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$5000 retrofit
« on: September 25, 2009, 02:59:00 PM »
Hi. Imagine that you have a big (14 ton (metric) milling machine in pounds I belive around 28,000), old machine that currently is driven by 1.1kw motors (induction ac motors) to feed each of the 3 axis. The machine is amazing rigid, type of contruction that nowdays is not done anymore.
I saw it machining 1.2cm height on each passage (1/2") with a 300mm diameter mill. Amazing power.

Moving with my hands on the existing motors I could move the machine without effort on a 50mm (2") pulley. I suspect that currently exists more or less a 1/10 reduction between the motors and the existing screw.

Now my question: Imagine that you have ballscrews at hand and that you want a professional close loop servo system (I want a trouble free thing) on all three axis to replace the current motors and existing reduction to a direct ball screw installation. And for all that you could spend $5000 in material.

It is possible? What you you recommend ?

Thanks for any idea.

Offline Hood

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Re: $5000 retrofit
« Reply #1 on: September 25, 2009, 03:07:19 PM »
 Why do away with the reduction? Doing away with it will put you into the territory of huge servos so $5000 is not going to do one axis let alone 3.

Hood
Re: $5000 retrofit
« Reply #2 on: September 25, 2009, 08:04:00 PM »
I agree with not doing away with the reduction, reduction allows the economical use of high rpm servo's, now 6000rpm+ is common for AC servo's.
If they were regular induction motors then the top rpm would have been 3500+ on 60hz.
If it is a very large machine, I would think it is only going to be economical if it already has ballscrews, which it sounds like it has?
Otherwise you may be over the $5k.
What kind of existing pulleys? you will need timing pulley type.
Nosmo.
Re: $5000 retrofit
« Reply #3 on: September 26, 2009, 02:52:48 AM »
Current motors I belive they are 1500 rpm 50Hz. Those motors are only for linear movement (user presses button and machine moves in one direction), so when it was done nobody was thinking in backlash. No timming pulleys there, only simple V pulleys where a little slip is no problem.

Putting the reduction away was to have a free backlash system, but I can also arrange timing pulleys reductions. Space is no issue in that machine.

About the ballscrews, it does not have it, but my client have lots of them, so no problem in putting the ballscrews. I am more concerned in motors/drives that would provide a closed system, free of troubles.

Thank you

Offline Hood

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Re: $5000 retrofit
« Reply #4 on: September 26, 2009, 05:42:57 AM »
 First off I should say I am no expert on this but am just going from my own experience of fitting out my lathe and mill.
 If you keep the gearing the same then it would be safe to assume that an AC Servo of roughly the same torque would do you. Putting ballscrews on will reduce the friction so make things easier to turn but it will also increase the ability of the the cutting forces to be transmitted back to the motor.
 The motors that are on at the moment should be approx 7Nm torque so you are probably looking about 1.5 to 2Kw AC Servo to get  similar continuous torque. AC Servos have a lot more torque in reserve (usually 3 or more times continuous)  but this can only be used for a short period of time, how long will depend on the drives/motors and lots of other factors such as temperature etc so it is always safest to base your calculations on the continuous ratings.
 You are probably looking at around $1500 plus per axis for the Chinese motors and drives in the 2KW range, the drives I use are Allen Bradley and for a 2KW drive it would be around £1000 ($1500) and then the motor about the same.

Because these motors will have high resolution encoders you will need to use an external motion controller to get the speed required, I use the SmoothStepper and it works well for me.

DC Servos may be something else to look at but I have no experience with them. Simpson36 has done a review of DC servo drives and it looks like the Dugong and Whale are about the best, you can see info here http://www.machsupport.com/forum/index.php/topic,12233.0.html

Hood
Re: $5000 retrofit
« Reply #5 on: September 26, 2009, 06:54:52 AM »
Thanks Hood

I will go for the AC servo/drive. I would like to avoid the Chinese motors and drives, and would love to try the granite drives (they are in Europe like I am). Unfortunately I find it hard to find a motor that will fit on the drive that would use all the 1.5Kw power that the drive can output. I belive I would need an AC 140V motor with continuous current of around 14A to take the most of the drive.
Fanuc motors (common for 140V) are one idea, but then I would need to but and encoder and so on... The best was to buy a suitable motor from the start.
Will ask granite devices.

But I would go for that.
Re: $5000 retrofit
« Reply #6 on: September 26, 2009, 10:47:09 AM »
Keep in mind that if you use an external motion controller, you do not necessarily need intelligent drives, i.e ones that close the loop to the drive, I use ones such as AMC etc.
There is a fellow by the name of Jon Elson that has made a commutation converter for Fanuc  Redcap AC servo motors, these make excellent large servo replacements and also spindle motors for smaller machines, I think he has also made a drive.
You may be able to use the 5S or 10S size.
Due to the proprietary nature of the commutation, you either need a converter as mentioned above , or change the encoder, which I have done by using the Renco commutation encoders. Other than the commutation, the original encoder is standard differential.
Even using them in the BLDC fashion results in a nice smooth motor as they are 8 pole motors.
You may be able to use the Granite drive on them, as I believe the Granite obtains commutation from the encoder pulses only?
Nosmo.

Offline Hood

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Re: $5000 retrofit
« Reply #7 on: September 26, 2009, 11:26:43 AM »
These AMC drives look interesting, dont see any pricing though, is there any place that has that info or does it need an email or dealer contact?
Hood
Re: $5000 retrofit
« Reply #8 on: September 26, 2009, 11:57:43 AM »
I usually go direct, although depending where you are they may refer you to a Repping agent.
Nosmo.

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Re: $5000 retrofit
« Reply #9 on: September 26, 2009, 12:07:34 PM »
Will have to drop them an email and see if they have any dealers in the uk, thanks.

Hood