Wednesday, 15 August 2018
This episode is inspired by Ted who was kind enough to share his problem and the solution.
I like a mystery so long as I find out the eventual solution. Probably why I spend most of my time listening to mystery Audiobooks. (no this is not an advertisement for audiobooks)
Teds issue is that despite using the Autozero script that I demonstrated in recient episodes he is having issues with the cutter cutting into the spoilboad even though it shouldn't. He explained how he is using the machine and everything was good so the issue had to be the machine but what?
When it comes to checking things I always check the easiest first.
1. Slop in the spindle / router bearings.
Don't take this for granted even if it is new. It takes a second to check and i have seen the issue on both old and new routers. I have seen people spend hours checking their machine trying to find lost steps when 1 second checking the routers bearings would have told the story.
2. Slop in the X Z carriage bearings.
This would equally apply to the Y carriages as well but that is even less likely and the test will highlight it as well. Again it is the work of a second to try and flex the Z axis and all will be revealed. You are not looking for flex in the X rail but abnormal movement in the axis. Some machines have more flex than others but you are looking for flex that doesn't belong there. In this case loose bearings. Vbearings such as I use are very susceptible to this as a small amount of wear on a V'ed rail will provide a lot of movement on the axis. Again it is is so quick and easy to check this why wouldn't you.
3. Unlevel Tabletop
All wood absorbs moisture and will swell when it does. MDF is a little worse than other woods but so long as it expands in a constant rate over the entire table you don't have a problem. It's when the MDF expands at different rates over the surface that the tabletop can become uneven. This usually takes many months but because the tabletop isn't constantly being scared and resurfaced the effect will accumulate over time. We all know what happens to MDF if you get it wet. It doesn't return to its original size when it drys out. Maybe it should be marketed as the grow your own tabletop. Just water occasionally and when it grows to the height you want just resurface. I of course jest!
The solution is to simple resurface to the lowest point on the table. It will flatten the table and your soilboard will remain the same thickness.
4. Lost steps
The curse of all home CNC enthusiasts everywhere. Fortunately it is easy to test for and well worth checking if all else fails.
5. Machine accuracy
This covers the touch off plate thickness and the Z axis calibration. If either of these is out the effect will be the same. Where possible use a dial indicator to calibrate the Z axis as it will give the most accuracy. The small movement of the Z axis will makes it very hard to calibrate unlike the X & Y.
Likewise the touch off plate thickness measurement needs to be correct. You would be surprised how a shallow cut in the tabletop looks like the grand canyon when the light gets on it just right.I can't help wonder how accurate the cheap calipers are but they always seem to be pretty good.
Well as you saw the solution was a combination of the touch off plate accuracy and Z axis calibration. These were always the most likely to be at fault but you must never take things for granted. Check everything but do it in a methodical manner or you will simply make things worse. If you find something wrong then fix it and retest. If you still have a problem continue searching but don't hop about like a flea in a fit. Remember that the aim of the exercise is to fix the problem you have, not create new and even more diabolical ones.
Change only one thing at a time.
Thanks to Ted for sharing his problem and more inportantly the solution.
Until next time