Tech Tip: How to Check Bracket Strength Using Custom Tools

In this example, an L-bracket holds a 300-newton load. That is about 30 kilograms hanging off the end. Is 6 mm aluminum thick enough?

This Tech Tip uses a custom feature and a custom table that does the math for you. Enter the size, material, and load. The bracket is built, and a table indicates whether it holds. If it does not hold, the number turns red.

What’s a Custom Table?

A custom table is one that you or your company writes instead. It reads the Part Studio every time the model rebuilds, pulls out the important numbers, and shows them in a panel on the right side of the screen.

Two pieces work together. In this example, a custom feature called Load rated bracket builds the part and determines how strong it is. The Bracket load report is a custom table that provides feature data. Together, they answer one question: will this bracket hold?

No code is required to use the feature. Enter information into the feature, and you and anyone else can read it and the table moving forward.

In addition, the FeatureScript MCP Server from Onshape Labs makes it possible to create a custom table with little coding knowledge using a text-to-code-CAD workflow.

This Tech Tip will go over how the custom feature works, and then how to create your own custom table. The FeatureScript code referenced in this Tech Tip can be found in this Onshape Document.

Add the Bracket Feature to Your Part Studio

Open a Part Studio. Click the custom feature button in the toolbar, then pick Load rated bracket. The feature dialog opens on the left.

Click the checkmark. The bracket appears positioned the way it would be used.

Look for the Warning Triangle

If a yellow triangle displays next to the feature name, the bracket may be built correctly, but something requires your attention. Hover over the triangle for an explanation. For example, in the screenshot above, a design warning is indicated.

A warning is not an error. The part is still there. The tool is telling you the design is weak, or a hole is too close to an edge. You decide what to do about it.

A red error is different. That means the numbers do not make sense, like a fillet that is bigger than the bracket. Correct the error by entering a valid number and the warning disappears.

Understand Safety Factor

Safety factor is a crucial number. It is a measure of how much stronger a part is than it needs to be for its expected load.

Type the number you want in the Required safety factor box. If the bracket does not reach it, the warning triangle is displayed.

Custom Tables: Open the Bracket Load Report Table

The feature provides a part. The table provides the numbers behind it.

Look on the right edge of the Part Studio for the table tab and click it. Choose the Bracket load report. A panel opens with checkboxes, and the table shows up next to it.

Pick the Columns You Want

The table can show 17 columns in five groups, and each group has its own checkbox. Uncheck what is not required. Checking a box adds those columns right away. The part is not changed, only what is visible.

Two resultant columns are important to note:

• Governing stress: The highest stress anywhere in the bracket, in MPa. MPa is a unit for stress, like PSI. Higher means the metal is working harder.
• Governed at: Determines where that stress is located. Root means the inside corner. First hole means the hole nearest the corner. Since this is the spot that cracks first, it is where to add metal.

Read the Red Cells

Red means that the number failed a check. Hover your mouse over a red cell, and a tooltip tells you exactly why.

There are two checks:

• Safety factor: Turns red when it drops below the target you set in the panel. This is under your control.
• Min edge dist: Turns red when a hole sits too close to the edge. The shop rule is that the hole center should be at least 1.5 times the hole diameter away from the edge. Get closer, and the metal can tear out around the bolt. This is not under your control. It is a rule, not a preference.

On the example bracket, the holes are 6.76 mm across and sit 6.76 mm from the edge. The rule wants 10.13 mm. So that cell is red, and the tooltip displays “Below the 1.5 x diameter guideline of 10.13 mm.”

Compare Several Brackets at Once

Add the Load rated bracket feature more than once in the same Part Studio, with different numbers each time. The table provides one row per bracket, with no extra work.

Here are three brackets, all 127 mm by 76 mm, with different thicknesses, metals, and loads:

Two rows are red, so two of these three designs fail. Read across and you can see why:

• The thin steel one fails even at the same 300 N load, because 4.76 mm is too thin.
• The thick aluminum one carries double the load and still passes. It is also lighter than the steel one.
• The stainless one is the worst, at 0.72. Notice it says Root instead of First hole. It has no holes, so the weak spot moved to the inside corner.

Set Rows to include to Below target only, and the table drops to just the two failing rows. On a big list that turns a wall of numbers into a short to-do list.

One thing to expect: Every bracket is built at the same spot, so they sit inside one another in the 3D view. That looks odd, but it does not break anything. The table still reads all of them. If you want them lined up side by side, move each one with a Transform feature.

Learn More About Custom Features and Tables

Want to know how this is put together, or build your own? Start with How to Build a Custom Table Using FeatureScript. The Learning Center also has Custom Tables in FeatureScript Fundamentals as a full course.

To put a table like this on a drawing, see the Tech Tip: How to Insert Custom Tables into Drawings.