Designing a Fixture After The Part Is Programmed

Have you ever set-up a job on your machine only to find that the setup causes an overtravel alarm or, even worse, a collision?  If so, you may have had to tear down your setup and replace it with a new setup or you may have had to reprogram your toolpaths.  You may have even had to transfer the job to another machine.  But is this really necessary?

When planning a new job, there are a lot of things to consider including the machine, stock, fixturing, cutting tools and holders, toolpath programming…just to name a few.  All of these components must be considered carefully in order to safely and efficiently produce the finished part to spec, but where do they fit in during process planning?

Designing a Fixture After The Part Is Programmed Image

In this article, we’ll be specifically discussing fixture design as it tends to be one of the components that is misplaced in process planning.

The primary objective of a fixture is to hold the workpiece with enough force to machine it, and the appropriate amount of force to not damage it.  That aside, the fixture is considered an obstacle in the sense that it takes up space in the machines work envelope which limits the safe, reachable space that can be accessed by the tool.  This is especially true in 5-axis machining.

In both 3-axis and 5-axis machining, the height of the fixture effectively reduces the usable range of the Z-axis.  In 5-axis machining, this goes a step further.  As the part is tilted and rotated, the height of the fixture gets projected onto the XY plane of the work envelope.  At the extreme, where the part is tilted at +/-90 degrees, the full height of the fixture is projected onto the X and/or Y axes greatly reducing the usable range of these axes.  This can cause unexpected overtravels that could have been prevented.

Poor fixture design does not only lead to overtravel issues, it can also result in collisions.  This is true of any 3, 4, or 5-axis machine as I’m sure we’ve all seen videos of tools being snapped by bolts holding down edge clamps on 3-axis machines.  In a 3-axis machining scenario where all of the machined features are above the fixture components, you have little to worry about if you’ve picked up your work offsets correctly, set your tools correctly and double-checked your NC program.  But, in 5-axis machining it’s a completely different story. 

In 5-axis machining, once the part starts to tilt and rotate you have to make sure that your tool and/or holder don’t smack the fixture.  That may seem obvious enough, but don’t forget to keep an eye on the spindle to make sure it doesn’t collide with your table.  If your fixture is too short or the effective diameter of your setup is too small, you could find yourself needing a new spindle.  Again, this situation could be avoided.  How?

As the title of this article states, all of these situations can be avoided by designing your fixture after you’ve programmed your toolpaths.  How is this possible?  Through the use of a virtual machine.

The goal of any machine shop is to keep their machines running.  Anytime you have to prove out a new program, or in this case a fixture design, you’re not making effective use of your machine.  To keep your machines running, you should have your entire setup and NC program verified before sending it to the machine.  This is where fixture design fits in.

Simulation packages, like CAMplete TruePath, allow you to see your part, tools, holders, and toolpaths so you can test different fixtures in a virtual environment.  If your process will be using a standard vice, you can quickly try different vices to find the one that best suits your process without any collisions or overtravels.  If you are designing a custom fixture, you could test different revisions of the fixture to make sure it does not cause any issues.  Your virtual machining environment will show you exactly where any issues are, and speed up the iterative process of fixture design, allowing you to get your job on the machine quicker than expected.

Being able to test your fixture design in a virtual environment is a great resource, but you have to make sure that what your testing matches what’s actually going to happen.  With CAMplete TruePath, machine models and data are provided directly from the machine builders, so you can feel confident that your setup in the virtual environment will accurately reflect what happens at the machine.

This article was originally published by Cutting Tool Engineering here. 
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