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View Full Version : Ball Screw Accuracy V Price



Wobblybootie
30-12-2009, 11:25 PM
Do any of the learned members have views on the accuracy/repeatability of C5 compared to C7 Ball Screws. Also, is the extra cost of C5 kit worthwhile for the sort of work folks produce? For myself it is something I may consider bearing in mind one of the things I am looking to do is the engraving of custom made Fountain Pens (I already have an interested party in such a project)

irving2008
31-12-2009, 12:57 AM
Do any of the learned members have views on the accuracy/repeatability of C5 compared to C7 Ball Screws. Also, is the extra cost of C5 kit worthwhile for the sort of work folks produce? For myself it is something I may consider bearing in mind one of the things I am looking to do is the engraving of custom made Fountain Pens (I already have an interested party in such a project)

For a machine doing fountain pens you wont have leadscrews longer than 300mm. C7 is 104um for error and fluctuation per 300mm length, C5 is 18um error + 23um fluctuation = 41um, these are all worst case figures.

If your leadscrew is a 5mm pitch, direct driven then you are looking at a resolution of .025mm per step, or 25000um. With a C7 screw your worst case error is 208/25000 = 0.8% per step, with C5 its 0.16% per step.

Spending your money on making the machine rigid and look at double preloaded nuts to remove backlash will improve the accuracy far more than going from C7 to C5!

Basically for engraving a fountain pen there is absolutely no point, as your engraving tip is massive compared to this error (and anyway the rigidity of the machine will introduce larger errors).

Wobblybootie
31-12-2009, 08:04 AM
I thought so, and deep down I probably knew it as well. Thanks for putting me straight. I have a nasty habit of trying to 'over engineer' things which normally leads to me making any sort of project far more complex and costly than I can cope with. Net result ... Frustration and failure.

Gary
31-12-2009, 08:16 AM
As pointed out C5 while nice, is not really needed.
I sell quite a lot of ball screws for cnc machines and have only been asked once for C5, and for such a small axis it does not offer any great advantage.
Zero backlash and making the machine solid is good though.

Robin Hewitt
02-01-2010, 01:41 AM
If your leadscrew is a 5mm pitch, direct driven then you are looking at a resolution of .025mm per step, or 25000um.

Uh? Irving? I think you'll find .025mm is 25um.

Sounds like you may have overdone the New Year celebrations :heehee:

irving2008
02-01-2010, 10:55 AM
Uh? Irving? I think you'll find .025mm is 25um.

Sounds like you may have overdone the New Year celebrations :heehee:B***er you are right...

Over a 300mm length C7 gives a overall worst case positional accuracy of .07% and C5 gives 0.013%

If absolute step position is paramount then C7 gives, worst case on 5mm pitch, an absolute step to step position error of 2% (of resolution) against C5 of 0.8%

Doesnt change the conclusion tho, C5 is an unnecessary expense