I have a CNC3040 from eBay and it works great. It has got me on the learning curve and given the confidence to try to make my own machine now :)
So I have a few questions if I may:
1) What are the pros and cons of ball-screw vs. lead-screw?
2) How important is supported rail vs. unsupported? I plan to build a machine that will have a 120cm x 60cm cutting area.
3) What factors dictate the max. speed of a machine? Is it the motors or the drivers, and how can I estimate the speed I'll get?
1) Ballscrews have far greater efficiency - about 90% vs 50% and significantly lower, but not zero, backlash however unlike leadscrews due to the use of rolling not sliding friction they can be preloaded to obtain practically zero backlash.
2) Extremely important. An unsupported rail will bend significantly more when a force is applied, ultimately leading to tool deflection and thus poorer accuracy. Supported rails are supported along the entire length, hence the force is largely transmitted to the frame which is significantly stronger than the rail and therefore deflects less. Does depend on the orientation, but either way a 12mm supported rail is probably stronger than a 25mm supported... huge difference.
3) Motors, driver voltage, ballscrew pitch, ballscrew diameter, drive ratio, mass of moving parts, type of rails used, preload of rails ... etc. You can estimate it using this calculator:
Thanks for the info Jonathan.
Could somebody please explain to me what is meant by 'fixed side' and 'supported side' with reference to ballscrews?
I don't understand why the same simple bearing can't be used on each end.
If you fix both ends axially you have to wonder what will happen when it expands and contracts due to temperature change.
Fixing at both ends is good if you can hold it in tension with springs, otherwise you have to let one end free to slide.
Pics below show ballscrew with fixed end on right and a floating end on left, you will see fixed end as threads on ballscrew and the block is wider than the floating end because it has 2 angular contact bearings working togehter to handle the axial loads. The fixed end has just a normal bearing which is allowed to move inside the mounting block if any thermal expansion happens, it is still attached to the ballscrew via a small circlip so it can't fall off.
Thank you, nice picture! This machine is getting more expensive ;) Still, good tools earn their value.
Which of these illustrations is the correct way to mount an X axis with supported rails?
(Yes I know there is no Z axis ;) )
Also a question I feel stupid asking, but what is commonly called the X and Y axis? On my CNC3040 the Y axis is the length of the table and the X axis goes across. I've seen people talk about it the other way though.
What you have pictured is generally called the Y/Z axis, although as you say it does vary. I always call X the axis the gantry runs on, so generally the longest axis.
The orientation of the rails in the image on the left is considered better since the deflection due to each bearing is more even. For forces in the direction pulling the bearing off the rail, not surprisingly, the bearings are weaker and will deflect more. If the open sides are facing, as you have drawn, this evens out so the deflection will be the same for all axis which is much better than having significantly different deflection which is what will happen if the rails are both the same way round.
Thanks for the feedback. You mean like this?
Hmm.. I think I'll go for unsupported rails on the Y axis to keep the gantry weight down, plus build simplicity.
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