More on manufacturing tolerances
by B.B. Pelletier
Today’s guest blog is so fantastic, I jumped at the chance to use it. Brian has been making precision aircraft and missile parts for over 30 years, so he knows quite a bit about the subject of manufacturing tolerances. Take it away, Brian.
by Brian Saada, aka Brian in Idaho
If you’re a regular reader of the Pyramyd Air blog, then you probably read BB’s article of May 21, Pellet Variation What Do You Do?
The simple answer to that question is — not much! Or, as BB noted, you can weigh, sort and package your pellets with some semblance of order as to actual weights. Still, some of us may want to know more about these types of manufacturing variation(s) and what causes them. Is there no perfection to be had out there? In production manufacturing, there’s no perfection — only allowable tolerance or deviation from the norm.
So, what is this tolerance thing anyway? Simply stated, a tolerance is an allowable range of dimension or weight or other measurable feature of a part or product. If the part or product falls within the “tolerance,” then it’s assumed to be fit for the application or use for which it was designed. As an example, if the head diameter of a .177 lead pellet can be functional or useable in a range of sizes (tolerance) from .175 through .178 diameters, then the manufacturing tolerance allows for pellets inside the tins we buy at Pyramyd Air to be .175 or .176 or .177…and also .178 in diameter. In plain, old-fashioned drawing or blueprint terms, we would see this written as .175 / .178 diameter for the nominal size .177 pellet. In practical terms, we feel these variations in our single-shot guns each time we push a pellet into the breech or barrel.
Wait a minute…what’s this nominal size stuff? A .177 diameter pellet or pellet gun barrel diameter is .177. Right? Not exactly. A nominal size simply means the size that can be normally expected or used in relation to some industry standard or design standard. As many of us know, the infamous steel BB that is called .177 is not .177 at all. It is typically .173 diameter but its tolerance allows it to be as small as .171 and as large as .177 (almost never at .177 diameter, per Daisy data). The nominal size of .177 is used in labeling and packaging for pellets, BBs and guns. It is not the actual or as-measured size of much of anything!
Weight and size relationship
As we know, a lead pellet is more or less a tube, depending on the style of the pellet. I say tube because, it has an outside diameter and an inside diameter too. Sure, it may not be totally hollow, but at least on the skirt surfaces we can see the inner and outer diameters. So, if a pellet diameter can vary (tolerance) does that affect the weight too? Potentially, it does. Both the inside and outside diameter can vary, and not always at the same time! Whew, more variation and larger tolerances? No, more often than not, it would be the changes in the forming die that squeezes the pellet into shape from lead wire that affects or controls the weight (along with the inside and outside diameters). Larger outside diameters or growth in the head size = greater weight. Given the density of lead, it doesn’t take much change in diameter to affect the weight of the pellet. Remember, we’re talking tenths of grams or less, and a significant change to the pellet diameter is a lot of additional surface area (weight) for an object as tiny as a lead pellet!
OK, enough on the math lesson and tolerance talk. What BB did not mention in his article about pellet weight variation was the somewhat-related topic of pellet sizing. Years ago, pellet sizing was a fairly common practice among serious airgunners. A pellet sizer was simply a hand-held steel die that forced the pellet into a specific diameter. Since then, these pellet sizers have lost their popularity except for certain die-hards (pun intended) who demand ultimate control over their pellets (wait, isn’t that us?). Now, unless the sizer sheared off some material when used, it did not affect weight. Then, as now, the only way to deal with weight variation was through sorting. Back then, we also sized the pellets after sorting out the weights. A lot of work for (arguably) little gain.
Summing it up
Manufacturing tolerances allow for mass production. Otherwise, we would handcraft each pellet to within .0001 grams of weight to each other, and we would probably manufacture two or three tins of pellets per day at a cost of, say, $150 per tin. Oops, can’t do that, now can we? Still, BB’s advice about weight-sorting your pellets is the most practical and useful piece of advice on this subject.
Enough for now, I’m off to send some of my pellets to the local testing lab to check on the standard deviation of the chemical and physical elements as relates to the density and weight of the lead alloy. Right! Typical airgunner!