Help please. Dip switches and mach 3 settings

[Radu_Andrei]

Self correction, updated for today. Although the rails seemed a while back paralel enough, they are not. I have a 1-1.5 mm error on a 1.5 meters run. Anyway, I.ll take the y axis apart in the next days, and assuming you are in my shoes, and you plan to allign the y axis from scratch, in what manner would you do it? Where to start first? Paralel, perpendicular, coplanar? In what order? And how would you do it? The method, please. :)

[Neale]

The requirement is for the two rails to be absolutely parallel, both horizontally and vertically. To deal with the second of these first - I ensured that the two rails both sat on "perfectly" level surfaces. This meant that I could adjust each side separately because "level" is a measurable quantity. I actually used a precision level bought from eBay for the purpose. Probably a bit more sensitive (and therefore more difficult to use) than necessary, but it needs to be better than you can achieve with a builder's level. I had used epoxy on my rails, but did not achieve a perfectly level surface all the way along so i ended up using shims to compensate. With the aid of the precision level, this was not too difficult. Note that I have talked about leveling each side individually. I deliberately built in the capability of dealing with two rail surfaces that were not coplanar. Other people talk about using a bridge between rails to achieve coplanar surfaces which is a good idea if you can do it; I preferred to allow for a small error in height (about 1mm, measured with a builder's level).

The other requirement is that the rails are parallel in the horizontal plane. Actually, getting them parallel is the easy part. The difficult part is bolting down the first rail absolutely straight. I used a straightedge as best I could, although it was not the full length of the rail. However, for a woodworking machine I decided that this was good enough. If I figure out how I can do better, I can always adjust it again. Once the first rail is in place, you can use the gantry to set the position of the second rail. Lightly bolt it in place so that it can move sideways, set up the gantry, and slide it along. This will bring the second rail into alignment and you can tighten the bolts as you go. This is actually one of the methods described in the Hiwin catalogue, if you cannot machine accurate reference edges into the machine structure. When you have finished, you should be able to move the gantry the whole length of the rails with very little effort. Note that the bearings actually move more smoothly if they have some load on them. These profile rails and bearings are amazing. When I was setting up my machine, at one point I had the bearings fitted to one end of the gantry while I was adjusting the second rail. I tried to support the gantry weight while the second rail was not in place but I realised that I had forgotten to do this. The end of the gantry (probably roughly 20kg?) sagged by no more than a couple of millimetres, while it was held at one end only by two Hiwin bearing carriages.

I also spent some time aligning the ballnuts and ballscrews. I started by ensuring that the ballscrews when held in the ballnut were parallel to the rails both vertically and horizontally. Gantry in the middle of its travel and ballscrew screwed into the ballnut so that it was more-or-less balanced, and no bearings fitted. I could then measure the level of the ballscrew (spirit level) and distance from the frame (as the rail was pretty much parallel to the side of the frame). I then converted these measurements into the size of shims I needed behind the ballnut mounting plate to bring the ballscrew into alignment. Once this was done I could fit and adjust the bearings at each end by turning the ballscrew (I actually used a cordless drill to wind it backwards and forwards) to bring the ballnut as close as possible to the bearing, then tightening the bearing mounting plate fixings with the bearing positioned by the ballscrew. Repeat at each end, then test. It took a while to achieve all this, but it did work.

[Radu_Andrei]

Hi Neale. Thank you for the comprehensive reply. It is so valuable, as I did think about this issue all day and could not come up with a method as yet. I was thinking of taking the time to build a large precise construction square, so I can figure out if the gantry is perpendicular with the y axis rails, but then, in order to get the most precise reading, I needed to have a tool that touches both x and y rails at the same time, and it is a quite a pain for me to build such a tool with enough accuracy, as there is a significant difference in height between this two sets of rails.

Regarding your post, ˇˇThe difficult part is bolting down the first rail absolutely straight.ˇˇ, before doing that, is it not necessary to set the y axis linear bearings/rails absolutely perpendicular to the x axis rails?
All holes for the bearings, rails, steel tubing, aluminium plates, are a bit larger than they were necesary, so sideways adjustement is not a problem, but I.m not sure if I should set the bearings in relation to the x axis first or the y axis rails and do the bearing afterwards.

A precision level cost here 100+ euro, and even if I.m tempted to discuss this option with Santa this year, I.m a bit concerned that, except for this job, I.ll never use it again. Thanks for the tip, though, I did not even know this tool exists until today.

[cropwell]

I don't know if this would work or even what level of accuracy you would get, but what about a water level ? You could make one out of clear plastic tubing https://en.wikipedia.org/wiki/Water_level_(device) it is easy enough to make one that will do 4 places, just by adding in T's and more tubing. That way you could put your X rails coplanar, for a start.

[Radu_Andrei]

Hi Rob. I had the same thought initially. A water level. Although old fashioned, for long distances, I do not know a better method today.
But still, the question remains, what do to first. Rails or bearings?

[cropwell]

Hi Rob. I had the same thought initially. A water level. Although old fashioned, for long distances, I do not know a better method today.
But still, the question remains, what do to first. Rails or bearings?

Work from bottom to top i.e Frame, X rails, Gantry, Y rails, Spindle, then you are not so likely to be re-iterative with your adjustments.

[Radu_Andrei]

Ok. Thank you.

[paulus.v]

An easy way to check the coplanarity of the two rails is by stretching two diagonal strings and check if they touch in the middle. With thin fishing line and eventually compensating the thickness of the line for the one above you could get very good accuracy. When you have the ends coplanar then you can straighten the rails with a straight reference (the hardest part).

For the perpendicularity of the gantry to the x axis, draw with the cnc the corners of a rectangle, and measure the diagonals which should be equal. The larger the rectangle, higher the measured accuracy.

[Radu_Andrei]

One more great ideea. Thank you paulus.v

[Neale]

Regarding your post, ˇˇThe difficult part is bolting down the first rail absolutely straight.ˇˇ, before doing that, is it not necessary to set the y axis linear bearings/rails absolutely perpendicular to the x axis rails?

Actually, getting the gantry square to the other rails is one of the easiest adjustments, and it was almost the last thing I did. I am assuming that you are able to adjust the gantry - in my case, I can loosen the bolts between the gantry and the plates which carry the bearings so that if I turn one ballscrew, I adjust the "squareness" of the gantry. So, get it as close to square as you can - I used a carpenter's square - and then cut a test piece. As someone has suggested, drill four holes on the corners of a square and measure the diagonals. I used the shanks of drills as pegs in a piece of MDF, having used the same size drill to make the holes. With a little bit of schoolboy trigonometry, you can then work out how much you need to turn just one of the ballscrews driving the gantry to bring it into square. Tighten bolts and check again. I wrote a short piece of gcode to drill the holes so that I could ensure that I always approached the holes from the same direction to remove backlash from the process.

My very final adjustment was to use the machine itself to skim the strips of wood I use as bed supports so the the bed was reasonably flat once I screwed down a sheet of plywood. Not good enough for precision metalwork, but this is a woodworking machine. I machine a smaller spoil board if I want more depth accuracy for a particular job.

Good luck!