Rough grinding the easy way: say goodbye to the drudgery of hogging out a curve in your mirror blank
I remember hearing famed telescope guru John Dobson proclaim, “Mirror making is caveman work–eat well, sleep well, and work like hell!” And there’s little doubt that the most caveman-like part of the process is rough grinding. That’s the stage in which you carve the initial curve into the surface of a glass blank. The traditional approach is to use lots of coarse abrasive (usually #80 or #60 carborundum) and plenty of muscle power. Depending on the size of the mirror and its focal ratio (which determines the depth of the curve), rough grinding can take many long hours. And if you have to prepare multiple blanks, the task can be daunting.
Such was the situation facing telescope maker David Groski in advance of the annual Delmarva Mirror Making Seminar in Maryland. Dave had to prepare several blanks for attendees and he needed a faster way to rough grind the curves. He decided to machine generate the blanks the way professional optical fabricators do. His approach is simplicity itself and worth considering even if you’re only going to make one or two mirrors. “It takes around 30 to 45 minutes to cut an f/4 curve in a 10-inch mirror and about the same amount of time to cut the corresponding curve on the glass tool,” Dave notes. “If you were to grind it by hand it would take six hours at least, and most likely much longer.”
The equipment requirements are minimal. You need a drill press–a piece of gear found in virtually every home workshop. You also need a diamond coring bit such as the ones available from THK Diamond Tools, which has an online store on eBay. The bits cost around $30 (depending on the size). Lastly you’ll need to fix the glass disk to some kind of turntable with a fine tilt adjustment. If the table on your drill press allows for precise tilt adjustments, you only need to mount a simple turntable to it and you’re up and running. But more likely you’ll need to make what Dave refers to as a “sine table,” shown in the photographs here. The tilt of the glass blank is precisely set by adjusting a nut and bolt at the far end of the table.
To generate a concave curve for a mirror, you position the blank under the coring bit so that the edge of the bit just touches the center of the glass blank. For a convex curve, the bit just touches the outer edge of the glass. The bit needs to be at least half the diameter of the glass disk, but it can be larger. For example, a 5-inch-diameter bit will suffice for mirrors up to 10 inches in diameter.
Crucial to the process is calculating the correct amount of tilt to generate a curve of the necessary depth for your mirror and tool set. The sine of tilt angle is the diameter of the coring bit divided by two times the radius of curvature of the mirror, or tool. For example, let’s say you’re using a 5-inch bit to generate an f/4.5 curve in a 10-inch mirror. The mirror’s radius of curvature is 90 inches and requires that the table be tilted 1.59[degrees]–the angle with a sine of 5/(2×90). You can look up the angle in a trig table, or use a scientific calculator’s arcsine key.
How do you achieve such a tiny, precise tilt? This is where the sine table comes in. “I set the table to the angle I need by using simple trig and measuring a known distance from the hinge,” Dave explains. “Then I set the vertical height by adjusting the set screw until I achieve the correct angle. This gets me close to the radius I want, but I use a spherometer or template to measure the curve after I make the cut and, if needed, adjust the angle and take a second pass.”
To generate the curve, set the drill press at its slowest speed (under 100 rpm) and lower the spinning bit until it engages the surface of the glass. As glass is ground away, you continuously, slowly rotate the mirror with the turntable (it may turn on its own), all the while ensuring that the mirror and bit are kept moist to prevent dangerous glass dust from becoming airborne. Grinding continues until the bit has ground the full diameter of the glass and the curve extends from center to edge.
The curves for both the mirror and its matching tool should be generated in succession without changing the angle of the sine table. It’s especially important to ensure that the mirror’s bevel doesn’t wear down and cause edge chipping. Dave finishes off the process by grinding the mirror against the tool with a few wets using #80 grit. “In most cases,” he notes, “the curves generated with the drill press will be close enough that some grinding with #80 will quickly get me to the curve I want. And because I need to mate the tool to the mirror, there’s no reason to try to get things dead perfect when initially generating the curves by machine.”
Readers wanting to know learn more can contact CanhPhuc at Wheezy, Big Wheezy, and Old Milwaukee are three of our trusted workshop friends here at Model Railroader. So is the S.S. Odee-guard.
Throughout late spring the staff has been finishing up work on our new project layout, Bay Junction, with the help of these friends. The layout will be featured in Model Railroader this winter, so be sure to renew your subscription.
Our friends aren’t human. They’re shop tools that the staff has nicknamed.
We named these tools to distinguish them from similar tools, or because the tools are homemade and don’t have names yet need to be called something.
No one on the staff ever declares this tool or that device should be called such and such. The nicknames just happen.
In a practical sense, the names help with staff communication when the staff is working together to build a layout, just like a model railroad club.
For the record, here are the background stories on our nicknames.
Wheezy is a small shop vacuum that uses a rechargeable battery for power. It was inside the box marked as an “extra bonus” when bought a new Shop Vac a few years ago. Because we aren’t diligent in keeping Wheezy’s battery charged, the small handheld vacuum tends to run down. Before the little guy’s battery dies completely, it wheezes.
Big Wheezy is the new, plug-in vacuum. It doesn’t wheeze, but it’s bigger. We could have called Big Wheezy the “shop vacuum,” but Old Milwaukee is a shop vacuum, too. While working on our project layout, the staff found a need to distinguish between the two vacuums since one has a much longer hose.
Old Milwaukee is an older, steel drum-shaped vacuum on wheels. It’s manufactured by Milwaukee Tool and looks like a big beer can.
The S.S. Odee-guard is a rolling workbench built by the late Gordy Odegard, one of Model Railroader’s memorable staffers. It was built at a time when storage space was tight in our workshop and we had to move things around to accommodate photography.
It’s got a 2% x 5 foot flat work surface. On one side of the workbench are eight heavy steel drawers and a steel cabinet to hold hand and power tools. The opposite side has pegboard hooks to hold carpenter’s squares and handsaws.
An ancient full-size drill press, a modelers’-size drill press, a bench vise, and a small Dremel sander are bolted to the top of the work surface. With its heavy construction and drawers packed with tools, it easily weighs 500 pounds.
Soon after it was built, the massive workbench was jokingly christened as the S.S. Odegard. Twenty years later, the name, with an exaggerated pronunciation, lives on.
I can’t say if other model railroaders nickname their tools or other devices, but I suspect many do, especially members of clubs. If you’ve got an amusing name for a workshop tool, drop us an e-mail at [email protected] or read some of article about drill press reviews type of drill press at here.
Contributing editor CanhPhuc has made numerous mirrors and is about ready to give up his caveman ways. Some of his scopes are featured at his website,
Begin by cutting the dowel stock to the sizes provided in the Parts List on the next page. (For mitered pieces, the dimensions given are for the long side.) Dowels have a tendency to rotate slightly when cut with a miter saw, which can lead to uneven or scorched cuts, so use a cradle that securely holds the workpiece. We built ours by cutting a V-shaped channel into a scrap piece with a table saw. To cut small parts safely, hold the longer waste side of the dowel and trim off the finished part . Never attempt to cut anything less than 6 inches long as this can cause the blade to launch the workpiece like a missile.
Use a drill press to bore a 5/32-inch hole through the center of each part. Prevent the drill bit from wandering by first marking the center of each part with a center punch. To ensure the part is positioned perpendicular to the drill bit, hold it against the square end of a scrap piece before tightening the hand-screw clamp or drill-press vise . Finish the parts by rounding their edges with sandpaper.
Using either a drill press or a cordless drill fitted with a 3/4-inch Forstner bit, bore a %-inch-deep hole in four 1 1/8-inch balls. To keep the balls from moving as you drill them, clamp them within custom-made cauls fashioned from scrap pieces with 3/4-inch holes bored in them . Ease over the edges of the holes to prevent them from marring the ball’s surface.
Head and Muzzle
Make the head in the same way you did the feet, but add another hole for the muzzle. Next, with the part still in the vise, bore a 5/32-inch hole that connects the centers of the two 3/4-inch holes. Add 1/8-inch holes for the eyes. For the muzzle, bore a 1/2-inch hole about 1/8 inch deep into the 3/4-inch ball.
Cut two leg sections, each 11/4 inches long, from the 2-inch dowel. Mark the centers of the leg sockets, which are positioned halfway along the body sections and about 1 1/2 inches apart from each other. Use a 1-inch Forstner bit to cut each socket 1/8 inch deep. Connect the centers of the leg sockets with a 5/32-inch hole. You’ll need to bore this hole at a steep angle.
Cut the center section at 5-degree bevels on both ends. Next, drill a 5/32-inch hole through the center of each body part. To find the center, take a 1-inch-wide piece of scrap–half the diameter of the dowel–and position it against the toy part. Make a mark, turn the part about 90 degrees, and make an intersecting mark .
Secure a 2-inch split ball in a hand-screw clamp or bench vise. Create the neck socket with a 1-inch Forstner bit. Position the bit so its edge is 1/8 inch from the edge of the part, and cut the hole 1/8 inch deep.
Find the center of the flat side of the split ball and bore a 5/32-inch hole at an angle so that it exits through the center of the neck socket.
Stain your toy with food coloring, which is nontoxic and can be mixed to create custom shades. Apply with a paintbrush and wipe off any excess with a rag . Note that water-based finishes swell the parts, making them difficult to fit, so let everything dry overnight before assembling. For added luster, wipe on food-grade beeswax with a rag. Avoid waxing areas that will be glued as this will prevent a good bond.
Using a utility knife, cut a piece of elastic cord 8 inches long and knot one end. Pass the cord through one set of leg parts, a body section, and the opposite set of leg parts . Pull the cord as tight as it will go, and tie it off. Trim off any excess. Cut another piece of elastic cord 16 inches long, and knot one end. Thread it through the head, the neck parts, the body parts, and the tail parts. Pull it tight and tie it off. Trim excess cord from the head, but leave an inch at the tail.
Drip PVA glue into each foot’s socket and attach the feet to the legs. Glue the muzzle so that its hole is positioned over the knot.
CONCRETE FASTENERS 60-WATT-BULB ALTERNATIVES BENCH GRINDERS
Miter saw / center punch / hand-screw clamp / drill press / 5/32″ and 1/8″ drill bits / 1″, 3/4″ and 1/2″ Forstner bits / cordless drill / 100- and 180-grit sandpaper / utility knife / paintbrushes / rags.
WARNING: This toy has small parts that can he a choking hazard, so we don’t recommend it for children under 3 years of age. As with all handmade toys, don’t leave your child unattended with it–and if it breaks, be sure to scoop up all the parts.
What could be more important to a gunsmith than his tools? Here’s how to maximize their use and service life.
From the high dollar lathe, milling machine, and bluing set up of a large shop, to the screwdrivers, punches, and bench vise of the hobbyist, tools are at the heart and soul of every gunsmith. Despite this, I find many folks just don’t know how to take care of their tools and why it is so important. I caught my 40-year-old son sawing back and forth with a file and realized that nobody really teaches us how to use and maintain even seeming-simple tools. Let’s take a look at some of the basic tools of the trade and see if we can find a few things you don’t know but should.
Screwdrivers are among the most basic and important hand tool of the trade. Many people will judge the quality of your work by first looking at the condition of the screws in the guns you have worked on. The typical screwdriver found at a “cheap Chinese tool store” has a keystone tip that tapers along all four axis of its length. These screwdrivers are responsible for more buggered screw slots than any other. Because the blade tapers to fit any number of screw slot sizes it only contacts the metal at the top edge of the slot. When you put pressure on the screw the first thing that happens is the metal at the top edge of the slot deforms. The more pressure, the more deformity. Good gunsmithing demands a parallel ground blade fitting snugly into the slot and the same width as the screw head. With parallel sides the pressure of installing or removing a screw is transferred to the entire interior face of the slot uniformly. This allows far more pressure with no damage to the slot itself.
Grace USA (graceusatools.com) makes a set of gunsmith screwdrivers that appear to have keystone tips but closer inspection reveals the very ends have been milled to present a parallel section inside the screw slot. Brownells and Wheeler Engineering offer sets with up to 85 bits to fit the most commonly found gun screw slots. Brownells goes a step farther offering Thin-Bit sets for the super skinny screw slots found on many high dollar guns. They are well worth the investment and will save you a world of trouble. You will also find that the tips of the best bits are made to break rather than bend when they reach their torque limit. It may be upsetting when a bit breaks but the alternative is for the bit to bend and cam itself out of the slot, damaging the slot on its journey to pop out of the slot and dig into the surrounding metal or wood. A good bit is engineered to break so it won’t ruin a stock or scratch the metal of the gun. Brownells has always offered free replacement of broken bits. If you over torque the bit and it breaks, send it back and they will replace it free.
If you find yourself with a tool box full of standard commercial screwdrivers the tips can be ground on a stone to the correct profile in just a few minutes. Brownells offers the Magna-Tip Bit Shaping Stone so you can use your drill press to grind any screwdriver to precisely fit any screw. There is just no excuse for buggered screw slots in our trade.
Files are a simple tool that are often misused. For safety s sake, please put a handle on every one of your files. In the heat of a long bout at the bench it is easy to run the tang of the file into your hand. The handle also gives you better control of the file and that is important when working in tight places. Far too many people seem to think files are indestructible; they are not. Depending on how you use and care for them, a good file can last a lifetime, or be irretrievably ruined the first time you use it.
Protect files from coming in contact with others. Either hang your files on a rack so they do not touch each other or make cardboard sleeves for each one before storing. Cut only on the forward stroke, not back-and-forth like a saw. Files remove metal with microscopic burrs located on the edge of the teeth. When you push the file forward those burrs scratch the surface of the metal and remove small pieces of it. Drawing the file back while maintaining downward pressure, those burrs are bent into the valley between the teeth. On the next forward stroke they take off less material until the burrs are removed completely or break off from being bent back and forth, leaving a file that does not cut. Use chalk (yes, the same stuff used on a blackboard) to prevent, or at least reduce, the build up of file shavings in between the teeth of the file that causes it to stop cutting and increases scratches on the surface of the work, known as pinning. Chalks helps stop sharp little metal chips from getting into the teeth, requiring less time with a file card cleaning out the chips, allows the file to cut just as aggressively, and gives the work a smoother finish. Standard chalk works fine or buy it in one-inch diameter sticks from Brownells (#080-705-006AK.) Finally, clean the chips out of the teeth after each use or whenever needed with a file card. Take care of your files and perhaps you can be like a friend of mine that has used his for twenty-five years without having to replace them.
In the case of hammers my advice is simple. Any hammer that will come in contact with a gun should have its face polished to perfect smoothness. Crazy perhaps, but every nick or scratch on the surface of the hammer face will be transferred to whatever it strikes with any force. Those little areas will have to be polished out before you finish the project. Isn’t it easier if there are no marks to polish out? Yes, this is picky and extreme but it is also part of a mindset. Approach every job as if each detail is important and your work will improve. If you ever present your work to one of the custom gunsmith guilds for judgment you’ll realize this level of detail no longer seems silly.
In the gunsmith shop there is rarely a need for a really heavy hammer. Heavy hammers move more metal faster but we are normally moving small amounts of metal in a precise manner. A four to six ounce ball peen hammer is all you need for the majority of gunsmithing jobs. A hammer should feel comfortable in the hand and the head should go where you want it to. If you’re having trouble hitting the head of a punch squarely or the hammer just feels like it is out of control don’t be afraid to try shortening the handle or modifying the grip. I know gunsmiths that make their own hammers because they feel their design idea does a particular job better. There is nothing like a tool you have designed and made yourself to give you pride and satisfaction in a job done well.
There are more different types of punches than you can shake a stick at and I have tried to collect a set of every one of them over the years. Long, short, pointed, or flat, they all need to be kept in good shape to do the job you bought them to do. Probably the most abused punch in the shop is the pin punch. We use them to remove pins, poke into tight corners and hidden places, move sights, open paint cans, pick our nose, and peen rivets. OK, I exaggerate … slightly. Punches have two business ends. One is struck by a hammer and the other transfers force to an object, usually a pin we are trying to remove. Both ends are subject to being mashed and the small end is also subject to being dinged, broken, or bent. Both ends should be inspected regularly and repaired when they need it. I have never been successful in trying to straightening a bent punch so as to again slip through a hole cleanly when driving out a pin. When a punch bends or breaks I write its size down so I can order a replacement, cut it off above the bend, dress it off, and use it as a starter punch. A battered tip is larger in diameter than it is supposed to be and should also be dressed back to original diameter and polished. The same goes for the striking surface. Clean it up, square it off, and polish it smooth.
The points on your center punches should be sharp and centered. A center punch that has been inadvertently used on hardened steel will have its tip flattened, making it difficult to precisely locate the center point next time it is used. Dress the tip and you’ll find it much easier to precisely locate the exact point to mark.
Every bench I’ve ever looked at has had a wide selection of pliers. The most common are needle nose, parallel jaw, and duckbill, with more lengths of needle nose than any other kind. A good set of pliers is like an extension of your hand with tiny little tips that can pluck debris out of places your fat fingers could never reach. A good set of gunsmithing pliers should never leave the shop and visit the garage where plier-destroying disaster lurks around every corner. If abused the tips on those delicate needle nose pliers no longer come together when closed and has an “S” bend in it. The parallel jaws aren’t after trying to hold on to a nut, leaving the jaws anything but smooth. If so damaged, assess if they can be fixed. Sometimes a little careful work can realign the needle-nose tips or resquare the parallel jaws with careful filing and polishing and save replacing a potentially expensive tool.
About all I used to care about drill bits was that I had the size I needed. Once I had completed the set up, turned on the drill press, and started to try to make a hole, I began caring very much that the dang drill bit was sharp. I hate it when they smoke and squeal but won’t cut. Most often that will happen on a popular size I use often, so I usually have several more bits available. I’m not going to sell you a drill bit sharpener. Unless you are working in a large, high-production machine shop, bits are too inexpensive to ever buy a machine to resharpen them. I solved this with a Titanium nitride coated high speed tool steel set of 115 bits purchased for $55. This set has every fractional, letter, and number size bit in it. As such a set isn’t the best quality of bits, I replace the often-used bits individually as needed from Brownells. Only a quarter of bits in my set are replacements because I rarely need the oddball sizes, but the day I need one I already have it and it will be good for at least a few holes before it needs replacing. This also keeps all the bits in one box nicely labeled and sorted. That saves a lot of time I used to waste trying to find the right size bit.
We will end our trip around the shop with a look at your bench vise. I think I have seen more guns ruined with a poorly prepared vise than any other tool. This isn’t the easily replaced damage like a buggered screw head that can generally be repolished and a wider slot cut. No, this is caused by serrated teeth on the vise jaws and an operator who either thinks nobody will notice that fancy work on the metal, doesn’t care, or never took the time to figure out a way to avoid it. Most vises come with a set of jaws that are something other than smooth. The serrations help plumbers hold pipes and other parts that will be hidden behind a wall or under a toilet. Unless your customer or you keep you guns under that toilet, people will notice. Most jaws are removable and the serrations can be milled, ground, filed off and polished smooth. For more delicate parts, a set of soft jaws made of lead, plastic, wood, brass, or any other material softer than steel. They should slip easily on and off the vise. I suggest having several for different jobs. For reworking screw heads I like jaws made of soft wood. I can tighten the vise around the screw threads and hold the screw while I work on it without damaging the threads. To drift sights dovetailed into the slide or barrel, brass or lead jaws hold the work firmly without marring the steel. That vise is your third hand and you’ll use it a lot. Don’t let it mark your customer’s gun.
Specialty vises secure barrels without digging into the metal. Action wrenches will hold the action without damaging it. For your own sake don’t let people know that you worked on a gun because they recognize the tooth pattern of your vise and/or the pipe wrench you used on every barrel you have ever changed. I promise, that kind of calling card won’t bring you more work.
This is by no means a complete list but covers some of the more commonly used hand tools and the problems that can be ignored. Good tools help to turn out good work and make the job easier. Keeping them in good condition is a mindset important to the gunsmith. Every customer showing a gun you worked on displays the quality of your work. The results will determine if you will get his next job or if he goes elsewhere looking for better quality workmanship. That quality starts with your tools.
There is no question that when someone has been in business for 12 years they learn how to use certain tools to get them by until they can afford to purchase whatever tool it is that they are working around.
To start this column off, this is my quick list of obvious stepping-stone tools. Not all of these examples have affected me personally, but I’m sure if you have a shop you can relate to at least one of these:
* Using a chopsaw until you realize that a lay-down bandsaw or a cold saw works so much better and makes a hell of a lot less noise
* Learning how to lay down a clean weld with your MIG until you can get a TIG and start learning how to weld all over again
* Tolerating the horrible bend of a Harbor Freight bottle jack tubing bender hoping to one day afford or fall into a nice air-overhydraulic bender with all the appropriate dies
* Wrestling the nuisance of a small vertical woodworking bandsaw until you find a nice metal saw that won’t kill blades every week.
* Finally realizing that a $200 Craftsman toolset is like a Fisher-Price set compared to the tools you get off the Matco or Snap-on truck (but, for the price you really can’t beat them)
This is just a small list of tools one would graduate to, and there are of course tools that you wouldn’t necessarily have an entry-level version of that you deal with while waiting for “the one” to show up. A lathe or mill is pretty much an all-or-nothing kind of tool. Finding bench-top models would be an entry point, but you really can’t use a drill press for now in anticipation of a mill. A drill press simply can’t mill, but a mill can certainly drill holes and anything else you can imagine (I guess this is a good reason why a mill is 10 times the cost of a drill press).
There is one graduation tool I deliberately left off the list that I’m sure at least one of you felt should be on it, but I don’t agree with this one. Using an oxy/acetylene torch awaiting a plasma cutter is not a step in a positive direction if you ask me. They are two tools that have different uses. True, they both can cut, and when used properly they can both cut well, but the plasma cutter is a single-purpose tool and can only do one thing: cut. A torch on the other hand can both heat and cut, but even beyond that the versatility of its cutting abilities are vast and extremely useful if you know how to use a torch properly. I fear that the torch is so quickly dismissed as a butcher tool that most folks don’t concern themselves with learning how to use one beyond lighting it and pushing the valve to make a cut. There is a line from the movie The Mask of Zorro where Anthony Hopkins’ character asks Antonio Banderas’s character if he knows how to use a sword. His response was, “Sure, the pointy end goes in the other guy.” That statement is perfect; I think there is so much to know about a tool as simple as an oxy/acetylene torch where most only see a “hot wrench.” I’ll tell you this about my torch: I have spent enough time with a plasma to know that my torch is a far superior tool. I can only think of one time in my 12 years in business when I looked at Steve and said, “I think a plasma might be nice right now.”
So go fill your bottles and give your torch another spin. You might even like it.