14mm ballscrew + nut for VMC260

Hi all,
I am looking for replacement ballscrew for my Boxford 260VMC. The screws curently fitted are OK but could be better...
The problem is these are 14mm with 2.5mm pitch, not really a common thing these days. Also the nuts are threaded type that screw into the machine.

I have a couple of questions that hopefully someone here could answer... :-)

1- Would I loose in torque is I use a ballscrew with a 4mm pitch instead of the current 2.5mm pitch ?
2- Anybody ever use these guys for ballscrew: http://www.mooreinternational.co.uk/...8/FGR14X4.html
3- Any other place I could find this kind of Ballscrew?

Ta,
RNR

1) Formula for force (F) developed on a ballscrew for a given torque (T), lead (L) and efficiency (e) is:

F=2*Pi*T*e/L

So as you can see if the pitch of the screw, L, is increased the force is decreased , so going from 2.5mm to 4mm you would have 62.5% of the original force (common sense really...) However that's not really an issue since F is already huge - say the motor provides 1Nm of torque for the 2.5mm screw, that's F=2*pi*1*0.9/0.0025=2262N which is clearly plenty.

The real problem with using 4mm pitch instead of 2.5mm is you will loose resolution. One turn of the motor with a 4mm screw would move 4/2.5=1.6 times as far so your resolution will be 1.6 times worse. However if the ballscrews are driven from the motors via timing belts you could put a smaller timing pulley on the motor to compensate and end up with the same value for step/mm overall. You need to decide if you require the original resolution?

2,3) Considering how much more it will cost you to buy the correct ballscrews perhaps it's worth seeing if you can fit the cheap RM series ballscrews from linearmotionbearings2008 on eBay? The RM1204 ballnuts are the smallest he does.

Quote:

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Originally Posted by Jonathan

The real problem with using 4mm pitch instead of 2.5mm is you will loose resolution. One turn of the motor with a 4mm screw would move 4/2.5=1.6 times as far so your resolution will be 1.6 times worse. However if the ballscrews are driven from the motors via timing belts you could put a smaller timing pulley on the motor to compensate and end up with the same value for step/mm overall. You need to decide if you require the original resolution?

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Hi Jonathan,
Thanks for the quick reply (and the Mechanic/Math lesson) :-)

There are defferent pitch available on 14mm ballscrew from this vendor.
They can also provide 2mm pitch. Woult that be better option then ? (I guess it would make the mill a bit slower... but I've got time, I am not runing a production line... ) :-)

Laurent

Quote:

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Originally Posted by rnr107

They can also provide 2mm pitch. Woult that be better option then ? (I guess it would make the mill a bit slower... but I've got time, I am not runing a production line... ) :-)

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Yes I forgot to mention, 2mm pitch will most likely be slower than using 2.5mm or 4mm pitch, however can't be sure without knowing the motor torque curve, mass and friction. The difference in speed between 2mm and 2.5mm will at worst be 20% slower. More likely somewhere between.

We also offer a 1202 ballscrew with a flanged type nut.
http://www.zappautomation.co.uk/r120...?cPath=1_59_60

http://www.zappautomation.co.uk/r120...th=1_59_66_188

both nut and screw are stock.

Quote:

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Originally Posted by Gary

We also offer a 1202 ballscrew with a flanged type nut.
http://www.zappautomation.co.uk/r120...?cPath=1_59_60

http://www.zappautomation.co.uk/r120...th=1_59_66_188

both nut and screw are stock.

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Hi Gary,
Flange nut won't do... They have to be threaded type.

one question so, the £70 you charge for the machining of the ballscrew, is it for standard machining only or can I provide a drawing of what I want?

Ta,
Laurent

as long as the machining is no more complicated than the standard machining like to fit the FK and FF end supports then the price you see on the website is correct.

One thing to remember with fine pitched ballscrews is they have a shorter life than courser pitched ones due to less ball being in contact with the screw and nut.

Quote:

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Originally Posted by John S

One thing to remember with fine pitched ballscrews is they have a shorter life than courser pitched ones due to less ball being in contact with the screw and nut.

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Hi John,
I am not too wory about the life time... I only use my mill once or twice a week...
If I replace the ballscrews with new ones, they will last a life time for me. :-)

But thank you for the info!

Laurent

Did anyone find a good source for threaded ballnuts?.. my boxford Z Axis Ballnut just spat its bearings out... so im looking for a source of threaded ballnuts myself that wont snatch my arm off...