In this Series of discussions, we will explore some past projects that involved some trouble shooting and some machining tips that were learned along the way. This might help you set your mind when certain problems arise in your next project.
Machining PTFE:
PTFE or its brand name Teflon®, and its filled grade Rulon® a group of over 300 unique formulas. Rulon PTFE materials cover nearly every industrial application, some formulas are NSA, FDA, USP Class VI compliant, and some serve in wet applications, some in dry conditions only and some are abrasive. Rulon is very versatile material with many design possibilities; the key challenge is in machining. Holding the component without deforming it, holding forces must be minimized and spread out over the surface area. It is often required to machine mandrels into Rulon tubes or Inner diameters of the work piece to reduce distortion from the jaws or clamps.
Common Applications for PTFE:
Aeronautical
Anemometers
Meat Processing
Medical Equipment
Gaskets
Packing materials exposed to chemicals
Bearings
Seals
Piston Rings
Electrical insulation
Unfilled PTFE can usually be machined using high speed steel or highly polished carbide tooling without coolants. Cutting tools must be sharp, and if using filled PTFE which is abrasive from the glass contents, carbide tooling must be used. PTFE responds well to coolants since it can easily resist water and is resistant to most chemicals.
Drilling Tip: Coolants are strongly suggested during drilling operations, especially with notch sensitive materials and glass or carbon reinforced products. The insulating characteristics of plastics require consideration during drilling operations, especially when hole depths are greater than twice the diameter.
Milling Tip: Sufficient fixturing allows fast table travel and high spindle speeds when end milling plastics. When face milling, use positive geometry cutter bodies. Climb milling is recommended over conventional milling. To ensure finished part flatness, always machine a plate flat to start. Do not force a plate flat with a vice or vacuum.
Threading and Tapping Tip: Threading should be done by single point using a carbide insert and taking four to five 0.001” passes at the end. Coolant usage is suggested. For tapping, use the specified drill with a two-flute coated tap. Remember to keep the tap clean of chip build-up. Use of a coolant during tapping is also suggested. Use of a coated tap will create radii at the root of the threads resulting in a stronger and tougher thread which is less prone to cracking from over-torquing.
Turning Tip: Turning operations require inserts with positive geometries and ground peripheries. Ground peripheries and polished top surfaces generally reduce material build-up on the insert, improving the attainable surface finish. Fine grained C-2 carbide is generally best for turning operations.
When turning PTFE try machining with a surface speed of 400 FPM, a feed rate of 0.004 to 0.006 inch per revolution with diameters of 1/8 or smaller. PTFE bar will have to be supported during machining with either a live center or with a steady rest set opposite of the tool. Small diameters may require larger diameter rods to form the desired diameter just prior to part cut off.
Case Study #Improve Wear, Performance and Noise Reduction: PTFE/Rulon
This component is simple in form as far shape goes but is subject to high wear in its environment, the client approached us to see if we can aid on improving the performance and maintenance requirements on a cowling cover inlet wear strips. Since day one of their current design, the wear limit strips were wearing out and being replaced at a costly rate. During speed up and during speed down, there are centrifugal components that gradually expand outward to create and maintain a certain mechanical function. The centrifugal parts are made of a composite material and the current Teflon wear limit strips are there to control the expansion these components while protecting the cowling cover. Although this rubbing is brief it occurs about 40-50 cycles per day, and after a few hundred cycles the wear strips must replaced.
Challenge: Improve wear, lower maintenance costs; reduce replacement frequency while maintaining existing cowling cover and protecting expensive composite components.
Solution: First thing to notice here is that we are dealing with rotating components that tend to rub while rotating at a certain rpm and force during a certain amount of time. This brings our attention to the Pressure Velocity (PV) of the currently used Teflon strips, note these strips are there to protect the cowling cover, guide and prevent damage or wear to the more expensive centrifugal parts which are made of composite materials. After studying the Pressure Velocity values of the current Teflon strips, we proposed trying a Rulon formula with an increased PV rating and improve temperature stability of the material. With several formula adjustments a custom machined Rulon wear limit strip outperformed the current Teflon version in this mechanical friction application. Client has reported a reduction on friction, lower replacement frequency rates and lower maintenance cost over the span of a month with just a material change while retaining their current design.
Resolution: With in the following year, we were awarded with a contract to revise the customer's original design and replaced the wear strips with Rulon roller wear strips. This design contained Rulon rollers at the various points of contact, this design change tripled the life of the wear strips with zero wear to the composite materials.
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