6' 20mm ballscrews...

[magick_man]

I am debating going with ballscrews for the router I am building.
Although a belt drive is still not out of the question.
There are some "inexpensive" ballscrews available on ebay and they sparked my interest.
I would be using a 68" x 20mm ballscrew for each side of the x axis.
The price for all 4 screws my machine would use is $485 including $140 shipping.
2x) 1750mm x 20mm X dual drive.
1x) 1000mm x 20mm Y drive.
1x) 500mm x 16mm Z drive.

Does anyone have experience with screws this long?
I am not sure if dealing with possible whipping would be worth the trouble.
I haven't priced out a belt drive setup yet but I would imagine it is quite a bit cheaper.

Any insight would be appreciated.

Thanks,
Joseph K.

[magick_man]

This is the seller if anyone is interested.
It is the same person selling the cheap 1.5 and 2.2kw spindles.

http://stores.ebay.com/linearmotionbearings

[tumutbound]

Originally Posted by magick_man This is the seller if anyone is interested. It is the same person selling the cheap 1.5 and 2.2kw spindles. http://stores.ebay.com/linearmotionbearings

Can't comment on whether the length of your ballscrews will have a problem with whipping but I can give the thumbs up to this eBay supplier. I ordered some ballscrews from him last year, good quality and quick delivery.

[irving2008]

lets say for sake of argument you wanted 200ipm on rapids so your 'end fixity factor' Fe for a 20mm (say 18mm (.7") root dia) screw, 68" long, with an 80% safety factor and 5mm (.2") lead is:

Fe = 200 * 68 * 68/(4760000 * .8 * .7 * .2) = 1.73

Because this is > 1.47 (Fixed-Simple config) and < 2.23 (fixed-fixed) this means you WILL need fixed supporting bearings at both ends of the screw (provding both axial and radial support)

Your actual critical speed is:

Cs = 2.23* 4760000 * .8 * (.7 * .2/(68 * 68)) = 257ipm

Above this your screws will whip....

So dont forget to add the cost of appropriate back to back angular contact bearings plus housing for both ends of the screw (or a normal bearing plus a thrust race) and the ballscrew clamps to set the required pre-load on the bearings.

[veteq]

balscrews are very nice offcourse, don`t forget when you want to use them that the planes where you mount the bearing block on do have to be precise on the same plane.
a offset of for example 0,05 inch isn`t possible.

For such a big setup the price goes up very high.

regards,

roy b.

[magick_man]

Originally Posted by tumutbound Can't comment on whether the length of your ballscrews will have a problem with whipping but I can give the thumbs up to this eBay supplier. I ordered some ballscrews from him last year, good quality and quick delivery.

Thanks for the heads up, I think I am going to order a screw and spindle from this guy.
So it is nice to hear some good feedback on him.

Originally Posted by irving2008 lets say for sake of argument you wanted 200ipm...

Thanks, that is what I needed to hear.
I wasn't sure about the math to figure critical speeds.
And ideally I want this machine do be able to do over 200ipm on rapids.
I have been pricing out belt drive and it looks like it would be about $300 for X and Y if I dual drive the X.
If I mount the x motor(s) on the gantry and fix the belt in place I can cut out another $60 or so in belt costs.
So it is looking like Belts might win out on the X and Y.
But I think I will still go with the ballscrew for the Z.
With the 20" screw I would use for the Z even with one end unsupported i should never have to worry about whipping.

Originally Posted by veteq balscrews are very nice offcourse, don`t forget when you want to use them that the planes where you mount the bearing block on do have to be precise on the same plane. a offset of for example 0,05 inch isn`t possible. For such a big setup the price goes up very high. regards, roy b.

Yeah I think I was aiming a little high for this being my first machine.
And being on a semi limited budget I think the complications the long screws will add just aren't worth it at this point.


I appreciate all the feedback so far.