If your miters aren’t as tight as they used to be, here’s how to get your saw singing on pitch again!
Every carpenter should know that when you buy a new chisel or hand plane it’s not razor sharp out of the box — you have to sharpen it before using it. Well, the same is true for miter saws. They don’t come from the factory in perfect tune.
Besides, after you’ve dragged your saw in and out of the truck a few dozen times, or jammed heavy stock against the fence, or maybe even had it flip off the back of a saw stand — or a tailgate — all those precise adjustments can get seriously out of whack. If you’ve noticed joints not quite closing up for you lately, it’s probably time to tune up your saw. Here are a few tricks to get that big investment dialed in just right.
Setting up a saw properly isn’t possible with a dull or bent blade. Deal with that first. If you don’t have a fresh blade, get a new one and install it before going any further. But which blade should you buy?
Do not use the same blade in your miter saw that you use in your table saw. Ripping and crosscutting blades have different grinds. For the miter saw, I prefer a thin kerf crosscutting blade with 60 teeth or less. This type of blade often comes on new saws. My reasons for this preference are:
- They produce less friction, requiring less motor power.
- These blades cut just as straight and flat as a 500-tooth Hackaboard. (Straight and flat are the most important requirements in finish work. I rarely need glassy smooth end grain that a 90-tooth blade might produce.)
Next, I frequently use both the chop saw and the sliding saw to cut with the grain, and these blades do that job best. And last, when these blades are sharp, they don’t flutter on a plunge cut any more than a 1/8-in. thick blade with 100 teeth. However, many carpenters choose thicker miter saw blades with the maximum number of teeth, 80 or more for a 10 in. or a 12 in. blade.
Manufacturers that seem to dominate the field of blade making are Forrest, Tenryu, Freud, Amana, and Ridge, to name a few. Plus, nearly all saw manufacturers offer their own brand of upgraded industrial blades. Once you’ve put a good blade on the saw, do some basic checks before you cut into that walnut mantle shelf.
Check the table
A machine shop checks a surface for straight and flat with a machinist’s straight edge or surface plate, and those tools can be expensive. However, a good framing square is adequate for checking the table and fence of a miter saw. Put the framing square edge down on the saw table to make sure it’s flat. You can use various types of paper to measure for irregularities. An index card is eight thousandths (.008) of an inch thick; notebook paper is .004 in. thick; and paper from the phone book is .002 in. thick.
I should mention that it’s unlikely that your saw table is out of flat, because modern saws are well supported at their centers. But if a strip of notebook paper can slide under the framing square blade, you may need to make some adjustments so that vertical cuts can be dialed in perfectly later on. If a piece of cardboard fits under the blade, then adjusting the saw head to cut square on either side of the table will be impossible.
There are two possible ways to flatten a sway back table. You either scrape it flat, or flip the saw over onto parallels and straighten the table on a press. Scraping is a job that requires special tools and skills. Pressing to flatten a saw table should be done in very small increments and with great care. Cast aluminum will break. If the space under the straight edge is more than .010 in. (thicker than an index card), you may just want to take the saw to a repair shop and have them flatten it.
Straighten the fence next
After confirming that the saw table is flat, check the fence next. A bowed fence is the most common reason that a miter saw doesn’t make accurate cuts.
Using a framing square and a piece of telephone book paper as a feeler gauge, press the paper against the fence with the square. You shouldn’t be able to withdraw the paper anywhere from one side to the other (see video, below). If there is a gap, you need to adjust the fence. I shoot for perfect with this operation.
To straighten a two-piece fence, loosen the screw closest to the gap and tap or pry the fence lightly towards the framing square. Stop when the two sides of the fence align, and snug the screw. Check the entire fence again as before, and then tighten the screw firmly.
To straighten a one-piece fence, loosen the screws closest to the gap and use a pry bar to straighten the fence. Keep tension on the pry bar as you tighten the screws. An extra person is a big help with this procedure. By yourself, you have to hold the bar in position, drop the straight edge, pick up the wrench and tighten the screw. It can be a real juggling act and you may have to do it two or three times to get it right.
A word about calipers
Calipers are very inexpensive these days, both dial and digital. Whether you are working with metal or wood, calipers can help you do very fine work. When you’re sizing the depth and width of dados and grooves, nothing works as well as calipers. Working in “thousandths of an inch” may sound funny to some carpenters, but it can save a lot of frustration and time in the long run. Besides, most routers have micro-fine adjustment knobs that operate in those tolerances. For miter saw adjustment, calipers can tell you precisely how much tweaking you need to do. You don’t have to own calipers to adjust your miter saw — unless you want it to be dead accurate.
Calibrating the miter gauge
Many carpenters make their living with miter saws that don’t cut accurately. While the “keep cutting ‘til it fits” method might work, it can waste a lot of time and produce a lot of sawdust. If a saw is adjusted perfectly, assembly time is reduced, and the enjoyment and pride of our craft is increased. Most of us chose the finish carpentry profession because of the pleasure of tightly fitting pieces together to beautify and complete a living space. Working with tools that don’t perform accurately can frustrate that process. To adjust a miter saw for precise miters, begin by squaring the blade to the fence.
A quick check
To check if your saw is cutting square to the fence, start with the widest piece of stock you can crosscut with your saw. The longer the cut, the greater the accuracy of the measurement. Plywood or MDF will work just fine for this test.
For a quick rough check, hold the piece snug against the fence on one side of the saw, and trim a little off (see photo, left). Then, with the same edge against the fence, flip the piece over to the opposite side so that the bottom is facing up. Lock the saw head down so that the teeth are below the saw base. You’ll probably have to use a bungee cord to pull the saw down far enough. Then slide the cut edge of the board up to the blade. It should touch the blade along its entire length.
If there is a gap in the front or the back of the cut, the adjustment you need to make to square the saw is only half of that space. So be conservative as you make the adjustment. What may seem to be a tiny adjustment can send the cut past square in the opposite direction.
A closer examination
|To find out exactly how much the saw is off, you have to use calipers. Start by making the same initial cut described above, but when you flip the stock to the opposite side, cut off a piece about 1/2 in. wide.
My DeWalt produced a piece that was off by .010 in., meaning that each cut would be out of square by .005 in. in a full length cut. Five thousandths of an inch might not sound like much, but a gap that size in a mitered casing joint is visible from four feet away.
For those of you who are after even greater readings, Festool describes a four-cut calculation method in the instructional PDF for testing the accuracy of their Kapex saw. Instead of the two cuts used above, four cuts are made on a piece of stock.
The final cutoff is measured, and the difference is divided by 4 instead of 2, hypothetically quadrupling the accuracy of the measurement. Festool also has a mathematical formula in their online instruction manual. You can plug in the measurements from your final cut, hit the ‘Calculate’ button, the find out exactly how much to adjust the angle and in what direction. But here’s a bit of irony: All of these careful measurements and formulas only determine the amount of error in the saw. Adjusting a saw (even the pricey Festool) is far less precise than these testing methods!
Adjustment is trial and error
Now that you know exactly how much to adjust your saw, it’s time for a little or a lot of trial-and-error — how much depends on your idea of perfection. Like I said earlier: the testing method is a lot more accurate than the adjustment system. No manufacturer yet that has come out with a mechanically controlled method for adjusting the miter cut on their saw. In other words, we can measure tolerances all day long, but no saw that I’ve ever seen has a micro-fine adjustment knob or screw to dial in those tolerances. Tight-tolerance adjustments just aren’t easy.
When it comes to adjusting the miter gauge on a saw, I know of only two types of miter saws: those that have movable fences, and those that have movable miter scales — move the scale and you move the saw head in relation to the fence.
Movable fence adjustment
The miter gauge on the Bosch miter saw doesn’t move — it’s cast into the base of the saw, along with the detent positions (see photo, right). To calibrate the angle, you have to move the fence. A good machinist’s square can make fence adjustments easier. In fact, some saw manufacturers, such as Milwaukee, say that a square gets the saw as precise as it needs to be. Still, a machinist’s square can get you close enough for making initial test cuts.
First, make sure the saw is secured in the 90° detent, then lock the head down with the teeth on the blade below the base of the saw. If the transport position isn’t low enough, use a bungee cord to pull the saw head down (see photo, left).
Slightly loosen the screws securing the fence, but leave them snug, so that the fence won’t move with your fingers. Press the square tight to the fence and place your feeler gauge (a piece of phone book paper) between the back side of the blade and the square (see Fig. 10).
Without moving the square, check the front side. Adjust the fence by tapping it lightly with a rubber mallet so that the feeler gauge rubs the same at both the front and back of the blade (see Fig. 11). When you’ve squared the blade to the fence, lift the saw head and check to make sure the fence hasn’t bowed from the squaring process. If it has, re-straighten the fence, and adjust the miter angle again. Repeat the process until the fence is straight, as well as square, to the saw blade.
Miter scale adjustment
For a saw with a movable miter scale, swing the saw head until the it clicks into the 90° detent. But don’t lock the handle down, or the scale might not move. With this type of system, the actual scale has the detents that hold the saw head in position. So moving the scale moves the saw head in relation to fence.
Any movement of the miter scale must be incremental and controlled. The slots for the screws that secure the miter scale are elongated to allow for a lot of adjustment parallel to the fence. But with many saws, there is enough play for the scale to move perpendicular to the fence as well. It doesn’t take much movement to throw off the 45° miter even when the 90° miter is right on.
With my DeWalt miter saw, I loosened the scale plate just enough to pry it over with a screwdriver (see below).
Then I made another test cut using the two-cut method. It took me 6 tries before I could get the error down to a .004 in. difference, near perfect for an 8-in. crosscut in wood. That meant that each cut was out of square by only .002 in 8 in., or .001 in 4 in. — more accurate than a framing square.
After adjusting your scale plate, always check that the saw is cutting perfect 45° miters, too. To check for perfect 45s, rip a piece of 1/4 plywood or MDF. You could use thicker stock, but it will offer more resistance as it’s being cut. The ripping should be perfectly straight, and as wide as you can miter.
Lock the miter at 45° to the right, and cut four pieces long enough to allow for a left hand miter. Set the saw at 45° to the left, then stack and cut the pieces in the same order as you cut the left hand miters. When the pieces are assembled you should have no gaps.
If you do have gaps in the miters, and if your saw has an adjustable miter scale, loosen the outer mounting screws and push or pull the scale toward or away from the fence to adjust the 45° miter without messing with the 90° cut. If the plate doesn’t have enough wiggle room, you can file the screw slot, but personally, I don’t care enough to do that.
If your saw doesn’t have an adjustable scale, you may have to adjust the miter each time you cut. This only matters when you are doing broad miters such as big casings, or landing treads, or any other wide pieces mitered on the flat.
Calibrating the bevel
Adjusting the bevel angle can be a little tricky on some saws, while on others, it’s actually easier than calibrating the miter. Like the miter adjustment, I start by squaring the bevel to the table. For some carpenters, and some manufacturers such as Milwaukee, that’s perfection enough. But for others, that’s just the beginning. The two-cut and four-cut testing methods work just as well in the vertical for checking the bevel as they did on the flat for the miter.
First, lock the saw head down, so the teeth of the blade are beneath the saw base. Then hold a good square against the saw table, just touching the blade so it doesn’t deflect. (Remember, the table must be flat.) Use a sheet of phone book paper as a feeler gauge to ensure that the blade is parallel to the square, and adjust the bevel as necessary.
Each saw has a slightly different mechanism for calibrating the bevel. Here are a few of them, but you should check the manual that came with your saw for precise instructions. If you threw away the manual, most tool companies provide manuals you can download from their websites.
Of all the miter saws I’ve used, DeWalt seems to have the most pragmatic and intuitive adjustment features. To adjust the bevel on the model 706 DeWalt saw in this article, I worked with three separate bolts: one for the 90° detent, and one for each of the 45° stops on either side of the saw. The bolts are very easy to access and the process is straightforward.
The 90° adjustment bolt is located on the top of the bevel hub. Simply turn that bolt clockwise and the blade tips to the left; turn that bolt counter clockwise, and the blade tips to the right (see Fig. 15). To adjust the 45° degree stops, just back out the stop bolts, or thread them in deeper.
On the Milwaukee saw, first remove the dust chute.
Here’s something to consider: if a screw has 20 threads per inch, it advances .012 in. for every quarter turn. So a little goes a long way with these adjustments.
Adjusting the Bosch saw is similar to the first two. Before you start, back out the main depth-stop screw so the blade can drop below the throat guard, then remove the back cover to view all the adjustment bolts — and the adjustment tools.
Before touching any of the adjustment bolts, lift the bevel lock lever and set the saw in the 90° detent. Now loosen the bolts labeled A and B in the photo below.
The wrench supplied with the saw works, but it’s easier with a 10-mm socket.
|Next, loosen the set screw labeled D using the 4-mm Allen wrench supplied with the saw. Back out the screw at least three full turns.
Adjusting Festool’s Kapex saw is a bit different. The Kapex isn’t equipped with a micro-fine bevel adjustment bolt or screw, which means that dialing in the tool isn’t nearly as accurate as the 4-cut calibration test they suggest. But there is a work around.
Start by locking down the bevel in the 90° detent. Next, loosen the two adjustment screws at the back of the motor. I found it easiest to remove the cord reel. You can even use the wrench supplied with the saw (see photo, left). Festool suggests two ways to adjust the saw: You can move the entire head or just the bevel plate. To move the entire head, keep the bevel locked in the 90° detent. To move just the plate, release the bevel lock lever.
Because there is no micro-fine adjustment bolt on this saw, the head and plate move freely, making it very tough to dial in a fine adjustment. But here’s a solution: Before loosening the two adjustment screws, cut two perfectly square pieces of stock. If you’ve adjusted the miter angle first, you can cut those blocks on the flat.
Clamp one block against the miter saw fence while sliding it snugly against the blade. Get the other block and clamp ready for the opposite side. Then loosen the two adjustment screws. Wiggle the saw head a little until the blade is flat against the first block. Now clamp the second block against the opposite side of the blade, tapping it gently to trap the blade between the blocks. When the blade is secured in a perfectly square position, tighten the two adjustment screws, then check your cuts again using the two-cut or four-cut testing method. Once more, trial and error is the only way to further refine the adjustments. With patience, you can dial in the bevel angle even closer.
Know your saw
When it comes to miter saws, the best piece of advice I can offer any carpenter is: Know Your Saw. When the saw cuts a perfectly square bevel, but the miters aren’t perfect, you may have to make miter adjustments each time you use the saw. Knowing your saw means practicing and perfecting your miter saw tune-up procedure.
One additional problem you may encounter with a miter saw — and especially a sliding saw — is blade tracking. The saw blade must be perfectly parallel with the rods. If not, the trailing edge of a saw blade will cut a little more wood as it passes through the kerf. The same type of problem can show up while doing tall plunge cuts with a standard miter saw. If the blade plate rubs and burns wood at the top of the cut, then the blade is not in the same plane as the arc of the saw head. But professional saws are machined on CNC equipment that maintains tolerances within .0005 (five ten thousandths!) of an inch. If your saw isn’t tracking perfectly, then it’s likely something happened to the saw after you bought it. The blade arbor may be a little out of whack from a sawing accident. The head may even be bent. Or maybe you didn’t see it fall out of the truck before your helper stuck it back in there real quick.
There are no adjustments for blade tracking problems. You either have to replace parts or buy a new saw. But before you send your saw to the junkyard, consider this:
When my Hitachi was brand new, a handrail fitting slipped out of its clamp and twisted the blade as it slammed between the fences. The head was bent so badly that the blade was out of perpendicular to the hinge pin 1/8 in. across its diameter. That brand new saw sat in my garage for a year before I decided that I had to figure out how to fix it.
I clamped the head in a vise, clamped a bar near the blade arbor and bounced on the bar — I mean, with all my weight — well, a lot of weight. It made a loud popping sound. I rechecked the blade/pin relationship and found that the error was only .010 in. over the radius of the blade. I guess I was lucky to get it that close. I’ve been using that saw for six years now, and I’m satisfied with it. I have never had the blade plate rub on a fresh cut, though I’m sure there must be cracks in the casting. You can’t bend aluminum castings much at all. Of course, the right way to fix that problem would have been to buy a new head casting. But, it wouldn’t have cost much more just to buy a new saw!
I hope these ramblings have been useful. I was glad for the opportunity to write this article because it pushed me to tune up my own saws. These modern miter saws are amazing. But just as that proverbial little girl who had a little curl: When they’re good, they’re very, very good; but when they’re bad, they’re horrid! Inaccurate cuts are rarely the fault of the saw, and most often they’re something that can be corrected with a little attention.
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Please don’t try anything you see in THISisCarpentry, or anywhere else for that matter, unless you’re completely certain that you can do it safely.
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David Collins has been making stuff all his life. At age seven, he carved swords, canoes, and all sorts of things with his own pocket knives. At age eight, he made popsicle-stick fences to go around the Christmas tree — he painted them silver. David’s first entrepreneurial endeavor was trying to sell those silver fences. His 81-year-old mother still keeps some of those things in her cedar chest.
David always took things apart to look inside and “make improvements.” He dismantled mini-bikes, old pieces of motorcycles, and a Victoria Bergmeister, which he bought at age 15. His most ambitious teenage project was rebuilding his parents ’57 Ford, although he did have plenty of help with that. After graduation, David and a couple friends drove that car to California to see what the “Height Ashbury” thing was all about—didn’t figure it out, but it sure was interesting.
While David messed with all of those things, he was also captured by music. His mother recognized the talent in David and his siblings early on, and she faithfully drove them to their weekly piano lessons. He didn’t like to practice, but the threat of mom’s pancake turner crackin’ his butt kept him at it. He thanks her for that discipline today.
In his early 20s David worked as a framer, and soon decided that he was going to need a college education. But music was his first love, so he signed up for the music program at OSU. He couldn’t take very much of it — he’d go to school for a while and then work for a while. After seven years, David finally graduated with a B.M. from Capital University.
In 1973 David married Kathryn Hartley. She endured about half of David’s education and a great deal of other stuff since. After graduation, the church where he was pianist hired him to teach choral music at their school. Never having considered the financial implications of a music degree, the ’70s and early ’80s turned out to be lean years. Construction work in the summers got them by. He worked for the great Dave Porter of Columbus every summer through the ’80s, trimming high-end houses. The work was always interesting and satisfying.
In 1983 and in 1986 Kathy and David adopted Hannah and Emily. In 1989, David became disillusioned with teaching. Students do what they want to do, and a piece of wood does what he wants it to do. David handed in his resignation at school and went into finish carpentry full time, a move he has never regretted.
A few years later David was running some 8-in. crown on a big job. He couldn’t cope that molding with a coping saw and got a little desperate. He’d always used the saber saw from the back side (right side up) and started figuring out a way to cope that large crown molding without banging around on the face of it. He needed to freehand the saw without the restrictions of the flat base. David hammered out sheet metal bases, and after 6 or 8 tries, he produced what is now called the Collins Coping Foot. That thing worked so well that he figured everyone would want one. David spent a lot of borrowed money on lawyers and tool and die makers and started the Collins Tool Company.
David hasn’t done any finish carpentry for-hire since 2006. He spends his early mornings with the Good Book, and writing music at his music work station. The rest of the day is spent in tool production, and tooling up for a new product called Mitertite.