Anyone used servo drives instead of stepper motors?

[Mason123]

I am looking to get a mechmate, and I would like to use servo
motor drives for it. Has anyone done this? How difficult is it and
do you have to cut the laser parts custom for the servo mounts?
Also on the mechmate how fast can you move the gantry without
danger of throwing off tolerances or damaging the machine.
Basicly is it the stepper motors limiting the speed or is it the gantry?
Thanks,
Mason

[bradm]

Mason, you'll need to research what servo motors you would use, and whether or not they are physically compatible with the existing mounts to answer that aspect of your question.

As for the question of the speed limits, it's actually a complex balance of factors, including the mass of the gantry, speed and power available at the spindle, depth of cut, material, desired cut quality. It isn't a single factor.

My MechMate with OM7.2 steppers and a G540 is well capable of moving faster than my 2.2kw spindle can cut at 24k RPM with a 1/4" end mill. It is arguably at the lower powered end of the spectrum, and is still plenty fast, as in 400-600 IPM depending on the quality of the computer I've had driving it. That's 2-3 times faster than I would normally cut any material, so any extra speed applies only to rapids, and enters the realm of diminishing returns quickly once you factor in acceleration and deceleration curves.

Perhaps you could share a little more about what your application is, how fast you want to cut and move, and why. We can then help you evaluate solutions that fit the need.

[Richards]

Servo motors are big, heavy and have less holding torque than steppers. I have an eleven pound brushed servo that is rated at 220 oz*in. It is driven by a Gecko G320 driver. It pulls 7 to 20 amps at 80VDC. It would need at least a 3:1 belt drive to give it the same torque as a PK299-4.5 stepper gives 1:1.

At one time I considered adding servos to my Shopbot, but after experimenting for several months it became very clear that brushed servos offered no real advantage over properly implemented steppers on a machine with the MechMate's mass.

[Mason123]

I am mainly interested in speed for rapids. It seems that servos might be more trouble than they're worth. Would you normally use and encoder for a stepper or are they primarilly used for servo motors? Mike, did you notice a significant
differance in noise and vibration with your servo vs stepper?
I am debating using a laser for engraving which typically
needs to move rather fast.
Thanks,
Mason

[Richards]

Servo motors "dither" when they are stopped, i.e., the encoder can cause the motor to "hunt" when stopped. If the encoder breaks, a servo will take off until it reaches the stops.

I would think that for a laser application, you would be moving much less mass than a regular CNC, in addition, there would be virtually no cutting force involved, so depending on the resolution required, you might get by with a 1:1 drive, so speed would not be an issue.

[KenC]

How fast do you want?

[danilom]

42.000 mm per minute (1600 ipm) rapids are not enough :) ?

[Mason123]

I think 1600 ipm will be more than fast enough.
Just out of curiousity.. --> Why do the
multi million dollar cnc machines use servo motors? Is it mainly for their
speed that they go for servos over steppers?

One last question..slightly off topic. How common
is it to use a multi tool mount? Here's what I mean,
Is it reasonable to mount a router and be able to interchange
the router with a laser or plasma torch?

Thanks,
Mason

[Richards]

The "big iron" machine are heavy. The more mass you move at speed, the larger the motors that are required.

If you build a dual-purpose machine, you'll need to design the machine to handle whichever task requires the most weight and the largest motors. The laser machines I've watched have required very little mass. They ran at substantial speed with moderate sized motors. In other words, with no cutting force, the machines only had to be "substantial" enough to handle direction changes without unexpected flexing. Ramping handled the expected flex.

[Mason123]

Could you build a gantry with each tool on opposite sides of it
and then just use your software to reset the origin based off of which tool you tell it
to use?

Thanks,
Mason

[danilom]

look here for a dual spindle setup

http://www.mechmate.com/forums/showthread.php?p=58438&postcount=54

[domino11]

I know Gerald talked about the possibility of even having two Y cars on a single gantry as well, but this has not been documented as far as I know.

[Gerald D]

You could even have two gantries on a longer table......

Double Z's in double Y-Cars on twin gantries could give you 8 heads.

Ant then the y-cars and gantries can be tripled!!......:D

[domino11]

Now that would be a Super MechMate! :):D:cool:
And if you had a super long table, like Chuck built,
you could run one smaller job on each end of the table at the same time.

[smreish]

I so want to do that! Just for fun.

[Mason123]

That would be Awesome!

[Gerald D]

Triple gantries are not that far-fetched. We have a big steel supplier that plasma cuts plate on a large scale. The x-rails are about 50 meters (yards) long and there are three independent gantries. An overhead crane, with electromagnet "hook" loads steel sheets in about 6 work stations and the nearest gantry sets to work for the cutting.

[KenC]

50yd...:eek:
and I thought my 5 x 20' plasma table is big... :o

[darren salyer]

In the beginning of my build I thought long and hard about a 5x20 table, with two gantrys working at opposite ends. Load one end, and while it is running, load the opposite end. Then keep alternating back and forth.
Then I realized about the only savings over building 2 5x10 machines would be a couple legs....
A 5x20 with twin gantrys would be impressive, but I'm quickly realizing my 5x12 is really bigger than I need most of the year.
ALTHOUGH, the jigs I've set up on the last 4 feet of table don't need removing to process a 4x8 sheet.
Its all tradeoffs, isn't it?

[domino11]

Ken,
So when are you going to add the second gantry? :)

[KenC]

Ken,
So when are you going to add the second gantry? :)

two gantry? no thanks... Just too lazy...
Will try out dual-Z when I over come procrastination. So far... procrastination win every time...