Mobile Edition
Print
Get the Mag
Weekly UpdatesWhy 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.
[ILLUSTRATION OMITTED]
The wear marks at the gage line (large end) of the taper shank are normally 1/4" to 1/2" in length and look as if the holders were fretted (an unusual wear pattern). The marks at the small end are where the threads have expanded the toolholder.
[ILLUSTRATION OMITTED]
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 from the tool flange, detecting growth in diameter of the toolholder as little as 0.000003".
[ILLUSTRATION OMITTED]
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 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 important to remember that toolholder tapers are ground to a total tolerance of 0.000080" in a class 3 fit (AT3).
Growth at the small end of the toolholder resulted in a poor fit in the large end of the toolholder with the spindle. This in turn resulted in erratic positioning of the toolholder when loaded into the spindle and inconsistent concentricity at the tool tip.
Testing with both generations of the fixtures proved that toolholders should be checked for expansion prior to their installation in the machines. The test results revealed that retention knobs started to expand the shanks of the toolholders at a much lower torque value, and that without exception, all toolholders returned to their original size after removal of the retention knobs.
[GRAPHIC OMITTED]
Using the first generation test fixture, technicians experimented with the retention knob de sign. It was found that standard retention knobs inserted into the toolholder with as little as 20ft/lbs of torque expanded the toolholder shank.
With the second generation of the test fixture, the toolholder began to expand with as little as 5 to 15ft/lbs of torque. It also was proven that the new High Torque retention knobs, when tightened to the same torque value as standard retention knobs, expanded the toolholder shank two or three times less than standard knobs.
Inclusion of a pilot on High Torque ANSI retention knobs gready cut the likelihood of retention knob breakage, especially when the Belleville washers were not performing at full strength.
High Torque retention knobs are designed to be balanced dynamically from end to end, making it easier to keep balanced toolholders within tight balancing tolerances after installation.
The graph shows the growth experienced while testing using the second-generation fixture and the High Torque retention knobs compared to the standard retention knobs.
The vertical axis on the graph is based on a toolholder grind tolerance of 0.000080". The greatest deformation reflected by the graph at 80ft/lbs, is approximately 10 times the tolerance, or 0.0008". While this number may seem miniscule, this discrepancy increases by a factor of 3.40 or 0.0027". This number represents the distance that the toolholder moves out of the spindle due to the deformation of the toolholder.
The resultant impact is that toolholder that measures 5.400" in length from the face of the spindle to the tool tip may run out up to 0.002400" T.I.R.
This same graph illustrates that the same toolholder, paired with a High Torque retention knob, may experience tool tip run out up to 0.000576" T.I.R., which is a significant difference.
Simply stated, the two numbers represent the diameter the tool would be free to move forward and back or side-to-side. Therefore, by implementing lower torque pressure during retention knob installation, along with the selection of the best quality toolholder and use of a High Torque retention knob instead of a standard knob, the distortion problem is solved.
It's important to maintain retention knob installation torque at an acceptable level. Machines with 40 taper spindies normally have draw bar pressure of 2,5001bs or less. Axial force of 1.25 times the draw bar force, or 28 to 33 ft/lbs of torque, is adequate to insure that the retention knob does not lose contact with the face of the toolholder.
Tool studies
A titanium slab mill, one of J&M Machine's customers located in Georgia, routinely made two or three tool tip changes per day using standard retention knobs.
After installation of J&M's retention knob, the same machine running the same job used only one edge of the tool insert with no tool tip change for five days. The slab mill realized an immediate savings of $458--standard knob tool cost was $48015 days; J&M High Torque knob cost was $32/5 days.
The same customer also runs aluminum at high speed. The slab mill reported that the tools run much quieter with the high torque retention knobs. Because the tools fit the spindle better, the machine no longer emits the whine that often ac companies aluminum milling.
One Ohio customer reported its carbide porting tools are holding better sizes and tolerances and are not chattering. There are fewer complaints about tool edge breakage since converting from the standard knobs to the high torque design.
Yet another J&M customer in Ohio mills aluminum forgings, making high-end wheels for sports cars. This company uses a 1" ball mill which extends 18" from the face of the spindle. Previously, there were problems with chatter and tool breakage. After switching out tools and installing the high torque knobs, the new tools ran for over two weeks and the chatter and premature breakage issues have been resolved.
J&M Machine Inc., www.rsleads.com/905tp-177
John W. Stoneback is president of J&M Machine Inc. of Fairport Harbor, OH, a manufacturer of retention knobs for 35 years.
FEED