The results? It worked! I think I will keep this vacuum press.
I did the following using new shop-resawn veneers; previously resawn panels will be used for the doors (i.e, the “wing”, and “Don Quixote”). Before doing any serious work with the vacuum press I wanted to assure myself it worked properly, and that I was indeed familiar with the process. The system did the job well. But not all was perfect, as you will learn toward the end.
Now, some details of the first veneer panel done with the vacuum press:
As I did the quality control of this first veneered panel I noticed a slight bow as I sighted down the top and bottom edges. To measure the amount of the bow, I placed the panel flat, with the concave spalted maple side up. With a straight edge placed across the surface I measured 0.030″ concavity (this is 30 mils, or 30 thousands of an inch).
I was puzzled about this amount of distortion, as I expected the veneered panel to be perfectly flat. One obvious area to look was the platen I fabricated, and showed earlier:
In retrospect, I now feel that the platen should not extend too far beyond the edges of the package being pressed. This is especially true if the “sandwich” is thick, as this one was (about 4 inches). I now believe what happened was that the outer edges of the platen lifted slightly during the time that vacuum was on, lifting them and causing the slight deflection I measured.
I must run one additional test, to determine if my thinking is correct. That is, unless anyone reading this can confirm my theory in the next 16 hours or so. I would appreciate your feedback, especially if it will save me doing another test glue-up.
The bottom line of this test: I achieved absolutely perfect adhesion, with no gaps anywhere along the visible glue line between the veneer panels and the substrate. In addition, the entire surface of the veneers is perfectly flat, totally free of bubbles or areas of poor adhesion.
I was off in my rough estimate of the total height, Neil. The image below illustrates the total stack height in the bag:
The total height is more like 2.8 to 2.9 inches. In detail: The platen, and the bottom and top cauls, are all 3/14-inch; the veneered panel is 1/2-inch, plus 0.120″ for both veneers.
Why 3/4-inch for the cauls? I had plenty of the 3/4-inch MDF to use as caul material, but only one sheet of the 1/2-inch. I ordered some more of the 1/2-inch yesterday.
Additional troubleshooting on Friday showed me that there is also a slight bow on the workbench top, lowest on both ends and highest in the center. I don’t have a long enough straight edge to measure this, but a perfectly flat piece of 3/4-inch, 4-foot ply shows the left and right edges above the bench top by about 1/16-inch.
So, not only is the bench top crowned front-to-back, but also along its length.
I called Sjobergs in Sweden too late – Friday afternoon all but one person were gone. They will call back on Monday.
Al:
“I now believe what happened was that the outer edges of the platen lifted slightly during the time that vacuum was on, lifting them and causing the slight deflection I measured.”
Not sure where you are but the 4″ thick sandwich has been bothering me for your reason above. What makes up the 4″‘s?? Also any particular reason why you went to 3/4″ panels for top/bottom platens in the sandwich?
Thanks, John. I will let you know how the next pressing turns out, using the same platen.
In the meantime, I have sent the manufacturer a note, requesting their advice.
It sounds like you did everything properly, Al.
Let us know if the bench is the culprit!
John
John,
1. The panel substrate was 1/2-inch thick
2. The veneers were the same thickness
3. Although I never thought much about it, within 5-10 minutes of pulling the panel out of the bag I went back and checked it for flatness, as part of the quality check I did; this was when I first noticed the bow, by sighting down the top and bottom edges.
4. I have the panel stored on the bottom edge. The amount of bow has remained the same.
An update:
This morning I started looking for possible sources of the concavity of the panel. This is what I found:
1. Out of the bag, the platen was quite flat – I used a 30-inch precision straight edge; but I did notice that the front and back corners were not flat on the workbench.
2. I started to turn the platen clockwise on the workbench, to see if I observed the same thing when turned 90°.
3. The platen spun easily, with much less friction than I expected out of a 4′ X 2′ piece of 3/4-inch MDF.
4. Since the platen *was* quite flat,the workbench was now suspect.
5. The workbench has a crown right down the middle, running just about the full length of the bench. It is 93 inches long; even the breadboards have a slight crown.
For now I will use the little 42″ X 42″ table I use in the finishing room. This morning I started to prepare some new veneers, to start another test press run tonight.
My thoughts: This winter has been the wettest in over 40 years in NW Missouri. I suspect the moisture level has remained high, sufficiently so to change the European beech cross-grain. The bench top is 3-1/4 inches thick, with a 4-inch apron all around. I don’t want to plane it level yet, as I don’t want to end with a trough when the humidity decreases.
I will contact Sjobergs in Sweden tomorrow; but I will send them an e-mail tonight, to alert them. NO contact for them in the U.S., according to Woodcraft Tech Support.
What do you think? I am interested in your suggestions, especially if there IS something else I should try. Tech Support suggested sealing the bottom surfaces of the bench top.
Al,
I have a few questions for you. How thick was this panel’s substrate? Were the two veneers the same thickness? Did you notice the “slight bow” immediately as you removed it from the press, or did you notice it later? If later, how was the panel stored from the time it was removed until you saw the bow?
John
Thanks, Neil! I had to jump right in, as the cabinet awaits.
The top and bottom cauls are 3/4-inch, and only 1/8-inch larger than the panel being pressed.
I now wonder if the platen itself might be a little concave, as it was a piece of 3/4-inch MDF I had in the shop for some time.
Oh – you were not late, Neil – the only time constraint was related to saving me from getting another test pressing ready today. But first I must define the cause of the slight concavity.
Thanks for helping me sort things out.
Hi Al………pretty cool. You didn’t waste any time…right into that press.
Correct me if I’m wrong….you think that your top and bottom platen extended beyond your layed-up panel too much. It’s hard to see the detail in the picture but, the top and bottom platen only needs to overhang around the perimeter of the panel 3/32…+/- 1/32. So many approaches to doing this. What’s in the sandwich that equals 4″‘s???? Are you using 3/4″ panels as your top/bottom platen of the sandwich????
I see the time constraint and I’m probably late, I know you’ll get correctly, let us know where you are.
Neil