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Thread: 4th axis help for smaller CNC machine

  1. #1
    JeffJo is offline Plastic
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    Default 4th axis help for smaller CNC machine

    Hello!

    I'm new to this forum and I am looking for some helpful suggestions.

    Our company recently purchased an ACT DMCIII tabletop milling machine. We purchased the optional 4th axis because we engrave dies and require rotation during milling. The rotary table was supposed to have zero backlash. We quickly found out after going through 2 of them that this is not the case. We even tried to use bungee cords to more tightly reduce the backlash.

    Now we are looking for a new rotary table with stepper motor to replace the one we've already purchased. I understand now that true zero backlash in a rotational axis might not be possible. We'd like to find one that has little to no backlash or that is adjustable down to zero.

    Frankly, I am no engineer. I don't have the experience to rebuild a rotary table. What I am looking for is a good, straight out of the box 4th axis that would work with our machine.

    Any suggestions?

    I appreciate any and all help that I can get!!

  2. #2
    RoboDriller is offline Plastic
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    This may sound obvious to some 'residents' here, but I rarely travel in two directions when programming something with a 4th axis. I typically set up, touch off, and check, all moving the rotary in the same direction. I find this absolutely critical for repeatability.

    Ive only ever worked with Hardinge and Richmill 4th axis rotary implements.

  3. #3
    RoboDriller is offline Plastic
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    This may sound obvious to some 'residents' here, but I rarely travel in two directions when programming something with a 4th axis. I typically set up, touch off, and check, all moving the rotary in the same direction. I find this absolutely critical for repeatability.

    Ive only ever worked with Hardinge and Richmill 4th axis rotary implements, but rarely experienced backlash that was not correctable by simple adjustment of the backlash adjusting screw inside the covers.

    In my experience, many 4th axis implements have their own standalone controllers which you program the angular moves, and it is triggered by an M-code from the NC controller. Try setting up, indicating, and programming your axial moves in one direction, if you need to go +45, -45, make it +45, +315 instead.

  4. #4
    Pete from TN is offline Cast Iron
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    Default Well....

    If I am thinking of the right machine here that mill is a very small table top machine that is more like a cnc router than a milling machine in design. It is quite small and you are gonna have to find a solution that is ALSO quite small. Most of the best rotary units use a harmonic drive system that does indeed have near zero backlash but they are two things, EXPENSIVE, and LARGE. They are designed to be installed into a full size VMC. You have two options I see. One is to purchase a small harmonic drive gearbox and build your own indexer using a stepper off of it and you said you did not want to build anything. Or you can purchase the Sherline Rotary table unit which is supposed to be pretty good and adapt it to your machine. I do not think that unit has perfect zero backlash either but it does get a lot of good reviews on the benchtop and hobby machinist forums and it is not too expensive. From what I remember it is also made to be a horizontal unit so you need to purchase a right angle plate to mount it to or maybe sherline has something now for it. Either way it should work pretty good and I understand the backlash can be adjusted in it carefully. Other than that there are a bunch of different companies making CNC rotary tables from inexpensive chinese rotary tables that are not that great from what I have heard. Another option might be to try to find a very small cnc 5c collet indexer somewhere. Good luck with your search and peace

    Pete

  5. #5
    CalG is offline Titanium
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    Obviously, for engraving letters, numerals and such, a single direction approach will not be useful.

    With that, I have, surplus to my needs, a NOS Design Components Incorporated 6R45 rotary table fitted with a Compumotor Stepper.

    This unit was purchased for a still born project some years ago. It's on a 6 inch square base that houses the gearbox, and the table is about 6 inches in dia.

    I just took a look at it, I couldn't feel any lost motion.

    If it's of interest to you, Send me a PM. I did have the specs and literature for it in an electronic reference, I'll see if I can dig it up.

    Cheers

    Cal

  6. #6
    CalG is offline Titanium
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    I just took a look at web site for your ACT DMCIII.

    Though the text description of that rotary device suggests lash free construction. Combining Motor coupling, 90 degree turn and a speed reducer in one unit is not the stiffest arrangement. A worm drive with adjustment for lash has a lot going for it.

  7. #7
    gregormarwick is offline Stainless
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    Not too long ago there was a chinese company on ebay selling very small 4th axis units built around a harmonic drive, I came across them while looking for a harmonic drive for a project.

    It was essentially nothing more then a stepper/servo, drive, and mounted chuck in an aluminium housing made to bolt to the table. I also recall that they were inexpensive although I can't remember an exact figure.

    There doesn't appear to be any listed just now, but the point is that they do exist and would seem to be what you're looking for.

  8. #8
    Edster is offline Titanium
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    What is a harmonic drive? I know about worm gear setups, roller gear setups, and direct drive setups, but what is a harmonic drive?

  9. #9
    CarbideBob is offline Titanium
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    Quote Originally Posted by Edster View Post
    What is a harmonic drive? I know about worm gear setups, roller gear setups, and direct drive setups, but what is a harmonic drive?
    Google it.
    An internal gear with a flexible spline that engages it using a set of rollers to deform the spline.
    Used for low backlash high reductions. Very common in robot joints.
    Kind of hard to describe with words alone. Goggle will give you some pictures.
    It does have some drawbacks, price tag and alignment of the parts being up there on the list. It is not "zero backlash" under load, but it is much better than a worm/wheel arrangement.
    I use them on grinder workheads for accurate indexing.
    Bob

  10. #10
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    Fasto is offline Hot Rolled
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    I've got a 200mm (8") Nikken 4th axis that quotes 15 microns backlash at the outer edge of the table. It's a pretty conventional worm and wheel arrangement. I think Nikken makes a 150mm (6") version, too. You can actually adjust the backlash down to 5 microns at the edge of the table at the expense of higher wear.

    That said, it weighs about 120 pounds and uses a lot of space on my smallish 14"x22" travel VMC. It also costs upwards of $10k last I checked, not including the servo amplifier.

    I think Tormach makes a 4th axis that might be suited to your needs a little better, though you'll need to figure out how much backlash is acceptable in your application. I would expect that "0" backlash isn't going to be readily acheivable. How much backlash did the 4th axis you used have?
    Laurentian likes this.

  11. #11
    doug6949 is offline Hot Rolled
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    I could not find a picture of the 4th axis on the ACT website. However, the specs page clearly claims that it has zero backlash.

    4th axis rotary table is a high precision table with zero backlash, 7.2 arc sec (0.002 degree) resolution.
    I would return it for a refund. Unfortunately, I've never seen a 4th axis for this grade of machine that was any good.

    Doug

  12. #12
    CarbideBob is offline Titanium
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    The big questions are how much backlash can you live with and how much force are you going to put on it?
    Do you have the budget and room needed to go with a NSK Megatorque or a Kollmorgen DDR?

    The Tomach is a rotary table the same as Enco sells with a stepper motor attached. Like a manual rotary you can adjust the lash down to a small amount. The internal construction is nothing like a Nikken but they do work.

    I use this style table which I retrofit with AC servos and fit thrust bearings on the worm shaft. Since they are so cheap I buy them 4 at a time and toss them when they start to wear. I go through one about every six-nine months per machine.

    You can't build a worm/wheel table with zero backlash. Even if you buy the $8000 sub-micron level gearsets you need some clearance for mounting tolerances and heat build up.

    Cone drive makes "split" worms that can be preloaded and my Agathons use a split wheel spring loaded to eliminated backlash. Both will open up with enough pressure and this style limits speed capability not to mention the price tag.

    The nifty designs use a split worm that is mechanically "opened up" during movement then clamps back shut at final positioning.

    The high end trend nowadays is towards direct drive rotaries. A linear motor wrapped in a circle or basically just a real high pole count AC servo motor.
    Bob

  13. #13
    mrprecise44 is offline Plastic
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    Thumbs up No backlash drive

    Hello Jeffjo:

    Have a look at DoughtyDrive

    They make a small 2-axis rotary table for a router, which makes a 5-axis CNC machine. The table is gear reduced by multiple timing belts, and is a true no backlash drive design.

    The whole unit is (2) belt-drive rotary tables, A & B, connected with an angle plate at 90 degrees. The router spindle mounts on the second table B. The Z axis carries the A & B, making a 5 axis router.

    They sell a single table, or the double unit. The tables are aluminum construction, sealed, pre-loaded thrust bearings and zero backlash.

    John

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