A broken tap is the most expensive tool failure in the shop, and it is not close.
Tapping happens at the end of the process. By the time a tap goes into a hole, that part has already absorbed hours of machine time, a stack of previous operations, and in a lot of cases material that took weeks to source. When a tap snaps off in a blind hole, you are not out the cost of a tap. You are out the part, and hardened tap fragments do not drill out. That job goes to EDM or it goes in the scrap bin.
The good news is that most tap breakage traces back to two decisions, and both are made before the tool ever touches the part. This post covers both: which tap geometry to use, and what size to drill.
Tap Selection Comes Down to One Question
Forget the catalogue categories for a moment. Every cutting tap creates a chip, and that chip has to go somewhere. Where it needs to travel is the entire selection logic.
- The hole goes all the way through. The chip can exit the bottom, so push it forward.
- The hole is blind. There is no exit at the bottom, so pull the chip back out the way the tap came in.
- The material is ductile and you have control over hole size and lubrication. Do not make a chip at all.
Those three answers are spiral point, spiral flute, and form tap. Everything else is detail.
Spiral Point Taps for Through Holes
A spiral point tap, also called a gun tap, has straight flutes down the body with the cutting face ground at an angle at the tip. That angled point drives chips forward, ahead of the tap and out the bottom of the hole.
Because the chips never enter the flutes, nothing packs and nothing gets recut. The flutes can also be shallower than on other tap styles, which leaves a larger core and a stronger tool. That combination makes spiral point taps the fastest and most stable choice for production through hole work.
The limitation is absolute. Never use one in a blind hole. The chips get driven to the bottom of a hole with nowhere to go, they compact, torque climbs, and the tap breaks. There is no technique that gets around this.
Spiral Flute Taps for Blind Holes
A spiral flute tap has deep helical flutes running the length of the body, working much like a drill in reverse. As it cuts, the flutes carry the chip backward and upward, out of the hole in the direction the tap entered.
That is exactly what a blind hole needs, since chips have to leave the way they came in. Spiral flute taps are also the answer when the hole is interrupted by a cross hole or a keyway, where the tap has to bridge a gap.
They come at a cost. Cutting those deep helical flutes removes cross sectional area from the core, so a spiral flute tap is physically weaker than a spiral point of the same size and more vulnerable to torque breakage. In long chipping materials such as low carbon steel and aluminum, chips can also wrap around the shank, which marks the part and can snap the tool. Peck cycles and good coolant delivery matter more with this geometry than with any other.
Helix angle is a secondary choice worth knowing about. Slow spirals in the 18 to 30 degree range and fast spirals in the 45 to 52 degree range behave differently on chip lifting force and rigidity.
Form Taps for Ductile Materials
A form tap, also called a roll tap or fluteless tap, does not cut. It has a lobed profile that displaces material and cold forms the thread into shape.
No cutting means no chips, which removes the entire failure mode this post has been describing. In a blind hole there is nothing to pack. The advantages go further:
- Stronger threads. The grain flow follows the thread contour rather than being severed, and the surface work hardens as it forms.
- Far longer tool life. With no cutting edge to dull, form taps commonly last several times longer than cutting taps in the same material, and manufacturers report multiples ranging from three to twenty times.
- Faster speeds. A common rule of thumb is roughly double the speed of a cutting tap.
- Better thread accuracy. Because material is displaced rather than removed, oversized threads are far less likely.
The constraints are real, though, and ignoring them breaks taps just as reliably as the wrong flute geometry. Form taps work only in ductile material that can deform without fracturing. Aluminum, brass, copper, low carbon steel, and austenitic stainless all form well. Cast iron, hardened steel, and brittle alloys do not, because they crack instead of flowing.
They also demand a larger pre-tap hole than a cutting tap, and this is where shops get caught. Using a standard tap drill chart with a form tap gives you a hole that is too small, which drives torque up and breaks the tool. Form taps are lubrication sensitive as well, with cutting oils generally preferred over water soluble coolant.
The Tap Drill Decision That Breaks More Taps Than Anything Else
Here is the one most shops get wrong, and it is worth more than the geometry choice.
Standard tap drill charts target roughly 75 percent thread engagement. That figure was set decades ago when thread stripping was the main worry and broken taps were treated as a cost of doing business.
The mechanics do not support it. Thread strength climbs steeply up to around 60 to 65 percent engagement and then flattens out. Going from 75 percent to 100 percent adds only a small amount of strength, with published estimates in the range of 5 to 10 percent, while roughly doubling the torque needed to tap the hole. Sources vary on the exact multiplier, but every one of them agrees the strength curve flattens while the torque curve keeps climbing.
The practical consequence: in a joint at 75 percent engagement, the bolt fails before the thread strips. Cutting deeper buys you strength you will never use, at a torque penalty that breaks taps.
So if you are breaking taps, drop the engagement percentage before you change anything else. Moving from 75 percent to 65 percent costs a few percent of thread strength, which almost never matters, and reduces tapping torque meaningfully while opening up chip clearance in the flutes. This applies with particular force to:
- Blind holes, where chip packing is already a factor
- Deep holes, past about 1.5 times the tap diameter
- Small taps, where the tool has almost no torsional strength to give
- Stainless, titanium, Inconel, and other tough alloys
- Any job where you have broken taps before
For difficult materials, many shops simply increase the tap drill slightly as standard practice. It is one of the cheapest reliability improvements available.
When Tapping Is Not the Right Answer
Some threads should not be tapped at all.
Large diameters, very hard material, tight thread tolerance, or a part that is far too valuable to risk a broken tap in are all cases where thread milling makes more sense. A thread mill is smaller than the hole it produces, so it cannot jam, and if it breaks it comes out. We work through the full comparison in thread milling versus tapping, and we supply ThreadSTAR threadmills for that work.
When tapping is the right call, we carry YMW Taps across cutting and forming styles, alongside our full tapping solutions range. For shops in Hamilton, Stoney Creek, and across the GTA, that means sorting out a tapping problem without a long lead time on the fix.
Choosing a Tap Without Guessing
The takeaways:
- Follow the chip. Through hole means push it forward with a spiral point. Blind hole means pull it back with a spiral flute. Ductile material means form the thread and make no chip at all.
- Never put a spiral point tap in a blind hole. Nothing about your technique will save it.
- Spiral flute taps are weaker in the core than spiral points. Respect that with peck cycles and coolant.
- Form taps need a larger drill than the standard chart. Using the cutting tap chart is a guaranteed breakage.
- Form taps need ductile material. Cast iron and hardened steel crack instead of flowing.
- Drop your thread engagement to 60 or 65 percent when taps are breaking. You lose almost no strength and you cut the torque substantially.
- Breaking taps and not sure why? Contact us and we will work through the application with you.
Breaking taps and not sure why? Contact us and we will work through the application with you.
Frequently Asked Questions
What is the difference between a spiral point tap and a spiral flute tap?
A spiral point tap has straight flutes with an angled cutting face at the tip that pushes chips forward and out the bottom of the hole, making it the right choice for through holes. A spiral flute tap has deep helical flutes that pull chips backward and up out of the hole, which is what a blind hole requires since chips must exit the way the tap entered.
Can I use a spiral point tap in a blind hole?
No. A spiral point tap drives chips ahead of itself, and in a blind hole those chips have nowhere to go. They compact at the bottom, tapping torque climbs sharply, and the tap breaks. Use a spiral flute tap for blind holes, or a form tap if the material is ductile enough to form.
Why do my taps keep breaking?
The two most common causes are the wrong geometry for the hole type and too much thread engagement. Standard tap drill charts target about 75 percent engagement, which requires roughly double the torque of a 65 percent thread for only a small gain in strength. Dropping to 60 or 65 percent engagement, and confirming the tap style matches the hole, resolves most breakage problems without any change to the machine or the program.
