NEW SERIES! Metaling Affairs: Workholding

January 13, 2012 No Comments
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Controlling the Variables

toc_columns50pxby Andrew Skoog

Containing controllable variables in your milling setups will make you faster, produce less scrap and give you more consistent parts. Setup includes workholding, toolholding and the cutting tool. In this article, I am focusing most on workholding in machining centers.

STEVENS_VerticalSpindleOne of the first controllable variables in milling is the setup. There are many setup solutions for machining centers. I am a big fan of subplates and consider them to be the foundation of the setup. They are inexpensive and pay huge dividends. Operators, programmers, process engineers, etc. are all on the same page, because the setups and coordinate offsets are predetermined. Most subplates have an alpha/numeric locating system so process sheets, floaters, and travelers will have the location of the fixture stated. They typically have a series of bushed holes in a standard pattern separated by tapped holes. I am mostly familiar with the Stevens Engineering grid. They have a few different grid patterns, 2.5” high density, 5” standard density and 5” cluster density. This density is relative to the centerline of the bushed/dowel pin holes. The tapped bolt holes are then on 1.25” centers between the bushed holes. (See Figure 1 above)

Subplates will locate fixtures, vises, and rotary tables. For those using automation, fixture locations are repeated making programming the automation faster especially for repeat jobs.

How many times have you had a customer call that wanted parts yesterday? Depending on the job, this could mean several hours of take down and setup to run the parts and then again, switch back to resume. Subplates give the ability to change jobs in minutes. Many shops have made their own. My suggestion is to obtain an industry standard so you are not captive to your own system. Some manufacturers offer angle plates, tombstones, and vises that are fully compatible with their subplates.

Once every machining center in the shop has the same platform, there is less need for large inventory of vises and rotary tables. Fixtures, once made to be compatible with the supplate, can be machined anywhere in the shop, and on vertical or horizontal equipment. Now the coordinates are known and they can be engraved in the fixture; the location will always be relative to the dowel pins, allowing them work in any machining center in the shop. Your table now works like a manual tool changer, only holding workpieces.

Raptor DovetailAnother controllable variable is the workholding—holding on to a part confidently knowing the part isn’t going to come loose. Dovetail fixtures are becoming popular, especially in 5-axis machining centers. In 5-axis work, a big challenge is holding a part solid while having access to all sides of the work piece. Most 5-axis vises have ends to work around, making tool holding a serious issue and secondly, these vises have gripping forces that are easily overcome. Dovetail fixtures waste very little material, have a death grip on the workpiece and give access to all sides of the workpiece. These also can be fixtured to work with subplates. They are not only for 5 axis use; they also are used in indexers/fourth axis, wire EDM’s, even built into tombstones and, since they bite on as little as 3/64”, I have seen shops use them simply for saving on expensive material .

Three and four jaw scroll chucks are a great way to hold onto parts in a machining center because there are a host of machinable jaws available. I have implemented Dovetail Jaws in 2, 3 and 4 jaw chucks to put that same death grip on the part. In lathes, dovetails are a nice advantage when you have the pressure dialed down. There are some nuances to making a good dovetail. The angle is important for strength. The part needs to pull down on top of the dovetail, not the root, for lateral stability.

When considering your setups, plan ahead, see how you can most efficiently utilize your equipment and time. There are solutions to shorten setups and controlling variables in these setups. Typically your machines are making you money when they are making chips. Taking advantage of these solutions will help you get more parts out the door—every day.

Andrew Skoog is with Hexis in Minneapolis, Minn. He can be reached at

Copyright © 2012 Minnesota Precision Manufacturing Association. For permission to use or reprint this article please contact Amy Slettum, publications manager for Precision Manufacturing journal.


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