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Work Surface (Fixturing)

In order to cut a part on a CNC router that part must be held firmly in place. The process is commonly referred to as fixturing and it has a considerable influence on part accuracy, finish, feed rates and tooling life.  The underpinning for fixturing is a system’s table-top (work surface) which should be rigid, flat, allow rapid part placement and removal; and in a job shop environment support a range of fixturing options.

Techno CNC Work Surface

All Techno CNC routers feature a heavy-duty, anodized, extruded aluminum table plate which is scratch/corrosion resistant and features an internal reinforced webbing to provide maximum stability.  The table surface is flat milled and features a T-Slotted design which accepts standard M-6 nuts to expediently secure side and top clamps, test fixtures, gauges and so on.  Additionally, the work surface supports the use of spoilboard in conjunction with T-Slot clamping.  (Spoil-board is to protect the table when you are cutting all the way through a part).  The spoilboard is attached by adhesion to the system table-top and milled parallel to the system spindle.  Optionally, all Techno models can be configured with universal vacuum hold down, vacuum cups, and pneumatic clamps which engage or disengage under system control

Mechanical Clamps

The advantages of mechanical clamps are that they are inexpensive, easy to understand and offer a highly rigid mounting when properly applied. They may be the only practical hold-down method for small or odd-shaped parts that do not have enough surface area for vacuum hold-down.

The graphic directly below depicts top clamps anchored in T-Slots which secure the work piece with downward pressure.  Further, the work surface supports spoilboard panels which protect the table-top when cutting all the way through the work piece.

 

 

The graphic directly below depicts side clamps and a backstop anchored in T-Slots, this combination secures the work piece with a (vise like) sideway pressure.  Further, the work surface supports spoilboard panels which protect the table-top when cutting all the way through the work piece.

 

 

The graphic directly below depicts spoilboard and clamps used in conjunction with a tool-path routine commonly referred to as tabbing.  Tabs are used specifically to hold a work piece in place when the cutter breaks through the bottom of the material.  Clamps hold the work block in place and small tabs of un-machined material near the bottom of the cut secure the work piece to the work block, thereby preventing the work piece from shifting position when the cutter breaks through the bottom of the material. 

 

Vacuum Hold Down

Vacuum is the most common means, and often the most convenient and effective system, for holding parts on a CNC router.

Vacuum hold down works by using the air pressure of the atmosphere.  The larger a part’s surface area the greater the atmospheric pressure applied.  As an example, a one foot square part has 144 square inches of surface area.  An effective vacuum system will cause approximately ten pounds of pressure per square inch to be applied to the work piece, exerting about 1,400 pounds of total holding pressure.  The effect is similar to stacking 1,400 pounds on top the work piece. Of course, as a part’s surface area decreases, holding pressure is reduced.  In the case of a 6 inch square part, the surface area is 36 square inches, and about 360 pounds of holding pressure will be applied.  Below a certain surface area, too little holding pressure will be applied and the work piece will move.  Work pieces with less than four square inches of surface area can present a problem for vacuum hold down.

The graphic directly below depicts a work piece being held solely by vacuum. This is an approach generally utilized when not cutting entirely through the work piece, making a spoilboard un-necessary.

 

The graphic directly below depicts a work piece being held solely by vacuum which pulls air through a spoilboard. This is an approach generally utilized when it is necessary to cut entirely through the work piece. 

 

Since, vacuum hold down is a mysterious area to many people, a brief explanation is as follows. When a vacuum pump is connected to the vacuum table, air is constantly pulled through the spoilboard.  This maintains a vacuum under the spoilboard and creates a very low-pressure area on top of the spoilboard.  If a part is laid in this low-pressure area, the amount of air on the underside part is less than the amount on top and the part is held in place. This generally works without the need for seals between the part and spoilboard

 

The graphic directly below depicts a work piece being held by a combination of vacuum and top clamps. This approach is utilized to handle tall parts and warped material that cannot be secured solely by vacuum.  The top clamps serve the purpose of exerting a downward pressure to add additional rigidity to tall part fixturing or can be used to flatten warped material enabling a vacuum seal to be established

Techno Vacuum Table

The Techno Vacuum Table is based on phenolic panels about ¾ inch thick.  Milled into the panels is a waffle iron pattern which exposes much of the underside of work material or spoilboard to vacuum chambers while still providing support for the material.  The phenolic panels feature a high co-efficient of friction mitigating part slide during the machining.

Additionally, to prevent air leakage, the vacuum table features a universal sealing system based on rubber gaskets, which are soft and pliable enough to prevent air leakage and yet strong enough to withstand repeated loading unloading of parts without becoming damaged.  To create the most effective seal the gasket material is placed in machined grooves cut into the vacuum table.

 

General Notes on Fixturing

Some materials require higher cutting forces than other materials.  These materials will require a more rigid hold down system. Some materials will vibrate or chatter when cut. The hold down system will need to support the part and eliminate the chatter.

Certain fixturing methods that are available from Techno are beyond the scope of this review, these include, but are not limited to: roller hold down, vacuum cups, pneumatic clamps which engage or disengage under system control, custom mechanical fixtures and 3-D vacuum fixtures for trimming applications.  For application specific information about these options, please don’t hesitate to contact us.