multi-axis machining, especially 5-axis, the workpiece needs to be secured in a way that provides clearance for movements without creating interference areas that will ultimately crash the machine. The most common interference areas occur between the table and the spindle housing of the machine. This is generally caused by the need to have a wide angle of attack to the workpiece from the spindle to the table. Elevating the workpiece with something like a riser chuck provides machining access and allows for the use of shorter tooling, resulting in better cutting parameters, faster feed rates and heavier depths of cut. Form and positional tolerances are also improved when multiple faces of a large workpiece are machined in one setup. Similarly, a quick- change workholding component can be directly attached to the underside of workpieces that need to be held without any side restrictions. Production and Mix Levels The frequency and complexity of part changeover should impact how fast you need your workholding to
be. Generally speaking, the higher the part mix and/or production level, the more changeovers. Seriously consider how much time a certain piece of workholding will require during locating and clamping. We find that it’s not typically until the setup cost reaches a tipping point, generally 20-to-25 percent of the total cost to manufacture a part, i.e., hours/day and minutes/ fixture, that we see customers inquiring about a change. That’s a long time to wait and a great deal of efficiency to lose. In any case, a zero-point workholding solution that maintains impeccable accuracies and strength while allowing for extreme modularity and quick changing of fixtures/ parts. Level of Operator Interaction While we often think of robots or assembly lines when we think automation, when it comes to workholding, the focus is largely on the ability to clamp and unclamp the part or fixture repeatably and accurately, typically with the help of air or hydraulics. There are three levels of automation for consideration in workholding:
Part Shape and Condition Choosing workholding that
accommodates the shape of your part really comes down to whether it’s round or prismatic (has flat sides). If the parts are prismatic, most often a vise will do just fine. If the part is round, you’ll need to hold onto an outer or inner diameter on the part. This may require a collet chuck, lathe chuck or V-blocks, if it needs to go into the mill and so on. Knowing if what’s coming through will be raw material, a casting or partially finished parts, will also indicate what the workholding will have to accommodate. For example, a round raw bar from the mill may have an OD tolerance of ±.005″, but if a part is cast, then the size may be ±.02″ — could be better, could be worse. If a part has already been machined, then the size is not a variable, it’s a known controlled value. Number of Axes Prismatic parts often require multiple machining axes. This introduces a different set of workholding considerations. For
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