Newbie- What would be the best approach?

[finaltheorem47]

Hello, I'm looking to put together a CNC machine for DIY home use this spring.

- I want to be able to cut large 4x8 pieces of sheet metal (Plasma would be best)
- I want to be able to cut smaller thicker pieces of metal into shapes for parts, like making a engine mount or some bracket that fits well onto an axle tube, etc. (A mill CNC style would be best)
- I want to be able to cut wood into various objects (A router style CNC would work best)

All three styles are 3-axis, and a mill and router is very similar... in fact what is the difference? Is metal cutting done at a slower speed than a router?

My manual hand crank mill cuts wood just fine.

Would a 3-axis mill style CNC machine fit all my needs? Speed isn't a problem, I'm not a business that needs to make hundreds of a certain part.

Would I encounter problems if I want to make a 4x8 table to cut large objects on, and then have a tiny 2"x2" object that needs intricate cuts?

Sorry if this has been posted before, I tried searching for a few key words, but was unlucky.

The main purpose of the CNC would be to cut metal. 45% sheet metal, 40% any other form of metal and 15% wood.

[thackman]

I can understand where you are coming from. Many who are new to this start out wanting a router because the few inches of travel on a mill are so limiting. The two are similar but a router and mill are at opposite ends of the 3-axis spectrum. I’d build the router table for the big wide tolerance stuff and CNC the manual mill for the real milling. A router trades accuracy and rigidity for higher travel speeds and a much larger work envelope. Wood is far easier to cut than steel so it doesn't need nearly as much rigidity. More importantly large projects that are measured in feet have much higher tolerances than parts that are measured in fractions of an inch. Let's say you had $100,000 to spend on a vehicle. For the money you could get either a Porsche or a tractor-trailer. Both have four wheels and a motor but are optimized differently. Even the techniques used on the machines are different.

Overall it depends on your definition of intricate cuts. To scale up to 4x8 you have to use different technology than a mill. Your machine isn't going to have 8' ball screws nor ground ways. Every tradeoff that you make to reduce cost is going to reduce your ability to mill the harder metals. If you switch from wood to steel and keep the same size cutter you would generally use 1/10th the RPM, 1/10 the feed, and have 10 times more cutting force.

Large commercial routers can cut sheet aluminum but I don't think there is any hope that you could mill steel on a 4'x8' DIY router. You might have some success if you are using some small cutters at very high speeds w/ light cuts but that is all. Decorative or low tolerance parts may be possible but you will not approach the precision of a mill. You will have less pulses per revolution on router, less accurate slides, and a less accurate screw. The large working area will make the machine so springy that the cutter deflects and bounces rather than cutting and it destroys the cutter. I started off with a CNC'd X3, 400lbs of cast iron w/ a work area around 15"x7". I was amazed at how solid all of that cast iron felt and how smooth the axis operated. It wasn't until I started cutting steel that I realized my concepts of rigidity were way off the mark. The Tormach that I have now has 1200 lbs of cast iron for an 18"x9" work envelope and it's still not a true production mill. Something that simply feels rock solid isn't enough for a CNC mill you need to start measuring deflections with a dial indicator. I'm not sure what the typical force on the cutter is when milling steel but I'm guessing it's on the order of 30-100 lbs. If your thinking about a 3/8" cutter in steel then you then need a structure that deflects less than a few thousandths with these forces. The column on my X3 would flex about .008" with 20lbs of force. Cuts that were .010" deep would make the mill shudder like a jackhammer. You can’t cut intricate inlay with an axe nor can you cut down a tree with a chisel. Each tool has its place. That said I'd love to own a CNC router since large projects aren't possible on my mill but I simply don't have the space

[finaltheorem47]

Wow thanks!!! That was a huge help!

Warmest regards,

-Joe