Material Hold Down
Accu-Router engineered Roller Assist holddown … with four steel rolls, urethane coated & fixed mounted to the machine base for effective single or stacked panel holddown.
Roller Assist supplied with quick release pins & ductile iron bearing hangers;
Accu-Router “wrote the book” on a roller holddown system, having a U.S. patent in force from September 1986 to June 2006.
Beware, not all rollers are created equal.
Most vacuum-only CNC’s use “down-spiral” cutters to keep the material and vacuum seal intact. Since rollers push down on the material, you can use up-spiral tooling with the following benefits:
Generate less heat, evacuate chips quicker rather than regrinding debris along tool path
Extend your tooling & spindle bearing life, plus achieve a higher quality edge finish
Given above, often faster cutting rates
Reduce concern with fires (heat + vacuum), especially when machining hardwood
Eliminate the cost, maintenance and noise of a vacuum pump. New liquid-ring vacuum pumps (our preference) can retail for $25,000 or more
Lower energy consumption, carbon footprint
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.
Accu-Router flow-thru vacuum systems incorporate a no-flow restriction plumbing design that includes:
System on/off butterfly valve,
20’ of 4” ID Tigerflex type hose from the pump to the machine,
4” ID rear table mounted manifold into which four 2” ID incoming lines come from four different table top areas,
Recessed table top providing for air clearance below the bleeder panel
Table top access holes are screened to help keep debris out (inset photo).
The vacuum system is powered by a Travaini 40 HP / 500 CFM liquid ring vacuum pump with motor starter and inlet filter. The Travaini pump is amazingly quiet and can be placed near the machine cell without concern for excess noise. It includes an attractive two year warranty plan.
Pioneered by Accu-Router in the mid-1990’s, double CNC holddown has become the standard in high production panel machining …
INCREASED OUTPUT. With the ability to machine stacks 1.5+” thick, Roller Assist will double or triple your output per cycle.
TIGHTER NESTS = HIGHER YIELD. Holding material securely from top & bottom virtually eliminates part movement, maximizing your yield.
LONGER TOOLING LIFE. Vacuum-only routers often use downspiral cutters to maintain seal, but they generate heat and oppose chip evacuation. Double holddown works great w/ 3-flute, soft upspiral cutters, addressing those issues at faster cutting rates.
GREAT WITH NESTING SOFTWARE. Plataine & SigmaNEST increase the profitability of a double holddown Accu-Router, particularly with variable center distance between spindles.
EVEN GREAT 1-UP! Even in complex cuts that can’t be stacked (bevel, roundover, dado joint, countersink, etc), you’ll still enjoy benefit from Roller Assist cutting “1-up” (yield, accuracy). Let us show you with a test cut!
DOES VACUUM REALLY HELP IN A STACK? Yes. Our studies have shown up to 50% holding efficiency on 2nd layer (thru MDF carrier panel & 1st layer). Ideally 40HP liquid ring pump with limited 90-deg plumbing turns.
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.
300-400 individual pods are manually flipped to 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.
Disadvantages? Seals must be replaced periodically, hard to securely hold small parts without enough pod coverage, and slick or dense material can shift during machining.
The pod panel should be constructed of UHMW polyethylene. This vacuum system is more expensive than the traditional vacuum.
In some cases, you’ll need to hold down a 3-D surface that doesn’t lend itself to flat vacuum or rollers. Thankfully Accu-Router has the experience and partnerships for almost any requirement.
Custom workholding fixtures are most common in aerospace, automotive, reinforced plastics, and some 3D wooden parts. Custom fixtures are commonly machined from aluminum and made to conform to the bottom side contour of the part including any desired locating points involved. Vacuum, mechanical clamps and other methods are specified for minimal setup to hold the part securely during machining. Some customers automate the load/unload of such CNC’s with robotic arms and custom end effectors.