# Seeking Best Practices for Long-Term Z-Axis Stability and Plenum Flatness **URL:** \ **Category:** General Chat\ **Created:** [November 3, 2024, 10:07am UTC](https://forum.avidcnc.com/t/seeking-best-practices-for-long-term-z-axis-stability-and-plenum-flatness/3887 "2024-11-03T10:07:20Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![nmathon](https://yyz1.discourse-cdn.com/flex027/user_avatar/forum.avidcnc.com/nmathon/32/2372_2.png) [@nmathon](https://forum.avidcnc.com/u/nmathon)\ **Post date:** [November 3, 2024, 10:07am UTC](https://forum.avidcnc.com/t/seeking-best-practices-for-long-term-z-axis-stability-and-plenum-flatness/3887/1 "2024-11-03T10:07:20Z")
Hi, I am trying to find the best way to maintain stability in the plenum over time. I need to machine a plastic board using the onion-skin technique, with the onion skin layer no thicker than 0.1mm (anything more is too difficult to cut with a utility knife). The issue I’m facing is that the Z-axis consistency deteriorates over time. After surfacing my spoilboard, the Z-axis stays consistent for a few days (sometimes just hours), but then differences of at least 0.3mm start appearing between the highest and lowest points. I initially used an MDF spoilboard, then switched to HDF, but the problem persisted. For the last six months, I’ve been using a PVC vacuum table, which I surfaced. It was slightly better initially, but I had to resurface it multiple times (which is challenging with a vacuum table, as it requires remachining all the slots). Now, once again, I’m seeing inconsistencies, with some areas 0.4mm higher than others. After some research, I decided to try an HPL board. I am upgrading to the EX controller with servos and will use this opportunity to create a new vacuum table with the HPL board. Given the cost of these boards, I want to ensure I’m following best practices for plenum stability over time. Sometimes I need to hammer materials on the table, so I’m concerned this might cause some movement in the aluminum table that affects flatness. I’m considering adding a 1mm thick metal sheet on top of the aluminum table, beneath the HDF layer, to increase rigidity by securing all the aluminum extrusion profiles together. However, I haven’t seen anyone else do this—would this be a good idea? What other solutions could I try? Is it unreasonable to expect 0.1mm accuracy on the Z-axis over time with an Avid CNC? It doesn’t seem too ambitious to me. ---
**Author:** ![JeffsDesign](https://yyz1.discourse-cdn.com/flex027/user_avatar/forum.avidcnc.com/jeffsdesign/32/5574_2.png) [@JeffsDesign](https://forum.avidcnc.com/u/JeffsDesign)\ **Post date:** [November 3, 2024, 3:34pm UTC](https://forum.avidcnc.com/t/seeking-best-practices-for-long-term-z-axis-stability-and-plenum-flatness/3887/2 "2024-11-03T15:34:10Z")
I think you getting the best results in our environment an thermal expansion and contraction ---
**Author:** ![nmathon](https://yyz1.discourse-cdn.com/flex027/user_avatar/forum.avidcnc.com/nmathon/32/2372_2.png) [@nmathon](https://forum.avidcnc.com/u/nmathon)\ **Post date:** [November 3, 2024, 5:17pm UTC](https://forum.avidcnc.com/t/seeking-best-practices-for-long-term-z-axis-stability-and-plenum-flatness/3887/3 "2024-11-03T17:17:54Z")
Do you mean the thermal expansion of the plenum? Because I don’t think the thermal expansion of aluminum is that high. It’s really annoying to have changes of 0.5mm when using a vacuum table. It means you can easily go through the material and end up with air leakage. From what I found online, with my 25mm thick PVC plenum, with variations of 30°C, I’m supposed to have a contraction/expansion of 0.0825mm. Which is good for me. Not 0.5mm. HPL should be 0.06mm, so I hope I’ll get better results but it’s not a so big difference. I would be happy with variations under 0.2mm.