Quite an Unusual one

[EddyCurrent]

The hammer is now mounted on a gimbal and can be made to work in a circular motion by moving the X and Y axes.

No need for a rotating base so it can be made cheap and strong.
Mach3 can operate the X and Y axes.



OR easier

A small X Y gantry mounted on top moves the end of the hammer so 50mm there would be maybe 300mm at the other end of the hammer.
Then Mach3 could be used in the normal way where you would use a 2D software to draw a reduced size pan with the required toolpaths.
A pneumatic proportional valve working on 0-10v (like a VFD) could be used to control air pressure to the hammer using M codes e,g, M3 to turn on the air
then for example S500 would tell the valve to deliver 5Bar, S200 = 2Bar , etc.
So the S codes could be put in the G code as required to change the hammer power.

[Boyan Silyavski]

Now back to reality. I need some help here please! With motor calculations. I read numerous threads here and at the zone, have done numerous calculations as far as i could but will shamelessly ask here again as i am stuck.Almost finished the drawings. The Z looks quite nice and now i am sure its possible, though some blocks will look like cheese due to the ridiculous hole placement

I though i had everything clear until yesterday i calculated the weights.

The Z weights 32kg and the gantry in total 120kg

Part of the problem lies that i don't know what exactly i want as till now i have experience only with small weak machines.

1. I would like the gantry and the Z to be rigid as they are in the drawings
2. I would like the machine to be in the middle between resolution and speed. What i am not sure is what speeds i want/not rapids-cutting speed/. I will cut wood and aluminum on that machine. I chose 1610 to move the z left right and 2510 screw to move the gantry. I thought of 1:1, now i am thinking of 2:1, otherwise not only i am jumping in the so called "servo territory" but will have to jump in the "expensive big servo territory"

I have decided on the 3kw spindle. I am not sure but i believe i will be happy with sturdy heavy machine that could do 250IPM /6350mm per min/- cutting in wood at good if not full cutter depth. The weight should help with the depth and i assume with the aluminum finish. Aluminum i will be happy if i do it at flute diameter depth. It seems the machine will be sturdy enough for this, having in mind the very extremely low overhang with my design, enclosed frame spoindle and strong gantry +the quantity of bearings.

3. I thought of driving the Z with hybrid closed loop Leadshine Nema 23 3Nm, same the Z left right and 2x the same that move the gantry.

That was when i did not know about the actual weight.

Now the question is:
-do i make my machine smaller / which i would not like much, cause the money will be almost the same and missing eventual job on the long run could be very undesirable/

or

Keep it like this and figure how to drive it.


So definitely i need some help in this mess in order to evaluate options and make no mistake here.

The servos i am looking at are these: my friend Fred's BST store
He seems to have the lowest priced servos and drivers around and as i will buy the rails and screws from him, seems quite logical choice

Questions:
having in mind that i don't want to compromise one axis due to small savings of $ and ruin the integrity of the machine. Obviously on the long axis i will go with servos. And here BST store llloking at the prices of servos i am almost ready to fit servos on all axis, as they are almost the price of the Leadshine closed loops

1. The Z weight will stay the same. 10kg spindle + some aluminum, there is nothing more to do except lower the weight about 5-6kg which i believe is not worth it . That is clear. RM1605 screw . Should i stay with 3Nm leadshine Nema 23 closed loop motor 1:1, change the relation 2:1, or upgrade the motor to servo?
If servo, what reduction and how big the servo motor?

2. Z weight again same. RM1610 screw. Should i stay with 3Nm leadshine Nema 23 closed loop motor 1:1, change the relation 2:1, or upgrade the motor to servo? If servo, what reduction and how big the servo motor?

3. Gantry 120kg , 2510, rotating ball nut both sides.
options i see:

- drive it with 2 servo motors, one each side. I am lost what power i need here. According to what i calculated if i am not mistaken could go with 2x 6N.M 1.8KW 3000RPM 110ST AC Servo Motor 110ST-M06030 + Matched Servo Driver / 310euro each . i am not sure though if the reduction here should be 2:1? Or if with 2:1 reduction i could go with smaller servos like the 4N.M 1.2KW 3000RPM 110ST AC Servo Motor 110ST-M04030 + Matched Servo Driver, where the price is 304euro, so as you see there is almost no price difference .

-drive it with 1 big motor and 25mm belt which i am almost sure i can fit to pass through the gantry beam
for a motor i believe the 130ST-M10025 servo motor and servo driver system 2.6KW 10Nm 2500rpm 10A / 375 euro/ would be ok. Again not sure if the ratio should be 2:1 as i assume.

4. Or i stay with closed loop steppers, but they arent cheap, i almost decided on servos.


5. I can not find what size in Nema are these servo motors so i could continue with the drawings, in fact no info on these cheap servos

PS. Found the documents at last for the servos PDF specs
it seems they are Xinje Servo drives and motors here PDF for the drives




What are your thoughts guys?? Please, don't be shy , tell me where i am wrong


here are some updated cheese drawings:

[Jonathan]

Answers to questions:

1) Ballscrews give a very large mechanical advantage. You do not need to worry about the weight of the Z-axis so much. See here. You'll be fine with a generic 3Nm motor and 70V stepper driver. The closed loop ones are a good concept, but cost a huge amount more for little gain.

2) Don't use RM1610 - it doesn't gain anything.

3) I wouldn't go with the long belt idea, especially when using servos as servos work best when they are rigidly coupled to the load - i.e. without a long stretchy belt.

You may well be fine with two stepper motors - have you tried putting the values in this spreadsheet?

If using one servo, the formula you need is this:

Je=M*(L/(2*pi))^2

Je is the equivalent inertia, L is the pitch of the screw (so 0.005m with 1:2 ratio) and M is the mass of the gantry.

Next find the inertia of the servo motor rotor (Jr), e.g. it says in the datasheet for 110ST-M04030 is 7.61kg-cm^2. You then need to find what 'inertia ratio' the motors will tolerate - typically it might be Je/Jr<5. So just use the above formula to calculate Je, divide it by Jr (7.61kg-cm^2) and if the result is less than 5 the motor is probably suitable.

e.g, M=120kg, L=0.5cm (1:2 ratio):
Je=120*(0.5/(2*pi))^2=0.76kg-cm^2

Je/Jr=0.76/7.61=0.1.
0.1 is much less than 5... so that servo is plenty big enough. Another way to look at it is rearrange the formula to find the mass a particular servo will drive, e.g:

M=Je/(L/(2*pi))^2
Je=Jr*5=7.61*5=38.05
M=38.05/(0.5/(2*pi))^2=6000kg(!)
Or if the ratio is 1:1, that's
M=38.05/(1/(2*pi))^2=1500kg
(So note the quadratic relationship - if you double the ratio the drive-able mass goes down by a factor of 4)

The rated speed of that motor is 3000rpm, so the gantry would go at 3000*0.01=30m/min with 1:1. You need to check the Je/Jr ratio - Je=Jr gets the most efficient energy transfer, but the drives do tolerate a mismatch so you need to find how much, as going with them equal generally results in an oversize motor. You should also include the inertia of the pulleys in the system - just work out their inertia (assuming cylinders is near enough) and add them to the Je value, remembering to take into account the drive ratio.

Either way, you can see that that servo is far bigger than you need... so I suggest you put the formulas in a spreadsheet and find which motor is best. Also try the stepper motor spreadsheet I linked to earlier, as I expect you'll find that there are suitable stepper motors available.

4) If you use stepper motors, and select the correct size motors, then they wont stall unless you crash the machine. The only time the closed loop servos help therefore, is if the machine crashes. For the same amount of money as the closed loop stepper you could get a much higher torque standard stepper motor and be certain that it wont stall.

[Jonathan]

I forgot to mention...I'm not keen on the placement of your Z-axis ballscrew. If a large force is applied parallel to Z, the axis will begin to skew (i.e. rotate in the Y-Z plane), due to the ballscrew placement being off center. The effect can be reduced by increasing the spacing of the Z-axis linear bearings, or go back to two ballscrews, or get the ballscrew closer to the center. That's why on the new machine I placed the ballscrew intersecting the Z-axis ram - it got it closer to central, but admittedly still not ideal.

[EddyCurrent]

Jonathan, good reply.

[Bart]

Hi,
my first helping post here but I was reading for quite some time...
silyavski
here is part of my excel spreadsheet that might be useful for you- - just type in your variables in green boxes and you should get all the date you need.
Attachment 11071
I do not guarantee correctness of the results but I'm fairly confident that they are correct.
It's slightly more complicated then most spreadsheets that I've seen here so if you have any questions ask here.

[Boyan Silyavski]

Hi,
my first helping post here but I was reading for quite some time...
silyavski
here is part of my excel spreadsheet that might be useful for you- - just type in your variables in green boxes and you should get all the date you need.
Attachment 11071
I do not guarantee correctness of the results but I'm fairly confident that they are correct.
It's slightly more complicated then most spreadsheets that I've seen here so if you have any questions ask here.

Thanks!
P[W] stands for calculated power per motor for X or for both motors?

[Boyan Silyavski]

Some updates:

Motors:
Thanks for the help! Now things are more clear there.

The main option i will go most possibly is 4 servos x 400w or 2x400w +2x750w.

Obviously from the calculations if 2:1 then 2x200w+2x400w will be sufficient, but as the price difference between 200w,400w and 750w is merely 5-10 euro, i dont see a reason why not going properly bigger.

The Hybrid stepper option i discarded due to price similar to servos.

DC Servos and motors from Leadshine are almost the price of AC servos and motors , so 220v seems a lot better choice

Steppers are still an option, except for the long axis that moves the gantry. However i would like the Z also to be closed loop, due to the vibrations. Plus the price of decent driver and motor and PSU is almost as the price of AC servo, hence the servo decision.

Another point there is i am starting to doubt about the cheap servos, so i am trying to get for the same price 4 x ACH750 Leadshine AC Servo motor drives + motors. Bargaining with Chinese people takes time :-)

The Z axis

The single screw design i am almost 99% sure will be without a problem. Hoewver i decided not to risque due to the impact hammer head and changed the design. Lost something of the beautiful simplicity and acquired annoying amount of pulleys and idlers, but i could not design it better for now. Best would be with 2 stepper motors on 1 drive, i know,not a very good idea, but works/my flimsy cnc is that way on the gantry without any problem, but is flimsy, not rigid like this design/
So, for now that's how it would be:



Rotating ball nut
At the end i found somebody help me with making it. Jeff, from the forum, who happens to live very near, here in Spain. Wow, what a work shop and machines he has, especially compared to my garage.

Now as i decided on Servo, may be even will go with double ball nut/all over the machine also/. 1.450kg said Fred yesterday, weights the 2510 double nut. I wonder what the inertia will be, he he.

I thought of integrating it into the gantry, but at the end decided to fix it aside, due to the possibility to lower significantly the screw length+ avoiding an accidental hit of the gantry to the screw mount




Breakout board
-ESS and PMDX126 seems to be the first choice. What i don't like of this combo is the fact that i have to buy additional spindle control board

So i investigated quite a bit even looking at some second hand Galil motion boards on ebay.

Another good option seems the KFLOP. In fact it seemed ideal/8 axis they say +usb/ until i found that i have to know C+ and that half of its IO are 3v so i have to spend another 240$ for accompanying IO board.

What i need is very simple: cheap ethernet digital 8axis board

A question ?????
There is something i don't understand about boards and servos. Why all speak of the encoders to go in the BOB? Dont they go to the servo drive and the servo drive controls the closed loop? I read at Galil and KFLOP that the BOB closes the loop? Or am i mistaken?

[EddyCurrent]

A question ?????
There is something i don't understand about boards and servos. Why all speak of the encoders to go in the BOB? Dont they go to the servo drive and the servo drive controls the closed loop? I read at Galil and KFLOP that the BOB closes the loop? Or am i mistaken?

I would always expect encoder feedback to go directly to the driver, you don't way any delay introduced, it should go directly to the summing junction that should also exist in the driver.
If feedback does go via the bob on any system, I would steer clear of it.

[Bart]

P [W]
Stands for maximum (instantaneous) mechanical power of a motor when going at full speed and utilizing full torque.
This wont happen in real life as with stepper motors you'll have a safety margin of 1/2 so you will get only half of that power from a stepper motor. It's also important to bear in mind that you will utilize half of this power only in two instances. First instance, when you're accelerating to full speed at full allowable acceleration during fraction of a sec just before reaching max speed. Second, when cutting at full speed with full allowable force. Otherwise you're using only fractions of that power.

Cutting a steel is done between 1 and 2 m/min so if your Vmax is 10 m/min than you will use only 10-20% of Pmax and only if the gantry had to be pushed with full allowable force which is unlikely to happen.

In a cell where you input a torque of motor it was required to input a combined torque of motors for given axis. Now you just input a torque of motor and number of motors.

I also see that you are interested in power of motors, so I've attached one more spreadsheet inside to calculate various mechanical powers of motor, as well as some information on power and current drawn from power supply - to help choose PS.

Attachment 11075

I don't know much about application of servo motors but from what i briefly read, the most important feature of a Servo Motors is a possibility to overload them up to 10 times which is not possible with stepper motors due to the nature of a driver.

If this overloading feature is possible than consider following.

For a 400 W servo motor that you've selected (assuming an overload factor of 5) you can get as much as 2000W for a fraction of a sec.
Now, if you open my spreadsheet (the new one) all the data there are for my design.
Equivalent Gantry Weight 290kg
Motors: 2x 1,5Nm -> maximum power of 150W per motor but steppers have to be used with some safety factor so as not to loose steps.

I've taken 1/2 so I'm Using only 75W of each motor. With that torque/power I can accelerate 290kg gantry to 10m/min (393 inches/min) in 55 mili sec. That's quite fast but with your servo and 2000W you will be able to accelerate it to the same speed in as little as 4,5 ms over a distance of 0,38 mm, ceteris paribus.

So what I want to say is: don't over-invest in motors.

Best regards