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Metric or Imperial ball screw

Discussion in 'CNC IN THE HOME SHOP' started by j ferguson, Jan 24, 2017.

  1. j ferguson

    j ferguson United States Active User Active Member

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    This should be obvious. but...
    (edit: it was obvious - I'm stupid)

    I'm using LinuxCNC on a 6040 router and have something screwed up with the setup. it makes everything a little small.

    I'd assumed since machine was sold in US, it has imperial lead screws. ... but maybe not.

    5 mm/rev not the same as 5 revs/inch. So if i have a metric ball screw, I can't use inch machine units.

    Can i simply correct the difference in my stepconf setup?

    my thought would be to enter 25.4/5 or 5.08 where you enter revs/inch and then actually check the results.
     
    Last edited: Jan 24, 2017
  2. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    Yes 5.08 is correct for turns per inch for the screw if it is a 5mm pitch but for the Mach3 settings you need to know steps per inch which is the sum of the screw pitch, stepper steps per rev, any micro stepping from the drives and any gear reduction.

    Here is a link to a steps per inch calculator. http://www.machsupport.com/forum/index.php/topic,16315.0.html
     
  3. j ferguson

    j ferguson United States Active User Active Member

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    Thanks Jay. I'm using LinuxCNC but each of these elements is accessible. this is actually a pretty simple direct drive so I'm spared some of the uncertainty.

    I also have a big pile MDF to try settings on in case of confusion.

    By the way, Jay, I liked your bit about things which cannot go wrong. I bought a new VW in 1968 to replace my 61. Very soon the turn signal blinker module failed - not under warranty. the replacement was $17. i asked the parts guy why they had made a part which cost $17 to do what the previous thermal relay which cost $1.79 had done perfectly well. "This one lasts forever" I was told.
    "Then why do I need a new one."
    "Oh it's not the one you had that lasts forever, it's this new one you are paying all this money for."

    They looked the same to me.



    best, John
     
    Last edited: Jan 24, 2017
    jbolt and TomS like this.
  4. j ferguson

    j ferguson United States Active User Active Member

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    25.4*8000/25 = 8128 which goes in the SCALE entry in LinuxCNC router.ini file. It's amazing how many alternate theories I invented to explain why parts made on the router didn't quite fit. Thanks Jay.
     
  5. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    Considering the pitch difference from 5mm to 5TPI is about .003" I'm sure it drove you nuts. Glad to hear you got it sorted out.

    Is it a DIY router, kit or ready made? You should post some pictures.
     
  6. j ferguson

    j ferguson United States Active User Active Member

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    This is it:
    [​IMG]

    It cost $1.4k on Ebay, is Chinese and except for not knocking the sharp edges off the parts very well made. I doubt if the vendor really understood what they were selling, but I did get an email from them about six weeks after it arrived asking if it was working, was I happy with it and was there anything they could learn from my experience. Wow, how often does that happen?

    The HomeDepot paint-bucket vac is catching dust from the dust-shoe in the photo. It also makes the vacuum for the MDF vacuum table (being made) as shown in the photo. It seems to be able to handle up to 6x12 inch 3-ply stock. I've made a new table from HDPE which will hold spoil-boards made of LDF.

    I use SheetCAM to generate the G-Code and LinuxCNC to drive the machine. The LinuxCNC runs on Ubuntu 10.4 and so far has worked without a hitch. The PC is an Intel 965 board based box with a motherboard parallel port and an add-on port. The other port runs the CNC box just to the left of the monitor which in turn runs my Sherline 2000.

    Designs are done in antique AutoCAD, output in DXF which is what SheetCAM accepts.

    I'll fill in more detail when I get the HDPE vacuum table debugged.

    I'm designing and making R/C Airplanes - now using 1/16 plywood, 3/32 balsa, and 1/32 balsa for skins. once i get them to fly and process better understood will be going to entirely carbon fiber.

    This is to discover whether I can actually do it, not any commercial interest.

    PS, the dust shoe was made on my Sherline. The hard part was taking the brush part off of two shop-vac brushes, straightening them out to fit the groove in my part and forcing them into the slots. I suppose there is glue which will work with HDPE, but I couldn't find any locally.

    So far this thing has worked fine. It's easy to put on and take off.
     
    Last edited: Jan 26, 2017
  7. j ferguson

    j ferguson United States Active User Active Member

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    Just realized that I'll need to switch back to old WRONG scale setting to finish present project because otherwise new parts won't fit.

    Boy you have to keep a clear head in this stuff!!
     
    Last edited: Jan 26, 2017
  8. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    Looks great!

    We run LinuxCNC on the big router we built for the high school and it runs great. Linux in general drives me crazy so I don't run that machine. I use Mach3 on my mill and router at home.

    It's nice to see someone still building R/C airplanes. I've been out of that for about 15 years now. I love scratch building my own designs. Things have changed so much with radios and electrics that most of my old gear is obsolete. I sold off a lot of my stuff a year ago. Still have a rafter full of kits and a few built airplanes I need to get rid of. I'm still keeping all my helis for some reason. It's hard to let that stuff go for the pennies on the dollar it will only bring these days.
     
  9. j ferguson

    j ferguson United States Active User Active Member

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    Hah, Mountainview I see. FWIW all the solid modeling is done on a 1992 Sun SPARCstation10 running 1993 SunOS4.1.4 For example, I make a solid-model fuselage aft of wing using first former and last former then cut sections at 2 inch intervals which become nested DXF arrays which then go to SheetCAM for the G-Code and then to the router. I'm using 1/16 inch carbide diamond cut bits. I have a little 3d printer and use the Sun to design and make stl files for parts like landing gear.

    There is a fair amount of fooling around with this because I'm not following any plan. I frequently have to do stuff over or stop model design and improve the 'manufacturing' setup. One thing that I've found really useful is ability to make jigs on the 3d printer to make it easier to assemble wings (1/16 3 ply ribs on carbon fiber spars with carbon fiber torque tubes for the ailerons running to servos in the fuselage). Span of first plane is 36 inch. I have no idea how it will fly, but it's getting close. My plan was to make cabin section, wings, rear fuselage and tail feathers, then make the nose section as long as it took to get CG. Thing's ended up looking like a Pilatus Porter (really long nose) so I used that as a guide to landing gear, made out of 3D printed hub and fittings, carbon fiber tubes, springs, and music wire.
     
    Last edited: Jan 28, 2017

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