Nesting Bleeder Board Material

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CNC users dissect the options. January 24, 2005

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.

From contributor A:
You can use a LDF. Try edgebanding the four sides to keep from losing vacuum.

From the original questioner:
Does using LDF make much of a difference, and how?

From contributor J:
I'm using LDF, and it works good for me.

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?

From contributor T:
LDF should work considerably better than MDF - the primary advantage being that it has a much lower density (this means that all else being equal, more vacuum will "pull" through LDF than MDF). Restraining the side vacuum loss with edgebanding, tape, etc. is a must.

From contributor A:
One more quick note. LDF is especially valuable when machining small parts.

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.

From the original questioner:
We do not have an edgebander in our operation. What if we brought the entire unit of LDF into the paint department and sprayed the edges with primer? Or could we quickly cover the edge another way? Tape seems like it would rip off when you surfaced the board.

From contributor S:
I start with 5/8 LDF, but it gets better as it gets thinner. When I place my sheet and the corners aren't pulling up, I apply masking tape where the spoilboard and sheet meet. It sucks right down and doesn't wrap or pull up when I cut it. This also works well when you're just machining a small blank. I used to edgeband, but it gets cumbersome.

From contributor B:
We usually take 6 or 8 sheets of MDF over to our paint room and while they are spraying the cabinets, they spray about 3 good coats of finish on the edges. Works well for us.

From contributor O:
There is no great secret about edgebanding. Before those big fancy machines came along, it was applied with a simple iron. You buy it preglued, turn the iron to high heat, and away you go. If it takes 5 to 10 minutes to apply, I would be surprised. They even make little hand held trimmers to remove the excess. That way, you are not lugging a sheet of material around. Make sure you get the melamine tape and remove all the glue from the face of the iron before giving it back to your wife.

From contributor Z:
Let me throw my two cents worth into the bleeder board discussion. When LDF first became available, I thought it would be the ultimate bleeder board material for flow-through fixturing. We immediately started using it and even had to lower the limit on which the machine control would issue a "no vacuum" alarm.

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.

From contributor J:

I was having the same problem. I'm using 18mm LDF, and it loses a lot of air every time it makes a cut. It's not like I'm going through… you just have to have your eye on the pressure gauge.

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.

From the original questioner:

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.

From contributor Z:
Contributor J, flycutting both sides of MDF removes the compacted layers on both sides and allows for the vacuum to flow through more easily. This is not necessary for LDF, but you will never be able to achieve the optimum 10-12 inches of mercury that will enable you to cut a sheet of smaller parts if you stay with LDF, unless it is thicker.

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.

From the original questioner:
Thanks for the input, and yes, I agree - 600ipm is slow by today's standards. But I get paid when I ship product, not cut parts on the CNC. At 600ipm we cut parts faster than we can process them, so our bottleneck is not the CNC. We have also found that if we keep our bleeder board clean, we don't have problems with chipout.

From contributor S:
Flycut and flip every time to avoid warp.

From contributor W:
I get pretty confused about results I have seen. I respect what contributor Z says, and a 40 hp pump would be awesome. I can't remotely afford it. I have seen 5 x 8 Flexicam cut a small piece out of a half sheet laying on a piece of MDF. Half of the table was open (2.5 x 8 was uncovered) and the part was held just fine. Surely that large open area must be greater than kerf openings in a nested panel? That was with a 7.5 kW Becker pump (high pressure).

From contributor Z:
Contributor W, did the Flexicam router have the vacuum turned off for the area of the table that was not covered? What you described does not seem remotely possible.

"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.

From the original questioner:

Contributor Z, we used a 3/8" three fluted compression mortising bit from C.R. Onsrud when we first started nesting for about 6-8 months. Great bit, but we were never able to achieve high speeds due to not having a large vacuum pump. We then tried a 1/4" down spiral from Bamtool and never looked back. More parts per nested sheet, less vacuum loss through the kerf, little to no chip out if we keep our bleeder board clean, greater feed rates (probably due to our smaller pump), which increased our tool life four times, and quieter cutting. Best of all, the bit cost $12.00, so instead of sharpening and worrying about employees setting the tool comp wrong, we throw it away. Is there something I'm missing here?

From contributor Z:
Sounds good to me!

From contributor W:
Contributor Z, I can't explain it - it's just what I saw. The Flexicam did not have any zones flipped off. The spoil board was flycut pretty low, 3/8 or so. I saw a demo with an Omnitech where the part, a 12" circle, was held pretty firmly with a good deal of the table open (maybe 25%). Both of those machines had 7.5kW Becker pumps. The part could be slid if you used both hands and tugged on it with all your weight (in my case, a portly 230 pounds).

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?

From contributor M:
To the original questioner: can you elaborate a little more on the 1/4 downshear? Are you just cutting Baltic birch or are you doing melamine, also? Are you cutting multiple passes and also tabbing, or just using tabs? Are you having any problems with the tool packing chips using the down shear in that tight of a kerf? Interesting solution… look forward to more info.

From the original questioner:
No, we have not cut melamine, however Bamtool also has an up/down 1/4" bit you might try. We are not making multiple passes, but we are tabbing. We make our tabs so that they cut off with the next part cutout if possible. As far as packing chips, yes, we do, but it works well for us. The chips are a nice size and warm to the touch and I believe the packing only helps keep the vacuum pressure up. Try it.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

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).