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17-03-2011 #1
Hello,
Despite some search, I haven't found anything on this forum about this technique described (in french) here: http://cncloisirs.com/Construction/E...ntVictorSendas
Here is a picture from this site which can help you to understand:
It seems very simple, a ball bearing rolling on a trapezoidal screw and quite efficient in terms of backlash reduction & transmission without friction.
Have any of you tested this technique? What are the feedbacks?
I am wondering what is this "nut" in which the ball bearing is maintained?
Thanks for your answers!
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17-03-2011 #2
My view is that most if not all leadscrews are soft "Not Hardened" the profile of the leadscrew is not made to be used in the scenario you describe.
So in the standard situation the most expensive part "The Leadscrew" runs in a bronze bush/nut "The Cheap Part" so the bronze bush is replaced at minimal cost preserving the integrity of the leadscrew.
The situation you describe is a bearing is used to run down the thread. Now for the problem, the bearing "The Cheap Part" is hardened and it is gonna wear the expensive leadscrew in no time at all as its harder than the leadscrew.
Now you could use a ballscrew which is hardened but then its not much more trouble to use a ballnut and get an even better solution.
Good English BTW
Phil
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17-03-2011 #3
Extensively discussed here:
http://www.cnczone.com/forums/linear...gotta_see.html
see also :
http://www.cnczone.com/forums/linear...ind_screw.html
Bill
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17-03-2011 #4
Well i looked at the 3 page thread, nobody mentions the downsides i brought up, i will not be looking at the 51 page thread.
Phil
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17-03-2011 #5
I have done it on my Y axis as 'Chip' said it's in my thread and videos on youtube. There's a good shot of it in the long video about wind turbine blade mould if I recall correctly.
http://www.mycncuk.com/forums/showth...ding.../page11
(I've measured the backlash and recorded it somewhere in the thread - pun not intended!)
I used 16mm threaded rod with a bearing that happened to fit - can't remember the dimensions offhand but I'll check if you want. I didn't put any sort of insert on the bearing - it's just running directly on the screw. At the moment it's working perfectly well, however the screw is visibly worn. Not much, but noticeable. Initially the backlash was very very low (practically zero - clearly you're never going to actually get zero), now it's a little more but still good. Better backlash than a cheap ballscrew at least.
Originally Posted by M250cnc
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18-03-2011 #6
If say you were using a bronze nut "You Don't Use Brass" that was 12mm long that would have a contact area on the thread of (12x3.142)x12 of 452mm approx, using a bearing this way would have a contact area of approx 20mm that's 22 times less contact area not to mention threaded rod is low grade with a poor surface finish so will wear out the bush in no time.
The fact that this idea is not commercially available says it all really.
Phil
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18-03-2011 #7
True, you're not going to be able to put anywhere near as much force on it - and the pressure is much greater due to the tiny contact area. Probably only a couple of mm, not 20, since it's only contacting at two lines (in theory). Could improve this with more bearings.
Yes it does. Except one thing - the sort of machines we're making are generally not used for heavy cutting (like commercial machines). I think this method excels when you're only cutting woods/plastics so the forces are low. ACME rod would certainly be better, but I just used stainless rod as I know that eventually I will probably buy a ballscrew.
It could be good for a PCB machine where low backlash is going to help with accuracy and the cutting forces are tiny.
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18-03-2011 #8
I was being generous, so its even worse than i thought it was.
To drive a small PCB machine a belt drive like they use on the RepRap machines would do the job better, be easier to implement, would last longer and be far stronger. You also do not need angular contact bearings for the fixed end of the screw either. That is the HD 10mm pitch belts BTW.
Phil
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18-03-2011 #9
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18-03-2011 #10
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