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02-09-2013 #1
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.Last edited by JAZZCNC; 02-09-2013 at 03:19 PM.
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02-09-2013 #2
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
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
VagelisLast edited by ba99297; 02-09-2013 at 09:33 PM.
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02-09-2013 #3
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.!!Last edited by JAZZCNC; 02-09-2013 at 10:04 PM.
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The Following User Says Thank You to JAZZCNC For This Useful Post:
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03-09-2013 #4
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?
VagelisLast edited by ba99297; 03-09-2013 at 08:05 AM.
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03-09-2013 #5
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 - YouTubeLast edited by JAZZCNC; 03-09-2013 at 03:51 PM.
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03-09-2013 #6
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
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03-09-2013 #7
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.
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