Build Thread The Design and Construction of a 'Backlash-Free' Rotary Table


Page 1 of 9 1234 ... LastLast
Results 1 to 12 of 104

Thread: The Design and Construction of a 'Backlash-Free' Rotary Table

  1. #1
    Gold Member
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3048
    Downloads
    4
    Uploads
    0

    Default The Design and Construction of a 'Backlash-Free' Rotary Table

    This will be a many-part series covering my efforts at building a CNC Rotary Table (RT). Please forgive any obvious mistakes. Yes, of course you are going to get my biased opinions. Yes, there will be photos and drawings. No, there won't be detailed plans: they are not really the point of this exercise. Updates will happen every few days.

    The background to all this is the rather long thread here at CNCZone about a Backlash-Free Rotary Table, started by Zoidberg
    from Sweden in Jan-2009. It went for 80 pages, which is not bad. There were some passionate arguments about technology, especially on just how you can get (make) 'backlash-free' reduction in a home shop or hobby context. Shall we summarise by saying 'opinions differed'? But we all had fun.

    The Design and Construction of a 'Backlash-Free' Rotary Table-rotary1_1-jpg

    Let's remove any doubts right up front: this is NOT about hanging a stepper motor on the crankshaft of a manual rotary table. Such tables do not have the bearings for continuous CNC operation, plus they usually rely on manual brakes to stop them from moving under load. As a manual indexer they may be fine, but they are NOT a full CNC Rotary Table.

    For the most part, the discussion was about the means for gearing the motor rotation down to to the table shaft. Arguments pitted ordinary gears against worm drives against toothed belts. There were some esoteric variants, but generally they were just
    'variants'. Some felt that all the options have backlash, while others (myself included) argued that modern toothed belt profiles
    do not. A few felt that gears could be made backlash-free, while others argued for special worm drives. Various rather expensive
    commercial variants were brought into the discussion, but it was generally conceded that they are unrealistic for a hobby
    construction. The issue was never really settled to everyone's satisfaction.

    The Design and Construction of a 'Backlash-Free' Rotary Table-rotary1_2-jpg

    The really big problem here is the rather large reduction ratio needed between any drive motor and the table itself in order that
    fine positioning can be had at the rim. Of course, if you don't want fine positioning at the rim then life becomes far simpler, as
    shown here. OK, maybe that is a shade too simple? Mind you, for pottery it may be just fine.

    The Design and Construction of a 'Backlash-Free' Rotary Table-rotary1_3-jpg

    If you have really simple requirements you can even dispense with any reduction, and just go with a chuck on a stepper. That might be adequate for woodworking for instance. In this context, the motor options we normally have for a hobby construction are either a 1.8 degree stepper motor or a DC servo motor with a 500 line encoder. There are others of course, but these would be the most common.

    To give a concrete example, let's consider a 10 micron accuracy at a 50 mm radius. This 10 micron accuracy 'sort of' matches the linear accuracy one might expect from the XYZ axes of a good (hobby) mill. To get that sort of resolution (not accuracy)
    requires an angular resolution of approximately 0.0115 degrees (0.69 arc-minutes) at the table. A 1.8 degree stepper would
    require a 157:1 reduction to get there (ignoring microstepping for now). That is a lot of reduction. It also places some very tight
    limits on allowable backlash, and that is where the trouble lies.

    The problems with ordinary gears are fairly well known. First, they have to have some clearance in order to turn, even with the
    very best design and manufacture, and this clearance makes for backlash. Second, practical size constraints within a rotary table mean that gear ratios of more than (say) 5:1 are unlikely. To get the required reduction with gears would mean a rather long gear train, and the concensus was that the backlash at the end of such a gear train would be ... bad. To be sure, not everyone agreed.

    This does not even consider the problem of getting a perfectly concentric pitch circle on each gear. To be sure one can get very
    close, but even 'very close' leaves room for backlash.

    A worm gear reduction of about that much is far more possible, but even so there are real problems with backlash. You still need some clearance so the worm can turn against the wheel, although the clearance could be very small. What might be more difficult is (again) the machining error found in the pitch radius of the wheel. Any slight variation in effective pitch radius would mean that the table could be rather tight in some positions and rattling around in others.

    It was suggested that you could have a spring-loaded worm driving the wheel. That is true, but there are now two design problems to deal with. First, you have to have the worm really tightly constrained along its axle. That means good miniature bearings.
    Second, you have to allow the worm and its support bearings to rotate slightly around some pivot point (or axle) without the
    slightest trace of 'rattle' in the rotation bearings. That puts a lot of precision bearings in a very small volume. It's possible, and I did spend some time on such a design, but it is difficult. Well, I thought it was difficult, anyhow.

    Of course, since you have a sliding movement between the worm and wheel, there are also problems of surface wear and lubrication. To get lubrication between the worm and wheel you need clearance, even if it is only a few microns. That means some backlash. No lubrication and you get much faster wear. Not easy. Yes, I know most small manual rotary tables use a worm and wheel, but they don't get spun as much as a CNC rotary table would (so there's less wear), and they all come with quite powerful position clamps at the rims, which have to be used when machining anything on them. In effect, they are really indexing tables, not full dynamic rotary tables.

    The Design and Construction of a 'Backlash-Free' Rotary Table-rotary1_4-jpg

    Finally, we have toothed belts. The old (or original' designs of 'timing belts' such as L & H and variants thereof had all manner of gaps between the pulley teeth and the lugs on the belt. Yes, that meant you could have backlash, and variable backlash at that.
    However, we have come a long way from the early L and H profiles. Modern profiles such as the HTD, AT and GT series are far better, and they have been designed expressly to zero out the backlash. It's all in the belt profile and how it meshes with the pulley teeth you see. Googling will get you lots of vendor explanations, so I won't expand any further.

    The Design and Construction of a 'Backlash-Free' Rotary Table-rotary1_5-jpg

    Belt stretch has similarly been improved with steel cores (the string inside the rubber), fibreglass cores, Kevlar cores and carbon fibre cores. The belts are meant to operate under some tension (to seat the belt teeth into the pulleys) and they can deliver a surpisingly large number of kWatts are very high speeds. This means you could string a couple of toothed belts together and
    still have negligable backlash on a rotary table. However, you would still have the problem of that 157:1 reduction ratio. A
    realistic limit for a single stage of reduction in the confines of a rotary table might be 6:1. Three high-ratio stages would be needed, and you would have to tension every one of them. That's not so easy either.

    Despite all these problems, I decided I really needed a rotary table, and it had to be better than a converted manual
    indexing table with lots of backlash. OK, make that 'I really WANTED to make one'.

    Before closing this chapter, I had better draw a distinction between resolution, accuracy, linearity and repeatability. I will use
    angular rotation here as it is most relevant.
    * Resolution is the minimum step the hardware is capable of doing. You might for instance have a resolution of 0.001 degrees.
    * Accuracy is effectively the difference between what you ask for and what you get. In this context it is a bit complex, so we will
    pass on for the moment. Suffice to say that resolution is often (usually) finer than accuracy.
    * Linearity is part of accuracy: does a command for a 1 degree rotation give you exactly the same movement when the start angle is 0 degrees compared to 180 degrees for instance? Usually the answer is 'close but not exactly'. An off-centre drive pulley can for instance cause cyclic non-linearity.
    * Repeatability is another part of accuracy, but it pulls in the concept of hysteresis. If you tell the system to go to +90 degrees, then have it wander off for a while doing things, then again tell the system to go to 90 degrees, repeatability asks whether it come back to exactly the same position as before. What the target angle is does not matter here; it's whether you get there or not. For instance, if you approach the 90 degree position from two different directions (CW and CCW), hysteresis usually means you do not get to exactly the same position. So repeatability is complex as well.

    In the next few Chapters I will discuss my options for bearings, housings, spindles and so on. After that comes a provisional
    design, a few major changes (which altered the entire project), further engineering details like motors, and so on. As part of the
    series I will go into measured performance specifications.

    Cheers

    Similar Threads:


  2. #2
    Gold Member
    Join Date
    Dec 2007
    Location
    Australia
    Posts
    2110
    Downloads
    1
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    Looking forward to seeing the progress on this!

    cheers, Ian

    It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!


  3. #3
    Registered
    Join Date
    Sep 2006
    Location
    Denmark
    Posts
    607
    Downloads
    0
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    This looks promising. What testing equipment will you use to make sure that you have reached your goal? Your test setup will have to be at least 10 times more accurate than the goal to validate the construction. If you don't have a solid way of testing your design, then how do you know when to stop improving on it?



  4. #4
    Gold Member
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3048
    Downloads
    4
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    What testing equipment will you use to make sure that you have reached your goal?
    Good question, of course. All will be revealed in the coming chapters.
    I have a PhD in physics and I have spent some 40 years doing research into measurement technologies for 'difficult' areas.

    Cheers
    Roger



  5. #5
    Gold Member daniellyall's Avatar
    Join Date
    Sep 2009
    Location
    New Zealand
    Posts
    1716
    Downloads
    3
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    good sum up of the old thread on my cnc router it has GT2 belts and pulleys there is almost no backlash it`s about 0.001mm but that might be the stepper

    http://danielscnc.webs.com/

    being disabled is not a hindrance it gives you attitude


  6. #6
    Gold Member
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3048
    Downloads
    4
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    Bearings

    This chapter is mainly theory. If a thing goes around for any distance, it has to have a bearing. We have lots of options for bearings, starting with a round bit of hardwood axle stuck through a greased hole in some more hardwood under a cart, and steadily improving from there. We will skip metal and plastic bushes and go on to some common rolling element bearings. Any number of good textbooks or Application Notes might say all this better. The web site for any bearing manufacturer or dealer will have lots of photos and drawings of bearings.

    Deep Groove Bearings
    These have round balls travelling between an inner and outer ring or race in a deep groove, invented (well, patented) by Philip Vaughan in Wales in 1794. There are a lot of companies now claiming to be the world's best producer of DGBs. I don't know what the world production of DGBs is, but I did read that one German company claims to produce about one hundred million DGBs a year. (Can you imagine what happens when 'they' give each of these bearings an internet connection to report on their reliability?)

    The first advantage of DBGs is that they can be cheap. The second advantage is that the balls contact the races over a moderate area (compared to a shallow groove). That improves the load rating a bit. But there is always some clearance and so some rattle (less rattle = more $$). To get tight position tolerance you can put two DGBs up against each other so the balls press on the sides of the track rather than the bottom. This iis called pre-loading, and it works, but it reduces the load rating.

    So how many steel balls are made each year? Again, dunno, but Wikipedia claims 'In 2008, the United States produced 5.778 billion bearing balls'. The accuracy of that claim, better than 0.02%, seems debatable to me, but it is still a very big number. I do wonder where they all go.

    Angular Contact Bearings
    These are similar to DGBs but the surfaces on the races are tilted at an angle to the centre line. This means that putting two of them in opposition actually improves the contact area. They are meant to be used like this with a pre-load. Naturally, this means they are more specialised, there is lower production, and they are a little more expensive. Double ACBs are superior to DGBs for spindles.

    Thrust Bearings
    These are quite specialised for forces along the axle, and not really suitable for an MYOG rotary table by themselves.

    Crossed Roller Bearings
    These are a very exotic sort of bearing using rollers rather than balls and usually having quite a large diameter. They can be adjusted for rattle and can give superb performance. Naturally, they are also quite expensive: say 100x the price of a cheap slightly smaller DGB.

    Spindle Length

    The Design and Construction of a 'Backlash-Free' Rotary Table-rotary2_1-jpg

    A single bearing is great for constraining a spindle at a point, but you need two of them (one at each end) to keep a spindle pointing in the same direction all the time. Since each bearing has a tolerance, the farther apart the two bearings are, the less the spindle can rattle around - up to a point.

    Both DGBs and ACBs can be used like this, with one at each end of a spindle. A little design cunning and you can squeeze the bearings together to remove the clearance in the two bearings. A large nut at one end is often used. However, it is not quite that simple: you have to make sure that the bearings are kept square onto the spindle. Some bearing suppliers sell specially machined nuts for this.

    A secondary consideration is that the spindle should not flex between the bearings. If you have a 6 mm shaft running through the armature of a motor, the armature had better be very well balanced or the shaft will bend in the middle. Thin shafts are not good. Fortunately, one normally wants a large bore through the middle of a rotary spindle, so a large diameter tube is used. Provided the wall thickness is reasonable, the spindle is 'stiff' and all is well.

    That's enough theory for now. The next two chapters will cover a design I put together for a fairly conventional 'spindle with two bearings in a box'. But don't jump to any conclusions just yet.



  7. #7
    Registered
    Join Date
    Aug 2004
    Location
    spain
    Posts
    618
    Downloads
    0
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    Glad you are doing this.
    Glad you are using real numbers.
    Very glad you are making clear the distinctions between resolution, repeatability and accuracy.



  8. #8
    Gold Member daniellyall's Avatar
    Join Date
    Sep 2009
    Location
    New Zealand
    Posts
    1716
    Downloads
    3
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    someone who use`s the correct words is always a cleaver person but someone who makes it so a nob can understand what is being writing about when they have a PhD is a practical person.

    looking forward to the design.

    http://danielscnc.webs.com/

    being disabled is not a hindrance it gives you attitude


  9. #9
    Registered
    Join Date
    Sep 2006
    Location
    Denmark
    Posts
    607
    Downloads
    0
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    Quote Originally Posted by RCaffin View Post
    Good question, of course. All will be revealed in the coming chapters.
    I have a PhD in physics and I have spent some 40 years doing research into measurement technologies for 'difficult' areas.

    Cheers
    Roger
    Then I will just sit back and enjoy the ride! Have you heard about the infinite monkey cage?



  10. #10
    Gold Member
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3048
    Downloads
    4
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    the infinite monkey cage?
    Er ... apart from the BBC show?
    Sorry - I don't think my brain has grasped the connection yet. But I don't watch TV, so ?

    Cheers
    Roger



  11. #11
    Registered
    Join Date
    Sep 2006
    Location
    Denmark
    Posts
    607
    Downloads
    0
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    It's a radio show, so no need to watch TV. Seeing as the show is about popular view on science and physics I thought you might enjoy it. I certainly find it amusing. Helps me get through long production runs and makes me wish I had chosen a career in theoretical physics.



  12. #12
    Gold Member
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3048
    Downloads
    4
    Uploads
    0

    Default Re: The Design and Construction of a 'Backlash-Free' Rotary Table

    Ah, I see. Not sure the show is available in Australia though. I haven't looked. Could maybe get it via the web. But I am not going to run Internet connections AND Mach3 on the same machine.

    A career in theoretical physics? A bit chancy these days. If you think the business world is tough, you haven't seen an academic cat fight. A whole new world of hurt.

    I worked in a Gov't research lab with external funding. It was a good place, until the management screwed up so badly that the funding body said stuff it and cancelled the whole deal. So the lab was closed. Mind you, the last few years there had not been pleasant anyhow. Those who can do research do research; those who can't try to get into research management, even though they know nothing about it and usually just make the researchers miserable. When the pressure comes on, they start back-stabbing. After that I went freelance as a research consultant, and earned twice as much per year with better holidays. My wife did ask why I had not done that 5 years earlier - not for the money, but for the better work environment.

    Cheers
    Roger



Page 1 of 9 1234 ... LastLast

Tags for this Thread

Posting Permissions

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


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

The Design and Construction of a 'Backlash-Free' Rotary Table
The Design and Construction of a 'Backlash-Free' Rotary Table