operation cnc

[Jonathan]

OK but surely it depends on the speed you need.

Yes, if it's sufficient and the motors can drive it at that speed you're fine. But that's the other issue, you will need significantly more torque from the motors to obtain the same feed-rate/acceleration as with the 16mm screw since it is not a linear relationship, power 4 in fact, so for a given acceleration you need (20/16)^4=2.44 times the torque. So if at the speed/acceleration you would have run the machine on with the 16mm screw you have that much spare torque then it would be fine with 20mm, but it's a shame because you would definitely have got better feedrates with the 1610 screw and it's added an unnecessary risk.

For a 1500mm 2005 screw the critical speed is about 1200 rpm with the standard bearing arrangement, so 1200*5=6m/min feedrate.
For 1610, same length etc, it's about 1000rpm, so 1000*10=10m/min feedrate.

From experience the formula used here seems to be conservative, so that is probably enough assuming the motors you have can actually output enough power to achieve that feedrate.

Plus you get greater mechanical advantage with a smaller lead?

True, but you can use the pulleys to get whatever mechanical advantage (i.e. ratio) you want, so it's a bit of a moot point.

[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:

[kingcreaky]

ok, bits ordered (warning: communication with chai is a bit difficult, generally a one day delay with every message due to time zone) then, when you do get a reply its questionable if he has undertood your meaning).

X Axis
* 2 x RM1610 screws(1603mm Total length) I need (1500mm) of screw, so 1500mm PLUS one end machined to type A (except dimension F to be 25mm instead of 15, and the other end machined again to type A, but without any F
* 4 x BK12 fully supported bearing with supported block mouting
* 2 x ballscrew nut
*2 x SBR25 1600mm fully supported rail with 4 bearing blocks

Y Axis
*1 x RM1610 Screw (This one, id like the TOTAL length to be 1000mm, but one end machined to type A (except dimension F to be 25mm instead of 15, and the other end machined again to type A, but without any F
* 2 x BK bearing with supported block mouting
* 1 x ballscrew nut
* 2 x SBR20 fully supported rail (1000mm) (with 4 bearing blocks)

Z Axis
*1 x RM1605 Screw (This one, id like the TOTAL length to be 300mm, but one end machined to type A (except dimension F to be
25mm instead of 15, and the other end machined again to type A, but without any F
* 2 x BK bearing with supported block mouting
* 1 x ballscrew nut
* 2 x SBR20 fully supported rail (1600mm) (with 4 bearing blocks)


So, I now have some momentum. Im a few hundred quid lighter. But most importantly the ball is rolling. So now not just out of my depth :D BALLS DEEP and out of my depth.

Just have to wait for my contact to return from South Africa, to make an order for the box section to commence making the table. (hopefully will have this by the end of next week (mon 19th)

I have soo many questions, Ive tried my hardest to research them before asking, but straight answers from the experienced is so much easier! as often you go off and read something, then come back here and read contradictions and end up banging your head against the wall and opening another beer.

So some questions gents. First one; what am I searching for in order to get videos, detail etc on the process of software behind creating a work-peace once the machine is made. Is there such thing as a machine simulator? I understand Mach3 simply takes an input file.. a dxf? So I can draw the workpeace in autocad {if i had a licence} -- what about sketchup?. What is and why do people use solidworks?

Second One;
Anybody want to face inevitable persecution and real off a list of recommended electronics? or maybe even just the electronics your using so I can copy!. I have previously mentioned what steppers I have,

Nema23 Stepper Motor 3.1Nm x 3

I understand the trick is more voltage to the stepper the better.

I read good things about these Leadshine AM882's for controllers.... anygood? thoughts?

can anybody suggest anything better? Realistically, im not going to build my own power supply to then plug it into the hundreds of pounds worth of controllers ive just bought.... il just spend a little bit more a buy a power supply aswell. Reccomendations?


then, the other big question is breakout board... /controller board. I like the idea of one of these Cat5 units (as my computer is firmly mounted in workshop and usb and parrelel has a finite distance)


Finally, if I use 3mm steel box section. Im going to need to drill and tap it to accept bolts that will affix the linear rails etc. Therefore is 3mm thick enough?


look forward to your replies.

Once my machine is done, and I have used it a few times. The next thing Is I plan to hold an open evening. All will be invited to my workshop for coffee and cakes. The idea being anybody who (like me) is just coming into this, feels ridiculously out of their depth, and wants to see a machine work, see the process from start to finish and ask limitless stupid questions and get straight answers can attend! You all Watch this space :-D