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Tap Dancing With Chips
Sidestep tapping troubles with the right tool, toolholder, and predrilled hole size
- By Sue Roberts
- April 25, 2014
- Article
- Management
It seems like it should be reversed. For many components flowing through a machine shop, one of the most difficult processes is the last to be performed. The part, already the owner of the majority of its production value, is ready for its tapped features.
Whether the machinist is creating threads in through or blind holes, chips can threaten damage to the threads or cutting tool. Chips can bind up the tool, cause it to snap off, and leave the machinist with the decision of spending precious time trying to remove the tool without damage to the component or scrapping the nearly finished part.
“Tapping is one of the more difficult things to be done,” said Cullen Morrison, business development manager for threading, Komet Group, Schaumburg, Ill. “On the cutting tool side you have a very small, angular face for a tool that has to cut a very accurate profile. So you have a lot of load and a lot of resistance. From a programming standpoint, all you can really change is the tool speed. You can’t change the feed rate or the depth of cut like you can with a drill or milling cutter. And, you usually have to go fairly deep—two to three times the depth-to-diameter ratio of the tool—and that can cause chip evacuation problems.”
It’s no secret that creating blind holes causes more chiptrapping frustration than through holes. The bottom of the blind hole prevents the chips from being pushed out in front of the tool during cutting and increases the risk of a clog.
“Through holes typically don’t present much trouble because the tap is winding up the chips and moving them forward,” said Pat Nehls, product manager, Walter USA, Waukesha, Wis. “In blind-hole taps, chips can’t go forward because they will be crushed between the tap and the bottom of the hole, so we have to have a tool designed to bring the chips out of the hole. The chips can be stringy and difficult to break and can end up wrapping around the tool and damaging the tool and the workpiece.”
Handling Long, Stringy Chips
Chances are a tapping tool that completes a job without chip chaos is available. But choosing from literally thousands of options can be a daunting experience. The right combination of length of chamfer on the front of the tool, number of flutes, variety of coatings, and coolant delivery can manage chips for just about any application.
“Originally, a tap was a completely straight-fluted tool that would try to break up the chips within the flutes. To some extent it still works, but now when you thread steels, aluminums, and most of today’s materials, you get long, stringy chips that just don’t work well with a straight-flute tool. With modern technology, coatings, and geometries, we’ve moved on to where we have exceeded the physical limits of what a straight-fluted tool can do for most materials,” said Morrison.
“For through holes the spiralpointed tool actually curls the chips and pushes them in front of the tool. That keeps the chips out of the cutting area,” Morrison added.
Online selection guides, manufacturer- specific software, and tooling engineers can help manufacturers shift through the myriad threading tool options for the best fit for an application. If component specifications present unique production challenges, a custom design may get the job done profitably.
Eliminate Drift With Toolholders
Accuracy of synchronization between the spindle and the feed rate, largely influenced by the machine tool’s age, can determine if a toolholder is needed to produce threads within tolerance.
“A machine has a lot of mass,” said Nehls. “If it is not fully synchronized, you can get some drift when you try to slow down or stop the rotation. That drift can give you some mismatch in the thread profile. A toolholder compensates for length variations caused by the machine movement.”
A tension-compression toolholder and the updated synchronous chuck act like springs, giving the tool a little play to protect against chip problems and broken tools when the speed of the threading process is turned up. Tension-compression holders typically allow ±5 mm of movement in length. The synchronous toolholders, specifically designed for taps, allow ±2 mm of play. This is just enough room to even out the load on the flanks to avoid tool or component damage when the tool reverses.
Rigid toolholding becomes an option as better encoders and synchronization tools have improved machine synchronization.
“In newer machines equipped with rigid tapping, the spindle RPM has become synced with the feed rate. This allows full rigid toolholding, where tools can be held directly in the collets or another tool system. Length compensation is not needed,” Morrison said. “We are, however, seeing where we might be growing out of the rigid toolholding again with some of the modern technologies. They run at such high RPM rates that there can be some problems with accurate tracking.”
Keep Cool and Lubricate
Tapping should not be performed dry, with the exception of tapping cast iron, which creates its own lubricant. Type of coolant can vary, but it should contain a 9 to 10 percent oil additive for lubricity.
“A lot of heat is generated during tapping because you have a small tool taking a large chip load, and there is a lot of friction,” Morrison said. “Anything that can be done to improve the lubricity of the chip coming off the tool to help it flow in and out of the hole easier is important. That flow is greatly improved with coolant.”
Coolant delivery can be accomplished in a variety of ways. Nehls said, “We can tap with semisynthetic, water-soluble oil, straight oil, or minimal-quantity lubrication via an oil mist that is very environmentally friendly. We look at the machine and the application to determine the coolant delivery options. It may be that the customer is willing to make adjustments with the machine depending on the application. We may recommend an axial coolant delivery in which the coolant comes through the end of the tap or an axial with radial exits so it is coming out through the flutes.”
Hold Tight Hole Tolerance
Preparing the workpiece for tapping is also important. The predrilled hole has to be correct for the type of tap to be used to remove material to the larger-diameter thread profile. “Typically, customers are looking to tap 70, 80, or 90 percent of total profile height,” said Nehls.
“People need to understand the thread strength and the percent of thread for what they are trying to accomplish,” said Morrison. “A lot of the drill and tap charts may have been developed for high-speed-steel drills from the 1940s or 1950s and could lead to drilling oversized holes. With high-speed carbide drills, we drill to a much tighter hole tolerance, and with that the percent of thread goes up significantly so the tool works even harder, tool life is reduced, and there is more breakage. The dynamic of the hole size is important.”
How about forming those threads?
Chip problems can be bypassed if formed threads are acceptable. Forming displaces the metal to create the threads rather than cutting it. It is efficient and cost-effective, and tools can last longer than those that chip away the material.
Since there are no chips to evacuate when forming threads, flutes are not necessary and the tool diameter is thicker. The greater cross section makes the forming taps less likely to break. So if thread cutting tool breakage has been a problem, forming may be a viable alternative.
Thread forming has similar requirements to cutting. The size of the predrilled hole needs to be correct. If the hole diameter is too large, threads will not be completely formed. If the hole diameter is too small, the tap will not be able to move all the material and it can get stuck and break. Coolant is critical to control the heat generated by the moving material.
There is, however, a downside. Formed threads are not accepted by some industries, for example, aerospace, medical, and some food equipment manufacturers.
“As a thread is formed, the material moves up to the tool crest, the point of the thread, and there is a slight void. There is no real guarantee that all of the manufacturing contaminants can be removed from that void. Those contaminants can play havoc if the part goes in a human body,” said Pat Nehls, product manager, Walter USA, Waukesha, Wis. “Forming is not allowed in the food industry for about the same reason. The void can trap substances that promote bacteria growth and cause problems. For aerospace, the issue is different. The void can become a fracture point and create a safety issue.”
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