I am often asked what tooling to use for drilling and finishing all of the critical holes of a Kalashnikova rifle. I’ve answered the question many times on different forums and decided to write it one final time. The following is not to be considered as a treatise on hole finishing.
Drill bits. The most basic cutting tools that exist and easy to purchase and use, for the most part. The “common” general purpose twist drill bit has a point of 118-degrees included angle. Why such an “odd” angle? It is a matter of easing shop floor calculation. With a 118-degree point, the full diameter of the drill bit is almost exactly (0.3)*(diameter) from the point and this makes calculating and setting depth very easy. While these type drill bits work fine for handyman projects, they are not well-suited to machine holes on hardened steel alloys. The 118-degree general purpose twist drill has a “chisel” point that performs no cutting at all – it swages metal from the center to the sides so it can be removed by the lips of the drill bit point. This works fine for wood and soft materials but causes problems when used on hardened steel or alloy steels.
A far better choice is the 135-degree included angle drill bit point that is referred to as the “split point” or “crankshaft grind”. These bits have an extra grinding operation to reduce the thickness of the non-cutting web and allow the “chisel” of the point to do some cutting, rather than just shoving material to the side. This allows the drill bit to cut more freely and with less heat and distortion, and this type of point allows the 135-degree split point drill bit tip to remain on center much better than that of the common 118-degree point tool.
Now, couple this improved point design with a short drill bit of heavy section known as the “stub” or “screw machine” drill bit and one has a tool that will stay on center very well and not work-harden alloy steel by spinning rather than cutting. In general, if one purchases “screw machine or stub drill bits with 135-degree split point” then one has the best commercial drilling bits for alloy and hardened steel. These are available from any good industrial supply firm such as McMaster-Carr. Hint – buy American, and buy Cobalt for the harder steels.
Regarding dowel pins; the engineering practice for just about every type of inch or Metric machine assembly is what we call “basic shaft” where the pin is produced as closely to nominal size as possible and the material for the interference fit is placed on the hole by finishing the hole slightly smaller than the nominal size. Conversely, the Kalashnikova design is what we call the “basic hole” system where the hole is produced as closely as possible to its nominal size, and the material for the interference fit is placed on the dowel pin. This means that commercially-available dowel pins will not work on these rifles since they will be too small. These rifles require specific dowel pins that have the additional material.
Kalashnikova rifles use dowel pins of 2,5mm, 3,0mm, 4,0mm, 6,0mm and 7,0mm depending on the Country of production, type of rifle, and location of the pin. All of these pins are larger than nominal size in order to generate the interference fit required.
For these rifles the best practice to control the size and shape of a finished hole is to drill slightly smaller than the nominal size, and then finish with a reamer of exact size. How much smaller should the holes be drilled prior to reaming? They should be drilled from 0,15mm to 0,3mm (0.006″ to 0.011″) smaller than the finished size with less material remaining for the smaller holes.
Equivilents: 2,5mm = 0.0984″. 3,0mm = 0.1181″. 4,0mm = 0.1575″. 6,0mm = 0.2362″. 7,0mm = 0.2756″. These are the sizes of the finished holes, and the sizes of reamers required. The drill bits to “rough in” these holes should of course be smaller. Which type of reamers? The straight-flute chucking reamer is quite adequate for assembly tasks of this rifle.
How much interference is correct for these pins? These pins can be safely assembled with 0,013mm (0.0005″) to 0,025mm (0.0010″) interference – smaller diameters require lower interference levels. If the levels of interference are higher, these pins can be deformed by bending, swelling, or galling. They are hardened – but not very much.
My practice has remained to finish the holes to correct size, and if necessary to alter the pin diameter to make a correct fit. This requires a method to “spin” the pin and carefully use a small file or oilstone while measuring with a micrometer – not a beam caliper. These pins must be able to be driven or pressed in without bending or swelling them. If one does not have a lathe, one surely has a drill press or even a drill motor.
If one has “used” pins from a demiled rifle, then one generally has pins that are damaged from the initial forcing in, and forcing out. It is very common to find that these pins are swollen on each end. These must be repaired prior to use, and this repair is possible by spinning the pins in a method at hand, and judiciously using a fine file and/or oilstone as well as a micrometer. The goal is to render each end of the pin the same diameter as the middle of the pin.
I used to assemble a lot of Zastava M85 and M92 barrels from virgin kits. The virgin diameter 3,0mm pins supplied with those kits are seriously oversized and must be reduced. I have not noticed any other virgin Kalashnikov pins with this much material.
Now, let’s get back on topic regarding cutters – how fast should these cutters spin?
Cutting speeds in the USA are often expressed in terms of “feet per minute” (FPM) using the circumference of the cutter (or part, i.e. a part chucked in a lathe) for the calculation. The proper feet per minute of any material is already determined by testing; for the most part. The values are based upon trials made with high-speed steel cutters.
So, what is the “quick and dirty” RPM calculation for spindle speeds? Easy. For roughing cuts, the spindle speed is: RPM = (FPM * 3) / diameter and for finishing it is RPM = (FPM * 4) / diameter.
A few common cutting speeds: Mild steel = 100 FPM, yellow brass = 200 FPM, aluminium = 300 FPM, alloy or high carbon steels = 50-75 FPM, hardened yet machinable steels = 25-50 FPM. The parts of Kalashnikova rifles can be classified as “hardened yet machinable” since they are exactly that. An entire chart of cutting speeds can be found on the web.
Let’s make a couple of practice calculations. The part is made of mild steel and the drill bit is diameter 0-1/2″. The cutting speed for mild steel is 100 FPM, and for safety we will use the “roughing” formula.
RPM = (FPM * 3) / diameter, so let’s work it out: RPM = (100 * 3) / 0.5, hence RPM = 600.
If a diameter 0-1/2″ mild steel part was mounted in a lathe and turned using high-speed steel tools the RPM would be the same.
What about Cobalt alloy tools? Cobalt alloy tools are excellent when (a) longer tool life is needed, (b) higher production speeds are needed, and (c) harder materials are being machined. When using Cobalt alloy the cutting speeds can often be increased from 15% to 25%.
What about tungsten carbides? On very rigid purpose-built machinery, tungsten carbide cutters allow the highest rates of metal removal and allow the machining of materials that are too hard for high-speed steel tooling. On general-purpose machinery, tungsten carbide cutters are often a waste of good money. Tungsten carbide removes material by “shoving” rather than cutting and light general-purpose machines have neither the rigidity nor the horsepower to effectively use this cutter material.