Selecting motors for the MechMate - some history
 

Our first MechMate has Oriental Motor Vexta-Step type PK299-01AA motors, directly driving the pinion gears to the racks, without any gear reduction. This is a relative low-cost, low risk option - I know it well, it works.

However, because Mach3 can pulse at higher frequencies, you can use a geared motor for better cut quality. In this case, ShopBot did suggest the standard PK296A1A-SG3.6 as being a close equivalent of the geared motors specially built for them. (reference). Better cut quality, higher price, and the risk that I don't know what I'm talking about http://www.mechmate.com/Forum/clipart/happy.gif. But, I have designed the MechMate so that this geared motor *should* screw straight on. Most ShopBots use the similar motor & gearing, but they were handicapped by poor drivers - now that the Gecko-type drivers are freely available, folk are squeezing amazing performance from these motors. This could be my first choice today.....? (Edit on 2 Feb 2007: Instead of a 3.6 ratio gearbox, I would now lean towards a 7.2 ratio gearbox instead. It is becoming easier to supply the step frequency required by the 7.2 gearbox, and Doc Tanner says it works well. He is using the PK296A1A-SG7.2 motor) (Edit on 9 Sept '07: The PK296A1A in series connection does not match to the G203V drive because the inductance is too high. If this motor/drive combination is used, it must be wired half-coil. The better motor choice for the G203V drive is the PK296A2A motor with gearbox)

There are motors within the Oriental Motor range, with the same frame size, than can produce more torque at higher speeds, consuming bigger currents, but I am a bit concerned that the higher torque makes them run "rougher", the higher current makes them hotter and that cut quality will suffer. Art Fenerty made an interesting comment on his forum recently about maximum torque for a stepper motor for a CNC router. (link). I had some PK299-F4.5A motors that I wanted to test sometime. (but Alan Conelly has them now)

Oriental Motor have worldwide representation. Their USA division has on-line shopping with public prices at these pages (page 1, page 2). The ungeared PK299-01AA is $205, and the PK296A1A-SG3.6 is $257 on that list. (Some of the international offices of Oriental Motor have differences in their model ranges and numbers) The MechMate needs a quantity of 4 identical motors.

Having sung the praises of Oriental Motor, it must be recognised that stepper motors (not talking of gearboxes here) are not high-tech devices and many other manufacturers will probably offer comparable quality at a wide range of prices. (if the motor is with a gearbox, then I would probably stick to the OM brand) When looking for alternative motors to those suggested above, the following need to be taken into account:

The shafts need to be at least 12mm [0.5"] in diameter to carry the spring load and not bigger than 13mm so that a small pinion gear can still be fitted. The shafts must be at least 32mm long to reach the rack, but not longer than 40mm otherwise they will collide with the rail supports. The motors must fit on the MechMate's mounting plates. They must be compatible to Gecko G202 drives

Added 22 March '07: The motor bodies should be of the modern square style (heavier, contains more iron, better for micro-stepping)

For the really forward-looking folk, consider motors with shaft extensions to the rear for encoders in the future, or consider that stepper motors may give way to servo motors. . . . .

Pretty good info on this topic. Check it out. http://www.cnczone.com/forums/showthread.php?t=52090

I am building a 4X8 router along the same lines a MechMate. With the proper gear, would the PK296A1A-SG7.2 be a viable replacement for the PK296A1A-SG3.6?

Apparently yes, but I am not so sure about its internal backlash. The 3.6:1 has only one pair of mating teeth (as far as I know) and this must have tiny backlash. The 7.2:1 might have 2 pairs of mating teeth with twice as much backlash?

Some people recently started using 7.2 gearboxes on ShopBots and they rave about them, but it is early days yet...

According to Oriental, the 7.2 has less backlash. It is still a single stage gear reducer. The standard gearbox sold on orientals site is a spur gear reducer, which can produce from 60 to 90 arc minutes backlash, the Tapered Hob is 15 arc minutes. This is all according to Oriental, others claim it is a very tight reducer. It may loosen up after running a while. I noticed John Forney had one of the original spur Gear reducers and I checked it and didn't notice any play.
I would choose the Pk296A2A over the PK296A1A. The torque curve is much better.

http://www.mechmate.com/Forum/messages/15/1909.jpg

The following is the torque curves, notice the B designation is just for a double ended shaft

http://www.mechmate.com/Forum/messages/15/1910.jpg


http://www.mechmate.com/Forum/messages/15/1911.jpg

Dirk, I wish I was within telephone distance of an Oriental Motor product specialist. The questions I would like to put to them are:

1. PK296A2A verus the PK296A1A. Are the motors in fact different? Isn't the 1 amp version just a labeled down 2 Amp motor? Who would be buying the 1 Amp? (presumably because of driver limitation - but surely the 2 Amp could be under-driven). Is the 1 Amp "smoother"? I can see the one has thinner wire in the stator coils (more resistance/inductance) what is the overall effect of this? (smoothness, detent, heating, etc.)

2. The "tapered hob" issue: A hob is a tool for cutting a gear, why is Oriental practically the only company in the world that makes an issue of the tool with which the gear is cut? Its like saying my house was built with a 2lb claw hammer - why is that supposed to make a difference?

Gerald
Your questions are valid. I think the motors are different. Running the higher amp motors increases cost both on the driver and the power supply (If you mounted the 3 amp motors on your machine you would need a 600 VA transformer and 2 more caps because the max voltage would be around 35 volts).
Some applications don't need the high-speed torque. I would think under driving the higher amp motor would kill the low speed torque. So my opinion is they offer several motors so it will fit the application and not create unnecessary expense. If there?s more to it (or less) I don?t know.

I have no idea as far as the tapered hob issue. They don't publish the spur gear spec, and I think the rep was guessing when I asked him. It would be interesting to break down both reducers and see what the real difference is.

On a side note, I?m still curious as to Mike?s theory that torque has as much to do with smoothness of cut as resolution. I?m mounting my 1300oz motors on my PR direct drive to see how it will cut. May be interesting.
Dirk

Be aware that the PK296A2A-SG motors are rated at 1.4V (unipolar) while the PK292-02AA motors are rated at 3V (unipolar). I didn't notice that difference until I touched a geared motor that I was running at about 70V. It was HOT! Reducing the voltage to about 30V works well and is within the 25X limit suggested by Gecko.

I'm still thinking that torque of the motor, while in standby mode, is very important when running a multi-axis machine. Using a 3:1 gearbox on my Alpha greatly improved the cuts. Since the resolution of the machine without a gearbox should have been 3.14/1000 = 0.00314 inch per step, it should have been adequate for cutting wood with minimal chatter. (The offending chatter marks were certainly much larger than 0.00314 inches!) My theory is that 3X the standby torque keeps the non-moving axes from being pushed around by the active axis.

The "taper hob gear" is actually a "taper gear". From their page:


I suppose the pitch diameter is an average across the width of the gear.

Hey guys, I'm new here, and I'm a little puzzled. Are the Steppers connected directly to the Rack&Pinion drives, or should I use one with a gearbox on it.???

Welcome Kim! The first post at the top of this thread is still applicable.

Hi Gerald should this stepper here be just as good as the Oriental stepper...

www.motionking.com

Motor Type
34HS5804

Kim, that is a tricky question. It might be as good, but I have no experience of it. You have selected a much bigger size than what I use, and I am a bit concerned that this may give problems such as rougher cuts.

Gerald.. Could you direct me to the model that is closest to the Oriental stepper..???
The Steppers you use is also Nema34 right..???

KM

From the first post on this thread, the motor that I am experienced with is the Oriental PK299-01AA. From the link, it is 85mm square, 96mm long, the mounting screw holes are 69.6mm square pattern, housing spigot 73mm diam by 2mm deep. (The spec does not say Nema34 exactly).

Physically, the MotionKing nearest this size is the 34HS980* group. The mechanical interface (Nema34?) is identical. (Nema is an American standard, not everyone uses it)

The holding torque is nearly identical at 430 to 450 N.cm.

Electrically, the nearest MotionKing is the 34HS9803 with 2.7A rated current, but after this point I have no idea how to pick an equivalent motor...http://www.mechmate.com/Forum/clipart/sad.gif

ok. Maybe motionking can give me an answer if they compare them.. The bigger motor there I could get for only $57 a piece plus postage and packaging... But still very cheap for that big a motor...

That is an excellent price!

The Chinese stepper motors seem to be okay in general. If they were bad, there would be a lot of people complaining, but I don't see complaints about their stepper motors. If you open a stepper motor (some people say you are crazy to do this) you can see that it is really a stupid/simple device. The magnets need to be a good quality, but China is doing good magnets....

Gerald:
regarding these, (or any) motors for the MechMate. Do you recommend 4 wire or 8 wire motors? and why?
Do you think an 8 wire motor affords the user more flexibility in setting up his router?

Regards,
Bob C.

Bob, 8 wire obviously gives more options than 4 or 6 wire, but I am still at a loss to understand whether we should be using unipolar, bipolar series or bipolar parallel wiring. Here is an exchange on the Gecko forum about 2 months ago....

Me:
Mariss, could you enlighten us why that Nanotec wiring diagram says:
8 lead wire parallel for high freqency > 1kHz, and
8 lead wire series for low freqency < 1kHz?
For CNC machines, the motor will spend most of its life in the low frequency range, no? I have difficulty deciding when to use series or parallel - the rule always seemed to me to use the parallel if the drive could cope, otherwise reduce current with series. Now there is another selection criteria....?
Thanks

Mariss:
You have "two motors in one" when you have an 8-wire motor.

1) "Both" motors have the same low-speed torque.

2) The series connection phase current is 1/2 the parallel connected
phase current and runs out of low-speed torque at 1/2 the speed of the
parallel connected motor for the same supply voltage.

3) The series connected motor runs cooler than a parallel connected
motor for the same supply voltage.

4) Use a series connection and/or a low power supply voltage for
low-speed applications. Use a parallel connection and/or a high power
supply voltage for high-speed applications.
Mariss

Me:
With a CNC router table, which can use the full speed range of the
stepper motor, what are the signs (or effects) that tell you it might
be wiser to use series rather than parallel? Only motor heating?
Roughness of cut? Etc. My impression is that parallel should be the
first choice if the driver can handle it, but then to drop to series
if the motor gets too hot. Is this simplistic logic faulty?

Another way of looking at it is to leave the motor parallel, but
change the resistor to limit the current at a lower value?
Thanks

Those questions are still in my mind...

Bob,
The GeckoDrive G202 - Step Motor drive that Gerald specs, doesn't care how many wire come with the motor. http://www.geckodrive.com/photos/Step_motor_basics.pdf
Ton's of good info there. I have had good luck using the http://catalog.orientalmotor.com/item/stepping-motors-/pk-series-stepping-motors /pk296a1a-sg7-2?&plpver=11&origin=keyword&by=prod&filter=0

Doc, did you try the two different wiring options (unipolar & bipolar) and would you recommend one version over the other? My experimentation in this area didn't give me a conclusive result.

I set mine up as 1 amp at 70volts and was so impressed by the power, speed and cut quality that I didn't even think of changing it!
Drives and motors produce no noticible heat. If the leds weren't lit up, you wouldn't know it was on.
When it gets back from the powdercoater tomorrow and I get it back together, I will give it a try.
I have an ncpod sitting on the desk, was thinking of giving it a try. But I'm not sure how it could be better! http://www.mechmate.com/Forum/clipart/happy.gif

Very good to have a satisfied customer! http://www.mechmate.com/Forum/clipart/happy.gif

But, in the interest of science, it would be interesting to get a definitive answer on the best way wire the motors. Would appreciate you experimenting a bit. (Remember to change the current limit resistors accordingly).

The Motionking discussion has been moved here.

Those who are actively pursuing this MotionKing option must consider the following:

1. Those motors have never been tested on a MechMate. They should work, but we don't know for sure.

2. Those motors are without gearboxes. They should work as well as my setup. But, the evidence is that my setup could work a lot better if I had gearboxes. Those geared motors mentioned at the start of this thread appear to be a much better (albeit more expensive) option.

3. If the Motionkings don't work, then please don't say you got bad advice from MechMate. I would love to be able to endorse a cheaper option, but I have to be cautious.

Gerald...

Of course youo won't be held responsible. This is just a way for me to cut cost a little, and I'm wiolling to give it a try, even though I have been advised not to... One has to learn from ones mistakes right. But then again, I could get lucky and maybe end up with a motor that works fine at one quarter the cost of an Oriental motor...

Kim, I would be very happy if you tested those motors and told us all about it. I am just a bit nervous about those folk already talking of bulk buying the experimental stuff.

I would never bulk buy something I haven't tested. But I would however buy them for my self only. I'm also not keen on being the fall guy if everyone suddenly didn't get what they wanted from these motors.

And on another note, if the motor seems to run a little uneven, the I'm going to build my own gearbox for it with belt pulleys and high tension belts.

I have made an edit to the first post in this thread - see the bit in red.

To summarise the choices of motors....

1. Best cut quality (highest step frequency) will be obtained with the most expensive $257/motor option from Oriental Motor. (The backlash factor of these gearboxes is not discussed much - that is a bit of an unknown)

2. A commercially acceptable cut quality (smoothed with a few wipes of sandpaper if necessary) could be obtained with direct drive motors from about $50 to $110 each (Chinese) or $200 (Japanese). (Backlash is not a factor with this option. I have years of experience with this setup and 99% of our customers are happy with the cut quality)

A lots depends on where you live and what the shipping charges will be to your destination.

Hey guys, I believe I'm reading the Motor specs right, and the PK296A1A-SG7.2 motor is 4.4 volts?

So I need to build a larger power supply than the example shown under power supply in order to have the recomended 25x voltage, right?