Stepper or servo moters (Lots of ?'s)

[Toolmaker]

Howdy all! I am in process of designing and building a cnc router.
I have researched this site and found a ton of information so far.
I would like to know what type of motors I should go with.
Servo Motors or Stepper?? And what size motors for cutting Aluminum (up to 1/2" thick) Wood, plastics etc.

I would like to purchase an entire kit consisting of the motors, Drives, etc. Where everything would be plug and play.

I have been looking at Turbocnc software, It looks real good with "G code" programming, any suggestions for other software that may be better?

Sorry for all the questions and I am sure you guy's have read and answered these type about a Ga-zillion times.

Thanks for any suggestions you can give me.

-Toolmaker-

[balsaman]

What are you building the machine out of? How big? What sort of spindle?

E

[Toolmaker]

It will be built out of aluminum, travel around 20"x20"x6"
I am not sure about the spindle, perhaps a high speed router.

[arvidb]

If you want to cut aluminum I'm not sure a high speed router is the correct choice.

You want to aim at a specific "surface speed" or edge speed of your cutter. This is different depending on what material you cut, the material your cutter is made of, and type of cutter. Also it depends on the type of cooling used.

Let's say your edge speed for milling a certain metal with a certain type of endmill is 0.5 m/s. This would mean that a 10 mm dia endmill would work best at about 955 RPM (edge speed / circumference of cutter * 60). This also means that a 1 mm dia endmill of the same type would need about 9550 RPM to work well!

But this is not all. You also want a certain chip thickness (you can find data on both optimum edge/surface speed and chip thickness of an endmill at the tool manufacturer). So, again let's say you are using the above 10 mm dia cutter at 955 RPM, and want a chip thickness of 0.1 mm. Let's also say this is a 4-flute mill.

The endmill will rotate 955/60 times each second, and each rotation means four cuts (four flutes) = about 64 chips/second. So you should use a feed rate of 955/60*4*0.1 = 6.37 mm/s.

With a router there's risk of melting through the metal instead of cutting. This works, I guess, but the cut will be ugly and you will destroy your tooling.

Actually, with a very fast spindle (perhaps 40000 RPM), special endmills, and a very high feed rate, you can cut aluminum without cooling and get a very nice finish. This is because all the heat is removed with the chips. But this kind of equipment is not within the budget of a hobbyist :-(

Arvid

[ger21]

http://www.onsrud.com has router bits, not endmills, for cutting aluminum with CNC routers. They have a catalog you can download that lists recommended chip load for all their bits in different materials. For aluminum, at 18000 rpm, they recommend .004-.006 for most 1/4" to 1/2" bits. The problem with this on a homebuilt machine, is that for a 2 flute bit @18K, you need to cut at around 180 ipm if I did the math right, and I don't think I've seen anyone here cutting aluminum at 180ipm. Now you could in theory slow your router down to 10,000 rpm and 100 ipm, which might be attainable, but you better have a very ridgid machine and a pretty powerful router. You can always get away with taking very light passes and going very slowly, but the bits are not designed for that and will dull very quickly. If you really want to cut mostly aluminum, build the strongest, fastest machine you can afford. (this probably goes for cutting wood as well, but stiffness is more important with aluminum). And everyone here knows, speed costs money. What kinid of budget for motors and drives are you looking at?

Gerry

[arvidb]

Originally posted by ger21 http://www.onsrud.com has router bits, not endmills, for cutting aluminum with CNC routers. *snip *

Thanks for the link, really cool homepage!

Originally posted by ger21 You can always get away with taking very light passes and going very slowly, but the bits are not designed for that and will dull very quickly. *snip * Gerry

If you go slow at high RPM, things will quickly heat up and the metal will melt, clogging the router bit or mill. You will get a very ugly cut with a melted "edge" or rim. Maybe with lots of cooling? It will still dull the bits though.

Arvid

[HomeCNC]

Toolmaker,

Glad to see you got registered here. Ger21 is right on about needing a very ridgid machine. That is why I use my milling machine for metal and my aluminum built router for wood and plastic.

[Toolmaker]

Thanks guys for all the information, I kinda thought the machine may not be rigid enough to cut aluminum so it will be used for wood and plastic only. I run a Haas mini mill at work along with a Clausing/Kondia knee mill, I can use them whenever I want on my own time so I will make my aluminum stuff there.

Thanks for you help Homecnc for getting me registered here.
Any suggestions on Steppers vs. Servo's?

Thanks again

[pack rat]

Check out the treadhttp://www.cnczone.com/showthread.php?threadid=2360 servers are better for accuracy due to feedback design.

[ger21]

Exactly where in that thread does it say that servos are more accurate than steppers? A properly set up stepper system can be every bit as accurate as a servo system. The feedback just tells you when the sevo loses position, with a stepper you have to figure it out yourself.

Gerry

[keithorr]

Originally posted by ger21 The feedback just tells you when the sevo loses position, ... Gerry

Question for servos then:

Doesn't the feedback error cause the driver to produce more current so that the error is corrected? I thought closed loop meant the axis deviation would only exist (under ideal conditions) for a short duration of time until the axis caught up.

Yes/No?

[HuFlungDung]

You are correct, Keithorr. IIRC, some of the less expensive controller softwares some of these guys use does not fully close the loop.

Personally, I couldn't be bothered to waste my time and money on a stepper system, but that's natural for metal workers