Universal spoil boards
Advantages of this method are:
From contributor D:
I have a customer (casegoods manufacturer) that was running part-specific spoil boards, over 100 of them. I was able to work out pod beds for their Shodas that eliminated all but two part-specific spoil boards.
The pod bed consisted of a sheet of 3/4" C grade phenolic bolted to the steel table. The phenolic was fly cut and machined to accept 2" and 4" vacuum pods. We had the pods made at a machine shop from delrin tube and stock o-rings. The pattern that we machined in the bed had a very high pod density. We actually overlapped pod locations, allowing up to 6 different pod positions for every vacuum port. The high pod density and the two different sizes of pods gave us excellent flexibility and a very high air flow for when we needed to use a bleed board. Finally we installed precision bushings in the table and used slip fit phenolic pins as part stops. Since the machines had four main heads, each bed had stop locations corresponding to each head. This allowed 1-up, 2-up, and 4-up machining as required.
The bed could be used with pods or without pods. Without pods you still had the choice of conventional or bleed board fixturing.
Below is a photo of one corner of one of the beds.
To get back to your original question, you could machine standard pin locations (defined by control work offsets) in all of your beds to locate universal fixtures. You could also create vacuum dams to fit a standard fixture size or set of sizes. The end goal would be to eliminate as many fixtures as possible. If a fixture could not be eliminated, it could be made universal.
I have been using a very simple system for the last 9 years on a Heian double table machine. Each table is 63" x 63" and we use 3/4" thick MDF with 1/4" wide by a scant 1/4" deep grooves cut every 2-1/2" on both x and y axes to create a grid pattern in these universal spoil boards. We rout 1-3/4" diameter x 5/8" deep pockets into the center of these squares created in the grid pattern where appropriate (we put them every other one or 5" apart). We cut a 3/4" diameter hole through the bottom of the boards in each pocket to pull vacuum through and make plugs covered with closed cell 1/32" thick foam mtl on the bottom and place these in all the pockets except under the area we want to pull vacuum. We run 1/4" diameter rubber cord inside the part profile and we have all the vacuum we want. We run small parts down to 5" x 5". Anything smaller than that we leave skin on bottom of part and flush trim out off the mach or use a bleed board on top of this universal spoil board.
An important key is to plane the spoil board with a large bit (for z axis accuracy) and put a couple of coats of lacquer sealer on the boards before fastening to machine. We also drill for steel or aluminum pins in case you need to position parts against pins if not doing a surround cut.
We typically get +_ .003" tolerances on our z axis using this system. We run these boards for approximately 2 years before discarding after they have been planed off several times and also because of wear and tear. The durometer of the rubber is also somewhat critical - if I recall correctly it was around 38 duro - you may have to experiment and see what works best in your situation.
Contributor D, where did you find the Derlin tube? What kind of stock o-rings did you use? How did you determine the depth of the groove for the o-rings? Are the o-rings on the bottom as well as the top? Do they stand proud of the Derlin tube and accommodate compaction when placed under vacuum condition? You mention the use of a C grade phenolic for the spoil board. Why did you choose this? Where can it be found?
From contributor D:
The delrin and the o-rings were purchased by the machine shop that made the pods. The o-rings were buna-n material, size -326 for the 2-1/4" pod and -342 for the 4-1/4" pods. The groove for the o-ring is .195" deep, which lets the o-ring stand proud of the pod .015". O-rings are on both sides of the pod. The C grade phenolic sheet was purchased from Piedmont Plastics. I selected this material because of its durability and dimensional stability. Some of the machining was pretty intricate, and I did not want pieces breaking off (they did not).
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
Would you like to add information to this article?
Interested in writing or submitting an article?
Have a question about this article?
Have you reviewed the related Knowledge Base areas below?