The CNC router dilemma: which hold-down system will work for you?

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At the 1994 International Woodworking Fair in Atlanta there was a multitude of CNC routers - inexpensive, expensive, light and heavy, small and large, domestic and foreign, one-twin shuttle pallet tables, one too many spindles. Each and every booth salesman advised that his CNC router was absolutely the finest available. Prices varied widely. One could certainly come to the conclusion that machine specification and selection are complicated issues.

The encouragement to purchase a CNC router is that it will lower processing cost given enough volume. It is flexible automation readily adapted to small lot sizes and a just-in-time manufacturing environment. In short, this technology becomes a must to become cost competitive in the global marketplace. Those businesses that are not cost competitive simply will not survive.

Across the spectrum of multiple choices mentioned above, there is another extremely important area to be addressed when buying a CNC router. This is one of the least understood aspects all too often overlooked in system decision making, yet it is crucial to productivity. Most first-time buyers make the assumption that competitive offerings are the same. This specific subject is the machine part hold-down system.

Beware! Not all hold-down systems are created equal. What you do not know or learn upfront will likely require much adjustment after a machine arrives. Beware of selling statements that present a "universal" hold-down system. Each of the many hold-down systems available have a specific application range where each is best suited. Perhaps a custom or combination system would be optimal for you. The purpose of this report is to differentiate between the various hold-down systems and offer guidance accordingly.

Traditional vacuum

A traditional vacuum system incorporates a multi-zone approach whereby vacuum flow is directed through a tabletop grid surface and fixture board to hold-down a desired blank(s). This is a very efficient hold-down method due to the use of gasketing to seal off the vacuum right near the part edge being machined. The fixture board is usually a piece of 3/4-in.-thick cabinetmaker's plywood for long life. The fixture can be quickly and accurately machined using the actual part program. For production jobs, this hold-down system is most economical. For best results we recommend hold-down zones every 12 in. of table length for ease of setup and a milled top pattern that covers the entire aluminum table top on a 1-inch square basis. A 3- to 5-hp rotary vacuum pump with a continuous duty rating is required.

Vacuum pods

Gaining popularity is the vacuum pod hold-down system, highlighted by an economical manual flip pod approach. Three hundred to 400 individual pods provide vacuum flow to panels being held. A 10- to 20-hp rotary screw vacuum generator is needed. This selection is recommended for applications where low lot sizes discourage the use of vacuum fixture boards, blanks are already dimensioned for machining, and horizontal boring or routing is required. The activated pods allow access to the part top, sides, and even bottom edge. The blanks are already positioned up in the air with adequate clearance for most horizontal machining. Another advantage of pods is that a conventional fixture board could be placed on top of selected pods to effectively hold-down small or three-dimensional parts. The pod panel should be constructed of UHMW polyethylene. This vacuum system is more expensive than the traditional vacuum.

High output vacuum

This choice involves taking a 40- to 50-hp rotary screw vacuum generator and directing a high level of cfm flow through a porous substructure to hold-down flat panel products. This is the most expensive vacuum option and most misapplied. It is not a universal, cure-all system. A purchaser considers this design to avoid the need for machine setup and vacuum fixture boards. This zero set-up time feature makes high output vacuum ideal for custom panel processing. There are however, the following system limitations.

* Approximately 75 percent of the activated table area or zones need to be covered with work material for an overall effective seal.

* Parts need to be 8 inch by 8 inch or larger to be held down. Thus it is better for panel processing than machining solid wood.

* Once every shift, production must be stopped for the top of the porous (usually MDF) substrate board to be machined off to eliminate the "dust plug" as well as cutter tracks. This represents lost production time as well as the need to replace the MDF panels as often as every week at an annual cost of up to $5,000 per operating table. There is also the cutter cost to perform this needed step.

There are several necessary design steps required so as not to "starve" cfm flow from the pump to your panel on the machine table. Failure to recognize or provide what is required could result in a machine not being able to properly hold down material.

Palletized stack/flotation balls

In July 1993, the ANSI 01.1-1992 safety guidelines code became public domain. This code mandates that an automated machine with moving parts, such as a CNC router, must have full perimeter guarding interfaced to the machine control to prevent an operator or third party from entering an active machine area. The onus of compliance is with the machine owner.

What this code means is that operators must stay well clear of an operating table. This strongly encourages consideration of a twin, shuttle, or pallet table machine as long as the second table is parked well out of the machining zone and can be guarded as such. Beware that most tables do not have such extra travel as designed.

A twin, shuttle, or pallet table machine is considerably more expensive than a single table version. An alternative is to pre-load your blank material on a carrier board the same size as the machine table. This is called palletizing. The single machine table can be then machined for a series of air-pressurized flotation balls. This overall approach allows for very fast loading and unloading of the palletized carriers maximizing the machining capability of the CNC router. As such, this specification provides an inexpensive alternative to multiple tables.

Three-dimensional vacuum fixtures

How does one hold down a 3-D surface such as a bent plywood chair seat? A vacuum fixture can be economically made to conform to the bottom side contour of the part including any desired locating points involved. Using normal gasket material and a 3-D groove line, piece parts can be most effectively held down by vacuum. The fixture is composed of molded plastic material that will last.

Custom hold-down systems

Hold-down systems can be customized to fit particular customer production needs especially those that include through-feed and continuous mode. Each individual requirement must be analyzed and addressed as such.

A part hold-down system is a major decision area with a CNC router purchase. There is much to be considered. Look at it carefully. Meeting your particular needs is what should guide the end selection. System productivity depends on your making the right choice.

Todd A. Herzog is president of Morrison, TN-based Accu-Router Inc.