Frequently Asked Questions in the Routing of Plastics #3
The following questions commonly arise during the setup and machining of plastics. These topics focus primarily on machine condition and fixture design and provide a starting point for further optimization of the cutting process.
Why Can Acrylic Still Show Heavy Knife Marks After Following Recommended Cutting Parameters?
Producing a high-quality finish in acrylic requires a large number of factors to work together correctly. A router bit may remove approximately 0.002 to 0.010 inch of material during each revolution, while the desired finished surface can require extremely small variations between knife marks.
Cutter geometry helps reduce the visible knife marks produced by the chipload, but tooling alone cannot compensate for movement or mechanical problems elsewhere in the system.
Fixturing must remain solid throughout the entire cutting cycle. Even very small movements of the part can create irregular surface finishes.
Machine condition is more difficult to evaluate because every component between the cutting tool and the machine structure can influence the finished surface.
Can Machine Condition Be Evaluated Without Expensive Alignment Equipment?
Yes. A number of relatively simple checks can provide a good indication of whether additional maintenance or inspection is required.
Check the Tool Clamping System
The first area to examine is usually the tool clamping system. Collets must remain in good condition and should be replaced regularly.
Collets do more than hold the cutting tool. They are also responsible for aligning the router bit along the spindle centerline.
Any out-of-round condition can cause multi-flute tools to remove different amounts of material with each cutting edge. When that occurs, producing a uniform machined surface becomes extremely difficult.
Single-edge tools are also affected by poor concentricity because chipload continually changes as the tool rotates.
Broken tool shanks or severe machine crashes should trigger a detailed inspection of the collet and may justify immediate replacement.
Collets should be treated as consumable tooling. Concentricity can be checked using a dial indicator with a plug gauge or cylindrical round.
Multiple readings should be taken, with the chuck disassembled and reassembled between measurements. This provides a better indication of both accuracy and repeatability.
Inspect the Spindle and Drive System
If the clamping system is in good condition, the spindle and drive systems should be evaluated next.
There should be no play in the spindle bearings, and the spindle mount should remain rigid against the back plate. Movement may indicate that the spindle needs to be remounted or that the bearing pack requires service.
Axis movement should also be checked. With the drive motors engaged, there should be little or no movement in any axis.
Excessive play may indicate worn ball-screw nuts and backlash in the system. Backlash can create unstable movement during acceleration and deceleration and appear as poor finish on the machined part.
Perform Test Cuts Across the Table
A useful final check is to make a series of test cuts at different locations on the vacuum table.
Straight cuts along the X- and Y-axes, a diagonal cut involving both axes, and both large- and small-diameter circular cuts can reveal how smoothly the machine travels.
If one type of cut appears significantly worse than the others, the axes involved should be examined more closely.
Repeating these cuts at different areas of the table can also reveal problems near the limits of machine travel or localized wear in the ball screws or ways.
How Long Should Spindles, Collets, and Other Machine Components Last?
There is no universal service-life figure for most machine components. Ways, ball screws, ball nuts, spindle components, and bearings continue to improve and their useful life varies considerably by machine and operating conditions.
Collets are more predictable because they remain the primary flexible component between the spindle and cutting tool.
Although this interval may seem short, collets are repeatedly expected to maintain tolerances measured in ten-thousandths of an inch while transmitting the power of a high-horsepower spindle to solid carbide tooling.
For premium finishes, keeping the entire clamping system in optimum condition is generally preferable to extending collet life until visible failure occurs.
What Is the Best Way to Fixture Small or Intricate Parts Such as Letters or Logos?
The best approach depends heavily on the application, but the first decision is whether to use a universal flow-through vacuum system or a dedicated vacuum fixture.
Universal Vacuum Systems
Universal vacuum systems require a relatively large surface area to develop holding force. Small-diameter tools can help because they create less lateral cutting pressure on the part.
Skin cutting or tabbing can also improve holding performance. In some applications, cutting through the plastic while leaving the masking intact preserves the vacuum seal and prevents small parts from moving.
Vacuum loss should be minimized throughout the cutting cycle. As a large sheet is gradually divided into smaller parts, available holding area decreases.
Scrap pieces can be placed over completed cutouts to reseal the cutting paths as the program progresses. The edges of the universal spoilboard and unused areas of its surface should also be sealed.
Dedicated Vacuum Systems
Dedicated vacuum systems can be more effective for small parts when designed properly.
The primary goals are to eliminate vacuum leaks, increase friction between the part and spoilboard, and distribute vacuum over as much of the part surface as possible.
Applying a sealant to the spoilboard surface can reduce vacuum loss. Enamel or latex primers and paints may work, while rubberized coatings can provide the additional benefit of increased friction.
Channels are generally the preferred method for distributing vacuum while maintaining contact between the material and spoilboard.
Spoilboards without channels, or gasket tape installed directly on top of the board without being recessed, can create long-term problems. The tape may wear quickly due to excessive compression and lateral forces.
In some cases, the part can also deform toward the vacuum source or partially seal the vacuum opening, reducing the effectiveness of the fixture.