Replies to This Discussion

Permalink Reply by Ellison Chan on August 7, 2012 at 8:19pm

Pre-made servos solutions cost thousands, which I suspect will be out of range for most here.  Volume will reduce cost, but also our own DIY will save tons, with no R&D markup costs.

Permalink Reply by Dustin on August 7, 2012 at 8:42pm

As I understand it, we've already identified some $160 or so per axis pre-made servo motor controls.  All I'm really trying to say is that we should find out how many people don't really care and simply want stepper motors because they are cheaper and how many people would prefer a servo solution.  This number would give us the ability to find out, for example, how much the real cost would be to creat our own servo motors.  $8 per bare board at 50 boards was mentioned earlier, but this is based upon pretty much everyone wanting servo motors.  Do we know this to be the case?  I'm not sure we can really know this without knowing how much the rest of the unit (everything but motors and electronics) will cost.

 

Sorry, I'm not trying to be difficult... just raising some questions that will, hopefully, help us solidify the options and what we are trying to accomplish.

 

Permalink Reply by R. D. Starwalt on August 8, 2012 at 6:19am

The parallel port is simply old and slow but *very* reliable.

It has been left behind  from a PC perspective. The hobby CNC boom was partially based on an abundance of old PCs laying around garages, victims of better, faster, video/software demands.

There are now several open source projects using USB interfaces to drive motors (stepper or servo).

The question then turns into, what will your CNC software support?

The mechanics of the machine must be built first.

The mechanics will determine how much motor torque is needed just to move an axis. The next torque hurdle will be determined by the type of materials you want to cut, how much you want to cut, and how fast you want to cut it.

Machinists live by Feed and Speed (right Monroe?).

The tendency is to put big-ass motors on a system just to make sure you can cut what you think you might cut. This is fine as long as the drive mechanics can handle the torque the motor produces.

There are many questions to ask on a project like this. Usually those questions are answered by *specifications* of the project.

Permalink Reply by Mike R. on August 7, 2012 at 6:39pm

I'm in as well :) - I've played with some small cnc stuff, but haven't completed my first build yet, so this would be a great one to get in on..

Permalink Reply by Franco Scipioni on August 7, 2012 at 8:15pm

Here is a quick survey for gauging interest on steppers vs servos for the prototype. You can state why you chose that, define a specific stepper or servo, or just add any comment you want. It should display results after you vote and I'll post the "why did you choose this" comments after a number of people have responded.

Click here for stepper vs servo!

Permalink Reply by Ellison Chan on August 7, 2012 at 8:17pm

Great stuff!  I voted.

Permalink Reply by Dustin on August 7, 2012 at 8:50pm

Thanks Franco. 

I'm searching through the forum to find what it was called, but there was mention of the servo along with another solution that actually provided feedback as to whether or not the actual linear motion was successful as addition to the standard servo motor feedback.  Does anyone remember what this was called?  Is it necessary?  Is it feasible?  Is it too costly?

Permalink Reply by Ellison Chan on August 7, 2012 at 8:56pm

No feedback = Stepper solution.

Feedback w encoders = Servo solution

Permalink Reply by R. D. Starwalt on August 8, 2012 at 5:50am

As Ellison mentions below Servo systems have feedback and will hold the drive mechanism at a specific point until the design limits are exceeded.

This does not mean ALL servo systems feedback to the controller PC circuitry exactly where they are in the X, Y, Z system.

A simple example is an RC servo. The RC receiver has no idea at what angle the drive arm is located yet the servo proper will hold the arm at that position until it is commanded otherwise or it cannot due to loading.

To achieve complete location information, an absolute position encoder system must be used. In this type of system, the encoder is read by the control PC and then move commands are sent to the motor drive to achieve 'balance'. This is usually not a hobby system.

The only time I have experience errors with my stepper system is when some limit was exceeded. By limit I mean the ability of the machine to cut a depth at a certain speed. This usually means the torque of the drive could not do what it was asked.

Another time I drilled through a part into the table bed essentially pinning the X & Y axis in place.

That was noisy and smelly.

Permalink Reply by Ellison Chan on August 8, 2012 at 7:21am

Wow, that's great news.  I think we have enough interest in both servo and steppers to justify both routes.  Again, I don't think it's an either or situation.  In fact shouldn't we also be able to do a hybrid where the drive system is stepper, but we include positional encoders?

Permalink Reply by Dustin on August 8, 2012 at 10:28am

Great news on the servo boards!  What are the servo/stepper results so far.  Is there a way to see them without voting again (don't want to skew the results)?

 

I'd love to see your harp design.  Would it increase possible project/cutting area sizes -- do you know the sizes of the cutting area?

DistributorPermalink Reply by Dany Thivierge on August 8, 2012 at 10:41am

looks like it will be very close.... 50% servo / stepper... 

We will need to offer the 2 options.... make sense... I am still all in for the most expensive, precise and powerful option!  ;) 

Dany

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