X3 mill with rails, ATC, new motor

[Jonathan]

Did you mention what size/grade ballscrews you are using?

Briefly in post #18 - I'll be getting TBI brand ballscrews with DFU nuts, C5 grade with preload. I have already put one of these on Z. For the diameters, likely 20mm on X and Y based on the stiffness calculations in the same post, but not certain yet.

Just wanted to chime in on the bearings front. I would suggest you go with angular contact bearings and ones specific for ballscrews or ones specifically made with medium preload as standard.

The ones specific for ballscrews are what I've been looking at (i.e. 60 degree contact angle, preloaded), but I'm somewhat put off by the pricing, even on eBay. This document compares the virtues of triplex angular contact bearings vs a pair of tapered roller bearings and seems to conclude that the main limitation is the additional friction. In terms of stiffness, they are pretty much the same. I don't really consider the additional torque to be an issue, as that is a secondary consideration compared to the bearing axial stiffness - in essence I can just use slightly higher rated motors, which wont be a concern if I use my own. I will use at least 15mm bore bearings, so the available sizes are not an issue.

Some time ago, I did buy some angular contact bearings from aliexpress with 12mm bore that were supposedly matched pairs. Although they have the markings of matched pairs, they don't actually seem to be so, hence I am a little wary of trying that again with more expensive bearings, e.g. BS1547TN1 or BS1747. Interestingly the load rating of ballscrew specific bearings is similar to that of similarly sized taper roller bearings - probably the stiffness requirement dictates a certain load rating.

From what I've read/calculated the stiffness of generic single row <60° angular contact bearings in matched pairs is low compared to the screw/not or simply unclear/unspecified. There's a formula floating around for calculating the bearing axial stiffness for given dimensions, which I can do from the data on the SKF site, but the resulting values don't give much confidence. I fear that if I use these the stiffness of the bearings would be noticeably less than the ballscrews, so when combined in parallel with the nut & screw stiffness, it would not be a well balanced system, hence I want to at least get 60° contact angle bearings, if not roller bearings.

[komatias]

The stiffness you get from the bearings is also dependent on the preload class you use. Just like in the ballscrews, you can have zero backlash (P0) but you can also have P3 and P5. And of course there is the precision class and clearance to them too. The more accurate, the stiffer the bearings can be claimed to be.

You are right about the price though. On My KRV I went with heavy preload and spent around £240 for 2 sets.

You should also think about the stiffness of rotation perpendicular to the axis of the ballscrew. this will govern which way round you put the bearings. I forget which orientation is stiffer (face to face or back to back)

In the end I bought IBC bearings with 60deg angle medium preload. You can find all you need here: http://www.ibc-waelzlager.eu/dmdocum...t_Bearings.pdf

The ones I would have bought had I not bought the ones I did are: http://www.qualitybearingsonline.com...ct-20x47x15mm/

Another slightly overlook aspect is the locking nut you use should be as good as you can get it. The squareness of the thread to the face is not great in some of the chinese nuts I have come across. In my case I also wanted to lock on the front face as opposed to on the thread. Too a while to find anyone in the UK what had them.

[Jonathan]

Another slightly overlook aspect is the locking nut you use should be as good as you can get it. The squareness of the thread to the face is not great in some of the chinese nuts I have come across. In my case I also wanted to lock on the front face as opposed to on the thread. Too a while to find anyone in the UK what had them.

My plan there was to make them - not been a fan of any cheap ones I've bought and making my own at least means it's under my control. Or maybe buy these.

Today's big news is that I collected the newly ground milling table. They did it really quickly - dropped it off at about 9am on Monday and it as done by midday. Will post in more detail later - for now, here are some shiny pictures:



Note the size comparison with the old X3 table :)

[komatias]

Jonathan,

I used these: http://www.spieth-maschinenelemente....ts/series-msr/
Much better than I would have been able to make or get made.

Can't quite see in the photos of the table. It seems that the marks are circular? Did they use a Lumsden grinder? If so then do check the flatness and parallelism they quoted. It is nowhere near as good as surface grinding that I would have thought they would do for a machine table. But it may of course be good enough for what you want....reminds me that my machine could do with a regrind this year.

[Jonathan]

Thanks for the link to the locknuts.

Regarding the griding, yes it was done on a Lumsden grinder.

For the parallelism I've checked with a micrometer at several points roughly down the centerline of the rails on both sides. The range of the readings was 49um, but most were within 20um or so. I'll post the readings tonight. That is worse than one would expect for a commercial milling machine, but I think it is very reasonable for what I need (being realistic about other tolerances in the system) and certainly better than the mill I have. Also consider they only charged £50.

Flatness is harder to check - I've tried bluing a parallel and seeing how the blue transfers to the bed at places across the width and length. It seems fine, but the parallel is only about 200mm long. I do have a ~600mm long surface plate. No idea of it's history though so I'm not sure if there's much value in trying to check flatness with it?

Jonathan,

I used these: http://www.spieth-maschinenelemente....ts/series-msr/
Much better than I would have been able to make or get made.

Can't quite see in the photos of the table. It seems that the marks are circular? Did they use a Lumsden grinder? If so then do check the flatness and parallelism they quoted. It is nowhere near as good as surface grinding that I would have thought they would do for a machine table. But it may of course be good enough for what you want....reminds me that my machine could do with a regrind this year.