Introduction: Precision Puzzlemaking Primer -- Volume 1
By Lee Krasnow
This instructable will guide you through all of the steps necessary to turn an ordinary table saw into a precision puzzlemaking machine.
Join me in this in-depth primer as I cover all of the key aspects of designing and building your own puzzlemaker's crosscutting sled like the one shown in this picture. I'll explain how to attach and calibrate special jigs and fixtures which allow you to cut wooden cubes accurate down to one thousandth of an inch.
From there I'll give you pointers on good gluing techniques, and then present you with ten fantastic puzzle designs which you can make on your own using the tools and skills covered here.
This primer will be the first in an ongoing series of puzzlemaking instructables -- each subsequent volume building upon lessons from the past ones, and each one using the puzzlemaker's crosscutting sled that I describe in this volume.
I've been getting a few emails from people wanting to know when I'll put out volume 2 of this series. I'm very glad to know some of you are finding this useful information, and please if you are eager for more of the same, then post some pictures of the puzzles that you've made from volume 1 of the primer!! Has anybody made all ten of them yet?
Step 1: Background Information
As you can see in the above podcast, most of my work involves cutting and assembling small and strangely shaped pieces of wood. This alone is enough to dissuade most people from ever getting involved in puzzlemaking as a hobby -- accurately cutting small parts can be a challenging task for even the most seasoned woodworker.
Most workshop machinery is designed to accommodate larger pieces of material, and is often inadequate for working with components that are smaller than a few inches across. Pushing projects to the limits of our machine's working capabilities and our own skills of engineering and craftsmanship often result in hastily thrown together jigs and potentially dangerous situations. Not only will the extra time and money spent setting up accurate and dependable equipment pay for itself many times over in improved safety and ease of use, but you'll also be much more likely to get the quality of results you're hoping for if you've got good gear to work with.
Step 2: Crosscutting Sled Basics
The table saw is one of the most important machines in any woodworking shop. In this regard, puzzlemaking workshops are no different... however most (if not all) puzzlemaking work is done using custom built crosscutting sleds rather than using the rip fence or miter gauge.
In its simplest form, a crosscutting sled is just a sheet of plywood which has rails underneath to keep it running straight in the miter slots of the saw. Typically sleds have a crosscutting fence attached the the back edge (the side closest to you) which is set 90 degrees to the cut line, and which is taller than the maximum cut depth (so that it doesn't get cut it half during use!) Most sleds will also have a fence on the front edge -- this adds rigidity.
On the other hand, my own crosscutting sled is anything but simple; the machined aluminum sled plates ride down ground steel rods on four linear ball bearings to ensure smooth motion with minimal linear deviation. The surfaces of the sled plates are peppered with holes which allow me to affix all sorts of special cutting jigs and fixtures. I've even got a CNC power feed unit which I attach when I'm running long batches of the same kind of cut.
I consider these two examples to be at extreme opposite ends on the spectrum of different ways that you can make a crosscutting sled. There is no limit to the number of variations that you can come up with to suit your own particular needs -- each variation with its own pros and cons. Different sled designs suit different tasks, and there is no such thing as the "best" way to make a sled, or the "best" way to make a puzzle. These are things for you to figure out on your own. I encourage you to think of this primer as a good starting point for your own journey, rather than a destination in and of itself... ;-)
Step 3: Machine and Blade Considerations
Although most any table saw can be adapted to work well for puzzlemaking, it's always helpful to have a nice new machine to work with. If you're in the market to buy a new table saw, I strongly recommend the SawStop. It's much safer to work with one of these saws than ANY other kind of table saw. Even setting safety considerations aside, the SawStop is an all around well engineered and well built machine. Man, I wish that I had one... ;-)
Regardless of what kind of saw you've got, you're going to want to get a high quality carbide tipped crosscutting blade for it. For best surface quality and least amount of bottom side tear-out, I recommend using a high tooth count crosscutting blade with 5-10 degrees of positive hook on the teeth and 30-40 degrees of ATB. I'm a big fan of the 10 inch 60 tooth Forrest Woodworker 1. I've got about 20 of them, and they're just about the only kind of blade that I ever put on my saw.
Regardless of what blade you're using, make sure to get it sharpened regularly. Also, make certain that the blade is aligned parallel with the miter slots on your table saw surface. This is usually an adjustment that you make inside the guts of the saw, and it can be a pain to fix sometimes, but it's important to get it right.
The key thing to understand about this adjustment is that your crosscutting sled will always travel parallel to the miter slots, and thus the cut that you make will also always be parallel to the miter slots. If the blade isn't parallel to the miter slots then the back side of the blade will rub against the material as it passes by.
Alternating Top Bevel (shown on left of diagram) is what gives a crosscutting blade that W-shaped cut profile. Hook (shown on right) is the amount that each tooth is tilted forward relative to a radial line coming from the center of the blade. The one that I drew has got 10 degrees of hook and 45 degrees of ATB.
A ripping blade has flat top "chisel" teeth (0 ATB) and typically has a very aggressive hook. These blades are useful for notching (to be covered in the next volume of the primer) but in my experience they're not any better than a good crosscutting blade (i.e. Forrest WW1) when it comes to ripping down puzzle stock.
Step 4: Taking Measurements
- the width and depth of the miter slots
- the distance between the miter slots, and where the saw blade sits between them
- the thickness of the blade itself (this is called "kerf"), and also its maximum depth of cut
- the thickness and diameter of any blade dampeners or stiffeners that you plan to use
- how far back the blade sits from the front of the saw
- how far the saw is front to back, and how much travel the sled will have
Making accurate measurements is an art form in and of itself. The thousandth of an inch (0.001") is a popular dimensional increment, and there are a wide range of readily accessible (and occasionally reasonably priced) tools which will take certain measurements accurate down to one thousandth. By far, the most versatile of these tools are four-way dial calipers. This picture shows my drawer of inspection (i.e. measuring) tools -- I've got three sets of calipers in there.
I strongly recommend that you get yourself a set of calipers. I will make several more tool recommendation throughout the course of this project, but this one is by far the most important of them. Although you won't actually need calipers to build this sled, you will need them to calibrate your saw and jigs, and to measure for error in the resulting cuts. Your cuts can only be as accurate as your ability to measure.
Step 5: Sled Building Overview
There's a lot of information covered in this step, so take your time to absorb it all. I suggest referring to the images frequently as your read through this.
cross support design considerationsEach cross support (front and back) is made up of three layers of plywood. When you're working out the dimensions for the cross support components, cut out the five middle layer blocks so that the four carriage bolts have enough room to move side to side about 1/4 inch in either direction; it's better to have too much slop here than not enough. Also, remember that the saw blade will cut most of the way through the middle of the thing, so make sure that the cross supports are tall enough to not get completely cut in half.
cross support assemblyThe three layers of the cross supports are held together using screws which come in from both sides. Use lots of screws -- you really want the cross support assemblies to be sturdy. Each screw should fall loosely through oversized holes in the first and second layers of wood, only biting into the third layer deep.
Make sure that you line up the cross support assemblies straight and smooth when you screw them together. Tighten down all of the screws and then back them off a few turns to be sure that all seven blocks of wood are free to rattle around. If this is not the case, you may need to go back and enlarge a few of the oversized screw holes. Align the loose assembly using the surface of your saw as a flat guide. When you do the final tightening down of the screws, go about it in a gradual manner, and (gently) use a hammer along the way to nudge everything flush.
sled plate constructionEach sled plate (right and left) is made up of two layers of plywood. Each layer gets many holes drilled through it before assembly.
linear motionRather than riding down the miter slots on rails (as do most sleds) this design uses ball bearings which run against the inside edge of the miter slots. Attach the bearings to the underside of the sled plates using heavy duty screws with necks that snugly fit the inner diameter of the bearings. The bearings need to spin freely from the underside of the sled plates, so put a couple of narrow washers between each bearing and the underside of the sled. If your miter slots are especially shallow, you may need to countersink each bearing assembly into the underside of the sled, or alternately grind down the screw heads a little bit.
The side-to-side placement of the bearings is dictated by the layout of the miter slots on the saw, however the front-to-back placement of the bearings is purely a matter of preference. Placing the bearings directly underneath the cross supports (rather than closer to the center of the sled) results in the most stability, but reduces the amount of effective sled travel because the sled will slip crookedly if the bearings are extended past the edge of the tabletop. A best-of-both-worlds solution to this dilemma is to mount another bearing (or a stack of washers) in the middle of each sled plate and adjust it so that it almost but not quite touches the miter slot wall. This will prevent the sled from jerking sideways if pushed too far, which in turn prevents the blade from getting all bound up in whichever cross support it happens to be passing through at the time.
thread pointsAs with my machined metal version, this design also has an evenly spaced pattern of threaded holes going through each sled plate. I recommend spacing these hole locations as shown where the distance from any hole to its horizontal or vertical neighbor (but not diagonal) is 1.5 inches. For my example sled (the one that I made to take these pictures) I used press in 10-32 threaded inserts (Servalite 628-E) which I hammered into undersized (6.5mm diameter) holes. Countersinking the threaded inserts without marring up my sled plates was easily accomplished by pounding down on a socket head cap screw which I spun down into each insert before hammering it in place.
Using these threaded inserts was convenient, but they're expensive and hard to find. In retrospect I wish that I had used tee nuts countersunk in between the layers of the sled plates. It would have been a lot more work to drill those countersinks, but I think that it would have been worth it in the end.
Regardless of whether you use tee-nuts (recommended) or threaded inserts, you'll want to get a whole selection of 10-32 threaded socket head cap screws to work with. Get several of each length available, and be sure to get extras in the 1" to 1-1/2" range because they're especially handy. Get a bunch of #10 washers. Get some 1/4" washers as well so that you can step up to clamp over a larger hole. I recommend getting a divider case to hold all of the screws separate from each other and from the washers and other little things that you'll frequently be reaching for as you build up fixtures onto the sled plates.
sled plate assemblyJoin the top and bottom plate halves together using screws which come up into the top plate from underneath. Place a few of these screws in between thread points, and also space them evenly around the perimeter of each plate. Countersink the screw heads so that the sled slides smoothly, and to maximize the amount of bite that each screw takes into the top plate.
blade clearancePlan out your cuts and hole patterns so that the inside edge of each sled plate actually extends into or even slightly past the other side of the saw blade. Once you've screwed the sled plates together and mounted the bearings underneath, trim that extra material off the inside edges of each plate by running them down the miter slots with the saw running. The benefit of doing it this way is that you are guaranteed to get a perfect zero-gap clearance between the inside edges of your sled plates and the saw blade. Getting this plate trimming cut started can be tricky -- lower the blade down so that it's not cutting anything and then hold the sled plate in place (pressed up against the inside edge of the miter slot) as shown in the main image on this page. With the plate firmly held in place like this, slowly raise up the running saw blade to get the cut started.
sled assemblyOnce you've built these four main subassemblies, it's time to mount the cross supports onto the sled plates using eight carriage bolts. Hand tighten the nuts and washers down onto each of the eight carriage bolts, and then switch to a wrench or ratchet to dig the square necks of the carriage bolts into the top layer of the sled plates. Now loosen it all up again in order to more carefully align things. Using a small square to line up the cross supports perpendicular to the cut line between the plates, tighten down the nuts on just the left-hand sled plate. Now push the two plates together (pressing the bearings up against the miter slot walls) and firmly hold them that way as you carefully tighten down the nuts on the right-hand plate.
Experiment setting different tensions between the two sled plates -- you want enough so that there's no side-to-side play, but you don't want so much tension that the sled is difficult to push. Don't worry too much about getting the cross supports lined up at exactly 90 degrees to the cut line -- in the next step I'll show you how to mount a secondary (more accurate) crosscutting fence.
Step 6: Setting Up a Secondary Crosscutting Fence
As already mentioned, you can use the back cross support as a fence for making rough cross cuts, but for making very accurate cross cuts you'll want to set up a secondary fence. You can make a reasonably straight set of fences from scrap plywood, but I recommend using a set of machinist's parallel bars instead. Get the cheapest set that you can find, making sure that they've got holes at regular one inch intervals. Mount the parallels onto you sled using cap screws and washers as shown.
Through trial-and-error you can align this secondary fence exactly perpendicular to the cut line by placing varying thickness of shims between it and the back cross support. If you want to get serious about things, I recommend getting a set of machinist's gage blocks for layout and calibration tasks like this. In the mean time though, you can just cut out a crude set of spacers using scraps of plywood. For fine tuning, you can add pieces of paper (0.003 thick) or pieces of aluminum foil (0.001 thick). Another good option is to get a five dollar set of automotive feeler gages.
Regardless of how you shim the fence into position, you need a way to verify that the cuts you're making are actually square. An extremely effective way to do this is by making a few cuts on a scrap of material and then measuring for deviation. Start out with a rough rectangle that has got an arrow drawn on it. Lay the block down with the arrow pointing towards the saw blade and cut a clean edge on it. Now rotate the block so that the arrow is pointing down towards the fence and make another cut. Make sure that you're holding the block (that edge you just cut) firmly against the fence. Now rotate the block a second time so that the arrow is pointing away from the blade and make a third and final cut.
If the fence is properly adjusted, then the block should measure the same width across both above and below the arrow. A block that measures thicker above than below indicates that your fence is tilted away from the blade. Fix this by adding thickness to the outside stack of shims. If it measures thicker below than above then you need to add thickness to the inside stack. Repeat this process until the above and below measurements match each other.
You can use this same technique to adjust the blade angle square to the sled plates. To do this, simply use a thicker piece of scrap material and compare measurements that are made closer to and further away from the sled plate surface. If the block measures thinner as it rises up from the sled plate, then your blade is tilted too much towards the side of the sled that you cut the block on. If, on the other hand, it measures thicker as it goes up, then the blade is tilted away from that side of the sled. Make adjustments and repeat the process until you get it just right.
Having properly calibrated one fence, it's much easier to mount and align a second one on the other side of the sled. To do this, simply clamp a straight edge up tight to both fences, and then lock the second fence down in position. Test the alignment of one fence to the other by sliding the straight edge back and forth between the two fences -- the straight edge should slide smoothly on and off of each fence without catching on anything or jiggling around. You can also do the cut-and-measure test on the second fence to verify its angle.
With your secondary cross-cut fences set square to the cut line, and the blade set square to the sled surface, your sled is now ready to make accurately square cuts. Congratulations! This is a point that many woodworkers never actually reach. Practice cutting out rectangular blocks of wood with every face perfectly square to its neighbors. Verify this by placing them next to each other on a flat surface and seeing that their angles meet correctly. Also practice verifying your angles by taking measurements with calipers.
Step 7: Setting Reference Points and Making Cuts to Specific Dimensions
Setting up the sled to accurately make cuts to specific dimensions requires you to first establish a lateral reference point. Cut out and clamp down a block on the rightmost side of the fence, making sure it's far enough out so that you can still fit in the longest cut that you're planning to make without bumping into it. Now install a second (adjustable) block to the left of the first one. Put a stack of shim spacers between them and tighten down the adjustable block so that the shims are held tightly in place. Varying the thickness of the stack of shims that you place between these blocks lets you fine tune the length of your cuts.
Setting up a ripping fence is done similarly to this. Build a pair of tall toggle clamp towers and mount them in place as shown. Not only is using toggle clamps a good idea for safety reasons, but it's also friendlier to your fingers and wrists, more convenient for repetitive tasks, and will generally give you much more consistent results than you would otherwise get by holding the wood with your hands. Moving the sled back and forth is also a lot easier (and more consistent) when you're not pressing it down onto the surface of your saw.
With these towers in place, you can use their edges as reference points for laterally adjusting blocks that you clamp down next to them. Pairs of independently adjustable blocks can be aligned with each other by pressing a straight edge up against them as you tighten them down. I recommend putting a small metal dowel pin in with these shim stacks (see the full-size picture for what I mean) so that each of these clamp-down blocks has rotational play. If you don't add dowel pins here, the clamp-down blocks will tend to stay parallel to their respective toggle clamp towers.
Make sure that your toggle clamps are long enough to reach well over the ripping fence. If the rubber tipped screws that come with your toggle clamps aren't long enough to reach the stick that you're ripping, then lay a scrap of wood in between. This is a good idea regardless of the rubber tipped screw length -- it's easier to use different sized scraps here than to get out a wrench and adjust the toggle clamps.
Once you've built your ripping setup, make a test cut and measure the front and back ends for consistency. Adjust the shim stacks and realign the ripping fences if necessary. Once you've got the rips set parallel to the cut line, you can add or remove equal amounts of thickness for both stacks to adjust the rip offset. Rough cut some wood into crude square sticks which are about 0.100" larger than your target stock dimension. Make a big pile of these rough sticks, and then take a pencil and mark all four sides of each stick. It's better to mark the sticks all at one end or the other, but not right down the middle. Marking them in this manner helps you figure out how to properly orient each stick when you load it onto the sled.
With a pile of marked rough sticks at the ready, set the rip fences so that they cut about 0.050" larger than your target dimension, and then trim down the first surface of just one rough stick. Rotate the stick so that the surface you just cut is facing down towards the sled plate, and with the stick clamped in this position, make your second cut. Now adjust the fences so that your third and fourth cuts hit your target dimension. Each subsequent cut on the stick is done with it rotated 90 degrees from its previous orientation.
Just as you were able to set the blade angle using measurements made from your cross cutting fence calibration cuts, so too can you fine tune the blade adjustment by measuring these square sticks, and compensating for any deviations that you may find. Extra effort spent getting this calibrated correctly will pay for itself later on down the line when you're depending upon the accuracy and consistency of the square cross section of these sticks. Run this whole process through one stick at a time, making adjustments when necessary, until you are satisfied with the measurements that you are reading. It is only at this point that you should run the rest of your sticks through this ripping jig as one large batch.
Once you've got some good square sticks to work with, you can use the cross cutting fence to accurately chop them up into cubic or rectangular sections. Before making any cuts on a new length of square stock, be sure to trim a bit off the end to ensure that it's squared off properly. For maximum accuracy, I recommend regularly (i.e. every few cuts) using compressed air to clean out the sawdust from the places where you load in parts. Also, I recommend setting up some toggle clamps to hold the sticks in place while cross cutting. This would be more easily accomplished without the ripping fences attached.
Lastly I want to point out that these photographs show the cross cutting fences spaced rather far apart from each other. This is bad. If I were actually going to use this sled regularly (rather than my precision metal one) I would modify the crosscutting fences so that they were closer to the saw blade. Rather than accomplishing this by modifying my threaded hole pattern, I would just grind those two parallel bars down so that I could move them each one hole closer together. If you're making your crosscutting fences from plywood then you don't need to worry about this -- just set them too close together and then trim them flush to the blade.
Step 8: Gluing Guidelines
Work on the flattest and smoothest surface possible. I recommend using a granite surface plate (as shown) for maximum accuracy, however a small scrap of glass or mirror will probably work well in the short term. The only other alignment tool that you'll need is some sort of straight edge to press your pieces up against. I recommend using a thick parallel bar or a machinist's 1-2-3 block, but you can easily make do with a leftover section of square stick.
Use a quick setting yellow woodworking glue -- the brand doesn't matter as long as it contains aliphatic resin. I'm fond of Titebond #1, which I put in a plastic syringe to help to keep the glue under control as I'm applying it. I keep some masking tape wrapped around the syringe to seal off the end when I'm done with it, and I use a needle to clean out the tip when I take the tape off next time.
Put a few drops of glue onto one of the surfaces to be joined and then smear it around with your finger to ensure that it coats the wood evenly. Choose a finger to be the dedicated glue spreader -- pick one that you don't need to use to hold the pieces -- that way you don't have to worry about cleaning the glue off your finger right away. I use my right pinky finger.
After spreading the glue, but before pressing the blocks together, I recommend using a different finger to wipe excess glue away from the perimeter of the face that you just put it on. Look closely at the full size version of this image to see what I mean. This trick helps to minimize the amount of glue that squeezes out of the cracks between blocks.
If you get beads squeezing out of the cracks anyway, then there are two good ways to get it off:
- use a wet brush (or a wet finger) to immediately wipe away the excess glue. Aliphatic resin glue is water soluble, so it should clean off completely if you get at it right away.
- wait about 2 minutes and then carefully scrape the semi-hardened glue away with the edge of a razor blade. There is a reasonably short window of time during which this technique is effective, but if you hit that window then it works extremely well.
Put some thought into the order which you glue the blocks -- some ways will be better than others. A good rule of thumb in this regard is to not glue up complex puzzle pieces one block at a time... instead build them as two or three smaller subassemblies which you then join together to form the larger shapes.
Step 9: 3x3x3 Cube Puzzles
There's something about having a bunch of cubes which makes you want to just pile them up into bigger cubes. Experience and observation has led me to believe that the 3x3x3 cube is one of the most naturally intuitive shapes for a puzzle to assemble into. Test this out for yourself by handing one of these three 3x3x3 cube puzzles to somebody in a disassembled state. Chances are high that they will automatically assume that they should build a cube, or otherwise will ask you if that's what they're supposed to do.
All three of these 3x3x3 cube puzzles have unique solutions. The second and third ones are interlocking. All three of them were designed by Stewart Coffin, who is a based out of the USA.
According to Stewart Coffin: "A puzzle is considered to be interlocking if the last step of assembly (or the first step of disassembly) necessarily involves the sliding of mating surfaces parallel to each other. Such puzzles tend not to come apart without deliberate effort, and must be cut and glued with the utmost of care in order to assure that sliding surfaces line up with each other properly."
A serially interlocking puzzle is one where the pieces must be assembled in a specific order because each subsequent piece added locks the previous ones in place.
Step 10: Cross 2
These four interlocking pieces will assemble to form the 2x2 triple cross shown here.
Vaclav Obsivac, the designer of this puzzle, is based out of the Czech Republic.
Step 11: Three-Piece Block
This three piece puzzle is deceptively difficult to put together, even if you've got a picture of the end result to work from. It's also difficult to glue the pieces together accurately. The best way to do this is by cutting out some spacer blocks which are exactly half as thick as the cubes which the puzzle is to be built from.
This puzzle was designed by Stewart Coffin
Step 12: Convolution
These seven serially interlocking pieces will assemble in only one way, and in only one specific order. To complicate things even further, piece #5 doesn't slide directly into place -- it must be rotated into position. A few edges of this piece will need to be rounded over slightly to accommodate this twisting motion.
This is another one of Stewart Coffin's designs.
Step 13: Wookey Hole
This puzzle requires four sets of each of these three pieces. That's 12 pieces total. The only way to get them together is by building each set into identical subassemblies which then twist together into pairs. These pairs slide into each other to form a 5x5x5 cube with a hole through the center of each face.
Yet another fantastic Stewart Coffin design. For more designs of his I highly recommend buying his new book "Geometric Puzzle Design" or alternately you can access much of the same information in his older book entitled "The Puzzling World of Polyhedral Dissections" which is freely available here:
Step 14: Knobbly Burr
Dic Sonneveld, the designer of this puzzle, is based out of the Netherlands. He's got other designs which you can make using the skills that you've already acquired -- try googling these other designs of his:
- Knobbly Box
- Sonneveld's 4 Piece Cube
- 3 Piece 2x2x2 Burr
- Sonneveld's Illegal Burr
Step 15: Oskar's Cubes
Oskar van Deventer, the designer of this puzzle, is based out of the Netherlands. He's got other designs which you can make using the skills that you've already acquired -- try googling these other designs of his:
- Oskar's Paperclips
- Little Paperclips
- Six Fingers
- Dutch Rings
- Big Knot
- Staple Cubes
- Sticks Cube
Step 16: Cube in Cage 333
Mineyuki Uyematsu, the designer of this puzzle, is based out of Japan. He's got other designs which you can make using the skills that you've already acquired -- try googling these other designs of his:
- Cube in Cage 222
- The Cube of Cubes
- Tornados in 4x4x4
Step 17: Closing Remarks and Safety Considerations
After building and trying to assemble a few of the more elaborate puzzles that I presented in the following steps, you may be wishing that you had been more careful during the ripping and/or crosscutting steps. If you find that your blocks aren't lining up properly then that usually indicates that something earlier on wasn't calibrated right. Sometimes sandpaper can help coerce poorly cut or glued blocks into their proper positions, but usually you'd be better off just cutting new ones. Be patient with this process -- don't expect to get it exactly right the first time through.
There has been a tremendous amount of material covered in this primer -- following along with me from start to finish will most certainly take up months of your free time. Do not attempt this project if you do not know how to safely operate woodworking machinery. These guidelines are intended for woodworkers with a moderate degree of prior experience, and who already know how to properly operate and adjust a table saw.
Be safety conscious when running long batches of repetitive tasks. Keep mentally aware of the running saw blade at all times! When working with this sled, keep both hands firmly braced on the sled plates or cross supports, and keep your body braced against the saw. Never put your fingers unnecessarily near the path of the saw blade. Use toggle clamps whenever possible. Never put your thumbs anywhere between the two center carriage bolts in the back cross support! Paint it bright red if necessary for you to remember this. Never reach your hands over or across the blade or behind the sled when the blade is running. Be sure that the miter grooves on the table saw surface remain free from debris which might jam up the movement of the sled. Wear safety glasses at all times. Be smart about what you do in the workshop. Have a plan for what to do if something goes wrong.
(BTW, that's not my hand... I just googled "cut finger image")