cheap rack and pinion

[lwill]

If you want a really cheap option check out Dynamic Automation http://dynamicgear.com/sg/web/index.htm

They sell worm gears that work on standard screw threads and they can be used with treaded rod as a rack and pinon. Could be an interesting option.

[Torchhead]

There is a lot to think about. Since you have the work envelope defined that is one step out of the way. In designing your drive train it's important to keep some goals in mind.

1. You need fairly high speeds for plasma cutting. Depending on the material and the plasma cutting you could be looking at 400 to 500 IPM to get optimum cuts. Most cutting is done at 100 to 300 IPM. Your table should be able to do 300 to 400 IPM. Most rack and pinion can do that easily but then you have to also have the torque available to sling the gantry around. Steppers loose torque at higher RPM. If you look at the curves they can be down to less than 25% of their rating at 500 RPM.

2. You need good acceleration to cut sharp corners and tight detail if your goal is decorative cutting. Acceleration is defined as the available torque to go from 0 to full speed on an axis in a short amount time. Acceleration is measured in In/min/sec or In/sec/sec. So an acceleration of 10 in/sec/sec means that if your top speed on the axis is 400 in/min (6.66 in/sec) it would take .66 seconds to reach full speed. You also have to deacclerate into a turn and that takes some time. If your design is under powered (plenty of speed but no torque) your acceleration will suffer and you will get rounded corners.

So what all of this means is that you want to compromise and build a drive system that has both speed and torque. Steppers have pretty good low end torque. A lot of builders direct drive the pinion with a stepper. You can get some mind boggling rapids that way but torque will be wretched. Most rack and pinion is geared down using reduction toothed belts to give them lower rapids but better torque and accelleration.

Find your rack, pick your pinion then do the math so that you are at the 400 IPM goal with the motors spinning at abut 400 RPM. That means you want to move one inch for each Revoution of the motor (also means your resolution will be .005 inches) Resolution is defined as the smallest PREDICTABLE distance you can move with one pulse from the computer. Things like microstepping would appear to increase the resolution but in fact the rotary position is not predictable (you can't count on it).

That's enough to chew on for a while. Then we can get into motor sizing, motor drives , rails, and things like control software and the importance of a THC.

In your planning go ahead and design a full 3 axis machine with motor drives for each axis. It will save you a lot of money in the end.

Tom Caudle
CandCNC.com

[Keith Dorsey]

Originally Posted by redneck79 Thanks for the info Jack, I am just in the research stage for a 4x8 plasma table right now, but I do have a design rolling around in the back of my head. I plan on using rack and pinion for the drive, and either v-groove or bearings and maybe some square stalk for the guide rails. I think I can get away with driving the long axis from one side as long as i spread out the guide bearings far enough on the drive side,and make the gantry light enough, there shouldnt be any chatter on the undriven side.I am just trying to get an idea of how much everything is going to cost before I jump into it with both feet.

Building my table and gantry. Putting all the pieces together on a budget takes time . Going with Dynatorch servos & software. Go to their website, then to Sitemap:Solidware Complete Units. This will give you a 3D model that you can rotate, invert, even turn it upside down. Very sophisticated. From there you can just about copy everything in their commercial gantries. Will save you many hours of design and redesign. Give serious consideration to building jackshaft dual drive system. My gantry with servos, bearings and gear rack, using AL extrusion will weigh about 90# Remember that with some fast cutting speeds on thin material, you're going to have inertia momentum. This yaw motion can affect accuracy.

[Torchhead]

Do you know if their "smart" motors can be driven with any other controller software besides theirs? Do any of the lower cost CAM packages have a Post for their software or do you have to use their toolpath creation as well?

[aghobby]

Tom, their Smart motor are simply animatics motors and their software is simply a version of Jencnc from animatics too, Dynatorch doesn't have reinvented the wheel!!

You will find all the info here:http://www.animatics.com/web/jen.htm

[Torchhead]

Thanks for the link. Their Smart Motors need special software (either theirs or some you can write in C). There is no mention of Step and Dir inputs so the RS485 serial is their interface to their software. None of the common open type software like EMC, TurboCNC, MACH3 or even DeskCNC will work with those motors. Writing a post for their software would not be hard. I guess in the commercial world where options are not as important they are well positioned. They are a little pricey for the average guy wanting a plasma cutter for a hobby or small business.

Tom Caudle
www.CandCNC.com

[aghobby]

You could program them with the SMI software to run in step and dir mode, but yes, they are expensive, their servostep series is a little less expensive.

We have studied their application on our commercial tables but we have selected standard brushless instead and on lower cost machine we are still using dc servos or sometimes when customer is on a real tight budget , we use stepper. We have designed a pretty well desing with a gearbox made from belt and pulley with a pneumatic tensionning system to prevent backlash.

[Ed from NY]

Originally Posted by Keith Dorsey Building my table and gantry. Putting all the pieces together on a budget takes time . Going with Dynatorch servos & software. Go to their website, then to Sitemap:Solidware Complete Units. This will give you a 3D model that you can rotate, invert, even turn it upside down. Very sophisticated.

Wow pretty amazing level of detail on the design of their machines. Thanks for the "link"!!!

[Torchhead]

At the risk of being redundant here. If the builder elects to go with the Smart Motors, it closes the door on other options. Only the one software package supports the motors, so if you wanted to try something else you are SOL. Since their software is written for a wide varity of uses it may have features you don't need and others you do that aren't there. Once you are painted into a proprietary hardware/software corner your options drop to one. Maybe for some that is best since there are no future decisions to make and you eat what you are given.

Seeing how tables are made gives you lots of food for thought but every design has things that are compromises. I see different designs all the way from high dollar commercial to DIY garage built units. There are common elements on the designs but like everything else there are lots of ways to do things. You find that one choice (i.e. rack and pinion VS leadscrew drive) will then dictate another aspect of the design. A lot of us build our tables from "found" components...things we can get cheap or that other builders point us too. Part of the fun of this is that there is no "perfect" design and that each use of a machine (plasma, routing, engraving, etc) brings a new set of parameters that have to be considered.

Okay, I've gone on too long.

Tom Caudle
www.CandCNC.com