. .
Page 5 of 8 FirstFirst ... 34567 ... LastLast
  1. #41
    Quote Originally Posted by Robin Hewitt View Post
    3 microns is an interesting number BUT what about the backlash?
    when I was testing the touchplate setup on my router, I was getting repeatability to within +-0.003mm. Quick back-of-envelope sums showed that this was equivalent to one microstep. Might this be why this magic number is there? Mind you, my backlash is a bit more than that..

  2. #42
    As it happens Neal I've just run some more tests. Prepare to fall asleep! Contrary what the fool in the vid says, the Y does seven cycles!

    Despite all the grunting (not mine) the steppers don't appear to be losing steps at 2500mm/min.



    Actually, I don't know what the backlash is yet. I would think it was small but I need to set-up a proper test.
    Last edited by Agathon; 10-07-2017 at 12:10 AM.

  3. #43
    Just measured the back-lash at 0.03mm on each axis. I'm a little surprised by this since I can't push/pull either axis by more than 0.01mm. The motors are coupled to the screws with flexible star-type couplings (see pic) - which are original and in good condition, but I have to say I've always been suspicious of. Now they are robbing me of precious microns!

    Seriously though I can live with 0.03mm.

    Click image for larger version. 

Name:	coupling.png 
Views:	155 
Size:	345.1 KB 
ID:	22152

  4. #44
    Quote Originally Posted by Agathon View Post
    Seriously though I can live with 0.03mm.
    I used to tell myself that I could live with a slight backlash but deep in my heart of hearts I knew it wasn't true.

    I move 5um per step. Could I watch it step 6 times without moving the table then sleep at night?

    Difficult question...

  5. #45
    Quote Originally Posted by Robin Hewitt View Post
    I used to tell myself that I could live with a slight backlash but deep in my heart of hearts I knew it wasn't true.

    I move 5um per step. Could I watch it step 6 times without moving the table then sleep at night?

    Difficult question...
    I'm not sure I can either! After I posted I took another look and the back-lash is at the feedscrew. I think I slackened the pre-load on the X too much and need to tighten up the Y. Watch this space....

  6. #46
    Decided to strip the ball-screws and repack with new balls:


  7. #47
    Clive S's Avatar
    Lives in Marple Stockport, United Kingdom. Last Activity: 12 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 3,333. Received thanks 618 times, giving thanks to others 78 times. Made a monetary donation to the upkeep of the community. Is a beta tester for Machinists Network features.
    Decided to strip the ball-screws and repack with new balls:
    Very nice vid. Would you mind giving a link to where you purchased the balls from. Tnx
    ..Clive
    The more you know, The better you know, How little you know

  8. #48
    I have my nuts sprung together one quarter ton. I hold the X screw in tension, Y and Z I crush angular contact bearings together. Everything is one quarter ton and feels glorious.

    I sprung my Roland mini-mill to 200 lbf, I don't have a magic formula, I just use whatever seems right.

    Do you have a way to inject oil in to the nuts or do you depend on that grease? I started fitting narrow bore nylon tube to carry oil but one of the push fit connectors didn't fit and I wanted to play cutting stuff so I left it out. I think I have to strip back and finish the job. Gravity feed is traditional but would that need wider bore piping? Such fun.

    For a moment I was worried that I might have put you off with talk of backlash. Glad to see you back.

  9. #49
    Quote Originally Posted by Clive S View Post
    Very nice vid. Would you mind giving a link to where you purchased the balls from. Tnx
    Hi Clive, I just ordered them through my local branch of Brammer - 1/8" Chrome steel balls, Grade 100 Ģ10.08 for 500. These nuts have about 0.004" clearance, so I did look into getting balls at 0.127" - but they were very expensive ranging from Ģ58 - Ģ82. As I say in the vid, I realised this wasn't the right way to go with this type of nut.

    Quote Originally Posted by Robin Hewitt View Post
    I have my nuts sprung together one quarter ton. I hold the X screw in tension, Y and Z I crush angular contact bearings together. Everything is one quarter ton and feels glorious.

    I sprung my Roland mini-mill to 200 lbf, I don't have a magic formula, I just use whatever seems right.

    Do you have a way to inject oil in to the nuts or do you depend on that grease? I started fitting narrow bore nylon tube to carry oil but one of the push fit connectors didn't fit and I wanted to play cutting stuff so I left it out. I think I have to strip back and finish the job. Gravity feed is traditional but would that need wider bore piping? Such fun.

    For a moment I was worried that I might have put you off with talk of backlash. Glad to see you back.
    The machine's central lubrication system is very comprehensive and delivers oil to the ball-nuts and ball screw in a couple of places.

    I'll put the x-axis back together later today and see if there's any improvement on the backlash.
    Last edited by Agathon; 14-07-2017 at 10:43 AM.

  10. #50
    Fwiw...
    I think You have one of the best manual tools of this type (c frame small mills) ever produced.

    Your explorations/experiences/results are similar to mine, on a decent-rigidity heavy chicom 12x" lathe.
    Except that mine is about 10x worse in terms of fit, finish, quality in general, of course.

    I would opine, while never having seen a Fehlmann in person (is a tour available if I happen to be in Your country ..?),
    - repeatability to about 1 micron is very achievable
    - resolution to much better than 1 micron is available to You, and probably relatively easy to do

    My experience for 0.03 € (I need to make a profit, You know..):
    I made treadmill-dc motor-servos with geckos (320) about 2005.
    So-so, at 10.000 counts, accurate but noisy, sparks, ozone, some jitter, heating.

    I used the treadmill servos (DC motor 180V/6000 rpm, at 68 V dc, so too many amps and not enough volts, but I never need speed anyway)..
    and at 1:3 via HTD, 5/15 mm, 1:3 or 16:48 teeth, could reliably index, or incremental-move, 1 micron at a time.

    The basic accuracy and stiction and smoothness of the chicom 12x is probably 2x-4x worse than Your mill.
    (But it is more rigid. Lathes always are, especially heavier ones, like mine. 350 kg/24" == 2000 KG on a 1.2 m long bed.Maybe not more rigid than a Fehlmann, one of the best ever made.Imho.)

    So I built very heavy supports, mounts, belts, using HTD8-30, taperlocks, 17 mm shafts on 220V ac brushless servos, of 750 W, 10.000 counts, at 1:2.
    Results are weak in accuracy.
    Pulleys are poor, relatively, and belts may or may not be poor, relatively.

    But..
    Using these relatively-expensive servos, 700€ landed, I do get 1 micron indexing or relative movement.
    But it is not smooth, consistent, steady, and no-way no-how a dial-to-size solution.
    But..
    Theoretical resolution is 0.2 microns, and the fact is, the screw itself *will* move by 0.2 - 0.n micron increments, every single time. Increments, not accuracy.
    Screw will always move in angular terms. Some waviness from belts/pulleys., ie it is not perfectly linear/smooth/accurate, transmitted to saddle x axis, with "some" bounces of uncertain size, perhaps 1-2-10-20 microns, depending on gibs, tension, position, oiling.

    With light (typical manual) gibs, oiled, imho, about 0.5 microns resolution is perfectly reliable for me. I cannot measure this, yet.
    I will, and may put in 0.1 micron glass scales (thats the plan, anyway).
    I opine You can easily get 0.5 microns, or better, resolution from one of the best mechanical mills ever made, via servos.

    Once pre-loaded on an axis,
    Led readouts, on the servos, show errors, which are always zero, at position, because the servo has == 10 Nm / 10.000 counts / rev, or 20 Nm at screw, about 10x more than any possible resistance.
    At 20 Nm the push force == 2000-3000 kgf.

    The saddle bounces because of these mechanical errors, none of which relate to stepper or servo as such,
    - the x mount is not rigid-enough and not mounted in-plane with the screw thrust
    - sticktion
    - gibs
    - poor screw (I knew this)
    - yoke connection to saddle is poor re:rigidity. Much worse than I thought, just saw this 2 days ago.
    With a heavy load/stiff gibs, everything bends noticeably on the x axis.

    Everything bends always, I/one just did not see it well enough in the past.

    I got great results with soft gibs, semi-heavy x axis lock, like manual turning, since 2005 or so.
    The current ballscrew on x is 14x more rigid than the original 16-17 mm acme screw (fixed-fixed mount, in tension, rigid for 2x, half free length for 4x, thicker for == 2x).
    So I expected 6x more rigid belt drives to do better.
    They did worse, or same, with much better servos, and more rigid/tighter gibs.

    Probably, softer gibs==manual, would deliver better results in resolution. I now think.
    All this is related.
    I got 20x, perhaps better, higher mrr or Material Removal Rates, than I have ever had, last week.
    Industrial level results.

    2.5" 63 mm ISO30 face mill, 4 inserts. Cutting thick tool steel, full width.
    3.9 mm deep, !!! 63 mm woc ie full face, 45 mm/min speed, 550 rpm.
    That is a relatively 63 mm thick end mill, 3.9 mm deep !! for milling terms, full width.

    The machine was worked hard, but quite happy, when I achieved balance.
    Balance was hard / impossible to maintain.

    New face mill, axxx something, new inserts, I just hold the facemill front in the 12" 4-jaw chuck, clocked to 0.01 mm or better tir.


    - gibs are now too tight, after adjusting
    X-axis Screw is now 0.750 " roton, rolled.
    The new x axis screw is TBI taiwan, 32 mm, 5 mm, with a 60 mm thick yoke.
    I need major work before it is in, ..
    new saddle plate (ends are now milled, hooray !), yoke all 6 faces, yoke bored, supports of no particular accuracy.

    My opinion(s);
    1. Go to servos.
    2. Go direct drive. There must be a reason all manufacturers use direct drive.
    3. Use a bigger coupler.
    4. Make anything used in the motion-control train very very heavy in steel.
    5. Use highest-resolution servos you can, while being relatively economical, and having sufficient speed in khz/mhz for your controller.
    Mine is a csmio-ip-s, 4 MHz.
    A 3000 rpm/10.000 count servo, is 50 revs/sec, = 500 kHz.

    I could not care less about top speed, and may use something near top acceleration, in the future, or not.
    Today I use about 1/5 top speed and top acceleration, and both greatly exceed the best stepper systems I used in the past.

    "Good" steppers as in fast nema 23 steppers, 68 v, gecko 203v, centipede hw pulser/controller (excellent hw and timing).
    At that, the stepper made in tests 10.000 rpm, over 4-6 secs acceleration, with no torque, no-load.
    Geckos fault 380 kHz-400 kHz+ or so.
    In use, best-optimum std nema 23 steppers make about 600-700 rpm, in == 0.2 secs (lathe, needs acceleration, as much as possible).

    Small nema 23 servos, and the bigger nema 34 servos on the lathe, deliver;
    == 0.02-0.04 secs to 3000 rpm, at load.
    About 10-50x better in acceleration, real-world.
    About 5x more accurate.

    Imho, accuracy is very important.
    Acceleration is very important, or important.
    Top speed is totally irrelevant.

    what happens is that servos are excellent at many small tiny moves, like 3-d contouring, or modern high-speed milling toolpaths.
    Or very,very accurate tiny moves, that steppers cannot do directly.

    A stepper at 1/10 microstep (2000 steps/r) has about 1/10 rated torque iirc (gecko, Mariss).
    So a 3 Nm stepper has 0.3 Nm at 0 rpm (best case) at 1&10 microsteps. It is, in essence, a spring.

    A servo at 10.000 steps has 3x rated torque, from 1 step at 0 rpm to 1 step at max rpm, say 3000 rpm (for the 3 secs max peak torque).
    The servo of 1.3 Nm (400 W, similar cost 290€ / axis), 68V DC/AC, has 1.27 Nm cont, == 4Nm peak.
    So the small servo has approx 4 Nm vs 0.3 Nm stepper torque of a similar size, about 13 times more, and over 50-100x more torque over 1000 rpm vs a stepper.

    All this with a cheap small economical Nema 23 400 W servo, of course.
    If comparing to more industrial-type stuff, ... well...

    My 750W - 220V ac driven servos .. are exactly the same in use.
    Just 10.000 counts vs 5000 counts,
    220V vs 68 V,
    3.x /10 Nm vs 1.27 Nm.

    My 0.2 micron (now) step size lathe has 10.000 x 2 / 5 mm = 0.25 micron resolution, theorical/electronic.
    = 7.5 m / minute, 0.125 m second.
    Typical free length is less than 100 mm = 10 cm, on z. 24" minus 12" chuck 180 mm, minus ts, == 220 mm, == 400 mm.
    About 250 mm free length between 12" chuck and ts at shortest extension.
    And plenty length for me, never needed to remove the gap from the bed, or the ts maybe once, minor stuff.

    I donīt usually do e. gunstuff, or long spindles, and when I do, easy mounts exist.
    My lathe is a technology demonstrator, mostly.


    When cutting anything, clearances are usually 2-3-5 mm, for me, now, and less when everything is dialed in.


    The absolutely only need for high acceleration /speed in lathes, for me, has always been leaving threading, at the end.
    And it is extremely important to be very, very, very consistent, of very high torque, at very high speed in both start-time and pullout-time, in ms, when pulling out.

    E.typical.
    Threading at 500-800 rpm towards hs, steel, about 1-1.5" D workpiece.
    Typical, maybe 8 passes.
    Every pass makes the end bit divot deeper, and the pullout point is always deeper, and more rigid, each pass.

    Any tiny error or delay, makes the threading tool dig-in, if there is any bend (there is always bend), slop, backlash in the whole x axis drivetrain.
    Any tiny dig-in, exponentially increases the error until failure of something, unless the tool is pulled out fast enough, strong enough, to avoid failure.

    In this scenario the great benefit of servos is,
    -vastly faster acceleration for pullout
    -vastly higher force of pullout
    -vastly more accurate/repeatable pullout point and action
    The relevant part of the pullout is perhaps 0.1 - 0.5mm in length, aka most of the thread depth, and takes maybe 0.1 secs with a stepper, and maybe 0.05-0.01 secs with a servo.
    But the servo exerts 10-50x the force, at 10-50x the acceleration, and 5x or more accuracy.

    It is of note that a very tiny 0.01 - 0.02 mm mechanical slop with steppers, or machine error, can snap the tooltip, as the dig-in is exponential and mechanical and increases exponentially.
    This does not usually happen with servos, because no matter what most of the tooltip is already out of the workpiece, with the same mechanical error condition, simply leading to a slightly rounded pullout edge.


    E.
    I used 50 mm / 2" thick tool steel for the mount plate for the Z axis ballscrew.
    About 140x200x50 mm.
    Not because it is "stronger" but because it is very much stiffer, and will repeat better.
    I think 10x more rigid than "typical", donīt really know, and it only cost == 20-30€, maybe 15 kg in mass for the mount plate alone.

    My z axis screw is 32 mm.
    Yours will be similar, I think.
    32 mm screw == 1600 kgf push force, rated, static (weakest rating).

    Example/anecdote.
    This push force is == equivalent to lifting a SUV with the screw.
    And You want to try to bend as few microns as possible, while lifting the suv.
    A typical 32 mm screw has 54 kgf/um or 540 N/micron rigidity.

    I am by no means a "servo zealot".
    Steppers are very easy and can provide excellent repeatability, accuracy, force, positioning, of relatively low rpm, very reliably and cheaply.
    But they have low dynamic range.
    This means either accuracy, or speed/power, but not both.

    So the ideal apps for steppers are
    e.g.
    telescope mounts of low rpm and very high geared resolution/repeatability,
    cutter grinders, similar,
    saw accessories,
    microscope accessories,
    cnc mills of typical hobby shops needs, some jobshop uses,
    cnc jewellery stuff,
    etc.

    E.
    I have made all my x-axis mounts/stuff behind the lathe .. so it looks stock from the front, and can be used manually.
    New screw connects to yoke of 60x120x120 mm.
    It will be about 100x more stiff than now (because I use a temp. crap lashup to bolt to the current saddle plate, (was better in the past, pre new servos) of temporary/test use).

    With direct drive, I will get 0.4 microns/step vs 0.2, but no wind/spring/bounce from belts, and vastly less from winding error / screw, and much less bend from yoke.
    And no pulley error, belt error.

    Belt drives with small belts are great for steppers - but the wrong choice for servos. Imho, Imhe.

Page 5 of 8 FirstFirst ... 34567 ... LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Toolbox Refurb - New Handle Fittings
    By Wal in forum Metalwork Project Showcase
    Replies: 4
    Last Post: 22-09-2016, 10:26 PM
  2. CONVERSION: Sx2 plus cnc conversion
    By Lewywri89 in forum Conversion Build Logs
    Replies: 11
    Last Post: 30-11-2014, 11:36 PM
  3. RFQ: CNC Myford Conversion
    By Jconway651 in forum Projects, Jobs & Requests
    Replies: 0
    Last Post: 16-10-2014, 09:52 AM
  4. CONVERSION: X2 conversion on a budget!
    By craigrobbo in forum Conversion Build Logs
    Replies: 1
    Last Post: 30-10-2012, 05:00 PM
  5. Sieg sx2 conversion kit
    By Goldigger in forum Milling Machines, Builds & Conversions
    Replies: 3
    Last Post: 24-04-2011, 07:42 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •