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Machine Shop Practice: Making Custom Metal Hardware For Basses
- Thread starter Bruce Johnson
- Start date
Following @Bruce Johnson 's post regarding squares, I'm gonna share this link. If you want some quality tools at a reasonable price from a small American company, check here -
Tools > Measuring > Machinist Squares
Tools > Measuring > Machinist Squares
I’ll second that. I’ve bought a couple of things from them after watching a Stumpy Nubs video. Good quality at decent prices.
@mapleglo I appreciate the formula and all but can I just jump right to n+3 or maybe 5? It just seems more accurate.
Yeah, for me the formula is: n + m, where m is also a variable, not equal to 0 or 1. The number of additional tools I need is not constant, but always a positive number greater than 1. Over the years my Total Number Of Tools becomes a large number, difficult to calculate. Or even estimate.
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An interesting company. I poked around their site a bit. It looks like they specialize in buying up overrun lots of specialty tools from larger manufacturers. Good quality tools, not cheapie junk. Their prices are reasonable, discounted some from the OEM price, but not super-bargains.
Easy-peasy. Take a walk in the woods near a golf course, file handles galore.
Or you can take up wood-turning.
Or you can drill holes in some scrap wood. With or without a reinforcing ring (ferrule) to reduce splitting (scrap pipe, wrapped wire, what you like)
They like to try to compete with Woodpeckers on quality, but beat them on price. It's a good source for some really good tools.
Okay, let's talk about drilling methods. Specifically, how to drill holes in accurate positions in metal blocks. Which eventually become hardware for basses. I think every piece of custom hardware we make has holes in it. And they usually need to be in fairly precise locations to line up with other stuff.
There are two basic ways to drill a hole in a specific location. I call them Spot Drilling and Machine Drilling.
Spot Drilling:
In Spot Drilling, you start out by making a small cone-shaped indentation in the metal, called a Spot, at the precise location where you want the hole to be drilled. Doing Spot Drilling in a drill press, you usually use a standard Jobber style drill bit. Clamp the block in a vise, preferably with some of those cool stepped jaws to keep it level and perpendicular. And the vise slides around freely on the table of the drill press. This is actually important. The vise should not be clamped down when Spot Drilling. The whole idea of Spot Drilling is that, when the spinning tip of the drill bit engages the Spot in the block, it actually pushes the block and vise into position, centered accurately under the drill bit and spindle of the drill press. The cone tip of the drill bit engages the cone of the Spot, and they line themselves up.
This is an important principle to understand. You slide the vise around so the Spot visually looks like it's directly under the tip of the spinning bit. But the bit itself does the final little kick to bring the Spot into precision alignment. If you try to lock the vise down to the table, any tiny misalignment between the Spot and the drill bit will cause the drill bit to deflect as it starts drilling. This can cause the hole to be off position, off perpendicular or even oval shaped. Jobber drill bits are surprisingly flexible, and they will deflect while spinning, trying to follow the spot. That's how holes end up "walking" off position, or breaking out of the back side of the block way out of place. You have to allow the drill to pull the Spot into alignment.
In the next post, I'll go over the process of Spot Drilling in more detail, including how to make a really good Spot. An accurate Spot is more than just a smack with a center punch.
Machine Drilling:
Machine Drilling specifically does not use a Spot. You solidly lock the block and the vise under the drill bit and spindle. Push the spinning drill bit straight down into the metal, and it will center the hole right on that location, right where you point it. This requires that you have some way of accurately locating the block, exactly where you want it, and holding it there during the drilling. It should not be able to slide around.
There are different ways to accurately locate the block for Machine Drilling. Here are quick summaries of a few:
In Machine Drilling, you also use a different type of drill bit. They are commonly called Stub Drills or Screw Machine Drills. They are shorter in length and have a thicker center web than the common Jobber drill bits. And the tip is ground more precisely. They are made specifically for Machine Drilling, to not flex or walk, and to put the hole exactly where you point it.
At the bottom are two standard Jobber drills. In the center are two Screw Machine drills. At the top are two Spotting Drills. These are special drill bits made specifically for...drilling Spots. I'll show how they are used when I go into Spot Drilling in more detail.
I keep a full set of Jobber drills in the metal shop, all the fractional, letter, and number (wire gauge) sizes. I buy the Screw Machine drills in only the sizes that I use most often in metalworking. I have maybe 25 sizes in stock. I buy them a couple at a time from McMaster.
Writing this, I realized that I should also add a sideline post in this thread about precision drilling in wood. These same general techniques as applied to wood. I'll do that a little further down.
There are two basic ways to drill a hole in a specific location. I call them Spot Drilling and Machine Drilling.
Spot Drilling:
In Spot Drilling, you start out by making a small cone-shaped indentation in the metal, called a Spot, at the precise location where you want the hole to be drilled. Doing Spot Drilling in a drill press, you usually use a standard Jobber style drill bit. Clamp the block in a vise, preferably with some of those cool stepped jaws to keep it level and perpendicular. And the vise slides around freely on the table of the drill press. This is actually important. The vise should not be clamped down when Spot Drilling. The whole idea of Spot Drilling is that, when the spinning tip of the drill bit engages the Spot in the block, it actually pushes the block and vise into position, centered accurately under the drill bit and spindle of the drill press. The cone tip of the drill bit engages the cone of the Spot, and they line themselves up.
This is an important principle to understand. You slide the vise around so the Spot visually looks like it's directly under the tip of the spinning bit. But the bit itself does the final little kick to bring the Spot into precision alignment. If you try to lock the vise down to the table, any tiny misalignment between the Spot and the drill bit will cause the drill bit to deflect as it starts drilling. This can cause the hole to be off position, off perpendicular or even oval shaped. Jobber drill bits are surprisingly flexible, and they will deflect while spinning, trying to follow the spot. That's how holes end up "walking" off position, or breaking out of the back side of the block way out of place. You have to allow the drill to pull the Spot into alignment.
In the next post, I'll go over the process of Spot Drilling in more detail, including how to make a really good Spot. An accurate Spot is more than just a smack with a center punch.
Machine Drilling:
Machine Drilling specifically does not use a Spot. You solidly lock the block and the vise under the drill bit and spindle. Push the spinning drill bit straight down into the metal, and it will center the hole right on that location, right where you point it. This requires that you have some way of accurately locating the block, exactly where you want it, and holding it there during the drilling. It should not be able to slide around.
There are different ways to accurately locate the block for Machine Drilling. Here are quick summaries of a few:
- Milling Machine (or Mill/Drill); The block and vise are positioned using the X and Y axes and the DRO's or micrometer dials.
- Drill press with an X-Y vise, setting the position using a Master. I'll show that soon.
- Drill press with a standard vise, clamped down, setting the position with a Master.
- Drill press with vise, with a Drill Fixture, using Drill Guide Bushings.
- Pin Drilling; A drill press fitted with a vertical pin aligned directly under the spindle. The block clamps in a fixture, which has a hole in the bottom that fits on the pin.
In Machine Drilling, you also use a different type of drill bit. They are commonly called Stub Drills or Screw Machine Drills. They are shorter in length and have a thicker center web than the common Jobber drill bits. And the tip is ground more precisely. They are made specifically for Machine Drilling, to not flex or walk, and to put the hole exactly where you point it.
At the bottom are two standard Jobber drills. In the center are two Screw Machine drills. At the top are two Spotting Drills. These are special drill bits made specifically for...drilling Spots. I'll show how they are used when I go into Spot Drilling in more detail.
I keep a full set of Jobber drills in the metal shop, all the fractional, letter, and number (wire gauge) sizes. I buy the Screw Machine drills in only the sizes that I use most often in metalworking. I have maybe 25 sizes in stock. I buy them a couple at a time from McMaster.
Writing this, I realized that I should also add a sideline post in this thread about precision drilling in wood. These same general techniques as applied to wood. I'll do that a little further down.
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Would it be a good idea if you started with a jobber bit, and once established, continue with a standard drill bit in order to get the most life out of the jobber bits?
I think you got the names mixed up. Jobber bits are the "standard" bits. I think you are asking whether you can start a hole with a Screw Machine bit, then continue with a Jobber bit? Yes, that's how you Machine Drill deep holes.
To get more life out of the Screw Machine bits? I'm not sure why? Good quality Screw Machine bits are about the same price as good quality Jobber bits. Of course, they are both more expensive than cheapo bits. Which I don't recommend for Fine Metalworking anyway.....
Check this one out..
.0086 (.22mm) drill.
Into 440c with a big ‘ol Bridgeport @ 4000rpm (should be going 20,000rpm).
.0086 (.22mm) drill.
Into 440c with a big ‘ol Bridgeport @ 4000rpm (should be going 20,000rpm).
One more.
2 - .0124 holes exactly .003 apart with the same old Bridgeport. .062 side hole goes exactly half way into .0124 hole on each side. Hardened and lapped face.
Other one is a .0086 hole offset exactly .004 from center. If it’s more or less offset I have to scrap it and make another.
I use a Mitutoyo tool makers microscope to measure the parts.
2 - .0124 holes exactly .003 apart with the same old Bridgeport. .062 side hole goes exactly half way into .0124 hole on each side. Hardened and lapped face.
Other one is a .0086 hole offset exactly .004 from center. If it’s more or less offset I have to scrap it and make another.
I use a Mitutoyo tool makers microscope to measure the parts.
Nice work! That's some of the Real Machining that I refer to on this thread. The Metalworking that we do when making up hardware for basses is one or two orders of magnitude lower in accuracy.
Drilling tiny holes in steel has its own problems and tricks. In making up truss rod components, I use 2mm (0.079") dowel pins to hold them together. I regularly have to drill 2mm holes through brass, aluminum, and 304 stainless. Even that size can get a little tricky. You have to be very careful of the feed rate and downforce. The bit is small enough that, if the chips don't flow up and out the spiral smoothly, they will jam up in the hole and snap off the bit. But, if the feed is too light, you can heat up the stainless down in the hole, work-hardening it and dulling the tip of the bit. Drilling tiny holes is a very Sensitive operation. You have to be able to feel what you are doing in your fingers on the controls.
I recently acquired a beautiful old 1925 Avey Sensitive Drill, a special drill press designed for drilling tiny holes. It's a joy to use, and I've learned a lot from it.
Spot Drilling:
Here's a closer look at the technique of Spot Drilling. This is the method used to locate a hole in a metal part, accurately, using hand tools and a drill press. We use it for making one-offs or a few parts, where we don't want to take the time to set up for Machine Drilling. Machine Drilling is used when you need to drill holes in accurate locations in a batch of identical parts. But setting up for Machine Drilling usually requires the use of a Master part. I'll explain that as we get into Machine Drilling. And that Master part is usually made by.....Spot Drilling.
The Spot in Spot Drilling is a small cone-shaped depression drilled into the surface of the metal. That cone is a guide for the tip of the drill bit. The part is normally clamped in a vise, which rests on the table of a drill press. The bottom surface of the vise, and the surface of the table are smooth. The vise is able to slide around easily on the table. That is very important in Spot Drilling. You slide the vise over to visually line the Spot up directly under the tip of the drill. Close by eye. As you bring the spinning drill down, and the cone-shaped tip contacts the cone-shaped Spot, it literally kicks the part into alignment with the spindle of the drill press. The drill bit pushes the part sideways, sliding the vise on the table a small amount, lining up the centers of the two cones.
This "kicking" movement is tiny, maybe only 0.005"-0.010", but it's very important to allow it to happen. If you clamped the vise down to the table with the two cones mis-aligned by 0.005", when the bit hits the spot, the drill bit will flex sideways as it tries to drill into the cone. The hole starts out 0.005" off position through the top surface of the part. That may not sound like much, but that angular deflection adds up as the drill goes down through the part. When it comes out the back, it may be off 0.060", a 1/16". That's a big problem. That's how you end up with holes in metal parts that don't line up, and screws that go in crooked.
When Spot Drilling, you should be able to feel that little kick in the vise as you are holding it. Same if you are drilling a flat plate, not in a vise. As you just start drilling into the surface, lift the bit up for a second and set it back down into the hole. Watch very closely. The bit should not be deflecting sideways as it goes into the hole. If it is, woah, don't keep drilling! Lightly tap the vise a little bit on the side to bring it into better alignment. Test it again, bringing the bit down lightly into the hole, watching to see if it is deflecting. You may have to help it like that if the vise and part are fairly big and heavy, and the bit is small in diameter. The bit may be too weak to kick the vise sideways.
Back to our discussion of types of drill bits, this is where a long standard Jobber bit can cause a problem. In smaller sizes, particularly under 3/16", Jobber bits are very flexible. You may put a center punch mark on your part, and hold tight on the vise. It sure looks like the bit hit the center punch mark as you drove it down in there. But the hole ends up "walking" out of position and being crooked. What happened? The bit deflected as it went into the punch mark, and wasn't able to kick the part into alignment.
This is part of the reason for using the shorter Screw Machine drill bits. They are shorter, with a thicker web, and take much more force to deflect. They have more muscle to kick the part when they hit the cone. Screw Machine drills also are sharpened to a more precise point, called a Split Point. It goes to a thinner section at the web, at the point. A Screw Machine drill will also do a better job starting its own hole, when you aren't providing it with a Spot. We'll see that in Machine Drilling.
The other important part of Spot Drilling is, of course, making a good Spot. The Spot needs to located exactly where you want the hole to go, and it needs to be a nice clean cone. A simple smack with a center punch isn't good enough. It takes several steps to make a good Spot.
This morning, I was making up a part for my Medium Green Pin Router. You may have seen the ongoing project over on the Pin Routers thread. This is an aluminum plate that needs a couple of holes drilled and tapped in it. It's a one-off special part, so there's no need to set up a milling machine to drill these holes. It's a good part to demonstrate Spot Drilling.
Layout basics: I decided that I want these two holes to be parallel to the edge, 3/8" from the edge. This is a job for the small Double Square and the Machinist's Scriber. I set the square to 3/8", hold it along the side of the plate at the approximate locations of the two holes, and draw the scriber along the end of the blade at the two locations. I'm fumbling here, trying to hold everything with one hand for the camera, but the scriber should be tilted at an angle from vertical, so the very point drags along the end of the blade. The square is being used to set the distance of the scribed line from the edge.
I also want the first hole to be 3/8" from the end of the plate. I turn the square around the corner, and scribe a cross line right there.
Now I use the Automatic Center Punch to make a very small punch mark right at the intersection of the two scribed lines. There's a trick to this. The point of a small automatic center punch like this is ground to a small narrow angle, narrower than what you want the Spot to be. This fine point is there to help you find scribed lines.
Lean the punch at a small angle and drag it lightly across the metal until you feel it click into one of the lines. Then, lightly drag it down that line until you feel it click into the intersecting line. If you hold the punch lightly, dragging it with just the weight of the punch itself, you can actually feel it fall into a 0.002" wide scribed line. And you can locate a precision punch on scribed lines to within 0.001". By hand and feel.
When you reach the intersection of the scribed lines, gently stand the punch up to vertical and push down on it. It cycles with a big click and leaves a tiny punch mark, right at the intersection of the lines. That's why we like these little automatic center punches. You can feel the lines and make the punch in seconds with one hand.
If you don't have a small Automatic Center Punch, the old-school way was to use a Prick Punch. Politically Incorrect these days I'm sure. A Prick Punch is a smaller version of a Center Punch, with the tip ground to a narrower angle, very sharp point. Same technique; drag it lightly to find scribed lines, stand it up vertically. Then use the other hand to pick up a small Machinist's Hammer and make a single tap on the head of the Prick Punch. A tiny little punch mark, in the exact right place. That's classic machine shop technique.
Kind of blurry, but there's the teeny punch mark at the intersection of the scribed lines.
I forgot to mention these before, but a good quality set of dividers is an important tool for layout on metal parts. I have dividers of all different sizes on my metalworking and woodworking benches. These four are the ones I most often use for metalworking. They are all Starrets, top quality. One of those tools to watch for at flea markets and when cleaning out garages.
Dividers are used for scribing a line a precise distance away from....a punch mark.
The Dividers are set using a Machinist's Ruler. I'm using the feel of the points of the dividers dropping into the engraved lines of the ruler. In this case, I'm setting the Dividers to 1.500".
I set one point into the groove at the 1" mark. Then adjust the Dividers until the other point just drops into the groove at 2.50". Good quality Machinist's Dividers can be adjusted very finely, and with a few seconds of patience, you can set them very accurately using this technique.
Then, set one point of the Dividers into the first tiny punch mark, and gently use the other point to scribe an arc, intersecting the other horizontal line. There's the other hole location.
Same technique with the Automatic Center Punch. Feel the lines, make the small punch mark.
I scribed and punched a third hole location using a similar technique with the square and ruler. I often use a Sharpie to make a circle around each punch mark, so I can see them in the next steps. A Precision Machinist's Sharpie. That's a joke around here.
The next step is the actual Center Punching. A Center Punch is larger, and the point is ground to a wider angle, usually 90 degrees. It gets smacked with a hammer. I move the plate over on top of a small anvil that I have right next to the machine shop bench. The technique is to gently slide the Center Punch around until you feel it click into the tiny punch mark. Then hold it vertically and give it one or two smacks with the hammer. I have an 8 oz ball peen hammer right there at the anvil. This enlarges the punch mark and widens the angle.
Be careful that the punch doesn't bounce between smacks. Feel that it's in the punch mark before hitting it a second time. Or, you'll get a double mark.
Here's what the punch marks look like after Center Punching. They are larger and wider angle, and still correctly right on the intersecting scribe lines. But, notice how there's a ring of metal raised up around the punch.That's the aluminum that was displaced when the punch made the cone.
That raised ring is a potential problem. If you push a spinning drill bit down into that punch mark, the cutting edges of the bit may catch on that raised ring, pushing the drill bit off to the side. It may kick out of alignment. And start drilling out of position. This is what can happen if you skip steps and only center punch the location, and go right to drilling.
This is what Spot Drills are for. A Spot Drill is a special drill bit that's very short and rigid, and its cutting edges are ground just for making center punch marks into Spots. They are an essential tool in precision metalworking. I keep one small drill press always set up with a small Spot Drill in it, and it gets constant use. This is a small 1/8" x 90 degree Spot Drill, which is the size I mostly use. I notice that McMasters calls them Hole-Starting Carbide Drill Bits; $14.35. Well worth the investment if you want to do precision metalworking.
Here's what the completed Spots look like. A one-second touch with the Spotting Drill cleans off the raised rim and makes a nice neat little cone. Now it's ready for drilling!
Drilling the holes. That's a #21 Screw Machine bit, because I'll be tapping the holes 10-32.
And note that I'm using that low-profile vise with its cool new aluminum jaws. Just the right size for this part.
That's the technique of Spot Drilling. Hand layout, and drilling in a drill press, to accurate locations.
Next: Drilling larger holes, and moving on to Machine Drilling technique.
Here's a closer look at the technique of Spot Drilling. This is the method used to locate a hole in a metal part, accurately, using hand tools and a drill press. We use it for making one-offs or a few parts, where we don't want to take the time to set up for Machine Drilling. Machine Drilling is used when you need to drill holes in accurate locations in a batch of identical parts. But setting up for Machine Drilling usually requires the use of a Master part. I'll explain that as we get into Machine Drilling. And that Master part is usually made by.....Spot Drilling.
The Spot in Spot Drilling is a small cone-shaped depression drilled into the surface of the metal. That cone is a guide for the tip of the drill bit. The part is normally clamped in a vise, which rests on the table of a drill press. The bottom surface of the vise, and the surface of the table are smooth. The vise is able to slide around easily on the table. That is very important in Spot Drilling. You slide the vise over to visually line the Spot up directly under the tip of the drill. Close by eye. As you bring the spinning drill down, and the cone-shaped tip contacts the cone-shaped Spot, it literally kicks the part into alignment with the spindle of the drill press. The drill bit pushes the part sideways, sliding the vise on the table a small amount, lining up the centers of the two cones.
This "kicking" movement is tiny, maybe only 0.005"-0.010", but it's very important to allow it to happen. If you clamped the vise down to the table with the two cones mis-aligned by 0.005", when the bit hits the spot, the drill bit will flex sideways as it tries to drill into the cone. The hole starts out 0.005" off position through the top surface of the part. That may not sound like much, but that angular deflection adds up as the drill goes down through the part. When it comes out the back, it may be off 0.060", a 1/16". That's a big problem. That's how you end up with holes in metal parts that don't line up, and screws that go in crooked.
When Spot Drilling, you should be able to feel that little kick in the vise as you are holding it. Same if you are drilling a flat plate, not in a vise. As you just start drilling into the surface, lift the bit up for a second and set it back down into the hole. Watch very closely. The bit should not be deflecting sideways as it goes into the hole. If it is, woah, don't keep drilling! Lightly tap the vise a little bit on the side to bring it into better alignment. Test it again, bringing the bit down lightly into the hole, watching to see if it is deflecting. You may have to help it like that if the vise and part are fairly big and heavy, and the bit is small in diameter. The bit may be too weak to kick the vise sideways.
Back to our discussion of types of drill bits, this is where a long standard Jobber bit can cause a problem. In smaller sizes, particularly under 3/16", Jobber bits are very flexible. You may put a center punch mark on your part, and hold tight on the vise. It sure looks like the bit hit the center punch mark as you drove it down in there. But the hole ends up "walking" out of position and being crooked. What happened? The bit deflected as it went into the punch mark, and wasn't able to kick the part into alignment.
This is part of the reason for using the shorter Screw Machine drill bits. They are shorter, with a thicker web, and take much more force to deflect. They have more muscle to kick the part when they hit the cone. Screw Machine drills also are sharpened to a more precise point, called a Split Point. It goes to a thinner section at the web, at the point. A Screw Machine drill will also do a better job starting its own hole, when you aren't providing it with a Spot. We'll see that in Machine Drilling.
The other important part of Spot Drilling is, of course, making a good Spot. The Spot needs to located exactly where you want the hole to go, and it needs to be a nice clean cone. A simple smack with a center punch isn't good enough. It takes several steps to make a good Spot.
This morning, I was making up a part for my Medium Green Pin Router. You may have seen the ongoing project over on the Pin Routers thread. This is an aluminum plate that needs a couple of holes drilled and tapped in it. It's a one-off special part, so there's no need to set up a milling machine to drill these holes. It's a good part to demonstrate Spot Drilling.
Layout basics: I decided that I want these two holes to be parallel to the edge, 3/8" from the edge. This is a job for the small Double Square and the Machinist's Scriber. I set the square to 3/8", hold it along the side of the plate at the approximate locations of the two holes, and draw the scriber along the end of the blade at the two locations. I'm fumbling here, trying to hold everything with one hand for the camera, but the scriber should be tilted at an angle from vertical, so the very point drags along the end of the blade. The square is being used to set the distance of the scribed line from the edge.
I also want the first hole to be 3/8" from the end of the plate. I turn the square around the corner, and scribe a cross line right there.
Now I use the Automatic Center Punch to make a very small punch mark right at the intersection of the two scribed lines. There's a trick to this. The point of a small automatic center punch like this is ground to a small narrow angle, narrower than what you want the Spot to be. This fine point is there to help you find scribed lines.
Lean the punch at a small angle and drag it lightly across the metal until you feel it click into one of the lines. Then, lightly drag it down that line until you feel it click into the intersecting line. If you hold the punch lightly, dragging it with just the weight of the punch itself, you can actually feel it fall into a 0.002" wide scribed line. And you can locate a precision punch on scribed lines to within 0.001". By hand and feel.
When you reach the intersection of the scribed lines, gently stand the punch up to vertical and push down on it. It cycles with a big click and leaves a tiny punch mark, right at the intersection of the lines. That's why we like these little automatic center punches. You can feel the lines and make the punch in seconds with one hand.
If you don't have a small Automatic Center Punch, the old-school way was to use a Prick Punch. Politically Incorrect these days I'm sure. A Prick Punch is a smaller version of a Center Punch, with the tip ground to a narrower angle, very sharp point. Same technique; drag it lightly to find scribed lines, stand it up vertically. Then use the other hand to pick up a small Machinist's Hammer and make a single tap on the head of the Prick Punch. A tiny little punch mark, in the exact right place. That's classic machine shop technique.
Kind of blurry, but there's the teeny punch mark at the intersection of the scribed lines.
I forgot to mention these before, but a good quality set of dividers is an important tool for layout on metal parts. I have dividers of all different sizes on my metalworking and woodworking benches. These four are the ones I most often use for metalworking. They are all Starrets, top quality. One of those tools to watch for at flea markets and when cleaning out garages.
Dividers are used for scribing a line a precise distance away from....a punch mark.
The Dividers are set using a Machinist's Ruler. I'm using the feel of the points of the dividers dropping into the engraved lines of the ruler. In this case, I'm setting the Dividers to 1.500".
I set one point into the groove at the 1" mark. Then adjust the Dividers until the other point just drops into the groove at 2.50". Good quality Machinist's Dividers can be adjusted very finely, and with a few seconds of patience, you can set them very accurately using this technique.
Then, set one point of the Dividers into the first tiny punch mark, and gently use the other point to scribe an arc, intersecting the other horizontal line. There's the other hole location.
Same technique with the Automatic Center Punch. Feel the lines, make the small punch mark.
I scribed and punched a third hole location using a similar technique with the square and ruler. I often use a Sharpie to make a circle around each punch mark, so I can see them in the next steps. A Precision Machinist's Sharpie. That's a joke around here.
The next step is the actual Center Punching. A Center Punch is larger, and the point is ground to a wider angle, usually 90 degrees. It gets smacked with a hammer. I move the plate over on top of a small anvil that I have right next to the machine shop bench. The technique is to gently slide the Center Punch around until you feel it click into the tiny punch mark. Then hold it vertically and give it one or two smacks with the hammer. I have an 8 oz ball peen hammer right there at the anvil. This enlarges the punch mark and widens the angle.
Be careful that the punch doesn't bounce between smacks. Feel that it's in the punch mark before hitting it a second time. Or, you'll get a double mark.
Here's what the punch marks look like after Center Punching. They are larger and wider angle, and still correctly right on the intersecting scribe lines. But, notice how there's a ring of metal raised up around the punch.That's the aluminum that was displaced when the punch made the cone.
That raised ring is a potential problem. If you push a spinning drill bit down into that punch mark, the cutting edges of the bit may catch on that raised ring, pushing the drill bit off to the side. It may kick out of alignment. And start drilling out of position. This is what can happen if you skip steps and only center punch the location, and go right to drilling.
This is what Spot Drills are for. A Spot Drill is a special drill bit that's very short and rigid, and its cutting edges are ground just for making center punch marks into Spots. They are an essential tool in precision metalworking. I keep one small drill press always set up with a small Spot Drill in it, and it gets constant use. This is a small 1/8" x 90 degree Spot Drill, which is the size I mostly use. I notice that McMasters calls them Hole-Starting Carbide Drill Bits; $14.35. Well worth the investment if you want to do precision metalworking.
Here's what the completed Spots look like. A one-second touch with the Spotting Drill cleans off the raised rim and makes a nice neat little cone. Now it's ready for drilling!
Drilling the holes. That's a #21 Screw Machine bit, because I'll be tapping the holes 10-32.
And note that I'm using that low-profile vise with its cool new aluminum jaws. Just the right size for this part.
That's the technique of Spot Drilling. Hand layout, and drilling in a drill press, to accurate locations.
Next: Drilling larger holes, and moving on to Machine Drilling technique.
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@Bruce Johnson It's interesting to me how much of this I picked up on through trail and error and how much proper technique I still have left to learn. As always, thanks for sharing!
Most of the technique that I'm showing here is classic Machine Shop Practice, as shown in the old textbooks and (formerly) taught in classes. But not all of it. Some things I learned from watching old Machinists. And they learned it from watching older Machinists.
The lesson is: Watch old Machinists at work. If they've been doing it for decades, they are full of clever little techniques and shortcuts.
Hey, don't go looking at me like that! Okay, go ahead....
Speaking of clever shortcuts:
In reference to the technique of using Dividers to scratch an arc, to mark the location of a hole.....
Take a look at your Digital Calipers......
Notice the two pointy things at the top. That you are always stabbing yourself with. They are supposed to be used for doing inside measurements. But they are awfully sharp. Suppose I were to set the calipers to 1.500" (per the job above) and tighten the lock knob.....
...And use them as Dividers! Digital Electronic Dividers! Put one point in the punch mark and gently pivot the calipers to scribe a little arc. Your old grey haired Machine Shop Teacher would be wagging his finger at you for doing this. But who cares? You are alone in your own shop now, with your own tools. And this actually works quite well.
Scribing arcs using the points of your Digital Calipers is a very quick way to lay out the locations of holes. Particularly when you want accurate dimensions between them. Like, most of the time.
In reference to the technique of using Dividers to scratch an arc, to mark the location of a hole.....
Take a look at your Digital Calipers......
Notice the two pointy things at the top. That you are always stabbing yourself with. They are supposed to be used for doing inside measurements. But they are awfully sharp. Suppose I were to set the calipers to 1.500" (per the job above) and tighten the lock knob.....
...And use them as Dividers! Digital Electronic Dividers! Put one point in the punch mark and gently pivot the calipers to scribe a little arc. Your old grey haired Machine Shop Teacher would be wagging his finger at you for doing this. But who cares? You are alone in your own shop now, with your own tools. And this actually works quite well.
Scribing arcs using the points of your Digital Calipers is a very quick way to lay out the locations of holes. Particularly when you want accurate dimensions between them. Like, most of the time.
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