Quite an Unusual one

[Boyan Silyavski]

I see no point in having 2 ballscrews to drive the Z axis.? Whats the thinking behind this.
Your just making the motors work harder and increasing costs for no practicle gain.? You'd be much better spending the money on a second ballnut and creating Zero backlash.!!

Hi Dean,

Yeah, what i was thinking, you got me there . I started the design half an year ago, and learned a lot during this time.The thing is that i wanted not only to have 2 ball nuts but also 2 extra ball screw supports, " 4 is better than 2 " to resist the vibration. Thats why the double ball screw. Also the double ball screw saves on the total Z length.

Now i am not so sure. Still its not too late. Now i see that i could have made the ball screw machined so that it accepts double support at the ends. I have an extra ball screw at home, longer, i mean. Then the only thing that needs changed will be to buy 2 longer Hiwin rails to compensate. Have to rethink that. There is still time.

Now going this way of thinking i start doubting if i should not make the whole z axis 100mm longer and have travel of 300mm instead of the 200mm as its now. Could just raise the vertical bars on the sides of the base with 100mm without compromising any integrity. They are ~80mm high right now. the thing is would i really need that extra 100mm and for what?

[JAZZCNC]

How does 2 screws same length give extra travel.? Edit: Ok re -read under stand now 2 ballnuts in parallel is less length than 2 nuts in series.! But still bad idea as it will be quite hard to ensure preload is correct to give zero backlash.

Unless your spinning the ballscrew at very very high speed then at this length vibration shouldn't be problem if supported at both end. If your really bothered then use fixed bearings at both ends and tension the screw.
Personally I would just use fixed at both ends with tension and double ballnuts, this will be more than enough.

John I don't think Boyan is trying to use ballnuts to increase strength just lessen backlash and vibration/whip on screws.!

[John McNamara]

Reflecting on the dynamic load applied to the ballscrew by the Impact tool.
For many reasons I would prefer to use timing belts instead of ball screws. They do not suffer brinell pitting damage
This search came up with some useful links.
https://www.google.com.au/#q=brinell+pitting

I built this router with timing belts (The Z axis only in this machine is a ball screw but that could be changed)

I know the budget is finite but would it be possible to make a belt driven machine Or two machines One for impact work and one for Routing? Maybe sharing the electronics?

This Is the Router I built with a mate using timing belts for a non profit Men's Shed. http://www.machsupport.com/forum/ind...?topic=23730.0 There is a small video. It works really well and is used by the group so much there is a waiting list.

Regards
John

[Boyan Silyavski]

For The Z or any other axis you cannot rely on a ballscrew to take any lateral load (at all) only a pure as possible long axis load is permissible.

You will need a pair of rails to carry the load. One ballscrew is fine sized according to the long axis load carrying capacity you will find on the manufacturers specification. Bear in mind the impact hammer will apply a large dynamic load to the ballscrew, You may need to use a higher rated screw to cope with this. I would contact the screw manufacturer and ask for advice.

When setting the rails and ballscrew up they must be perfectly collinear or you will be applying a forbidden lateral load as the paths interfere with each other.

Regards
John

Thanks John,

-Yes, we are talking all the time of long axis load. There will be no side forces or if there will be, they will be smaller than say cutting aluminum.

- the dynamic load of the hammer is not big at all, as the head of the hammer will be small, anyway, the hammer can not produce such a force as one can suppose.

-The collinearity will be way under specifications of ball screw or rails manufactures. I can easily make them collinear to something like 0.00mm

How does 2 screws same length give extra travel.? Edit: Ok re -read under stand now 2 ballnuts in parallel is less length than 2 nuts in series.! But still bad idea as it will be quite hard to ensure preload is correct to give zero backlash.

Unless your spinning the ballscrew at very very high speed then at this length vibration shouldn't be problem if supported at both end. If your really bothered then use fixed bearings at both ends and tension the screw.
Personally I would just use fixed at both ends with tension and double ballnuts, this will be more than enough.

John I don't think Boyan is trying to use ballnuts to increase strength just lessen backlash and vibration/whip on screws.!

Ok, thanks for the input.


The design as its now and i have the elements already at home, implements that the both screws have fixed bearing supports at both end.

So you say there will be no sense to make the design as you suggest with one ball screw, 2 nuts and 2 and 2 fixed bearing supports at both ends? I mean only one fixed end will be enough? The bearings doesn't seem very big to me.

Reflecting on the dynamic load applied to the ballscrew by the Impact tool.
For many reasons I would prefer to use timing belts instead of ball screws. They do not suffer brinell pitting damage
This search came up with some useful links.
https://www.google.com.au/#q=brinell+pitting

I built this router with timing belts (The Z axis only in this machine is a ball screw but that could be changed)

I know the budget is finite but would it be possible to make a belt driven machine Or two machines One for impact work and one for Routing? Maybe sharing the electronics?

This Is the Router I built with a mate using timing belts for a non profit Men's Shed. http://www.machsupport.com/forum/ind...?topic=23730.0 There is a small video. It works really well and is used by the group so much there is a waiting list.

Regards
John

I know about the ball deformation. It doesn't bother me though. The hammer head will be half 10mm diameter circle . The air in the hammer dampens the hit, the material is very stretchy too/1-1.2 mm normalized sheet/ , the whole sheet stretches also, not only at the point of hit. More or less the hit will equal to 1 bearing ball hitting the material. This would be spread to all the bearing balls in 2 ball screw nuts. I am not sure how many are they. means that its not like a hit on hard rigid surface


I was contemplating this belt driven. What i want right now is the following. To finish the machine as faster as possible. To try the hammer thing at least once. to draw conclusions and not break the precision of the machine in that process. To use the machine right away to make a new one, simpler, with homemade ridgid elements, belt driven as you say and most importantly 5-6 axis. I almost have all elements for a new machine, apart of this one.

[JAZZCNC]

So you say there will be no sense to make the design as you suggest with one ball screw, 2 nuts and 2 and 2 fixed bearing supports at both ends? I mean only one fixed end will be enough? The bearings doesn't seem very big to me.

You miss under stand me I think.? . . . . . I would use 1 x screw with 2 fixed ends and tension the screw and if backlash was important then I'd use 2 ballnuts.
Correctly sized screw will have no issues with this setup if the forces are small like you predict.

[Boyan Silyavski]

You miss under stand me I think.? . . . . . I would use 1 x screw with 2 fixed ends and tension the screw and if backlash was important then I'd use 2 ballnuts.
Correctly sized screw will have no issues with this setup if the forces are small like you predict.

Dean thanks for your time!

I started already redesigning it to go with one screw and 1 or 2 nuts as you suggested. That will save me for a beer or two. I understood you correctly, but messed with my English.

What i meant is if there would be additional benefit if i fix that screw both ends with double fixed end supports, meaning 2 supports each side chained together and then tightened by the nut at the end.

[John McNamara]

I have seen Pre tensioned ball screws in large CNC machines, Also many where one end is fixed in preloaded angular contact bearings While the other is allowed to float in to allow for changes in temperature. The floating setup may be a less difficult setup to use, particularly if the frame is lightweight. If you pre tension you are imposing a substantial load on the frame, It would have to be stiff or it may be deflected out of alignment by the preloading stress, and the deflection would be likely to vary with temperature.