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    Member peteeng's Avatar
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    Default Re: feed rate mismatch between planes

    Hi Wizard - What are the issues with AT belts? I'm exploring different belt profiles at the moment. My first machine used T and has given very good service. I have moved to AT as they are twice as stiff per width but they have not been used enough yet to comment. Comments appreciated. As you say the peripherals of a belt drive do add up in cost. I do not like U drives as they reverse bend the belt and shorten their life and the idlers and mounts have to be made then this is $$$. So I use a single run system, but this doubles the belt length so belt stiffness is important. To combat this I intend to run a stepper at each end of a simple belt system. Some people think this is a bad idea others don't know. The cost of the stepper is the same as the cost of the idler, mount etc.

    I agree with your bullet list... especially about preload. If it's correct then the system once settled in is very stable. Cheers Peter S
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    Default Re: feed rate mismatch between planes

    Quote Originally Posted by peteeng View Post
    Hi Wizard - What are the issues with AT belts? I'm exploring different belt profiles at the moment.
    First off remember that my experience is with fast moving gantry type extractors.

    In any event the belts can wear significantly in areas of high acceleration causing position errors. I suppose any belt will do this but the design or mate up of the pulley and lugs seem to impact AT belts more. Now remember we are talking large gantries with high acceleration and saddles with more mass than the average router.

    The interesting thing is that this localized wear is not as noticeable on smaller HTD derived belt systems. Here I’m talking pick and place, buffers, and the like. Of course the acceleration profiles can very widely and at times the drive is overmatched for the load.

    On another note we have a few Fanuc/Cincinnati electric mold machines using very large Fanuc motors driving leadscrews and extruders. These use HTD type belts that are in some cases very wide. I’m actually surprised at the lifetimes on these belts. The axis strokes are relatively short and obviously on these sorts of machines the motion is back and forth constantly.


    My first machine used T and has given very good service. I have moved to AT as they are twice as stiff per width but they have not been used enough yet to comment. Comments appreciated.
    It might not even be a problem. There are all sorts of factors when it comes to belt wear as you know. It could be an issue with the design or the high acceleration. Note we have engineers that love to “speed things up” so that is a factor. What I find interesting is the high wear at points of acceleration but almost nothing in the other sections. This might indicate fewer problems with “random motion” machines like routers.
    As you say the peripherals of a belt drive do add up in cost. I do not like U drives as they reverse bend the belt and shorten their life and the idlers and mounts have to be made then this is $$$.
    Unless one can find the parts prebuilt it can be expensive. If a person has the ability to DIY the drives that can save money too. Perhaps the best bet is to look for used plastics machinery as parts donors. Even going the used equipment route has its pitfalls.

    Little things like idlers do impact belt life. Having an idle of reasonably large diameter can make a big difference.
    So I use a single run system, but this doubles the belt length so belt stiffness is important. To combat this I intend to run a stepper at each end of a simple belt system. Some people think this is a bad idea others don't know. The cost of the stepper is the same as the cost of the idler, mount etc.

    I agree with your bullet list... especially about preload. If it's correct then the system once settled in is very stable. Cheers Peter S
    .
    This is what I’ve found belt drives designed for positioning service can position reliably. Usually if there is a problem something else has happened. What is always the question is the belt drive good enough for the intended usage, usually that is a question of stiffness

    Note:

    Used or old machinery can have its negatives. Special gear boxes that are no longer produced have caused us issues at work. This is where making sure the hardware on used equipment supports standard mounting interfaces is important. I recently had to source new gear boxes, come up with an adapter plate, and a custom pulley simply due to a custom gearbox no longer being produced. So while retrofiti g old plastics machinery has its positive it also has some negatives.



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    Default Re: feed rate mismatch between planes

    Hi Wizard - The place of high acceleration means that's where the peak loads are especially if its changing direction at the same spot so that's the spot where wear should occur and it does. So Planck you have a lot to ruminate on. For your first machine and it has been said many times thru this forum, KISS. These thoughts will be put to the test in Maximus the machine I'm designing at present. Search for it if your interested I've just started on the motion parts...Good Luck Planck Peter



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    Default Re: feed rate mismatch between planes

    Hi All - I'm researching belt profiles and timing belt design at present. The Optibelt design manual is filled with great info. They have an ATL profile (which runs on AT pulleys) designed for precision applications that has less clearance and a negative tolerance to accommodate the extra pretension required in this application. Maybe it costs more but it's worth a look to see if it's available. Better to have the right stuff for the job... Peter



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    Default Re: feed rate mismatch between planes

    Hi all,
    Hard to follow up the pace, great answers all along. Thanks!
    I am gently heading to make the ultimate tuttifrutti CNC : Rack and pinion for the Y axis, ballscrew on the Z and belts over X (Everman or not, undecided yet). At least I will have learnt a few things along the road.

    Quote Originally Posted by peteeng
    So Planck you have a lot to ruminate on
    That's an understatement ... I couldn't imagine my innocent question would bring me so far, no choice but to undertake it now ¯\_(?)_/¯.
    Sure I will go take a deep look to the Maximus. And by the way thanks for posting the belt profiles, I was looking for them.

    Thanks wizard for your comments and good advices. Just wanted to rebound a little on this one:
    Quote Originally Posted by wizard
    The first thing that came to mind seeing the drawing is jam ups and glitches due to material getting caught in the belt drive.
    I also had this kind of thoughts, but concluded that with a decent air compressor, found in most workshops, it is plain easy to add a few well positioned "blowers" in between the 2 belts aperture and chase out the chip and dust. But yes, just another avoidable tinker one could argue.

    Quote Originally Posted by wizard
    By the way it would help if you could define usage a bit better.
    I think it can be classified as "unflat Interior design" applications. On top of the usual sheet drilling and forming, there will be hardwood and medium carving, but rather simple smooth surfaces as well as articulation parts. Nothing barocco although with respectable carving depths. At some point I intend to append a lathe axis to do "colonnades" type carving. For this to work I should have a good feed rate range and be able to mount heavier tools. Also on the list is thin surface works for insertions and veener, with 0.1ish accurate milling of relatively large surfaces while keeping very clean edges (speed + accuracy + stiffness required). If possible a little soft metal milling too, for assembly parts. Overall nothing especially demanding for a CNC machine I think, but in any case I don't expect to be able to do all I imagine. It is more about opening new possibilities in my everyday work.

    Quote Originally Posted by wizard
    While I don’t have a formal name for the design a similar approach is to drop the glued down belt.
    If I understand well this is the straightforward U approach with a single loose belt right, or is something else?

    And yeah, I will try to KISS this as much as possible.

    Cheers,



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    Default Re: feed rate mismatch between planes

    Hi Planck - soft metal milling... Hmmm you really need a very stiff machine to do this. It's a big step up in stiffness unless your depth of cut is very small. But you will figure that out. Peter



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    Default Re: feed rate mismatch between planes

    Hi Peter,
    Did you look into Optibelt Delta Chain Carbon products ? It seems to mesh pretty well for face to face applications. Only available in 8mm and 14mm width though, but they advertise that "Carbon cord achieves unmatched length stability and outstanding breakage resistance". However I am not sure the price will be any lower than a helical rack.

    Cheers

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    Default Re: feed rate mismatch between planes

    If you're going to do a "ServoBelt" type drive, stick with T5 belts.
    Bell-Everman probably spent years of R&D to determine that T series belts worked best for that application.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  9. #29
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    Default Re: feed rate mismatch between planes

    Hi Planck - Yes I looked at them but they are not recommended for the application. In regards to the Everman system the T5 belt is needed as it is less stiff then the other belts. Everman states that the T5 is a matching profile, this is incorrect by the way. If you use a stiff belt you cannot stretch them far enough over the short idler distance to remove the tooth clearance. Personally I think there are enough unknowns in the concept to stay away from it. Have a look at Maximus I have just put up some calculations on belts & motors for it.

    On another track. I mentioned elsewhere I was considering putting a motor at each end of a drive to remove the short belt/long belt problem. Thinking thru it its not a good idea as the "pushing" pulley has to transfer load through the pulling pulley and its unlikely they will be in perfect sync so this will create an elastic conflict. So KISS wins again. I suggest you look at a simple static motor with a motor drive pulley and a return pulley. This puts the 15-20kg you will have in your motors and their associated stuff off the gantry. Or use the U drive and then your in known territory either way. Its your first machine and it should use reliable solutions otherwise you will be in for a big redesign if something does not work out. Cheers Peter S

    Last edited by peteeng; 05-22-2019 at 08:53 PM.


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