Question
We have been using standard 3/4" MDF for bleeder board material for nesting parts. Can we do better?
Forum Responses
(CNC Forum)
From contributor K:
I am using 3 mm MDF for spoilboard all the time. It's very easy to handle by one person (for cleaning), has more consistent thickness, and is cheaper. It can't be used to fix down the edges of an unwilling sheet of (birch) ply with a screw, though, but that's the only disadvantage I can think of.
I usually put layers of tape around the sides - first two layers are white tape, other two layers are the clear wrapping tape we use to wrap our furniture. This works good, but takes awhile to do, and can get quite messy.
I'm not too familiar with edgebanding. I'm in the sofa/furniture industry. How does edgebanding work?
Edgebanding is the finish applied to the edge of material to accomplish a finished look. Take a look at your furniture. It more thank likely has edgebanding glued to the side edge. Please let me know if you have success with the LDF.
Two things would happen. When we were machining MDF doors (using a big vacuum pump, 40 hp), the bleeder board would actually begin compressing under the vacuum load. This caused inconsistent tool blends, but the main problem with the LDF we termed "kerf leakage".
Vacuum pumps produce about 28-29 inches of mercury for vacuum depending on the pump technology. Your parts are being held only with the differential. For instance, if your bleeder board shows a reading of 8 inches of mercury, and the max vacuum is 28, 20 inches of mercury is available to hold parts. You can see how LDF looked promising since an LDF bleeder board with nothing on top would show 4 inches of mercury.
The problem occurs when you start machining parts and the router bit kerfs begin to let vacuum leak. You can imagine what happens when you are running a sheet of drawer parts. Even a giant vacuum pump could not overcome the problem.
The magic solution is as follows. We would first take a standard piece of MDF, place it on the machine and flycut approximately 1/16" off of one side. It seems that when the product is pressed, the outer surfaces compress. This flycut operation removes that layer. Then flip the sheet over and flycut the other side while watching the vacuum gauge. When the reading reaches 10-12 inches of mercury, stop. The bleeder board is ready for use. 10-12 inches of mercury seems to be the optimum reading. Enough differential to hold the parts but enough resistance to overcome the kerf leakage issue.
Finally, a big table requires a big pump! Don't even consider anything less than 40hp for a 5 x 10 table (or larger) if you expect to cut sheets of drawer parts.
IWF 2004 is next week and if you are planning on purchasing a machine at or around the show, and plan on using a bleeder board, make sure you can cut a sheet of drawer parts. That will bypass the salesman's usual response that: "15HP is plenty. See, those cabinet end panels did not move." And by the way, the "return onion skin" method of toolpathing will not make up for a small pump.
So you're saying that if we fly cut both sides once, it's better? What can we do about the LDF getting warped? It gets warped and bends after about the 3rd day, depending on how much you use it. I just tape the ends down to the table.
I guess I've got to start edgebanding the ends instead of taping them. I'll talk to the guys across the street and see if I can come down and try it out with a few sheets.
We changed to using a 1/4" down sheer bit. We love it. There's less bleed-through after cutting parts. We cut at 600ipm through 18mm Baltic ply using only 10HP vacuum, and yes, we need more vacuum, but we do okay by thinking out our tool paths and tabs. Small parts first, and cut tabs off as you go while cutting the next part.
To the original questioner: the 1/4" downshear does reduce the kerf, therefore vacuum loss, however, 600 ipm is very slow by today's standards. Also, the downshear bit should tend to avoid "lifting" the parts, however it will chip more on the bottom side of the panel. A better bit choice would be a 3/8", 3 flute, compression spiral "mortising" bit. The upshear part of the "mortising" designation is only about 1/8". The remaining cutting edges are downshear to help hold the part to the table. Also, the third flute allows you to cut faster because the chip thickness is reduced. You can run out of spindle horsepower in the spindle if you feed too fast.
"High pressure" means the vacuum that is produced by the pump will support a column of mercury 29.89 inches tall. That describes the amount of hold the pump can produce in a sealed environment. There are other vacuum systems that are based on blower technology that produce huge volumes of vacuum at about 15 inches of mercury.
7.5 KW is about 10HP.
The pump's CFM describes the volume of vacuum it can produce. With flow-through, this volume is required to make up for the vacuum that is lost through leakage, kerf, etc. Before flow-through was developed, small vacuum pumps were very common. A 40 HP does not hold any better in a sealed (gasketed system) application than a 3 HP. 29.89 is 29.89.
To the original questioner: you made a very good point when you referred to the bottleneck. If I have a machine that will cut 100 sheets per day (roughly 6 average kitchens), then I must be able to sell 6 kitchens per day, assemble 6 kitchens per day, finish 6 kitchens per day, and install 6 kitchens per day! Most smaller shops are not able to do the above.
I would recommend that you try the 3/8" three-fluted compression mortising bits, however.
I am certainly not trying to contradict anybody, just learn. It may well be that the demos I saw were not typical of the kind of conditions I will see with my future machine in use.
Using the 1/4" bit sounds like a good precaution. Will that bit work in veneer core plywood?
Comment from contributor N:
A point of clarification on how vacuum technology works relative to your CNC router table... Contrary to popular belief, router table vacuum technology does not hold components to the table by pulling or sucking. While it is true the vacuum pump is attempting to achieve a pure vacuum by sucking air away from an area, in actuality, the pump is merely removing atmosphere from below the work material. Parts are not held in place by vacuum, but are pressed onto the table by the weight of the atmosphere above. Your vacuum gauges express the amount of vacuum in inches or millimeters of mercury (29.92 or 760 max respectively), and some show it in kilopascals (101.325 kPA). Due to loss and the nature of most vacuum pumps used in the woodworking industry, you will never see these numbers represented. These are representations of one atmosphere pure vacuum, or the maximum measurable if one pure atmosphere of vacuum could be achieved. A one inch square column of air, the height of Earth's atmosphere, weighs approximately 14.7 lbs. This is, in fact, what holds your parts to the table. A table with a working surface area of 10' X 5' has 7,200 square inches of surface area. Atmospheric pressure in an area that large is approximately 10,584 lbs! If you think about it, though, you would never experience this, due to internal pump loss, cut path loss, loss through uncovered areas of the bleeder board face and lateral bleed through the sides of the bleeder board (which incidentally are the areas of greatest loss in bleeder board as they are the path of least resistance).
