Now that we have a baseline on the vacuum table performance, time to start performing modifications to see what happens.
The easiest thing to reduce air resistance is to remove layers – we don’t strictly need the spoilboard in this setup, so it is removed. We then added some rubber gaskets to improve the seal between the plenum and the fixture, which should reduce air leaking past that particular junction.
These modifications did not drop the vacuum of an empty fixture – in fact vacuum was boosted by 2 inches to 25. This implied having the spoilboard in the setup was letting a lot of air slip around the fixture. Removing it was a good call.
When the work pieces are in place, the vacuum went up another inch to 26 inches. Less air is leaking past the work pieces, and they are now held by about one inch of mercury (roughly half a pound per square inch.)
We haven’t put any effort into improving the sealing between the work pieces and the fixture. How much gain can we realistically expect from the effort? In order to get a rough estimate of how much more we might gain, we draped a plastic sheet over the fixture.
Looks like we have about 1.5 inches of mercury we can gain from better workpiece-to-fixture sealing.
This is a promising start, as this tells us we’re in reach of a decently high level of vacuum for work-holding. We now need to put some effort into the other side – improve the path for the vacuum to reach the work pieces and hold them in place.
The CNC router at Tux-Lab has been under-utilized partly due to its under-performing vacuum table. It has a poor track record on an existing project, and we want to understand why (and hopefully fix the problem) before doing more projects on the CNC router.
To narrow down the cause, we will record the pump’s vacuum gauge reading at various configurations. We use a phone to take a picture identifying the vacuum configuration. We then hold that picture up next to the gauge and take a picture of the phone and the gauge together.
Establish the bounds
First, we get the upper bound: once the pump is up and running, close all the zone valves. The reading – nearly 30 inches of mercury – confirms the pump itself and the majority of the vacuum piping is in good working order.
The lower bound is obtained by opening all zone valves and place nothing on the spoilboard. When in working configuration, the vacuum will never be weaker than this 7.5 inch reading.
Most of the tests confirmed that the vacuum setup itself appears to be in good working order. We only started seeing problematic numbers once we started involving the spoilboard and the project fixture. Good news since these are the easiest pieces to fix.
Past runs of the existing project has been done with the fixture mounted on the spoilboard. The vacuum reading of this configuration is surprisingly high at a hair under 23 inches. Indicating a lot of air resistance despite being carved from low density fiberboard.
We then added the work piece blanks on top of the fixture and measured again.
The vacuum barely changed, to just a hair over 23 inches. This is a problem: it tells us the air can easily find a path around the work pieces so very little of atmospheric pressure is applied to hold pieces in place.
Now the objective is to modify the setup to (1) reduce the vacuum reading of an empty fixture and also (2) increase the vacuum reading of the populated fixture.
Increasing difference between these two readings should increase the holding power.