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Does a 20x2 ballscrew backdrive?

Luke Rickert

Hot Rolled
Joined
Oct 24, 2007
Location
OSLO
I know that all normal ballscrews will backdrive and therefore are not used in manual machine tools without brakes. That said I have a Schaublin 135 lathe which is a manual lathe that uses a ballscrew for the Z axis (long), I believe it is a rotating nut and as far as I know it does not have any sort of automatic brake. It does not backdrive under moderate loads (the DRO shows no movement when facing etc without any manual lock) This would indicate that perhaps not all ballscrews backdrive or it could also be that Schaublin did something tricky to prevent it from happening.

At some point in the near future I would like to replace the Y axis screw and nut on that machine as it is both rusted and inch pitch, buying a new screw and nut from Schaublin is an option (a very expensive one) but it is also possible to get a ballscrew with a very low helix angle and correct pitch and "hand" (such as the 20x2 in the title). I would need to squeeze the nut in there but that is manageable I think. I don't really want to spend the money on the screw and nut just to find out it is not a workable solution so I am curious if anyone has first hand experience with this sort of thing?

What about a Satellite Roller Screw?

thanks

Luke
 
Whidbey,

A 20x2 preloaded ball screw is very unlikely to backdrive when combined with any kind of dovetail drag and drivetrain drag on the bearing mounts which are also preloaded at least on one end.

A roller screw is far less efficient and less likely to back drive and likely a better choice for a cross slide (X-axis) from a resistance to backdrive due to high forces combined with vibration from tool chatter or interrupted cut.
 
5mm pitch ball screws on a bridgeport mill won't back drive. I do agree about chatter causing them to move though, both ball and acme are prone to that, but its not so much the loading but the vibration effectively nearly totaly unloading then loading the screw that seams to do it, even a very light chattering cut can some times move a acme on good free running slides.

My best advice is add some kinda brake or lock to the slide.

i have been toying with the same issue with my harrison lathe, probelm is space is bonkers tight on that thing ad would have to drop to a 12mm screw to stand a chance of it fitting.
 
There is a manual lock already that I can use when needed so that is fine, I just won't want to have to use it all the time.

Now if I can fit a nut in there is a whole different question, the nut on a 20mm ballscrew is going to be tight at 36mm. (20 is also massive overkiill) For a 16x2 it is only 25mm at the nut OD so that would probably fit. The original screw is 16mm trapezoidal so even a 14mm ball would be fine from a capacity standpoint, the helix angle is just getting shallow as the diameter goes down.

I will add it to the long list of projects along with finding or making a new dial.

L
 
Depends on the static friction/resistance to rotation of the connected mechanism. Sometimes seal friction alone will be enough others may range from changing to a stiffer grease to a purpose made friction element may. Back drive against an energized stepper? I doubt it unless the lead is long and the cutting force proportionately large.
 
Some months ago I talked to Aratron AS regarding ball screws for a lathe. (Aratron distributes Eichenberger and Hiwin ball screws in Norway.) They mentioned that Eichenberger only makes rolled ball screws, and that although they offer them with preload they said that this configuration might yield friction variations as you turn the screw. (Ostensibly due to dimensional variations along the screw.)

If I understood the salesman correctly that ground ball screws do not come with this risk. Me having no experience on the matter and this coming from a salesman it might well be imprecise.

Hiwin seems to offer 20x2mm ground ball screws; GBS-R20-2T4-FSIE2, found it in this Hiwin ball screw configurator.
 
It sounds to me like they were trying to upsell, for Eichenberger anyway the rolled screws are very nice, consistent etc. The junk from China not so much although Hiwin rolled screws are probably also just fine (they are from Taiwan anyway) A ground ball screw would be overkill for this sort of application I think. The original trapezoidal screw was ground as was the Z axis ball screw, but 50 years ago I don't think there was any other choice. I have a few Eichenberger screws and they seem very consistence to be sure. The only way roundness could be an issue is if you ran a super high preload which really isn't necessary on a manual machine tool with a DRO. Ground screws are needed for VMC's etc that depend on the screw for their exact position, run high preload and see much higher loads and much more travel.

L

Some months ago I talked to Aratron AS regarding ball screws for a lathe. (Aratron distributes Eichenberger and Hiwin ball screws in Norway.) They mentioned that Eichenberger only makes rolled ball screws, and that although they offer them with preload they said that this configuration might yield friction variations as you turn the screw. (Ostensibly due to dimensional variations along the screw.)

If I understood the salesman correctly that ground ball screws do not come with this risk. Me having no experience on the matter and this coming from a salesman it might well be imprecise.

Hiwin seems to offer 20x2mm ground ball screws; GBS-R20-2T4-FSIE2, found it in this Hiwin ball screw configurator.
 
Ground screws are not really esential for a cnc as most controls have comp tables, simply measure real axis movement compared to commanded and you can easily have far more precise movement than a ground screw gets you. This is very very common on long travel machines.
 
Ground screws are not really esential for a cnc as most controls have comp tables, simply measure real axis movement compared to commanded and you can easily have far more precise movement than a ground screw gets you. This is very very common on long travel machines.

Yes for CNC, linear encoders are going to be better than any ground screw but before that was doable very accurate screws were important. I am sure still are for certain applications as ground screws must have other advantages given their very high costs :)

For a manual lathe the linear encoder (in this case my DRO) means the screw accuracy isn't particularly important. Although for very small movements being able to use the dial is much nicer than depending on the DRO. I find it easier to dial in small cuts with my pure manual schaublin 102 than on my 135 with an inch screw and DRO (I only work in metric so the dial on the 135 isn't useful)

L
 
It sounds to me like they were trying to upsell, for Eichenberger anyway the rolled screws are very nice, consistent etc. The junk from China not so much although Hiwin rolled screws are probably also just fine (they are from Taiwan anyway) A ground ball screw would be overkill for this sort of application I think. The original trapezoidal screw was ground as was the Z axis ball screw, but 50 years ago I don't think there was any other choice. I have a few Eichenberger screws and they seem very consistence to be sure. The only way roundness could be an issue is if you ran a super high preload which really isn't necessary on a manual machine tool with a DRO. Ground screws are needed for VMC's etc that depend on the screw for their exact position, run high preload and see much higher loads and much more travel.

L

Ah, I should have guessed as much. Good to get an idea of the utility of rolled screws.

Might I ask from whom you purchase Eichenberger screws, do they also sell directly?
 
nitromarsjipan I sent you a PM with more details but I got the Eichenberger screws from a dealer in the UK although I would check the local dealer first as shipping was expensive. I think Eichenberger might sell directly but perhaps not for one screw.

Luke
 
nitromarsjipan I sent you a PM with more details but I got the Eichenberger screws from a dealer in the UK although I would check the local dealer first as shipping was expensive. I think Eichenberger might sell directly but perhaps not for one screw.

Luke

Hi Luke !

Did you ever installed those?
 








 
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