Thread Whirling - What is it, and How Does it Work?
What is Thread Whirling?
Thread Whirling is a form of the Thread Milling process with the exception that the cutters are mounted on the inside of a Cutting Ring or Cutter Holder rather than the outside of a milling tool.
Advantages of Thread Whirling
• Deep Threads - Bone Screws usually have a deep thread formation like Acme forms and other increased pitch threads which can be machined in less time by whirling.
• Long Length to Diameter Threads - When used on a Swiss Type machine the Thread Whirling Spindle works close to the Guide Bushing for increased support and rigidity on long length to diameter screws. The process is usually performed in a single pass from stock diameter which maintains constant material support.
• Faster Setup - By eliminating the need to match rough and finish insert forms the thread whirling process reduces setup and debugging time. It eliminates special support devices and expensive startup development costs.
• Increased Productivity - A result of cycle reduction and increased up time is greater productivity. Thread Whirling is usually performed in a single pass from stock diameter. This eliminates multiple passes required for single point threading. By eliminating unnecessary support devise and multiple thread passes a single cycle can be reduced by minutes.
• Increased Tool Life – Whirling inserts have a stronger cutting edge than single point tools because cutter side clearance is achieved by rotating the whirling spindle not by relieving material under the cutting edge.
• Cutter Clearance - With a single point threading insert the side strength of the cutting edges are weakened with the additional increased relief. If the helix angle of the thread is 7° then the cutter clearance must be greater 7°.
Lay down inserts may use an anvil under the insert that will accommodate some of the clearance issue. However, 6° is the usually the maximum angle found on standard anvils and the remaining clearance must be made up with relief on the insert itself.
Top notch or side screw inserts would required special holders or would need all of the clearance to be built into the cutter; this would considerably weaken the edge strength. Chip load for whirling is considerably less than single pointing and combined with the increased edge strength it equates to longer tool life and increased production time.
Compared Precision of Threading Methods
The Thread Whirling process offers many advantages including precision results comparable to other high quality threading methods such as Thread Grinding and Thread Rolling.
|Lead accuracy||Very good||Medium; controlled by subsequent heat treatment||Medium; depends on machine accuracy and stress relief|
|Diameter control||Very good; direst through steadies and infeed axis||Medium; indirect through bar diameter||Poor; direct, but depends on centering of bar under machining load|
|Lead wobble||Very good||Not controlled||Good|
|Thread roundness||Verygood||Good||Poor; whirling cannot produce round threads|
|Surface finish||Good; depends on wheel used||Depends on polishing process||Good; depends on cutter quality|
R2 - Source of table: August Steinmeyer GmbH und Co. KG, (Bedford, MA) www.steinmeyer.com
In a comparison to thread milling a 0° thread wall which is commonly found on the trailing side of a bone screws thread, the finish pattern of thread whirling is improved due to the tangential radial entry and exit of the cutter which leaves a uniform pattern.
In the thread whirling process the cutter path enters and exits the cut in a radial arc which is similar to the thread radius direction. As the heel of the insert exits the thread form the resulting finish leaves a cross hatch pattern. This cross hatch pattern would not be seen on any thread wall with a positive wall angle. This is because the cutter only contacts the work piece at the full tangency of the cutting path.
In the thread milling process the cutter path cuts on the way into the cut and up to the full tangent point. As the heel of the cutter makes an exit path the direction of the finish pattern changes as the cutters pull up material as the part rotates into the cutter path; the result is a cross hatch pattern.