Fixturing always seems to be one of those items that most shops forget about until the last minute. This is unfortunate because using the most efficient and cost effective fixturing can make a world of difference in your product cost structure.
Over the years we have seen CMM operators using just about whatever they can get their hands on to use as fixturing. Things like duct tape, putty and glue are commonly used to secure parts for inspection. While these types of media can be sufficient, a structured, repeatable fixturing system can reduce set-up times and costs, making you more competitive.
Building a fixture for inspection and building a fixture for machining are not too different. Both need to position, locate and hold the workpiece.
Where the difference comes in is the amount or mass of the fixture. In machining your goal is to secure the workpiece during the machining process.
The main fixturing goal in inspection is to allow the probe to have access to as many features of the workpiece as possible.
There are many options available when it comes to fixturing for a coordinate measuring machine (CMM). The first task is to determine what fixturing system works best for your applications. Some systems work best on large workpieces and some are designed for fixturing medium to small sized workpieces. For the purposes of this article I would like to focus on the latter.
Building a fixture for inspection and building a fixture for machining are not too different. Both need to position, locate and hold the workpiece. Where the difference comes in is the amount or mass of the fixture. In machining your goal is to secure the workpiece during the machining process. During that process many forces are being applied to the workpiece in many different ways.
For inspection applications, the less fixturing, the better. Also during the inspection process there is very little force being applied to the workpiece, therefore eliminating the need in some cases for bulky clamps and positioners. The main fixturing goal in inspection is to allow the probe to have access to as many features/parts of the workpiece as possible. Also often times it is required that the workpiece be positioned in the same orientation as it will be in the finished assembly. Therefore it is important to have access to a wide variety of components to adequately fixture those items.
This can be easily and quickly done with a standardized CMM fixturing kit and fixture plate. The fixture plate becomes the base for the entire system. Plates can be purchased with an alpha-numeric grid pattern that allows the user to document the location of the components of the fixture for future use. This is ideal for shops that often need to inspect the same part at different times throughout the year. When a past job comes around, the user simply references the last set-up and rebuilds the exact fixture within a few minutes. This way the part can be accurately re-located into position, clamped and inspected. Fixture plates are available in many standard sizes with various threaded hole types and configurations. Custom size plates and grids are not uncommon and can be manufactured easily and cost effectively. Users will often order multiple plates and gang them together on the granite surface. This is sometimes a better option than getting one large plate to cover the surface area. Large single plates can be bulky and difficult to manage. With the entire surface area covered with a fixture plate(s), users can now have the option of building multiple fixtures on the inspection table at one time. In addition, users can be building one fixture while the CMM is inspecting another fixture in another location of the plate.
Documenting a fixture for future use is an important feature to this type of system. Users can simply indicate the part number of the fixturing component(s) used and then document where that component is located on the fixture plate via the alpha-numeric grid pattern on the fixture plate. This can be as simple as a handwritten document that is filed away for future reference or the data can be stored in a spreadsheet software program and saved on a computer system in the inspection area. Fixtures can also be documented with a CAD software package as well.
Most systems offer a wide variety of positioners and locators that allow the user to position the workpiece in almost any orientation. By using fixture towers, parts can also be oriented into a vertical or upright position if needed. Fixture towers are often available in a variety of sizes and configurations. Fixture plates can even be mounted in a horizontal position with the use of fixture towers.
Due to the need for light clamping pressures, clamps are available that only require a “finger touch” to apply clamping pressure. Again the only force needed is to simply lightly secure the workpiece during the probing process. These clamps are available in different sizes and configurations, including with a rubber coating to minimize workpiece marring. These clamps are really the key to this overall system. These clamps are designed to minimize any part distortion or marring with their light touch clamping pressure. This is very important when inspecting finished parts where cosmetic appearance is vital.
Standoffs are offered in many different sizes and heights to raise the part off the table surface. Also some components include a pin stop that allows quick positioning and locating of the workpiece.
Magnetic components are also available that are ideal for round or flat workpieces. Magnetic vees allow the user to fixture round, steel parts quickly and accurately without the need for additional clamps.
In applications where more clamping force is required, you can use either plastic or steel clamps to securely hold your part. The advantage of the plastic clamp is that the clamp does not harm the surface of finished parts. This is critical for applications where the finished surface must be free of any marks or scratches.
Many of these systems can be purchased in various kits and configurations. However, individual components can also be purchased if needed, and to use special or non-standard components. Additional components can be added to the kits to expand the flexibility.
When it really comes down to it, you are only limited by your imagination when using today’s CMM fixturing systems. There are many options available. The key is to find out what fixturing system is best for your application.
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