1. ## Aluminium machine 1.5mX1m

Hi,
I have started to design my own Cnc machine...
I decided to build it with profile aluminium and aluminium sheet...
Below I attach two photos of it designed in Google's SketchUp (very nice program).
I would like to use stepper motors in all 3 axes and 4 Hiwin linear guideways (size 15) in every axe.
Could you tell me what should be the stepper motors' power?
I am not a professional, so I want it to be not too fast and not too slow...I need a middle satisfying speed and I want lot of accuracy too...
Also what controller should I look for the motors you suggest?

Thanks in advanceand nice to found you...
Kostis.

2. The motor power required relies on a lot of factors, and some of the most important ones it looks like you haven't decided on yet (although if you have, we'll need to know to help you).

The force required to move something, at a minimum, is (ideally) equal to the coefficient of friction multiplied by the normal force (gravity*mass) of the object you're trying to move. What is the weight of your gantry? Calculate this in pounds. Try to overestimate, not under. Then look at the specifications for the linear bearings you intend to use. I imagine they come with specifications on their friction, along with maximum load etc. which you'll want to check.

Saying your mu (friction coefficient) was 0.05, and your gantry was 100lb, the needed force would be 5lb-force. Now you need to look at what is supplying that force - a lead screw of sorts, let's say a ball screw (smaller diameter screws will not work at this size very well, FYI). Find the efficiency of this screw from its documentation. A good ball screw has an efficiency of around 90% at minimum, so let's say 0.90. Divide your needed force by this (5lb-force / 0.9 = ~5.6lb-force). Now you need to do calculations on the force supplied through the ball screw (before losses due to efficiency) by the stepper motor.

Imagine a ball screw of diameter 2". That means the point at which force is being supplied by the motor at a radius of 1". Motor torques are rated usually, in our case, in ounce-inches. This means it is the number of ounces the motor could move if attached an inch away from the center of the motor's axel (as you can imagine, if you put the weight on a lever 20" long, it can only hold 1/20th the weight. Holding something on the end of a long beam will be more difficult than on a short, as it works as a lever, with the fulcrum at the point you're holding it).
Anyways: Now you take the required force, and take into consideration the number of turns the screw must undergo to move the gantry a single inch. Say there was 2 turns/inch. That means each turn of the screw 360 degrees (2*pi radians) advances your gantry by half an inch.

You multiply the needed force by the distance the gantry progresses per turn (0.5") and divide it by the radius that the torque acts on (1"). Remember that one turn is 2*pi radians. All physics formulas work in terms of radians (it's a lot simpler), as when you multiply a radius by a distance in radians you get an arc length, and so on, without any need for conversion. Since the force is acting over the diameter of the circle that is the cross-section of the ball screw, you divide the number you have so far by 2*pi to get the equivalent torque needed for your linear force.

So finally, that's your 5.6lb-force * 0.5" (inches/turn) / 1" (radius of ballscrew) / (2*pi radians) = 0.44lb-inches or just over 7oz-in. (Remember, we started with pounds, not ounces, so we have to convert!)

This number is the amount of force required to _keep your gantry in constant velocity_. That means applying this amount of force won't move it anywhere unless it's already moving. So your motor needs to be rated a good bit above this. How much? That depends on the acceleration you want. Go back through all the calculations above, except this time leave out mu (friction coefficient). This gives us the torque required to accelerate the gantry at 1G (9.8m/s^2) after friction is taken care of. So that's 7oz-in / 0.05 = 140oz/in. So to accelerate (from rest) at 1G you'd need a stepper capable of ~148oz/in.

These are all best case numbers, so now you have to add a bit of fudge room. The faster a motor turns, the lower torque it can output (in general, for steppers). Steppers are usually rated for their _holding torque_, that is, the torque they can apply at stall (zero motion, simply holding a weight, fighting some force, etc. without any turning). As soon as your gantry starts to move, the motor is speeding up and losing torque. Very quickly the acceleration you started with goes down to 0.5G's or 0.25G's. You'll need a full data sheet on a motor to figure out exactly the rate at which its torque decreases in relation to its speed (If you know enough calculus: You can solve dV/dt = (torque - friction)/mass for V (conversion of units is very likely necessary) and watch its limit as t goes to infinity to find the ultimate top speed of your router).

I'd say at least double the calculated required torque for the acceleration you want, and remember, in reality you'll never reach that acceleration due to all manners of losses of energy. Even if you could, it'd only last for a small fraction of a second, as the rate of change of acceleration is decreasing over time.

(Hopefully someone can double check my math?)

3. Hi,
and thanks very much for your scientific answer, although it was difficult quite difficult for me (as a newbie) to understand, but in the end I managed so...
I understand now (after reading in forums) that ballscrews, and not motors, do make the cnc accurate and also the solid and good design and assembly of the Cnc structure.

Well, in my case, my gantry will weight ~55 pounds.

I plan to put:

1) Linear guides 15mm on all axes (linear guides like this) and their trails

2) This motor in axe X and this drive for motor in axe X.

3) Ballscrew hiwin like this.
A question here: What is a logical ballscrew diameter in my case,so as to have accuracy and satisfying speed?

4) Are the motor and its drive enough and compatible for this job?

5) Is there any point in the design of the Cnc, as shown in the images below, that might be wrong or needs extra care, according to your point of view?

Thanks again.

4. ## Can a cnc router cut such a figure in wood?

Hi again,
some questions:

1) Can the router I 'll build cut a figure like this of image below?
Its overall dimensions are: 5cm(height) and 14cm(width).

2) Are there router bits of 0.3-0.5mm diameter, which can cut such thin lines?

I 'll use stepper motors and ballscrews.
Stepper motors/drivers/power supply etc from http://www.kelinginc.net/CNCNEMA34Package.html

Thanks.

5. Cjmovie,
Nice analysis, but you missed a few things. First, friction is still present while accelerating, so don't leave that out. Second, you forgot the force of cutting. Since that is the whole point, it should probably be included. ;> This is usually the big question - what are cutting forces? Personally I used 100lbf in my design and the end result seems to work well. Also, your steppers must have enough torque to generate this cutting force at the speed they need to turn during cutting.

6. Kostis,

Here's a link to my own build - maybe something in there would be useful to you. There are several 8020 machine logs and they're all worth searching for.

http://www.cnczone.com/forums/showthread.php?t=27527 (In progress - 8020, linear rails, ball screws, steppers)

I would download the 8020 deflection calculator and play around with it. Using those 2 pieces of 40/50mm? extrusion across a 1100mm (45in) span may not have the rigidity you need when you factor in the weight of the Z axis and router/spindle. The rails will help here as well, but you may need some type of backer plate to add some strength to your wide gantry.

You've chosen decent screws, but make sure they match your needs for speed and accuracy. With 5 TPI ballscrews, the NEMA 34 stepper packages will have plenty of torque to move your gantry at lower speeds (under 150 IPM, 750 RPM). Perhaps even faster than that, but it's hard to say. I don't think you'd need anything bigger than the 640 oz/in size, and the larger steppers may actually hurt your high end performance. Someone with more experience can help you here.

If you want to hedge towards speed/acceleration, you may want to consider a coarser thread, something like a 2 TPI or a 12-15mm pitch. This will still give you excellent accuracy, but allow you to keep the motors performing in their lower RPM range for potentially much higher speed. May people here are using Acme threaded 1/2"-10 5 start to get higher speed at lower RPMs.

The THK style 15mm rails are what I used, and they have been just fine. If I was starting over, or buying new components I might consider 20mm rails just to have a little extra beef. You can get long lengths of 8020 nut stock and have them drilled and tapped to match your rails. Makes installing them quite easy and very solid.

And yes, there are plenty of small bits for engraving and milling in a wide range of materials. The sizes you ask about .3-.5mm are fairly common, and with a well built machine will run very nicely. I use .5mm (.02") very frequently and I generally break them from careless handling, not during cutting. Doh.

Steve

7. Hi,

I am new to the CNC area (about 1,5 months) and find your thoughts interesting, however it is quite difficult for me to decide what should the ballscrews I have to get because I hear many thoughts on this.

For example, a technician from zapp automation told me the following:

"The larger the diameter ball screw the faster you can turn it before
it starts to whip (Bend)
5mm pitch is what most people buy, and because we offer 16 and 20mm
ball screws already machined, these are the best option.
1500mm is quite long for a 16mm ball screw and will have a max speed
of 750 RPM (62 mm/sec) but a 20mm ball screw at 1500mm long can
turn at 960 RPM (80mm/sec)
But with stepper motors running at low voltage will find it hard to
get to this speed quick.
What you need to know is what speed you need?
I think SY85STH118 (this) is a good choice for the X and Y axis, and the
SY60 (this) is good for Z, but before I can give you any more advise on the
motor selection, I need to know what linear speed you need?"

Stevespo you made a good machine and rigid. I agree with your opinion of something more rigid in y axe, so I am going to put solid aluminium(backer plate) on which rails will be put to finally make it more rigid and solid.
Do you think 15mm is a good thickness number for this plate?

Also, I want to have both speed and accuracy, so 2 TPI or a bigger pitch would do the work better,right? What are the maths for calculating speed and accuracy, given the pitch size, diameter size and TPI?

8. Kostis,

I think the 15mm backer plate will add a lot of stiffness up/down. This is similar to the K2 design. If this is bolted to your extrusions, it will add stiffness up/down and front/back as well. The calculations would not be too hard, but it feels like a good design to me.

I used 2 pieces of 1530 bolted edge to edge for a 1.5" x 6" main beam on the gantry. I calculated that my 35 lb load (Z axis + spindle) would create almost no downward deflection. In fact, a 200 lb mass would only create .001" downward deflection - so it's probably over built. More mass, but better safe than sorry.

The only thing I would change on my machine are the screws, or possibly the motors. If I used steppers again, I would probably choose a NEMA 34 and definitely use a 2 TPI equivalent screw on my X/Y. If I was using servos, I would keep the 5 TPI screws, as the servos run at much higher RPMs. Even with the higher screw pitch, the servos typically have to be reduced via timing pulleys to drive the screws effectively.

Assuming steppers, and a 2 TPI ballscrew (or acme) and an upward RPM limit (on motor and screw) of 750-960 RPM that gives you the potential for 375-480 IPM rapids - quite fast on a benchtop machine. I don't imagine that you'll need more performance than what this provides. Honestly, 150 IPM seems pretty fast on a benchtop machine, especially when something goes wrong and you're reaching for the eStop button!

As far as accuracy goes, that's based on the microstepping accuracy of your drivers and the pitch of the screws. The Keling drivers have a lot of microstepping options. If you chose to use 10:1 microstepping, that gives you 200 base steps/in * 10 microsteps * 2 TPI = 4000 steps/inch, or .00025" potential resolution. There has been some discussion on the true accuracy of microstepping, but let's assume that it works as advertised. This is far more precision than necessary for woodworking applications.

In reality, the rolled screws have no where near that accuracy, and the material you're cutting may change more than that during the process of machining, so the 2 TPI screws will give you great potential for speed as well as great accuracy (with microstepping). The screw diameter doesn't influence the machine accuracy. It allows for faster RPMs (less whipping), and changes the torque calculations somewhat modestly.

As I mentioned - if I was starting over, this is what I would choose for my machine. Either 1/2-10 5 start acme, or try and find some affordable 2 TPI ballscrews for the X and Y (perhaps 20mm/.625" diameter). The 5 TPI on the Z axis works out very well. I think this gives a great combination of speed and accuracy on a mid-sized machine.

Steve

9. Hi again and thanks for your help,

For the sturdiness of Y gantry, I think of putting only this 15mm back plate and bolt directly my two rails on it, avoiding to extra-put the aluminium profiles. Is this enough for the gantry to be sturdy? I know that I optionally could put two pieces of 1530 in Y gantry edge to edge, but I don't know where to cut them accurately with mm accuracy.(Do you have any idea on this?)
But, if I put a back plate on Y, then I can sketch it 3D in pc and go to a Cnc store to cut it accurately.
So, do you think that the 640 oz-in kelign's 34 nema package in combination with 2 TPI ballscrews do the job well?

Will I need microstepping in any case if I want accuracy? Gecko's 203 pdf site says the following for the microstepping:
"Conclusion: Microstep at low speeds, Full-Step at high speeds. This means morphing from a microstepping to full-stepping when the motor speed is above where microstepping is of any benefit (above 4 revs / sec.)"

What if decide to go to 5 TPI ballscrews, which are found more easily than 2 TPI ballscrews in my city?

I think that, thanks to you, I am starting to clear out how this CNC stuff perform and I appreciate it....

Thanks Stevespo.

10. ## What is the max depth?

I have a question for you.

Taking the specifications of a bit for example the following:
"0.0200 in (0.51mm) dia. DeepReach Fish-tail End-mill, 3-flute, 0.125 in. max DOC, 1.5 in. OAL"

could you verify if maximum cut depth is 0.125 in.?

Thanks.

11. Kostis, I'm not an engineer - so my answers are strictly opinion based on my own experience building my 8020 machine.

The 15mm back plate will provide good stiffness up/down, handling the weight of the Z+spindle. When you bolt the rails to it, it will help strengthen it laterally as well. I think it will work well. I understand the appeal of having a machine shop cut this part and having everything accurately laid out, square, parallel. For this application, I would lean towards the 20mm rails to get some extra strength as well.

Q: If you are taking a deep cut with the gantry moving forward quickly, will it resist twisting as well as an I beam or larger extrusion? I don't know. There are commercial routers that use this design successfully, but generally with smaller spans (25"). Your 1150mm (45") span may need more support than just the plate/rails. It seems unlikely, but this is going to be a great machine and you want to have everything work well.

If you can draw/model this, it would be worth asking in one of the engineering groups. Let someone who knows what they are doing answer this question for sure. You could always built it, and then reinforce later with an extrusion bolted to the back. It's not elegant, but on the off-chance that the 15mm plate isn't enough, you still have options.

The 640 oz/in motors, microstepping drivers and 2 TPI screws sound like a great combination to me. That is what I would favor. Selectable microstepping on the Kelings is nice, but the Geckos are also excellent. I have the 203Vs. You will need some form of microstepping for accuracy, but it can be fixed via a jumper of morph depending on speed. Total accuracy also depends on the screw you choose, so you have to consider the entire package.

The 5 TPI screws are a good option, but the tradeoff is (speed) performance. You will be somewhat limited by the top RPM of the motor/driver/power supply you choose, and with steppers this will probably be 750-1000 RPM or 150-200 IPM. If you can work with this, no problem. If you're concerned about higher speed, then you should find a source for the 2 TPI screws and you'll never have to worry about speed.

http://www.roton.com/Mating_Componen...family=7059834

I guess the screw choice is the #1 decision as far as speed/accuracy goes, and once you have made this - everything becomes a lot easier. Do you need rapids of more than 150 IPM? Use the 2 TPI. Less than 150? Use the 5 TPI. This is a hard decision to make without having the machine operating in front of you. 30-60 IPM sounds fast, until you start cutting wood and then you feel like it's crawling.

I'm really looking forward towards your build. It will be a great machine whichever route you choose, it just depends on your goals/needs.

>Taking the specifications of a bit for example the following:
>"0.0200 in (0.51mm) dia. DeepReach Fish-tail End-mill, 3-flute, 0.125 in. max DOC, 1.5 in. OAL"

DOC = depth of cut, or maximum depth
OAL = overall length

Steve

Page 1 of 9 1234 ... Last

1. ###### ????????? CNC ?????????? 1,5? ? 1? - Page 8 - ??? ????????? - cnchobby.gr
Refback Post #0
05-08-2013, 02:26 PM