[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?s=&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

[pack rat]

The purpose of the tread above is to provide the source of servos and inside that tread is "cheap servos" tread that takes you to SOURCE for 10$ apiece. You have to add encoder from US digital, that too is explained in the treads. As for accuracy of steppers verses servos it is based on reading of treads in this forum. I should have posted it as my opinion, sorry.

[ger21]

I misinterpreted your post, pack rat. I was assuming that since this is the DIY forum that no one here is using a closed loop system that can make corrections for errors. I should have stated that. Does anyone have a homebuilt machine that can do that? If so, what software/hardware are you using?

Back to the original intent of the thread, I think steppers will be fine for a machine that small. You just need to get the right size steppers for your application.A lot of people here are using smaller steppers. 80-100 oz-in steppers might work for some here, but I think the prevalent use of small steppers here is why you see so many "servos are better" posts. But it seems like everyone with servos has 300+ oz servos. Apples and Oranges. I read a post from Mariss somewhere where he said up to a certain power level (100 to 200w maybe?) that either would work fine. Once you get above that level than servos are the better choice. Hopefully Mariss will chime in and maybe post the power formulas I saw over at CCED. I just looked, but couldn't find them.The question you need to answer is how fast do you want to cut and how much do you have to spend? Do you want to cut 3/4" hardwoods in 1 pass? The more info you give, the better answers you get.

Gerry (all this is just my opinion, btw)

[Toolmaker]

Thanks Gerry, I guess I wood like to cut and rapid as fast as possible
I am thinking of going towards the servo motors, more and more because of the speed they deliver. All modern cnc machines use servo motors and I think there is a reason for that.

My old Clausing knee mill at work can go 300 ipm's I think I would be happy with 50+ ipm's.

I would like to cut woods up to 1" thick with multiple roughing passes, so I was thinking of a servo motor with 300+ oz/in
peak.

Yes I am finding out that one hobby of making things, has turned into another hobby, of making a thing, to make a thing

[balsaman]

They both are very accurate provided you don't go over thier rated output. Servos are usually faster, because they are able to provide thier torque at rated rpm, whereas a stepper provided thier rated torque when stopped. I think if you spend as much on steppers as you would on servos, you probably would get close to the same performance, as long as you are not needing extreme power. Also with servos you know when you are having a problem, whereas with steppers you may not notice a few missed steps. Servo's and thier drives are more complex and more expensive as a rule.

For a first machine, steppers are a great way to go.

E

[Toolmaker]

Where can a purchase a good set of steppers that are the equivilant to the servo motors I spoke of in my earlier post.
And can steppers be mounted directly to the lead screw?
Thanks

[ger21]

Most people here mount their steppers direct with some type of coupling. How much torque are you lookin for, I don't see what servos you mean? You can get steppers here:

http://sanyo-denki-online.com/
http://new.kimcontrols.com/category/...fic-Scientific

What do you plan on using for drives? Geckos? You still haven't shared your budget. Do you want to cut or rapid at 50ipm? I've seen machines here with a Xylotex and 100 oz steppers that could rapid over 40 ipm for under $200 for 3 motors and the drive. Geckos will set you back at least $350. with servos you're probably looking at $600-700 MINIMUM, unless you find some eBay deals. A Xylotex and some 200-250oz steppers with new motors will be $400 to $500. Maybe $300 to $400 on eBay.

Gerry

[ynneb]

Toolmaker,

I just bought these servos on ebay.


He has 150 of them for sale at $450 for 3
I havnt taken delivery of them yet. (Still been shipped from the US to Australia )

The only thing I can vouch for is the seller and his willingness to answer heaps of my stupid question. He seems to know what he is talking about.

Here are the specs

He is willing to fit the encoder of your choice onto them included in the cost. ( I think that is right ? ) Best to ask him

His email is [email protected]

Torque Constant 44oz-in/amp
Voltage Constant 32.5 V/1000RPM
Nominal Voltage 40 VDC
Maximum Voltage 60 VDC
No-Load Current <.51A
Demagnetization current >20A
Terminal Resistance 2.60 ohm
Mech Time Const(max) 18.3 msec
Armature Inertia .09 oz-in-s^2
Max Friction Torque 15 oz-in
Damping Loss Torque 12 oz-in/krpm MAX
Thermal Resistance 22deg C/watt MAX

I have also bought the controllers for
the above servos


3 x R990H (100V/20A servo drive) @ $148 = $444
1 x R990mb (Mother Board)@ $58 = $58
1 x R990io ()@ $68 = $68
1 x delivery @ $10 = $10
Total (before tax) = $580
GST = $58
Total incl. GST = $638 (USD)

The only other thing is the transformer that I am told I will need is a 40 volt 36 amp transformer. Probably $200-300 bucks will buy the tranny, diodes and capacitor

I hope this gives a guide to costs to set up a servo system.
These prices are currnet as of Feb/20/2004

Oh and you mentioned software........um I am so undescided I am sure other could help with that. I would also be interested to hear opinions.

[HomeCNC]

Ah yes, Mr. KevenCNC and his Amatec windmill motors he converts. He used to have encoders that were not compatable with the Gecko drives.