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View Full Version : 2m+ travel lengths with a ballscrew, using linear rail to support the ballnut



Desertboy
25-01-2018, 12:00 PM
If I use 2m+ long ballscrew with 15mm Hiwin's 2 carriages supporting the ballnut will I be able to spin the screw faster without whipping? Rotating ballnut seems unfeasable and expensive so need a way to get speed and accuracy am toying R&P but would be better if I could use a ballscrew of course.

JAZZCNC
25-01-2018, 05:20 PM
If I use 2m+ long ballscrew with 15mm Hiwin's 2 carriages supporting the ballnut will I be able to spin the screw faster without whipping? Rotating ballnut seems unfeasable and expensive so need a way to get speed and accuracy am toying R&P but would be better if I could use a ballscrew of course.

You haven't stated size of ballscrew so not easy to say. However supporting the ballscrew with linear rails is going to get expensive quickly and require lot messing about.
There are ways around this thou. Use ballscrew with higher pitch and put ratio on it. This will lower the screw speed but still give desired speed.

IE: 20mm pitch with 2:1 ratio will give 10mtr/min @500rpm where 10mm pitch at 1:1 would give same 10mtr/min but screw would be spinning at 1000rpm.
This approach also gives the advantage that torque is aprox doubled so less chance of stalling motors.

This machine use the same method with 25mm 25mm pitch screws 3000mm long. As you can see in this video around min 1:50 there's no whip at all and it's doing 15mtr/min at some points.
https://www.youtube.com/watch?v=4STVbn_EMoc

Desertboy
26-01-2018, 11:12 AM
Unsure how long I want to do go, at least 1.8m travel but I was looking at 2700mm ballscrew and Hiwin's last night.

Amazing work on that cnc machine, given me a lot to think about.

magicniner
27-01-2018, 01:23 AM
Adding a guide with the mass and damping at the right place to stop whip, if not implemented with the same primary slides as the axis, does not need rails with the same accuracy ;-)

Desertboy
27-01-2018, 09:17 AM
I was thinking these
https://www.ebay.co.uk/itm/15mm-linear-slide-guide-shaft-SBR15-1500mm-2-rail-4HSR15CA-bearing-block-CNC-set/232585268762?hash=item3627298e1a:g:8UcAAOSwNWxaJP-~

I could butt 2 together to get lengths I desire so 240 would support 2 3m lengths.

The choices so far
1. 2* 2525 ballscrew 2.7m with decent quality BK/BF fittings ~1000
2. 2* 3232 2.5m with fittings ~1500
3. 2* 2010 with fittings 2.7m length fully supported with the clone rail and 2 carriages per ballnut leaving me 4 spare carriages in total, I could then use 6 carriages to support the Z axis which has the same rail on it given me 50% more strength without affecting travel. ~600

I would think the 2 biggest factors in critical speed are how straight is the ballscrew and how good are the bearings in the BK/BF fittings, I might even play with type C mounting method but I believe this needs super precision although doubling your criticial speeds.

Resolution is not so relevant as I'll use servos, not sure how powerful but was told 400w should be ok I think 2k will buy the servos and the mesa controller so I can use linuxcnc.

I really want to direct drive with oldham coupling but it can be belt driven I suppose, the most important things are large travel, good accuracy and good speed.

Desertboy
27-01-2018, 10:23 AM
I think I need an experiment to test the validatity of supporting the ballscrew. The rail I'll have 1.4m 1605 ballscrew I can use with BK fittings have the carriages need a 1.5m length of rail hmm I might just buy another set of 15mm after all they can used for something interesting (Thinking CO2 laser cutter) and 120 is a bargain.

I have this clone rail on my gantry at moment and original Hiwin's on my Y axis I honestly cannot tell the difference between them except for the Hiwin's being like 4 times the price lol.

How fast can a nema spin? I suppose theirs no easy answer to that and going to depend a lot on inductance. Since 2 of the motors I use was recovered from other machines I have no idea of what their inductance or even nm is. I can only guess based on weight and chassis dimensions.

I'm going to budget 7k for the build I hope I can do it cheaper but I think 7k is realistic if I buy new.

JAZZCNC
27-01-2018, 11:50 AM
Resolution is not so relevant as I'll use servos, not sure how powerful but was told 400w should be ok I think 2k will buy the servos and the mesa controller so I can use linuxcnc.

I really want to direct drive with oldham coupling but it can be belt driven I suppose, the most important things are large travel, good accuracy and good speed.

You're in Cuckoo land if think direct drive 400w servos are going to spin 2.5mtr long 25mm or 32mm ball screws at high speeds.

On the rail idea why spend 250 plus on rail system which will be a pain to implement when there are much easier ways to avoid or deal with the whip.
The best way to deal with the whip is not to have it in the first place. So choose the method and size the components correctly and you won't have the whip.

Would be better spending 250 on building rotating nut system and take whip out the equation.!

Desertboy
27-01-2018, 01:07 PM
You're in Cuckoo land if think direct drive 400w servos are going to spin 2.5mtr long 25mm or 32mm ball screws at high speeds.

On the rail idea why spend 250 plus on rail system which will be a pain to implement when there are much easier ways to avoid or deal with the whip.
The best way to deal with the whip is not to have it in the first place. So choose the method and size the components correctly and you won't have the whip.

Would be better spending 250 on building rotating nut system and take whip out the equation.!

What size do I need? 1.2kw?

Rotating nut is considered as well ;) It's just I only vaguely understand how to make it work.

JAZZCNC
27-01-2018, 02:21 PM
What size do I need? 1.2kw?

Impossible to answer without knowing more about the machine and Masses you are trying to move plus what speeds your looking for.

Jonathan
04-02-2018, 06:50 PM
Would be better spending 250 on building rotating nut system and take whip out the equation.!

On that note, I do have 6 partially made rotating nut assemblies left over. I never seem to get round to finishing them, so if DesertBoy is keen then it's a cheapish way forward. I've made the motor mounts and partially made the bearing mounts (bored both bearings and center). Shafts need making. I've got the pulleys in stock. I notice we're both in Leicester...