Tooling & Production - May/June 2009 - (Page 14)
toolholding The taper test Prototype ﬁxture ﬁnds why vexing toolholder wear marks appear By John W. Stoneback hy toolholders exhibit wear marks at the threaded area and at the gage line is a question that’s been asked for some time. J&M Machine Inc. of Fairport Harbor, OH, a manufacturer of retention knobs, decided to give the matter a look, even if it meant designing and building a test ﬁxture. But the gamble provided answers — and a new retention knob design. The wear marks at the gage line (large end) of the taper shank are normally ¼" to ½" in length and Wear marks (gray areas) were causlook as if the holders were freting machinists to seek a solution. ted (an unusual wear pattern). The marks at the small end are where the threads have expanded the toolholder. J&M technicians examined measuring instruments available in the market place, and determined that there wasn’t an inexpensive method for measuring taper shank diameter changes. This led to the design and building of a prototype Taper Shank Test Fixture (patent pending). The prototype gage measured the distance the test ﬁxture moves away A Taper Shank Test Fixture was from the tool ﬂange, detecting growth designed and built for the tests. in diameter of the toolholder as little as 0.000003". Protocols developed Three toolholders and three retention knobs of the popular brands were purchased, inspected, compared to national standards, and then tested: • First, prior to installation of the retention knob, the test ﬁxture was positioned on the toolholder shank and all three indicators resting on the ﬂange of the toolholder were adjusted to 0. • Second, the test ﬁxture was removed and the retention knob installed to a predetermined torque value. • Third, the test ﬁxture was reinstalled, and May/June 2009 W the average movement of the three test indicators was recorded. Technicians removed the test ﬁxture and the retention knob from the toolholder. The test ﬁxture was reinstalled on the toolholder without a retention knob, and the three test indicators were checked to make sure they were still set on 0. This insured that the indicators readings were correct and had not moved during the testing. Tests were started at 20ft/lbs of torque with retention knobs lubricated with light oil. Three retention knobs of the same manufacturer’s brand were checked in a row, and all the toolholders were checked in the same order with each retention knob. In subsequent tests, two from each manufacturer were checked. The retention knobs were checked at torque values of 20 through 160ft/lbs. Later tests were checked at torque values of 20ft/lbs through 80ft/lbs. J&M Machine hired a company specializing in rebuilding and regrinding spindles to grind a second generation hardened Taper Shank Test Fixture. The ﬁxture mirrored the toolholder taper, detecting any increase in size over basic, no matter how small the increase or where the distortion occurs along the length of the toolholder. This is signiﬁcant in that most toolholder manufacturers use air gages to check the rate of taper at three points of the holder, but not the full length of the taper. The ﬁxture took an out-of-round condition into account, unlike the air gage. Test results Technicians found that a toolholder, when checked with an air gage, indicated one AT3 grind limit out of tolerance. Checked again with the Taper Shank Test Fixture, the same toolholder was three AT3 grind limits out. The J&M ﬁxture also self-compensated as it would not show expansion or growth in size until the toolholder expanded or deviated enough to register as movement above the grind limit. It is www.ToolingAndProduction.com A High Torque Retention Knob. 14
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