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How to Save $100K in Your Inspection Fixture Clamping

Virtual Clamping Digital Engineering Tutorial


To begin, open a new project in GOM Inspect software and add in the project data by dragging and dropping the CAD file into the workspace and selecting "New Part." When you pull in the CAD file, remember to click reset zoom, so the CAD data enters and remains in the frame. Next, bring in the mesh, add it to the part, and give it a pre-alignment. Then, add the nominal geometries to the part.

Next, add the deformation model to the part. A deformation model is what makes virtual clamping possible. You'll notice it looks similar to the mesh structure, but it differs in that it's uniform all the way across. Unlike the CAD model, it includes a lot of information, including volume, mass density, the Young's modulus, and the Poisson's ratio. This information comes from a volumetric CAD model that you input into GOM Inspect software. The software then turns it into the deformation model. The deformation model is essentially a digital representation of a physical piece of sheet metal. Virtual clamping is possible because the deformation model has a bend, flow, and strain, just like a real piece of sheet metal would.

First, rename the points "RPS Z" and "RPS Y," for example. You can rename several points at once by highlighting them and selecting "Rename Element." This function allows you to rename multiple points the same name, while also automatically numbering them to keep them in order. If you want to rename a single point, you can just highlight it and select the hotkey F2.

Once all the points are named, give them measuring principles and checks. Hit CTRL and right-click to bring up the I-Inspect window. Highlight all the Z-points and give them a measuring principle of "Intersection with Mesh." Also, check them in the normal direction. Since the X-point and the two Y-points are not surface points but are edge points, give them the measurement principle of touchpoint edge, and instead of checking them in the normal direction, check them in the trimming direction. Since the clamping point is a surface point, give it the measuring principle "Intersection with Mesh," and check it in the normal direction.

Next, set an RPS by highlighting all points from the RPS, and bring them all in. Then, with all points set to zero deviation, all deviations are allocated to the designated clamping point. The result represents what the part would look like clamped in the RPS position.

Go to "Workspace Selection," then "Inspection," then navigate to "Create Surface Comparison on CAD" to pick a surface comparison on CAD and select OK. You'll then see the deviation displayed throughout the part. The next step is to clamp the other section of the part to even out the surface. That's where virtual clamping comes in. While GOM Inspect software has always provided the ability to put a part in RPS, the ability to clamp it in RPS plus other inspection clamps is a new feature. The first step is to add a clamping point. If you're using GOM Inspect Professional software or ATOS Professional software, Virtual Clamping is an addon. To access the Virtual Clamping function, go to "Operations," "Mesh," and select "Virtual Camping." There are two available selections: "Gravity Compensation" and "By Clamping Points." This tutorial discusses "By Claiming Points" first.

To utilize Virtual Clamping with the "By Clamping Points" function, choose that selection, then select the actual mesh, and then the deformation model. Next, bring in the holding points by selecting "Insert elements from selected alignments," which are the RPS points. Since each point only controls one direction, and it's not like a cylinder that controls x, y, z, you must change the direction of the Z points to normal instead of letting them have their respective axis. Then, change the edge points to trimming.

Next, add the clamping point by clicking on it and then click on the button in the bottom left corner of the "Clamping points" section. A clamping point populates in the "Observation point" column that represents the XYZ value of an adjacent point. This point would be a helping point if you had a physical clamp in your CMM, and you were unable to get to the clamping point. You can choose to keep the direction set to normal, or you could change it to the individual direction. In the dropdown menu, there are two choices for the "Movement-in-plane" column: allow or block. If you are working with a very rigid part, such as a welded part, you would need to choose to block the movement-in-plane because you shouldn't have movement in a welded part. If it's not, then you should allow movement-in-plane.

Next, set expert parameters by inputting the number of iterations to allow in the "Expert parameters" section. The variable you enter in the "Iteration tolerance" field indicates when it will stop iterating. When it reaches below that point, it stops iterating. If you want the point clamped to zero, enter 0.001. In the "Clamping mode" field, there is an option for consecutive or simultaneous clamping. If you choose "Consecutively," it clamps in consecutive order of the holding points, or, if you choose "Simultaneously," it clamps everything at once. Then, hit "Create and Close."

You will then see the first iteration beginning at the bottom of the screen, as well as the current deviation. In this process, it's virtually clamping, while also making another CAD model separate from the original CAD model. You will see the difference between each generated CAD model before and after clamping. As it completes the process, it creates a CAD model to compare with the newly created clamped mesh. You will now see the virtually clamped part, with each point successfully clamped at zero deviation.

An important feature to note is the "Check Clamping force" feature. This feature simulates the human feedback that you get from physically clamping something tough that would require extra pressure to push down on, such as a piece of metal or plastic that's warped, or has an issue that needs attention. Since haptic feedback is not possible in a virtual setting, GOM Inspect software allows you to visualize the clamping force in Newtons. The table on the clamping point labeled "cf" contains a number that represents how many Newtons it took to clamp the part at that point. You can put tolerances on the clamping points and observe the force that it takes to clamp the part and relay any alarming observations to your engineers so they can check the process for errors.

Current State-of-the-Art is the Qualification of Parts in Fixtures

The current state-of-the-art is the qualification of parts in fixtures. The virtual clamping feature was developed for toolmakers who are trying to produce a part with as little warpage as possible, meaning just coming out of the press, the dye stamp, or otherwise. Warpage with an acceptable range of force, such as Newton clamping force, is uncritical and is eliminated during subsequent assemblies like welding or bracing and is pulled into position. Still, the part manufacturer only wants to qualify his or her parts, not the fixture clamps.

Disadvantages of Fixture Clamps

The problem with fixture clamps is the high cost of construction, production, calibration, and storage. The high investment of time comes in with multiple measuring cycles for different clamping situations and iterative adjustments of the clamping points. If they're bent, you must fix them. Also, there is a certain amount of conflict change management involved whenever someone tries to change a part, including by modifying clamping points or when modifying CAD states. A new revision level part comes out, causing you to change the clamp to meet the new iteration of that part and adjust the CAD model. Other disadvantages of fixture clamps include limited accuracy, non-perfect supporting points, limited repeatability, user influence, person to person interference, different clamping orders, and unidentified friction at the clamping points. Another disadvantage of using a fixture clamp is the limited accessibility of hidden measuring points that get covered by an inspection clamp. Also, since the direct clamping point where the inspection clamp touches the part is often a datum, you must take an adjacent point to the side of that datum instead of the datum.

Virtual Clamping Workflow

The workflow beings with the CAD model, then you scan the part, compare the mesh in a normal CAD comparison, and collect the deviation at the clamp points. Then you add in the deformation model, which you will use for virtual clamping. Then, create the displacement field, make another CAD model comparison, and then you will have the assembled and final quality version of that part. This workflow allows you to scan the part just once, then give the toolmaker what he needs and give final quality what they need all with one part.

The Benefits of Virtual Clamping Using a Deformation Model

The deformation model is essentially the computational basis for deformations and forces. It's what allows the digital part to bend and flex in a virtual world as a real part would. It describes the deformation behavior during clamping and under the influences of gravity. The properties are the discretized geometry, Young's modulus, Poisson's ratio, and density. It requires a volumetric CAD model (this prerequisite also applies to sheet metal or a CAD model that has a thickness). Once you input those components into GOM Inspect software, you get the deformation model. When you compare a virtually clamped part to a mechanically clamped part, you'll see they're nearly identical. However, with Virtual Clamping, a variable indicates precisely how difficult it was to clamp, rather than a vague explanation from a worker who experienced the difficulty. The added benefit is that you can record and track that variable to observe how the part is riding out.

Unclamped Measuring Using the Universal Pneumatic Device

GOM also offers another Virtual Clamping feature that comes with peripheries, which is a pneumatic clamping device with a gravity compensation function. This function is useful for very flexible parts, such as plastics or single part sheet metal. Inside the pneumatic clamping device, lay the part flat on a rotation table and measure it. Then, the software performs gravity compensation for the actual bow and bend in the part from gravity, essentially taking the gravity out of it. Then, the software rotates it from the horizontal measuring position into its normal position inside of the software, reapplies gravity, and then clamps it.

The pneumatic clamping device has extremely flexible holders with three pneumatic suction elements. There's a smart valve control for the complete free state. It clamps the part, then puffs air out, relaxes, and clamps back in a sequential order allowing it to be in a perfect free state. It has photogrammetric position identification for each one of the inspection clamps so the software can identify where they are in space. A benefit is excellent accessibility for all your measurements, so there's no more blocking of points. Other benefits include reliable mounting and a variety of part possibilities because of its mobility around the measuring plate.

Gravity Compensation Makes Non-Rigid Parts Weightless

The functions of gravity compensation include measurements in an unclamped state, location of the support points (pneumatic clamping device), the directions of gravity, and your deformation model. The results you get are gravity compensation or measuring in a weightless state. You would want to measure in a weightless state when you are measuring something that is not supposed to be measured lying down flat because it's being boded flexible, such as a window frame. A window frame would need to be propped up in a car position in just past a 90-degree position to look like it is supposed to look in assembled position while also clamped.

To access this feature in the software, first put your part in the pneumatic clamping device lying down horizontally and measure it in the unclamped state. Then, the software takes gravity compensation and makes it weightless. From there, it rotates it up into a car position, then re-adds gravity, then virtually clamps it, and gives it the assembled location, fully clamped, even though you measured it completely horizontal with a bow in it due to gravity. This process integrates into scanning template concepts of the GOM Inspect software. You only need to do this once for it to become a template function.

Process Capability Analysis

In this process, the part is mounted, measured, and unmounted several times using the universal pneumatic device. The data processing steps involved in this method are the removal of gravity, the addition of gravity in the mounting position, and then virtual clamping. Deviations and variations at the holding clamping points are zero. The variation of surface measurement is low (less than 50 microns) compared to corresponding mechanical fixtures, which vary. A part with mechanical fixturing with physical clamping compared with a universal pneumatic device with virtual clamping yields identical results within the scattering of mechanical processes. It must be uniform material meaning it can't be various multi-layered welded materials. Currently, the software feature only supports single parts smaller than two meters.

The software feature provides the ability to cut costs dramatically, saving space and time spent reloading and unloading fixturing by performing everything inside the software. If you're interested in gaining the benefits of this software feature for your process, contact a Capture 3D team member today, and schedule a demo with an expert who can give you more information about this revolutionary feature.

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