New CNC Mill on the horizon

[fvfdrums]

Quick question that I was trying to get answered earlier in the year but never got a definative so I thought I would get "paint" involved.

I was wondering if the BK12 bearing units have a rubber seal in them before the 2 bearings in the houseing and if so does the machined end of the ballscrew stop at this seal or the black part at the back of the unit or doe it fit right up to the bearings inside the unit. Picture attached is it A or B or even C and I am way off!

On a separate note I am awaiting the arrival of some profile rail and bearing carraiges, managed to find a good deal in the end and they are now in the country according to Royal Mail and out for delivery so there is a chance they may turn up today which would be great.

[Clive S]

If I am not mistaken it is more like B but there is a little spacer at each end of the bearing so that the ball screw does not squash the rubber seals. Be in mind that you will have a bit of free movement at the BF end as the bearing can float in it, say about 3mm. ..Clive

[Jonathan]

If I am not mistaken

You're not mistaken :)

[fvfdrums]

Thanks for the replies, I also had one from Chai and confirmed the same thing I am actually planning on using the bk12 on both ends so wanted to make sure where the ball screw ended in relation to the placement of the bearing blocks.

[MikeyC38]

Hi

Thanks for the replies, I also had one from Chai and confirmed the same thing I am actually planning on using the bk12 on both ends so wanted to make sure where the ball screw ended in relation to the placement of the bearing blocks.

Glad to hear that you have restarted your project. So have I and I've just ordered my aluminium plate for it. I noticed that you are going to use BK12/BF12 bearings - I have 2 pairs surplus to my build as I am using FK12 and FF12 instead on the Y and X axis. If you are interested, pm me andwe can come to some arrangement.

I will be officially opening my build log soon!

Regards
Mike Campbell

[fvfdrums]

Ok, so today started with me making a 5 mile trek with these linear bearings on my shoulders all the way across nottingham becuase the other half has the car and is away for a week and there was no one around to grab a lift from (I know I could have got a taxi but that would have been some of the cnc fund ;) ) Anyway I would not recommend it.

Having said all that when I finally got back home and inspected the package it was extremely well packaged and I would not hesitate from ordering from FA Systems from ebay again or recommending them. They were extremely easy to deal with even had a bit of negotitations with them for a better deal on price and postage and they arrived in 5 days from Korea. I got stung for a little Import duty but overall it was a great deal.

The bearings themselves hardly look used, they are well greased and run smoothly up and down the rails which also are mark free so all in all very happy indeed.

@ Mike - Thanks for the offer but I have just placed an order with Chai so I already have these parts on the way otherwise I would have been in touch. I look forward to seeing your build come together.

So due to the dimensions of the rails it has meant we have a slightly different sized machine now, its 1130 x 1060 with a cutting area of 940 x 940. The redesign of the machine is going well and even though I would have rather had the original files to work from its bee refreshing to tackle the design from the ground up and incorporate some of the great ideas people have come up with over the last year since I put the original together.

[fvfdrums]

More ordering done from overseas!!

We got a great deal on the motors and drivers so now we have 1 x nema 34 and 2 x nema 23 plus 3 of the 7.8amp drivers coming in the next 5 days or so.

Also the order from Chai has been processed now and is also enroute so once everything is here and checks out I think its time to get on to Aluminium warehouse and start the gantry build!

btw: Chai was extremely patient and helpful even pointing some things out I missed and I did not feel the language was a barrier at all. Hopefully what arrives is as good as the service

[fvfdrums]

Whilst i am here, this machine will have an adjustable bed, I was wondering what peoples opinons are on a sensible distance between adjustment holes? 50mm 100mm? That is how far the bed moves up and down between one set of holes and the next

[Jonathan]

Thanks for the replies, I also had one from Chai and confirmed the same thing I am actually planning on using the bk12 on both ends so wanted to make sure where the ball screw ended in relation to the placement of the bearing blocks.

There's two reason to use angular contact bearings (bk12) on both ends of the screw. One reason is it increases the axial stiffness of the screw, since you have twice the number of bearings supporting it. The other reason is it raises the critical speed of the screw, so you can spin it faster without it whipping. For the vast majority of DIY builds, the stiffness of the screw is at least an order of magnitude greater than other parts in the axis, so you're unlikely to see a tangible gain from doing so. The critical speed however is important, but for the 1610 screw you've chosen you'll be able to get good feedrates so long as the screw is no longer than 1500mm. According to an earlier post ('1200x900 footprint area with approx 1000 x 800 cutting area'), your machine is well below this length so using two BK supports on each screw will only increase cost with no useful gain.

(Search on google for 'Ballscrew stiffness calculation' and 'ballscrew critical speed calculator' if you want a reference for the above.)

Whilst i am here, this machine will have an adjustable bed, I was wondering what peoples opinions are on a sensible distance between adjustment holes? 50mm 100mm? That is how far the bed moves up and down between one set of holes and the next

It depends on your Z-axis travel. Also no need to have them evenly spaced, why not have more holes nearer the place you expect to be using the bed most often and less at the limits? In the past I've gone for 60mm spaced an 50mm spaced, simply because that what the size of the box section I used. You'll probably only move the bed about once a year anyway! I'm not sure I've even moved mine that often, and the latest machine is so much stronger that it doesn't matter where you put the bed.