Tapping Drill Size Problem #2: Form Taps vs Cut Taps In deeper holes, we run the most risk of breaking a tap and in sheet metal we need the strongest threads because there is much thread depth for holding power. For hard or tough materials, a lower percentage can really help tap life and those materials have lots of strength even at the lower percentages. The material matters because we can afford higher thread engagement in softer materials and we actually need it to increase thread strength. Try this approach, which is based on material and type of work: So what’s the right way to choose a thread percentage and how do we get from there to a proper drill size? General Guidelines for Choosing Thread Percentage Is it any wonder the darn taps give up and break on us after that kind of abuse? When We accept the 75% percent of thread the average drill chart hands us, we’re subjecting our poor taps to a huge amount of additional torque for very little extra strength! What stands out is the relationship between the torque required to tap (solid black line) and the strength of the tapped hole (dashed line). Red marks the typical 75% percent of thread drill recommendation. Redmarks the typical 75% percent of thread drill recommendation. Let’s take a closer look at the relationship between torque and thread strength based on thread percentage: That’s why these lower percentage are a better idea for your tap’s tool life! Thread Percentage vs Strength Chart Companies like Guhring actually recommend 60% to 70% percentage of thread for most applications because it significantly lowers the torque force required to tap without giving up much thread strength. For most applications, you can get longer tap tool life and lower likelihood of breaking a tap off in the hole by using a lower percent of thread. Most tap drill charts call out only one tap drill size, and that will produce an approximate 75 percent thread. These specifications also provide a minimum value that varies from approximately 53% to 75%, depending upon the diameter and pitch of thread.
For example, American National and Unified thread specifications provide for a maximum of 83 1/3% thread. But is it okay to have less than 100% threads? In fact, many standards bodies insist on it. Now we all know what happens when we break a tap, right? Sailors would go running out of the shop if they heard the language we use in that case. Getting 95% of the strength with 1/3 the force on the tap means you’re dramatically less likely to break the tap off in the hole. But here is the real kicker: That 100% thread requires 3 times the power to tap! Why do you care? because it is the power to tap that breaks taps, for one thing. Kennametal says a 100% thread is only 5% stronger than 75% thread. In fact, they contribute surprisingly little strength. They’re pretty delicate anyway and will wear off quickly. Now suppose you ran a twist drill down the hole and shaved off some of the peaks. Imagine the fully formed internal thread. To know which drill bit size is right for your particular tapping application, you need to consider what thread percentage you want when you’re done tapping. How can that be? Well, it’s actually pretty simple. Anytime you see just one size being recommended, you know you can do better–one size does not fit all when it comes to tapping. The same is true of the size recommended on the tap and even the recommended size from the tap manufacturer. Moreover, the size recommended in most drill bit size charts is very often not the best size to use. Let’s start with an important fact–there isn’t just one drill bit size to use for a given tap size.
Newsflash: There Isn’t Just One Drill Bit Size for a Tap But in this age of computers and the Internet, you can do better. If it has information on which twist drill size to use for tapping, so much the better. Make no mistake: having a good drill bit size chart (sometimes called a “drill index”) is a very handy thing indeed to have around. What’s better than a drill bit size chart?
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#Printable metric tap drill chart pdf#
If you just want a simple tap drill chart in Metric and Imperial sizes, or a pdf you can print and post in your workshop, scroll on down.īut, before you do, wouldn’t you like to hear about something that’s much better in every way? Something that’ll make it much less likely you’ll break a tap? Something a little more modern than a simple chart?
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