Messages

371

General Mach Discussion / Re: How to handle servo faults

« on: September 25, 2013, 05:56:00 AM »

There is a lot to consider when setting up an E-stop scheme. I would advise that before copying an existing scheme, make sure that whatever hardware you are using has the same features and behavior as the setup you are copying or you may get a very different result.

A well designed E-stop setup will stop the motors as fast as possible, but also retain the machine coordinates, otherwise every E-stop is going to require rehoming the machine.

I make a 4th axis product and often put customer's motors (and drives) on them, so I have exposure to a lot of different stuff and have read a lot of manuals and had conversations with manuf reps and engineers to the point where it all starts to blend together so I try not to quote specifics on a particular brand. That being said, IF my memory serves, the Allan Bradley drives that Hood uses will retain their location and continue to track the encoders while disabled. My Mitsubishi drives and I think also Yaskawa and some others do not.

So, to use Mistubishi as the example, when the drive receives ENABLE, it considers whatever the actual position is  . .  to be the current commanded position. Depending on the conditions present when the drive was disabled, the difference between where the drive IS and where it should be will vary from a lot to a tiny bit, but the error will be there.

Before I became aware of this phenomena, I was using disable/enable to avoid faulting when the 4th axis spindle was locked while cutting a gear or pulley tooth. The error was tiny per tooth, but after 360 degrees and 72 teeth, the error has acculated to a few degrees. Not even visible to the naked eye sometimes, but enough to ruin a gear if more than one pass is required to cut the teeth.

This took a long time, a lot of ruined parts and a lot of phone time with Mitsubishi engineers to finally figure out what was hapenning, so I just pass it along FWIW.  I use ENABLE/DISABLE all the time on the spindle and the 4th axis (when it is in LATHE mode), but there can be pitfalls to using ENABLE/DISABLE on an axis drive, depending on how the feature is implemented on your hardware.

Speaking of FWIW, for my E-stop setup, I have the error lines from each drive 'daisy chained' thru a bank of opto isolators  and then a single wire from the end of the chain to the Kflop E-stop input. This arrangment takes care of the voltage difference, the isolation, and multiplexing the inputs in one step in one device. If any drive faults, it breaks the chain and the Kflop gets the E-stop (think christmas tree lights). It is set up active OFF so a broken wire or power problem will also cause the E-stop.

Also consider recovery from the E-stop. Make sure that unexpected instant restarts are supressed (spindle primarily). Another difference between drives comes into play here again. My Mistubishi axis drives require a reset after a fault whereas my Copley Controls spindle drive need only a disable/enable cycle and they are ready to rock again.

372

General Mach Discussion / Re: New Build Suggestions

« on: September 25, 2013, 04:45:13 AM »

I kept the original gearing of 2.5 :1

Gears or rubber belts?

373

General Mach Discussion / Re: New Build Suggestions

« on: September 24, 2013, 07:10:01 AM »

Oversizing a servo can result in all sorts of problems including making the system un-tunable, defeating built in auto tuining and/or having the motors break into uncontrolled self exciting harmonics (given an elastic connection to the load).

Interesting, I found the exact opposite on my Bridgeport when I swapped it over to servos. I had some 1.5Kw motors and they were extremely easy to tune, in fact they were the first ones I have had any real success with the Auto Tuning in the Allen Bradley drives.

Would I be correct in guessing that you have a big bridgeport with dovetail slides and the orignal leadscrews directly coupled to big low RPM servo motors with nice heavy armatures?

374

General Mach Discussion / Re: How to handle servo faults

« on: September 24, 2013, 03:19:36 AM »

The emergency relay I am using is a timer relay (in addition to the other relays) I use this because I have a class 2 E-Stop set up and what happens is before the Enabels are taken away from my drives they get a fault signal which tells them to stop under power. This means they stop almost instantly and there is no chance of them coasting. Once stopped the enables are taken away, all under 0.5 seconds if I recall.

Hood

I agree with this method.


NOTES FOR NEWBEES:

Servo drives stop the motor by powering then in the opposite direction. Absent that power, the drive has no way to quickly stop the motor and the result is 'coasting'. If you are very lucky, there will be enough energy in the caps to stop the motor. If not, then you will have made a bad situation much worse by removing the power from the drives, in my opinion.

On first encounter with higher level drives it seems logical to assume that all of the features are there because you need to use them, but that is not the case. in any given application, most of the features will not be used. Only the handfull needed for that particular application. For example, it may seem logical that the drives own built in E-stop would be useful with a Mach setup, but it would be redundant at best and most likely counter productive.

If the drive is set up in a mode where you give it coordinate or vector and it proceedes on its own to the commanded location, then you need some way to 'break in' to that process if something goes wrong. Hence, the drives built in EMG (emergency stop) or equiv. This has no useful purpose in a MACH setup if you already have the drive error lines connected to MACH. On E-stop, MACH simply stops sending pulses to the drives and they will consequently 'stop instantly under power' which is what you want to happenn.

The bottom line is that you should choose ONE method. Internal to MACH as descreibed above, or external to Mach as described by Hood, or any number of other ways. The important thing is to pick ONE way and not have redundancy. Otherwise you will be pulling your hair out trying to track down false E-stops.

375

General Mach Discussion / Re: New Build Suggestions

« on: September 24, 2013, 02:46:30 AM »

In addition Servos have a big overhead for short periods, usually 2 to 3 times the constant torque.
 For  a Bridgeport sized machine I would think 750Watt AC servos would be more than adequate at 2:1 reduction, some people reckon even 400W would suffice although I am not so sure about that, suppose it depends what you will be doing and cutting  and what accel and Vel you are wanting.
 Hood

These days it is difficult to compare machines by 'size' so I suggest focusing on the weight (mass) of the load and not the size. For example, I just finished a servo conversion on the largest asian bench mill, namely the Industrial Hobbies mill with the 'oversize' table. While the table is close in length and width to the a bridgeport table, it is only half the weight. Another difference betewwn servos and steppers is that you can oversize a stepper without negative consequences. Oversizing a servo can result in all sorts of problems including making the system un-tunable, defeating built in auto tuining and/or having the motors break into uncontrolled self exciting harmonics (given an elastic connection to the load).

The IH conversion has 400 watt motors moving the 150lb table. An actual Bridgeport table is over 300lbs and, as you suggested, would require the 750watt motor to achieve similar performance.

On the IH conversion, the head was scratch built and weighed in at over 300lbs so the Z axis in fact has a 750 watt motor.

Mitsubishi publishes a software specifically for sizing their AC servo motors and that is what I use to spec the motor size for a particular application. It is a bit complicated and requires a lot of technical details about the application including things like the coeficient of friction of the sliding members and stuff like that, but one could also use a 'rule of thumb' approach and probably not get into too much trouble.  If one accepts that 400 watt is good for a 150lb table (with dovetails) and 750 watt is good for a 300 lb load (on ball slides), then it might be reasonable to extrapolate a rule of thumb like 1.25 to 1.35 watts per lb. of total mass to be moved (for a machine tool application).

Note that for calculating the horizontal movements of the table, the weight of the table (but not the mass) can be discounted. A 'live load' however shoudl be included since this is the actual typical operating condition. You might consider 1x the table weight as a reasonable WAG. So in our examples, the IH table would carry a 150lb load for a total mass of 300lbs to be moved. 400 watts / 300lbs = 1.33 watts per lb.  Similarly the 300lbs bridgeport table carries a 300 lbs live load (mill vice, 4th axis, fixtures, workpiece, etc) for a toal weight of 600lbs. Therefor 750watts /600lbs = 1.25 watts per lb.

The Z axis is a bit different in that ther is no live load . . other than the head itself . . which has 'weight' to be lifted in addition to the mass to be accellerated. So if the head is fully counterbalanced (air springs, aka struts, or equiv) Then just use the all up weight of the head as the load to be moved. If the head is not cojnterbalanced, then add the 'weight' of the head to the mass of the head and use that.

DISCLAIMER;   the proper calculation method for motor sizing are complicated and the 'rule of thunb' described above is not presented as the correct nor the preferred, nor accurate method to calculate servo motor size. It is a theoretical context without warranty express or implied  . . . use at your own risk . . .  batteries not included. 


   

376

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: September 24, 2013, 01:09:56 AM »

Ok here are some numbers
30 taper
Max RPM 6000
Cont torque 18Nm
Peak Torque 80Nm

Hood

Are these BT30 maximums or specs on your spindle?

Numbers indicate approx 11.5kw.  How does that compare with your spindle motor?

377

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: September 23, 2013, 03:22:02 PM »

Looks like its working well
I am not a fan of low voltage signalling and is one of the reasons I always shied away from the kFlop, that and also I am no programmer but looks like its working well for you
Regarding the programming, thats why I tend to use PLCs for things like this, ladder logic is easy for me, code is not.

Hood

I can think of a lot of reasons to shy away from the Kflop, but it is a very powerful board and given some programming experience, it can be made to do some impressive stuff. The biggest obstacle with the Kflop is lack of documentation. There is no programmers reference, etc. so the learning curve is practically infinite. I will say this for the Kflop; while it can be challenging to get it to do what you want, once you have it working, it is a rock. It just rolls on day after day without a hitch. Smoothstepper was always flakey and had 'moods', but the worst problem was that if there was an issue, it was pretty much permanent because the user had no way to fix it (regardless of programming knowledge) and the manuf was in no hurry (as in months, years) to correct anything.

To comment on 'low voltage signals' I think pretty much all modern processors are 5V or less, more recently 3.3v or less, so there is always going to be a conversion somewhere between the processor and the 'outside world' where signals need to run hotter to cope with massive intefference generators like high voltage wiring and electric motors and solenoids all over the place. Differential signals are actually low voltage if you compare them to the typical 24V industrial control signals. In any case, once you have the isolation and voltage conversions in place, it really becomes a non-issue and you don't even think about it.

There is still a lot of improvements and feature additions to be done to the ATC controller, but I went with a separate processor for the ATC.  The Kflop is primarily doing the motion control duties and the programming for that device is pretty much done . .  .  thank goodness.

I would liken 'programming' vs 'laddering' to your lathe having tool posts both in front of and behind the spindle. I suspect such an arrangement would be mind boggling to most people, but you have gortten used to the complexity over time and have learned how to make use of the capabilities that come along with that complexity. Likely you would feel quite handicapped if you had to make due with a single toolpost after having two. Programming is the same way . .  all that is needed is an inexhaustable supply of tollerance and patience . . . .   and a Whisky Sour . .  or two . . . 

378

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: September 23, 2013, 02:42:17 PM »

Question for spindle experts: Are there any specs on the various tapers which define the power or torque capacity of each size? I have been unable to locate this info.

The time has finally arrived for me to build my own mill. The IH conversion that I had been working on  . . . . for a while  . . . is completed and delivered and I have only a couple of projects to finish up and then I will be  . . at long last . .  turn my attenetion to my own project mill.

Previously the spindle and ATC was BT30. Thus far, I have built several spindles but only one ATC. The converted machine is completed and shown in this video changing tools and cutting steel and aluminum with a 3.5K spindle motor (close to 9k peak).

http://www.youtube.com/watch?v=k2F6100RKOE

For my own spindle, I am leaning toward BT40/CAT40 because my new spindle motor peaks at 15,200 watts and I think that is going to be too much for BT30. Since I have not found any specs, I started looking at the power ratings of available spindles and it seems like 5.5kw (unknown peak) or thereabout is the high end for BT30.

In addition, I have in mind to mount a tool carousel in the spindle to rotate tools toward the InTurn™ 4th axis (this will be in addition to the existing gang tool plates) and again I am feeling like BT30 might be a bit light for that application. Overall the new mill should be impressive. Built on a new cast iron frame, it will have massive ball slides and ball screws (all high precision ground and pre-loaded)on each axis with travels of 38" X, 26" Z and 14" Y. Each axis will be powered by a 750watt Mitsubishi AC servo motor and be capable of about 700IPM. ATC of course and like the previous project, it will have 5 axis capability. I am looking for repeatability and accuracy measure in tenths for this new mill.

The spindle will feature precision pair of angular contact bearings and labarynth seals with a positive pressure purge on the spindle cartidge this time around (9,000 + RPM spindle) and each ball screw will also have pairs of ABEC3 angular contact bearings. People have been asking for more detail and for some design data so I am planning to spend more time with the video camera on this build and show the actual design and fabrication of some of the parts and pieces, including the bearing blocks for the ball screws and some other interesting stuff. The IH conversion has a lot of this stuff, but I did not have time to cover any of the details in that build.

So, if anyone has some info on the capacities of the various taper sizes, please post them.  

379

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: August 20, 2013, 07:29:23 AM »

The ATC is now operational. Below is a quick video fragment that I grabbed before the machine ships out in a couple days. The fragment will eventually be part of a larger video covering the last stages of the mill conversion, but for now, you can get to it with the link.

This video fragment shows a tool CHANGE . . i.e. grabs and stores one tool and then selects another tool, retrieves it and puts it in the spindle. And no more 'smoke and mirrors' trick using MACH axis to drive the motors. What you see is being run entirely by the all new and completely autonomous ATC controller. This video does not show the spindle homing becuase that operation, while finished, was not linked into the tool change macro (M6). The macro now homes the spindle as the first task, then converts the tool numer to binary and presents that at MACH outputs and then signals the ATC controller that there is a tool change needed.

The ATC controller them retrieves the binary number from the MACH outputs and acknowledges to the macro that it will accomplish the tool change.
The ATC controller then operates independently from MACH, including generating the step.dir pulse stream for the drives, and at the end of the tool change, it the sends a 'Done' signal to MACH (to the waiting macro) .

It sounds a lot more simple that it actually is. There are about 1,200 lines of code so far in the all new ATC controller and it is using the newest (4 to 5 times faster) processor. The new processor is 3.3V so that eases the interface problems with the Kflop, which is also 3.3v . . .  mostly.

This scheme is a bit unusual, but it does not rely on Modbus or a plug-in and it will work with any CNC that has 6 available outputs . . . i.e. not tied to MACH. My 4th axis controller is already using the modbus and I decided not to try to have them (the 4th axis and ATC controllers) communicate with each other, nor did I want to attemp multiple simultaneous modbus devices.

In this way, the BT30 spindle and ATC are not tied to the 4th axis.

Also not shown in the video fragment, but now operational, are the two primary safety interlocks. The drawbar cannot release accidentally and the claw cannot release (read 'throw') a tool. Until electric solenoids are retracted, the drawbar actuator cannot move and the claw cannot open. The speed seen in the video fragment is something like 2/3 of the full speed. The claw interlock was not installed yet and I did not want tool holders launched across the shop. But rest assured, both the arm and the carousel can run much faster than shown here.

There are quite a few sensors to integrate (hardware is completed, just a matter of adding the sensors to the software), and of course. This was a particularly difficult project, but it is just the first iteration of the ATC. I have lots of other goodies to add like a dual arm, a much larger fixed carousel and perhaps even a tool belt.





http://youtu.be/QmlAgfIbscQ

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General Mach Discussion / Re: Check this out, I am about to be published in Digital Machinist!!!

« on: July 24, 2013, 07:19:34 AM »

Congrats Scott!

Not to take anything away from your tech prowess, but really I think you also qualify to be published in Digital Comedian. That's some funny $h1+ !!

Farm Animal Addition?   Too funny . . .

- Steve