A vacuum platen is a great accessory for your CNC router. The platen serves two important purposes. It provides a near perfect flat surface that is true to the X/Y plane of your router. The vacuum pulls the work material tight against the reference surface of the platen. This provides excellent control of the Z axis.
This platen was built from scrap material found in the shop. It is a small platen, only 6" x 6", but it can be built top any size needed. This particular platen was built for milling circuits boards and small name tags. Milling circuit boards require Z axis positioning with a tolerance of +/- 0.001". The platen makes this possible.
An interesting feature of this design is that it exploits the fact that MDF is very porous through it's edges. Rather than using hundreds of drilled holes in the top. this platen uses the thousands of pores in the MDF to bring the vacuum to the workpiece. No holes required.
Step 1: Materials Needed
The platen is made in three layers, the Base, the Mid-section, and the Top. I made the Base using 1/4" pre-finished birch plywood. This is commonly called drawer bottom plywood. It has a durable epoxy finish on one side. The amount of wood required for the project will be determined by the size of the platen built. The Mid and Top sections were made from 3/4" MDF. Also needed is a 3/8 tubing to 1/4 NPT adapter and some 3/8 ID vinyl tubing. Both of these item can be found at Home Depot. Also some slow curing epoxy and some wood glue will be needed.
Step 2: The Vacuum Pump
You will need a vacuum pump. The pump I used was from an old medical suction machine (I really don't know the official name). It's an oil-less diaphragm type pump that rated for about 4 amps at 120VAC. The oil-less diaphragm type pumps are the best choice for this sort of work. They can be found on the web at eBay and simialar sites. You should be able to find one for $40 or less. BTW, these pumps are a good thing to have around the shop. They can be used for vacuum veneering, de-gassing casting resin, and other uses. You may want to use a small automotive fuel filter as a filter on the input of the pump. This inexpensive filter keeps sawdust and other dust out of the pump. You can find them at auto supply houses such as Auto Zone.
The high vacuum rotary vane oil type pumps, used for air conditioning service, should not be used. They are designed to pull a very high vacuum and when moving high volumes of air they tend to produce a lot of oil fog on the output which can be messy.
Step 3: Cut the Wood
Unlike plywood, MDF comes in exact size, that is, 3/4" MDF is really 3/4". For my 6x6" platen I cut the following pieces:
1/4" Plywood Base - 1 piece 6"x8"
3/4" MDF Mid-section - 1 piece 6" x 6"
3/4" MDF Top Section - 8 pieces 6" x 1/2" x 3/4", These were cut from 3/4" material.
Step 4: Drill and Tap the Vacuum Connection (port)
Drill through the edge of the center section with a 1/2" drill. This should be done in a drill press to insure the hole is straight. I had a 1/2 NPT tap handy, so I tapped the hole to accept the brass tubing adapter. If you don't have a tap, just enlarge the hole using a reamer, or file the thread off the brass adapter. The adapter will be epoxied into the midsection, so the fit can be a bit loose. Screwing the adapter into the mid-section without tapping of enlarging the hole may cause the MDF to split apart, so be careful.
Step 5: Glue the Mid-section to the Base
I used epoxy to glue the Mid-section to the Base. This was mainly because I use pre-finished plywood for the Base and I was concerned that wood glue would not stick well to the smooth finished surface. If you use normal plywood, wood glue should work fine. Note that the edge of the Mid-section at the vacuum port is flush with the Base. This makes it much easier to connect/disconnect the tubing when in use. A large sucker-rod connector works well for pressing the parts together while the glue sets. If you don't live in an oil-field, you might need to use a brick or large rock.
Step 6: Assemble the Top-section
The secret behind this platen is that it takes advantage of the porous nature of the edge of MDF. MDF is made from layers of fibers pressed together. The space between these fibers act like an open cell sponge. Cutting the MDF into narrow pieces, rotating them 90 degrees, and gluing them into a panel, produces a surface with thousands of tiny pores.
Put a small amount of wood glue on each piece and assemble on a flat surface covered with wax paper. Lightly clamp and allow the glue to dry.
Step 7: Machine the Vacuum Channels in the Mid-section
When the glue holding the Base and Mid-section has cured, it's time to cut the vacuum channels. The design of the channels is up to you. In my small platen I used a 1/4" end mill and cut channels 0.3" deep and 0.7" center to center. The channel going up the center connects the vacuum port to all channels. Leave enough wood to make sure the top is well supported. If the top does not have enough support, it may sag when vacuum is applied.
Step 8: Attach the Top to the Mid-section
Using a small amount of wood glue, glue the Top to the Mid-section. The granite block worked well to press the parts together. A block of wood and some weights would work as well.
Step 9: Coat the Edges With Epoxy
Apply a liberal amount of epoxy to the tubing adapter and screw or press it into the vacuum port. Then coat all edges of the assembly with epoxy to seal the vacuum. Lacquer or shellac could be used, but I found epoxy works best.
Step 10: Machine the Top Surface Flat
Mount the platen on your CNC router. Choose a position where you would normally want to place it. It's a good idea to mark the bed, so you can always mount it in the same place.
Machine about 0.020" from the top, more if you need to. After cutting with the router, lightly sand the surface with 220 grit sandpaoer. This helps remove the fuzzies left from the machining operation. At this point you have a reference surface that is perfectly aligned with the X/Y plane of you CNC router.
Step 11: Using the Platen
Connect the vacuum pump and turn it on. Place the material for machining on the platen. You should feel it pull down a bit. Cover the area around the material (work piece) with blue tape. This concentrates the suction on the work piece. Lastly, cover the area around the workpiece, overlapping the workpiece slightly (about 1/8").
Machine the piece. In this case I was V-Carving some two-layer engraving plastic called Laserables. This material has a thin (about 0.003") top layer with a bottom thick layer of a different color. Another common supplier for this material is Rowmark.
Maintaining the platen:
The platen is used as a spoil-board. Eventually it will require another top-leveling machining operation. When the top become too thin, just machine it down to the original Mid-section and apply a new top. Make sure to seal the edges of the new top. The platen should be usable for many years.