Mark Harris wrote:Check the extrusion, even precision aluminium extrusion isn't even remotely close to precision.
For linear cross section dimensions use ±.008″ per inch of the dimension
For wall thickness use ±10% of the specified dimension
For twist use ½ degree per foot of length
For straightness use .0125″ per foot of length
Angularity will vary with wall thickness and size of the surface but are generally between ±1 degree and ±2 degrees
I think anyone will agree that is pretty awful.. mounting the gantry on a good steel linear guide rail will tend to pull extrusion straight, and give you very precise motion
Yep, those are awful figures for a precision machine. But I think these specs describe the worst case scenario - and that is fair as an engineer should probably always expect the worst
In reality, the precision of the extrusions will vary depending on the manufacturer. I got my maker slides from a shop in Germany and I am pretty pleased with the tolerances. I had no problem with wobbling or unsmooth movement whatsoever. Until now, the accuracy of my LitePlacer has been good enough
Without a doubt, precision-made commercial linear rails from steel are much better in terms of tolerances - but they come for a steep price.
Btw, have you seen Anthony's pick and place for OpenPnP which he is constructing around linear rails?
Check it out here:
https://hackaday.io/project/9319-diy-pick-and-place
However, I have the feeling (without being able to prove it by measurements yet) that the slack of the belts on my LitePlacer is worse than the tolerance of my linear guides (makerslide + V-wheels + excentric nuts to adjust). So if I were to upgrade to linear rails, I would also need to change the drive system and that makes things much more complex and expensive...
So for now, I will stick with LitePlacer's approach of MakerSlides and V-wheels.
kvide wrote:So I measured the diagonal of the pnp frame and was able to get it marked down to a 3 1/4 digit precision. Tightening the frame into place helped but there was still some wobbling. By skewing the diagonal of the frame off about 0,5 mm of measured points I can get the wobbling to almost disappear, but there is still some wobbling.
Christian, you should try and get the machine as square as you possibly can. I had no way to measure the diagonals with good enough precision - so I did the following:
- To make things simpler, I added precision cut (Misumi) 2020 profiles to the left and right side of the machine - basically I built a rectangular base frame first.
- I constructed right angles on the table top using pencil and an improvised dividers (Zirkel in German) to align that base frame in right angles. I then measured and adjusted the distance of all extrusions to get the thing exactly square and the extrusions parallel.
- All the right angles and other connections will have a bit of play. I found that - after perfectly levelling the table surface - a water level gives best precision to make the vertical extrusions orthogonal and all horizontal parallel to table surface.
- Once the base and Y-extrusions are nice and square in all dimensions, it still took some time to adjust the gantry.
- For the gantry, I suggest you interconnect the two maker slides by 3D-printed pieces or screws + T-slot nuts to make the assembly much stiffer. This thing works nicely, if you have a 3D printer:
http://www.thingiverse.com/thing:74301
- Also, the left + right wheel plate (Y axis) are a great source of error. I loosened the screws and moved the gantry along the Y-axis (before belting) to make sure it runs smoothly along the whole range of movement. I then fixed the screws one by one making sure both the X-carriage and ganrty ran smoothly along X- and Y-axis.
- I disliked the the self-tapping screws for fixing the gantry but rather cut a proper M5 thread into the profiles and used normal cylindrical screws; which gives better control (less torque needed) when fixing them.
- I believe it is key to get the mechanics of X and Y straight, Z-axis is less critical then.
Good luck!
