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  1. #13
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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Thanks, can you recommend a budget Galil model capable of this? I have researched and some seem very expensive.

    This would be ideal if I could set up my mill in this fashion with my DRO with plain open loop steppers to start with, and later upgrade them to servos. All the while enjoying improved accuracy due to actual backlash / feedback correction.


    From Galil:
    "Dual-loop The dual-loop (DV) feature enables the controller to compensate for mechanical backlash. Typically, dual-loop systems use a rotary encoder on the motor and a linear encoder on the load (Galil controllers accept inputs from two encoders per axis as a standard feature). Dual-loop control changes the standard PID control and closes the position loop with the load encoder (“PI”) and derives the damping terms (“D”) from the motor encoder. This method provides smooth and accurate control along the motion path regardless of backlash."



  2. #14
    Gold Member Jim Dawson's Avatar
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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    To buy a new Galil board is very expensive, but there are a lot of them on Ebay. Some good deals, and some sellers are just crazy in their pricing. The ones you want are 18x0,18x2,18x6, 20x0, 21x0, 22x0, 41x3, 40x0. Where x = the number of axes. The newer firmware revisions have what is called Stepper Maintenance Mode (SM), that will error correct when using open loop steppers with encoders on the load. If you find some that you are interested in, you can post here or PM me for advice. I've been installing Galil products for over 20 years.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    I do not *know* what Jim Dawson is using .. nor doubt his numbers,

    But his 3 um accuracy is a very high nr for accuracy (not res. or feature/local accuracy), and has multiple caveats.
    Is the surface finish that good ?
    But all the advice re: servos, scales, etc. he made is spot on.

    As Jim intimated imho, accuracies of 0.02 - 0.01 mm in mechanical resolution are perfectly reasonable.
    That is resolution, not accuracy.
    And very much better is easily done, for little more money.
    Using a secondary measurement, like glass scales, easily allows one to gage to much better positioning//accuracies, 1 micron being quite possible.

    I use AC brushless servos on my 12" lathe with gibs (so far).
    750W, 220V, 10.000 counts, 1:3 belt drive via HTD8-30 mm belts.
    Theory - 0.2 microns step size. Approx 2000 kgf push for one step of 0.0002 mm. 32 mm screw on z, now, hiwin clone (TBI).

    It bounces, is not reliable.
    The belts and pulleys are not good enough - they are not linear.
    The gibs cause windup, sticktion causes tension, the bearings in the mounts are stock and are, in a word, crap.

    The x axis screw system I made is 18x more stiff than stock. Rigid-rigid mount in tension. 0.750" vs 15 mm or so.
    But it is loose, not linear.

    1.
    Yes. It is perfectly feasible to get about 0.01 mm or better in resolution and calibrate accuracies to better than that.
    As Jim said.
    2.
    The 400W servos are the sweet spot. As Jim said.

    All that I consider problems, are mostly less than 0.01 mm local errors in incremental positioning.
    You would not notice them, especially on a mill.

    Fwiw..
    I use an optical encoder DTI, electronic, external, to verify positioning.
    It is easy, and quite reliable, to position to 1 micron via manual MPG.

    This works well, but is useless for me as a business.
    I need the machine-co-ordinates to closely match the theoretical position.



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    Registered MikeC8's Avatar
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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    This is just my opinion, but with todays market I would only look at doing your own conversion if taking on a project is something that you really enjoy. So the real question is do you want to start making CNC parts right away, or do you want to take on a project that might challenge you, and make it your own?

    I bought my machine already converted. However I have slowly been redoing just about everything on it. And if I did not have the machine I have today, I would probably buy a tormach 440, as that seems like the perfect size for the parts I make. I have yet to make use of my 19" x travel. And I have only used the 8" y travel once or twice.

    Work: Hurco VMX42/VMX50 - Shopsabre 4896 - HSMworks with Solidworks
    Home: RF45 with Ajax CNC Controller - Fusion 360


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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Jim,
    Do you have a build thread or any more details on your linear scale set-up. I'd be very interested in learning a bit more about it.

    to the OP, for what it's worth I have a PM45 that I've converted over with some closed loop steppers (these ones specifically), have mapped my x and y ballscrews and I can hold +/-.001" without much effort at all. But like ninefinger said, it's all about what you can afford to get away with.



  6. #18
    Gold Member Jim Dawson's Avatar
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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Quote Originally Posted by CS900 View Post
    Jim,
    Do you have a build thread or any more details on your linear scale set-up. I'd be very interested in learning a bit more about it.
    I really don't, but I can create a thread and go into some depth. Mechanically it's pretty simple, mount the sensor and slap on the magnetic tape. To the controller it looks just like any encoder.

    Here is a picture of the one on my mill X axis. Mounted on the front of the table, been there 3 years with no problems. This particular sensor is a Renishaw, but I have had excellent luck with the Ditron units also. The Ditron units are identical in size and performance to the Renishaw units, and considerably less expensive, about $120 / axis.

    Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?-img_0126-jpg

    Attached Thumbnails Attached Thumbnails Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?-img_0126-jpg  
    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Quote Originally Posted by CS900 View Post
    Jim,
    Do you have a build thread or any more details on your linear scale set-up. I'd be very interested in learning a bit more about it.

    to the OP, for what it's worth I have a PM45 that I've converted over with some closed loop steppers (these ones specifically), have mapped my x and y ballscrews and I can hold +/-.001" without much effort at all. But like ninefinger said, it's all about what you can afford to get away with.
    Thanks CS900.

    I am very interested in what Jim recommended and I've been researching it since. It certainly takes a bit to get your head around, I am currently looking into Kflop (~$250) as it seems a more economical alternative to Galil ($ much more ?).
    Steppers in Aus are $200 / axis compared with servos which are $550/axis (starting). I would like an electronic system to start with steppers and linear encoder feedback, possibly upgrading to servos later.

    Mechanically what is necessary for good accuracy? I am thinking of C7 ballscrews with anti backlash mechanism + Hiwin rail CLONES. Genuine Hiwin I hope I can avoid, unless I win some lottery.
    That's just to get me up and running, later on I will probably go for a new spindle and accessories.



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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    You can get genuine HIWIN for pretty good prices (I use "BST automation" on aliexpress).

    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)


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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Quote Originally Posted by pippin88 View Post
    You can get genuine HIWIN for pretty good prices (I use "BST automation" on aliexpress).
    Wow thanks! That's so cheap. Do you know of any recommended ballscrew suppliers?

    The nice thing about Alliexpress is us Aussies can access the same stuff as US folk.



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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Quote Originally Posted by Jim Dawson View Post
    I really don't, but I can create a thread and go into some depth. Mechanically it's pretty simple, mount the sensor and slap on the magnetic tape. To the controller it looks just like any encoder.

    Here is a picture of the one on my mill X axis. Mounted on the front of the table, been there 3 years with no problems. This particular sensor is a Renishaw, but I have had excellent luck with the Ditron units also. The Ditron units are identical in size and performance to the Renishaw units, and considerably less expensive, about $120 / axis.
    thanks jim, I'm actually very familiar with the renishaw once as i use them at work. I was unaware they had the same signal output as many of the common encoders though. That certainly does make things a bit easier. I assume these have a response time much better than the cheapo magnetic scales?



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    Gold Member Jim Dawson's Avatar
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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Quote Originally Posted by CS900 View Post
    thanks jim, I'm actually very familiar with the renishaw once as i use them at work. I was unaware they had the same signal output as many of the common encoders though. That certainly does make things a bit easier. I assume these have a response time much better than the cheapo magnetic scales?
    Not sure about the response time vs other mag systems. They are rated at something like 4 m/s with 1 micron resolution, 4 MHz. That's fast !

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?

    Quote Originally Posted by j3dprints View Post
    Wow thanks! That's so cheap. Do you know of any recommended ballscrew suppliers?

    The nice thing about Alliexpress is us Aussies can access the same stuff as US folk.
    I'm very happy with my 1616 (16mm diameter 16mm lead) ball screws from BST. They seem to be higher quality than my other chinese ballscrews. They are on a router though.

    BST sell the standard cheap Chinese 1605 1610 2010 etc screws. I expect they can get HIWIN screws if you want.

    Most good sellers will custom order things, don't go with the stock listing. If they won't custom order, they are probably the wrong seller for this stuff. E.g. I order custom length HIWIN rails with higher precision, and higher preload. Cost a bit more (for the high precision really).

    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)


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Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?
Accuracy I can expect from a RF-45 or similar cnc conversion? Bang for $$ point?