CNC Conversion Fundamentals

[kevindsingleton]

I know this has been covered, but following the discussions has lead to mucho confusion.

I'm in the planning stages of converting my square-column mill to CNC, and there are some gaps in my knowledge.

For instance, if I go with Gecko drivers, do I need two drivers to control three axes? If so, how do I go about splitting the control from the PC to the two drivers?

Also, raising the head on my mill takes some real muscle. Is it reasonable to think that I can get a stepper motor to do that much work, even with appropriate gear reduction?

I've got lots more questions, so if this is all stored in one place, please feel free to direct me to the knowledge base.

Thanks!

[skullworks]

Gecko Drives - Will require a (1) breakout board to connect to the PC.

1 Gecko Drive is required for EACH axis (sometimes 2 on gantry type machines)

Counterweights can help and hurt - using a weight to balance the weight of the head makes for less effort to "lift" the head, but it DOUBLES the total mass to be moved which could cause motor inertia issues. A gas spring might be a better choice.

Choice of the right motor and right gearing can move anything - just maybe not at speeds you would like.

[kevindsingleton]

Thanks, Skullworks!

So, if I understand, correctly, I need a PC with appropriate software, a breakout board, three drivers, a suitable power supply, three motors with the required gear reduction, ballscrews with nuts, and all the mounting/wiring/enclosures, and, probably, some type of counterbalance for the head.

Is that enough to get me started, and keep me busy for a few months?

[acondit]

Kevin,

Whether you need gear reduction or not depends on the mass of the components that you need to move, the type of motor you use (servo vs stepper) and the pitch of the ballscrews.

Alan

[kevindsingleton]

Acondit,

Which type of motor is better suited for moving the mass of the table, vise, rotab, and workpiece? Why would you choose one type over the other? I'm not looking to do this on the cheap, so if there is ample reason to select one type of motor over the other, I'd certainly be interested in going with the better choice.

This will never be a production machine, so I'm more interested in precision and accuracy than speed or hogging ability.

Thanks!

[acondit]

Kevin,

The stepper motor is the simplest to hook up and the cheapest to implement.

The stepper has its maximum torque at slowest speeds and decreases in torque as the speed increases, the servo torque increases or maintains torque as it speeds up but it requires more complicated hardware to maintain position and accuracy. So if you want the highest speeds servo is the way to go. On my router, I use direct drive and I get rapids of about 120 ipm (inches/minute) with my old Superior Electronics 200oz/in steppers. Some other guys are getting 200ipm with more modern steppers. That seems fast enough for me.

Most people using servos require some form of gear reduction to bring the speed of the motor down to something useable. A lot of people use direct drive with steppers. The idea is to match the motor power with the desired speed and gearing (screw) whether direct drive or gear reduced.

On my lathe, I am going to use 4:1 belt reduction to achieve more steps per revolution (greater precision). The trade off is that I will have less speed.

Alan

[skullworks]

Both sites below will help alot to show you what your getting into and how/why things are done.

Check out the movies under "images" on the http://cnckits.com.au/ site and also the pages at http://www.industrialhobbies.com/

The product instructions are a great example of what you will have to do.

The movies - well - watch and judge for your self.

Also the Tip and tricks section has handy info.

++++++++++++++++++++++++++++

BTW the other areas for you to think about are the base and/or enclosure for your machine - once you start cutting metals, coolant or cutting oil containment maybe become a problem - think this part thru and much of your conversion plans depend on these choices.

[kevindsingleton]

Acondit,

Thanks for the excellent info. That's the sort of thing I hadn't gathered from my research. Since I'm less interested in speed than accuracy and torque, I think I'll go with steppers. Does it make sense to use motors that are stronger than the minimum required (given that the power supply and drivers will also have to accomodate the increased current)?

Skullworks,

Terrific links, and some much-needed information. This is actually the larger, RF45-type mill, so I'm probably looking at a more robust installation than with the Seig mills, I expect. That'll mean more money, and stouter motors. Is it reasonable to drive the Z axis from the existing shaft, or will I need a ballscrew and nuts for the head, as well. I'm a little uncomfortable thinking about the weight of the head on ballscrews, with little mechanical restraint.

I'm already using the mill, manually, so I've dealt with some of the coolant recovery issues, but I understand your point about coolant or cutting fluids and all the electrical/electronics. That's something I haven't had to address, as yet.

Thanks, again!

[Al_The_Man]

A little clarification on steppers/servo's, both generally have maximum torque at zero speed, stepper torque will drop off drastically as rpm increases, but this can be offset somewhat in the drive design to enhance torque as speed increases, Servo's on the other hand have a fairly level torque range from max at zero with a taper off slightly as max speed is reached, the advantage of servo's is that due to the higher RPM possible 6000~10,000 rpm.
Using gearing, if this top speed is not required in the design, allows the more economical use of smaller motors and drives.
Al.

[acondit]

Acondit,
<snip>
Since I'm less interested in speed than accuracy and torque, I think I'll go with steppers. Does it make sense to use motors that are stronger than the minimum required (given that the power supply and drivers will also have to accomodate the increased current)?
<snip>
Thanks, again!

Up to a point. Mariss of Geckodrive says that if your motors are excessively powerful (i.e., not matched to the load) you get cogging which affects the machining finish. I think what this means is that when they are way more powerful than needed, they sort of snap from one step position to the next rather than moving smoothly.

A slightly more powerful motor will allow a little higher speeds. Even though their torque is falling off as the motor speeds up, the point at which the minimum required torque intersects the speed torque curve will be at a higher speed.

Alan

[kevindsingleton]

Thanks, Al.

That clarifies my understanding, somewhat. So, if I decide that 60 ipm is all I require, then I can calculate the gear ratio from the motor speed and screw pitch, and use servos that are rated for lower torque than I would need with steppers to achieve the same rate. There is so much to learn!

Acondit,

I think I understand your comments. I wouldn't go too far with motor size, but I'll do some math, and see if I can't produce a spec that gives me a little margin. Do you configure the movement in the CNC software, so that the computer knows how much to turn the motor to move the table or head a specific distance? I'm planning on using Mach3, and I'm assuming that I can configure the software to control the motors, using the gear reduction and screw pitch to determine movement per revolution. Am I way off base?

[project5k]

my understanding of mach 3 is that you will have to set it up so that it knows how many steps per inch.. on each axis... it takes care of all the rest of the math.. you will have to spend some time tuning it for the motors as well, a heavier load will take a longer accel and decel time....
so just a short example, if you have a 200 step per rev motor, and go through 1:2 gearing so that the motor now turns 400 steps per rev of the screw, and your using a 10 TPI screw, then thats 4000 steps per inch.. thats what you'd put into mach3...