Motors Minimum RPM ( wrt rigid tapping )

Wow, Spumco, Jim and Tom, again thanks for sharing your knowledge. Spumco, I'll pm you today about the encoder!

So, as far as the spindle bearings, this is what I have already upgraded my spindle with, supposedly 15K plus on both :

qty 1 - VXB Brand 7207C P5 High Precision Angular Contact Bearing 35x72x17 ABEC-5
qty 1 -VXB Brand 7206C-P5 7206C P5 High Precision Angular Contact Bearing 30x62x16 ABEC-5

Here is a link to my thread for more specifics on my mill project but by now you know most of everything about it already :)

https://www.hobby-machinist.com/threads/my-pm-940-cnc-modifications.57976/

I have moved from the Mesa 7i76e / LinuxCNC to Acorn and I have to say, I'm really loving it comparatively but haven't made nearly enough chips on either to be an authority on it in action, but as far as simplicity and peace of mind, the Acorn has been really a night and day improvement already. I have stopped hovering over the E-stop button and the thought of installing a pre-emptive kernel in linux again is really daunting. There is a great group of guys that helps people install linux CNC on Mint but it really is a difficult chore for some configurations, like mine. The Mesa hardware is very very nice, but the documentation is, well, lets just say, Linux Like, a bit over my head... by a lot, actually :)

Regarding parameter 36 using bit 4, thats a nice find. It says high range is required, however. So far as I can tell, the option to change "gear" ratio, is only available on the medium and low range settings. High gear is 1:1 and cannot be altered, but I should verify that by looking further in the manual. So far as I can tell, high gear is the "reference" for the medium and low gear ratio to be set as a ratio of it.
[edit : Parameter 33 allows this change, HOWEVER, the Centroid Acorn Wizard application that has this option greyed out and a stern warning is in Parameter 33 to not make a change to this unless you know what you are doing. I suppose to ensure runaway doesn't occur in a servo driven drive situation... which would include encoder driven too ? just an assumption ]

By the way, sadly it appears my ratio of 2:1 is actually 1.9xx or something / thereabouts. I guess I should have expected it wouldn't be perfect, its a MicroV belt and two different brand pulleys I noticed the rpm on the VFD itself showing noticeably more than 1/2 the rpm than what the spindle encoder was showing. So my pulley sizes are bit off a bit. If I go 1:1, however, the pulleys are from the same manufacturer. Hopefully tolerances are good and no slippage occurs. One of the advantages of the belt system is that if I ever crash an end mill, hopefully the belt will fail instead of doing excessive harm. Its a 10 groove. The relatively small diameter of the pulleys is a bit of a concern because they may not have adequate surface area to avoid slippage in normal use. I try not put excessive side load on the spindle, which was my main concern using a pulley and belt drive system but the part of the spindle shaft I'm using is pretty beefy at 28mm. I may try to implement a top bearing to help carry the side load. For now, it seems to be ok.

spumco, I'll pm you about the encoder. Thanks you. Perhaps a typo. You are now using a 100 ppr encoder ? or 1024 ppr ? I'll also check the voltage of the Acorns 0 to 10v signal and try the 5v index signal test too and reply in the next day or two with results.

Cheers,

Jake
 
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cut2cut said:
One of the advantages of the belt system is that if I ever crash an end mill, hopefully the belt will fail instead of doing excessive harm.
Click to expand...

No no no no no. NO. Bad Jake.

Do not try rigid tapping with a v-belt, micro-rib or not. Tiny bit of slip and you're toast on retract.

Get or make a timing pulley set. You're going to a lot of trouble getting the spindle & motor and controller synchronized, don't cripple it all with low-efficiency belts prone to slipping. You can buy them pretty cheap and use a tapered bushing system to mount them on your spindle and motor shaft so they use the same series bushing/pulleys. They also put less side-load on the upper spindle bearings.

Jake, you've got champagne spindle bearings, Gucci controller, and a Bugatti spindle motor. Don't use Bud Light pulleys. I know there are tons of folks using V-belts and micro-rib pulleys successfully. But just don't.

If you crash an end mill, that's why the VFD has an overload protection circuit. Besides, crashes are usually side-loading the spindle, not over-torquing the end mill in a rotational way. Mostly. If you plan on crashing, use carbide end mills so they just snap off and not bend and beat the heck out of everything.

Don't compromise the spindle drive mechanics just 'cause you might dig a flycutter in with an M3 S2500 G0 Z-1.0 oops.
 
I installed a timing belt on my mill spindle drive in anticipation of rigid tapping. I also went from a 1:1 micro-V belt drive to a 1.5:1 overdrive on the new drive. I'm using a 8mm pitch HTD system.
 
spumco said:
No no no no no. NO. Bad Jake.

Do not try rigid tapping with a v-belt, micro-rib or not. Tiny bit of slip and you're toast on retract.

Get or make a timing pulley set. You're going to a lot of trouble getting the spindle & motor and controller synchronized, don't cripple it all with low-efficiency belts prone to slipping. You can buy them pretty cheap and use a tapered bushing system to mount them on your spindle and motor shaft so they use the same series bushing/pulleys. They also put less side-load on the upper spindle bearings.

Jake, you've got champagne spindle bearings, Gucci controller, and a Bugatti spindle motor. Don't use Bud Light pulleys. I know there are tons of folks using V-belts and micro-rib pulleys successfully. But just don't.

If you crash an end mill, that's why the VFD has an overload protection circuit. Besides, crashes are usually side-loading the spindle, not over-torquing the end mill in a rotational way. Mostly. If you plan on crashing, use carbide end mills so they just snap off and not bend and beat the heck out of everything.

Don't compromise the spindle drive mechanics just 'cause you might dig a flycutter in with an M3 S2500 G0 Z-1.0 oops.
Click to expand...

But.... but.... but..... Bud light is the only beer I can drink as its easier on the gut ! :) Ok, ok, Bad Jake... lol Ok, but the damn timing belts are not nearly as quiet :-( bummer... well, I can enclose them anyway where the gearbox used to be. :)

so the "learning" never stops. I even was making my own J profile pulleys on my HF lathe.... it was fun though. Damn lathes scare me a little.... all that spinning mass.

Ok, onto plan B. Haas here we come. Just joking!

Cheers,

Jake
 
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It occurred to me that, in a 1:1 *timed* :) pulley arrangement, that using a single encoder to drive both the VFD and a control card would be an interesting way to negate any "tug of war" that might occur between the VFD and the CNC contol card. I don't know if its technically possible, but maybe something to consider ? Guessing but the negative attribute might be the slack in the timing belt and also the speed each device processes the encoder signal.

Jake
 
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So, I looked int he Centroid manual with regard to changing the high gear ratio. In the Acorn Wizard application ( a little separate GUI app for setting parameters ) The option to change the standard high gear ratio is "greyed" out. However, ( I am not at the mill currently, to check ) my guess is, as outlined int he manual, Parameter 33 can be changed via opening the parameter configuration panel within Centroid CNC itself ( not the Wizard ). A stern warning is included in the manual to take care when making a change to this. I suppose in a closed loop scenario, a runaway could occur. And technically, this is closed loop, so if the encoder is enabled to maintain a certain speed and the ratio is off by more than a little bit, it could runaway ?

Jake
 
It's not going to run away. It's just going to command and report one speed, and the actual spindle speed will be different. If there's no encoder signal on the spindle, then there's no feedback to the Acorn which might cause it to go crazy.

You can check it with a cheapo laser tachometer from Amazon on the spindle itself.

cut2cut said:
using a single encoder to drive both the VFD and a control card would be an interesting way to negate any "tug of war" that might occur between the VFD and the CNC contol card
Click to expand...

That's how mine is set up:

- Encoder on motor only
- 1:1.56 timing pulley arrangement
- Encoder A/B/Z +/- to terminal block.
- Terminal block A+/B+/Z+ to VFD
- Terminal block A-/B- to controller BOB encoder input pins
- Spindle proximity sensor to controller BOB encoder index pin

There's no conflict between the controller PID and the VFD. If there is, it's not noticeable and is only little 'nudges'. It holds +/- 1RPM unloaded and +/- 10RPM under load unless I bog it hard with a huge facemill or drill. Commanded speed is within 50RPM of actual spindle RPM, but it's a bit further off at the low (<400RPM) and high end (>7kRPM).
 
I don't have a ton to add to this conversation, but this is excellent tech! I will be watching this like a hawk...

Only constructive thing I have to say is watch your motor temps at slow speeds. I've seen quite a few motors cooked from running them too slow, that was in an industrial setting though where they were racking up hours unlike a hobby mill. I wonder if you can fit the externally powered fan from your old motor to your new one?? Might be worth a shot, it's louder than a flock of geese flying into a windmill, but it will keep the motor from cooking. Adding a simple temperature switch would eliminate the noise 90% of the time anyway.

Looking forward to more!

PZ
 
That Marathon is an inverter duty motor; 100% duty cycle @ 100F. Those motors are rated at 1.5HP, but they're built (and weigh) like a typical 5HP 'farm duty' motor. He isn't going to hurt it.

My 2HP Marathon weighs about 75lbs.

It's not like he's some clown at the plant smoking a motor by fiddling with the the VFD on a 24/7 pump 'cause he can't be bothered to buy a smaller impeller.

Besides, that motor has a thermal fuse in it that'll blow before he cooks the windings.
 
She's a stout motor. I've actually put my hand on it regularly to see if she is getting warm. I'll admit I haven't pushed it hard but it barely gets warm.... on the "can". So far so good but I have to say its surprising that it has no vent holes. Good to know if has an internal thermal fuse though.... but I certainly am not the type to push the limits on something so heavy :) If anyone is interested in the motor, the best deal is here at the link below, but the shipping is on the order of $100, at least to my location. For some unknown reason the 1.5 hp version is less than the others, even smaller ones. I suppose they accidentally built too many of this model. I think I paid around $100 less last year.
the fact it has a nice 1024ppr encoder is really quite amazing.
http://dealerselectric.com/145THTR15540.asp
Cheers,

Jake
 
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