Hi boys,

It has been awhile since I have posted here and it is nice to see some new faces mixed and with ol' regulars!

It seems that the forms have been rather slow of late, but that is understandable considering the summer affords us outdoor types an opportunity to get outside and do what we love the best whenever that may be... in my case that is a little shooting and some fishing and thinking of new projects to work on; as though I don't have enough of those!

Yesterday, I was speaking with a friend concerning some specifications and tolerances for a custom copper CPU heat sink that he is having fabricated for one of his U1 rack mount servers. This is the airgun form, not a computer form, so I won't talk about the details surrounding the computer issues... but one thing that struck me during the course of my conversation with my buddy was his lack of understanding and ability to visualize what .015 really was (measured in inches).

That got me to thinking in terms of measurements for airgun parts. Many times, we see people quote measurements that are expressed as decimals and it struck me that perhaps a good post would be concerning decimal measurements and what they really equate to in terms of real world objects that we see everyday... so that's what this post is going to be about.

Before I get started giving some examples, I would like to make mention of top hats and spring guides as that is where we see a good deal of use of decimal measurements are concerned... Springs fall into this category to do I don't want to throw too many topics into the pot... let's just keep it simple for an easier understanding of some things that we all hear and talk about.

Some people advocate using top hats and spring guides that are tight...
Some people advocate using top hats and spring guides that are snug...
And on the rarest of occasions we will see people who advocate using top hats and spring guides that are not necessarily lose but most certainly are not snug or tight in their fit.

I don't want to this to become a debate over how top hats and spring guides should be fitted to the spring although I will say that having them tight has never proven to me to be beneficial over having them fit snug within the inner diameter of the spring. However, a looser (not to be read as LOOSE) fitting spring guide will allow for a slight increase in the overall power output in a well tuned gun, but it is really just one of many elements to look at if a person is tuning a gun purely for the purpose of power.

I have never advocated using extremely lose and sloppy fitting spring guides even though I am one of those outcasts which have come to be known as "Magnum Junkies" - a badge which I WEAR PROUDLY! As with all things in life, moderation is the key, and a reasonable and logical balance should be the goal for most things we try to accomplish in life...tuning airguns included! But again, I will say with the utmost respect that I don't wish to turn this into a debate on the subject of power and proper tuning techniques...

So getting back to the topic at hand... decimal measurements of some real world items that we see on a day-to-day basis...all measurements listed below are in inches....we'll do metric conversions a bit later on in this post.

Listed smallest to largest:

Human Hair - .001
Newspaper Page - .0025
Standard White Printer Paper - .0035
Yellow "Sticky Notes" - .0045
Single Edge Razor Blade - .009
Glossy Business Cards - .015
My Thumb Nail - .028
Credit Card - .0295
Back Edge (dull) of a Single Edge Razor Blade - .038

So there are some things to think about.....it makes ya kind of wonder if arguing about making the O.D. for the barrel (shaft) of a spring guide being .570 as opposed to .573 is really worth arguing about....quibbling over the difference of (3) human hair thicknesses is a bit silly in my humble opinion...yet, I see these kinds of arguments all of the time on various forums and in talking to people.

It would be more productive to focus on parts being grossly out of spec. OR to talk about specs and tolerances of piston seals or breech seals, etc...places where a few thousandths REALLY DOES make a tremendous difference in the performance of a spring powered airgun...again, that is just my humble opinion!

Personally, I am more concerned about deviation in the tolerances held in the skirts of pellets - not to mention the lengths of pellets....this does have an affect on the weight of the pellet and thusly, the performance as well...you know, (more mass vs. less mass ||| mass = weight ||| sectional density, etc.)....rolls eyes....ok...LOL

For example I have in my hand (3) .22 caliber Beeman Kodiaks that I had sorted out per their weight. I sort all pellets into 3 weight categories in an effort to create more consistent batches of pellet that I shoot...ya know – 3 weight categories of Kodiaks, 3 categories of Crow Magnums, etc...I do this because there IS a good bit of variation from pellet to pellet even when they are the same make, model, and caliber...just throw some on your grain scale and see for yourself!

Let’s look at the variation in the spec. tolerances from the three pellets that are from the same weight batch (21.1 – 21.3 grains):

Pellet (1) --- Length = .3320 --- Head Diameter = .2165 --- Skirt Outer Diameter = .2240
Pellet (2) --- Length = .3355 --- Head Diameter = .2120 --- Skirt Outer Diameter = .2215
Pellet (3) --- Length = .3335 --- Head Diameter = .2158 --- Skirt Outer Diameter = .2290

Review:

Greatest Deviation in Length: .0035 (= to) 3 and a half human hairs

Greatest Deviation in Head Diameter: .0045 (= to) 4 and a half human hairs

Greatest Deviation in Skirt Outer Diameter: .0085 (= to) 7 and a half human hairs

Conclusion:

The variations in pellet specs and "tolerances held" are a lot more important than the difference in specs for the O.D. of the barrel (shaft) of a tophat or spring guide...the variations in the I.D. of a main spring's specs and "tolerances held" can vary by up to .009 (which is almost a full 1/100 (.01) to over .045+ (per the ones that I just spec’ed out tonight).

Areas of an airgun where I think more attention needs to be paid to specs. and tolerances.

1. Bore/breech inner diameter
   
2. Trigger blade and internals (especially the holes for the pins in the internals and in the trigger housing)
   
3. Piston seals and breech seals
   
4. Finally....the pellets that we shoot....
   

Final Thought:

Holding tight specs is important, but we need to try and remember that specs and "held tolerances" of our barrel's Inner Diameter (I.D.) and the pellets that we shoot are of a considerably greater importance than holding to the closet .001 on a tophat or spring guide...I would like to see a greater level of attention paid to the specs and "held tolerances" of our trigger blades and trigger internals...that is just a secondary thought, but never the less and important one.

Here is a fraction to decimal conversion table that you can copy and print...it’s handy to have at hand.
(select the table and copy and paste it into a text file [.txt] [notepad if U R on a Windows based computer]...all computers regardless of OS type can generate a .txt file...)

Eights
1/8 = 0.125
1/4 = 0.250
3/8 = 0.375
1/2 = 0.500
5/8 = 0.625
3/4 = 0.750
7/8 = 0.875


Sixteenths
1/16 = 0.0625
3/16 = 0.1875
5/16 = 0.3125
7/16 = 0.4375
9/16 = 0.5625
11/16 = 0.6875
13/16 = 0.8125
15/16 = 0.9375


Thirty-seconds
1/32 = 0.03125
3/32 = 0.09375
5/32 = 0.15625
7/32 = 0.21875
9/32 = 0.28125
11/32 = 0.34375
13/32 = 0.40625
15/32 = 0.46875
17/32 = 0.53125
19/32 = 0.59375
21/32 = 0.65625
23/32 = 0.71875
25/32 = 0.78125
27/32 = 0.84375
29/32 = 0.90625
31/32 = 0.96875


Sixty-fourths
1/64 = 0.015625
3/64 = 0.046875
5/64 = 0.078125
7/64 = 0.109375
9/64 = 0.140625
11/64 = 0.171875
13/64 = 0.203125
15/64 = 0.234375
17/64 = 0.265625
19/64 = 0.296875
21/64 = 0.328125
23/64 = 0.359375
25/64 = 0.390625
27/64 = 0.421875
29/64 = 0.453125
31/64 = 0.484375
33/64 = 0.515625
35/64 = 0.546875
37/64 = 0.578125
39/64 = 0.609375
41/64 = 0.640625
43/64 = 0.671875
45/64 = 0.703125
47/64 = 0.734375
49/64 = 0.765625
51/64 = 0.796875
53/64 = 0.828125
55/64 = 0.859375
57/64 = 0.890625
59/64 = 0.921875
61/64 = 0.953125
63/64 = 0.984375


Well....I hope this helps in some small way....I am not going to spell check this as I am tired and need coffee and a smoke!

Good luck and good shooting to all!!!!

-Curtis

Folks always seem to be looking for closer and closer tolerances. And, like everything else in this world, they kind of expect perfection. Pellet measurements fall into this category. I view bulk pellets in the same light as non-match firearms ammo. They are produced within a range of tolerances, much the same as the chambers of the guns that take them. Sorting ammo will produce better accuracy but it's the price in manpower you pay for not spending more for higher grade fodder. And I'm always pointing out that guns like a Diana 350 are not designed for 10 meter work, so you kind of have to adjust your expectations. Such guns usually do what they are intended to and with surprising efficiency, even stock.

I think the one piece of an airgun that needs the most tweaking is the nut behind the trigger. No, not the little one that adjusts the sear, but the one supporting the buttplate.

Now, if you could just explain liters and pints, I would appreciate it?

How many shots does this thing hold????

While I mostly agree with you Curtis, there is one aspect that you did not touch upon that merits more discussion:

Individuality of each gun and each component.

I've made many a spring guide, and none have needed exactly the same size. Some thousandths in the spring guide does make a remarkable difference in the gun's behaviour, especially with Bronze guides.

Because the guides depend on each spring is why I decided to make the guides in two sections: the rear washer and the tube.

I've often wrote that you need to LISTEN to each gun, that each gun will tell you what it needs.

My rule of thumb is to measure the inside of the spring and make the guide to that measurement + 0.005" being that springs increase their internal diameter when they are compressed, that interference is not too large and does not reduce power noticeably. The spring does not touch the guide when the the most power is needed and only gets damped when the piston reaches the end.

Good post!



Un Abrazo!




H�ctor