Identifying sources of prism
October 8, 2005
I. Using a dial test indicator.
II. Using a gage pin to check collet condition.
III. Using a micrometer to check blocking arbor (chuck) condition.
I. A dial test indicator is a specific instrument suited for reaching into small areas to measure movement. It is usually mounted on a magnetic base which has a fine adjustment to position the indicator. It is not the type of indicator found on older manual lathes that have a larger 2” (50 mm) face. In working with technicians, I ask what the increments of the indicator are. While contact lens manufacturing has been conducted in .01 mm increments, testing and calibration needs to be conducted in .001 mm increments. Thus, I recommend a .002 or .001 mm increment dial test indicator. The .002 indicator is adequate as long as the technician is aware of the actual “line” value.
I have worked over the phone with people who talk in terms of “lines” on the indicator. In these situations, I make the technician identify the size of the lines by telling me what the dial face reads.In the lab, the basic use of the dial test indicator is to check runout of the lathe collets and blocked lens blanks. Runout is the actual movement of a rotating element as measured on its surface. This reading can be taken on the side or end corner of the element (spindle, chuck, blank). On side runout, the reading amount is twice the eccentricity of the element. That is, the center of the element is offset from the center of rotation half the “Total Indicated Runout” (T.I.R.). In terms of prism, the T.I.R. is twice the prism value. So, if you have .30 mm runout that is equivalent to .15 mm prism which is about one diopter of optical prism.
II. A gage pin is a precision round pin 12.7 mm in diameter (for our purposes). It is a standard which is traceable to the (US) Bureau of Standards and can be used as a standard for ISO quality protocol. It is used in lens manufacturing as a guide to check the condition of the lathe collets and to calibrate other measuring tools. It should be stored with the dial indicator and related instruments.Our main use of the gage pin is to load it into a lathe collet and check the runout with the dial test indicator. The runout indicates the condition of the collet. Readings of less than .005 mm are acceptable, but a new collet in a good spindle can be as good a .001 mm. Readings of over .005 mm T.I.R. will add to prism problems and should be addressed (replaced).It is not uncommon to have an old lathe with an exceptionally true running collet that you can use for other calibration procedures. I have an old Hardinge lathe that has .001 mm runout at 20 mm from the face of the collet.. Also, with multiple insertions and readings, you want the readings to repeat. If they do not, examine the collet 12.7 mm bore for defects. I recommend checking new collets before putting them into production.
III. A micrometer (not calipers) is a good instrument to measure diameters and check the surface quality of those diameters. Two important items are blocking tools (chucks) and lens blanks. It is imperative that the 12.7 mm shanks of the blocking tools (chucks) be 12.695 -12.705 mm in diameter. Shanks sizes beyond this range will not properly fit in a collet. The most common problem with metal arbors is that they become expanded at the bottom with age. This prevents the collet from holding them in a repeatable way and causes prism. The micrometer set at a “snug” fit over the shank of the blocking tool will tell you if the tool is OK. Try to pull the tool out of the micrometer anvils. If it stops before the end, then the tool is expanded. You can also measure the diameter at the end. If it is larger, then the tool is bad.You can also check the surface of the tool by rotating it in the anvils. If it is smooth, it will turn consistently. If not, it will be jerky from the roughness. Most lens blanks have a ground diameter of 12.7 mm. Since almost all manufacturers locate off of this diameter at some point on their process, it is important that it be the right size and round. An out of round blank will be strained (crimped) in the collet which will result in poor optics.Place the blank in the anvils of the micrometer and lightly tighten them so that the blank does not fall out, then rotate the blank. It should have a consistent resistance while you turn it. If it stops, it is not round and you may have an optics problem with it later on.