Tips for Tapping Hard Materials

There are sensations and experiences that must be experienced before they can be appreciated. Believe it or not, machining can provide sensations that only a machinist can truly understand.

For me it is the trepidation that I feel when a tap is about to enter the hole in hard-to-machine materials like nickel alloys. While I have never bungee jumped, I think tapping in difficult materials is probably similar. The tap going in the hole is like taking the first step, you have to push through the fear and let yourself go. The second level of fear is during the fall, hoping the rope will hold and the tap will get to the bottom. And third is the exhilaration of having survived the fall and the tap coming back out of the hole in one piece.

To reduce the anxiety of tapping holes in difficult materials I have developed my own rules of thumb. These are the things I do to give myself the best chance of success and minimize the fear of jumping.

Rule Number One

Get a good tap. You can use almost any kind of tap to thread holes in aluminum, brass, plastics and most soft steels

with little worry of the tap breaking. That’s not the case with stainless steels or high nickel alloys like those used in aerospace applications. If the material is difficult to mill and drill, then tapping it can create nightmare scenarios. Getting a tap with geometry created for the material being machined is paramount to successful tapping of difficult materials. Having the correct geometry for exotic alloy materials will exponentially improve your chances of success.

Rule Number Two

Drill the correct size hole. Sounds like a no-brainer, right? Find the tap drill size on the chart and drill a hole, then tap it. That works fine — until it doesn’t. I suggest you drill the biggest hole you can get away with and still conform to the engineering requirements. It’s simple, a bigger hole creates less stress on the tap, which reduces risk. I will make the tapped hole as large as I can, even resorting to reaming holes when a drill is not available in the size I want.

Rule Number Three

Drill it deep. Through-holes or holes that go all the way through the material are the least risky when tapping because chips are less of a problem. Blind holes, holes that have a bottom, should be drilled as deep as possible and tapped as shallow as possible. This approach creates the maximum amount of clearance between the bottom of the hole and the end of the tap.

Rule Number Four

Lube it up! Tapping dry will cause problems in the easy materials like aluminum. So, tapping materials like stainless without proper lubricant is obviously more difficult. Oils and petroleum-based lubricants are always the best when they can be used. Unfortunately, using oils for production machining in 2024 is not ideal, so we must rely on water-based cutting fluids. Make sure your coolant is mixed correctly and it is not too lean. Coolants not only absorb heat, they lubricate the tool and the part. Most importantly, coolants reduce build up on the edge that would otherwise result in a broken tap. If you are having trouble with taps breaking or tearing threads, then make the coolant richer — 1% to 2% can work miracles.

Also, make sure coolant is getting to the tap. Position the nozzle so that the stream of coolant is going into the hole; sometimes a dedicated nozzle for the tap is needed to ensure the tool is never starved. Through the spindle coolant should also be used if available. Although I haven’t used one yet, I suspect through-coolant taps are effective. If I were making a new machining process for stainless or Hastelloy, I would plan to use through-coolant taps to ensure proper fluid delivery.

Rule Number Five

Use the right toolholder. It is always best to use a toolholder made for tapping. A tap holder allows the tool to move or float, minimizing the stresses and strains that can occur when the tap and machine are not perfectly synchronized. Modern machine tools are far less likely than their predecessors to suffer from these errors, but programmers occasionally make mistakes that a tap holder might correct. One of my previous employers tapped thousands of holes with the wrong pitch programmed, but the tap holder absorbed the error preventing breakage.

Rule Number Six

Tap holes as soon as possible. Taps are more prone to failure than any other tool, and broken taps rarely come out of the hole without additional work. Many times, it results in a scrapped part. By tapping early, you reduce the cost of scrap parts because fewer operations have been completed when the tap goes into the hole.

Rule Number Seven

Don’t use a tap. Thread milling is the best choice whenever possible. Avoid the use of taps at every opportunity. Thread milling is the most accurate way to produce threads in general machining. It is also the least risky as a broken thread mill will be easy to pick out of the hole unlike a broken tap, which is usually only removed after extra work and voluminous profanity. The accuracy and flexibility of modern machine tools makes thread milling easy, and it should be the first choice for threading difficult materials. Some manufacturers are making thread mills that also drill, allowing the user to consolidate machining operations and reduce cycle times.

These tricks do not remove the risk associated with tapping, but they will improve your success rate and may ease some of the anxiety of threading difficult materials. 

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