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View Full Version : BUILD LOG: Need some final decisions before start building…



msacnc
21-09-2014, 06:07 AM
Some most helpful people (espically JAZZCNC and Silyavski) answered me in my previous thread about many questions that make me rethinking deeply in everything in my machine (design, electronics, linear guide…)

After investigations i decide to reduce my steel machine dimensions to be 2000mm * 1500mm frame to fit MDF sheet because I concerned mainly with cutting and engraving all type of wood and may be thin plastic only. Of course I am looking for high precision with reasonable speed.

Working area: 1700mm*1220mm*200mm

Machine design:

All machine frame is 100*100*3mm

the machine will be the same main design like this but with some differences
http://www.mycncuk.com/attachment.php?attachmentid=13396&stc=1




Gantry will be two welded steel tube 100*100*2mm to reduce the weight, length 1500mm. like this
http://www.mycncuk.com/attachment.php?attachmentid=13395&stc=1

Gantry side is 100*100*3mm steel length 250mm like this image
http://www.mycncuk.com/attachment.php?attachmentid=13394&stc=1


Gantry attached with gantry side using steel angle brackets in the front for more rigidity and stiffens like this.

http://www.mycncuk.com/attachment.php?attachmentid=13397&stc=1


Linear guide: (I choose profiled rather round rails for rigidity and easy to align)

Profiled linear rails 25mm for X axis HIWIN
Profiled linear rails 20mm for Y and Z axis HIWIN

Electronics:


4 Nema 23 JK57HS112-3004 (NM 3 - 425 oZ 3A 6.8mh)
1 Power supply 70~90V adj. 12A
4 M880A Leadshine micro step drivers 80V 7.8A

Spindle:


2.2kw Chinese spindle air cooling 5.5kg 80mm

What about choosing 3kw Chinese spindle water cooling 9kg 100mm. Is this best for my machine?
Motion: I replace R&P with ball screw for cutting and engraving wood with high precision.


Double 2510 Ballscrew 1750mm on X axis
1610 Ballscrew on Y 1250mm axis.
1605 Ballscrew 400mm on Z axis.

All screw will be attached to belts and pulleys for flexibility to increase torque and resolution of X and Y axis Esp for 3Nm Nema 23. I can test 1:1 or 1:2 ratios to achieve the best speed with high possible precision.

THE BIG PROBLEM
Note: (there is no 2005 or 2010 screws just 1605,1610,2505,2510)
I think no problem with (critical speed, moment of inertia and fade rate) for Y and Z due to their length. The problem is in X axis. I read many threads that taking about this issue. First rotating ball nut (to eliminate whipping) for 1610 is not an option. So rotate the 1610 screw of that length 1750mm may be a problem (critical speed). I know that 1610 screw has small around 3times moment of inertia than 2510. Also with 1610 screw i can get high speed especially with 3Nm steppers (need lower torque for achieving high fade rate).
Due to this information
Critical speeds with BK/BF bearings (approximate):

16mm dia, 10mm pitch, 1600mm long - 800rpm = 8m/min
16mm dia, 10mm pitch, 1500mm long - 910rpm = 9.1m/min
25mm dia, 10mm pitch, 1600mm long - 1220rpm = 12.2m/min
25mm dia, 10mm pitch, 1500mm long - 1400rpm = 14m/min

I think I can achieve 800~1000 rpm
What about cutting speed?


There is many options:


2510 with 3Nm steppers with 1/2 gearing will give extra resolution and torque but this will reduce speed to half.
2510 with 3Nm steppers with belts and pulleys 1:1 ratio
1610 with 3Nm steppers with 1/2 gearing will give extra resolution and torque but this will reduce speed to half.
1610 with 3Nm steppers with belts and pulleys 1:1 ratio

May be Nema 34 651oz 5.6A 3.4Hm is an option with 2 PS and Leadshine drivers

Rely I need advice in this problem…

irving2008
21-09-2014, 08:53 AM
By going for 2mm wall section you might have reduced the weight by 30%, but you've reduced the rigidity too by 30% so where you had roughly .045mm per N deflection you now have .067mm per N in the Z direction and 0.27mm per N v 0.19mm in the X direction. Yet another reason why you can't make arbitrary decisions without understanding and considering all the implications.

Clive S
21-09-2014, 10:19 AM
By going for 2mm wall section you might have reduced the weight by 30%, but you've reduced the rigidity too by 30% so where you had roughly .045mm per N deflection you now have .067mm per N in the Z direction and 0.27mm per N v 0.19mm in the X direction. Yet another reason why you can't make arbitrary decisions without understanding and considering all the implications.


Also 2mm does not give any thickness to thread into. ..Clive

msacnc
21-09-2014, 05:11 PM
thanks for replay irving2008 (http://www.mycncuk.com/members/125-irving2008)
i face some difficulty to understand these calculations. but do you mean that i should use two 100*100*3mm welded steel or one 150*100*3mm for gantry.

irving2008
22-09-2014, 12:32 AM
Your original plan was to weld 2 bits of 100*100*3 together to give 200h*100w gantry. You then decided for some reason to use 2mm wall section which is lighter but not as rigid. I'm not saying anything about what you should do, just that you can't make random choices without understanding the impact of that decision. You reduced the weight because of limitations elsewhere (although weight can be beneficial against resonance) but you did it in a way that reduced rigidity and it's not clear you understood that.

Boyan Silyavski
23-09-2014, 12:22 AM
3 kw spindle better but not with that gantry sides. Look at my build log #70 (http://www.mycncuk.com/threads/6619-Quite-an-Unusual-one?p=61499#post61499), same simple gantry sides but much stronger.

Gantry should be at least 2x 100x100x3 , 150x100 is not a good measure to fit the bearings and at the same time have long Z travel. , try to use only one type of profile, at least thats what i do here, cause i buy 6m lengths.



Now to your "big" problem.
Dean told you already. Dont go with big screws cause you will not be able to rotate them.



So Definitely go with 2 x 3nm/1610 running 70V. Will warn thou that at 1750mm length with 16mm screws your at the point when whip at higher feeds can cause problems so you may still need to limit motor tuning to lower rapid speeds.?
Also the mounting and alignment will need to be perfect, which it should be anyway, but if your off just a little then this is amplified at this length which can introduce whip and resonance.!

If you feel adventurous mount the long screws both sides fixed mounts , but it must be perfectly aligned. It will lower the eventual whip even further.


I dont know what calculations you make but if you do it correctly it will move even faster than you think.