Due to time constraints, I have not had the resources to get a proper ShopBot Web Column article properly done...So until I do, here's a quick & dirty write up of what I call the 'Vacuum Film Technique'
I have been using this technique for the past few years to hold tightly nested, small and/or thin parts securely to the vacuum table without the need for tabs or other labor intensive band aids. I have used it to machine all types of plastics, non-ferrous metals (silver nickel, brass, AL) and even thin wood veneer.
(pics to follow at some point...I'm not at the shop at the moment)
I'll explain the basic concept using a 4 X 8 X .1875" Cast Acrylic sheet as our material to cut. Let's say we have an entire sheet completely nested with parts with little or no scrap, in order to get this job we desperately need to keep afloat.
I'll start by cleaning one side of the acrylic paper with a dust brush. Then I will adhere two 24" wide strips of High Tack transfer tape, taking care to avoid bubbles. Use a J-roller to firmly adhere the tape to the paper on the acrylic sheet. Then I take 6" wide mounting tape, which is very aggressive & double sided, and stick it to the entire back of the transfer tape I just put on. Then I take a $7 4X8 sheet of Coroplast (corrugated plastic) and laminate it to the mounting tape to give me a layup that looks something like this:
Acrylic Layer
High Tack Transfer Tape Layer
DS Mounting Tape Layer
4mm Coroplast Layer
I run the J-roller over the whole deal and make sure everything is bonded really well. Then I place the whole layup right on my universal vacuum bleeder & pull vacuum. I cut my parts, keeping an eye on things to make sure I am not asking too much from this hold down system by cutting too aggressively. If I get part movement, I make adjustments, but it is rare that I have to do this. I cut all the way thru the acrylic, tapes and well into the coroplast. This allows me to cut all the way thru the parts without ever breaking vacuum. The vaccum holds the coroplast...the coroplast holds the tapes to the acrylic.
When cutting is complete, I simply peel off the acrylic along with it's protective paper. The super aggressive adhesive remains attached to the coroplast and the transfer tape easily peels off the paper backer on the acrylic without a hitch. The mounting & transfer tapes are so thin that adhesive build up does not typically present a problem.
That's it! Now you can cut parts you only dreamed of cutting before!
Everything I mention here is from HarborSales.net:
Mounting Tape
6" x 72 yd | White | Double Side Adhesive
216 Double Coated Poly Film Mounting Tape
Price: $64.77 ea
Transfer Tape
#4076 | Conform/RLA | Higher Tack
24" x 100 Yards
Price: $58.00 ea
OK - This needs to be said to many of you. You MUST charge for part fixtures since there is a consumable cost - the coroplast and tapes, plus your labor in laying everything up. If the customer balks at this, then you might want to think about getting some new customers. There is no reason why you should have to pay for this out of your own pocket. I prorate the cost of the tapes and coroplast and charge shop rate for labor in the layup.
You are essentially giving the customer superior edge quality for just a few bucks more than a hack job and tabs. Something for you to consider.
-B