Messages

741

General Mach Discussion / Re: High speed spindle recommendations

« on: August 20, 2010, 05:38:13 PM »

"Timing belts are just not designed for such high speeds"   This statement is inaccurate. For those who prefer observation over engineering specs, note the black thing on the front a blown top fuel dragster engine  . . . 10,000 RPM 6" dia pulley . . do the math . . . or the primary drive of a custom Chopper (motorcycle), or the dry sump oil pump on indy cars or perhaps just the cam drive on many modern automobile engines. In machine tools, some surface grinders have 'timing belt' drives on the grinding wheels.

And . . .  all of these real world examples use the OLD trapezoid tooth profile. The newer curvilinear profiles are far better. Flat belts have been used for years for high speed applications like tool post grinders and so on because they are less prone to whipping like V belts which have a much larger tangential cross section. A 'timing belt' is similar to a flat belt in dynamics, except that there is no slippage, which coincidentally is actually where the heat comes from in v-belts, round belts and flat belts.

Round belts come in two types; elastic and corded. Example of elastic are vacuum cleaner belts. Belts with chords don't stretch (much) and you must follow the manuf specs for the type of belt . . specifically the type of chord.

In any case, no matter what belt drive setup is chosen, the amount of heat contributed by the belt is minuscule. The heat comes from the bearings.

742

General Mach Discussion / Re: High speed spindle recommendations

« on: August 20, 2010, 07:06:48 AM »

Round belts ('O'-ring, doughnut, etc) are very good for extreme speed, but I would be hesitant to use them where any swarf or cutting fluid could get on them. The newer tooth belts with curvilinear teeth (GT3, etc) run quiet and cool and have stupidly high power ratings for their size. i.e. you can use a belt that looks (to my old eyes, anyway) far too small to handle the power, but it clocks along for years even with exposure to contaminants. The older trapezoid teeth are a different matter and I don't recommend them for high speed.

The terms 'collet extender' 'collet adapter' and 'collet holder' seem to be used somewhat interchangeably. The item on PreciseBits appears to be 1/2" and not 20mm and also looks too short to use the shaft as an axle . . .  unless I missed something.

O1111 (off topic rant)  
Something that was impossible to miss on the PreciseBits site was the prominent blurb on the company's Christian roots and belief in equality of all people and in the US constitution . . . . . on the same page with a big red banner supporting the new Arizona immigration law which is being challenged as unconstitutional and which clearly presumes all Mexican people to be fundamentally inferior, unwelcome and having criminal intent. Arizona will undoubtedly next pass another new law bringing back the Inquisition.
M99 (End of Rant)
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743

General Mach Discussion / Re: RPM readout showing negative numbers . . suddenly . . . WTF??

« on: August 19, 2010, 07:07:06 PM »

My experience with the tuning is just my usual, alter a number and see what it does The real time scope in the software really helps with that.

Mistubishi has tuning software also with all manner of cool stuff. I have not played with it yet as I need to make up a special RS232 cable for which the special connectors for the drive just arrived yesterday. I followed your lead and converted to differential at the BOB. You know . . any excuse to make another little PC board. I'm still fascinated by that whole process . . . .  

744

General Mach Discussion / Re: RPM readout showing negative numbers . . suddenly . . . WTF??

« on: August 19, 2010, 06:59:14 PM »

FYI, the solution to the RPM readout is going to be a new computer. The problem developed rather suddenly and seems to be only with that particular computer. I re-loaded Mach from scratch to an empty folder and re configured from scratch . .  same thing.

Putting MACH on a couple of other computers did not duplicate the problem, so it seems to be a goblin that took up residence in the CNC computer. I've been around computers a long time and never seen anything quite like this. It could be a variety of things, but I'm not inclined to dig into it. New computer is the quick fix.

745

General Mach Discussion / Re: High speed spindle recommendations

« on: August 19, 2010, 06:51:13 PM »

My design process starts usually with what I have lying around or can get cheap    I had a  1/3 sheet of 19mm thick 5083 alu left from a job and the belt and pulleys so that's where I started my design. Not the best way I know but........

Actually, you have just described probably THE most common design criteria. GM used the same front suspension and steering in full size cars from I think 1955 or so until 1965 and then continued the same setup on the Corvette all the way to 1982. Somebody somewhere was definitely saying "use what we have!". I just finished a very large project that had the same criteria up front. It takes an act of congress to get approval for a new component.

The body was made of Alu and this may actually be the reason I feel the heat so quickly and as said above I really need to test a bit more as its possible that it ma not get much hotter with time, or is that a stupid thing to hope for?

Not at all. The aluminum getting hot is exactly what you want. That means it is drawing the heat out of the part. All you need to do then is take the heat off the aluminum, hence my comments about the exterior arrangement. Fins, fans or fluid, as they say.




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746

General Mach Discussion / Re: RPM readout showing negative numbers . . suddenly . . . WTF??

« on: August 19, 2010, 12:01:55 PM »

Yes AC servos are the mutts nuts, auto tuning is good but I find I can still get things sweeter with a further manual tune. Not sure of your drives software but the scope feature in mine makes tuning a lot easier, can monitor up to 4 things at a time although I usually only monitor the three.

With your experience, tweaking to perfection is an option. For me at this point, well . . .   not so sure 

Actually in my application, I could have a heavy chuck with a heavy workpiece one minute (and I am currently making a trunnion table as well) and then a piece of 1/2" 2011 in a 5C collet spinning 3,000 RPM the next, so tweaking, even if I was competent to do it (in anything approaching a reasonable amount of time), would be quite a task indeed. I have so far tried two AC servo drive series. The first auto tuned over a 'period of time'. Now I am working with the Mitsubishi J2S series and it tunes within maybe 6 moves. Pretty friggin' amazing really.

747

General Mach Discussion / Re: RPM readout showing negative numbers . . suddenly . . . WTF??

« on: August 19, 2010, 11:42:44 AM »

AS a rule you should keep a backup copy of the XML stored away somewhere safe (off of the computer) just in case you need it

Actually I have the entire drive 'ghosted' to a second drive in the machine, so I can get the XML right from the Mach3 dir on the backup drive. Thanks for all the good info!

PS:  YEP AC servos RULE (;-)

Not sure if I mentioned it, but ultimately the Granite Drive's 'Position Reached' signal turned out to be non functional. Something that they advertise, but does not actually work. However 'It might be a good idea for the next firmware version'. This MO unfortunately has proven to be typical for most if not all of the 'hobby level' drives as well as similarly positioned products like smoothstepper, etc.  Anyway, as I did with the 'swap axis' function, I implemented the needed feature in hardware. This time with a little 'sniffer' that snoops on the pulse stream and locks the 4th axis when the 'A' axis pulses stop for more than a (programmable) certain amount of time and then unlock it at the next pulse. This 'auto lock' can be enabled or disabled via 5V TTL from Mach and the lock can still be engaged via script or a manual button (for changing collets, etc). That's all wokring fine.

Now I have just installed the Mitsubishi J2S drive and it has a functional 'position reached' that even has a selectable proximity to trigger the signal. Works great running an led (so far)! Not used it with the lock yet, but will try that soon. It also has an externally variable torque limit so I could drop back on the torque during the lock if needed. Should be fun to dink around with it. The nice thing is that (so far) everything this drive is supposed to to, it does . .  a refreshing change.

748

General Mach Discussion / Re: High speed spindle recommendations

« on: August 19, 2010, 10:49:14 AM »

May be worth a try:

http://cgi.ebay.com/Water-Cooled-CNC-Spindle-Motor-1-5KW-2HP-Engraving-ER11-/170513456929?pt=BI_Tool_Work_Holding

749

General Mach Discussion / Re: RPM readout showing negative numbers . . suddenly . . . WTF??

« on: August 19, 2010, 09:38:41 AM »

IF you want to try something try reloading a backup XML, not the autosave version but an original copy.

That's a good suggestion. Is there a way to distinguish between an autosave and and original backup?

750

General Mach Discussion / Re: High speed spindle recommendations

« on: August 19, 2010, 09:33:41 AM »

There is an amusing engineering expression: 'A Camel is a horse designed by a committee'.

In engineering a mechanical assembly, you should start with objectives and parameters and then design to satisfy your targets. For example, you don't make a soup spoon out of titanium just because titanium is 'better'. Running hot is not automatically bad, either. Heat is actually beneficial in many cases. Just don't exceed the specs.

If there is an unlimited budget, then the smart move here it to simply plunk down the cash and purchase a high speed spindle, electric or air, as desired. However, the primary parameter here has been stated by the 'customer' as cost, so living with the tolerances offered by 'cost effective' solutions is simply a necessary part of this particular project. It is unrealistic to expect .000000000001" runout for 25 bucks. 

A good example is the heat issue. Aluminum is obviously better at getting rid of the heat, so you would want to make the body from aluminum . . .  but only if you can tolerate the accuracy loss generated by the clearance you would need to design into the bearings to accommodate the expansion difference between the aluminum casing and the steel shaft within. If you are using high clearance ball bearings, probably not a problem over 4" distance. On the other hand, if you have a precision angular contact set, the expansion difference will erase your pre load . . not good.

Bearings have a temp spec. Hybrid ceramics can take a lot more heat and are not nearly as expensive as full ceramic. Simply using a cutoff tool to make  . . oh, say 20 or so radial grooves .100" x.100" cut into the casing OD may provide adequate cooling for hybrid ceramics, which run hot, but thats perfectly OK.  Clamping aluminum mounting blocks right at the bearing sites will draw off a great deal of heat as well. Then you could get fancy and cut cooling fins in the mounting blocks . . etc, etc.

A left a question unanswered some time back, so here is the answer: For my previous little X2 mill, in order to run the spindle at 7,500 RPM, I simply attached an old Pentium 3 aluminum heat sink to the side of the head. That was enough to keep the beargins in spec. Low tech, cheap, ugly, but completely effective.