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  #1  
Old Fri 27 October 2006, 03:34
Gerald_D
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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 :


Geared motor

Most MechMate builders use this motor and are happy with it. (has some backlash). It works well with Geckdrives 201,202,203,250,251,540 However, it is an expensive motor, and difficult to obtain outside the USA. The equivalent motor in Europe is the Oriental Motor PK296AE-SG7.2

Second choice is to use an un-geared Nema34 size stepper motor directly driving the pinion on the rack. Have past personal experience with Oriental Motor Vexta-Step type PK299-01AA motors ($205). Less expensive than the first choice geared motors, but less shaft torque and a slightly rough cut quality. Still produces a very passable CNC router. The smallish price difference does not make an un-geared Oriental Motor attractive, unless you want to start out with a well-known motor to add a non-backlash belt reduction later. If you want to go this route, the Oriental Motor Vexta-Step type PK299-F4.5A is probably a better choice than the PK299-01AA that I used before.


Un-geared (direct) motor

Third choice is still to use an un-geared Nema34 size stepper motor directly driving the pinion on the rack, but to go for something much less expensive than the premium "Oriental Motor" brand. MotionKing 34HS9801 is being used happily and can be imported direct from China. The price over there, without shipping/tax charges is under $60. One of these inexpensive motors, with a belt reduction added at a later stage, will rival the performance of the "first choice" geared Oriental Motors mentioned right at the top. Even before adding belt reductions, you will have an income producing CNC router. You will be spending a little more time on sanding the slightly rough edges of your cuts though.

What to look for when selecting a motor:

1. Only consider a square body motor - the round body motors are older technology with a lot less torque.

2. Use at least a Nema34 size, meaning that the square body is 85x85mm [3.4x3.4"]. Smaller sizes will be under-powered.

3. Do not be tempted to use bigger than Nema34 motors - they will not fit the standard mountings provided for in the design. Besides, bigger motors bring other problems.

4. Nema34 motors are available in different lengths, depending on whether the rotor magnetics are single, double or triple stacked. For geared (or beltdrive) motors, a single stack (motor length about 66mm [2.6"]) is enough. When used un-geared, you will need a double-stacked motor at about 96mm [3.78"] long. Avoid motors much longer than this - see point 3 above.

5. For the MechMate mechanical design, the motor shaft must be at least 12mm [0.5"] in diameter to carry the spring load and not bigger than 14mm 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.

6. The best wiring configuration for our router driving application is "half-coil". (this gives the most torque at high speed while keeping the motor temperature within limits). To wire a motor half-coil, it must have 6 or 8 lead wires.

7. If you want the motor to be compatible with a certain range of drives, the motor inductance must match. For Geckodrives, the inductance must typically be somewhere between 1.5 and 6mH. (The Gecko 250/540 wants around 3mH). Very low inductance motors cannot be driven properly by Gecko's, while very high inductance motors need very high voltage drives that are not easy to obtain.

8. If looking at a motor already integrated with a gearbox, watch out for the backlash. The "first option" geared motor above has an acceptable backlash for general board cutting and cabinet making work. It will not pass for fine precision clock-making (but then you shouldn't be looking at a MechMate anyway). Belt reductions give the fine resolution without backlash.

9. Consider getting motors with rear shaft extensions, so that encoders could be fitted. Geckodrive has a drive under development that will pull more power out of a stepper motor if it has an encoder (ala servo motor). Not a new technology (Oriental Motor has the Alpha-Step series, for example), but there is a talk of a retro-fittable feedback /drive system at an economical price, within a year or so. . . .

10. Bear in mind that geared motors have a higher detent torque. ie. they can hold a higher load when power is removed. If you are going to have a very unbalanced z-axis, a geared motor might be able to hold it up when you switch off the power.
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  #2  
Old Thu 09 November 2006, 11:30
Dirk Hazeleger
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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

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  #3  
Old Thu 09 November 2006, 12:16
Gerald_D
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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?
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  #4  
Old Thu 09 November 2006, 13:12
Dirk Hazeleger
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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.
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  #5  
Old Thu 23 November 2006, 21:48
Gerald_D
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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.


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  #6  
Old Sat 03 March 2007, 19:00
eric thomas
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http://www.xylotex.com/4AxSysKit-425.htm
can you use this system for the mechmate?
why, why not
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  #7  
Old Sat 03 March 2007, 20:37
Gerald_D
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Sorry, too small and underpowered.
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  #8  
Old Tue 06 March 2007, 05:25
Thomas M. Rybczyk
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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
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  #9  
Old Fri 09 March 2007, 20:52
Mike Richards
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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.
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  #10  
Old Thu 29 March 2007, 05:25
glenn Crawford
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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.
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  #11  
Old Thu 29 March 2007, 06:18
Gerald_D
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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.
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  #12  
Old Thu 29 March 2007, 08:33
Shopteacher
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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
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  #13  
Old Sun 13 May 2007, 10:04
Frank D
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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!
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  #14  
Old Mon 14 May 2007, 09:32
Gerald_D
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Hi Frank

I was hoping that somebody else would chime in here since you all must be very tired of hearing my biased opinions.......

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.
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  #15  
Old Mon 14 May 2007, 10:06
Loren Gameros
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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.
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  #16  
Old Mon 14 May 2007, 10:29
Frank D
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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.
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  #17  
Old Mon 14 May 2007, 15:54
Robert Masson
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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
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  #18  
Old Mon 14 May 2007, 16:51
DocTanner
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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
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  #19  
Old Mon 14 May 2007, 19:56
Robert Masson
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Thanks doc,
It?s what I was hopping to get!!
Later, Robert
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  #20  
Old Tue 22 May 2007, 08:21
IN-WondeR
Just call me: Kim
 
Randers
Denmark
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
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  #21  
Old Tue 22 May 2007, 09:12
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
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.
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  #22  
Old Tue 22 May 2007, 10:21
IN-WondeR
Just call me: Kim
 
Randers
Denmark
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...
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  #23  
Old Tue 22 May 2007, 10:37
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
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.
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  #24  
Old Sun 27 May 2007, 13:09
Alan_c
Just call me: Alan (#11)
 
Grabouw (Western Cape)
South Africa
Send a message via Skype™ to Alan_c
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.
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  #25  
Old Mon 20 August 2007, 08:59
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
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?
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  #26  
Old Mon 20 August 2007, 09:39
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
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".
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  #27  
Old Mon 20 August 2007, 10:31
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
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?
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  #28  
Old Mon 20 August 2007, 11:09
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
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.
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  #29  
Old Mon 20 August 2007, 12:20
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by domino11 View Post
Gerald,
Do you think the 3.6 geared motor provides better resolution and better cut quality than the direct motor?
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.
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  #30  
Old Mon 20 August 2007, 12:22
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
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?)
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