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  #1  
Old 09-02-2019, 06:45 PM
hefty hefty is offline
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Default Spoilboard surfacing mystery?

I noticed my onion skins were inconsistent so went hunting for the culprit and found my spoilboard had swollen around the perimeter. Laid a long straightedge across spoilboard and with a light, could see that it was high all around the perimeter. I had put multiple coats of white glue on the surface and edges but obviously it did not stop the humidity from seeping in.

I decided to surface the spoilboard in small increments so I could visualize how much swelling took place over the last couple of months. The pattern I see is confusing and I was thinking this was a good mystery. Just so you know, when I replaced the MDF about 9 months ago, I tightened down all the bolts holding the main board down to the frame. I up-sized them to 1/4- 20 and locktighted in place. Then the MDF was glued down 100% on the main board.

If you look at the pics, you see after the first surfacing pass, there is a valley on the right / middle that is still lower than the fresh surface. This was a 10 thousandths pass.
The Second pass removed another 5 thou. The third pass removed another 3 thou. And then I needed one more pass at 1 thou.
So.... 19 thousandths to flatten everything up!

I expected the perimeter to be high all around but the dip on one side has me baffled. Under the table, the main board is painted consistent all across.

Anyone seen this on your machine?
This time I'll be using shellac to seal the MDF.

Thanks, John
Attached Images
File Type: jpg First pass 10 thou.jpg (90.9 KB, 73 views)
File Type: jpg Second pass 15 thou total.jpg (83.8 KB, 63 views)
File Type: jpg Third pass 18 thou total.jpg (86.9 KB, 63 views)
File Type: jpg Under table.jpg (78.8 KB, 56 views)
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  #2  
Old 09-02-2019, 07:48 PM
Ger21 Ger21 is offline
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We sometimes see changes like that in a few days, as our humidity can change a lot between a few rainy days and then a few very low humidity days.
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  #3  
Old 09-02-2019, 09:26 PM
rcrawford rcrawford is offline
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That's very normal. That is why I surface my spoilboard before any job that needs exact thicknesses.
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  #4  
Old 09-03-2019, 07:42 AM
Doug S Doug S is offline
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Very normal and very frustrating.
The higher the humidity, the more you see this movement.

In central Texas, the humidity in the mornings will be brutal. I can surface first thing in the morning and have a perfect bed. As the vacuum pump pulls more moist air into the table, it will swell more. Then, as the humidity falls as the sun comes up, the table will dry and shrink and I will start leaving behind plys in my cut.
I think an aluminum or phenolic top would greatly help this, as right now, I have two MDF boards together-plenum and spoiler.
The support below the table is also in play here. See these pictures of my table and what it took to get it parallel with the gantry. These are .020 passes each. All the low spots are between the steel frame supports.
Attached Images
File Type: jpg IMG_4897.jpg (101.5 KB, 43 views)
File Type: jpg IMG_4898.jpg (99.4 KB, 45 views)
File Type: jpg IMG_4899.jpg (98.5 KB, 41 views)
File Type: jpg IMG_4900.jpg (97.3 KB, 40 views)
File Type: jpg IMG_4901.jpg (100.1 KB, 41 views)
File Type: jpg IMG_4902.jpg (98.8 KB, 39 views)
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  #5  
Old 09-03-2019, 07:50 AM
Ger21 Ger21 is offline
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That looks like the spoilboard is sagging, and not necessarily changing thickness. Or, a bit of both.
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Old 09-03-2019, 09:51 AM
Jim Becker Jim Becker is offline
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Moisture and gravity do not always cooperate in predicable ways. I think there are many of us who don't re-surface our spoil boards nearly as frequently as we should for any number of reasons including dreading having to replace it with a new one sooner than we'd prefer. I know I'm very guilty of that!
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  #7  
Old 09-03-2019, 10:04 AM
hefty hefty is offline
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Doug,
I don't feel so bad now seeing yours. 0.100" is crazy. I would have never imagined that much swelling could occur.

I have been tossing around the idea of replacing the machine board with jig plate (precision cast AL) and then replacing the MDF with phenolic (I don't use vacuum). Not only would this stop the shrink/ swell but should add some rigidity to the top (I still feel a resonance in the middle even with the bolt upgrade). So maybe this is a question for someone like Gary..... How thick of a jig plate would you use for the base plate? I'm not worried about cost; just would not want to go thicker than needed due to weight (and handling the plate).

Thanks!
John
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  #8  
Old 09-03-2019, 11:03 AM
Logan Y Logan Y is offline
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You will still need some type of sacrificial board for your top board though. You wouldn't want to do through profile cuts into phenolic unless you are really keen on replacing bits all the time.
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  #9  
Old 09-03-2019, 02:32 PM
Doug S Doug S is offline
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This sounds crazy but in this side view, you can see they MDF plenum raised up between the supports.

Even crazier, this second picture shows after a fresh surfacing then running about 5 sheets on a very wet morning.
You can see on the left side, the cutter barley scratches the surface, but 6 inches in, it is deeply gouging the spoiler. The is in the middle of one of the support sections and clearly the material is raising up to hit the cutter so.
Some days are a head scratcher.
Attached Images
File Type: jpg IMG_4876.jpg (88.2 KB, 31 views)
File Type: jpg spoil.jpg (97.1 KB, 31 views)
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Existing:-2014 Cobra 5x10 ATC w/ 10 hp spindle, Mozaik, 25 hp Becker High Vacuum Pump, 5 HP Oneida DC-Central Texas
New:-2014 Cobra 5x12 ATC w/ 10 hp spindle, pop up pins, phenolic top, lathe, vacuum relief valve, mister system, 25 HP Blower.
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  #10  
Old 09-03-2019, 03:35 PM
guitarwes guitarwes is offline
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Doug S- It looks like your plenum is not flat and looks to be rising up from the frame a bit. Could be my eyes playing tricks on me. Could this be causing the spoilboard to take the form of it making everything out of level? Also, do you seal the edges of your spoilboard with glue or paint?

For the OP- If I'm reading your depths correctly and you just had to surface your spoilboard down .019 to make it flat, count your blessings. if it was .19, that's a different story.
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