Aircraft Fabrication and Assembly Technician Program

- AFAB 115, Aircraft Structures.

- Chapter 5: Hole Preparation, Drilling and Special Fasteners.

Image summary: A photograph shows a close up view of the nose and cockpit windows of a large white commercial aircraft parked on an airport tarmac.

Purpose:

- The purpose is to provide mechanics with the necessary knowledge and skills, through hands-on practice, to be able to generate accurate holes in various materials.

- The goal is to focus on defect prevention by identifying hole generation defects, their causes, and how to prevent them.

Objectives

Upon completion of this course you will be able to:

- Explain why accurate hole drilling is critical.

- List and describe factors that affect hole accuracy and defect prevention.

- Identify, describe, and use required documentation including Assembly Line Order Operations, Blueprints, Process Work Instructions, Visual Aids, and Process Specifications.

Objectives Continued:

Upon completion of this course you will be able to:

- Describe the principles of accurate hole preparation for different materials, including:

- Identification, verification of accurate selection, use, and care of drilling tools, aids, lubricants, and hole inspection.

- Identification and use of appropriate feeds and speeds for the material or materials being drilled, including aluminum, high-strength steel, composite or graphite, and stack-ups.

- Identification, causes and prevention of defects when generating holes in different materials.

- Describe, perform and inspect the drilling process for close tolerance and non-close tolerance holes.

Objectives Continued:

Upon completion of this course you will be able to:

- Generate holes in shop projects that meet process specifications and drawings for location, angularity, dimensions, and hole quality.

- Use kitted tools and approved feeds and speeds.

- Work in the following materials: aluminum, high-strength steel, composite or graphite, and stack-ups.

- Generate accurate holes with an emphasis on defect prevention.

Hole Generation

- Hole generation is the Technician or Mechanic's most critical job.

- The single largest number of defects or errors are associated with hole generation.

- The ultimate responsibility for accurate hole generation rests with the individual Technician or Mechanic.

- Preventing defects when generating holes.

- Product quality and cost.

Importance of Accurate Hole Generation

The capability of an aircraft to withstand aerodynamic loads depends upon the strength of thousands of fasteners which hold it together. Strength of these fasteners depends, in turn, upon the people who drill and prepare the holes in which those fasteners are installed.

Image summary: A whimsical illustration shows a small person standing on a red step stool, reaching up to place a tiny toy airplane onto a cloud, set against a background image of a large commercial aircraft. This suggests the precision and care required in aviation maintenance.

Hand Drilling Defects

- Defects can include being over or under tolerance.

- Issues with edge distance may occur.

- Holes may be mislocated.

- There may be extra or missing holes.

- Holes can be out of round.

- There can be problems with spacing.

- Angularity may be an issue.

- Burrs and breakouts can occur.

- There may be issues with drill starts.

- Cracks and fractures can develop.

- The finish may be defective.

- Areas may be discolored.

- Contamination may be present.

- And general damage can occur.

What should you know before drilling a hole

- Use a three step approach when performing a process.

- The first step is pre-drill.

- The second step is drill.

- The third step is post drill.

Pre-Drill

- You must be certified for the process of drilling holes on a production part.

- Identify the location of the hole.

- Identify the materials that will be drilled.

- Verify what process specification, aloo, Assembly Process Work Instruction (A.P.W.I), Visual Aid, and work instruction to use, as found in the documentation.

- Determine what drills, tools, and aids to use.

- Determine what lubricants to use.

- Follow the step by step process for drilling an accurate hole.

- Verify the fastener type and diameter.

- Understand how to prevent defects when drilling holes.

Drill

Perform the process per requirements.

- Keep the drill turning throughout the drilling process.

Post Drill

- Inspect your work.

- Review quality requirements and inspection criteria.

- Rework to specification when required.

Factors that Affect Hole Accuracy

All holes must meet the following criteria:

- Location involves edge distance, mis-located holes, missing or extra holes, and spacing.

- Angularity refers to the perpendicular or tangent angle of the centerline of the hole relative to the part surface.

- Dimensions and Tolerances include size, finish, and geometry.

Factors that Affect Hole Accuracy

All holes must meet the following criteria:

- Other Defects

- Discoloration is overheating or color that differs from that of the surrounding material.

- Burrs are excess material that protrudes into or from the hole.

- Drill starts are incomplete holes or indentations into the part surface caused by a drill.

- Delaminations are the separation of composite materials.

Factors that Affect Hole Accuracy-Location

- Check P.S's, Visual Aids, P.W.I's, Blueprints, and ALOOs for specific information on location of holes, such as minimum edge distance, hole spacing, and alignment.

Image summary: Two diagrams illustrate hole location accuracy. The first shows 'Incorrectly located' holes with callouts pointing to a hole that is too close to the edge and another that is misaligned. The second shows 'Correctly located' holes, which are evenly spaced and properly aligned in a grid.

Factors that Affect Hole Accuracy-Angularity

- Holes must be drilled at a 90 degree angle, perpendicular or tangent, to the surface being drilled. Drilling at any angle other than 90 degrees cannot produce round holes.

- Use drill aids to keep the angle straight.

- Ensure the aid is flush with the surface.

- Ensure the aid is the correct size and undamaged.

Image summary: Two photographs of drilling aids are shown. On the left is an Egg Cup tool and on the right is a Drill Block.

Factors that Affect Hole Accuracy-Angularity

- Tangency

- Line Tangent to an Arc.

- Perpendicularity

- Use of Drilling Aids

Image summary: A geometry diagram showing a line segment A B tangent to an arc centered at point C. A vertical dashed line drops from point C to the point of tangency, labeled D.

Image summary: A second geometry diagram showing a curved arc starting at point C and ending at point B. A line segment E D is drawn passing through point B, and a dashed line connects point B to point A.

Factors...Hole Accuracy- Dimensions and Tolerance

- Fasteners determine hole dimensions.

- Engineers determine acceptable tolerance, including plus or minus allowances, in hole dimensions.

- Find information on the above in Blueprints, Visual Aids, P.S's, P.W.I's, and ALOOs.

Review-Hole Accuracy

- First, why is hole accuracy important?

- Second, what is the mechanic's most critical assembly function?

- Third, what are the factors that determine an accurate hole?

- Fourth, name at least three things you can use to help ensure hole accuracy.

- Fifth, hole dimension and type is determined by the blank to be installed.

- Sixth, non-perpendicular drilling can or cannot produce round holes.

- Seventh, what must be done to prevent defects when drilling holes?

Drilling Tools & Equipment

- Drilling Tools, specifically Motors.

- This includes Hand Tools and Accessories.

- This also includes Power Feed Tools and Accessories.

- Drill Bits and Cutters.

- These are categorized by Types.

- They are also categorized by Material.

- Reamers and Core Drills.

- Deburring Tools.

- Drilling Aids.

Drilling Hand Tools

- Hand tools motors.

Image summary: A series of three images showing pneumatic drill motors: a pistol grip hand motor, a right angle drill motor, and a 45 degree drill motor.

- Hand tool accessories.

Image summary: A series of three images showing drilling accessories: angle attachments, an angle drill, and a flexible angle drill.

Power Drilling Tools

- Power Feed Tools

Image summary: A series of images showcasing different power drilling tools including a Right Angle Positive Feed, a Piggy Back Positive Feed, an Inline Positive Feed, an HT4 Peck Motor, a Spacematic Type Motor, and a Nutplate Drill Motor.

Drills

Drills

Image summary: A collection of seven different types of industrial drill bits. These include Twist Drill, Spacematic Drill, Nutplate Drill, Drill-Reamer, Dagger Drill, Threaded Twist Drill, and Double Margin Drill.

Reaming Tools

Reamers and Core Drills are used to obtain proper hole wall finish.

- Close tolerance holes use 125 R H R.

- Non-close tolerance holes use 250 R H R.

Image summary: A set of four images showing different reaming tools: a piloted reamer, an unpiloted reamer, a threaded reamer, and a piloted core drill.

Drilling Tools and Aids

- Defect prevention for oversized holes.

- Follow Work Specifications and directives.

- Check tools for the following:

- Check for tool life and sharpness.

- Sharp drills produce large, uniform chips and small burrs.

- Dull drills produce powdery chips and large burrs.

- Check for bent, discolored, or galled parts.

- Check for protection, such as chipped tools or obviously dropped or misused tools.

- Replace tools as needed.

- Verify the correct drill size.

Drilling Aids

- Drilling Aids.

Image summary: A photograph of a drill stand egg cup and bushing.

- Drill Stand (Egg Cup) and Bushing.

Image summary: A photograph of drill blocks.

- Drill Blocks.

Image summary: A photograph of a fixed nut drill bushing.

- Fixed Nut Drill Bushing.

Image summary: A photograph of a taper-lok bushing.

- Taper-Lok Bushing.

Image summary: A photograph showing multiple drill stops.

- Drill Stops.

Image summary: A photograph of a strap duplicator tool.

- Strap Duplicator.

Review-Drilling and Tools

Identify the following drilling tools:

Image summary: A photo labeled 'a' showing a metal drill bit.

Image summary: A photo labeled 'b' showing a handheld power drill.

Image summary: A photo labeled 'd' showing four different sizes of drill bits with springs.

Image summary: A photo labeled 'e' showing two different types of drill bits, marked as Type A and Type II.

Image summary: A photo labeled 'f' showing a pneumatic drill.

Image summary: A photo labeled 'g' showing an orange handheld power tool.

Image summary: A photo labeled 'h' showing a flexible drill extension cable.

Image summary: A photo labeled 'i' showing a long metal drill bit.

Lubricants/Coolants

- Use P.W.I's and P.S's to verify selection and use of lubricants for the following:

- Drilling Methods, including hand-held and power-fed portable units, as well as stationary drilling equipment.

- Materials to be drilled, including aluminum, titanium, steel, composite or graphite laminate, and stack up or not stack up materials.

Deburring Holes

- Why is it important to deburr fastener holes?

- Burrs cause a stress concentration at the edge of the hole.

- Burrs prevent metal-to-metal contact to mating surfaces.

- The presence of a burr could increase the stress load by 20 to 50 percent.

- A burr acts like a very small crack.

Deburring Requirements

- P.S's contain deburring requirements, as shown below:

Image summary: A text box displays a snippet from a Manufacturing Process Specification document for drilling, reaming, and countersinking fastener holes in metal and composite stack-ups. Section 5.1.5 on Deburring specifies that the hole edge must be surface free from defects, with a maximum deburring depth of zero point zero one zero inches. It also sets speed limits for deburring at 50 surface feet per minute for high speed steel cutters and 100 surface feet per minute for carbide cutters.

Burrs on Exit side

Image summary: A close-up photograph shows a small, dark circular hole in a metallic surface. Two orange arrows point to small protrusions, identified as burrs, on the edges of the hole.

- Burr.

- Burr.

Deburring Tools

Deburring Tools

Image summary: A photograph showing a variety of Weldon 90 degree deburring tools in different sizes.

- Weldon 90 degree deburring tools.

Image summary: A photograph of a Cogsdill Burr-off tool, which is a long, slender metal cylindrical tool.

- Cogsdill Burr-off.

Image summary: A photograph showing two Cogsdill Burraway tools, labeled as Type A and Type B, which are elongated metal deburring instruments.

- Cogsdill Burraway.

Image summary: A photograph of a Chip Chaser tool, featuring a thin metal rod with a specialized cutting tip.

- Chip Chaser.

Image summary: A photograph of an Edge Deburrer, consisting of a handled tool with a curved metal blade.

- Edge Deburrer.

Image summary: A collection of various Rotary Files, showing several differently shaped small cutting heads on shafts.

- Rotary Files.

Hole Inspection

- Visual Inspection—check the following:

- Location alignment.

- Perpendicular hole at a 90 degree angle.

- Cracks.

- Check for burrs, champers, and edge breaks.

- Tool marks, drill starts, and chatter marks.

- Inspection Tools.

- Use a Ball Gage with a micrometer to check hole diameters.

- A Ball Gage can also be used to measure belled holes.

Hole Preparation Specific to Different Materials

- Read and understand the P.S's or P.W.I's to:

- Select or verify proper drilling tools, feeds, and speeds.

- Select the proper cooling method for the material.

- Read and understand work instructions on aloq's or Blueprints including:

- Layout, which includes location and hole diameter.

- Drilling.

- Pre-drill to ensure location.

- Use piloted core drills to enlarge the hole, also known as step-up drilling.

- Ream to final size.

- Finish, which includes deburring, chamfering, edge breaking, and cleaning.

- Inspection.

- Perform inspection as directed in P.S's.

Hole Preparation in all Materials

Vital check, extremely important.

- Refer to A.P.W.I's and P.S for drill or cutter sizes and step drilling sequences.

- Always completely check the first hole.

- This is essential to verify that all tools, equipment, and the process and methods are correct.

- This helps prevent drilling a lot of bad holes.

- Always completely check every tenth hole if there is no sampling plan or S.P.C data requirement.

Part of a Step-up Table

A.P.W.I's lists the step drilling selections.

Image summary: A table titled Recommended Step-Ups for Hand Drilling shows the sequence of drill sizes required to reach a final core drill diameter. For example, to reach a nominal diameter of three eighths of an inch, the recommended sequence starts with a pre-drill of twenty sixty-fourths of an inch, followed by ten sixty-fourths, fifteen sixty-fourths, nineteen sixty-fourths, and finally twenty-three sixty-fourths.

Hole Preparation specific to Aluminum

- Hole preparation in aluminum involves several considerations.

- Since aluminum is a soft metal, fast drill feed rates may be used.

- You should check the tables in program specific P.S's and A.P.W.I's.

- When using a 10 degree countersink on an exterior aluminum skin, be careful not to produce too deep a chamfer or bevel.

- Some drilling in aluminum may require lubricants, provided the temperature at the hole head does not exceed touch temperature, which is plus or minus 130 degrees Fahrenheit.

Use of Drilling Tools and Aids

- Using P.W.I's and P.S's, review accurate selection of tools.

- Using P.W.I's and P.S's, review key points on proper use of tools. For example:

- Drill using firm, steady pressure.

- Drill at a 90 degree angle to the material being drilled.

- Use drill aids to ensure accurate hole angle or depth.

- Once the hole is drilled, remove the drill promptly.

- Keep the drill rotating during withdrawal from the hole to prevent tool marks.

- Use correct feeds and speeds.

Hole Preparation Specific to High-Strength Steel

- Because high strength steel is a very hard metal:

- Special cobalt twist drills are recommended.

- Slow drill feed rates should be used.

- Recommended rates are 500 to 250 R.P.M.

Hole Preparation Specific to Titanium

- Titanium is a high strength and heat resistant material. When drilling holes in titanium:

- Slow drill feed rates should be used, specifically from 500 to 250 R.P.M.

- Use of incorrect feeds and speeds causes discoloration and brittle parts.

- Feed should be applied immediately upon contact with the metal.

- Use a lubricant if the temperature of the hole exceeds touch temperature, which is 130 degrees.

- Special cobalt twist drills are recommended.

Hole Preparation Specific to Graphite / Composite

- Graphite and composites are both lightweight and strong materials, so when drilling holes:

- Special cobalt twist drills are recommended.

- Slow drill feed rates should be used, depending on hole diameter, step drilling, and reaming.

- Composites require specific processes to minimize delamination:

- Always keep the drill rotating before it touches the surface and during withdrawal.

- Always support the material from the backside.

Hole Preparation Specific to Stack Up

- Stack ups are a combination of dissimilar materials such as aluminum, steel, titanium, graphite, or composites. Therefore, when drilling holes:

- Use drill parameters for the hardest material in the stack up, which means the slowest drill speed.

- Use clamps to hold stack ups together.

- Use a back plate when drilling a stack up with composite laminate on the exit side.

- Make sure a constant supply of lubricant reaches the drill point or reamer.

End of Presentation