Degrees of Freedom in space
A work-piece free in space can move in an infinite number of direction. For analysis, this motion can be broken down into 12 directional movements or "degrees of freedom". All 12 degrees of freedom must be restricted to ensure proper referencing of a work-piece.
As shown in Fig 1, the 12 degrees of freedom all relate to the central axis of the work-piece. Notice the 6 axial degrees and 6 radial degrees of freedom. The axial degrees of freedom permit straight-line movement in both directions along the 3 principle axis shown as X, Y & Z. The radial degrees of freedom permit rotational movement, in both clockwise & counterclockwise radial directions around the same three axis.
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| Fig 1: The twelve degrees of freedom |
Degrees of Freedom of a Rigid Body
Consider the rigid body shown below which is situated in space. It can have 12 degrees of freedom.
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| Fig 2 : A rigid body on a space |
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| _Fig 3: A rigid body on a plane |
(2) X-
(3) Y+
(4) Y-
(5) ZCW
(6) ZCCW
Now try to understand 3-2-1 principle of fixturing. The purpose of Fixturing is
- Resting - Stability
- Locating - W.r.t datum
- Orienting - W.r.t secondary datum
- Clamping - Ensuring above not disturbed.
Primary Locators
First the three locators or supports are placed under the work-piece. It will be positioned on the primary locating surface also known as datum. It will restrict 5 degrees of freedom.
- Axial Movement downward along Z- axis ( No.6 in Fig 2 )
- Rotation about the X & Y axis ( No. 7, 8, 9 & 10 in Fig 2 )
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| Primary Locators |
Secondary Locators
The next two locators are normally placed on the secondary surface, restricting an additional 3 degrees of freedom.
- Axial movement along the Y+ axis ( No.3 in Fig 2 )
- Rotation about the Z axis ( No. 11 & 12 in Fig 2 )
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| Secondary Locators |
Tertiary Locator
This locator is positioned at the end of the part. It restricts 1 degrees of freedom
- Axial movement along the X axis ( No.2 in Fig 2 )
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| Tertiary Locator |
All these 6 locators will restrict a total of 9 degrees of freedom. The remaining 3 degrees of freedom ( 1, 4 & 5 ) will be restricted by the clamps.
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ReplyDeleteHi,
ReplyDeleteIt's a great explanation, could you please share any video references for DOF.
Also 321 principle
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