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UKRobotics
12-22-2003, 06:45 PM
Hi everyone,

I was hoping somebody here might be able to help...

I was introduced to CNC machining several years ago at my school/college which has this machine - Here (http://www.suregrave.co.uk/2516.htm)

Although they do let me use it a lot for my own projects, it simply isn’t enough any more, especially as I have to keep an eye on it all the time while it's milling in case some other kid comes along and fiddles with it.

My hobby of combat robotics (Battle bots/Robotwars) requires ever increasing amounts of CNC'd parts but after finding out the price of the things i thought about building my own, which is how I found this site.

I'm not too worried about the mechanical side of things. I'm confident it would not take me long to construct a machine suitable for the job but the electronic side of it has already got me confused.

I need motors which will happily mill through thick mild steel plate, or possibly even stainless. So where do I start?

Servo motors or Stepper motors? What controllers and where from? How to connect it to the computer and what software to use?

Also, a rough idea on how heavy I ought to make the machine, the thickness of steel/ali i should use to construct it, and the size/type of threaded rod to move the cutter I should use would also be very usefull.

I've read a lot on these forums about converting CAD files to G-Code so they can be milled out. Does this have to be done with all home built CNC mills? - Despite using the college CNC mill for several years, I have never come across any G-Code. You simply draw what ever needs cutting in the CAD software (Techsoft 2D Design) set the cutting depth, tool, and speed for each line and shape, click 'plot' and the machine cuts out what I’ve drawn on screen.

What motor would you recommend for the spindle motor ? I assume this doesn’t have to be a stepper or servo motor.


Also, I know this may be a stupid question, but what is the difference between a CNC mill and a CNC router?

Apologies in advance if some of my questions are rather vauge but I am new to this stuff! :D Any help would be much appreciated.


Regards,

Dominic

http://www.ukrobotics.com

HuFlungDung
12-22-2003, 09:26 PM
Hi Dominic

It sounds to me like you need a real milling machine for what you want to make. However, you would need to determine the maximum size of your parts, in order to determine if a "standard" milling machine is large enough to do everything. The common sizes of milling machines (most readily available) will likely be about 28" X travel by 12" Y travel, or 32" X by 17" Y. In the form of a common "vertical turrent mill", you would likely have a 5 or 6" Z quill travel, but the table will also hand crank up and down to give you another 8 inches of vertical work space.

A router is generally not as heavy duty of a machine (but it depends on how much iron was put into it of course). A router is more generally used for 2d profiles, since it may have quite a limited Z travel on the spindle.

Consider that a good machine vise takes up 5" of height on a milling machine table, and you will see you don't have much room for a tool hanging down from a router with a 6" Z space.

You can hardly go wrong buying a vertical milling machine. They are easy to get and easy to sell. You can buy a manual one and convert it to cnc or buy one that is already cnc ready, maybe in need of a new control or something. There is lots of good 1980's iron around, with crappy old controllers, just begging for a new control to be retrofitted on.

RE: gcode: you may not have seen gcode, but most likely it was there in the background. You may have been using what is called a "conversational controller" which, at first glance, seems to be more user friendly to program, because you just input geometry that you already understand, and it writes the program from that.
I've never run a conversational controller, because real men prefer gcode, and women really like a guy who knows gcode :D
Anyways, gcode is nothing to fear. Its like the difference between writing and shorthand. Is one better than the other? No, but one is a hell of a lot more succinct than the other. With a little practise, gcode means exactly what you want to say, with very little redundancy. From what little I have experienced of conversational controllers (via the posts required to make code from a modern cadcam program), I wouldn't want one if you gave it to me.

Modern cadcam programs take into account the method of tool entry into the work, as well as cleaning out the bulk of material safely, from any shape of workspace.

kenbeth
12-23-2003, 01:57 AM
going through the same process my self Dominic i have
thk slides and ballscrews (Jenetec) 12inch travel and the stepper motors with xylotex drivers,
but no mounting supports yet for the ballscrews,
cheapest made up supports so far i have found
are the INA ZKLR series at approx 60 GBP
waiting for some more quotes from the USA also looking at making my own with angular contact bearings ,
Questions :
would Abec1 angular contact bearings be ok
or do you have to go to the high precision type,
Anyone have sources for made up bearing units or mounting ideas
would be appreciated ;
My project is to make a 12inch 12 inch 3.75inch cnc machine for glass and stone carving using diamond burrs, Have all the bearings ,ballscrews driver and motors at the moment.

HuFlungDung
12-23-2003, 10:00 AM
The main reason to use ultra precision or super precision bearings is for cool running at high speeds, with minimum clearance.

So far as a thrust bearing for a screw, I think a normal grade of bearing is sufficient. A normal precision bearing is still a highly accurate device, and in angular contact styles, you must adjust the preload yourself anyways, so it is not as though you are locked in to whatever internal clearances the manufacturer decided upon, as is the case for a regular deep groove ball bearing.

You will notice if you compare, that the inner race of the angular contact ball bearing is very loose in one direction, and that a standard deep groove ball bearing is not so loose (but still not rigid). This is because there must be a thrust shoulder pushing against the inner race of the angular contact bearing, to seat it against the balls. To achieve this preload, a threaded adjustment is a common method, or, custom shim packs can also be used with a "try and fit" method, to achieve preload.

However, if you use a double row angular contact bearing, then the preload is built in. Lots to know :)

HomeCNC
12-23-2003, 12:20 PM
Dominic,

You are at the same place I was a few years ago. I wanted a CNC mill to do metal cutting. I did not have the space or budget for a Knee type mill. I decieded to get a Drill/Mill and convert it to full CNC. (which I did). After this I found that some parts I wanted, like sheet parts was too large for the table of a milling machine. So I built a CNC router.

I would not cut steel on my cnc router, but it can do aluminum. It's all in the feed and speeds you set. If you can't afford a nice ridgid milling machine then you will need to use what you can. Just take lighter cuts at slower feeds to get the job done.

kenbeth
12-23-2003, 04:52 PM
Hi HuFlungDung
thanks you have cleared up a few misconceptions i had to do with the type of angular contact bearings to use , will go the way of making my own housings with 2 angular bearings with locknut for preload, is the cheaper option, should have posted this in the how do i mount acme/ballscrew thread

ken