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  1. #61
    Member dharmic's Avatar
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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    it skips like a kangaroo and lacks sufficient torque to pull the skin off custard
    I had to laugh. Good one.
    At least one person got it, not sure how well it translates for those living in the real world (ie not in oz)



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by dharmic View Post
    Milling calculators (HSMAdvisor, FSWizard, G-Wizard etc) often tell you the expected cutting force based on work material, tool material, spindle speed, feed rate and stepover. They need this number to calculate tool deflection anyway.

    FSWizard is telling me (for example) that a 1/4" two flute HSS cutter running flat out (8881 RPM) in 6061-T6 with a stickout of 16mm, DOC 11mm, WOC 2.78mm, feed rate of 1200mm/min will use 500W, 0.54Nm and cutting force of 17.5 kg (172N).

    From there, it's just maths and basic principles - torque is force times distance from axis, reduction in speed almost equals increase in torque, etc.

    Following the example, worst case, that 172N applied across transverse to the axis at full radius of 3" (say 75mm) from a diameter of 6" is going to generate 172*0.075 = 12.9Nm of torque. If it's along the axis it'll be zero. But for that example I'd want the 4th to be able to deal with at least 25Nm (or in stepper terms, 3500 oz-in). This is achievable with a 5:1 reduction pulley and 700 oz-in NEMA 34 stepper, for example.

    Then, as Roger mentions, resolution becomes an issue. Again working on the max radius, if you had 6" (150mm) diameter pieces that's pi*150 = 470mm circumference. If you want a resolution of a thou at that size workpiece ie 0.025mm, you need at least 470/0.025 = 18,800 steps per revolution. Which in this example is going to be a bigger ask than the torque requirement.

    Whether you're going encoders or steppers, you need the resolution. And nigh on 20k divisions or steps per rev is going to need some reduction. Can you get away with a belt? Do you need something like a harmonic drive?

    You can say "Ok, I don't want to go nuts, give me a 1.8º step and run microstepping at 4" ie 800 steps per rev, means you need a reduction of 18800/800 = 23.5:1 or call it 25:1 which is in the realm of a 30:1 HD or a two stage 5:1 pulley and belt rig. At that kind of ratio, your 3500oz-in torque requirement becomes 140 oz-in and any old NEMA 23 stepper can run it.

    Or, if you want to stay at a single 5:1 belt rather than two stages, you'd need to run 18800/5 = 3760 steps per rev which is a squeak over 16 microsteps on a 1.8º stepper or 8 on a 0.9º stepper - doable if it and its driver can manage 700 oz-in per step at 8 microsteps.

    Don't forget that stepper torque seriously drops off as
    - the speed goes up: what might be fine for holding indexed work might be rubbish for continuous fast work, and this gets worse the higher your gear/belt reduction ratios are; and
    - the microstep count goes up.

    So you'll need to do some iterations working out feedrates - in the example FSWizard gives us a linear feed of 1201.4mm/min which, at 150mm diameter, is 2.5rpm - or 62.5rpm on the stepper if you're running 25:1 reduction.

    Get the idea? Calcs are a PITA but you only need to do them twice. Once, now, when you're planning your build and purchases. Then once again, when you put it all together and it skips like a kangaroo and lacks sufficient torque to pull the skin off custard and you need to work out what you got wrong the first time so you can do it again

    Damn, you went and made me do maths over my first coffee on a Saturday morning. So the numbers are probably wrong.

    Thanks. That actually helps. I usually get overly vague and confusing answers to questions like this but your worked example is something I can follow. I appreciate the Saturday morning math!

    If your numbers are right (and let's assume that they are), it doesn't seem like I would need a particularly big motor or a very high gear ratio for my needs. If backlash issues increase with gearing ratios then it seems like it's better to stick to a 20:1 (or maybe smaller). A lot of even small gearboxes can handle the sort of forces you mentioned.


    How much of an issue is resolution for a 4th axis indexing set-up? I can understand that "resolution" in this context means the motor (and therefore the part) can stop in a greater number of positions. But... are we likely to be able to take advantage of being able to index to micron level accuracy with a 1/4" or 1/8" end mill? Also, I assume that any resolution limitations from the other axis still apply.

    It looks like, even with a fairly average stepper and relatively low gearing, you would still be able to index to fractions of a degree. What sort of part design would require much more than that?



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    Default Re: A and C axis for a 5 axis milling machine

    What sort of part design would require much more than that?
    GEARS!
    And anything which has to mate with another part.

    Cheers
    Roger



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    What sort of part design would require much more than that?
    GEARS!
    And anything which has to mate with another part.

    Cheers
    Roger
    You are saying that making gears requires a higher resolution than you could achieve with an average stepper with a 20:1 gearbox? That sounds wrong unless we are talking about some tiny gears.

    Also, same point for the "anything that mates with another part".



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    Default Re: A and C axis for a 5 axis milling machine

    Is anyone here currently using a servo based 4th axis with an encoder? If so, does the encoder make backlash irrelavent when the 4th axis is used for indexing (because it provides positioning feedback to make sure it stops in the right place)?

    Is it necessary for a 4th rotary axis to change direction? While it might not be the most efficient use of power, can't it rotate to the correct position without changing direction (which is where backlash becomes an issue).



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    Default Re: A and C axis for a 5 axis milling machine

    a servo based 4th axis with an encoder?
    Not me: stepper-based.

    does the encoder make backlash irrelavent when the 4th axis is used for indexing
    IF you can ensure that critical rotation is always in the same direction AND you have a lot of drag AND you have a brake which does not perturb the chuck position.

    Is it necessary for a 4th rotary axis to change direction?
    Well, an RT which cannot reverse direction and work in full 4-axis mode is not much use as a 4th axis. So I would say 'absolutely yes'.
    Do I use my RT in both directions? All the time.


    Cheers
    Roger



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by Goemon View Post
    How much of an issue is resolution for a 4th axis indexing set-up? I can understand that "resolution" in this context means the motor (and therefore the part) can stop in a greater number of positions. But... are we likely to be able to take advantage of being able to index to micron level accuracy with a 1/4" or 1/8" end mill? Also, I assume that any resolution limitations from the other axis still apply.

    It looks like, even with a fairly average stepper and relatively low gearing, you would still be able to index to fractions of a degree. What sort of part design would require much more than that?
    Well, in the example we looked at resolution. It's not so much the angular resolution that matters as the linear resolution it maps to at the circumference. If the rest of your machine is good to a thou (damned imperials, but useful sometimes) then it makes sense to have your rotary good to the same level. I don't believe there's much point having more resolution than the rest of the machine because, if nothing else, it would mean finer steps around the part than along it which seems wrong.

    On a 3 or 6mm cutter remember that (so long as the cutter's fresh) once you have it dialled in it's accurate to the total runout of the tool mounted in the spindle. The diameter doesn't affect the accuracy, only the runout and tram. You could have a 4" cutter making finishing passes down to micron level if the runout is good enough.



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    Default Re: A and C axis for a 5 axis milling machine

    If the encoder were on the output shaft of the rotary table, then yes, backlash would be recognized. But if the encoder's on the motor, as is more common, then backlash that happened downstream in the gear train wouldn't register.

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


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    Default Re: A and C axis for a 5 axis milling machine

    If the encoder were on the output shaft of the rotary table,
    but angular resolution would be lousy. It's a hard world.

    Cheers
    Roger



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    If the encoder were on the output shaft of the rotary table,
    but angular resolution would be lousy. It's a hard world.

    Cheers
    Roger
    There are high-resolution encoders. Not sure what actual angular accuracy is on the 16-24 bit encoders on some of the higher end servos, but the resolution is certainly there. Even a 16-bit encoder would get you 1 thou resolution at a 20" diameter.



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by skrubol View Post
    There are high-resolution encoders. Not sure what actual angular accuracy is on the 16-24 bit encoders on some of the higher end servos, but the resolution is certainly there. Even a 16-bit encoder would get you 1 thou resolution at a 20" diameter.
    Just depends on what you are willing to pay, Renishaw Resolute encoders will get you to +- 1 Arc second accuracy (and 32 bit resolution)...



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    Default Re: A and C axis for a 5 axis milling machine

    Oh yeah, there are so-called 24-bit encoders on the market. They use microprocessors and some rather sophisticated algorithms to do lots of interpolation on the basic analog signals coming from the optical disk or mag strip. And you can program them to do all sorts of things. Just don't expect many of them to provide simple A&B signals back to the motor driver, because most of them communicate by serial data string.

    As for accuracy or linearity - ho hum, rarely above 12 bits. They rely on an optical disk or mag strip, and these have limited accuracy. The signals coming from the analog sections are not all that linear. Oh, they can be quite good, but they are not that good. Whether all the sales reps even know what they are talking about ... questionable.

    Then there is pricing. Going from a simple reliable 500 line encoder to a 24 bit unit, expect to pay at least 10x as much, or more, even 100x .
    Great marketing spin in a highly competitive world.

    If you really want sub-micron accuracy, you will need to look at linear motors, piezo steppers ("inchworms"), and laser interference gauging. And temperature control to better than 1 C.

    Cheers



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    Default Re: A and C axis for a 5 axis milling machine

    Sub micron level accuracy is only going to be required for very specialist applications. Maybe if you were making molds for nano-imprint lithography and were cutting with a laser instead of an end mill but, I doubt anyone is going to be attempting this with a diy machine.

    Almost nobody would be able to measure such accuracy at home and certainly would not be able to see it. I use 30 and 50 micron glass micro-lenses in one of my businesses and they are so small that you can't see them individually. They just look like a bag of white or gray powder.

    I'm sure we could all agree that, as a general point, more accuracy is always better than less when all else is equal but, for people working with diy builds and a modest budget, there has to be an acceptable margin of error that can be measured with your average affordable digital calipers.

    You don't see many threads here with people complaining that they can't make what they want due to the resolution limitations imposed by stepper motors. People seem to run into torque / holding torque limitations with cheap belt drive 4th axis first. With micro-stepping and gear reductions we are already down to tiny fractions of a degree.

    It would be interesting to see what sort of accuracy tolorances are offered on 4th axis kits for expensive VMCs as some of them offer very large rotary tables.



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by PCW_MESA View Post
    Just depends on what you are willing to pay, Renishaw Resolute encoders will get you to +- 1 Arc second accuracy (and 32 bit resolution)...
    Call me crazy but.... I believe that before choosing the technology, a person should try to have some understanding of what they actually need. It seems like one of those topics where forum envy could have you driving yourself mad looking for a level of precision that is well beyond what is needed while blowing the budget.

    In addition to forum advice, I like an additional sense-check such as looking how it's done on commercial machines that are already making similar parts. In my case, I'll be making rifle stocks, rifle stock molds and rifle chassis systems. These Laguna guys have YouTube video of their machines making rifle stocks using a 4 axis process. Their 4th axis appears to use one of those fairly average Chinese wormgear boxes:

    https://m.youtube.com/watch?t=7s&v=P5_nT1hsQf8

    So... I am going to assume that a little backlash can be managed and that I don't require a hyper fine resolution capability or a super precision set-up. Good job too as it's slim pickings when the requirement involves both precision and affordability.

    I sent a note to those Skyfire guys to ask how much their 5" and 6" harmonic drive 4th axis costs just in case. I don't expect to receive an answer though. They rarely (or never) answer any emails. I don't even know why they bother with a website with pics of CNC products. They operate as if they died.



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    Default Re: A and C axis for a 5 axis milling machine

    Hi, just read all the posts on this thread as it's a subject I'm interested in......about the only truly 100% backlash free drive I've ever seen is on the old fashioned analogue tuning dial of a radio from the good old days before digital readouts became the rage.

    With that info it wouldn't take much to replicate that drive design for a CNC set-up but with more robust (thicker) gears etc.

    However, the drive is a pure back force counteracting method and can be over ridden by a similar force applied against the spring loading.

    My preferred method for backlash elimination...….that is to have NO backlash at all....is to have a worm drive with the worm spring loaded into close contact with the worm wheel.

    It works on the principle that if you push the worm tightly into mesh with the worm wheel you will have metal to metal contact and no backlash, but due to wear on the worm wheel you need to have the worm spring loaded or resilient to cater for the tight and loose spots.

    Most if not all worms on rotary tables and dividing heads are adjusted with a cam action to the worm, but that is a fixed adjustment that is not continuously variable around the worm wheel.

    A resiliently mounted worm is constantly variable so that it can ride up and down on the worm wheel a thou or two and will be backlash free all the time.

    It does mean redesigning the way the worm is attached to the RT instead of having it in a cam type housing which cannot be constantly variable.

    if a 4th axis is being built up from the word go, using a worm and worm wheel drive design, then it is quite easy to mount the worm in a spring loaded swing housing to keep it tightly meshed but free to drive smoothly and so constantly variable.
    Ian.



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by handlewanker View Post
    Hi, just read all the posts on this thread as it's a subject I'm interested in......about the only truly 100% backlash free drive I've ever seen is on the old fashioned analogue tuning dial of a radio from the good old days before digital readouts became the rage.

    With that info it wouldn't take much to replicate that drive design for a CNC set-up but with more robust (thicker) gears etc.

    However, the drive is a pure back force counteracting method and can be over ridden by a similar force applied against the spring loading.

    My preferred method for backlash elimination...….that is to have NO backlash at all....is to have a worm drive with the worm spring loaded into close contact with the worm wheel.

    It works on the principle that if you push the worm tightly into mesh with the worm wheel you will have metal to metal contact and no backlash, but due to wear on the worm wheel you need to have the worm spring loaded or resilient to cater for the tight and loose spots.

    Most if not all worms on rotary tables and dividing heads are adjusted with a cam action to the worm, but that is a fixed adjustment that is not continuously variable around the worm wheel.

    A resiliently mounted worm is constantly variable so that it can ride up and down on the worm wheel a thou or two and will be backlash free all the time.

    It does mean redesigning the way the worm is attached to the RT instead of having it in a cam type housing which cannot be constantly variable.

    if a 4th axis is being built up from the word go, using a worm and worm wheel drive design, then it is quite easy to mount the worm in a spring loaded swing housing to keep it tightly meshed but free to drive smoothly and so constantly variable.
    Ian.
    Most manual rotary tables use worm gears and the higher quality ones sometimes have adjustable backlash. I am assuming that this is achieved with some kind of variable pre-load (although I'm not sure). I bet that, with enough tinkering (in hardware and software), they can be made to perform well enough to perform with your average hobby machine.

    The term "low backlash" seems to be a matter of opinion. Parker advertises their table with 7 arc minutes of backlash to be "low back". For some people, any backlash at all is apparently too much. These guys call their product "non-backlash gears":

    https://www.ebay.com/itm/Toyo-Seimit...6ArDw92lbwh5Tg

    I am still trying to figure out what a backlash rating of 7 arc minutes actually means in terms of real world results.

    A lot of low to mid-end CNC manufacturers seem to be fine just attaching a stepper or servo to a manual rotary table. I would like to see the sort of results that can be achieved using those converted rotary tables from Tormach, Sherline and the older Haas 4th axis assemblies vs converting a high quality manual table yourself.



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    Default Re: A and C axis for a 5 axis milling machine

    These are some of the no-backlash alternatives to harmonic drives that I found on my travels and also found semi-affordable deals for on eBay. There were other types but so far I haven't found them on the used market:

    Cycloidial gear box

    https://www.onviollc.com/cycloidal-g...peed-reducers/





    Spring loaded worm gear:


    https://conedrive.com/products/produ...pecials-etc-3/


    Nordex fixed / floating type:





    Of these, the cyclo gearboxes seem to be the most readily available on ebay if you know what to search for. They don't always advertise them as cyclo gearboxes so you have to search for brands that make them like IMT and those named above.

    Um... now that I posted this... hopefully prices won't shoot up like they did when word got out on harmonic drives... so.... let's just keep it between us guys who use the internet...

    The second / middle type looked to be the easiest to work with for making diy gearboxes.


    - - - Updated - - -

    These are some of the no-backlash alternatives to harmonic drives that I found on my travels and also found semi-affordable deals for on eBay. There were other types but so far I haven't found them on the used market:

    Cycloidial gear box

    https://www.onviollc.com/cycloidal-g...peed-reducers/





    Spring loaded worm gear:


    https://conedrive.com/products/produ...pecials-etc-3/


    Nordex fixed / floating type:





    Of these, the cyclo gearboxes seem to be the most readily available on ebay if you know what to search for. They don't always advertise them as cyclo gearboxes so you have to search for brands that make them like IMT and those named above.

    Um... now that I posted this... hopefully prices won't shoot up like they did when word got out on harmonic drives... so.... let's just keep it between us guys who use the internet...

    The second / middle type looked to be the easiest to work with for making diy gearboxes.




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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by Goemon View Post
    I am still trying to figure out what a backlash rating of 7 arc minutes actually means in terms of real world results.
    An arc-minute is 1/60 of a degree, so 7 arc minutes is about .12 degrees or .0003 of a rotation. So times pi D and you get about .001 per inch of diameter of your part.



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    Default Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by skrubol View Post
    An arc-minute is 1/60 of a degree, so 7 arc minutes is about .12 degrees or .0003 of a rotation. So times pi D and you get about .001 per inch of diameter of your part.

    I know that part. I just don't know how different a part would look if it was made using a 4th axis with 7 arc minutes of backlash vs one with 10 or 15.

    I was reading the specs of a $30,000 Haas rotary axis and it stated accuracy of 30 arcs minutes which is a lot higher than what you see in the specs of "precision low backlash" gearboxes. It was a very large rotary table though... so.... maybe you can get away with more backlash when making larger parts. Or.... maybe any backlash is ok as long as it is consistent and predictable so you can have the software compensate.... or something.

    I just ordered a Fanuc Robot arm 118:1 cycloidal gear head off ebay. They sell new for over $5,000 and I got a new one for $200. I'll post about how well it works once I have had a chance to test it. It has an 8" flange output with built in cross roller rotary bearings so it should be convenient to attach a face plate and chuck.



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    Default Re: A and C axis for a 5 axis milling machine

    Here is a link to the info on the gear head I bought in case anyone is looking for alternatives to harmonic drives and particularly for those looking to make larger 4th axis:

    https://www.rmhoffman.com/cycloidal-...-nabtesco.html

    The claim of less than 1 arc minute of backlash and the fact that they are made for precision robots makes me feel good about what I bought. I believe mine is an Rv-e series.

    This is a video showing how the RV series can be used for 4th and 5th axis application:



    Last edited by Goemon; 08-20-2018 at 02:05 PM.


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A and C axis for a 5 axis milling machine

A and C axis for a 5 axis milling machine