new build - Rails & ballscrews & motors

[CNCBlug]

Thx Gary for the chat. I now know where to start.

SPEED!

I have had a look around and here are some of my thoughts and assumptions:

It seems for a 20/5 (1500mm) screw you should stay under 800rpm to avoid whipping so it would go max 4000mm/min. Yet a lot of projects/threads claim 6000mm/min and above with 5mm pitch which makes me think that it is project/screw dependant and some people just get more because they are pusing it a bit. Oh and 800rpm is the "industry very safe limit".
I am sure there is a calculator somewhere which will tell me i am completely wrong about all this...:confused:

So I think realisticly, and this is my 2 hour research conclusion, is that you can probably do 5000mm/min without any issues and you can double that if you move to a 10mm pitch.

I think i'd like to stick to the 5mm pitch.

This is a symilar sized project which is where i got some ideas from.
[ame="http://www.cnczone.com/forums/showthread.php?t=30751"][/ame]
And here is a video of it in action
[nomedia="http://www.youtube.com/watch?v=ecWNTGk8AP4"]YouTube- DIY CNC Rapid demo[/nomedia]

Quote:
"The motors I am using are 1.8 deg 640oz in, these are running at 60v 5.5A which
is pretty close to their limit. This is deliberate so I can get better acceleration. At 13.5m/min the stepper
would be going 1350rpm, any faster and i'm sure the torque will drop off very quickly. "

So i have this figure of 4000-5000mm/min in my head and i hope it is not unrealistic.

Saying that i'd also like to mention that i intend to use a DeWalt 2kw for a spindle and the gantry will be from extruded aluminium and profile rails. I dont know how that affects the speed, but i am sure a heavy gantry/Z has all sorts of influences.

Thanks

[Gary]

Yes there is a calculator that will tell you the critical sceep of a screw and this is based on the root diameter the lenght and the type of mounting.

based on 1500mm and a 20mm diameter screw with a root of say 17mm and fixing with a fixed end and a floating end the critical speed is just over 900 RPM.
If people are claiming faster speeds this may be the case but this will not be within the specification of the screw and over time will damage the screw.

Use this link
http://www.nookindustries.com/ball/BallCalculators.cfm

I am off out now, but will have a look at the rest of you post tomorrow.

Thx Gary for the chat. I now know where to start.

SPEED!

I have had a look around and here are some of my thoughts and assumptions:

It seems for a 20/5 (1500mm) screw you should stay under 800rpm to avoid whipping so it would go max 4000mm/min. Yet a lot of projects/threads claim 6000mm/min and above with 5mm pitch which makes me think that it is project/screw dependant and some people just get more because they are pusing it a bit. Oh and 800rpm is the "industry very safe limit".
I am sure there is a calculator somewhere which will tell me i am completely wrong about all this...:confused:

So I think realisticly, and this is my 2 hour research conclusion, is that you can probably do 5000mm/min without any issues and you can double that if you move to a 10mm pitch.

I think i'd like to stick to the 5mm pitch.

This is a symilar sized project which is where i got some ideas from.

And here is a video of it in action
YouTube- DIY CNC Rapid demo

Quote:
"The motors I am using are 1.8 deg 640oz in, these are running at 60v 5.5A which
is pretty close to their limit. This is deliberate so I can get better acceleration. At 13.5m/min the stepper
would be going 1350rpm, any faster and i'm sure the torque will drop off very quickly. "

So i have this figure of 4000-5000mm/min in my head and i hope it is not unrealistic.

Saying that i'd also like to mention that i intend to use a DeWalt 2kw for a spindle and the gantry will be from extruded aluminium and profile rails. I dont know how that affects the speed, but i am sure a heavy gantry/Z has all sorts of influences.

Thanks

[irving2008]

...
So i have this figure of 4000-5000mm/min in my head and i hope it is not unrealistic.

Saying that i'd also like to mention that i intend to use a DeWalt 2kw for a spindle and the gantry will be from extruded aluminium and profile rails. I dont know how that affects the speed, but i am sure a heavy gantry/Z has all sorts of influences.

Thanks

5000mm/min on a 5mm pitch screw is 1000rpm or 16.67rps = 3333 steps/sec. On 40v at that pulse rate those motors (~4.5Nm holding torque) are going to give approx 1.2Nm of useful torque (there is no exact way to work this other than actually measure it, but the datasheets for similar motors on 120V/5.5A show 2.7Nm at 3kpps its a rough extrapolation)

This useful torque is whats available to overcome the static inertia of the load (including friction loses etc) and, often forgotten, the ability to decelerate the load (remember it will push back on the motor as it can drive the ballscrew in reverse) and will determine the maximum gantry weight under cutting forces that can be sustained without losing steps...

[Gary]

As Irving has said 1000 RPM from a 40V driver is asking a hell of alot.
If it were me, i would look at our motors and our drivers and this is what i would offer.

First off are you driving the lower axis with two ball screw or just the one from the center?
If you have two, you dont need two large motors and can get away with using two smaller motors, but the price differnece between this motor and the 80mm long motor is quite small, so i would most likely stick with two of the motors i am going to suggest.
A machine of this size would be better driving form both sides but you may get away with one, if your machine is very rigid, but be prepared to add another ball screw if it crabs.

ok, so our motors would be the SY85STH118-6004 and the driver would be the 2M880N and a 70V power supply (PS705)


As you most likely know we also do the ball screws and rails, so if you want me to quote you email me and i will be happy to send you the quote.