Closed loop vs. open loop

[pelesl]

So I'm interested in getting a dekstop CNC. Primarily, I'll be making small aluminum parts, and maybe very thin steel parts---think camera modification and RC car parts. I also want to have the 4th axis, at least as an option. As a secondary goal, I would also like to make ``free form'' 3D shapes; that is, curvy blob-like shapes of aluminum and plastic/foam.

I'm not really interested in building my own CNC; I'd rather get one ready to go and start using it. I've done enough hand machining to know that I don't like it, and I need to get this up and running quickly for a project.

I saw the thread on the guy who wanted ``all the desktop mills'' (http://cnczone.com/forums/showthread...light=benchtop) but he chose the Grizzly pretty quickly and that became very specific fast.

As an example of price range and size, I have two mills to consider: the X2 CNC kit from SYIL, and the MaxNC 10 CL-EC

http://www.syilamerica.com/product_X2.asp#X2package
http://www.maxnc.com/page2.html

both of which have 4th axis capability.

I have not found much information on the MaxNC's, and what I saw in this forum didn't look promising; it doesn't seem people trust them much. The Syil is chinese; I guess in the end every component is made in china but I would feel better if at least some of the heavy stuff was made here.

Aside from moral issues, the MaxNC has only two advantages I can see:

1. It has a closed loop system. Seems to me that this would be really useful in a small mill that may get ``stuck'' somewhere.
2. The built-in computer seems like a better idea than getting an old lump running DOS to run the machine.

The Syil, aside from looking much nicer, has software (in the package) for going straight from CAD to program (ala Gibbs CAM, I suspect). This Bob CAD/CAM seems to cost as much as the machine! I suppose the version included by SYIL is limited somehow, but it's better than nothing. Also, the package seems to be ``ready to go'' with it having the vise, etc.---though LittleMachineShop is here in Pasadena so I could get that stuff pretty quick. The MaxNC is much less clear about the software included, but I suspect if it's CNC from the factory it at least comes with controller software.

So now the series of questions:

1. Is closed loop really worth it? Obviously any stepper big enough will never slip so there's no need for encoding.
2. If you don't get something like Bob CAD/CAM, how do you go from SolidWorks to program? A brief overview-style tutorial would be nice, if someone can provide it.
3. It seems to me that other mills around this size/price look like three traverses tied together with a bracket and a drill chuck on one of them. What other machines are there out there for less than, say, $4k?
4. Is there anything else that I missed? Sometimes feature lists discuss interpolation schemes, simultaneous multi-axis movement, etc., but this all seems to me to be obvious; that is, any CNC should have such capabilities. Should I be more careful?

Any suggestions would be appreciated.

[Legion99999]

I have owned both machines from maxnc. Do yourself a favor and find a different route. I have Syil machines now. Much better machine and company support.

I cannot give Maxnc a low enough grade. They should be arrested for fraud. The machines are not what they advertise them to be.

Open loop has to do with steppers with no feedback or computer correction where as closed loop tends to be with servos and recieves feedback from the motors...as i understand it.

I ahve been waiting 3 weeks on a replacement cable from maxnc.

Take a look a syil or smithycnc.

So to anwer your questions.

1. If you do buy a maxnc yes buy the closed loop because they are under powered and often need correction.

2. You will need a cam program to convert Solidworks to a usable g code.
Maxncs cam is very primative. Buy Mach 3 and a decent cad/cam program. Bob cad cam is great and worth every penny. But the tutorials with it though.

3. Syil, Smithycnc, Microkinetics. Seem to be in your range and a much better choice based on my personal expierence.

4. The cnc learning curve is endless, there will always be something you have missed. Read, and listen, tons to learn and enjoy.

[Stepper Monkey]

The X2 and the MaxNC 10 are two entirely different classes of machine side by side. The X2 is much larger and MUCH more of what you are looking for. For one, the X2 can cut metal.

The MaxNC 10 looks pretty solid in pictures, but it is a Sherline-sized toy with even less rigidity. Don't try cutting metal with it. The 1/4-20 allthread for leadscrews, the rivets, and the plastic components do not make for a reliable or worthwhile machine no matter how cool the closed loop system may seem.
I for one like the idea of a closed-loop system, but;
1) It usually isn't necessary for small hobby machines unless you are really trying to push out that last few IPM for production work, or you have a machine so weak and sloppy it is always missing steps anyway like the MaxNC 10.
2) It is a feedback system, which is a big step up over nothing but isn't really a true closed-loop system.
3) No high-tech control system - when attached to a MaxNC 10 - means a lot. Putting $5000 rims on a Pinto doesn't make it a Ferrari.

You also might want to take a second look at some of the other machines that "look like three traverses tied together with a bracket". At the end of the day, what is a vertical mill other than that? Unless you need a quill, something like a Taig might be a great solution for you. It might look crude, which is misleading - all the money went straight into the parts that make a difference, and all the engineering went into what makes it work, with not two seconds thought put into what it looks like. Just good engineering with no frills, and extra helpings of ugly.
It is still not quite a full size or professional grade machine, but it kicks the living daylights out of a MaxNC 10 in every conceivable metric of performance outside of a beauty contest.

[pelesl]

The MaxNC 10 looks pretty solid in pictures, but it is a Sherline-sized toy with even less rigidity.

Actually, the Sherline is what I meant when I said Taig. The Taig does look pretty solid, but it doesn't seem to have a fourth-axis option.

Thanks for the clear comments. Evidently the initial feeling I got about the MaxNC's was on the right track.

You also might want to take a second look at some of the other machines that "look like three traverses tied together with a bracket". At the end of the day, what is a vertical mill other than that? Unless you need a quill, something like a Taig might be a great solution for you.

What is a quill?

[pelesl]

3. Syil, Smithycnc, Microkinetics. Seem to be in your range and a much better choice based on my personal expierence.

It seems you are happy with the Syil. That is definitely looking to be the most attractive, if it does indeed come with Bob CAD/CAM, which you also recommend.

The SmithyCNC also looks nice, but it doesn't seem to have a fourth axis option, and if it does, it looks like it will put the price a little out of reach.

The Microkinetics I had seen before, but they look so dinky---looks as though the whole thing would twist just from turning it on. Any experience with this machine?

I also looked at the Rolands, which someone here in the university has (http://www.rolanddga.com/asd/products/milling/). I don't know which model exactly they have, but it cost about $20k, and I was hoping they had something cheaper, but it doesn't look like there is much. Are the more expensive machines just going to have more software and higher reliability for production purposes or what?

[Stepper Monkey]

Any machine that has a place to bolt one on does have a fourth axis option! Any rotary axis from any company will go onto any other machine.

My Taig has a Sherline rotary. Even my $15,000+ Model Master has a factory installed 4th axis that is really just a modified Sherline rotary as it was the best choice for what they wanted to do. Very few manufacturers bother making thier own from scratch, there is no point. There are already some good ones out there.

Don't let the 4th axis enter into your choice of mill, treat it is a separate accessory just as much as a mill vise or coolant system.

-------------------------

BTW, I think the Rolands, as well as similar higher-end machines like my Model Master, are great units FOR THIER INTENDED PURPOSE. Thier purpose is just not what we want to use them for. For us they usually turn out to be less flexible, less capable machines! The extra money really goes into reducing variables we take for granted, not increasing machining accuracy, speed, or capability.

These machines were NOT designed for machinists. They are essentially designed as an appliance, or as close to one as a CNC mill gets. They were designed for people like engineering firms or jewelers to turn out prototypes, who only call in a machinist to look at it when it breaks in the same way they would call in a technician when the copy machine stops working.

If you need a fault-tolerant and bulletproof unit to run for years without adjustment, in a production environment, dependent on one machine to perform a critical function, purely as an appliance, in the hands of non-machinists or bumbling students, it is worth every penny.
Other than that, don't bother!

[pelesl]

Any machine that has a place to bolt one on does have a fourth axis option! Any rotary axis from any company will go onto any other machine....Don't let the 4th axis enter into your choice of mill, treat it is a separate accessory just as much as a mill vise or coolant system.

I was actually worried about controlling the fourth axis. If I get a CNC from the factory, I figured it wasn't understood that the controller provided would have the extra channel. I can always find out, of course. But since I do want to get this thing up and running soon, the X2 kit is sounding better and better.

The guy that has the Roland here must have spent a fortune. The machine he has is $25k without the rotary axis. He also got the automatic tool holder, and has two CAM packages. I'm sure he went over $30k.

The X2 sounds like a nice one to get started with since it comes ready to go. Then if I need to expand, I will know more about CNC and how it all works; at that point it will probably be better to just do my own conversion.

[Stepper Monkey]

The guy that has the Roland here must have spent a fortune. The machine he has is $25k without the rotary axis. He also got the automatic tool holder, and has two CAM packages. I'm sure he went over $30k.

It is really easy to do. I also went over 30k on the Model Master/Artcam setup, and while it was worth every penny for what it was intended for (a production appliance), and paid for itself handsomely, it is no where near as capable or utilitarian in our hands, for our hobby uses, as the sub-$2k Taig on my desk.
Much the same as a 4WD pickup has more utilitarian uses than a Rolls. Both are worth the money but don't do the same things.

This is why I have both and still recommend even if you won the lottery tomorrow stick with an X2 or a Taig over the Roland or a Model Master for hobby use.

[pelesl]

By the way, I just contacted Syil this morning asking some questions about the X2 kit with the fourth axis, etc. Long story short, they answered my questions within 2 or 3 hours by email, and offered me a free upgrade to a faster spindle transfer (belt-driven).

The key answers that I got:

1. The controller supports up to seven axes.
2. The Bob CAD/CAM which is included in the package is a full version.
3. PC connects to the controller via parallel port. Still waiting to hear, but I assume the program that sends the instructions is Windows compatible. Still it's probably best to have a dedicated el-cheapo computer for that type of thing so that a nicer CAD workstation doesn't have to be tied to the machine.

[NC Cams]

For $20K you can buy a very decent and well equipped Bridgeport V2XT - I've seen many priced in the $12K range w/o 4th axis.

These are factory fitted with the wiring for a supplemental 4th axis (all you add is amplifier and external axis) and you'll have a REAL mill based VMC machining center albeit something less than a Haas or Hardinge/Bridgeport VMC.

I know of a recent 'find' where a member found one WITH a 4th axis and only paid a little over $10K!!!.

I wasn't as lucky as my "combo" package of the same device has run me nearly $17K and I had to buy it in installments (first the VMC part, then the 4th axis).

For $25k-$30k, you can go to a Haas TM1 or so and have a state of the art package that will easily adapt up to a tool changer and 4th axis.

EDIT: all the above systems are closed loop servo systems

[awerby]

If you're planning to make relatively small parts in metal, then the Taig is definitely worth looking at. It does come, optionally, with a 4th axis (based on the Sherline rotary table) and the newest model does have a closed-loop feedback system, which stops motion if the machine can't keep up with commanded position. The control box handles 4 axes, even the one shipped with a 3-axis mill. For a machine that's made in the USA, the price is very reasonable, and support from the manufacturers is good.

Roland makes a range of milling machines which vary in price considerably, but none of them, with the exception of the most expensive current model (the MDX-540), are intended for milling metal. However, if you're planning to make parts in wax or plastic, the lower-priced models are viable contenders, and they include CAM software that is easy to use. The MDX-15, MDX-20, and MDX-40 models can also be used as scanners, and they do an excellent job of this, using a piezo-electric touch probe to produce very finely detailed models down to .002" granularity.

Andrew Werby
www.computersculpture.com