Thread Engagement Calculator —
How Deep to Tap, and the Tap Drill for Any Percent of Thread

Tap Drill for Any Percent of Thread

A tap does not have to cut the full thread height. The tap drill sets how much of the theoretical sharp-V height the tap engages, and the standard shop practice is about 75%, which gives nearly the full strength of a 100% thread at roughly half the tapping torque. Going below 60% is common in tough materials like Inconel and titanium to save taps; going above 80% rarely adds strength and breaks taps.[1]

The calculator then picks the closest standard drill from the drill size chart and reports the percent of thread that drill actually gives, so you can see whether it landed slightly above or below your target.

Thread Class Sets the Hole Limits, Not the Drill Chart

A percent of thread is a shop convenience. The drawing does not say 75%; it says 1/4-20 UNC-2B, and ASME B1.1 turns that class into limits a gage can check. Classes 1B, 2B and 3B are the internal thread classes (A is external). What the class changes most is the pitch diameter tolerance: 3B is 0.75 times the 2B band and 1B is 1.5 times it. None of the B classes carries an allowance; only external 1A and 2A do. 2B is the default on nearly every commercial and aerospace drawing, 3B appears where a close fit is needed, and 1B is rare.[5]

The minor diameter is where the tap drill meets the standard, and it behaves in a way most people do not expect. The minimum minor diameter is identical for all three classes: the basic minor diameter, D − 1.0825 P. Only the maximum differs, and 1B and 2B share the same maximum; 3B is a little tighter. B1.1 gives the tolerance by formula:

The calculator applies these formulas to the selected thread (the values were checked against B1.1 Table 2 for eight sizes from #10-24 to 1-8) and converts the limits into a percent-of-thread band. Because the Machinery's Handbook percent uses D − 1.299 P as 100%, the fixed minimum minor diameter works out to 83% thread for every class. Drawings do not call out a percent of thread; they call out the class, and the percent is the shop's tap-drill choice. A shop that drills for "100% thread" has drilled below the B1.1 minimum, and the GO plain plug gage will not enter. Nothing above about 83% is a legal Unified internal thread.

Read against the drill chart: #9 (0.1960) sits on the minimum and gives 83%; #7 (0.2010) gives 75% and lands in the middle of the band; 13/64 (0.2031) gives 72%; #5 (0.2055) gives 68% and is still inside 2B; #4 (0.2090) is above the maximum for every class and produces a thread the NO GO plain plug will enter. The drilled hole also tends to run a little larger than the drill's nominal size, so a drill near the top of the band is the risky end, not the bottom.

Tap H-limits and thread class: how they connect

The class band for the pitch diameter belongs to the hole. The tap has its own pitch diameter, and tap makers state it as an H-limit under ASME B94.9: H1 means the tap's pitch diameter is basic to basic plus 0.0005 in, H2 is plus 0.0005 to plus 0.0010, and each step adds another 0.0005 in (L-limits run below basic). Sizes above 1 in step in 0.001 in. A GH prefix means a ground thread.[6]

So an H-limit is not a class and a class is not an H-limit; they are two numbers measured on two different parts. The connection is that the tap's pitch diameter has to sit inside the hole's class band with room on both sides: room above, because a tap under normal conditions cuts a hole slightly larger than its own measured size, and room below, because a tap loses size as it wears. Tap catalogs resolve this by recommending a limit about 40% of the way into the class band. For 1/4-20 that is H5 for 2B (the band is 0.0049 wide, and H5 puts the tap at 0.2195 to 0.2200) and H3 for 3B (band 0.0036, tap at 0.2185 to 0.2190). Every H-limit from H1 to H9 lies inside the 2B band for this size, which is why an H3 tap can legitimately cut 2B holes; it simply has less margin for wear before the GO thread gage stops entering.[6]

Two consequences follow. First, a tap cannot produce a class of thread; it produces a hole that the gages accept or reject, and the class is only met when both members are within their limits. Second, changing the class on a drawing from 2B to 3B changes the tap the shop reaches for and the gages it sets, but not the drill: the minor diameter band moves by a few ten-thousandths at most. The calculator prints the pitch diameter band for the selected class and the H-limits that fit inside it; the choice within that range belongs to the shop, its material and its tap wear history.

How Deep to Tap: Length of Engagement

A bolt in a tapped hole can fail three ways: the bolt breaks in tension, the bolt's external threads shear off, or the internal threads in the hole strip out. Good design makes the bolt break first, because a broken bolt is obvious and replaceable while stripped threads in a housing are neither. The engagement length that guarantees it comes from comparing the thread shear areas with the bolt's tensile stress area.[2]

Shear strength is taken as 0.6 times tensile strength for every material in the tables, a common engineering approximation. The calculator uses basic thread dimensions; class 2A/2B allowances make the real shear areas a few percent smaller, so treat the result as a design minimum and round up to the next practical depth. It also assumes uniform load sharing along the engaged threads and does not cover thread inserts, fatigue, or fine-pitch threads in soft materials, which strip more easily than the areas suggest.

Behind every engagement answer are two loads, and the calculator prints both: the fastener's tensile capacity, A_t × UTS, and the load at which the engaged threads shear, thread shear area × length × 0.6 × UTS of the weaker member. The method sets the thread capacity at 1.2 times the bolt (twice the tensile area at 0.6 times the strength), so at the computed depth the bolt breaks first with a 20% margin. If you tap shallower than the computed depth, the threads become the weak link and the tool tells you at what load.

When the method asks for three diameters or more. Put a 1/2-13 alloy socket head cap screw into Delrin and the correction factor passes 12, so the formula returns about 3.5 in of thread, seven diameters. That number is arithmetic, not advice. Load is not shared evenly along a long engagement; the first few threads carry most of it, which is why B1.1 caps its own tolerance formulas at 1.5 diameters and why no real design taps seven diameters deep. Above two diameters the calculator still shows the depth, marked reference only, and says what is true: in that material the fastener cannot be made the weak link, so use a threaded insert (helical or solid), a larger or coarser thread, or a lower-strength fastener. It also shows the load the threads would carry at a practical 1.5 diameters so the shortfall is visible. Across every thread and material in the tables, only the two plastics trip this ceiling; every metal, even 6061 with a 170 ksi socket head cap screw, comes in under two diameters.

Worked Example: 1/4-20 Grade 5 Bolt in 6061-T6

The result lands on the rule of thumb machinists already use, one diameter of engagement for a steel bolt in aluminum, and shows where it comes from. The same bolt in a 4140 pre-hard housing needs only the 0.14 in equal-material value, and in Delrin the multiplier climbs past 8, which is why plastic parts get threaded inserts.

Threads in the Calculator

Basic major diameters and pitches per ASME B1.1 (unified inch) and ISO 261 / ASME B1.13M (metric). The calculator reads this table when the page loads.[3]

Material Strengths Used

Fastener strengths are the specification minimum tensile strengths; tapped-material strengths are the typical values stated in this library's material guides, so the calculator agrees with the articles. Shear strength is taken as 0.6 × tensile throughout.[4]