A very long time ago I was pulled over by a state trooper for driving 72 MPH in a 65 MPH zone. I knew the car’s speedometer had been reading a steady 65 MPH so I was less than calm about the stop. After a quick look around the car, the trooper noticed I had new and very oversized rear tires on the car, so he spared me the ticket and offered the advice that I get the speedometer calibrated. I learned that things are not always what they seem; instruments and state troopers.
As a guy who is still learning machine shop operations, I don’t have that intuitive feel that comes with experience, the one that keeps an operator out of trouble and allows little metal curls and chips to be read to know if speed, material and tooling are operating in harmony.
For me, a subtle clue that all is not well might be a piece of bar stock or a cutting tool bursting into flames, or a twenty pound chunk of steel excusing itself from a chuck and becoming recessed art in the shop ceiling. So I have to depend on machinist’s guides and speed and feed tables for the time being, and the little operating speed plaques that grace most metal cutting machinery. The only problem is that all calculations and assessments are predicated on the notion that the RPM matrix on the machine is correct
– While cutting speed, expressed as fpm, comes from tables referencing type of material, forming process and hardness, spindle RPM is equal to the cutting speed, multiplied by four, with the product divided by the diameter of the material as expressed in whole and decimal inches. In my case, some A36 carbon steel with a 112 Bhn had a recommended cutting speed of 140 feet per minute. So I multiplied the 140 by 4, got 560. Then I divided the 560 by the diameter, 0.500″ which resulted in a quotient of 1,120 RPM. As pictured above, it seemed I only had to select pulley II and I, using the B groove and running at 1,000 RPMs… but not really.
A tattletale tachometer without shift points… interesting
The question of how fast something is spinning comes up a lot; machinery, engines and motors, grandchildren, etc. So I bought a laser/optical tachometer for $139, rather than install $200 magnetic pulse tachometers on each piece of machinery. Originally, tachometers selling for half the price or even less were tried, however, they were unreliable in reading and it became necessary to reach at least to the upper end of the low end range of tachometers. Extech makes this type of cost effective and reliable equipment.
A laser/optical tachometer reads RPM by counting each time the reflection of the unit’s laser light is interrupted. A piece of reflective tape that is packaged with each tachometer is stuck on the face of the unit to be timed. The tachometer picks up the reflective tape every time it passes through the unit’s projected laser and the light from the laser is reflected back into the meter. Because reflected light triggers the reading, it is important that all other reflective surfaces surrounding the reflective tape be eliminated. In the picture, above left, the three pulley faces that were selected to hold the reflective tape were masked off and painted flat black. The attempt at Roman numerals coincides with the spindle speed plaque affixed to the machine.
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With the planetary drive pulley III being driven with the timing belt from the motor shaft and connecting directly to the spindle pulley I, the RPM readings at motor, planetary drive and spindle were close enough to be considered right on. Once the belt was routed from the motor’s V belt pulley set II to the spindle pulley set I, the RPM readings were 25% higher than indicated on the machine’s spindle speed plaque.
When I took a physical measurement of the motor and spindle pulleys I found the ratio was approximately 2.18:1, which tracked to the 1740 RPM read at the motor shaft with the tachometer. The motor speed was OK, but the motor V belt pulleys were larger than standard, which drove the spindle in excess of the RPM indicated by the machine documentation. The 160 RPM the tachometer read at the planetary drive center shaft resulted in a 58 RPM spindle speed with a 2.7:1 pulley ratio, which is pretty much as it should have been. So the culprit, if a pulley set could be labeled a culprit was the oversized aluminum pulley that had been installed on the motor shaft.
Conclusion
The speeds indicated on the matrix and the actual RPM were off by 25% and FPM was off 14%, something more appropriate for an entirely different type of steel. As a minimum, tool wear would increase and wrong impressions would have been formed and stored away as experience. It happens. Chinese and other imported machinery often are subject to running design changes and piece part inventory on hand. It is part of the package that gives us basically good machinery for a fraction of the cost of higher end professional shop equipment. In this case the problem will be solved with a replacement pulley set and I will be the better for the experience and on my way to learning something more.
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