Countersinking Guide

Tools Needed:

Drill Press and Bits Cutting Oil

Countersunk bolts are bolts that are recessed with their heads flush to the surface.  This is done for several reasons, the most common being to gain clearance or for aerodynamics.  You have probably seen countersunk fasteners in wood decking or the runners on drawers.  The steps are basically the same no matter what material you are assembling.  

Whether it's for a HID mounting bracket or for an end-plate, at some point you will probably need to countersink/spotface a fastener. Using these steps, you can produce clean professional results.  No matter how tempting it is, do not partially drill a hole with a larger bit to countersink a bolt head.  Take the time and obtain the correct countersink bit.  When purchasing a bit you need to know two numbers.  The diameter of the fasteners you are working with and how many degrees the bolt head is.  82°, 90°, and 100° are all common.  NAS fasteners used in aviation are 100 degrees while most common machine screws are 90 degrees.  Make sure you purchase the correct taper.  A microstop cage like the one shown below is not needed, but it makes production faster and the results cleaner.

Countersink Microstop Cage

Start by drilling the correct sized hole in your stock.  If you are working with metal use a little cutting oil to speed drilling and preserve your bits.  For most thicknesses of stainless steel and inconel keep the drill speeds under 300rpm.  For other steels 500-800rpm, and aluminum around 1000rpm.  

Drill hole for your bolt

Now comes the countersinking.  If you are using a microstop pre-set the depth so that when tightened the bolt head is flush with the stock.  Drilling a few practice holes will assure the final piece is correctly drilled.  The countersink bit will cut the correct taper and debur the stock for you.  

Use the microstop to cut your countersink

Below you can see the results.  This photo is under magnification and you can see there was some slight chattering.  More oil will help prevent this.  If you still experience low speed chatter, reface the hole at a higher speed.  If you have a high speed chatter, reface at a lower speed.  If the above methods do not work you may need to try a different countersink.  A tulip or a high speed seven flute bit would be an option for soft metals.  

Countersunk hole de-burred

Below is a close up of the installed fastener.  If correctly installed the head of the bolt will sit completely flush.  If it does not, you need to reset your stop and reface the hole.  If it is recessed, you set the stop too deep.

Flush bolt

In this guide I used Hi-Lok titanium bolts which have no grip on their heads.  They are tightened from the bottom with a hex socket and wrench as shown below.  These bolts produce a very smooth unique finish.  In addition to looks, they also offer the weight savings of titanium.  When used correctly on brakes, rims, or other rotating assemblies, the pounds saved translate into horsepower.

NAS titanium bolt
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