on an old PC on dos.
    It is a closed loop system and I can hold
    .0005 tolerence. It uses fanuc type programing with standard G and M code.
    I have a cadcam program that I use for
    my 2 axis Prototrak full sized knee mill so
    I am able to do a drawing and the cadcam
    generates the cnc program.I put it on a
    disk and load it into the pc that controls the little machine.It works very good for a
    low dollar system. I have done thread milling
    with it and got a perfect job.It works great for engraving also.
    Any way..MaxNc has a cnc lathe that runs
    on the same software as the mill.
    It can do threading as well as turning.
    The machine they sell ready to run is small
    But for some jobs would be perfect.
    They also sell a lathe kit similar to the mill kit that I got.
    I am going to build a lathe sometime
    and will probably use their stuff.
    If any one wants to email me I could give more detail.
    Gramps dsearles@ncounty.net

  6. #6

    Post

    John,

    Firstly, an excellant reply which is thought provoking, which leads to my second point, which is just a couple of questions:-

    1. Do you keep track of the X,Y positions with the steppers? or do you need a secondary positioning system like O/P ported linear DRO units.

    2. If you use just the steppers as the feedback for positioning do you have to initialise the units at each start up (like drive them to a known/datum position)?

    3. For a shaft encoder, is not a Hall Effect pick up a better proposition? It has the same zero speed resolution of an IR P/U, but is more resiliant to dirt contamination than the latter (the reason you should never use IR P/U's in automotive applications!)

    RR

  7. #7
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    Thanks for the replies and information. Lots there to digest but that was what I was hoping for.
    As I said, the lathe and ball screws are just machine work, so now to work on the other problems.
    So many variables and changing so fast that I don't know where to start but I'll be back with questions. Thanks, John

  8. #8
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    RR,
    All the low end steper driven controllers except Maxnc are open loop. That means it says go to xxxx and it sends the nessesary number of pulses out to do this but there is no check. 99.9% of the time this happens. What causes lost steps is trying to go too fast or taking to heavy a cut so it stalls..
    If you keep between limits it isn't a problem.

    As regard feed back for positioning it depends on the controller and the way you work. Some controllers can remember the last position when they are shut down. It writes this to a file which is reloaded on start up.
    Some always start on 0,0,0 so you park the machine at it's zero point and when you switch on you are still there assuming you don't move anything.
    Most of the low end controllers only update the screen when it's reached that point. They don't count along the move. Main reason for this is to concerve computer power for where it's needed. This is changing fast.
    If you go to http://www.artofcnc.ca/ and look at a program there called Mach1 you will see whats on it's way. This is still in beta but some people are actually using this to cut metal. Art, the writer, has found a way to get windows to run in real time. Mainly due to the vast procesing power of modern computers this looks a very interesting home project. This one updates its displays as it moves. The demo on the site is a fully working demo, only limited by a 1,000 line code limit. Due to the modern way this proceses the program it is only for Windows 2000 and XP.

    The hall effect switch is far better. I just used the IR switch as an example as the general description was that wide reaching I though maybe more people could understand this principle.

    John S.


    [This message has been edited by John Stevenson (edited 11-18-2002).]
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.




  9. #9
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    Sep 2001
    Location
    Birmingham, AL
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    Post

    FWIW the current issue of Ciarcia's Circuit
    Cellar (Nov 02) has an article on a controller of sorts based on the Gecko motor driver. It makes assumptions that require some study but is aimed at DIY types. The web version will be available for extended
    periods but the paper version will evaporate over the next 10-12 days. The local Barnes and Nobles buries the magazine in the computer section. Steve

    [This message has been edited by sch (edited 11-19-2002).]
    Steve

  10. #10
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    Question

    John S
    Thanks for the info.It's a start.
    If threading is not that hard to in stall then I will go that route.
    Will it be able to do taper theards?
    ie pipe.
    What type of computer? An old 486/will that do.Will it run in windows/
    eddie
    please visit my webpage:
    http://motorworks88.webs.com/

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