Ready Steady Eddy

[EddyCurrent]

That's Fig2 then, thanks.
I also decide to get some new bearings for the blocks because the backlash on the China bearings was serious. I fitted a shim between the outer races but with the locknut fitted there was still slight play, the shim was the thickness of a Guinness can which in this application is thick. I also took a skim off the block to make sure the outer races were being clamped.

Also I'm sure I have a bent ball screw on the Y axis, it's not something I've done myself, on the last 1/3rd of it as you rotate it by hand it goes, slack,tight,slack,tight,etc. and using inside calipers between the screw and the gantry the gap goes big,small,big,small,etc. I'm not 100% yet but it might be the turned down section on the end is off centre.

I may have to get into this

http://www.roton.com/screw_shaft.aspx

[Clive S]

I also decide to get some new bearings for the blocks because the backlash on the China bearings was serious. I fitted a shim between the outer races but with the locknut fitted there was still slight play, the shim was the thickness of a Guinness can which in this application is thick. I also took a skim off the block to make sure the outer races were being clamped.

I think I remember Dean saying some time ago that the tubular spacers on the BK bearings are not always long enough and that he had made fresh ones. It is there to enable the lock nut to push the inner races together.
I might be talking crap though ..Clive

[EddyCurrent]

As requested layout info for panel, please refer to schematics posted previously.

Starting top left.

Panel isolator, AM882's, ABB Inverter
PSU for stepper drivers, FL2, FL1, CB2, 5V PSU, 24V PSU
CB1, K1, K2, K5, Breakout Board with spindle board and K6, Smooth Stepper (mounted on panel side (aargh I hate that))
Terminal rail, K3, K4, PILZ Safety Relay



On the close up photo you can see 3 wires (77,75,24N) going to a piece of vero board located on a stand-up on the bob, that's K6 the transistor relay for flashing the lamp (see schematic)
It got a bit crowded and I've not got the fans in yet ! the grey wire hanging down is just the spindle I have lashed in temporarily.

Added a limit override switch (it's a spring return type so you can't accidentally leave it on), still need to label up everything on front.

[EddyCurrent]

Clive, you're not talking crap but in this case the spacers are okay, it's the bearings themselves, the inner and outer races are dimensioned wrong so there is no gap for preload. I've been reading that the bearings should be obtained as a pair.

[njhussey]

After reading this I've just been looking closer at one of my fixed end bearing blocks and it has play in the bearings. The floating end has a NSK Japan bearing in, not sure about the fixed as I've not taken it apart yet but I can bet my bottom dollar that they'll be cheap Chinese bearings...

[njhussey]

Yep....chinese bearings (ungreased) and face to face so the inner races were not being tensioned...need a shim and some grease and see how they fare...

[EddyCurrent]

Looks like Z axis ball screw has issues too. Noticed while driving it up and down, with the front plate off, the ball nut and bracket were fine at the bottom but the more they reached the top the more they oscillated side to side. I took it all to bits and found two things, the turned down section at the driving end was slightly off centre and the the whole turned down section was bend to one side. With the ball screw resting in two V blocks it was rotated and a DTI measured a deviation of 0.8 mm at the end where the pulley sits, this reduced to 0.35 where the BK12 bearing sits. So I did what any self respecting person would do, put it in the vice and knocked seven bells of shite out of it ! to say I was upset would be an understatement. Actually I put it in my mini lathe chuck with a DTL fitted and 'coaxed' it back straight. It's a lot better but not perfect due to the off centre turned down section. Next I'll have to see what can be done with the Y ball screw. Who's idea was it to get stuff from China ?

[Lee Roberts]

Who did you buy from ?

[EddyCurrent]

Who did you buy from ?

I bought them from Chai

[Jonathan]

Whilst we're close to the topic...for a while now I've been dismantling new ballnuts I've obtained from Chai, as it's not uncommon to find debris in them - be that just general dirt or bits of metal.

On the machines I've made to get zero backlash (lathe, mill & router), I preloaded the angular contact bearings using disc springs and shims between the bearings. That way there's less risk of damaging a bearing by over-tightening the nut as you know how much force you're applying from the spring deflection and it avoids buying matched bearing pairs. I also use bearings with a bigger outer diameter than the ones in the BK blocks - i.e. use 7201. They're less delicate...

If you want an easy option for a standard CNC router, then I don't see much wrong with just using double row angular contact bearings (52xx series), as although they have a small amount of axial play, the amount is much smaller than the single ballnut backlash so overall the effect isn't very significant. If you start pre-loading the nuts, then naturally the bearings need pre-loading too.

Also, be very careful with opening out the bearing blocks to better fit the bearings, as going too far may introduce backlash (depending on quite a few factors).