Beginner Questions

[dfranks]

I'm new to all of this and have a few questions that I hope someone can help me with. (If I'm in the wrong forum I'll appologize now). Also please excuse me if I don't use the correct terms as I am still learning.

1.) I would like to build a cnc table that would be interchangeable between a router and a plasma cutter. Am I off base by thinking that I can do this? After reading up on cnc tables it appears that they are basically the same except for maybe the size of the motors required and that you obviously can't run a ballscrew down the center of the table as it would be damaged by the plasma cutter. How do they deal with that anyway? Is a single motor used to move the table/gantry in the Y direction or is there a motor on both sides of the table?

2.) I have noticed that some use a 3 roller bearing system in an upside down"Y" configuration running on top of a steel bar while others use a steel bar running through the center of a linear ball bushing (incorrect term?) (http://cgi.ebay.com/ws/eBayISAPI.dll...520821545&rd=1) ? Are there advantages of using one over the other?

3.) Direct coupling versus using a timing belt. If I have read correctly direct couping is 'harder' on the stepper motors whereas coupling with a timing belt and different gear ratios is easier on them? I may be way off base on that one. Anyway, advantages, disadvantages.

Thanks in advance for anyone's input and for putting up with my rudimentary questions.

[mxtras]

First - you have to completely complete your profile. Completely.

I think almost everyone on this forum has poured over these same aspects of building a machine. I think all of the methods have their respective applications (here we go) depending on the exact design of your motion platform - how much mass are you moving, how fast, how rigid, what are you cutting, etc, etc. ...you know the drill. I am an Engineer - can you tell? Seriously - it DOES depend - I get the feeling you already know this.

I also think that picking a direction is harder than making the chosen direction work - and it can take just as much time.

1) Router/Plasma question - I would suggest that you build for rigidity for routing with considerations for the plasma cutting as you mentioned, then you can do both. There is a guy in Spain that built a really nice plasma table and posted photos recently. The machine resides in his living room....it's the nicest piece of furniture he has! jk..

2) Guiding question - the 3 bearing method is an inexpensive, DIY method that works well. The round bearing thingy is more expensive and maybe a bit better in some respects (I don't like it, though). The linear rails are the way to go if you have deep pockets and want perfect guiding - but I think it depends on what you are looking for and how much of it you want to take on yourself. Many, many people have built pretty accurate, reliable machines with little more than plywood, steppers, UHMW and threaded rod. Others use welded frames, servos, linear rails and ground ballscrews. It depends on your wallet and your ambition.

3) Coupling - when in doubt, I would recommend that you go with the option that offers the most options. The timing belt method is a relatively inexpensive way to get a lot for a little in my opinion. This is my method of choice.

As you know - there are a ton of people on this board with a wealth of info. The most important thing is to take pictures of your progress and keep us posted!!



Scott

[dfranks]

Thanks so much for your reply. That does confirm some of my ideas that I have had for designing this table. I will continue to do more research on this site as it is the most informative site that I have found so far. And I have completed my profile I will definitely take pictures and post them once I begin building my table.

[johnt]

Hi Dfranks,
I was guided by the design at http://super-tech.com. To me it just seemed simpler and something I felt I could take on.
Direct rack and pinion coupling.. seems ok.. I dont know what kind of resolution I'll have when I finally get to cutting but I think it will be fine for my purposes, which is just some metal art stuff.
One motor for X, 2 for Y.
I love watching it move.

[dfranks]

JohnT,
Thanks for the response. Looks like some great information on that site. I am still researching various designs since I am wanting to build a table to can convert from a router table to a plasma cutter table. Guess I want it all... hahaha. But seriously, since both tables are very similar I think that's the route that I will probably go. The base design that I have been thinking about using is very similar to that on the site that you posted above. So I think I will go ahead and order my steel tubing and get that started, now that my Internet connection is back up. (My internet was down for 4 days and it was killing me!!!) I was thinking about using 8020's tubing but I think for the base that I can save some money by just doing a little welding which I enjoy anyway. Keep us posted on how yours is going. I know that it keeps me motivated!

[anthony]

dfranks,
the only real differences i see between a plasma and a router table are
1. rigidity. plasma torches don't (shouldn't!) push down on the material while they
work.. that means there is no lifting force on your bearings. if you use an inverted Y with only two bearings, then when you plunge with your routher, you'll lift the gantry right off the machine!
2. clearance. plasma machines can be lower profile. not many people load 2" plates under their cutting torch. with a router, not only do you need to consider your material thickness, but also the length of your cutting tool. the gantry needs to be higher on a router table.

this of course, doesn't make them mutally exclusive. make as rigid a machine as possible with enough clearance and adjustment under the gantry to accomodate both close-working plasma torch and a tall router -- the trick is to make it as light as possible. the trade-off here will be stepper/servo sizing.

make sure to put some cardboard down on the kitchen table before working on anything, as i've found this to be the biggest safety hazard.

have fun, post pictures.
-anthony

[dfranks]

Anthony,
When I first read your reply I was just nodding my head and then I re-read your 1st comment a little closer. Your absolutely right! I had even thought of that. (I can envision my 1st cut and that gantry lifting right up! ) And that is why I keep reading and researching all of this. As I have gained more knowledge about CNC I went back last night and started looking at Torchmate's site again on their 4x4 table. This is the type of table that I am looking to build, multifunction. It does appear that the Y axis does have bearings that hug both the top and bottom of their rail system. Although the gantry clearance does appear extemely low. Material wise I am looking at cutting wood a maximum of 1.5 - 2 inches and metal would be no more than .25 inches. As far as the cardboard goes I have been cleaning the garage out and have added a drill press. I think the wife is starting to wonder what's up...ssshhhh