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AN Bolt

Drilling Technique & Cleco's

A large part of the activities when building aircraft is drilling holes and manipulating bolts, which usually go into the drilled holes. A big advantage is that aircraft kits are nowadays pre-punched. This means that all (well almost all) holes are already in the skins or ribs.

These holes are undersized meaning that cleco's can be installed but rivets do not fit. You will have to enlarge the holes during assembly of the separate parts.

Wait with enlarging of the predrilled holes until you are completely done with aligning, deburring and preparing the part. Just before you are ready to close it, is the right time to finalize these holes. Make sure to have enough cleco's on hand.

How to Drill

Aircraft Drill

Creating a hole, drilling, can seem a little daunting at first, don't worry, after a couple of holes you will get very good at it! And of course there will be mistakes along the way. If you are not sure, try on a piece of scrap material first.

Drilling holes

It is good practice to use a #40 (2.5mm - 3/32") drill when doing any assembly, if possible never drill to final hole size until the parts are completed for primer preparation and final assembly. This allows for any movement between parts or adjustment you may want to make that may cause misalignment of the holes during assembly.

Always use a sharp drill bit as:

  • A sharp bit will not wander as easily as a dull one
  • You can drill faster with a sharp bit
  • You don't have to push as hard, risking bending flanges etc

And remember to:

  • Always be careful to drill squarely to the work surface (a drill press works great)
  • Always make sure when drilling through multiple parts or layers that they are clamped together tightly so as not to move around while drilling and that drill shavings do not get in between the parts
  • Always debur holes on both sides of each part before final assembly
  • Work in a clean area as not to scratch the parts (and the Alclad layer)
  • Leave the protective plastic film on as long as possible
  • When drilling large holes to use a step drill when possible
  • Drill slowly as you do not want the drill to 'bite' the material and create square holes

Drill hole location

Holes must be drilled more or less on an exact location. You will notice every deviation from the true line especially on a wing where the stringer and rib are aligned. To help avoid these minor errors use these guide lines:

  • Use a template like a hole spacer or hole finder, check your tool supplier for these items
  • The use of pilot holes punched or drilled prior to final drilling is highly recommended

Drill speed

Rivet spacer from Aircraft Spruce

Drilling speeds depends on the material you are going to drill. Aluminum conducts the heat (from drilling) very good and can be drilled at high speeds (Between 1200 and 4000 RPM, use lower RPMs with larger drills. 5/32 and smaller: drill with 2500 - 4000 RPM. If drilling with bits larger than 5/32" use 1200 - 2500 RPM).

Stainless and normal steel will harden if you drill with higher pressures and RPM (thus more heat). Be sure to use low RPMs and use a cutting coolant to cool the drill. Some basic guidelines are: For normal sheet metal work use 135° drills, With plexiglas it is best to use 90° drills and cobalt (140°) drills for high heat treated steels.

Often used drills

Four common drills (and spares) are usually needed for the majority of holes to be drilled: 3/32" -> #40, 1/8" -> #30, 5/32 #21 and for 3/16" -> #11 drills. I also recommend to obtain two overlapping size step drills (up to at least 1" hole size), they come in handy at times too.


ClecoCleco plier

After drilling a hole, a cleco is temporarily inserted to attach/hold the parts together. Image on the left shows what they look like. You need lots of them!

Cleco's are installed with a special tool: a cleco plier. Clicking the image to the right will show to handle them.

It really pays to have more than one cleco plier, especially when you are working with more than one person on this project as this will keep everybody working instead of waiting on each other for the plier.

Written by EAI.

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