Basic gunsmithing skills: DR lling holes accurately: no gunsmithing project will work if you can't drill holes the right size in the right spot. Here Matthews shares a few tips that will help you drill holes right.
learning. You won't know everything when you start, and you'll never stop learning. When I started hobby gunsmithing, I was in my early 20s and knew little of the subject. I did, however, know I wanted to learn. Thirty years later, I'm still studying and learning.
I have read countless books and magazine articles on the subject of gunsmithing and metalworking. They let me learn without having to experience my own failures.
Just as other gunsmiths' experiences helped me learn, you may find something in my experiences.
Until now, I haven't written much about the basic procedures you need to know in order to complete a lot of my projects. This time, I'll make some recommendations on how to do these basic gunsmithing chores. Keep in mind that one little paragraph in a magazine article is scant information on a procedure. Obtain gunsmithing and metalworking textbooks for more in-depth information. Since this article is intended for the novice, I will try to describe things in basic layman's terms and not get too technical. Many machining operations are described in minute detail in textbooks, but these tend to be too technical for casual reading. Here I'll give you some basic tips that'll help you get started in gunsmithing.
One of the most basic gunsmithing skills is drilling a precisely sized hole at a precise location. This is not as easy as it sounds. While a hole location and size in carpentry may be fine if it is within a 1/32" (.031"), this tolerance is almost unusable for most gunsmithing needs. Hole location and size for gunsmithing may need to be within a few thousandths of an inch. In fact sometimes they need to be within a few ten-thousandths of an inch.
The first thing to realize is that drill bits are not intended for forming precise diameter holes. A drilled hole is usually only within a couple thousandths of the specified size. Drill bits tend to drill a couple thousandths (even more on large bits) over their actual diameter. That's why a 1/8" (.125") bit is actually sized at about .123" to .124". If you need a precisely sized hole, drill initially a few thousandths undersize and then use a precision reamer to bring it up to full and precise size.
The tendency of a drill bit to drill over size is known as runout, and can be minimized by step drilling to within about 1/64" (.015") of full size, then using the full-sized bit to finish the hole. If you're OK with a hole slightly over actual bit size, drill the hole all at once with a full-sized bit.
Use this knowledge to size holes for tight or loose fits. How you run the drill can also affect final hole size. Spindle speeds, rate of feed, material being drilled and lubricant use can all impact final hole size.
Experience will teach you how to compensate for all these factors when drilling. Always measure the bit if you need a precise hole. Drill bits, especially cheap ones, are made to wide tolerances, and size can vary a lot. You can't drill a .125" hole with a .130" bit. Also use only quality drill bits made from good material such as M2 steel. Inexpensive low-quality bits tend to be made from poor steel and will perform poorly.
Drilling holes For gunsmithing projects really requires a drill press or milling machine. You may think you are great at drilling by hand, but there is no way you can drill to within a few thousandths of an inch with a hand drill. When hand drilling, you'll tend to push the drill bit away from the starting point as it engages the workpiece. Leave the hand drills for woodworking.
To drill straight holes, your drill press table must be square to the quill travel. A quick and easy way to get it close is to simply use a square between the table and drill chuck. This is adequate for general purpose drilling but not for precise drilling.
Once you have the table squared, up you need to do one more thing; install a drilling vise. Hand holding parts while drilling is an invitation to injury! I can't begin to count the times I haven't followed my own advice and got bit by parts that took off spinning after the drill bit dug into the work and grabbed. I recommend an adjustable compound drill vise to help precisely locate holes. Be sure to solidly clamp it to the table.
Once you have the part in the vise, it must be aligned with the quill travel to obtain a straight hole. A quick and easy method is to install a straight long pin in the chuck and align the part parallel to the pin. This pin method is also adequate for general purpose drilling, but not precision drilling.
Now that the parts are set up square, it's time to select your drill bit. Bits come in a bewildering array of styles, shapes, materials, lengths, and diameters.
Most general purpose drilling is done with common twist bits made from M2 or High Speed steel. Be sure to only use sharp bits, as dull bits just make the job harder to get right. Bits can be easily re-sharpened at home but the process is beyond the scope of this article.
The novice is best served by buying new bits when they become dull.
Careful drilling can greatly extend bit life. When you get a few dull bits, they can be sharpened by a knowledgeable friend or commercial tool grinders.
Drill bits need to be run at the correct speed range for best results. There are charts and formulas for determining the correct and optimum speed for each size and type of bit. Most non-production users simply guess at an approximate speed.
Small bits can be run very fast, but larger bits need to run fairly slow. While 2000 rpm may be fine for a 3/32" bit cutting soft steel, a 3/4" bit may only turn at a few hundred rpm. If you try to run a bit too fast, it will quickly overheat and become dull. Holes can be drilled with or without cutting oils. Cutting oils will allow for higher speeds and feed rates, but the oil is messy to work with. Cutting oils also tend to result in a better finish on the drilled hole.
To drill accurate holes, the bit must start in the correct location. You can't move the work to align the location once the bit starts cutting, as this will just result in a crooked hole. You should start all holes at a center punched location. This will prevent the bit from wondering off location when it is trying to cut into the workpiece.
The bit must be aligned with the center punch location. If you try to force the bit off center to align it with the center punch starting point, the bit will drill the hole at a slight angle. For general purpose drilling, [find that a pointed pin in the chuck can be used to align the center of' the chuck with the center of the center-punched starting point.
For accurate hole sizing, always drill in steps. Small holes, such as 1/4" or under, can be drilled in two or three steps. Start the hole with a bit that is sized to fit the center-punched starting point. You can't accurately start a 3/4" bit in a 1/32" center punch dimple.
Use a bit that will easily fit in the center punch dimple for the first step of drilling. Once that hole is drilled, then increase bit size till you get to the final size. On a large hole, you may need to drill in several steps to get a reasonably accurate hole. Besides allowing for more accurate hole sizing, step drilling will allow the drill press to be stressed less during drilling. Trying to cram a 7/8" bit all at once through steel with a small drill press can easily overstress or damage a drill press.
ONE OF THIE MOST BASIC GUNSMITHIING SKILLS IS DRILLING A PRECISELY SIZED HIOLE AT A PRECISE LOCATION.
Milling Machine Use
A perfect companion to a drill press is a small milling machine. While a small mill is great for milling parts to size, they are also something of a precision drill press if set up correctly. Precise drilling is mandatory for any kind of deep hole that needs to be accurately located and sized.
A milling machine can be set up for precise location on the surface, but if the workpiece is not set up correctly, the hole will exit at a point that is not aligned precisely with the starting point. To precisely align your workpiece with the travel of the quill, you will need a tool known as a test indicator. These measure only a few thousandths of an inch, since that degree of accuracy is required for correct set up.
Start by lightly clamping your workpiece in a milling machine vise that is secured to the mill's table. Get it as close to square as you can by eye. Place your test indicator in the quill of the mill and then place the measuring end of the indicator against the side of the workpiece.
Adjust the workpiece so that as the indicator is being moved up and down the sides of the workpiece, it stays on zero. When you get the indicator to stay zeroed as the quill moves up and down, this will indicate that the side of the workpiece is exactly in line with the quill travel.
Once the workpiece is aligned, the vise can be turned down tight. Depending on the squareness of your milling machine vise, you may have to adjust the front side also. This may have to be done with shims. This procedure applies to small milling machines with fixed heads.
If you have a mill with an adjustable head, the head needs to be adjusted square to the milling machine's table before you set up your workpiece. This setup procedure can be frustrating for novices, since you're trying to align parts to within only a few thousandths of an inch, but it does get easier with practice. Once you have the work set up precisely to the quill travel, any hole you drill should exit at the same location on both the top and bottom of the work.
When machining parts, the dimensions are often specified from some edge point. So how do you find the exact edge of the part? Simple: use a tool called an edge finder. An edge finder is a shaft of steel with a spring loaded extension on its end that wobbles as it turns. When machining parts this tool is worth its weight in gold.
The edge finder is placed in a collet (don't use a chuck) and spun at several hundred rpm. As it spins, the spring-loaded end wobbles since it is not centered. As the wobbling end is slowly moved up to contact the work-piece, the gap between them shrinks as they get closer.
At some point, the gap will be completely gone and the wobbling end will be perfectly aligned with the shaft of the edge finder. At this point, any further movement will cause the now smooth-running end to deflect sideways away from the workpiece and indicate that you've gone too far.
The difference between being right at the edge and spinning smooth and having the end piece deflected is only one half thousandth of an inch. This tool is very accurate if used correctly.
Since the end piece is .200" wide, it is still .100" from the actual edge. Lifting the edge finder up and moving the workpiece over with the table feed .100" will result in the center point of the quill being precisely in line with the edge of the work. Once you know the quill is precisely centered on the edge, other dimensions can be referenced from this point.
Before you can drill or machine parts, the dimensions need to be laid out on the workpiece. While most industrial users have digital readouts for locating dimensions, most hobbyists don't have that luxury and have to do it the old fashioned way with layout dye and scribes.
The layout methods used here can be accurate to about .005" if you're careful and skilled. I guess you could call this semi-precision layout work. Tape measures are totally worthless for this degree of accuracy. Dial calipers or precision scales with graduations in 1/64" (or squares) are the preferred measuring tools.
Use layout dye and a fine point scribe to obtain any type of reasonable accuracy. Felt tip markers, no matter how fine the point, are not useable. First apply some layout dye to the approximate area to be measured. Layout dye, sometimes called toolmaker's ink, is available in spray or brush forms.
Use your measuring tool to locate the dimension and then use the fine point scribe to mark it. Squares are especially useful for this job, since you can set the dimension on the square and then just scribe along the end of the scale.
Calipers can be used to scribe the lines from the edges, but be aware that this will quickly wear the points and affect accuracy later. You also have to be aware of any angle induced variations. The fine point of the scribe means this layout method can be very accurate.
If your lines were drawn out to locate a center punch location, getting the punch right at the intersection of the lines is a matter of skill and good eyesight. I have to use powerful reading glasses to see well enough for accurate placement.
When you make the first strike with the center punch, use a very fine point punch. This will make seeing the mark easier. Also only tap it lightly and then check for alignment. If it's off a little, you can move it slightly if the punch mark isn't deep. Once you have a good location and have it punched with a fine point punch, use a wider point punch to deepen the punch mark.
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|Date:||Mar 10, 2014|
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