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<p itemprop="name" style="font-size:14px;font-weight:700">How to Choose The Right Size Countersink Diameter</p>
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<li style="font-size:13px;">A simple rule of thumb is to choose a countersink which is 50% larger than the diameter of the hole</li>
<li style="font-size:13px;">Countersink diameter = 1.5 x bolt size (hole) diameter</li>
<li style="font-size:13px;">Ex: For a 1/4-20 bolt, multiply the diameter (.250) x 1.5 = .375. This equates to a 3/8” countersink</li>
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<p itemprop="name" style="font-size:14px;font-weight:700">How to Choose The Right Size Countersink by Angle</p>
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<li style="font-size:13px;">Angle is based on the screw or bolt type being used</li>
<li style="font-size:13px;"> 82º is most common</li>
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Calculate Countersink Speeds
Countersinking is usually performed at 1/3 the RPM of drilling
Speed Formula
SFM (surface feet per minute) x 3.82 / Countersink Diameter (decimal equiv) / 3
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<th>Material</th>
<th>Surface Feet / Minute (SFM)</th>
<th>Sample Calculations for 1/4” Countersink Speed (RPM)</th>
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<td>Aluminum</td>
<td>200 - 250</td>
<td>(225 (SFM) x 3.82 / .2500) / 3 = 1,146 RPM</td>
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<td>Mild Steel</td>
<td>100 - 110</td>
<td>(115 (SFM) x 3.82 / .2500) / 3 = 586 RPM</td>
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<td>Stainless Steel</td>
<td>30 - 100</td>
<td>(65 (SFM) x 3.82 / .2500) / 3 = 381 RPM</td>
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