Selecting motors for the MechMate
 

There has been some evolution on this topic. A number of posts in this thread were edited and/or deleted Nov 14, 2008. The earlier version of the thread is archived here.

Today's (Nov 14, 2008) first choice in motor is a geared stepper motor from Oriental Motor Corporation available as a simple secure on-line purchase at around US$250.
It is the Vexta-Step type PK296A2A-SG7.2 :

According to Oriental, the 7.2:1 gearbox has less backlash than the 3.6:1. It is still a single stage gear reducer. The standard gearbox sold on Oriental's 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.



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

That "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?

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.

The "taper hob gear" is actually a "taper gear". From a newsletter:




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

http://www.xylotex.com/4AxSysKit-425.htm
can you use this system for the mechmate?
why, why not

Sorry, too small and underpowered.

Loren,

For a very good motor from an excellent supplier between $69-$89 follow this link. http://www.kelinginc.net/

I have been using this model KL34H280-55-4A on my current cnc machine http://accurate-inspections.com/pr%20cnc%20upgrade/ for about six months with excellent results.

These are not geared so you will have to go the gear box route for better resolution, but even at 1-1 I have done some very nice v-carving with these.

Tom

Generally, current relates to torque and voltage relates to speed. For a given motor, the more current that it has available (up to the maximum rating for the drive) the more work it can do. And in the same way, the more voltage available to a motor (up to the maximum allowed by either the stepper driver or the maximum recommended by Gecko, i.e. 20X or 25X of the rated voltage), the faster the maximum speed will be.

Recent tests on my test bench have shown that a moderately sized motor (PK296B2A-SG3.6) can be spun faster than a larger motor, so, if gearing is available, the PK296 (300 oz*in) or PK299 (600 oz*in) seem ideal for the job. Even without gearing the PK299 size seems to be a very good match.

Hi,
New member here trying to gain some knowledge and insight on building a CNC and converting 2 convential machines o CNC. My question is what size motor would you think I would need for a 9" South Bend Lathe and a small Enco knee mill? I'm not well vrsed in electronics so this is pretty much all new to me. Also is there a means of compensating for the backlash in the lead screws without going to ball screws?
Thanks to all you really got an interesting site here and I hope to learn a lot from it.

Hi Glenn, I'm sorry to tell you that we specifically want to cover only "big" CNC routers on this site.

However, having looked at your profile, I feel jealousy (you have a Multicam) and sympathy (you are a schoolteacher(I think)).....

Non-CNC routers are tools that need 2 hands to push around. Small non-CNC lathes and mills can be operated with one hand. Therefore you are needing smaller motors to replace the humans for small lathes/mills than for routers. (Big lathes and mills are a different story). Where we are using NEMA 34's for routers, you should be okay with NEMA 23.

www.cnczone.com would really be the right place for you to get all your answers. Good luck.

Hi Gerald,
Sorry to tell ya, you ain't getting rid of me that easy. LOL I have a second Multicam coming in the next school year (yes I am a school teacher and I appreciate your sympathy). In addition to converting the two machined above I intend to build a mechmate for home. I downloaded your plans and haven't had time to fully review them, but what I have seen and read really look great.
My biggest weakness in in the setup and understanding of the motors and the controls, but have been getting a better understanding from sites like yours.
Glenn

AHHHHHHHHHHH!!!! whew, i feel better now. All this stepper vs servo stuff is making me go bananas. After i posted the above message, i read the servo vs stepper thread. Ok, I need to know a little more before committing to a motor.

How much hand finishing would be required on a circle cut using a mechmate with properly sized and configured stepper motors?

I am believing that a geared stepper motor can hold the stationary axis stationary. Is this correct?

Ok, originally i was typing the desired accuracies of the machine, then i divided out the fraction and realised that even the most basic control system will be more than accurate enough.

So I guess as long as i'm not gonna hafta spend an hour filing or sanding cut parts, it's steppers and little green lizards for me!

Hi Frank

I was hoping that somebody else would chime in here since you all must be very tired of hearing my biased opinions.......http://www.mechmate.com/Forum/clipart/happy.gif

To answer the easy question first; We have un-geared steppers on two tables and we don't have problems holding the stationary axis stationary. My impression is that the people who have reported problems in this area are using Oriental Motors Alpha controller's with feedback from the motors, and that these motors "hunt" to hold a fixed position (The I in PID?).

The tricky question, "how much sanding on circles". It won't help you if I honestly replied that we never sand any circles, because we simply don't do any post-cutting finishing service. Our clients do that themselves and we don't compete on their turf. The few times that I have made things for myself, and have wanted to remove the chatter marks that occur on parts of circles only, I reckon that it took about 10 strokes with 80grit paper?? Can anyone else give a better answer to this valid question?

Realising that we have direct drive motors, the general client feedback is that they are happy with MDF and softwood cut for them, but that they want better for acrylic plastic (perspex). If we had gearboxes, we could probably also satisfy the plastics guys.

Hi,

Here is my 2 cents.
I don't really know how much "mill mark" there will be when cutting circles using ungeared motors since I haven't used them yet so in that respect I can't comment.

How about someone posting some pics with wood and plastic?

I do know this, if was to cut circles say with my bandsaw, I would have a considerable amount of sanding with a power sander then alot of hand sanding. If all I have to do is sand 10 strokes with 80 grit sand paper, I will be most pleased. I have purchased the ungeared motors from Oriental (as recomended) and will using them. I will play around with them when I get them moving. I would think that no hand held or hand propelled power tool would be able to hold any kind of smooth transition through a radius. I don't know if this is a better answer but I will take a router cut over my bandsaw any day much less my jig saw.

Guys, thanks for the info. I agree with the notion that 10 strokes of sandpaper is nothing, I always dress cuts anyway. And Gerald, your opinions are what got me thinking i could build one of these things. And the guys in this forum reaffirm that. So anyhoo, I guess now I'm on the hunt for some motors. Any thought on the gecko drive? 202 v 203. Can't figure out what the differences are from the blurbs on there site. I'll keep reading and can't wait to start posting pics.

Hi all, Gerald.,

Can?t seem to find posts on the MechMate speed ( ipm) subject.
I?d like to know more on the possibilities of its cutting & transient speed capabilities of a tipical MechMate set up. Don?t get me wrong here, I?m not hopping to get anything like this ?Tim Allen type? worthless supersonic speed !!
No?no, just curious on what to expect and hopping to find out more & the how on the ipm achievable?
Thanks, Robert http://www.mechmate.com/Forum/clipart/happy.gif

Robert,
A search of (ips) will find the topics.
Never have been sure which to quote - IPS or IPM

I use 7.2 reduction and a 35 tooth pinion.
It will surface the spoilboard at 400 ipm
At 575 ipm it loses steps.
The Mechmate is able to cut much faster than I am comfortable with.

DocTanner

Thanks doc,
It?s what I was hopping to get!! http://www.mechmate.com/Forum/clipart/happy.gif
Later, Robert

This motor
 

Is this motor enough to drive the Mechmate... it's rated at 3.33Nm so it should be strong enough, or am I missing something...
http://motioncontrolproducts.co.uk/p...products_id/83

KM

Kim, the smallest one is 3.33Nm which is a bit light. You should be looking at FL86STH80-4208 at 4.51Nm minimum. They are calling it Nema 23, but the sizes look like the Nema 34?

Frank, if you are still reading this, you may want to look here for pics of various cut qualities with different machines and speeds. We normally get much better than the best of those with the MechMate.

Thx Gerald....

I will go with the 4.51Nm version then, it's only £42 a piece, which is very cheap... Maybe I'll even go with the 6.5Nm version to get bigger torque... And possibly more control of the gantry...

I don't know what the shipping costs are like, but it might be worthwhile to buy only one of each now and test them. Put the "worst" motor on the z-axis where it doesn't work so hard, and get the better motors for the other axes. :)

Doc
Have you any further comment on your setup with the 7.2 geared motors? Is the 1A sufficient or do you see the possibility of needing a 2A motor? What sort of jog speeds, and move speeds while cutting are you achieving?
I am getting close to making a decision on motors and would appreciate your input.

In browsing the oriental motor website, I noticed that they now have 9, 10, 18 and 36 to 1 gear ratios available in this motor line. Gerald, or anyone for that mater, what do you think about going higher than 7.2 : 1? The higher torque might give a better cut quality.

Also, the 2 amp motor seems to have a lower voltage coil and a lower inductance. Seems to me that if your driver can handle the 2 amps, then maybe this would be a better choice than the 1 amp? Lower inductance should mean better acceleration and torque?

Any comments anyone?

High ratios could be useful if:
- There is a true higher torque. Even the 7.2 gearbox doesn't allow you to use 7.2 times the torque of the ungeared motor. Mechanical limitations on bearing loads, shaft strengths, etc.
- There isn't any additional backlash.
- Your computer/controller/drive can run at the higher frequency required.
If you want to do some experimenting, let us know what you find :)

The 1 Amp motor with 3.6:1 gearbox developed sufficient torque to break 1/2" cutters. Can't see an obvious reason why a 2 amp motor would be "better".

Gerald,
Do you think the 3.6 geared motor provides better resolution and better cut quality than the direct motor? I was thinking of the 10 : 1 motor as it would be a nice divisor and provide more resolution for better cut quality?

The main problem with high gear ratios is the number of steps per second that it takes to move along an axis. For instance, using the popular 30 tooth pinion that has a pitch diameter of 1.5 inches, moves the axis 1.5 X pi or 4.71 inches. Because a Gecko G20x stepper driver requires 2,000 pulses to rotate the shaft one time, each stepper pulse on a 1:1 (non-geared) motor would move the axis 4.71/2000 or 0.002355 inches, which is also 424.628 steps per inch. If we were to use a 3.6:1 geared stepper motor, it would take 424.628 X 3.6 = 1528.6608 pulses to move an inch. Mach 3 has a top speed through the parallel port of 45,000 pulses per second (on my computer). That means that Mach 3 could move/jog an axis at 29.43 inches per second with a motor geared 3.6:1 (if the ramping was perfect and no gremlins were active on that particular day; however, getting 1/2 that speed would still be a notable feat).

Going to 10:1 gear reduction would require 4246.28 steps per inch. Top jog speed would be reduced to about 10 inches per second (with 5-ips being more realistic).

More importantly, as Gerald pointed out, the gearboxes used on the PK296A1A-SG series motors are NOT famous for being particularly strong or accurate. The are limited in the amount of torque that they can handle and they have some backlash. For most normal CNC work, I would pick the 3.6:1 gearbox or the 7.2:1 gearbox. Personally, I have four of the 3.6:1 gearboxes on my test bench. If and when I ever get around to building a MechMate, I'll most likely use those motors.

Going by all reports, and following common logic, the 3.6 geared is better than direct, though I have no personal experience of it. The 7.2 seems to be the even better option (again, going by reports). 10:1 is not spoken of, and it is not even slightly important to use "nice" numbers.

Added Nov 14 2008: Have since converted from direct-drive to geared 7.2:1 motors. VERY happy with the results. Cuts are now smooth.

Mike, Art Fenerty recently found a way to get much higher speeds out of his Mach engine, which is apparently standard now. (Something like 100khz without changing the PC?)

Gerald,

Thanks for the info about the higher Mach 3 speeds. I haven't downloaded anything from Art since the stable G100 plugin became available. (I receive dozens of Mach forum messages every night, but usually I just quickly scan the headers to see if there's anything I need - somehow I totally missed the higher speed pulse rate.) At any rate, I've just downloaded the latest software and will soon be testing it at my test bench.

Mach 3, at 100kHz, is wonderful! For the first time, I can make the motors on the PMDX/Gecko G202 test bench act as though they were the motors on the G100/G203 test bench.

At 100kHz, gear ratios higher than 7.2:1 could be used, but, in my opinion, either the 3.6:1 or the 7.2:1 would still be my preference.

Edited: We need to remember that, even though Mach 3 at 100kHz can spin the motors much faster, the torque requirements on a motor will probably dictate that the motor be used at moderate speeds, the speed range where a stepper motor has the most torque. What I am thinking about, however, is that higher ratio geared motors could be used if good quality gear boxes were used (expensive, more than $800 per gear box) or that brushed servos could be used (also expensive because the ones I've tested are useless without a high ratio gear box). The advantage to using servo motors is that the torque curve seems to be constant throughout the motor's speed range, but the available torque is only a fraction of that available from a smaller and lighter stepper motor. For instance, I have a 34-size brushed servo (ID33004) that is long and heavy and has only 170 oz*in of torque at 70V. However, at 70V, that motor can run all day long at 2,200 RPM. So, with a 7.2:1 or 10:1 gear box to multiply the torque and to divide the speed, that particular motor might be useful on a CNC router. The problem of course is that even though the motor and Gecko G320 servo driver costs almost the same as a Gecko G20x stepper driver and a 600 oz*in stepper motor, the servo requires a gearbox - meaning that a MechMate with servos would probably cost at least $4,000 more than a MechMate with steppers.

Hi All,

How about http://www.linengineering.com/site/products/8718.html , i have a quote from them for model 8718S-05P around 250 USD each. Do you all think vexta is better, i think i should find a dealer for vexta here in bangalore

rgd
IRfan

Suggest you first find the Vexta price before you decide which company is the better supplier. On paper, the motors are very similar.

I got the quotes from Vexta distributers its almost 300USD for the ungeared and around 365USD for the geared (3.2) without taxes.

ho ho.............

Have you considered importing from Motionking, China? They might even tell you they have a stockist/agent in India.

5 axis design & power supply questions
 

Gerald,

I think that I have read through every post on motors, gearboxes, controllers, and drivers in this forum over the last few days - and though it may not be conflicting - it can get confusing :confused:.

I am looking at building a MechMate with a BC head. Ultimately I would like to build a 4x8 machine with a 24" Z axis. I have designed and am about to begin building the BC head. I have selected the following hardware/Software so far:

(1) Gecko GRex-100 Controller (6 axis)
(6) Gecko G203V Drivers
(4) PK296A2A-SG (3.6 or 7.2) Series Oriental Motors (X,X,Y,Z Axis)
(2) PK264A2A-SG (7.2 to 36) Series Oriental Motors (B,C Axis)
Mach 3 Software with G100 Firmware Upgrade

I beleive that this configuration will "work", but I would like to have your input.

Also, with this configuration, can you recommend a "pre-built" power supply? At a minimum, I would like it to be able to handle the power requirements of the controller, drivers, and stepper motors.

I do plan on adding some other "bells and whistles" in the future (vacuum table, motor on/off, etc.) so a little extra would power for the control relays wouldn't hurt.

I am much more "mechanically" inclined than I am "electrically" proficient - hence the hesitation to build my own power supply.

Again, your wisdom & knowledge would be greatly appreciated.

Joe,
I know that you addressed your post to Gerald, but, if you don't mind, I'll give you my input (and it certainly won't hurt my feelings if Gerald advises something different).

First off, you've made an excellent choice of motors, Gecko stepper drivers and pulse generator. I have almost the identical parts on hand on my test bench and have had absolutely no problems with any of the parts.

To determine which power supply will work best, you must decide whether you want to wire the motors Bipolar Series or half-coil. I strongly suggest that you choose half-coil to get much better response at higher speeds, but you will lose about 30% of the low-speed torque if you choose half-coil. Assuming that you decide to use half-coil wiring, the PK264A2A-SGxx motor will determine the maximum voltage. That motor is rated at 1.4mH (half-coil), so 1000 * SQRT(0.0014) = 37VDC MAXIMUM. I've had excellent results with a 27VDC power supply with the PK295B2A-SG3.6 motor (which is just the PK296A2A-SG3.6 motor with dual shaft). Current requirements would be ((4 X 3A) + (2 X 2A)) * 0.66 = about 11A MINIMUM. And 11A X 30VDC = about 350VA. I would select a 500VA power supply to give some margin.

If you decide to wire the motors Bilevel Series, then the forumula for the PK264A2A-SGxx motor is: 1000 * SQRT(0.0056) = 75VDC MAXIMUM. Personal experience dictates that a power supply voltage of 50V to 70V will work fine, with my personal preference being a power supply that is closer to 50V than to 70V. Current requirements would be ((4 X 2.1A) + (2 X 1.4A)) * 0.66 = about 7.5A MINIMUM. And 7.5A X 60VDC = about 500VA. I would select an 700VA or 800VA power supply to give some margin.

AnTek has a PS-5N30 power supply (30V @ 500W) for $100. They also have the PS-8N54 (54V @ 800W) for $120, which is only $10 more than the PS-6N54 (54V @ 600W).

The G100 comes with its own wall-wart power plug to generate the necessary voltage, so you won't need any other voltages unless you add other unspecified parts and pieces.

Mike is the right guy to speak to regarding the electrics. :)

Hi Guys,
This is my first post after reading everything on the forum about stepper motors.
My brain is fried for the moment,so I will make some unqualified statements.
If confused,should I just use a 4 wire motor?
Different wireing schemes result in half voltages or current or whatever.Perhaps the variable is how big a power supply you build.
I read many times in posts of the power supply pushing current to the motors. I thought power supplies provided current to the motors and the motors drew current from the supply,with the gecko's limiting the current.
The geared oriental motors have less torque than the un geared motor?
For the moment my brain is wired bi-polar series,100volts/20amps/80c.
After cooldown things may be clear.
BTW great forum,Thanks
Larry