Selecting motors for the MechMate - some history - Page 5

turnerseng · #**121** Tue 13 May 2008, 03:58

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.

Alan_c · #**122** Tue 13 May 2008, 07:03

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.

Doug_Ford · #**123** Tue 13 May 2008, 16:40

Andy,

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

Marc Shlaes · #**124** Tue 13 May 2008, 16:52

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.

Doug_Ford · #**125** Tue 13 May 2008, 19:23

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.

smreish · #**126** Tue 13 May 2008, 19:41

...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.

Kevin · #**127** Tue 13 May 2008, 21:11

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

Kevin

Marc Shlaes · #**128** Tue 13 May 2008, 21:46

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.

Richards · #**129** Tue 13 May 2008, 22:43

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.

Kobus_Joubert · #**130** Tue 13 May 2008, 23:19

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.

Thanks Mike, we were typing at the same time.

Doug_Ford · #**131** Wed 14 May 2008, 19:28

Sean,

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

smreish · #**132** Wed 14 May 2008, 20:02

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.

Marc Shlaes · #**133** Thu 15 May 2008, 09:04

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

Doug_Ford · #**134** Thu 15 May 2008, 20:47

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.

Robert M · #**135** Fri 16 May 2008, 05:49

Marc, thank you for sharing this valuable link.
Robert

PEU · #**136** Fri 22 August 2008, 13:08

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!

Richards · #**137** Sat 23 August 2008, 05:18

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.

domino11 · #**138** Sat 23 August 2008, 16:54

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?

Gerald D · #**139** Sun 24 August 2008, 08:13

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.

Richards · #**140** Sun 24 August 2008, 09:33

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.

sprayhead · #**141** Tue 26 August 2008, 23:11

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

Gerald D · #**142** Tue 26 August 2008, 23:21

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

which is 20 kilogram at 25mm

sprayhead · #**143** Wed 27 August 2008, 00:17

20.41Kg

Recalculated using G accel.

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

Thanks
Francis

Gerald D · #**144** Wed 27 August 2008, 00:31

Sounds like you are starting to believe us

sprayhead · #**145** Fri 10 October 2008, 06:58

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

domino11 · #**146** Fri 10 October 2008, 07:39

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

joepardy · #**147** Fri 10 October 2008, 08:30

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.

gmessler · #**148** Fri 10 October 2008, 08:41

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

Gerald D · #**149** Fri 10 October 2008, 08:50

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.

sprayhead · #**150** Fri 10 October 2008, 21:52

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.