Chasing Accuracy – Part 2

Part one was running the Circle Diamond Square test, which indicated I had a problem. Part two is starting to solve some of the accuracy issues, and discovering some more.

I posted the results of the CDS test on the ShapeOko forum and another member suggested that the results indicated that my X and Y axis were not square.

I attempted to square them up by squaring the Y axis rails to the rear plate, and then making sure that the back edges of the X carriage plates were the same distance from the rear plate. Then I ran the CDS test again. Same results.

Today I investigated further.

The straightest edge that I have is an aluminum circular saw cutting guide. I’ve been meaning to pick up a real straightedge for some time, but have not done so yet. I did purchase a set of engineering squares the other day, so I used the blade on the 6″ square to check the straightness of the aluminum. It looked acceptable for the time being, so I used it.

I clamped the straight edge across the rails and used the 6″ square to square it up to the front rail. Then I discovered that a 1/4″ rod is a nice slip fit in the slots of the Makerslide, thus making it easy to mount my dial indicator!

Straight edge.
Straight edge.

Taversing the indicator along the straightedge showed that the axis were unsquare by about .020″ over 5.5″ That was not acceptable.

I ended up unbolting the X rails from the end plate and using a file to widen the bolt holes. Then I squared it up. Now it’s within .002″ over 5.5″ and I’ll take that.

The next issue to address is the Y travel. It was mentioned on the forums that the belts stretch under tension and that the .08″ pitch might open up a little bit. So I decided to test it out.

I have three 1-2-3 blocks and a gauge block set, so I made up a 10″ test length. Well, almost 10″.

First I measured all my blocks. The 2 block precision set measured 2.9995″ each, the cheaper block measured 2.9990″, the 3/4″ gauge block was .7490″ and the 1/4″ gauge block was .2490″. So the total length was really 9.9960″. But that’s close enough to do some figuring.

I clamped them all together with a square on the end for the indicator to touch off on and bolted them to the table.

Gauge blocks.
Gauge blocks.

I set the indicator to zero on the end, lifted up 1/2″ and sent it 10″ down the table. The indicator read .007″ short. I backed up 11″, dropped down 1/2″ and moved forward 1″ into the end. The indicator was off by .0015″. Backlash I suppose.

Repeating the test showed .007″ again. That number, plus the .004″ short that the gauge block stack is, told me that the X axis needed tweaking by .011″. Since I was moving 10″, that means I can take the steps/mm and multiply it by 1.0011 to get the new value. I set up, ran the test again and the indicator showed .004″ the other way. Success.

However, I really need to build a torsion box table. Just leaning on the table while the indicator was against the end of the 1-2-3 block would move it .002″.

I also don’t know if the X and Y axis rails are parallel to the surface of the table (probably not) nor whether the Z axis is perpendicular to the table in either direction. So I have some work to do still. But I’m getting closer now.

It’s close enough to use for the things I want to do with it at the moment, but I’ll still keep tweaking it.

Oh, and my Z axis motor is shot. When I mounted the coupling to it I had to shorten the shaft. I used a Dremel cut-off wheel and I believe that I overheated it and ruined the bearings. And maybe something else inside. The Z axis motor makes a terrible noise and gets very hot to the touch, even when it’s not used very much.

I ordered a new motor today.


Running total costs.

All Shapeoko CNC Mill Posts.

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