Reasonable speeds for a cnc?

[Oce]

Hi everyone, I'm in the process of building my first machine...It has all taken a good while so far, and I haven't started building anything yet...Like to do a good bit of research before I do.
Sounds like a hardcore machine for a first-timer, I know, but I like to do things big when I first take the time to do it...

So far, I've ordered 25mm bearing blocks and support rails, breakout board from PMDX, 4 1200 oz-in steppers from Keling Inc, and are about to order drivers, probably Gecko or some with same specs.

My target machine has a working area of about 2.5x2.2x0.6 m (XYZ)
(Open for discussion and reduction on this point)
I bought 4 steppers to ensure that they can move the gentry properly, so I will have one on each side of it.

The router will be used to process mostly wood, and the occasional aluminium / steel pieces...

I will use mostly steel and aluminium for construction, as I'm quite familiar with working metals, and have a fairly good place to do the necessary work.

But then comes the tricky part; what should I use to drive it?
I've looked at a lot of different solutions, and the Nexen RPS system really got my attention. I've made contact with a local distributor, and they will give me a quote as soon as they get the necessary data.
I know, ballscrews are an option also, but I've read that the RPS can, in many cases, match or have a lower price than that of ballscrews, while at the same time be faster and more precise...
(Sounds like I'm taking the factorys word for it, right?)

And this is were you guys come into the picture; any idea what a "reasonable" speed, acceleration etc would be for a machine this size?

The information requested;
Total system weight kg
Max speed m/sec
Acceleration time sec
Max acceleration m/sec2
Guide System Friction µ
Degrees from Horizontal
Desired run length m
Cycles per day

I assume the weight will exceed 200+ lbs, no doubt about that, so let's just say 400+ to be on the safe side.
Guide system friction; I didn't find anything at the dealers site (marchant dice in the UK)
Max speed, acceleration time, etc, I'm open for suggestions?

Just give me YOUR honest opinion, so I have something to work with.
Any videos from youtube etc demonstration estimated speeds would be great!

[DocTanner]

Oce,
You might want to look at www.mechmate.com <--- Fantastic Machine Free Plans and Gerald is willing to "coach" you through it. I'm 99% complete with mine

EZ-Router.com and Shopbottoolscom for specs on machines similar to what you want

DocTanner

[Oce]

Yes, I have seen the plans, but I like the idea of building something on my own... maybe I will post my own plans here in the future as well
Thanks for the tip, I'll look into it!

[Zumba]

First things first. You need to draw your machine using a CAD program and really figure out exactly what you're going to be making, right down to the last bolt. You should really consider building a small machine since this is your first. Large machines are exponentially heavier and exponentially more expensive. Anyone who says otherwise hasn't done the math.

So what are the exact components you've purchased so far? What are these "25mm bearing blocks" you speak of?

I noticed that you want 0.6m of Z-axis travel. That much travel will require a Z-axis that's around 1.5m tall. For your first machine, try something more along lines of 0.15m of Z travel.

Nexen RPS... don't think you'll be saving any money with these. They claim to be cheaper than ballscrews. But they're comparing them to precision ground ballscrews, not the rolled screws that most of us use.

The questions you ask regarding friction, speed, etc. are impossible to answer. Kind of like asking, "how fast will my project car go? I've purchased a 400bhp motor so far."

[Oce]

Hehe, I realise that they are difficult to answer, I just want opinions, not definite answers (thats pretty impossible I understand.. )

I've done some small drawings so far, but nothing big.
I use AutoCAD at school, so I think I wil draw a simple 2D drawing.
Haven't started drawing in 3D yet, and I've heard it's pretty hard, vs that of Solidworks... Learing another program isn't the favorable option, although I might have to...

The price & weight doesn't bother me too much, I plan on using a lot of time and money into this, making it a great machine


Parts-list:
http://www.kelinginc.net/KL34H2120-42-8A.pdf 4x
http://www.pmdx.com/PMDX-122/index.html 1x
http://www.marchantdice.com/linear/SBR/index.htm 12x + a lot of rails in 100mm sections.

About the ballscrew VS RPS; I haven't looked too much into the advantages/disadvantages, but I like the idea of Nexen having 0,5m and 1,0m racks.
However, IF I had some estimates for the "recommended" speed etc, I could get a price quote, and maybe adjust accordingly...
(or just build-as-I-go.. )

Thanks,
Anders

[ger21]

The price & weight doesn't bother me too much, I plan on using a lot of time and money into this, making it a great machine

If you're going to spend the money, I'd shoot for 400-500 ipm cutting speeds and at least 1000 ipm rapids. You'll need a good spindle to handle those cutting speeds, though.

[Oce]

Thanks Gerry, sounds like a pretty high speed machine..
My vision is to start with something extremely good, fast, reliable etc...So the dissappointment of the final performance isn't too big...

Any suggestions on a price for ballscrews for a machine this size?

(It's not like price doesn't matter, it always does, but if I have to pay a little extra to get better overall performance, I probably will.)

[JerryFlyGuy]

Well I can offer some opinions and some of my experiances.

The steppers seem large enough for the job at hand, however you'll never run those steppers through the Nexen Rack's and keep the pinion's from blowing up [check the torque spec's]. I also doubt that they are more accurate than Ballscrews. I guess depending on the grade of ballscrew they might be but.. don't matter as they'll just bust on ya if you use them. Note also that this is assuming your using some type of reduction as direct drive would be too course of a resolution [distance per step] to be of use. For a table that size Ballscrews are the order of the day. Check out some websites like Nook or Misuimi [spelling] and see what they recommend for sizes to counter the whip issues that are going to crop up. You'll need to figure out what reduction you'll use as well as the needed power [at stepper rpm] to cut at the speed you want. Then know what lead to use to get that speed, then you can figure out screw RPM and figure out what size will not whip. I'd think a 1.5 to 2" ballscrew is going to be needed [if not larger].

I think that you could do it in 400 lb's but that would be lite. I'd shoot for more like 1000. If you really want to cut Alum/steel, if its just wood you'd be fine at 4-500.

I'm building a fairly large mill and I'm over a couple thousand already. I'm using rack and pinion for the X and Y axis, but then they are 20 and 10 ft respectivly.

As Zumba said, get it designed as its a complex pc of equipment and you'll having nothing but headaches if you don't know how its going to go together before you start. Either that, or it will look like it has warts all over it from just 'hanging' things wherever they 'look about right'.

Ger21 has pointed out that larger rapids and feeds will make it a real workhorse, also he mentioned that it will take a pretty good spindle, at these speeds I don't think a 3hp plunge router is up to the task, do you have a spindle picked out? I would first decide on what I want to spend, then find a spindle in the range and design towards that, that way you won't have a machine w/ a 2 hp spindle that could [ in theory] at 400ipm, but effectivly can only cut at 100 because of the spindle. 7-10hp is probably going to be required, that is unless you only want a DOC of 1/16" then the 2-3 hp would probably do it!

Keep at it! It's a terribly addicting past time, as well as expensive but I think it has its rewards for sure.. nothing like building something on a machine you've created and built. [ I've not felt that feeling yet, but in the next few weeks I hope to! ]

Jerry

[Zumba]

Thanks Gerry, sounds like a pretty high speed machine..
My vision is to start with something extremely good, fast, reliable etc...So the dissappointment of the final performance isn't too big...

Any suggestions on a price for ballscrews for a machine this size?

(It's not like price doesn't matter, it always does, but if I have to pay a little extra to get better overall performance, I probably will.)

You need to realize that "thinking big" is not necessarily a good thing here. If you really have the money and the time to do things right, take my advice and build a small 0.5m x 1.0 m prototype first.

Those linear rails... they're not bad. But they're not what you'd see on a machine that's "extremely good". Not even close. Those types of rails were never meant to carry heavy loads. They were meant to be very accurate, used to carry moderate loads in a clean environment, e.g. medical. I believe MRI machines use those in 50mm sizes. Then China came along and made junk versions of them that aren't very accurate. For a heavy duty CNC application, the only rails you should even consider are the square rails/blocks from THK/Hiwin. You'll noticed that the spec charts for all linear motion components have a "Dynamic Load" capacity column. If the dynamic load is 900lbs, that doesn't mean you're "safe" with anything under 900lbs. This number is used to directly calculate the life expectancy of your bearing blocks. The round rails you linked have a dynamic load capacity somewhere in the neighborhood of 10-25% that of THKs/Hiwins... far inferior. Another problem is that they need to be mounted upright to retain their load capacity. Once you turn them sideways or upside down, the load capacity goes from mediocre to pitiful. They're also more prone to dust contamition, as the seals that come with those bearing blocks are much thinner than those found on THKs/Hiwins. If you already bought em, sell em on ebay and recoup some of your money.

As for what size ballscrew... a prototype .5m x 1.0m would only require 20mm and even 15mm screws. But the machine you described would require 35-50mm screws on the long axes unless you put the screws under tension. Take a look at the prices for the big screws and compare.

The 1200oz-in motors have lots of torque, but the high inductance due to the length of the rotor/stator limits the top speed. Big, fast machines cannot be powered by steppers... servos only. There is a reason why machines such as the Shopbot use steppers.... they're very lightweight. But mass = rigidity. If you want a heavy, accurate machine that flies, you'll need the right drive system.

Don't let any of this discourage you. But I can tell you that jumping into a large machine and wanting the best will lead you to disappointment.

[Oce]

Thanks for the advice Jerry, I have also figured that there will be a need for some sort of reduction due to the torque...But I will take a look at the Nexen RPS none the less.
Might come up with some useful info for others as well...
(And that I know is a huge plus to other users!)

Ballscrew size; I imagine something along the lines you said, therefore I'm a bit worried about it getting awfully expensive.
But then again, IF and WHEN if turns out great, I'm sure it will be worth it...

Spindle; I have prepared a space in my garage / workshop, with 3-phase power socket for the router.
Still haven't thought about which one yet, suggestions are welcome

Zumba; wow, you really know how to put it to a layman
I think I will survive, better with the cold hard truth, than working my ass off and getting crap in the end.
I will consider the possibility to buy hiwin or THK, I just haven't found any decent shops here in Norway, that's one terrible part about all of this, customer service and availability in Norway SUCKS.
I found some used ones in Germany though, what do you think about that?
Is this type of rails generaly the best choice, or just THK / Hiwin?
Here are a selection of rails availible in Germany:
http://search.ebay.de/search/search....e=search&fgtp=


Thanks again, all of your opinions mean a lot to me and my work, I will never accomplish this without you!

[JerryFlyGuy]

Zuma said it VERY well. I didn't actually look at the rails you'd linked to, but he's right, they aren't what you want. You want a THK or Hiwin or INA or SKF or.. you name it, there's lots. I'd think not less than 20mm and probably 25mm rails and blocks to suit, if you want really good precision you should get a larger, pre-loaded block over the 'standard'. I personally stear clear of Ebay rails as I don't know their history, they could have been run on a grinding machine and I wouldn't know and won't be able to tell until they are set in place and checked for wear. Unless they are NIB on Ebay, I'd pass [again the precision thing].

Cost, I found it very suprising [as have others] but again as Zuma stated.. large machines cost more money [exponentially] than small. In reality I doubt you'd be able to make the machine your thinking for less than $20,000 USD, if you do you've skipped a step someplace in the 'precision' or the 'large' part of the equation.

You won't need 3 phz as you can get VFD's which take single and switch it for you. However, since you've got it so much the better. I'd keep an eye on ebay as higher HP Columbo spindles do come around from time to time. You should be able to pick up one for $2500-3000USD if your lucky, sometimes less. If you want to rebuild one, then less than even that.. The only reason I'm ok w/ Ebay for things like this is you CAN get them rebuilt, rails can't be. New spindles are lots of cash, I priced a 2hp HKS [ I think it was] spindle w/ pneumatic tool changer and it came out to $9kUSD for one the size your looking for, I'd expect $10K+USD [new].

You'll be able to get a VFD off ebay as well [Hitachi is a good name], keep in mind that you should buy your spindle first, as a VFD is NOT a one size fits all, you've got to spec it and the reactors to match the spindle.

There is a great bunch of guy's on here, who make the 'zone' what it is [ I'm not talking about me ] I know I wouldn't be where I am in my project w/out their help, thats a certainty..
HTH
Jerry

[Oce]

Uhm, you 're making me nervous about the whole project by saying this...But I've given it some thoughts together with a friend of mine...
Let's say we DO use the rails mentioned, and take the chance of them breaking relativly soon...What do you think about changing rails later on? Very much work, or can it work out?

20.000 sounds like a lot of money, where do you think most of it will go? Sounds like an awful lot right now, but I guess costs add up quickly enough...
(Looks like I won't be changing cars during this year after all...)

Thanks again!