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Thread: Need help brainstorming a solution for this mill turn spindle

  1. #61
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    Default Re: Need help brainstorming a solution for this mill turn spindle

    Quote Originally Posted by dharmic View Post
    Unless you changed the digital gearing on the fly - you don't care about positioning so much when you're using it as a spindle, and you don't care about speed so much when you're doing fiddly precision. How you get Mach3 to spit that out is another question altogether...
    He could only do that if he had to separate Servo Drives and motors which is what some do, you can't change electronic gearing on the fly, if he used a motor that is big enough for the job, one setup will work as well too 0- 3000RPM and any increment that he wants to index the spindle there is no complication needed, this is very basic stuff that is done everyday

    Look at the machine you are waiting for I don't know what the indexing increments are for that machine but they are doing the same thing without anything that is complicated

    Mactec54


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    Default Re: Need help brainstorming a solution for this mill turn spindle

    It is easy to achieve the resolution and RPM goals without any gymnastics using off-the-shelf hardware, but not in keeping with the additional goals of cheap and Mach3.

    Delta servo drives will accept step & direction inputs up to 4 mHz and have 20 bit encoder resolution. The problem is that to get into the power range required for the spindle the cost is in the $1000+ range.

    There are several free or inexpensive alternatives to Mach3.

    The better system would be to use analog control of the drive, but this requires more sophistication (and cost) on the control end and of course a drive that will accept an analog input. There is no really good compromise for the overall system.

    Jim Dawson
    Sandy, Oregon, USA


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    Quote Originally Posted by mactec54 View Post
    He could only do that if he had to separate Servo Drives and motors which is what some do, you can't change electronic gearing on the fly, if he used a motor that is big enough for the job, one setup will work as well too 0- 3000RPM and any increment that he wants to index the spindle there is no complication needed, this is very basic stuff that is done everyday

    Look at the machine you are waiting for I don't know what the indexing increments are for that machine but they are doing the same thing without anything that is complicated
    If you're talking about a different drive with 3 or 4mhz max frequency, then yes, you can step .005 degree and have 3k rpm. We are talking about a 500khz dmm drive and you can not drive it with step/dir to 3k rpm if its set for .005 step (full encoder resolution). Its not up for debate, it's the way it is. If you believe otherwise, fine, let's agree to disagree. I'm done arguing about it. Please allow those of us that understand the frequency limitation to continue discussing it.



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    Quote Originally Posted by Jim Dawson View Post
    It is easy to achieve the resolution and RPM goals without any gymnastics using off-the-shelf hardware, but not in keeping with the additional goals of cheap and Mach3.

    Delta servo drives will accept step & direction inputs up to 4 mHz and have 20 bit encoder resolution. The problem is that to get into the power range required for the spindle the cost is in the $1000+ range.

    There are several free or inexpensive alternatives to Mach3.

    The better system would be to use analog control of the drive, but this requires more sophistication (and cost) on the control end and of course a drive that will accept an analog input. There is no really good compromise for the overall system.
    Yep, exactly. Either higher frequency drive, or something other than step/dir. In my case, it will be fine for what I want to do within the limitation of 500khz and step/dir control. With 2-1 mechanical and 8-1 electrical I'll have 1500 rpm at the spindle with a step resolution of .02 degrees and 8000oz/in of torque. This will allow turning and milling in the same program without stopping to change pulleys or settings. If I need higher res for hobbing gears, all I have to do is plug in the servo usb to take out electrical gearing and have .0025 step with max rpm of 187.5. If I have a batch of smaller diameter parts to turn at higher rpm, I can swap pulley for 1-1 and get 3k rpm with step res of .04 degrees.



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    Default Re: Need help brainstorming a solution for this mill turn spindle

    Finished with the design and components are picked out. Decided the 1kw servo is a bad decision. The whole thing is costing over 2500, so cutting the horsepower and torque in half to save 130 bucks is pretty pointless. Going with the 1.8kw. Pulley system is 8m polychain 36mm wide with carbon gt belt. Pulley on the spindle is a 50 tooth (about 5 inch diameter), pulley on servo is optional 25 or 50t. The 2 pulley options will use 2 separate belts to avoid the use of a tensioner or long mounting adjustment with servo. Pulley system is not cheap. After a lot of reading, the gt2 8m setup with the carbon reinforced belt should be about as good as it gets for rigidity, torque, and backlash. To buy all this stuff new, it would be about 500 bucks for the 3 pulleys and 2 belts with taperlock bushings. Found all of it on ebay, unused old stock for about 200 bucks total.

    After thinking about it, I decided this setup will live most of its life at the 1 to 1 mechanical ratio, 8 to 1 electrical, 3000rpm turning. Here's what i realized, 90 percent of the work I do will be turning, then 90 or 45 degree indexing with an irrelevant starting azimuth. As long as my mechanical ratio is a whole number (1-1,1-2,1-3, etc), and my electrical gearing is a binary multiple (1,2,4,8,16, etc), i will get precise indexing of 180deg, 90deg, 45deg, 22.5deg, etc., even though step res is .04 degree. Those indexing points will land exactly on full steps. A couple limitations to this, I can't load a part that already has a flat surface and dial it in to sweep perfectly flat, it would have to be dialed in to nearest .04 degree, or ease up on the drawbar and tap it into flat. This is not something I'll be doing though. All indexing happens at once starting with a round part. Other limitation is hex features. The binary gearing won't put full steps on 60 degree divisions. The workaround is easy, when I need a hex pattern, or dial in an existing flat feature, or anything else that needs higher res, I just drop the electrical gearing out and have .005deg step with 375rpm limit. Plenty of resolution for anything I would need to do including hobbing. If for some reason I want more, throw the 25t on the servo and get .0025deg step. As for holding torque with 1-1 mechanical, it's way more than enough after calculating machining forces at given radius with the work I do. I think the setup should work quite well, and since I work with almost exclusively aluminum, 3000rpm will definitely be an advantage over 1500rpm.



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    Default Re: Need help brainstorming a solution for this mill turn spindle

    Sounds like you've got a solid plan. Please keep us in the loop as it progresses - I may pirate some of your work if I get around to motorizing my tailstock spindle!



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    Default Re: Need help brainstorming a solution for this mill turn spindle

    Second guessing the belt style a bit now. After talking with a gates representative for a while, it seems there's a trade off with the carbon gt belts. Apparently the regular gt3 belts have the least amount of backlash, but will stretch a bit more than carbon gt, carbon gt has a bit more backlash than regular gt3, but no stretch. Hard to quantify which is gonna lead to more overall deflection when applying loads in alternating directions. I would say minimal deflection during finish passes is most important so gt3 might be better. Probably just gonna have to do some testing to get actual deflection numbers and mill some test parts to compare surface finish.

    Edit: easy solution. Going with original design using 36mm wide pulleys which is direct fit for carbon gt. Gt3 belts use same type of pulley but available widths of belt are different. Goes from 30mm to 50mm wide. Ill order 50mm belt and build a jig with razor to cut it to 36mm width. That way I can test both belts.

    Last edited by QuinnSjoblom; 03-21-2019 at 10:43 AM.


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    Default Re: Need help brainstorming a solution for this mill turn spindle

    If you don't have to move the spindle during a cut, a disk brake works really well for holding in position.

    Jim Dawson
    Sandy, Oregon, USA


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    Quote Originally Posted by Jim Dawson View Post
    If you don't have to move the spindle during a cut, a disk brake works really well for holding in position.
    Yep, it's definitely possible i could end up adding a brake, but I'm hoping I won't need it as code kind of becomes a mess. I'll need a custom post of I don't want to dig through every gcode file and add enable/disable before and after every A move.
    The servo definitely has enough torque to hold position for the type of machining I'll do, so it just comes down to how rigid the belt is. That's why I'm spending so much time making sure i have the best possible belt style for this.

    Everything is ordered. Should be starting with the build on a week or so



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    Default Re: Need help brainstorming a solution for this mill turn spindle

    If you put an idler pulley on the belt OD to take up slack rather than a swining motor mount, you'll wind up with more teeth engaged - especially on the small pulley.

    Should cut down backlash and an idler is easy to machine yourself. Just make sure to check the minimum pulley OD for back-bending installations.

    EDIT - regarding the brake...

    The gent who posted the youtube videos how me makes the Mega/Ultra 4th axis has some interesting tidbits on brakes buried in his videos. He mentions that while servos can hold pretty well, servo dither can cause surface finish problems and recommends a brake. He also mentions adding a brake - at least for some drag - to the tailstock as well (assuming you're using a trunnion table or tail mounted chuck).



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    Quote Originally Posted by spumco View Post
    If you put an idler pulley on the belt OD to take up slack rather than a swining motor mount, you'll wind up with more teeth engaged - especially on the small pulley.

    Should cut down backlash and an idler is easy to machine yourself. Just make sure to check the minimum pulley OD for back-bending installations.

    EDIT - regarding the brake...

    The gent who posted the youtube videos how me makes the Mega/Ultra 4th axis has some interesting tidbits on brakes buried in his videos. He mentions that while servos can hold pretty well, servo dither can cause surface finish problems and recommends a brake. He also mentions adding a brake - at least for some drag - to the tailstock as well (assuming you're using a trunnion table or tail mounted chuck).
    Good point on the idler. Would indeed give more tooth engagement. As for swinging mount, I don't think I would need that even without the idler. My 2 gearing options only differ by 4mm center distance so was just going to have slightly elongated mounting holes, but that would mean through bolts with nuts. Tapping bolts directly into the mount would be easier and could do that if using adjustable idler. Ill consider that for sure.

    I've been talking back and forth with dmm and specifically mentioned servo oscillation and it's affect on surface finish with not using a brake. The representative said he's never seen that issue with rotary tables using direct drive with one of their servos, but I guess we'll see. I'm definately keeping the addition of a brake in mind while designing this.



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Need help brainstorming a solution for this mill turn spindle

Need help brainstorming a solution for this mill turn spindle