Larry,
Keep reading, just keep reading.

Mike....funny how you mention reading...just keep reading. I was soldering the harness connections on my motor's last night while the kids were busy watching Nemo....just keep swimming...
The kitchen table project has so many parallels :)

- Almost ready to put power to the box for a test run. Waiting for paint to dry so I can reassemble the cabinet.

Sean

Information overload.
Sorry my brain is still overloaded but I will try to make some assumptions.
From what I have read here,.66 is OK as all 4 steppers are never fully on at one time.the others act as a current dump.
I like the low current and sealed cabinet.For higher current you could cut a hole in the box and mount the Gecko's to an external heat sink.
Larry

Larry,

Although 66% is generally accepted as enough current, I always go 100%. Power supplies are cheap compared to the material that might be wasted if the motors can't do their job.

As far as heat goes, the G203v doesn't generate much heat. A slightly oversized power supply doesn't generate much heat. I run G203v all the time on my test bench. They've always stayed at, or very near, room temperature. The various toroidal transformers that I use also stay at or very near room temperature.

As to whether a device 'pulls' current from the power supply or whether the power supply 'pushes' current through a device is kind of a loaded question. Sometimes it's easier if we think of a power supply as a device that can produce pressure (voltage) and volume (current). If you've ever played with water, you know that the higher the pressure, the more volume - in a fixed amount of time. So, a power supply can 'push' electricity out to various devices, as long as the various devices can't allow passage of more current than the power supply is able to produce. That brings us to the device. If we use the water analogy again and think of each device as a certain size of hose, it is easy to visualize what is actually happening. Lets say that device A has a 1/4-inch hose and device B has a 3/4-inch hose. Using that example, it is easy to understand that device B will allow more current to pass at a given pressure than device A allows to pass. The same thing is happening with power supplies and electrical components. The power supply is able to push out electricity at a certain pressure as long as the various devices connected to that power supply allow less current to pass than the power supply is able to supply. So, to bring this long analogy to an end, if a power supply can supply 10A of current and all of the devices connected to that power supply can only allow 8A of current to pass, then the power supply can be considered adequate to handle the job. On the other hand, if the power supply could only handle 6A of current and all of the devices connected to that power supply can allow 8A of current to pass, then the power supply would be considered inadequate to handle the job (just like running too many sprinklers off of one water line causes pressure to fall to the point that the sprinklers can't do their job).

Mike, an early New Year's resolution of mine is to do some measurements of these lovely vague concepts we spend hours talking about. . . . .

1. Put a logging amp-meter on the output of the power supply driving motors which are doing various types of cutting (3D, cabinet doors, etc.). This will tell us if the 0.66 factor is okay.

2. Take a variable transformer powering a recommended motor via a g202 and g203V and log motor temperature versus voltage. This will tell which voltage is the limit for the motor.

But, if Santa has been kind while I am away on holiday, there will be lovely graphs and test reports waiting in my stocking when I get back. ;)

Ok, I am about to commit to the build and start buying hardware etc. I have read this thread 3 or more times and agree with many of the posters before me that this can be quite confusing and thankyou Gerald for updating your postings and keeping everything current. Is it a possibility that someone that has tested different motor control combinations could possibly post a spreadsheet to make it easier to compare and list pros and cons. Maybe if possible leave it open for people to add their specs of what they have working on their machines in the future so everything could be in one consise location. I know this may be asking for alot for somebody to take the time for this as time is my most precious asset but it would help all of us with less experience to make good decisions.

Who is a good source for servo motors in South Africa? and what would the model number be for the replacement to the Oriental steppers PK296A1A.

Try CNC Direct in JHB, their prices are not bad, but nobody on this forum has used their motors. The OM motors are available from Varispeed but their pricing is truly laughable if not downright criminal.

Andy,

If you are planning on using them on a MM, a servo won't work. It spins too quickly.

Doug,

I am surely no expert, but, it seems that with transmissions such as those that JR and I designed and built, they would work.

Today, JR and I should be posting the DXFs as well as a description of just how the cutting of the 1/2 inch aluminum worked.

You may be right Marc. I hadn't thought of that. Sure sounds like a lot of work and extra money especially when we're not sure it is needed. My machine has power to spare. If I changed my pinion from a 35 tooth to a 20tooth, I don't think I would ever lose a step and it would still probably go fast enough to scare me.

...Doug...it doesn't go that fast with 20Tooth......but it does a fine job.
I seem to cut in the 180-225ipm band comfortably all day long. I do think I am going to "upsize" slightly next month when I move the machine. I need a "little" quicker fast moves between parts. I need to shave about 4 minutes off each cut cycle to get a better part/price yield.

I think you will need a lot more than 3:1 reduction for a servo motor to work on a MechMate...

Kevin

Kevin, please elaborate. This is something I wanted to learn about for a while. Maybe it is just curiosity. Maybe it won't go anywhere but I would love to know more about servos.

Thanks in advance.

The servo motor that I've had for several years is an ID3004. With the Gecko G320/G340 type servo driver, it spins at about 2,200 RPM and gives about 225 oz*in torque. The motor is about 10-inches long.

As you can see, there are two problems, 2,200 RPM with a 20/20 pinion and rack would mean that the axis would move 2200 / 60 X 3.14 inches per second, or 115.13 inches per second. The second problem is that 225 oz*in is only about 1/3 the torque that I would consider adequate.

Most of my cutting is done at 8-inches per second or slower. My preferred jog speed is 12-inches per second. So, 115/12 = 9.58:1 gearing. With belt-driven transmissions, that means you would need a multi-stage transmission, and that means you're going to have a very bulky transmission. The smallest practical pulley on the 5/8-inch motor shaft is 20-tooth and the largest commonly available pulley on the drive shaft is 72-tooth, which gives 3.6:1 reduction, meaning that you would have to use a two-stage transmission. Remember that a transmission will also multiply the torque, so a 9:1 or 10:1 transmission will give about 2,000 oz*inches of torque.

Servo motors are quieter than stepper motors. They have constant torque. Their resolution depends mostly on the encoder that you use. (I use a 500 line encoder that gives 4 X 500 lines or 2000 "steps" per revolution.) However, if something happens to the encoder, the servo is going to run away. A servo also has jitter at rest. And, the servo that I've listed would require a separate 70VDC power supply per motor.

If 7.2:1 to 10:1 gearboxes were available at a reasonable price, I would switch to servo motors, but when the decent quality gearboxes cost around $1,000 each, using a servo motor with a rack and pinion drive is too expensive for my use.

Every CNC router that I've seen that uses servo motors also uses ball-screws. Ball-screws introduce many more possible problems. Dirt and dust are a big problem. "Whip" can be a problem at CNC router speeds. Cost is much higher than rack and pinion. Maintenance is also an issue.

On the other hand, when compared to servo motors, stepper motors are inexpensive, easy to use, maintenance free, and easy to use with rack and pinion. They can be used direct drive or with gearboxes/transmissions. From what I've seen on the forum, the PK296A2A-SG7.2 motors are an excellent match with the Mechmate.

On my machine (Shopbot), I started with direct drive 600 oz*in motors, then added 3:1 belt-drive transmissions to the X and Y axes, and finally switched to 7.2:1 gearbox motors on the X and Y axes. Using either the 3:1 or the 7.2:1 geared motors gave much smoother edges than the direct drive motors that came with the machine; however, the 7.2:1 motors gave hardly any improvement over the 3:1 motors.

Yes PLEASE..more servo info. I can lay my hands on some MOOG G4x3 servo motors. If these servo's can be used on a MM, this will be great.:D

Thanks Mike, we were typing at the same time.

Sean,

What are you upgrading? Bigger motors or larger pinions? Or something else?

Doug,

An associate of mine is building a servo based CNC plasma machine at this time and I am paying close attention to his progress to learn a little. The real reason is I plan on building a plasma table in the very near future. I haven't fully decided if I will convert the Mechmate that I just finished to the plasma since it's a classic version. I would prefer to build another MM with Mamba parts and spindle. Just easier to start from scratch then retrofit the existing machine. The cost is about the same!

With regard to your earlier post

"I am going to "upsize" slightly next month when I move the machine. I need a "little" quicker fast moves between parts. I need to shave about 4 minutes off each cut cycle to get a better part/price yield

I will change the pinion size next month to a slightly larger for quicker jogs and fast moves.

Came across this just a few moments ago. Pretty good info on this topic. Check it out.

http://www.cnczone.com/forums/showthread.php?t=52090

I'd love a CNC plasma cutter too but I don't have the room in my garage for another machine. Good luck with it.

Marc, thank you for sharing this valuable link.
Robert

I have a question regarding the Vexta PK292A2A-SG7.2 motor, as suggested in the 1st post Im thinking about purchasing them with a rear shaft for an encoder.
Reading the vexta catalog for series PK2 page C231 I read that the doubleshaft model is the PK296B2A but next to the part number it says its unipolar, am I reading it correctly? or what should be the doubleshaft equivalent of the PK296A2A-SG7.2?

Or it refers on how one wires them? in this case the PK296B2A-SG7.2 should be the motor I need.

Im confused :)

Thanks!

The PK296B2A-SGxx is the motor that has the shaft extended out the back. I have four PK296B2A-SG3.6 motors on my test bench. The PK296A2A-SGxx motor is the standard motor without the shaft extension.

Both the A2A and the B2A have the same electrical specifications and the same six wires per motor. Either can be wired full-coil or half-coil, depending on your needs.

Mike,
I have looked on the oriental motor site for an equivalent pk296a2a with 8 wires but dont see any. I was wondering if they had one so you could run a paralell combination. Or do you think parallel is not needed?

My guess is that since that family of geared motors is limited for torque by their gearboxes, it doesn't make sense for them to give you 8 wires so that you can increase the torque with parallel coils.

Gerald is right.

A six wire motor can be wired series or half-coil when connected to a Gecko stepper driver. You need an eight wire motor if you need parallel coils.

Someone able to tell me how much force/Weight a 5N.m motor can hold (holding torque) @ 25mm from the center of it's shaft?

The PK296A2A-SG7.2 gives out 5N.m from 0rpm to ~100rpm.

I am wondering what kind of push (in kilograms) this thing is able to put on the racks, so I can have a better idea of what's really going on.

thanks
Francis

5 N.m = 5 Newton.meter = 0.5 kilogram.meter = 500 kilogram.millimeter

which is 20 kilogram at 25mm

20.41Kg :p

Recalculated using G accel.

They are quite good these little motors huh!!! makes me feel good.

Thanks
Francis

Sounds like you are starting to believe us :)

For a heavier Z unit... say it is an extended version, or it is carrying a heavier spindle of a different kind or just some gadget of a different nature...

What are the thoughts of the people on using the PK296A2A-SG18 ?? It has a lot more holding torque then the PK296A2A-SG7.2.... It might even be interesting to use this motor and not use a gas shock on the original project? Proceed?

Francis

Francis,
Dont forget the reason most people dont go to the higher gear ratios are the fact that to get any decent speed, the computer and paralell port cannot keep up. Mach3 is also limited to the pulse stream it can handle. I would look carefully at the calculations and limitations of the hardware and software first. See Mike Richards post in this thread for more info.

http://www.mechmate.com/forums/showt...8&postcount=64

Heavier Z
 

As it so happens, I am not in the process of working on my Z axis. My machine is not the conventional Mechmate design, however, I may have some insight. My Z axis is about 24" long, and weighs about 80#. I originally tried the PK296A2A-SG72, and it would not lift it. I also tried the PK296A2A-SG36 - again, without success. In both cases, either the spur gear slipped on the gear rack, or (when locked into place), the motor bogged down - lifting it a fraction of an inch - then dropping it. I am therefore working with a combination of gas springs to counterbalance the weight. When I installed a single spring (and limited the travel to 13") the SG72 model worked great.

In my case, to offset the 80# weight, I am installing 100# springs.

Hi Francis,

Take a look at Mike's response to my post in this thread. Valuable information on holding torque.:)

http://mechmate.com/forums/showthread.php?t=56

When your motor is de-energised, the holding torque disappears and the router falls . . . . .

A typical solution for loooong z-slides is a pneumatic cylinder, adjustable pressure regulator and a compressor.

Jeopardy,

Do you use any parts of the MechMate on your Z axis? Same spider plate and Z slide perhaps? or it all had to be modified to fit both the gas springs? I am scratching my head thinking how to use 2 springs and get them to work together to be effective for the whole length.


Boys,

As far as speed and lack of pulse frequency that should be fine because I intend to use the smoothstepper anyways. My application won't require much in terms of speed from the Z axis, so no visible problems there yet.

Gerald,

I obviously didn't realize that when you turn the motors off the Z slide slides and goes baang, if there is nothing extra to hold it. :p

There apparently are big systems with electric/spring brakes. . . when the system is de-energised, the springs apply brakes to fight the gravity that never de-energises. :)

like a Stepper Motor with electromagnetic break? I think I have seen that on Servos.

Another thought I had why the PK296A2A-SG18 would be good is when the Gecko drops to 70% the torque... recirculating.

If one could attach an electromagnetic break to the PK296A2A-SG18 that would be a good solution, I guess.

This thread then drifted to other counterbalance methods:
http://www.mechmate.com/forums/showthread.php?t=1026

Selecting motors for the MechMate - Revised Nov. 08
 

A newbie opening this current thread for the first time was confronted with a confusing first post. Two years have taught us a lot about motors and we have settled onto some comfortable choices. I have rewritten that first post and edited/pruned some of the subsequent posts. However, the earlier version of this thread is here in the Archive.

Gerald,
That is a great summary. I especially like the links to other threads on motors, belt reduction and the like. Excellent. :)