Messages

361

General Mach Discussion / Re: Constant Velocity problem with rotary 4th Axis

« on: November 05, 2013, 08:23:30 AM »

You might investigate G93 to control feed rates for coordinated linear and rotary axis moves.

362

General Mach Discussion / Re: New Build Suggestions

« on: September 27, 2013, 06:07:31 PM »

Only times I had trouble with tuning a servo were when the servo was undersized.

'trouble' is not a useful description. Anyone who knows how to tune servos can certainly be more specific than that.

363

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

« on: September 27, 2013, 05:51:51 PM »

I really think it s more likely the springs are broken or weak as I do know they have never been replaced in at least the last 10 or so years.

Bellevilles loose tension over time (sometimes a failrly short time) that is a fact and is one of the disadvantages that I was looking to get away from with the coil. I agree with you and BR that most likely you will need a new stack, but you won't know that untill you get inside. If the gripper is coming off, you can check that easily  from the outside. It would be sort of a bitch if you tore the whole thing apart and then found out the gripper had unthreaded.

Dont really want to have to dismantle bearings etc but I have found a PDF from McBroom Industrial Services that mentions removing bearings   . . . .

After reading the scary tech stuff from the bearing manuf, I made sure that there is room behind the bearings to get a puller in there and reach the inside ring. Some AC bearings can be separated, but if the OD of the inside ring is sitting against a larger OD, you still can't get a puller on it, so you are left with heat/cold as the remaining option for getting it off undamaged.

Please post back here if you do dig into that spindle so that I will get notified. My new spindle project starts in a few weeks, but I may take it to a bigger forum since there is not very much participation here. In any case, I would want to follow your 're-springing' effort.

364

General Mach Discussion / Re: New Build Suggestions

« on: September 27, 2013, 04:30:01 AM »

In hindsight, I think a 'rule of thumb' is a bad idea for this topic, but I already stuck my foot in it, so this post is a final attempt to clarify some of it before I sign off the thread and hopefully it will prove genrally useful in selecting and purchasing Servos for new builds. 

UNDERSIZED:

*Stepper motors ratings and Servo motor ratings cannot be compared directly as they are different measurments made in different ways. For the same reason, industrial AC servo ratings cannot necessarily be directly compared to DC servo ratings, particularly the typical hobby level DC servos and drives.
  for example a Keling 90V 40A motor would theoretically be 90 x 40 = 3,600 watts peak. But paired with a Gecko drive which only puts out 20A max, that number gets cut in half. A typical linear power supply drops voltage as more power is drawn, so you will never see full voltage at full amps. Therefor take away for various losses and you might have 1,200 watts peak. Industrial AC servos are rated very conservatively and typically have several times their rated torque as a useable reserve.
 Bottom line is that a 400 watt industrial AC servo motos is roughly equivalent to the 3,600 watt hobby DC servo/Gecko combination . . . so direct comparisons are not meaningful.

* Given the vast difference in 'rated' vs real world performance, it should be clear that a 'rule of thumb' must be taken in context and not applied across the board. In the case of my rablings, they apply to Industrial AC servos.

OVERSIZED:

*Not all 'Auto Tuning' is created equal. Newer models can be vastly improved over previous drives and if you are buying 'whatever' brand used on Ebay for example, do not assume you will be getting the same level of capability as the setup you just read about on a foruim (unless you buy the same parts).
for example:  Mitsubishi J2S series servos are very susceptable to the harmonics issue that I described earlier (elastic connection of an oversized motor to a large load). The newer J3 series drives can measure the machine's resonance and has input filters and smoothing and can detect a variety of harmonic issues and compensate automatically.
 Bottom line is that the J3 is nearly immune to a common issue that will absolutely confound an earlier model drive. Don't assume that buying a 'Mistubishi' will erase a bunch of problems. Make sure you are getting the same model as are reading about.

*'Oversize' in the context of my 'rule of thumb' means that the motor falls outside the drives inertia ratio as published in the drive manual. This varies quite a bit between brands and between models and also with the types of motor.

*'Inability to tune' in conrtext means that the tuning can only be accomplished with the gain set very low, which is a typical bandaid for a host of problems. Prozac for Servo Drives. In particular this is one of the solutions to the harmonics problem I described. With the gain (responsiveness, frequency, etc) set low to overcome vibration issues, the overall performance envelope suffers and the super responsive 'stop-on-a-dime' behavior without overrun or underrun is not achievable.

*'defeating AutoTune' in context means that if the overall system responsiveness (gain, stiffness, etc) has to be set so far from the default (to counteract instability) that the auto-tune cannot achieve the performance as described above, then the Auto-Tuning has been defeated, in my opinion. 

Bottom line; tuning in general for ALL servo drives and Auto Tuning (which comes in m any flavors) are able to work withing a certain range of parameters. This 'window' is pretty much a guessing game with the hobby level drives, and you find out it won't work when it doesn't work  .  . usually after many frustrating hours of trial and error. However, on the commercial/industrial side, the envelope is well defined and published in the documentation.  Ultra low inertia motors are targeted at super-response applications like pick-and-place machines and similar ultra-resonsive, light-load robotic applications. Given a simiar ineria ratio, 'oversize' for these motors (moving the same load) would differ from the same size, same power 'standard' motor by a factor of 2.

That's a wrap and I'm outta here.



Either way, good luck with all projects and endeavors. That's a wrap. I'm outa here.

365

General Mach Discussion / Re: New Build Suggestions

« on: September 27, 2013, 03:24:34 AM »

This thread seems pretty much dead, but before I sign off, I want to mention a few caveats and clarify some terms in case there is confusion about the 'rule of thumb'.

*Stepper motors ratings and Servo motor ratings cannot be compared directly as they are different measurments made in different ways. For the same reason, industrial AC servo ratings cannot be directly comapred to DC servo ratings, particularly the typical hobby level DC servos and drives.

366

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

« on: September 27, 2013, 03:06:10 AM »

A scary possibility is that the gripper is unthreading itself from the end of the drawbar (assuming it is threaded on).

If that were the case, then it *might* be possible to screw the gripper back onto the drawbar using a conically shaped tool thru the spindle nose and into the gripper. A 'pinecone' shaped carbide burr with a long (6") shaft (as used in porting work on automotive engines) would do nicely. The gripper should be hardened (either finger or ball type) and if you turned the tool shaft with your fingers, no damage will occur and you might be able to feel whether the gripper was turning or not. Likely you would need to try with the drawbar set On and OFF and ideally somewhere in between if that is doable on your setup. Objective here is to assure that the gripper is not being used as a stop (or in some other way being forced against a surface internally) which would prevent it from turning.

In case you do find the gripper loose and are able to get it screwed back on, a 'Q'-tip saturated in GREEN locktite and touched to the end of the drawbar should get some of the stuff wicked up into the threads. I would place this stuff somewhere between blue and red in holding power.

In any case, good luck with the spindle. My bet would be that (if the spindle is a 'cartidge' arrangement) you can get to the drawbar and spring stack without disturbing the spindle bearings.

And you really don't want to disturb those guys. Here's why;

I have a precision option for the InTurn™ 4th axis now and my next mill spindle will have precision bearigns as well, so I have been spent a lot of time studying spindle designs, perusing white papers on vibration analysis and reading the precision bearing specs and handling proceedures as published by SKF, NTN, TIMKEN, and so on. Typically they say 'In the event you have to remove a bearing, it is best to replace it'.    $300 - $400 bearing . . . times 2 . . .   JUST replace it?   Yikes!  

Oh yes . .  and . . . the replacement bearings are slightly smaller than the OEM, so the spindle will need to be 'reground to achieve the correct fit.'  Gee, suddenly the bearing cost seems almost trivial. . .

How's this for a tollerance (EOM bearing supplier for Excello) Quote:  '  .0001" tight to .0001" loose '      . . . .  bizzarre.  Staying within two tenths is doable for me . . barely . .  but it takes a lot of time because I have to 'sneak up on it' as my machinist buddies used to say. Here is my special one-off hardened A series tool steel spindle. It is currently cut for a standard press fit to a ABEC3 deep grove ball, so there is meat on there to re-cut it for precision bearings. I just need to grab a couple while I'm out getting bread and milk . . . . .




 

367

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

« on: September 26, 2013, 11:07:00 AM »

I only have 400 lb-f . I do not have any specs on what it should be but I feel it should be higher.
See pic below, it shows almost 1200 PSI but I deliberately made my piston 1/3 inch square so that the value is trippled and thus the 1200PSI is actually only 400PSI.
Hood

I do not understand the math. 1/3" square piston is .11 SqIn.    

368

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

« on: September 26, 2013, 01:09:39 AM »

You did not say what your drawbar force measured out to be, but FWIW, in my early research on BT30 (I did not look at any other sizes at that time), what I unearthed was a dual spec for BT30 with significantly different tension specs above and below a certain RPM.

The tension specs were 600lbs and 1,000 lbs (above a certain RPM), but I don't recall the RPM break point. I *think* it was 6,000 RPM, so you might be OK as is.

In any case, one would imagine the drawbar having a threaded collar that bears against the spring pack . If so, then it would simply be a matter ot tightening that collar down a bit. Keep in mind that tightening a collar in this arrangement is also taking out travel, so it would be prudent to make sure (after such an adjustment) that the sping stack is not 'bottoming out' (binding) before the pull stud is completely released.

369

General Mach Discussion / Re: New Build Suggestions

« on: September 25, 2013, 07:16:51 PM »

Mr. Hood, sometimes you just get lucky, I guess. Probably your proximity to Ireland . . 

370

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

« on: September 25, 2013, 06:11:13 AM »

I think the challenge will be to find a planetary gear set that is compact enough and can handle that much power.

Speaking of power handling, it is encouraging to know that BT30 can handle the power that Chinon is pushing thru it. Doubling the power handling changes the landscape considerably. I would still like to find some hard engineering data on the capacities, but the Chinon specs are a good start and the drawbar tension chart you provided is priceless  . . especially considering the endless debates that took place on that topic a while ago. I run the BT30s at 1,000lbs  . . excuse me 1,000lb FORCE to keep the author happy . .     so I think that's in the ballpark.

Meanwhile, I have completed the new design for a BT40 spindle and located heavier springs (coil, not belleville) to meet the spec in your chart. I can see now where the bellevilles come into play. It looks as though I would not be able to go any larger than BT40 using an off-the shelf die spring.  Good to go with BT/CAT40 though so now I have a quandry as I have already started collecting BT30 tooling.

Its always something . . .