Three Ways to Thread

Threading can be a very challenging operation. Cutting forces are high, tolerances are tight and tools wear quickly. Two vital areas to consider are selecting the best threading tool possible, but also using the right tool for the thread-making method you have chosen.

The three most popular options are tapping, single pointing, and thread milling. In this article, we look at the benefits and drawbacks of each method and provide you with technical advice on best practice. 

High-Performance Threading: Tap and Die Sets

For as long as people have been making screw threads, they’ve been using taps and dies to cut them. While dies have pretty much gone the way of carbon steel tools, taps are alive and well. However, tapping methods and thread-making technology have evolved to keep up with the times. Here’s how you can use tapping to your advantage:

Always use a plug tap when possible. It is true that bottom taps can get closer to the bottom of the hole, but thread quality is usually not as good — nor is tool life. Now that you’re using a plug tap, be sure to drill the minor diameter as deep as possible, or all the way through if you can. This provides more room for chips.

Never settle for a general-purpose tap; these days it is likely that a tap exists specifically for the metal and thread style you are machining. For problem threads, add a small amount of machining wax or tapping compound into the hole before tapping – in fact, some machine tools can automate this step. Many taps are available with through-the-tool coolant. If your machine has this feature by all means use it.

Also consider using roll or form tapping in ductile materials such as aluminium and stainless steel, to provide a strong, smooth thread. Be sure to drill the hole according to the tap manufacturer’s recommendations, and towards the upper limit if possible.

Be aware of tap holder types. If your machine, like most modern ones, is equipped with rigid tapping be sure to use a “synchro” tap holder specifically designed for this. Otherwise, use a floating (mill) or self-releasing holder (lathe). For high-volume threading on a machining centre, a self-reversing tapping head might be just the ticket. 

CNC Machining: Single-Point Threading

Anyone who has cut threads on a lathe knows that single-point threading can be challenging. What angle should you tip the compound slide? How many passes should you take? What is the right number of spring passes?  Thanks to CNC lathes, it is all programmable. But while the threading process has become simpler, finding the right tool has become more complicated. Here is what you need to know about single-point threading tools:

Full-profile threading inserts cut the entire thread form. They eliminate the need for a separate turning operation to trace the thread’s major diameter - or minor, for internal threads. However, each insert can only cut one specific thread pitch and profile, increasing tooling inventory and costs.

Partial-profile inserts can cut multiple thread pitches with a single insert. They offer greater flexibility on size control, but they do increase cycle time slightly over full-profile “topping” inserts.

Double-sided laydown-style inserts offer the lowest cost per edge. A special anvil is required to tip the insert at the required pitch angle. However, take warning — use the wrong anvil and you’ll end up with a pile of scrap. Also be careful with spindle rpm, as it’s easy to overfeed the machine to a point where you’re pushing too fast for repeatable thread accuracy. 

Still using a G92 or G32 canned threading cycle? It may be time to pick up the programming manual and master G76 as it’s far more powerful. To increase tool life and part quality, use the compound feed option.

Tool coatings may be less effective in threading work, leading to built-up edge (BUE). Because cutting speeds during threading are often lower than turning or boring operations, you need to watch out. Uncoated or TiN inserts are the best bet, especially on high-temp alloys and very coarse threads.

Watch your Z-axis positioning. The Z-axis starting position should always be three to four times the thread pitch in front of the part. Also, turn off constant surface speed when threading in G96 by using a G97 command.  

Thread Milling

Traditionally found in machining centres, thread milling is becoming widespread on turn-mill lathes and multitasking machines. It’s a bit slower than tapping but offers far greater control and flexibility.

Where taps are limited to hole threading, thread mills can cut internal and external threads alike, often with the same tool. Thread milling is an excellent method, but you need the right tool and the right program to make it work. Thread milling is the best way to go for lower part volumes, difficult materials or where thread quality is critical — and where single pointing is not possible. Here’s what you need to know:

Solid carbide thread mills are the fastest, most rigid, and most productive tool available. They can cost more than other types but often offer the lowest cost per part. Alternatively, indexable thread mills require more time to cut the thread. Typically, this is due to having fewer flutes than their solid carbide cousins. For low production quantities or where the tooling budget is limited, however, they are often a good choice. Thirdly, single tooth, multi-flute thread mills should be used on delicate or thin-walled parts, or where long reach is required. Start the thread from the bottom and work your way out of the hole.

Most CAM systems today generate excellent threading routines — check with your software partner if you need help programming one.  Don’t have a CAM system? Most thread mill supplier websites offer program generators for free. Plug in the values, then cut and paste the code snippet into your program.

Always use cutter compensation to adjust the thread size, as this is one of the big advantages of thread milling.​

Right approach

Whatever approach you take, always buy the best cutting tool possible. Use a high-quality neat oil or clean, properly mixed cutting fluid. The set-up should be rigid, the tool should be centred on a lathe, or with no runout on a mill - and held securely in the appropriate tool holder. Plug in the correct program values and get threading.

For further advice on maximising productivity through efficient threading or to source the right tools for the job, visit www.mscdirect.co.uk