6.4: Lap Joints
- Page ID
- 67193
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\dsum}{\displaystyle\sum\limits} \)
\( \newcommand{\dint}{\displaystyle\int\limits} \)
\( \newcommand{\dlim}{\displaystyle\lim\limits} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\(\newcommand{\longvect}{\overrightarrow}\)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\ket}[1]{\left| #1 \right>}\)
\(\newcommand{\bra}[1]{\left< #1 \right|}\)
\(\newcommand{\braket}[2]{\left< #1 \vphantom{#2} \right| \left. #2 \vphantom{#1} \right>}\)
\(\newcommand{\braopket}[3]{\left< #1 \vphantom{#2}\vphantom{#3} \right| #2 \vphantom{#1}\vphantom{#3} \left| #3 \vphantom{#1}\vphantom{#2} \right>}\)
\(\newcommand{\qmvec}[1]{\mathbf{\vec{#1}}}\)
\(\newcommand{\op}[1]{\hat{\mathbf{#1}}}\)
\(\newcommand{\expect}[1]{\langle #1 \rangle}\)
\(\newcommand{\dfn}[1]{\emph{\textbf{#1}}}\)
6.4 Half-Lap Joints
The corners of the picture frame are joined using half-lap joints.
A half-lap joint is made by removing approximately half the thickness from each of two overlapping pieces.
When the parts are assembled, the combined joint returns to approximately the original thickness of the stock.
Learning Outcomes
After completing this section, you should be able to:
- Explain how a half-lap joint is constructed.
- Identify the shoulder and lap surfaces.
- Explain why the joint is stronger than a simple end-grain butt joint.
- Lay out a half-lap based on the actual stock dimensions.
- Evaluate the fit of a completed joint.
How the Joint Works
Imagine two pieces crossing each other.
If they were simply placed on top of one another, the joint would be twice as thick as either piece.
Instead, remove approximately half the thickness from each piece.
The remaining sections overlap.
The two parts then fit together at approximately the same overall thickness as the original material.
Figure: Half-Lap Joint
Why Use a Half-Lap?
A half-lap joint provides:
- a large glue surface;
- mechanical support from the overlapping parts;
- good alignment;
- a relatively simple construction process; and
- a visible expression of how the frame is joined.
Compared with simply gluing the end of one board to the face or edge of another, the half-lap creates much more useful glue area.
Layout From the Actual Stock
Do not assume every board is exactly the nominal thickness.
Measure the actual thickness of the frame stock before laying out the joint.
The goal is for each lap to remove approximately half the thickness.
For example, if the stock is 3/4 inch thick, each lap will be approximately:
3/8 inch deep
But if the actual stock is slightly thinner or thicker, the joint should be adjusted accordingly.
Joint Length
The length of the lap generally corresponds to the width of the mating frame member.
Transfer the actual part width carefully.
Accurate shoulders are important because they establish the location of the joint.
A Good Half-Lap Fit
A properly fitted half-lap should:
- seat fully;
- have shoulders that meet cleanly;
- lie reasonably flush across the faces;
- remain square; and
- not require excessive force to assemble.
A joint that must be hammered together is too tight.
A joint with large gaps is too loose.
The goal is a close, controlled fit.
Test Before Gluing
Dry-fit the frame before applying glue.
Check:
- all four joints;
- overall squareness;
- whether the faces sit flush;
- whether the frame lies flat;
- and whether the opening is the correct size for what you are framing.
Correct problems before glue is introduced.
Glue adds time pressure.
The best time to solve a joint-fitting problem is during the dry fit.
Check Yourself
- How much thickness is approximately removed from each member of a half-lap?
- Why does the completed joint return to approximately the original stock thickness?
- Why should the actual stock thickness be measured before machining the lap?
- What determines the length of the lap?
- What should you check during a dry fit?
- Why is a half-lap stronger than a simple end-grain butt joint?