--- name: WeldConstraint last_updated: 2026-06-10T23:09:12Z inherits: - Instance - Object type: class memory_category: BaseParts summary: "Connects two BaseParts together such that their relative position and orientation remain the same." --- # Class: WeldConstraint > Connects two [BaseParts](/docs/reference/engine/classes/BasePart.md) together such that their relative > position and orientation remain the same. ## Description **WeldConstraint** connects two [BaseParts](/docs/reference/engine/classes/BasePart.md) and ensures they stay in the same relative position/orientation to each other, meaning that if one part moves, the other moves the same amount. Even if the two parts are not touching, they can be welded together. The most common way to create a weld constraint is by selecting **Weld** through Studio's **Create** menu in the toolbar's **Model** tab. Note that this tool behaves differently depending on how many [BaseParts](/docs/reference/engine/classes/BasePart.md) are selected when the tool is activated: - If no [BaseParts](/docs/reference/engine/classes/BasePart.md) are selected, the next two [BaseParts](/docs/reference/engine/classes/BasePart.md) clicked will be connected by a new [WeldConstraint](/docs/reference/engine/classes/WeldConstraint.md). If the same [BasePart](/docs/reference/engine/classes/BasePart.md) is clicked twice, no constraint will be created. - If one [BasePart](/docs/reference/engine/classes/BasePart.md) is already selected, the next [BasePart](/docs/reference/engine/classes/BasePart.md) clicked will be connected to the selected one with a new [WeldConstraint](/docs/reference/engine/classes/WeldConstraint.md). - If multiple [BaseParts](/docs/reference/engine/classes/BasePart.md) are selected, those which are touching or overlapping will be automatically welded together by new [WeldConstraints](/docs/reference/engine/classes/WeldConstraint.md). #### Repositioning Behavior Moving a welded [BasePart](/docs/reference/engine/classes/BasePart.md) behaves differently depending on whether the part was moved through its [Position](/docs/reference/engine/classes/BasePart.md) or through its [CFrame](/docs/reference/engine/datatypes/CFrame.md). - If a welded part's [Position](/docs/reference/engine/classes/BasePart.md) is updated, that part will move but none of the connected parts will move with it. The weld will recalculate the offset from the other parts based on the moved part's new position. - If a welded part's [CFrame](/docs/reference/engine/datatypes/CFrame.md) is updated, that part will move **and** all of the connected parts will also move, ensuring they maintain the same offset as when the weld was created. ## Properties ### Property: WeldConstraint.Active ```json { "type": "boolean", "access": "ReadWrite", "security": { "read": "None", "write": "None" }, "serialization": { "can_load": false, "can_save": true }, "thread_safety": "ReadSafe", "category": "Behavior", "capabilities": [ "Physics" ], "simulationAccess": true } ``` True if the WeldConstraint is currently active in the world. If the WeldConstraint or one of its parts is not in [Workspace](/docs/reference/engine/classes/Workspace.md) the weld will be inactive. Rigid joints like [Weld](/docs/reference/engine/classes/Weld.md), [Snap](/docs/reference/engine/classes/Snap.md), [WeldConstraint](/docs/reference/engine/classes/WeldConstraint.md), [Motor](/docs/reference/engine/classes/Motor.md), or [Motor6D](/docs/reference/engine/classes/Motor6D.md) may also be disabled due to conflicts with other rigid joints, such as joints between the same two parts or indirect cycles in the weld graph. Joints disabled this way may be re-enabled later when another joint or part is added or removed. Duplicate WeldConstraints do not conflict because WeldConstraints derive their internal CFrames from the relative positions of their parts when they are enabled and all update when [BasePart.Position](/docs/reference/engine/classes/BasePart.md) or [BasePart.Orientation](/docs/reference/engine/classes/BasePart.md) is set on a part. The spanning tree may still disable them if they are redundant or form a cycle. ### Property: WeldConstraint.Enabled ```json { "type": "boolean", "access": "ReadWrite", "security": { "read": "None", "write": "None" }, "serialization": { "can_load": true, "can_save": false }, "thread_safety": "ReadSafe", "category": "Behavior", "capabilities": [ "Physics" ], "simulationAccess": true } ``` The **Enabled** property of a [WeldConstraint](/docs/reference/engine/classes/WeldConstraint.md) sets whether the constraint is active or not. When this property is set to true, if the constraint's [WeldConstraint.Part0](/docs/reference/engine/classes/WeldConstraint.md) and [WeldConstraint.Part1](/docs/reference/engine/classes/WeldConstraint.md) properties are set, then the constraint will ensure that its two connected parts will be locked together. ### Property: WeldConstraint.Part0 ```json { "type": "BasePart", "access": "ReadWrite", "security": { "read": "None", "write": "None" }, "serialization": { "can_load": true, "can_save": false }, "thread_safety": "ReadSafe", "category": "Parts", "capabilities": [ "Physics" ], "simulationAccess": true } ``` The `Part0` and [WeldConstraint.Part1](/docs/reference/engine/classes/WeldConstraint.md) properties of a [WeldConstraint](/docs/reference/engine/classes/WeldConstraint.md) set which two [BasePart](/docs/reference/engine/classes/BasePart.md) the weld connects. As soon as both properties are set and the weld is [WeldConstraint.Enabled](/docs/reference/engine/classes/WeldConstraint.md), the weld will lock the two parts together. If `Part0` or `Part1` are ever set to new parts, then the `WeldConstraint` will instantly link the new part. The old part will no longer be constrained. ```lua local Workspace = game:GetService("Workspace") local partA = Instance.new("Part") local partB = Instance.new("Part") partA.Position = Vector3.new(0, 10, 0) partA.Parent = Workspace partB.Position = Vector3.new(0, 10, 10) partB.Parent = Workspace local weld = Instance.new("WeldConstraint") weld.Part0 = partA weld.Part1 = partB weld.Parent = partA ``` ### Property: WeldConstraint.Part1 ```json { "type": "BasePart", "access": "ReadWrite", "security": { "read": "None", "write": "None" }, "serialization": { "can_load": true, "can_save": false }, "thread_safety": "ReadSafe", "category": "Parts", "capabilities": [ "Physics" ], "simulationAccess": true } ``` The [WeldConstraint.Part0](/docs/reference/engine/classes/WeldConstraint.md) and `Part1` properties of a [WeldConstraint](/docs/reference/engine/classes/WeldConstraint.md) set which two [BasePart](/docs/reference/engine/classes/BasePart.md) the weld connects. As soon as both properties are set and the weld is [WeldConstraint.Enabled](/docs/reference/engine/classes/WeldConstraint.md), the weld will lock the two parts together. If `Part0` or `Part1` are ever set to new parts, then the `WeldConstraint` will instantly link the new part. The old part will no longer be constrained. ```lua local Workspace = game:GetService("Workspace") local partA = Instance.new("Part") local partB = Instance.new("Part") partA.Position = Vector3.new(0, 10, 0) partA.Parent = Workspace partB.Position = Vector3.new(0, 10, 10) partB.Parent = Workspace local weld = Instance.new("WeldConstraint") weld.Part0 = partA weld.Part1 = partB weld.Parent = partA ``` ## Inherited Members ### From [Instance](/docs/reference/engine/classes/Instance.md) - **Property `Archivable`** (`boolean`): Determines if an Instance and its descendants can be cloned using - **Property `archivable`** (`boolean`): *(deprecated, hidden)* - **Property `Capabilities`** (`SecurityCapabilities`): The set of capabilities allowed to be used for scripts inside this - **Property `Name`** (`string`): A non-unique identifier of the Instance. - **Property `Parent`** (`Instance`): Determines the hierarchical parent of the Instance. - **Property `PredictionMode`** (`PredictionMode`): - **Property `RobloxLocked`** (`boolean`): A deprecated property that used to protect CoreGui objects. *(hidden)* - **Property `Sandboxed`** (`boolean`): When enabled, the instance can only access abilities in its `Capabilities` - **Property `UniqueId`** (`UniqueId`): A unique identifier for the instance. - **Method `AddTag(tag: string): ()`**: Applies a tag to the instance. - **Method `children(): Instances`**: Returns an array of the object's children. *(deprecated)* - **Method `ClearAllChildren(): ()`**: This method destroys all of an instance's children. - **Method `Clone(): Instance`**: Create a copy of an instance and all its descendants, ignoring instances - **Method `clone(): Instance`**: *(deprecated)* - **Method `Destroy(): ()`**: Sets the Instance.Parent property to `nil`, locks the - **Method `destroy(): ()`**: *(deprecated)* - **Method `FindFirstAncestor(name: string): Instance?`**: Returns the first ancestor of the Instance whose - **Method `FindFirstAncestorOfClass(className: string): Instance?`**: Returns the first ancestor of the Instance whose - **Method `FindFirstAncestorWhichIsA(className: string): Instance?`**: Returns the first ancestor of the Instance for whom - **Method `FindFirstChild(name: string, recursive?: boolean): Instance?`**: Returns the first child of the Instance found with the given name. - **Method `findFirstChild(name: string, recursive?: boolean): Instance`**: *(deprecated)* - **Method `FindFirstChildOfClass(className: string): Instance?`**: Returns the first child of the Instance whose - **Method `FindFirstChildWhichIsA(className: string, recursive?: boolean): Instance?`**: Returns the first child of the Instance for whom - **Method `FindFirstDescendant(name: string): Instance?`**: Returns the first descendant found with the given Instance.Name. - **Method `GetActor(): Actor?`**: Returns the Actor associated with the Instance, if any. - **Method `GetAttribute(attribute: string): Variant`**: Returns the value which has been assigned to the given attribute name. - **Method `GetAttributeChangedSignal(attribute: string): RBXScriptSignal`**: Returns an event that fires when the given attribute changes. - **Method `GetAttributes(): Dictionary`**: Returns a dictionary of the instance's attributes. - **Method `GetChildren(): Instances`**: Returns an array containing all of the instance's children. - **Method `getChildren(): Instances`**: *(deprecated)* - **Method `GetDebugId(scopeLength?: int): string`**: Returns a coded string of the debug ID used internally by Roblox. - **Method `GetDescendants(): Instances`**: Returns an array containing all of the descendants of the instance. - **Method `GetFullName(): string`**: Returns a string describing the instance's ancestry. - **Method `GetStyled(name: string, selector: string?): Variant`**: Returns the styled or explicitly modified value of the specified property, - **Method `GetStyledPropertyChangedSignal(property: string): RBXScriptSignal`**: - **Method `GetTags(): Array`**: Gets an array of all tags applied to the instance. - **Method `HasTag(tag: string): boolean`**: Check whether the instance has a given tag. - **Method `IsAncestorOf(descendant: Instance): boolean`**: Returns true if an Instance is an ancestor of the given - **Method `IsDescendantOf(ancestor: Instance): boolean`**: Returns `true` if an Instance is a descendant of the given - **Method `isDescendantOf(ancestor: Instance): boolean`**: *(deprecated)* - **Method `IsPropertyModified(property: string): boolean`**: Returns `true` if the value stored in the specified property is not equal - **Method `QueryDescendants(selector: string): Instances`**: - **Method `Remove(): ()`**: Sets the object's `Parent` to `nil`, and does the same for all its *(deprecated)* - **Method `remove(): ()`**: *(deprecated)* - **Method `RemoveTag(tag: string): ()`**: Removes a tag from the instance. - **Method `ResetPropertyToDefault(property: string): ()`**: Resets a property to its default value. - **Method `SetAttribute(attribute: string, value: Variant): ()`**: Sets the attribute with the given name to the given value. - **Method `WaitForChild(childName: string, timeOut: double): Instance`**: Returns the child of the Instance with the given name. If the - **Event `AncestryChanged`**: Fires when the Instance.Parent property of this object or one of - **Event `AttributeChanged`**: Fires whenever an attribute is changed on the Instance. - **Event `ChildAdded`**: Fires after an object is parented to this Instance. - **Event `childAdded`**: *(deprecated)* - **Event `ChildRemoved`**: Fires after a child is removed from this Instance. - **Event `DescendantAdded`**: Fires after a descendant is added to the Instance. - **Event `DescendantRemoving`**: Fires immediately before a descendant of the Instance is removed. - **Event `Destroying`**: Fires immediately before (or is deferred until after) the instance is - **Event `StyledPropertiesChanged`**: Fires whenever any style property is changed on the instance, including ### From [Object](/docs/reference/engine/classes/Object.md) - **Property `ClassName`** (`string`): A read-only string representing the class this Object belongs to. - **Property `className`** (`string`): *(deprecated)* - **Method `GetPropertyChangedSignal(property: string): RBXScriptSignal`**: Get an event that fires when a given property of the object changes. - **Method `IsA(className: string): boolean`**: Returns true if an object's class matches or inherits from a given class. - **Method `isA(className: string): boolean`**: *(deprecated)* - **Event `Changed`**: Fires immediately after a property of the object changes, with some