584,863 active members*
4,938 visitors online*
Register for free
Login
IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > The Design and Construction of a 'Backlash-Free' Rotary Table
Page 4 of 6 23456
Results 61 to 80 of 106
  1. #61
    Join Date
    Sep 2009
    Posts
    1856

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

    ZrN`s last a long time and you can push them very hard
    http://danielscnc.webs.com/

    being disabled is not a hindrance it gives you attitude
    [SIGPIC][/SIGPIC]

  2. #62
    Join Date
    Apr 2009
    Posts
    5516
    Quote Originally Posted by RCaffin View Post
    Hi Louie

    They weren't all that cheap, but they were definitely 'long' 6 mm ones. I was just testing them out.
    Didn't see any welding at any stage. I use kero+olive oil pulsed misting: it's pretty effective.
    I will chase up the brands you mentioned - thanks.
    The coatings - are they suitable for aluminium alloy? I will have to check.

    Cheers
    Roger
    Yes, they have very low coefficient of friction and uaed for aluminum machining.

    For Onsrud bits, I use their 65 series Super "O" flute especially for smaller endmills (come in 1/16" and I believe 2mm.)

    For SGS their S-Carb bits are excellent.

    For Widia, their Hanita endmills have I believe the most polished edges I've seen in and endmill.

    The ones I use with a coating are SGS, as well as some locally manufactured endmills with ZrN coating in sizes the others don't have.

    Also the closest your worj to the collet nose, the more ridgid the tool is, the better the finish. I cringe when I see a skinny rool sticking out and ridiculously shallow cuts are taken... a recipe for tool failure. Use as much axial DOC so you use as much the flank of the tool as possible, not the bottom which is the most inefficient part. That causes the tip to heat up and wear faster than the flank regardless of lubricant.

    On the flip side, the bits are still great for side milling if can program peel-milling toolpaths...

    Another point is the less the stick out the less the effect of any errors in the collet.

  3. #63
    Join Date
    Jun 2010
    Posts
    4252

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

    Hi Louie

    Interesting, thank you. I did not know of Onsrud or Widia.

    > the most polished edges I've seen in an endmill
    Yes, I did notice somewhere an ad which offered a super-polished verson of a standard cutter. Lower losses were claimed.

    I only have 2 'long' end mills - the rest are all quite short ones. I forget why I even had the long one which chipped. Must have got it for reaching into something.

    The chipped cutter - I can dress the end to have slightly less 'peel' on my T&CG. I will try that. It should be interesting. I recently dressed a very old imperial cutter someone gave me - it was I think it was just about pre-HSS vintage. Once sharpened and polished it cut very nicely, on steel.

    Cheers
    Roger

  4. #64
    Join Date
    Apr 2009
    Posts
    5516

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

    Just a few of the many brands out there.... Niagara, Kennametal, OSG... I use 2 flute primarily for roughing and 3 flute for finishing, though this is not set in stone. I prefer single spiral "O" flute bits for smaller diameters , as I feel they work the best with high-speed spindles. For 3 flute endmills, the Widia Hanita VariMill has uneven flute spacing which is designed to mitigate vibration/resonance. I believe SGS uses variable helix on some of their tools.

  5. #65
    Join Date
    Sep 2009
    Posts
    1856

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

    I have found SGS to be the best to use for small cutters 3 mm coated with ZrN last a long time and when the coating is worn of they are still sharp so good to use on plastic`s or wood
    http://danielscnc.webs.com/

    being disabled is not a hindrance it gives you attitude
    [SIGPIC][/SIGPIC]

  6. #66
    Join Date
    Jun 2010
    Posts
    4252

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

    The Motor Mount

    Where to put the motor? Ideally it should be tucked in compactly, if only to avoid a sprawling mess. Fairly obviously, in a design like this, it will be driving the shaft at the back end, so the motor shaft will have to be level with the back end. Since I was going to start off with a direct connection, it was also obvous that the motor would be sticking out backwards. In this case that presented no problems at all: there was plenty of room 'out back'.

    This is where I had a small comedy of errors. My idea was to allow for two positions of the motor: one for direct coupling, in line with the HD shaft, and one off to the side for a belt coupling. The bit of channel to be used for the motor mount was shown in a previous chapter. I chose channel like this for stiffness, although I could be accused of going overboard. Preliminary drawings showed that the motor would need to be at two different distances from the rear of the HD: one for the direct coupling and one for the belt drive. OK, that means two sets of mounting holes for the channel: one near and one further away. But that part of the design was done before I had the flexible coupling in my hand and knew the dimensions. Mistake.

    Why both versions? Because it was going to be easy to set up the direct drive, which I could use to make pulleys with the right numbers of teeth for the belt drive. It was fairly obvious right from the start that I could not buy the right pulleys from eBay, and the USA vendors wanted an arm and a leg, plus many of them wouldn't ship to Australia.

    Attachment 274418

    OK, the errors. First of all, I did not allow for the full length of the direct coupling. I needed more space between the motor and the HD than I had allowed. So I cheated and quickly bolted a small extension on the back of the side plates. You can see where the old holes are on the side plate and where the new holes are in the extension. So far, so good.

    Attachment 274420

    What is not visible in the previous photo is that the motor is one centimetre too low. That would not matter for the belt drive, but it sure messes up any idea of a direct drive. Another small design error. You can blame my using a 2D drawing package for that: I did not update all the projections consistently every time I made a change. (I still say AutoSketch is faster than anything else though.) So the rear extension was relocated to the top of the side plate, as shown here, and some redesign of the belt and direct couplings was also done.

    Yes, I could have machined up two whole new side plates, but that would have required extra material. Hey - this stuff does cost money, plus a lot of shipping time. Anyhow, I can always redo the side plates later with fewer holes. And I could fill the excess holes with aluminium epoxy too I guess, and maybe paint the lot. All the plates were very solid anyhow.

    Click image for larger version. 

Name:	Rotary12_3.jpg 
Views:	0 
Size:	94.3 KB 
ID:	274422

    Leaping ahead a little, here we have the belt drive version. The motor has moved to the left hole, the belt has been fitted over the pulleys (but is out of sight), and the motor pushed left a bit to tighten the belt. The angle bar and silver cap head masking the first motor hole (the through-bore) do the belt tensioning. With a highly geared design like this, you could tighten the belt too much quite easily. That either damages the bearings or the belt. I tightened slowly while checking the deflection with my finger. (It's a bit like tightening the chain on a chain saw.)

    Yes, the tensioning bar does block the through-bore slightly, but that does not matter. Once the motor mounting bolts have been tightened up, I can remove that bar completely. The motor won't move when the bolts are done up: they are good cap head bolts after all. What you may notice in the photo below is that I am not using nuts on the bolts; instead I am using two steel strips. There's a perfectly good reason for this: getting nuts in there turned out to be really tricky, but holding a strip of steel there from the outside was dead easy. Yes, the bottom ends do protrude.

    Click image for larger version. 

Name:	Rotary12_4.jpg 
Views:	0 
Size:	51.1 KB 
ID:	274424

    Here we have the belt and pulley details - and the third slight oversight. Yes, it all fits together very snugly. Now try removing or refitting the belt. There's not a lot of room down there for fingers, is there? Sensible people point the motor the other way so they can acccess the pulleys easily. Oh well. I moved the motor way over to the right (long slots for bolt holes) and then used a Bondhus allen key with a bent end to hook the belt up and over the motor drive pulley. It would have been easy except for the rims on both pulleys. While not essential, because in practice the belt tracks very well, having rims on both pulleys is a nice safety feature to have. Yes, both pulleys were fabricated for this.

    Now for the third 'oversight'. No, it was not the problem of getting the belt on the pulleys. There are 16 cap head bolts holding the HD to the front plate, aren't there? You can see the heads here, all done up tightly. Well, no, actually there are only 15 in practice. The 16th bolt head interfered with the motor drive pulley! So with some reluctance I removed it - while making sure all the other 15 were done up tightly. There has to be one 'defect' in the system.

    You may also note that the motor-holding bolts stick out 6-8 mm on the left but are invisible on the right. I had to grind the bolts on the right down a bit to clear the HD pulley, so I made them flush. That's not an error, just a design constraint.

    In a future reincarnation I would make the pulley on the HD stick out a bit further, making it easier to get at the belt and clearing the head of the missing bolt. That is unlikely to happen tomorrow though.

    In the next chapter I will discuss the chuck.

  7. #67
    Join Date
    Jun 2010
    Posts
    4252

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

    Chuck and Backplate

    My initial assumption had been that a 100 mm (4") 3-jaw chuck would be just the thing for this project. However, I had not really done much research to see what was available - I mean, everyone sells 100 mm chucks, don't they? And they are inexpensive, aren't they? I thought it would be easy to fit a front-mount chuck onto the HD.

    Well, yes, they do sell 100 mm chucks, but the prices vary. When I went looking it seemed that the front-mounting 100 mm 3-jaw chucks were about three times the price of the conventional rear mount chucks, which was a bit of a shock at the time. To be honest, that did not include a proper search on ebay - which was a mistake.

    Attachment 275148

    Well, OK, why not use a rear-mount chuck? The diagram here shows the problem with great clarity: the M8 mounting bolts for the 100 mm chuck sort of clash with everything else around the backplate, which bolts to the green parts. That is just not going to work! In hindsight, I should have probably shrugged my shoulders and bought a front mount 100 mm chuck despite the extra cost, but the sticker shock was still a bit high in my mind at that stage.

    Attachment 275150

    If the 100 mm chuck had the M8 bolts clashing, would a larger chuck have them clear? So I looked at a 125 mm chuck, shown diagrammatically here. Ah yes - the M8 mounting bolts will fit just nicely. The price for a Chinese unit from an Australian supplier was a very nice $96, and the chuck did come with engineering-type specifications and an inspection certificate. It almost seems that once you go above 100 mm (4"), you enter a more 'professional' market. So that's what I bought. In the event, while it turned out to be just a 'basic' chuck, I was not unhappy with the quality. It would do.

    However, did anyone explain that 125 mm chuck weighs almost double what a 100 mm chuck weighs? That did create some hesitation when I first picked it up. Oops! Oh deary, deary me! Perhaps ... maybe, one day.

    Also creating further hesitation was the vexed question of just how does one do up the M8 cap heads in that position? I hadn't thought about that when making my decision, had I? I could reach in with some surgical forceps to grab the head of the bolt and turn it a little way, and 10 minutes work saw the bolts all 'done up', but they were not tight at all. But in that space conventional bent allen keys would not fit. 'Some assembly required.'

    I have commented elsewhere on the way the Chinese make 'hex keys' out of little more than mild steel. They really are a bit pathetic (and equally so are Chinese cap head bolts). I had accumulated a stack of unbranded hex keys (mostly Chinese I imagine) over the years, and I got an idea. I grabbed one from the drawer, sawed off a short bit of the hex shape with a conventional hacksaw blade (it was not hard), drilled a suitably-sized hole in some steel strip, and force-fitted the short bit of hex into the hole. This gave me a custom hex key which could get into the space available - just, and I could tighten the three M8 bolts.

    Um - just checking: it turns out that was not a Chinese hex key I casually chopped up. It was branded C&K - a USA brand. All I can say is that it sawed up like a Chinese one. I could NOT do that with either a Bondhus or Unbrako brand key. Oh well. I have noticed that some brands stay with their quality, while other brands tend to slide down the quality scale in the pursuit of cheaper-cheaper-cheaper. I am not impressed by the latter.

    Attachment 275152

    Well, with the chuck mounted, the first thing you do is check the wobble (TIR) when the chuck spins. A bit of ground 15 mm HSS-Co rod was placed in the jaws and a dial indicator was applied. The TIR was not wonderful: maybe 100 microns peak to peak a little out from the tip of the jaws. That was noticeably worse than the specs on the chuck. Hum: not good enough. Fortunately I had remembered a suggestion from somewhere: when machining the backplate, don't make the boss which fits into the back of the chuck to the exact size, just in case ... Well, I hadn't.

    OK, slacken off the three M8 bolts a little, and gently tap the chuck sideways in the required direction. Gently means gently, and with a bit of wood, not a steel hammer. You don't want to ding the very expensive Crossed Roller Bearings bearings inside the HD after all. Then tighten up the bolts a bit and check.

    Two tries got the TIR down to better than the supplied specs for the chuck - down to about 30 microns. Not perfect, but I had my doubts about trying to go any further with this chuck. 30 microns with a 15 mm rod, but how much with a 100 mm rod? (I haven't checked.) After all, this is not a precision Swiss chuck: just be grateful.

    Well, having got it all together, I put it on my mill bed - see above photo. Um. It is big, isn't it? The extra size of the 125 mm chuck had eaten into the limited Y axis space on the mill table. It may be one day that I will change the chuck over to a shallow front-mount 100 mm unit, just to get the extra Y axis space. Or maybe I will redesign the housing to put the front plate further back again. But not yet: I haven't even used it seriously!

    In the next and final chapter I will detail some of the measurements I did once the unit was assembled.

  8. #68
    Join Date
    Jun 2010
    Posts
    4252

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

    Performance Measurement, again

    Before I mounted the chuck on the RT backplate I had opened up the chuck back cover to look inside. I was reasonable happy with what I saw. Yes, the basic guts are either very simple or slightly crude, depending on your point of view, but the parts looked solid, the interior seemed clean of any swarf and there was plenty of grease in the right places. So that seemed OK - just heavy.

    OK, so I mounted the chuck up on the RT, and try chucking a few bits of ground rod to see how much they wobbled during rotation. The first few did wobble - and rather variable amounts too (this is before the measurements reported earlier). That was a bit odd I thought. So I ran the jaws in all the way and out all the way several times, against as much load as I could manage with my hands, and then tried again. Aha - much better: less TIR and more consistent. From which I infer that the teeth on the jaws or the faces on the scroll (or both, more likely) just needed a little bit of bedding in, or maybe they were not totally free of very fine machining dust? It is cast iron after all. Only then did I do the TIR testing reported in the last chapter.

    Now that I had the outputs mechanics done it was time to check the input motor parts. I carefully cleaned the teeth on the two pulleys and the face of the toothed belt and assembled the drive. Then I carefully tighted the belt using the screw mentioned earlier. A loose belt is an invitation to all sorts of problems, but so is a belt under too much tension. I aimed for just a few millimetres of deflection under 'medium finger load'. Yeah, real technical! But the teeth on the bought GT2 belt fitted the teeth on the DIY pulleys very nicely - once the belt was tensioned. If you don't tension the belt it does not seem to fit 'into' the teeth too well.

    A few quick spin commands from the Mach keyboard, and it was clear that <i>it runs</i>! Good. (Sigh of relief.) So now to test it a bit more thoroughly.

    Attachment 275464

    I reused the optical sensor from before for further testing. First I attached a thin bent shim to the backplate with a small self tapper - tightly. That made the blocking vane. (If you must know, it was the sliding cover off a dead 3.5" floppy disk. Nice hard alloy.) Then I mounted the sensor block so the vane went through it without touching. Yes, the arrangement in the photo does not look 'industrial-strength', but it is not meant to be a permanent fixture. It is there just for these tests. By monitoring the output of the sensor I could measure tiny movements of the RT - and I do mean tiny. So now several trials could be run.

    Incidentally, I will mention here that the data logging had to be done by hand. Neither Mach nor a Smooth Stepper have the facility of reading in analog voltages - although I see that the UC-300 interface and UCCNC SW do. Interesting, and noted. I thought of using a USB data logger in parallel with Mach, but running two 'real-time' systems together on one Windows-based machine ... I thought maybe 'not today'. Actually, if you don't run the data logging very fast and you have a reasonable look-ahead buffer in mach3, it should work. Most of the traffic will be by DMA after all.

    The first trial was to calibrate the sensor again - for this arrangement. The previous calibration was no longer valid as I had changed the radius to the sensor beam. This let me convert the mV read by the multimeter into degrees of rotation.

    Previously I had ran a number of loops back and forth, which gave the hysteresis curve shown before. I had also measured the slope of the response curve for both sides of the hysteresis loop. They were close enough together, given that they were done with the direct drive at 283.333 steps per degree. But those results were fairly coarse. Now I could do better, since the toothed belt drive gave me 1,000 steps/degree. Just to remind you, that means a single step gives 0.06 arc-minutes of rotation, or 3.6 arc-seconds. The aperture of the sensor used is 0.1 mm or 100 microns, but you can't use anywhere near that range of movement.

    Attachment 275466

    Here we have a hysteresis loop with readings taken every 0.001 degrees - every step pulse. You can see a slight rounding over at the tips of the loop - that may be the sensor non-linearity. But the gap between the two parts of the curve - that's the HD hysteresis. Overall the sensor sensitivity is 0.000347 degrees/mV (or 0.347 degrees/V). The hysteresis is about 0.008 degrees.

    You can also see a few bumps. Some of those are due to rounding-off the DVM readings, while there may also be some slow time-constants involved in the sensor: it was working at a faily high impedance, and I was maybe a bit impatient sometimes.

    Attachment 275468

    This is a strange-looking graph, but it does make sense once explained (I hope). It shows what happens when the RT starts at 0 degrees (set to mid-range on the sensor, at about 0.033 degrees), rotates to +45 degrees and then back to 0 degrees, then to -45 degrees and back to zero, a number of times. The zero positions are shown; the 45 degree positions are way off-scale of course. Basically, it shows that the RT always returns to the same zero position, after allowing for hysteresis. Going in one direction you get the lower 'zero point' at 0.033 degrees, while coming back in the other direction gives you the other 'zero point' at 0.045 degrees. So the hysteresis is about 0.012 degrees for a 45 degree swing. But the reproducibility is far, far better.

    There are a number of cycles plotted here, and they mostly overlay nicely, but you can see what looks like a slight 'widening' of the line at the lower right. That widening is actually a number of plotted lines just faintly offset from each other in the vertical direction. I believe that is really a slight thermal drift in the sensor, as it is below the resolution of the stepper motor.

    I have seen a video of a large and very expensive RT doing just the same, against a dial indicator. When the RT came back to 'zero', the DI went back to zero too. I was impresssed at the time, but it seems an MYOG effort with a Harmonic Drive and a zero-backlash toothed belt can match some of the performance of very expensive commercial units.

    Click image for larger version. 

Name:	Rotary14_4.jpg 
Views:	0 
Size:	25.7 KB 
ID:	275470

    Another interesting graph which needs explaining. This time the RT is cycling under significant load (or torque). This is meant to give some guide as to what the RT might do when machining forces are placed on it: will it creep? The torque was created by a lump of steel hanging off an arm, giving a static torque of about 0.46 Newton.metres (N.m). What that means, in more practical engineering terms, is a 3 kg lump of steel hanging off at 150 mm from the axis of rotation. This is a fair lump of steel at a fair distance, and is significantly more torque than a 6 mm cutter would give on a 100 mm diameter bit of aluminium (I think).

    The RT was oscillated back and forth a few steps. A small range was used to stay within the scope of the sensor. The left hand bunch of points show the RT being driven from 0.06 degrees to 0.09 degrees and back: the repetition of end points is very good. For the middle third of the data the positions were moved up by 0.02 degrees. The repetition between points stays good. Then for the final one third the positions were brought back down to what they were at the start. There is very little difference in sensor reading between the start and the end, and what there is is probably thermal drift in the sensor anyhow. The RT is holding its position under load: it is not creeping.

    Zero Backlash

    Can I measure any backlash this way? The smooth continuous nature of the hysteresis curves under no-load conditions suggest there is no classical backlash: there are no flat spots in the response curves. That means the hysteresis which can be seen may be due to slight movements in the harmonic drive spline engagement, or maybe flex in the spline. It's hard to say, and you would need a solid read of the detailed technical explanation of how the HD really works. We are talking about some very fine elastic distortions here. However, I doubt I will see much effect in practice. After all, 0.001 degrees represents 0.87 microns at the periphery of a 100 mm diameter object. Did I go overboard with this? Not a bit: it's nice to have the resolution.

    Do I need a brake on the unit? At this stage, with these results, I do not see the need to add a brake. It would have to be quite a powerful brake, and you can't use a brake on a moving rotary table anyhow. You can use a brake on an indexer, but that was not what I want.

    Well, there it is. Questions are welcome.

    Roger

  9. #69
    Join Date
    Apr 2009
    Posts
    5516

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

    Quote Originally Posted by RCaffin View Post
    Performance Measurement, again

    Before I mounted the chuck on the RT backplate I had opened up the chuck back cover to look inside. I was reasonable happy with what I saw. Yes, the basic guts are either very simple or slightly crude, depending on your point of view, but the parts looked solid, the interior seemed clean of any swarf and there was plenty of grease in the right places. So that seemed OK - just heavy.

    OK, so I mounted the chuck up on the RT, and try chucking a few bits of ground rod to see how much they wobbled during rotation. The first few did wobble - and rather variable amounts too (this is before the measurements reported earlier). That was a bit odd I thought. So I ran the jaws in all the way and out all the way several times, against as much load as I could manage with my hands, and then tried again. Aha - much better: less TIR and more consistent. From which I infer that the teeth on the jaws or the faces on the scroll (or both, more likely) just needed a little bit of bedding in, or maybe they were not totally free of very fine machining dust? It is cast iron after all. Only then did I do the TIR testing reported in the last chapter.

    Now that I had the outputs mechanics done it was time to check the input motor parts. I carefully cleaned the teeth on the two pulleys and the face of the toothed belt and assembled the drive. Then I carefully tighted the belt using the screw mentioned earlier. A loose belt is an invitation to all sorts of problems, but so is a belt under too much tension. I aimed for just a few millimetres of deflection under 'medium finger load'. Yeah, real technical! But the teeth on the bought GT2 belt fitted the teeth on the DIY pulleys very nicely - once the belt was tensioned. If you don't tension the belt it does not seem to fit 'into' the teeth too well.

    A few quick spin commands from the Mach keyboard, and it was clear that <i>it runs</i>! Good. (Sigh of relief.) So now to test it a bit more thoroughly.

    Attachment 275464

    I reused the optical sensor from before for further testing. First I attached a thin bent shim to the backplate with a small self tapper - tightly. That made the blocking vane. (If you must know, it was the sliding cover off a dead 3.5" floppy disk. Nice hard alloy.) Then I mounted the sensor block so the vane went through it without touching. Yes, the arrangement in the photo does not look 'industrial-strength', but it is not meant to be a permanent fixture. It is there just for these tests. By monitoring the output of the sensor I could measure tiny movements of the RT - and I do mean tiny. So now several trials could be run.

    Incidentally, I will mention here that the data logging had to be done by hand. Neither Mach nor a Smooth Stepper have the facility of reading in analog voltages - although I see that the UC-300 interface and UCCNC SW do. Interesting, and noted. I thought of using a USB data logger in parallel with Mach, but running two 'real-time' systems together on one Windows-based machine ... I thought maybe 'not today'. Actually, if you don't run the data logging very fast and you have a reasonable look-ahead buffer in mach3, it should work. Most of the traffic will be by DMA after all.

    The first trial was to calibrate the sensor again - for this arrangement. The previous calibration was no longer valid as I had changed the radius to the sensor beam. This let me convert the mV read by the multimeter into degrees of rotation.

    Previously I had ran a number of loops back and forth, which gave the hysteresis curve shown before. I had also measured the slope of the response curve for both sides of the hysteresis loop. They were close enough together, given that they were done with the direct drive at 283.333 steps per degree. But those results were fairly coarse. Now I could do better, since the toothed belt drive gave me 1,000 steps/degree. Just to remind you, that means a single step gives 0.06 arc-minutes of rotation, or 3.6 arc-seconds. The aperture of the sensor used is 0.1 mm or 100 microns, but you can't use anywhere near that range of movement.

    Attachment 275466

    Here we have a hysteresis loop with readings taken every 0.001 degrees - every step pulse. You can see a slight rounding over at the tips of the loop - that may be the sensor non-linearity. But the gap between the two parts of the curve - that's the HD hysteresis. Overall the sensor sensitivity is 0.000347 degrees/mV (or 0.347 degrees/V). The hysteresis is about 0.008 degrees.

    You can also see a few bumps. Some of those are due to rounding-off the DVM readings, while there may also be some slow time-constants involved in the sensor: it was working at a faily high impedance, and I was maybe a bit impatient sometimes.

    Attachment 275468

    This is a strange-looking graph, but it does make sense once explained (I hope). It shows what happens when the RT starts at 0 degrees (set to mid-range on the sensor, at about 0.033 degrees), rotates to +45 degrees and then back to 0 degrees, then to -45 degrees and back to zero, a number of times. The zero positions are shown; the 45 degree positions are way off-scale of course. Basically, it shows that the RT always returns to the same zero position, after allowing for hysteresis. Going in one direction you get the lower 'zero point' at 0.033 degrees, while coming back in the other direction gives you the other 'zero point' at 0.045 degrees. So the hysteresis is about 0.012 degrees for a 45 degree swing. But the reproducibility is far, far better.

    There are a number of cycles plotted here, and they mostly overlay nicely, but you can see what looks like a slight 'widening' of the line at the lower right. That widening is actually a number of plotted lines just faintly offset from each other in the vertical direction. I believe that is really a slight thermal drift in the sensor, as it is below the resolution of the stepper motor.

    I have seen a video of a large and very expensive RT doing just the same, against a dial indicator. When the RT came back to 'zero', the DI went back to zero too. I was impresssed at the time, but it seems an MYOG effort with a Harmonic Drive and a zero-backlash toothed belt can match some of the performance of very expensive commercial units.

    Click image for larger version. 

Name:	Rotary14_4.jpg 
Views:	0 
Size:	25.7 KB 
ID:	275470

    Another interesting graph which needs explaining. This time the RT is cycling under significant load (or torque). This is meant to give some guide as to what the RT might do when machining forces are placed on it: will it creep? The torque was created by a lump of steel hanging off an arm, giving a static torque of about 0.46 Newton.metres (N.m). What that means, in more practical engineering terms, is a 3 kg lump of steel hanging off at 150 mm from the axis of rotation. This is a fair lump of steel at a fair distance, and is significantly more torque than a 6 mm cutter would give on a 100 mm diameter bit of aluminium (I think).

    The RT was oscillated back and forth a few steps. A small range was used to stay within the scope of the sensor. The left hand bunch of points show the RT being driven from 0.06 degrees to 0.09 degrees and back: the repetition of end points is very good. For the middle third of the data the positions were moved up by 0.02 degrees. The repetition between points stays good. Then for the final one third the positions were brought back down to what they were at the start. There is very little difference in sensor reading between the start and the end, and what there is is probably thermal drift in the sensor anyhow. The RT is holding its position under load: it is not creeping.

    Zero Backlash

    Can I measure any backlash this way? The smooth continuous nature of the hysteresis curves under no-load conditions suggest there is no classical backlash: there are no flat spots in the response curves. That means the hysteresis which can be seen may be due to slight movements in the harmonic drive spline engagement, or maybe flex in the spline. It's hard to say, and you would need a solid read of the detailed technical explanation of how the HD really works. We are talking about some very fine elastic distortions here. However, I doubt I will see much effect in practice. After all, 0.001 degrees represents 0.87 microns at the periphery of a 100 mm diameter object. Did I go overboard with this? Not a bit: it's nice to have the resolution.

    Do I need a brake on the unit? At this stage, with these results, I do not see the need to add a brake. It would have to be quite a powerful brake, and you can't use a brake on a moving rotary table anyhow. You can use a brake on an indexer, but that was not what I want.

    Well, there it is. Questions are welcome.

    Roger

    Interesting.... I wonder if there's some correlation with the hysteresis graph and maybe the fact that the unit is somewhat used? With probably no record of its previous life of the loads put upon it, it could factor for some error - enough maybe for the maintenance engineers to deem the part "lifed out." Still the results re impressive even for a used unit that won't see nearly any of the loads it probably once did.

    Or I wonder how much is affected by the belt?

    The question: Let's see the parts you made with the rotary!

  10. #70
    Join Date
    Jun 2010
    Posts
    4252

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

    Hi Louie

    My understanding from the vendor is that the drives were stripped out of obsolete or superceded equipment - like robotic handling systems which had been replaced. I forget the exact details, but I don't think the drives were the reason for that.

    The design of a harmonic drive relies on the torsional strength of a thin shell of spring steel and the mating of teeth between the shell and the housing. There will always be some compliance in the shell: steel is elastic, and the specs give a figure for what is expected. The measured performance or compliance matches the specs, so I don't think the unit is worn out. The hysteresis is also due in several complex ways to the elasticity in the shell and compliance between the inner drive bearing and the outer housing, but again I think it is well within specs.

    There's a 51:1 reduction between the input and the output, so any belt effect is going to be much reduced. The GT2 belts are known to have very little backlash, due to their design. There was little difference in measured performance between the direct drive and the toothed belt drive anyhow. I doubt I could effectively measure it and trust the results.

    I have made some toothed belt pulleys with the RT, and I have to say it was fascinating to watch it 'doing its thing'. I was using all axes for the pulleys: X, Y Z and A, in order to get a close match to the GT2 profile. That worked beautifully.

    But since then I regret to have to say I have been busy making parts without the RT. The joke is that I was intending to use the RT to make these parts, also making up a tailstock for it so I could 'bar feed', but I found that making them one at a time in the RT was a lot slower than making them 10 at a time using custom machining jigs (like pallets etc). An ATC would speed up using an RT, but I am using ER25 collets for tool holding. I have yet to see an ATC for ER collets.

    Must keep an eye open for something to make on the RT later. Any suggestions (with programs)?

    Cheers
    Roger

  11. #71
    Join Date
    Apr 2009
    Posts
    5516
    Quote Originally Posted by RCaffin View Post
    Hi Louie

    My understanding from the vendor is that the drives were stripped out of obsolete or superceded equipment - like robotic handling systems which had been replaced. I forget the exact details, but I don't think the drives were the reason for that.

    The design of a harmonic drive relies on the torsional strength of a thin shell of spring steel and the mating of teeth between the shell and the housing. There will always be some compliance in the shell: steel is elastic, and the specs give a figure for what is expected. The measured performance or compliance matches the specs, so I don't think the unit is worn out. The hysteresis is also due in several complex ways to the elasticity in the shell and compliance between the inner drive bearing and the outer housing, but again I think it is well within specs.

    There's a 51:1 reduction between the input and the output, so any belt effect is going to be much reduced. The GT2 belts are known to have very little backlash, due to their design. There was little difference in measured performance between the direct drive and the toothed belt drive anyhow. I doubt I could effectively measure it and trust the results.

    I have made some toothed belt pulleys with the RT, and I have to say it was fascinating to watch it 'doing its thing'. I was using all axes for the pulleys: X, Y Z and A, in order to get a close match to the GT2 profile. That worked beautifully.

    But since then I regret to have to say I have been busy making parts without the RT. The joke is that I was intending to use the RT to make these parts, also making up a tailstock for it so I could 'bar feed', but I found that making them one at a time in the RT was a lot slower than making them 10 at a time using custom machining jigs (like pallets etc). An ATC would speed up using an RT, but I am using ER25 collets for tool holding. I have yet to see an ATC for ER collets.

    Must keep an eye open for something to make on the RT later. Any suggestions (with programs)?

    Cheers
    Roger
    You can download DeskProto for free for 30 days... Though it only does X, Z, A simultaneously or A indexed.

    What to make? An impeller would be cool. Some videos cutting would be real cool

  12. #72
    Join Date
    Sep 2009
    Posts
    1856

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

    Fusion 360
    http://danielscnc.webs.com/

    being disabled is not a hindrance it gives you attitude
    [SIGPIC][/SIGPIC]

  13. #73
    Join Date
    Jun 2010
    Posts
    4252

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

    Hi Louie

    > Though it only does X, Z, A simultaneously or A indexed.
    I could not find that on their web site - but would they say so?
    Does this mean that the cutter is ALWAYS over the centre line of the A axis? No Y axis movement? just checking.

    Cheers
    Roger

  14. #74
    Join Date
    Jun 2010
    Posts
    4252

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

    Hi Daniel

    Can I run Fusion 360 totally stand-alone without any internet connection? Need to do that for me. No internet in the workshop.

    Cheers
    Roger

  15. #75
    Join Date
    Apr 2009
    Posts
    5516
    Quote Originally Posted by RCaffin View Post
    Hi Louie

    > Though it only does X, Z, A simultaneously or A indexed.
    I could not find that on their web site - but would they say so?
    Does this mean that the cutter is ALWAYS over the centre line of the A axis? No Y axis movement? just checking.

    Cheers
    Roger
    I had downloaded the manual. I believe this is also mentioned in the subforum here as well. But, the software is a lot cheaper than most for fourth axis work.

  16. #76
    Join Date
    Jun 2010
    Posts
    4252

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

    > But, the software is a lot cheaper than most for fourth axis work.
    Yeah, but sometimes that's a warning. Will it do what I want?

    I have seen some very nice smoothing done by using the side of ball cutters off centre - using X+Y+Z for contouring. Or maybe it was using round diamond-coated burrs, with a very fine grit. Just carving out the GT2 pulleys needed the Y axis as I was using the side of the ball-nose cutter.

    Fun stuff.

    Cheers
    Roger

  17. #77
    Join Date
    Apr 2009
    Posts
    5516
    Quote Originally Posted by RCaffin View Post
    > But, the software is a lot cheaper than most for fourth axis work.
    Yeah, but sometimes that's a warning. Will it do what I want?

    I have seen some very nice smoothing done by using the side of ball cutters off centre - using X+Y+Z for contouring. Or maybe it was using round diamond-coated burrs, with a very fine grit. Just carving out the GT2 pulleys needed the Y axis as I was using the side of the ball-nose cutter.

    Fun stuff.

    Cheers
    Roger
    Well that's the difference between a few hundred and a thousand or thousands of dollars... What you refer to as swarf cutting. Wouldn't be too difficult to hand code even a GT2 pulley... But an impeller would. I don't think XZA woul be enough to reach all surfaces.

  18. #78
    Join Date
    Jun 2010
    Posts
    4252

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

    I know, I know.

    That said, if I can get the equation for the impellor surface, I should be able to program it parametrically.
    Might take a while tho ...

    Cheers
    Roger

  19. #79
    Join Date
    Sep 2009
    Posts
    1856

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

    Quote Originally Posted by RCaffin View Post
    Hi Daniel

    Can I run Fusion 360 totally stand-alone without any internet connection? Need to do that for me. No internet in the workshop.

    Cheers
    Roger
    yes and no, yes you can run it with no net but you need to put it on line every 20 or 30 days not sure what one it is, it has 3 + 2 capability, you can do 3 + 1 but at the moment that's indexing only, yes it can do what you wont as you can do the GT2 pulleys with a form tool and index each cut or do it as a 3 + 2 cut. I will ask on the fusion form so I know what is available
    http://danielscnc.webs.com/

    being disabled is not a hindrance it gives you attitude
    [SIGPIC][/SIGPIC]

  20. #80
    Join Date
    Jun 2010
    Posts
    4252

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

    but you need to put it on line every 20 or 30 days
    NOT possible. No network link out there.

    Cheers
    Roger

Page 4 of 6 23456

Similar Threads

  1. Backlash free rotary table
    By Zoidberg in forum Linear and Rotary Motion
    Replies: 1026
    Last Post: 03-16-2022, 09:48 PM
  2. Design vs construction body
    By tcs118 in forum Community Club House
    Replies: 0
    Last Post: 10-31-2013, 08:42 PM
  3. Phase II 4" rotary table backlash.
    By TXFred in forum Benchtop Machines
    Replies: 14
    Last Post: 02-12-2011, 01:27 AM
  4. Rotary Table Backlash
    By fc911c in forum Uncategorised MetalWorking Machines
    Replies: 4
    Last Post: 04-18-2010, 10:11 AM
  5. Typical Backlash on a new Yuasa 8" Rotary Table
    By Roy Norris in forum MetalWork Discussion
    Replies: 5
    Last Post: 08-10-2005, 10:33 AM

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
  •