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Step 6Setting 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.
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.
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