Best bearings for BK12 support

[karnalta]

Hello,

I am building my first CNC and start recieving linear motion pieces from China.

I have got one of my SFU1605 kit and I noticed that I have got nearly 0.1mm of axial backlash on the BK12 support.

This is due to the bearings that are "of course" cheap chinese one simply branded as "6001Z".

I'd like to replace them but I have a few interrogations about them :

- Angular bearing seems to be the best for axial forces but only one SKF angular bearing is over 50$.. it would means 300$ for the 3 axis (BK12) which is way too much for my build. Have anyone already tried chinese BK12 mentionned with angular bearing in it ? They only cost 15$.. I am a bit suspicious..

- If I want to replace them with tradiationnal bearing (but quality one), should I take normal one or C3 one ? Because as I understand it, C3 is for additionnal clearance for high speed running. So it sound better to take standard one without clearance but on another hand "C3" bearings often seems to be mentionned as an advantage in ball screw kit. I am a bit confuse here..

Thank if someone can help me.

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[joeavaerage]

Hi,
angular contact bearings are the way to go.

If you get matched pairs, and you would ideally, then yes you will pay big bucks. If however you buy just bog-standard angular contact bearings
the price falls to about the same price as your 6001's. Of course they won't be matched but the chances are if you buy a few more than are needed
you'll be able to get pairs that match well enough.

Craig

[karnalta]

Hi,
angular contact bearings are the way to go.

If you get matched pairs, and you would ideally, then yes you will pay big bucks. If however you buy just bog-standard angular contact bearings
the price falls to about the same price as your 6001's. Of course they won't be matched but the chances are if you buy a few more than are needed
you'll be able to get pairs that match well enough.

Craig

Thank you for your reply Craig.

I was aware that angular bearing have a direction of mounting, like face-to-face, back-to-back, .. But I didn't know they had matching pair.

What do you mean ? They are specially machined in pair or something like that ? Thank you to light me up.

[joeavaerage]

Hi,

They are specially machined in pair or something like that ?

That is correct. They come from the factory as a pair such that when they are placed either face to face OR back to back they have the right
preload. This is achieved by the inner and outer races being ground in just the right manner to take up all the lash and just a little bit more called preload.

You can buy a single 7204 P4 grade bearing, P4 is the highly concentric grade specifically for spindle use, and a premium grade for about $40. If you want a matched
pair the you'll pay $300.

Craig

[karnalta]

Hi,



That is correct. They come from the factory as a pair such that when they are placed either face to face OR back to back they have the right
preload. This is achieved by the inner and outer races being ground in just the right manner to take up all the lash and just a little bit more called preload.

You can buy a single 7204 P4 grade bearing, P4 is the highly concentric grade specifically for spindle use, and a premium grade for about $40. If you want a matched
pair the you'll pay $300.

Craig

What about these :

https://fr.aliexpress.com/item/32847...rch-mainSearch

7001 / DF / P5 : 20$ the pair. It's pretty cheap.

Probably not top notch but maybe a lot better than the cheap 6001 I have in my BK12.

[joeavaerage]

Hi,
yes they look suitable. Note they are P5 grade and so that makes them much cheaper than P4 grade to start with. You don't need P4 grade
for a ballscrew, for a spindle yes, for a ballscrew, no.

Note also that they are DT types. You can get DF types which are matched for face-to-face use, DB types which are matched for back-to-back use and then DT types
which can be used either way. The important point is that they do genuinely appear to be matched, so you'll be fine.

You are fitting these to rolled (C7) ballscrews so the highest precision is not warranted. If you were fitting to a C3 or even a C1 ground ballscrew that you'd
find in a million dollar VMC then you'd be using matched ceramic P4's at $500 plus a pair!!!

Craig

[mactec54]

Hello,

I am building my first CNC and start recieving linear motion pieces from China.

I have got one of my SFU1605 kit and I noticed that I have got nearly 0.1mm of axial backlash on the BK12 support.

This is due to the bearings that are "of course" cheap chinese one simply branded as "6001Z".

I'd like to replace them but I have a few interrogations about them :

- Angular bearing seems to be the best for axial forces but only one SKF angular bearing is over 50$.. it would means 300$ for the 3 axis (BK12) which is way too much for my build. Have anyone already tried chinese BK12 mentionned with angular bearing in it ? They only cost 15$.. I am a bit suspicious..

- If I want to replace them with tradiationnal bearing (but quality one), should I take normal one or C3 one ? Because as I understand it, C3 is for additionnal clearance for high speed running. So it sound better to take standard one without clearance but on another hand "C3" bearings often seems to be mentionned as an advantage in ball screw kit. I am a bit confuse here..

Thank if someone can help me.

Yes, they are just a deep grove radial bearing, which is not suitable for a Ballscrew fixed end, Angular Contact is your best choice for the same housing, these want to be 30-degree not the standard 15-degree, 30 degree is the minimum to use for Ballscrews fixed end, normal cnc machines use 60-degree.

They can be matched pairs which makes it easier for installation, but any regular 30-degree angular contact Bearing can be used, you will just have to make a shim to create the preload needed

If you can't find 30-degree AC Bearings your size and amount you want to spend, then the standard 15-degree would work with light loading

[karnalta]

Hi,
yes they look suitable. Note they are P5 grade and so that makes them much cheaper than P4 grade to start with. You don't need P4 grade
for a ballscrew, for a spindle yes, for a ballscrew, no.

Note also that they are DT types. You can get DF types which are matched for face-to-face use, DB types which are matched for back-to-back use and then DT types
which can be used either way. The important point is that they do genuinely appear to be matched, so you'll be fine.

You are fitting these to rolled (C7) ballscrews so the highest precision is not warranted. If you were fitting to a C3 or even a C1 ground ballscrew that you'd
find in a million dollar VMC then you'd be using matched ceramic P4's at $500 plus a pair!!!

Craig

Of course, all proportion must be kept. I am building a small desktop CNC to mill aluminium so I try to avoid 0.1mm of backlash but I am not trying to go down to 0.01.

[karnalta]

Look like in the size I want (12 / 28 / 8) they are always 15°. The bigger they becomes the bigger the angle (30° than 60°)

[mactec54]

Look like in the size I want (12 / 28 / 8) they are always 15°. The bigger they becomes the bigger the angle (30° than 60°)

Yes, that is small, 12mm shaft size, there won't be a lot of loading, unless you crash, your machine must be quite small.

[joeavaerage]

Hi,
I use 32mm THK BNFN double nut C5 ballscrews with FK25 supports. THK list 7205 P5 bearing pairs for the supports. They have a 25⁰ contact angle.
If its good enough for THK to specify them its good enough for me to use them.

Craig

[mactec54]

Hi,
I use 32mm THK BNFN double nut C5 ballscrews with FK25 supports. THK list 7205 P5 bearing pairs for the supports. They have a 25⁰ contact angle.
If its good enough for THK to specify them its good enough for me to use them.

Craig

(...) THK would only specify a Bearing by the information they were given. 25° is used for spindle Bearings also, so would only be light duty for a Ballscrew thrust Bearing. regular machining centers use 60°.

[karnalta]

Yes, that is small, 12mm shaft size, there won't be a lot of loading, unless you crash, your machine must be quite small.

Yes it's a small desktop CNC (320x220 working table). Commercial one I see generally use 12mm thread with 16mm linear shaft. I used one size above for security (16mm thread with 20mm linear shaft).

I think I have all informations I need about bearings (once again, budget is the master), I will give a try to cheap 15° bearing I have seen and see how it will work. My project has still long to go before milling the first piece

Thank you.

[mactec54]

Hi matec,



THK have been in business long enough for me to trust them, not you.

Craig

This has nothing to do with THK, 25° is not far off of 30° in terms of thrust loading, both are light duty, in terms of thrust Bearings.

[mactec54]

Yes it's a small desktop CNC (320x220 working table). Commercial one I see generally use 12mm thread with 16mm linear shaft. I used one size above for security (16mm thread with 20mm linear shaft).

I think I have all informations I need about bearings (once again, budget is the master), I will give a try to cheap 15° bearing I have seen and see how it will work. My project has still long to go before milling the first piece

Thank you.

Yes, they will work fine for what you are building, so if you use regular 15° AC Bearings, mount them face to face, this you will need to shim the inner race only, so just assemble and measure the backlash and then make a shim just .001" or .025mm thicker than the backlash, you may need a little more depending on how much thrust loading there is on the Screw.

Make sure the outer Bearing race is clamped tight in the Bearing Housing, or this will give you a false shim thickness required (Backlash number)

[dazp1976]

I got my mill basically close as I could with basic parts.
Then cut all my own bearing blocks. They don't need to be circular perfect as long as they are truly flat for mounting to machine.
Once you get the preload right and ballscrews fitted the AC bearings are solid.
I did this to fit larger AC bearings of 10mm thickness.
Installed 2 single back to back in there and they made a massive difference.

[mactec54]

I got my mill basically close as I could with basic parts.
Then cut all my own bearing blocks. They don't need to be circular perfect as long as they are truly flat for mounting to machine.
Once you get the preload right and ballscrews fitted the AC bearings are solid.
I did this to fit larger AC bearings of 10mm thickness.
Installed 2 single back to back in there and they made a massive difference.

Which way is best, for Ballscrew mounting, depends on the design of the housing and if spacers are used.

Normally for Ballscrew fixed Bearing end, the AC Bearings are mounted DF Face-to-Face, they can of course be mounted DB Back-to-Back this just depends on what works better for your build, most manufacturers only show the Bearings being mounted DF Face-to-Face configuration for Ballscrew mounting.

[karnalta]

Hello,

I just leave a quick reply here to share my feedback.

I received today my cheap chinese 7001 angular bearing (https://fr.aliexpress.com/item/32847...425a6c37Hb0PSN) and they didn't change anything

I still have the exact same amount of backlash in the screw than with the originals bearings. I have ordered 0.1 mm washer shim and I will try to put one between the two bearings to see if it helps !

If it's not, I will just follow the advices here and just go with them to later machine my own BK12 support with proper bearings.

[mactec54]

Hello,

I just leave a quick reply here to share my feedback.

I received today my cheap chinese 7001 angular bearing (https://fr.aliexpress.com/item/32847...425a6c37Hb0PSN) and they didn't change anything

I still have the exact same amount of backlash in the screw than with the originals bearings. I have ordered 0.1 mm washer shim and I will try to put one between the two bearings to see if it helps !

If it's not, I will just follow the advices here and just go with them to later machine my own BK12 support with proper bearings.

Check the depth of the BK12 housing the the 2 Bearings and that the end plate is clamping the outer Bearing race together in the housing, this is somewhere else that the backlash can come from.

[crjohnson]

If it's not too late for you, try dy-global on ebay:

https://www.ebay.com.au/itm/201599233474