PDA

View Full Version : BUILD LOG: New build.Stell frame cnc 1500X1000.Ball screw and gearing calculations



ba99297
01-09-2013, 09:14 PM
Hi everybody
My name is Vagelis and I am from Greece
Sorry for my english. I use google translation in order to find some words, so be tolerant with me.

Last year I am trying to plan and build a cnc router for cutting wood,plastic and ali if possible. The machine dimensions ( of the rails ) are
X : 1000mm
H : 1500mm
Z : 300mm
As a key element of the frame is steel, with compounds mainly by screws
I have already bought the motors which are three Yaskawa 400 watt Sigma II with 3000rpm. The servos and a 2,2 Kw spindle ( without the inverter) are th eonly things that I have already bought..
Thumbnails of drawings I made in the spring are the following

9938
9939
9940
9941


These are not the final plans. I just upload them in order to give you a general idea of what i am going to build. The gantry will be about 60 kgr heavy

Do you think the motor are suitable for such a machine?
What cutting speed is possible to be achieved with these motors?
The servos and a 2,2 Kw spindle ( without the inverter) are the only things that I have already bought

In the original design I was planning to move the bridge with a single motor, and a ballscrew under the table in the center , which is why I ordered set of three motors and not four . Lately I'm a little skeptical that my choice and I think maybe I'm wrong . On the other even if I am wrong I cannot spent at least another 500 euros to buy extra motor and driver. Given this limitation of scenarios is

1. Nothing changes from the original design and move the bridge a ballscrew center

2. Buy extra ballscrew and placed on the Y axis two ballscrews ( right and left), and give motion by a motor with belt and pulleys

3. Move the axis Y with a ballscrew in the center and use the following technique
4. Use the rotative nut.. That means we keep the ballscrew stable and we rotate the nut
rotative nut - YouTube (http://www.youtube.com/results?q=rotative%20nut&bav=on.2,or.&bvm=bv.51495398,d.Yms,pv.xjs.s.en_US.M4-36_38X9A.O&biw=1280&bih=885&safe=active&safeui=on&um=1&ie=UTF-8&gl=GR&sa=N&tab=w1)

second topic
----------------------------------------------------------------------------------
Now it is time to buy ball screws and linear motion equipments
The basic question that i have is what ball screw is better to buy
I mean of what diameter and mm per revolution

Also it is important to decide the gear ration between the motor and the ballscrew

The basic restriction is that i already have the motors (3 Yaskawa sigma 2 400 watt)
These motors are rated at 3000 rpm
At fist i was planning to connect them directly to the ball screws .

After reading this topic

Need Help! Ballscrew speed (http://www.cnczone.com/forums/diy_cnc_router_table_machines/191270-ballscrew_speed.html)

i realize that the ballscrews have a maximum speed limit , depending on the diameter , length and mode stabilization at both ends. So based on the following calculator

Metric Critical Speed Calculator | Nook Industries (http://www.nookindustries.com/EngineeringCalculator/MetricCriticalSpeed)
my own ballscrews (if i buy the 2005) could run

X axis length 1000mm maximum rpm 2478
Y-axis length 1500mm max rpm 1075
Z axis length 350mm max rpm 19739 ( this axis forget him )

That means with 1:1 gear between the motor and the ballscrew, the Y axis ballscrew would never run with 3000rpm because of the 1075 threshold.
I have many thing that i am thinking about but first i think it is better to hear your opinion
I repeat my questions

First question
Are the motors suitable for such a machine?

Second question
What cutting speed is possible to be achieved with these motors?

Third question
What gear ratio should i choose between the motor and the ball screw?

Forth question
What ballscrew should i choose?

Fifht question
Which of the following scenarios is the best choice for me

1. Nothing changes from the original design and move the bridge a ballscrew center

2. Buy extra ballscrew and placed on the Y axis two ballscrews ( right and left), and give motion by a motor with belt and pulleys

3. Move the axis Y with a ballscrew in the center and use the technique with stell ropes and pulleys
4. Use the rotative nut.. That means we keep the ballscrew stable and we rotate the nut
rotative nut - YouTube (http://www.youtube.com/results?q=rotative%20nut&bav=on.2,or.&bvm=bv.51495398,d.Yms,pv.xjs.s.en_US.M4-36_38X9A.O&biw=1280&bih=885&safe=active&safeui=on&um=1&ie=UTF-8&gl=GR&sa=N&tab=w1)

I remind that the dimensions are 1000X1500X350, gantry weight 60 kgr, steel frame

Have in mind that i want to get as much as possible from my motors. I would aprecciate if you analyze your answers
Thank you for you time


Vagelis

ba99297
01-09-2013, 10:06 PM
I just merge two topic in one

Greeny
01-09-2013, 10:22 PM
Hi Vagelis,

I am a beginner here, so take my advice lightly!!
I have done a lot of reading and looking at other peoples designs.
Option 2: using two ballscrews with one motor sounds best to me. You can use the pulleys to gear the motor to your needs.

The other options means you have a ballscrew under the frame in the centre. this means you have to have 1500mm unsupported to allow the bridge to move.

people have sucessfully used both methods, but for a fixed amount of steel, option 2 allows for a stiffer frame. the relatively low cost of one extra ballscrew and pulleys/belts for me would be worth it.

Hopefully, more experienced people will offer their opinion for you.

good luck
greeny

JAZZCNC
01-09-2013, 11:51 PM
Hi Vagelis

Option No2 is by far your best choice.

Buy 20mm Dia 10mm Pitch ballscrews and Gear the Motor 2:1. This will allow you to easily reach 13-15mtr/min rapid speeds even with a heavy gantry and the 20mm ballscrew will help reduce whip. Because of the 2:1 gear ratio your torque will double and also because servos give linear torque they will easily handle the higher inertia of 20mm ballscrews.

You will need to increase the frame strength with some cross bracing to achieve these speeds though has it's very weak how you have it drawn now.
Also with the Z axis looks all wrong.? You will only be able to reach down past the gantry bottom by the length of the tool or how much the Spindle extends out of it's bracket. This will limit your cutting ability and the gantry could or will catch work in some cases.!!

Better if you switch the bearings onto the back plate and put Rails on the moving front plate. This gives you maximum strength and will allow reach down and clearance when needed.

ba99297
02-09-2013, 12:02 PM
Greeny thank for your answer

Jazzcnc also thank for your calculations. I want to say that the plan that i upload are not the final plans. I just upload the images in order to give a general idea about what am i going to build
You told me

" Buy 20mm Dia 10mm Pitch ballscrews and Gear the Motor 2:1 "

My motors have 3000 RPM ( according Yaskawa ).

According this calculator

Metric Critical Speed Calculator | Nook Industries (http://www.nookindustries.com/EngineeringCalculator/MetricCriticalSpeed)

for 1400 mm distance between bearings and 20mm ballscrew, i will have 1234 RPM max speed for my ballscrews. Even if i gear them with 2:1 ratio i will reach 2468 rpm to the motor when the ballscrew turns with 1234 rpm. What will happen if the motor turns with 3000( or even more) ? that means that the ballscrew will turn with 1500rpm >1234 limit ( according the calculator )
You think i will have problem at this speed for my ballscrews?
Is the rotative nut a solution ? ( at this case i cannot move two ballscrew with one motor )
Is 3:1 gear a choice ( and the same time increase the mm per revolution at the ballscrew )
I think that the key factor is to know what cutting speed and rapids is possible to achieve with the 3 Yaskawa 400 watt motors for a machine that will be made for cutting wood plastic and aluminium if possible. I have no sense about my motors abilities. If i knew cutting speed and rapids, i think will be more simple to make calculations and combinations between ballscrews and pulleys

Anyway, so far thanks everybody for the time that spend to read, calculate and answer to my post.
Any other advice will be welcome.

Vagelis

JAZZCNC
02-09-2013, 04:15 PM
Hi Vagelis,

Don't worry to much about what the Spec's say regards critical speed it's mostly a guide.
I can tell you from experience that you will easily run 1.5mtr length 20mm screws @ 1500rpm with out any problems provided they are aligned and use correct end bearings.

Regards the speeds and feeds then this is mostly determined by the Strength of machine, Material, cutter quality and spindle being used.
For the stronger DIY machine using 2.2Kw spindle then you'll probably cut woods and at around 6-8mtr/min, plastics maybe slightly less depending on plastic type and cutter used.
For aluminium then you'll be much much slower rarely above 1500mm/min with 800-1200mm most common.

Rapid speeds only really matter if you have a large table area and plan to do lots of drilling or point-to-point work, other than that then they are mostly just for showing off.!!
When doing 3D work then you'll want higher accelerations rather than high velocity.

My suggestion is go 2:1 and if you find it too fast then it's easy to change a pulley. Remember you set the Speed of the machine in motor tuning and just because it can run at 1500rpm doesn't mean you have to go that fast.
My machine which uses 1.5Mtr 5mm pitch 20mm screws geared 1:2 can reach 11 Mtr/min (screw spinning at 2200rpm.!!) using steppers but I Don't have it tuned that high because I don't need rapid speeds. So I sacrifice speed in favour of better acceleration and higher torque, also often on smaller work having high acceleration over high velocity is actually quicker due to amount of short movements.

Either 2:1 or 3:1 will work fine for your needs but I'm sure 2:1 will give you a better balance.

ba99297
02-09-2013, 10:25 PM
I have made some calculations with motor speed at 3000 rpm ( max according Yaskawa ). Tell me if i am wrong

motor speed 3000


case 1-with 2010 ballscrew and 2:1 gear ratio

motor gear ratio 1
ballscrew gear ratio 2
ballscrew length 1400
ballscrew diameter 20
ballscrew pitch 10
ballscrew rpm 1500
ballscrew rpm threshold 1234
rapids mm 15000


case 2-with 2015 ballscrew and 3:1 gear ratio

motor gear ratio 1
ballscrew gear ratio 3
ballscrew length 1400
ballscrew diameter 20
ballscrew pitch 15
ballscrew rpm 1000
ballscrew rpm threshold 1234
rapids mm 15000

Both solutions i think they have the same torque and the same rapids. The differense is that case 2 reduce the ballscrew bellow 1234 threshold.


If i use one motor and two ballscrews i have to tension the timing belt
The follwing us a 2d plan i made in order to tension the belt

9951
The magenta cyrcles are the ballscrew pulleys
The yellow cyrcle is the motor pulley
The green cyrcles are pulleys with no tooth
The motor and the green pulley above the motor will be movable in order to tesnion the belt.
The red colore represent the timing belt

And finally my questions

1. Do you think the above plan is a good solution for my case?
2. can you tell me what pulley width should i buy?
3. Do you now any suppluyer with good prices an quality for such pulleys and belts?
4. Does the timing belts have rpm threshold?
5. If i tension the belt that way ( above plan ) would that reduse its life
6. Cast iron or aluminium is better material for pulleys
7. which case ( case 1 or case 2 that i discribe at the begining ) is better?


Thank you for your time

Vagelis

JAZZCNC
02-09-2013, 11:04 PM
Option 2 would be best in this case but you may find locating 15mm pitch screws difficult and expensive.?

#1: Really You don't need the upper tensioners just make the Motor adjustable to tension belts. But yes it would work.!
#2: 15mm belt width will work fine.
#3 Sorry no I buy local.
#4 Yes but much higher than 1500rpm . . Think about Timing belts in Car engines.!!
#5 Can't see why it should provided the tension and alignment is correct, most common cause of belt failure and premature wear is incorrect tension or alignment.
#6 Aluminium will reduce inertia on ballscrew but wear quicker than Steel but again if correctly tensioned and aligned then not an issue. I've had aluminium pulley on my machine for over 5yrs and it's still fine.
#7 See above but Like I said before 2:1 with 10mm pitch will be fine I tell you this through lots of experience not theory.!!

ba99297
03-09-2013, 09:01 AM
Jazzcnc thanks for everything
Sorry for insisting to ask but i am trying not to make mistakes ( at least to make as little as possible).


First question
This gear 2:1 and 2010 ballscrew should be applied at all 3 axis ( X,y,Z)
I think Z axis has not issue because of its small length, X also has rpm threshold 2400+rpm

Second question
In order to reduce inertria at Y ( considering that i will use 2 ballscrews ) is it logical to go with 1610 ballscrews instead of 2010 ?

Third question
You told me
"I can tell you from experience that you will easily run 1.5mtr length 20mm screws @ 1500rpm with out any problems provided they are aligned and use correct end bearings."

Are the "made in china BF and BK" that chai sells, the correct end bearings or i should go to something else?

Forth question
A well aligned and tensioned pulley ( really how we mesure tension?) does it have backlash?

Vagelis

JAZZCNC
03-09-2013, 04:49 PM
Jazzcnc thanks for everything
Sorry for insisting to ask but i am trying not to make mistakes ( at least to make as little as possible).

No problem and you are doing the correct thing asking questions. Just be careful not to get too hung up on specs and calculations they are helpful guides only not hard fast rules.!

Question #1 X & Y use 10mm pitch but for the Z axis you may want to use 5mm pitch for higher resolution/torque.

#2 If you where using steppers then I'd say yes possibly, but because you have Servos geared which give torque across the full rpm range and also geared 2:1 so doubling torque then No you will be better with 20mm to help reduce chance of screw Whip.

#3 The BK/BF are one type of correct end bearings but depending on your design you may prefer the FK/FF type both work the same just suit different mounting locations.

#4 Well you can buy belt tension meters but they are expensive I believe. After that then it's mostly experience and judgement I'm afraid.
So long has the belts isn't guitar string tight or washing line loose then with common sense you will tell if it feels right or wrong.

Regards Backlash then with the 15mm HTD belts I use then I see very little to none, certainly not enough to show in 99.9% of the work I do.
If you look at this Aztec Calendar you'll see the level of detail that can be done with timing belts and if any backlash was present this level of detail wouldn't be possible.!! . . . .(The timing belts were 4yrs old when this was done.! . . .and the screws are 1550mm long.)
VIDEO0021.3gp - YouTube (http://www.youtube.com/watch?v=wjDvimSdH8M)

ba99297
03-09-2013, 11:02 PM
So jazzcnc you sujest

Y axis: 2 X 2010 ballscrews 1500mm long with 2:1 gear
X axis: 1 x 2010 ballscrews 1000mm long with 2:1 gear
Z axis: 1 x 2005 ballscrews 350mm long with 2:1 gear

15mm HTD timing belts
Is that correct?
All three axis will be moved with 3 Yaskawa sigma II 400 watt ac servo motors

Jonathan
03-09-2013, 11:22 PM
No point using a 20mm diameter screw on Z as it's so short that the critical speed of a 1605 screw is plenty. Use 1605 for Z and you'll get better acceleration.

For the other axes I agree with the choice of 20mm screws if you want to get the highest speed from your servo motors. However if you want the best acceleration then using 16mm screws would be better, but this will of course lower the top speed.

JAZZCNC
03-09-2013, 11:37 PM
Agree with Jonathan regards the Z axis screw 1605 is correct choice.

Regards the 1610 on X & Y then I don't agree it's the correct choice at this length. Yes it will allow slightly better acceleration than 20mm but with using servos you'll have plenty enough anyway. 20mm screws will handle whip better and give a good balance of both allowing higher speeds if needed, with 16mm higher speeds will be limited due to potential whip.!!

Both would work but given servos not steppers then really think 20mm are the best choice at this length.