operation cnc

[kingcreaky]

Gentlemen.

I think hope that I am nearing the end of the design stage. I genuinely need to start ordering in order to finish the design now. Can I trouble you with casting your eyes over what I hope to be my final table and gantry design.
I have at present omitted the Z axis, or properly finished the Y. Mainly because until I have the bearing houses and rails in my hand, to measure them.(I appreciate the sites give dimensions. But) until I can bite it, Im not confident enough in my drawings.

Therefore I need some help before I approach linearmotionbearings2008 is it 'Chai?' for a quote on the bits I need.
Based on what ive previously explained. I wish to have spinning screws, rather than spin the bearings. (for now). This is mainly due to my present ability. I'm going to go for RM1610 screws (under recommendation). I never did understand irvings point within his post on p.21 when he refers to

"Another option is to upgrade the bearings to be double angular bearings (BK) at both ends. This will increase the critical speed to 880rpm (4.4m/min), better but still not great."

so, BK is like a bearing (next to the driven end of the screw) and BF is designed just as a support. I assume the BK end needs to be machined to fit the bearing, and the BF is non-machined.

His point is simply it would be better if I have BF style at both ends. and therefore get each end screw machined to suite?

So, based on my design. I will need
(X Axis)
* 2 x RM1610 (1500mm) screws (machined at both ends, one side will need to be machined to fit through the BF bearing and also so I can fit a driven pulley, and the other needs to be machined simply to fit the BF bearing. Im assuming I just let Chai know that I want to fit a pulley so he leaves enough shaft after the bearing housing. then I just by a piloted pulley and bore it to fit the machined end of the screw?
* 4 x BF bearing with supported block mouting
* 1 x ballscrew nut (quite tempted to make my own housing for this)
* 2 x SBR20 fully supported rail (1600mm) (with 4 bearing blocks) I assume SBR20UU


Y Axis
*1 x RM1610 Screw (1000mm) (same question as above)
* 2 x BF bearing with supported block mouting
* 1 x ballscrew nut (quite tempted to make my own housing for this)
* 2 x SBR20 fully supported rail (1000mm) (with 4 bearing blocks)

Z Axis
* 1 x RM1610 Screw (300mm)
* 2 x BF bearing with supported block mouting
* 1 x ballscrew nut (quite tempted to make my own housing for this)
* 1 x SBR16 fully supported rail (300mm) (with 4 bearing blocks)

Right, now I have satisfactorily embarrassed myself with incorrect terminology can somebody tell me if the above sounds about right? Or have I completely misinterpret ted anything?

while im asking the question.

assuming Im not bothered about speed initially. Therefore Id prefer to gear everything at a higher resoloution 1 turn of the stepper, means 1/2 a turn of the ballscrew. Therefore the pulleys on the ballscrew need to be twice the diameter of the pulley on the stepper.
so something like
10XL037 Flanged Timing Pulley :: BearingBoys.co.uk
for the steppers
and
16XL037 Flanged Timing Pulley :: BearingBoys.co.uk
for they the screws?

The steel on the frame is mainly 60x60x3mm which I may get ordered so I can begin building the frame.

Id appreciate your thoughts.. negative or positive.

[Jonathan]

RM1610 is good for X and Y since the higher pitch means you can get a higher feedrate with limited critical speed, however since the Z-axis screw is so short there's no need to do this, so stick with RM1605 on Z to maintain good resolution.

I advise using bigger rails on X and Z. Particularly Z since the forces on Z are a similar magnitude to Y, so you shouldn't use smaller rails.

For a 1:2 ratio you don't want 10:16 pulleys for two reasons - for a start 16/10 is 1.6 not 2, and also it's not good to have such small pulleys as you will have less teeth engaged in the belt which can lead to premature wear and possibly reduced accuracy. It's best to use at least 15T, so 15:30 would be good. Also HTD pulleys are better than XL.

The BK and BF mounts both hold bearings, the difference is the type and quantity. BK has two angular contact bearings inside, which means it can resist both radial and axial loads, whereas BF just has one 'standard' bearing in it, so only supports the ballscrew radially. If I were you, since you have the lathe, I'd make my own bearing mounts from aluminium. It's not that complicated and you'll be able to do a much better (and cheaper) job of it than the ones from China. They are often too tight a fit, or the seal rubs on the nut or other annoying thins to correct. I've never bought the BK/BF blocks myself, but I know people who have and they've all needed tweaking.

Also making your own ballnut mounts is a good plan simply because it's so much cheaper. Here's one I made earlier: