My Bridgeport came with this optical system for reading the location of the table. This is listed as a Bridgeport accessory but i've never seen one before and there is limited info on the web about these. There is a small box mounted on the side of the mill that coverts 110V to 6V and there are small 6V bulbs inside the x and y readout boxes that illuminate the graduations that are magnified inside the window (sidetone: the bulbs look like small motorcycle taillight bulbs). The graduations are actually on small glass/mirror? like scales that are fastened to the x and y. The surface is protected by small rubber flaps that keep the scales clean. The graduations are in .0001 increments....CRAZY! Think of this as something similar to a Digital Readout or "DRO".
For more on this read below and check out images I found of one of these mounted on someone's machine.
Before the advent of electronic measuring systems, milling machines commonly had two methods of precisely locating work - mechanical and optical.
Optical Measuring System for longitudinal and cross feed.
Another pre-electronic era device for the precise setting of work that involved no mechanical contact - and so avoided wear and consequent inaccuracy. The unit bolted to the machine using a majority of existing holes - and was protected in use by a neoprene guard. The longitudinal travel was 20 inches, cross travel 9 or 12 inches, the magnification x17 and the reading accuracy 0.0001 inches on a direct, single line scale. The scale had just one line to read - and no vernier estimations to be made.
Another pre-electronic era device for the precise setting of work that involved no mechanical contact - and so avoided wear and consequent inaccuracy. The unit bolted to the machine using a majority of existing holes - and was protected in use by a neoprene guard. The longitudinal travel was 20 inches, cross travel 9 or 12 inches, the magnification x17 and the reading accuracy 0.0001 inches on a direct, single line scale. The scale had just one line to read - and no vernier estimations to be made.
This is an Bridgeport Milling Machine optical measuring system. It operates like an old school DRO. It “reads” the table position in both the X and Y axis using optical lenses to view the glass scales, then transposes them measurements into the viewing screen. The viewing screen looks similar to a ruler, but is graduated in .001, with a smaller scale below for .0001 It is original Bridgeport equipment. The viewing devices are mounted on aluminum dovetail brackets the have thumbscrew adjustments. The whole system bolts up to the milling machine using the factory Bridgeport holes. It has a power supply with it that converts 110v power to 6 v DC, that powers the lamps for viewing. It uses little 6v motorcycle bulbs, readily available at auto parts stores. The only real drawback to this optical system is that you can not zero out the scales after you find and edge, bolt center, etc. You will have to manually keep track of your starting point, but it is not hard, and it works really well. There is a rubber protection strip that mounts above the scale as well to help protect from chips and fluids. These are very cool, work well, and have a certain Bridgeport coolness to them that any Bridgeport aficionado should appreciate.
The item “Bridgeport Mill Optical Measuring System, old school cool DRO”.
2 comments:
An overlooked benefit of such optics is not just reading direct .0001, but the repeatability. Accuracy of a machine dial [discounting temperature variances] is primarily; subject to wear of the lead-screw assembly overall, thread pitch of lead screw, constant wear of the nut, dividing accuracy of the dials, whether each change of position stops on the same 'side' of backlash, and fit of gibs to axis, condition of position lock screws, to name a few.
An electronic DRO removes lead screw and dial issues, and makes observing displacement of table movement; gibs or locks possible. As such DRO function can be thought reliable, not perfect.
There are two factors in this kind of measurement interpretation, DRO, digital caliper, dial height gauge, anything using other than a engraved scale.
A vernier has different impediments. Certainly, while a vernier is subject to mis-reading and variances in marking process, many people discern measurements to .001 dependably, occasionally to .0005!
Those two factors are 'accuracy' and 'precision'. Accuracy, or true size depends mainly on calibration of instrument used determining that size, like a conclusion.
Precision is determined when each test provides identical result under identical conditions, or 'repeatability'.
DRO's early limitations were 'resolution', the fineness of a measurement [not same as precision]. Compare a common steel faced micrometer graduated .001 to a carbide faced micrometer, that have TWO verniers reading .00005. That increase of resolution is useless un-calibrated, while repeating perfectly, lacks accuracy.
Common now are electronic DRO's reading .0005, some at .0002, and very high end versions at .0001.
.0001 and other fine optics have been around far longer, including protractors and other measurements.
Not being able to 'zero out' can ADD position accuracy if the starting or reference position is used continually. This is 'absolute' positioning, where each position uses the same reference. The lesser alternative 'incremental' measurement, adds each move to previous. While correct arithmetic says XX.xxxx, the stack of individual measurements will assuredly not land in same position found 'absolute'.
Another optic advantage is in the single vernier, compared to laying down 30" worth accurately. Many [most?] optic scales were photoengraved larger than finished size, accurately reduced, instead problematic creation of such fine lines at size.
The development of DRO's followed quite the same sequence, needed parallel advancements of circuitry, displays, and limiting interference.
https://en.wikipedia.org/wiki/Accuracy_and_precision
Thanks! Great info you shared!
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