Sunday, April 15, 2007

Measuring TIR Spindle Bearing Runout On Your CNC Machine

Spindle Runout Problems Can Be Your Hidden Quality Issues In Building Reliable Printed Circuit Boards

The simple definition of spindle radial runout is how much wobble a spindle produces at the nose. Axial runout is the measurement of how much play there is perpendicular to the axis of rotation. The reading is represented by Total Indicated Runout, TIR, which means the distance measured between the largest plus direction and the largest minus direction for a total indicated amount. A spindle can be measured either dynamically (at speed) or statically. The dynamic measurement is by far, the most accurate and requires special non-contact measuring devices.

Today's air bearing spindles have gotten faster with higher precision than the ball bearing spindles of the past. Air bearing PCB spindles can typically be found running in the 120K RPM through 180K RPM ranges, with some spindles running up at 300,000 RPMs. Since the spindle air bearings are floating on air, no mechanical contact is made with the rotor. Radial and axial runouts can be maintained with very tight precision. Westwind, a manufacture of high speed air bearing spindles claims total indicated runout accuracies of their PCB drilling spindle of less than 10 microns: "Typical synchronous radial runout values: < 10 microns (PCB drilling spindle, high speed)," which equates to approximately, less than .0004 inches TIR.

High Speed Spindles
High Speed Spindle Chart Courtesy of Westwind Air Bearings
Chart shows maximum spindle speeds in different market sectors

Many of today's CNC PCB drilling machines incorporate laser measurements for checking runout of the spindles at operational RPMs. However, this does not always tell the whole story. Many times the machine's software is set at a very nominal level of warning for telling an operator that the runout of the spindles is too great a value. The result is the machines may be drilling with less than desired accuracies for the hole locations and even the hole quality can be affected by runout related problems. This is becoming an even greater issue as today's circuit boards are becoming much denser, with tighter tolerances required between the holes and the traces, and the continued direction toward smaller carbide micro drills (.008 .20mm - .004 .10mm) and more concentrated holes. Other times, spindles with excessive axial play (end-to-end) will result in depth problems when trying to drill in a controlled depth mode, such as drilling blind vias where a certain depth has to be maintained within a thousandth or two, and hole quality issues when drilling standard holes.

The overall runout is not only affected by the quality of the spindles themselves, but factors such as collet wear, collet cleanliness, and collet adjustment will greatly affect the TIR reading. Collets wear out due to constant opening and closing movements which does produce wear upon the rubbing surfaces. These wear marks become increasingly more visible over time. Additionally, circuit board vacuum debris will cause wear and dirt issues which will result in poor TIR readings. Collets that have reached the end of their life cycles can sometimes be spotted by looked at with the naked eye or a small scope. This however, will not always show a problem with a collet. Sub standard collets are a particular problem today. As PCB manufacturers have attempted to cut costs, they have gone with cheaper collets that do not always measure up. Operator collet adjustment and cleaning maintenance training is another area where many circuit board manufacturers' personnel are lacking. In an effort to constantly cut costs, new drilling operators are hired with limited knowledge of how to properly clean and adjust spindle collets. This results in a total reduction in overall quality throughout the whole circuit board manufacturing processes.

Additional factors that contribute to spindle runout and spindle air bearing failures are the air supply quality and the stress spindles receive due to excessive drilling forces, through improper feeds and speeds. Excessive chip loads for large diameter holes can cause the spindle air bearing surfaces to collapse and become internally damaged. Spindles that turn at very high speeds produce lots of air friction. This friction generates heat which can cause early failure of the spindles. Clogged coolant passages, because of improper coolant system maintenance, can also be a factor for excessive heat generating issues and spindle failure. Ambient air temperatures in the drilling rooms can also greatly affect the life of the spindles. Extreme temperatures, both on the cold as well as the hot side can result in spindle failures. Manufacturers that insist on running their spindles at maximum RPMs are likely to see a reduction in the life of the spindles. Just because a drilling spindle can run at 180K RPMs does not mean that it should run constantly at those speeds.

Equipment Used To Measure Total Indicated Runout, TIR

Lion Precision is a company that manufactures a dynamic non-contact capacitance runout measurement meter that is ideal for measuring spindles at all speeds. With a dynamic indicator, a technician can precisely tell the condition of the spindles, collets, and the runout, at the typical distance from the nose of a spindle, that a drill tool would actually be developing at spindle RPM.

TARGA III Dynamic Runout System Lion Precision

The non-contact measurement from Lion Precision is top of the line equipment. Probe Industries has used Lion Precision equipment for many years on all types of drilling, routing, PCB, milling spindles, various machine shop equipment and other applications for measuring the total indicated runout for high precision equipment. We have seen equipment that was suspected to have all types of positioning problems with linear scales or rotary encoders, turn out to be a spindle runout problem all along.

Circuitree MagazineBe sure to sign up for the PCB industry's Circuitree magazine subscription in which a more in-depth spindle runout article is published in the August 2007 Circuitree issue, with additional information and a detailed description of how to measure spindle runout, along with all the procedural controls for QC to implement for the production floor. Don't miss it, your quality control depends upon it.

What Probe Industries Can Do For You

Probe Industries can evaluate your current machines and let you know what the status of each of the drill and rout stations are. We can check the spindles at all RPMs and produce detailed charts of the condition of your spindles. The result of getting a complete spindle inspection will let you know which spindles are good and which ones are bad. You will no longer be drilling in the dark. Your scrap rate will be reduced and your quality will improve. We can provide technical training for your quality control department. Probe Industries can also help out the CNC machining company that is interested in finding out the overall runout of their ball bearing spindles on their milling machines.

Contact Probe Industries for a detailed Spindle Runout Report for your machinery and equipment today. We also provide maintenance and repair services for all types of equipment.

Probe Industries, Inc.

We have over 28 years of service experience in maintaining and repairing Excellon Machines, Kennard, PDA, Pluritec, Hitachi, Mania, Klingelnberg, Advanced Controls, Trudrill, ESI, Dynamotion, ATI, and other PCB drilling and fabrication equipment, plus a wide variety of service experience on other types of machinery and equipment used in various industries.

Also, be sure to have us check out that piece of used equipment before buying from an auction or other company. Read about The Tale of the Uninformed Used Equipment Purchase.

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