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IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > A and C axis for a 5 axis milling machine
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  1. #21
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    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    Small ones work very well with Harmonic Drives. Larger ones have to use worm and wheel to get the torque capacity, but there are quite a number of sophisticated (and of course expensive) ways to get the backlash out of a worm&wheel. I am not saying that any of them are as good as an HD.

    A lot of RTs are really only used as indexers, and as such can use both a drag brake and a solid brake/clamp to lock the position. Not all CAM packages and not all CNCs can actually handle full simultaneous 6 axis machining anyhow.

    Cheers
    Roger
    Please can you explain how you are classifying small vs larger ones? I.e. What is the maximum size you can use a harmonic drive for and what prevents you from going larger with one?

    I guess that for my needs, I could manage quite well with an indexing solution instead of a simultaneous 4 axis capability. My interest would be in milling an aluminum rifle chassis in one set-up. My main concern is accurate indexing so I could machine both sides of a stock or chassis with accurate inletting. I have seen plenty of low quality stocks with more material on one side than the other due to poor indexing.

    I have been wondering if I might be better off with a decent manual rotary table (with a good break) and turning the part twice. As I don't have an ATC spindle yet, it's not like I could leave the machine unattended and if I have to be there for tool changes anyway, maybe manual indexing wouldn't add much burden.

    A lot will depend on the availability of quality used parts to make a 4th axis with acceptable performance. It sounds like the cheap Chinese belt-drive 4th axis won't cut it (literally) for aluminum. I occasionally see used rotary axis from old VMCs going for less than $1,000 so it's either that or a harmonic drive.

  2. #22
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    Re: A and C axis for a 5 axis milling machine

    With a 100-1 ratio, even a small harmonic drive will have plenty of torque for positioning a spindle-head or trunnion. What it won't have is speed.

    Inaccuracies in indexed parts are usually due to improper zeroing of the rotary axis.

    Yes, there are plenty of old rotary tables on the market; they used to be necessary for making round features, but modern CNCs are good enough to do that with G2/G3 commands. As long as your indexed toolpaths don't require changes of direction, the backlash in their gear trains won't be an issue. You can move it with a motor or a crank; it doesn't make much difference if you're monitoring progress and can step in to do it.
    Andrew Werby
    Website

  3. #23
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    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    Hi dixdance

    Read the full thread I cited earlier in this thread.
    Note that a full Harmonic Drive already includes a large crossed roller bearing, which is enough. As for torque: enough to tear your arm off. The HD I bought was in good shape.

    Cheers
    Roger
    Thanks for the reply Roger. I did read your entire post with great interest a month or so ago. That's where I discovered harmonic drives and crossed roller bearings., been fascinated by them since. Your post gave me the idea and courage to give this a try myself.

    As to the larger crossed roller bearing, I was concerned that with a 250mm table, there would be an awful lot of table hanging out there, and wanted it to be supported as close to the circumference as possible. For instance, with your drive unit, the support surface is only 90mm diam.

    Am I being overly concerned here? Is there a problem with having 2 CRB's, assuming I can get them concentric within a reasonable tolerance. And what would that tolerance have to be?

    Thanks, Richard

  4. #24
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    Re: A and C axis for a 5 axis milling machine

    Ah, what fun!

    how you are classifying small vs larger ones?
    Ah well, there is no hard borderline of course. It may be more a matter of where things get too expensive!
    For a horizontal axis of rotation, an overhang of the same size as the diameter of the bearing should (I think) never present any problems at all. For a vertical axis of rotation you could go a bit further. Others might argue that I am being wildly conservative here.

    Inaccuracies in indexed parts are usually due to improper zeroing of the rotary axis.
    I agree entirely with Andrew here. Getting everything square and lined up is crucial.

    Is there a problem with having 2 CRB's, assuming I can get them concentric within a reasonable tolerance. And what would that tolerance have to be?

    Yeah, well, what tolerance is ever needed?
    But in fact I don't think you would need two CRBs if they were of similar sizes. If the two bearings are offset spatially, then I would not hesitate to use one CRB and one DGB, of similar sizes.
    Even so, alignment would still be critical. I think I would mount the second bearing, probably the DGB, with a tight fit in a plate and then bolt this plate to the frame at the last minute - possibly with the shaft rotating. That way the DGB would be almost perfectly aligned to the shaft and the CRB. A large number of well-torqued bolts with washers would be appropriate.

    The alternative would be to machine both bearing cavities in one hit on the lathe or mill, but if the CRB is already in an HD you can't do that.

    If you want a 250 mm table, that is a lot bigger than mine. Can you get an HD that big? I forget. It would be kinda $$. I think ... I would use a large CRB close to the rim of the table (that is $$$) and a smaller HD to rotate it, and I would consider mounting the HD in a slightly compliant manner. The CRB would give you the strength needed while the HD would then give you the angular resolution and accuracy. More complex, but a 250 mm RT is going to be expensive anyhow!

    For 250 mm diameter I might also be looking at a worm&wheel solution with fancy worm using balls rather than sliding surfaces. I forget the brand, but that design might allow you to reduce the backlash to acceptable levels. The worm&wheel would also be $$ if you want accuracy.

    If this project of yours flies, we would all love to hear about it! I am toying with the idea of a slightly smaller ultra-thin RT (180 mm) with somewhat lower specs, but that is some distance off.

    Cheers
    Roger

  5. #25
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    Re: A and C axis for a 5 axis milling machine

    Is it really possible to avoid having an overhang on the table if you aren't going to limit the machine to only making very small parts?

    I was wondering about this for those 30" rotary table / round pallets you see on larger 5 axis VMCs. I was assuming that, under the hood, those large rotary tables are rotating on a far smaller spindle shaft and bearings. An over-hang seems unavoidable.

    It starts to make sense why some people choose to rotate the spindle instead of the work piece.

    I was planning to use a horizontal 4th axis with two lathe chucks. I read (somewhere) that the part can be double the diameter of the lathe chuck in this type of set-up. A typical part for me would be 6" x 20" or 6" x 30" so I was thinking that a pair of horizontal rotary tables would be best.

  6. #26
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    Re: A and C axis for a 5 axis milling machine

    Goemon - are you talking about an RT at each end of the workpiece? I doubt that is needed: I just use a tailstock at the other end.

    Cheers
    Roger

  7. #27
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    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    Goemon - are you talking about an RT at each end of the workpiece? I doubt that is needed: I just use a tailstock at the other end.

    Cheers
    Roger
    I was thinking I might need two chucks because I have doubts about how well a 30" long aluminum part will be held with a regular tailstock. I am not sure if I need two full rotary tables if I could attach a chuck to the tailstock. If I set it up as a manual rotary indexing model, I would benefit from having a break at each end with a second rotary table though.

    When I look at the typical trunnion tables you see on larger VMCs, they appear to have two rotary tables. I figure that if I am going to copy a design for my diy build, I might as well try and copy a machine that is made to do the sort of work I want to try. I haven't seen many CNC mills with a 30" lathe style 4th axis so I am not sure how they deal with work holding for longer work pieces (or if they are even viable for metal work).

    I have done a little searching for harmonic drives and there seems to be a lot of them on ebay but... the prices for "good" ones don't seem to be far off buying a used VMC rotary axis like this one:

    https://www.ebay.com/itm/10-TSUDAKOM...from=R40&rt=nc

    I am fairly sure that even an old VMC rotary table like that would be better than anything I could build myself, even with a harmonic drive. maybe that will change once I finish reading up on them and get my head around how the HD would connect to a rotary table and chuck.

  8. #28
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    Re: A and C axis for a 5 axis milling machine

    I doubt you'd want a full rotary table on both ends. I think on the big VMC trunnions the second end of the supporting axis is just a bearing setup, not sure what kind, but I really don't think it has any drivetrain. It just doesn't look like a tailstock because that form factor doesn't suit a trunnion. Maybe they do have a second brake in the other end, maybe another encoder? I've seen some that have the motor for the rotary table on the trunnion in the second end.

  9. #29
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    Re: A and C axis for a 5 axis milling machine

    This is the sort of 4th axis set-up I want / need:

    https://advantagemachining.ca/unders...nter-4th-axis/

    It looks like there is some kind of extra work-holding gizmo on the tailstock but I can't see what it is. It seems to have a face plate that bolts onto the part on the tailstock which is what I want to copy.

  10. #30
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    Re: A and C axis for a 5 axis milling machine

    I think the gizmo on the tailstock is just a solid live centre. Between the live centre and the RT there is a solid core - a sort of horizontal tombstone in fact.

    That 10" unit you mentioned - that is a LOT bigger than mine! I have a 5" chuck on mine and just maybe it should have been a 4" one instead. I have found that having a big chuck on the RT can actually be a DISadvantage at times: it gets in the way.

    As an aside: I have never worked out why second-hand RTs always really bad tatty paint jobs. You would think the vendor would get a better price by adding a nice coat of paint.

    Cheers
    Roger

  11. #31
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    Re: A and C axis for a 5 axis milling machine

    Ah, what fun! If this project of yours flies, we would all love to hear about it!

    Yes, I'm really looking forward to it, but it will be at least 6 months off, an awful lot on my plate right now. But fun to think about it and accumulate parts as I get more info.

    If you want a 250 mm table, that is a lot bigger than mine. I think ... I would use a large CRB close to the rim of the table (that is $$$) and a smaller HD to rotate it, and I would consider mounting the HD in a slightly compliant manner. The CRB would give you the strength needed while the HD would then give you the angular resolution and accuracy. More complex, but a 250 mm RT is going to be expensive anyhow!

    That is what I was thinking to do. I found a "lightly used" 8" (aprrox) CRB on ebay for $45, and from the US no less. I've been watching the HD's on ebay, but I still have to figure out the difference between the different configurations and models they offer. I'm confused by the component sets they offer, with 3 parts that get assembled. Do they need to go into some sort of precision frame or chassis? I'm also wary of any HD where I can see the bearings, seems like it would be very vulnerable to dust, or is it easy to create a seal? I'm obviously pretty ignorant about these, when I have some time I'll dig in and figure it out. Maybe even call the company and get it from the horse's mouth, altho I hate to waste their engineer's time as I'll be buying a used unit. As to other expense, I haunt the scrap yard and craigslist and have lots of cheap jig plate. Just need the HD, stepper and drive, and the time to play around. Oh, and I have to build the router that this will be used on too...

  12. #32
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    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    I think the gizmo on the tailstock is just a solid live centre. Between the live centre and the RT there is a solid core - a sort of horizontal tombstone in fact.

    That 10" unit you mentioned - that is a LOT bigger than mine! I have a 5" chuck on mine and just maybe it should have been a 4" one instead. I have found that having a big chuck on the RT can actually be a DISadvantage at times: it gets in the way.

    As an aside: I have never worked out why second-hand RTs always really bad tatty paint jobs. You would think the vendor would get a better price by adding a nice coat of paint.

    Cheers
    Roger
    I am not so worried about it getting in the way because I designed my (fixed) gantry and frame with enough room to add a larger 4th axis. My only concerns on that are if it is large enough to properly hold a 6" x 30" part and if it is sturdy enough for aluminum milling. A smaller one would clearly be easier and cheaper to buy chucks for though. I think a larger heavier one would provide better holding and vibration damping.

    I also wondered why used rotary tables look so weathered. Some of them look like they were recovered after being lost at sea. As long as they work though, I don't mind doing a little clean-up.

  13. #33
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    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by skrubol View Post
    I doubt you'd want a full rotary table on both ends. I think on the big VMC trunnions the second end of the supporting axis is just a bearing setup, not sure what kind, but I really don't think it has any drivetrain. It just doesn't look like a tailstock because that form factor doesn't suit a trunnion. Maybe they do have a second brake in the other end, maybe another encoder? I've seen some that have the motor for the rotary table on the trunnion in the second end.
    I wouldn't be looking for two drivetrains. I want two chucks to hold longer aluminum parts properly. I don't have much faith in those tailstock that pin the part in place with just pressure. If a part that long is not held securely at both ends, I can see it becoming a source of inaccuracies.

    A lot depends on the quality of the parts I end up buying. If I ended up with one of those 150lb rotary axis from an old VMC with an equally heavy tailstock, I would probably find less need for a second chuck. If I end up with a Sherline 4" 4th axis, I have zero confidence in a one chuck set-up.

  14. #34
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    Re: A and C axis for a 5 axis milling machine

    I'm confused by the component sets they offer, with 3 parts that get assembled.
    The core of the HD is the thin SS cup held between inner and outer toothed assemblies. That is what does the backlash-free reduction. They are available as a set for people (companies) who want to do the entire integration themselves.
    From there on you can get assemblies of different completeness, up to a stand-alone unit with all bearings. The latter is what I bought to make my RT.
    There are PDFs on the HD web site which explain.

    Cheers
    Roger

  15. #35
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    Re: A and C axis for a 5 axis milling machine

    Thanks Roger, that's pretty much what I thought. So the easiest way to go is with the complete stand alone unit, which is what I'll keep looking for. I've seen them with a 5 phase stepper motor attached, any reason to avoid those? From what I've read they give twice as many steps/rev, and smoother motion. Are the drives for them more expensive or difficult to find? I'm planning to use a Gecko 540 drive for my router, but it will have dual y steppers, so I'll need a separate drive for rotary anyway.

  16. #36
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    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by dixdance View Post
    Thanks Roger, that's pretty much what I thought. So the easiest way to go is with the complete stand alone unit, which is what I'll keep looking for. I've seen them with a 5 phase stepper motor attached, any reason to avoid those? From what I've read they give twice as many steps/rev, and smoother motion. Are the drives for them more expensive or difficult to find? I'm planning to use a Gecko 540 drive for my router, but it will have dual y steppers, so I'll need a separate drive for rotary anyway.
    I think the 5-phase actually give 2.5x as many steps per rev (500 full steps,) from what [little] I've read, but the drives are much less common, so I'd expect pricier.
    Assuming the stepper on the drive you get has a standard frame you could always replace it with a standard one or even a basic servo.
    Actually, a quick search shows they aren't that expensive:
    https://catalog.orientalmotor.com/vi...-motor-drivers

  17. #37
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    Re: A and C axis for a 5 axis milling machine

    5-phase is in many ways an older technology. They are still available, and so are the drivers, but I would expect them to be a bit more expensive as there is not the competition. Are they any better? Again, I doubt that as if they were there would be more of them. But I imagine the vendors will hotly dispute all of that.
    As skrubol says: if you stick to standard frames or mechanical interfaces, it is not a problem. You could easily swap later, although you would need to swap both motor and driver.

    Cheers
    Roger

  18. #38
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    Re: A and C axis for a 5 axis milling machine

    I am also finding the shopping experience for HDs to be confusing, or maybe it's more problematic....

    At a high level, I can see that, like other types of gear reduction, HDs are available in a range of sizes and levels of reduction but.... I haven't yet found any used ones with standard sized inputs for use with a stepper motor, or a convenient way to output to a rotary table.

    It seems like the best way to buy one is with a motor attached already, as long as the motor is fit for purpose and drives are available. Most of the "good" ones seem to be paired with AC servos which is not necessarily a problem except that there is a whole lot of extra cost involved in drives and cables. Annoyingly, nobody ever sells used AC servos with everything you need to start using them. What happens to all the cables from used AC servos???. With all the extra cost (and hassle), I would rather buy a ready made one from an old VMC.

    The outputs of the HDs I have seen also look problematic. For example, with this HD, how would it connect to a rotary table or chuck?

    https://www.ebay.com/itm/Mitsubishi-...from=R40&rt=nc

    The other types of gear reductions I have seen go from motor shaft input to a male shaft output which can be easily coupled with ball screws and rotary tables etc. with HDs, the output looks more like a rotary table face plate, or just a hole in some cases. Are you meant to attach a lathe chuck to the HD directly?

  19. #39
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    Re: A and C axis for a 5 axis milling machine

    Seems like a lot of them come out of robot arms, which explains the unusual output configuration. If an arm is normally hanging off the end of one without additional bearings, a chuck should be easy.

    Although that Mitsubishi one looks like it may not have an output bearing on it. And not sure how you would connect to the output, the holes don't look tapped and might not have enough clearance to for bolt heads inside. Maybe just meant for pins to drive the output and the spline would be left floating?

  20. #40
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    Re: A and C axis for a 5 axis milling machine

    My HD came complete with bearing and housing. Yes, I think it came out of a robot arm.
    Good lord: there is now a recycling market for obsolete robots! Time flies.

    If you get the drawings from HD, you will have all the necessary details for diameters and screw holes.
    I machined up a GT2 toothed pulley to match the input end and another one for the motor with the right numbers of teeth so I could get a step resolution of 0.001 degrees.
    For the output end I machined up the equivalent of a chuck back plate to fit securely on the HD.

    In theory, precise concentric machining is all you need for the backplate. In practice we often make the fitting between the backplate and the chuck a little loose. Then we centre the chuck on the back plate with a DI off a shaft held in the chuck. That way you should be able to get the whole thing aligned to a TIR of less than (your preferred tolerance).

    So yes, 'some assembly required'.

    Cheers
    Roger

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