Advice for squaring my compound miter saw
November 1, 2020 4:53 PM
I think I need 1) a good way to finely and precisely maneuver my miter saw while I'm adjusting both the miter and bevel alignments (or a way to make this unnecessary), and 2) the best right angle squaring tool(s) for this job.
So, I have a Bosch 12" compound miter saw. It's great! But I want it to be super-precise in its angles -- at least as precise as it can be -- and I haven't achieved that yet.
I know how to align it, basically, using the mechanisms provided by the saw (the two bolts in the back for adjusting the bevel, setting up the fence, etc.). However, when I do this, I can't get the adjustment _just_ _right_. It seems to move more than I want to when I nudge it, or else I want to nudge it so finely that it's not really making it easy. Especially the miter setting! The saw is big and heavy.
I also know that careful technique when cutting -- not applying lateral pressure when bringing the saw down, for example -- is important for this kind of saw, and that some people recommend using a radial arm saw for precision. I still think this saw can give better results than I'm getting.
Tutorials I've found focus on the mechanics of the saw's adjustment mechanisms. Does anyone here have tips, though?
I also know how to check whether it's aligned (e.g. cut 4 lengths of wood with 45-degree angled ends, put them together, see if it makes a square; check against a square).
What I'm hoping for is the mechanics of actually physically adjusting it -- something better, I hope, than nudging one direction, checking, then nudging the opposite direction, only to find that the angle is off approximately the same minute amount, greater or lesser, at each step.
Also - apparently having a really good metal square for aligning might be helpful. I have a square that's also a "stair tool" - does anyone recommend a particular square for aligning a saw like this?
I also saw one demonstration using tiny thin, 1/1000th inch gauged strips of metal to check the amount the saw angle was off. Would something like this help me? Any other helpful tools?
Thanks! You can help a lot of future shelves/tables/frames be that much better!
So, I have a Bosch 12" compound miter saw. It's great! But I want it to be super-precise in its angles -- at least as precise as it can be -- and I haven't achieved that yet.
I know how to align it, basically, using the mechanisms provided by the saw (the two bolts in the back for adjusting the bevel, setting up the fence, etc.). However, when I do this, I can't get the adjustment _just_ _right_. It seems to move more than I want to when I nudge it, or else I want to nudge it so finely that it's not really making it easy. Especially the miter setting! The saw is big and heavy.
I also know that careful technique when cutting -- not applying lateral pressure when bringing the saw down, for example -- is important for this kind of saw, and that some people recommend using a radial arm saw for precision. I still think this saw can give better results than I'm getting.
Tutorials I've found focus on the mechanics of the saw's adjustment mechanisms. Does anyone here have tips, though?
I also know how to check whether it's aligned (e.g. cut 4 lengths of wood with 45-degree angled ends, put them together, see if it makes a square; check against a square).
What I'm hoping for is the mechanics of actually physically adjusting it -- something better, I hope, than nudging one direction, checking, then nudging the opposite direction, only to find that the angle is off approximately the same minute amount, greater or lesser, at each step.
Also - apparently having a really good metal square for aligning might be helpful. I have a square that's also a "stair tool" - does anyone recommend a particular square for aligning a saw like this?
I also saw one demonstration using tiny thin, 1/1000th inch gauged strips of metal to check the amount the saw angle was off. Would something like this help me? Any other helpful tools?
Thanks! You can help a lot of future shelves/tables/frames be that much better!
I use a speed square for this sort of thing. The plastic ones are accurate, cheap and easy to verify for an everyday checking tool. I also have some assorted machinist squares/blocks/levels etc. but to get quality stuff can cost as much as your saw. EG: I have versions of this Starret three head square and this Engineer's square for precise tool setup. I also have a 12" machinist level for checking to see if things are flat but it's really over kill for that purpose.
Chinese versions of both squares are available at 1/10 the price and the engineer's squares should stay in square but you have to verify they are actually square. I once checked dozens of cheaper squares in a hardware store and found only two that were acceptably square. I don't really trust the cheap removable head stuff because who knows how quality the material is and whether it'll actually stay accurate.
Lee Valley has house brand and Bridge City stuff for more reasonable prices than the Starret stuff that should be good out of the box. I've heard good things about both and Lee Valley at least has bullet proof returns.
Feeler gauges (the thin strips of precise thickness metal) may be of use when used with a straight edge but your starting to talk about a precision that the saw can't support. Especially with anything other than 90 degree cuts the saw itself is pulling the angle open/closed by a minute amount.
Step one is to make sure your table and outfeed tables are flat. There isn't much you can do about the saw itself not being flat besides taking it apart and making sure it is clean if the swing part is proud of the side pieces. A 4' machined or extruded aluminum level can help here and is a good trade off between price and accuracy.
Step two is to adjust your fence to be flat and square to the right angle cross cut. Your Bosch looks like it has seperate left and right fences which are easier to get flat than one peice designs but also will go out of flatness easier. Flatness should be checked regularly and the fences tend to go slightly acute with use. Your 4' level will be handy here too. You can clamp the fences to the square when loose to keep them flat relative to each other while you adjust for square 0 degree cuts.
Step three is to make sure your fence is at 90 degrees to your table in the verticle direction. Hopefully it is machined bang on but you can use brass shim stop at the front or back of the clamp surface to make adjustments if necessary. This is where a set of feeler gauges might be useful. Honestly though if this is a new saw and it wasn't square I'd return it.
I don't really have any hints on making adjustments but I will note I've drilled and tapped the moveable part to accept a small piece of angle and the fixed part to accept a bolt that I could use as a stop block when I was making a lot of 45 degree cuts with a slider. The bolt lets you dial in the angle and then it can be set with a jam nut. The base pieces are aluminum so you need a good cutting fluid when tapping them. The Bosch has that big arm sticking out so you probably don't need to modify the moveable part.
If you securely bolt the saw to a piece of plywood you can do a similar thing with a piece of angle that you bolt to the plywood. Tap the upright part of the angle for a bolt. Secure the angle to the plywood with T-Nuts and you can move it around for different angles but you'll have to re adjust it each time (but that should be easier because of the bolt).
Typing this out I don't see any reason a person couldn't just clamp a piece of angle to the plywood at the initial set point and then use the bolt to make the fine adjusts.
BTW a carriage bolt works good for this sort of thing because the head is already smooth and the mushroom shape ensures minimal contact area.
One of the saws I've used had detents for the common angles that could be adjusted with set screws from underneath so something to check.
posted by Mitheral at 7:24 PM on November 1, 2020
Chinese versions of both squares are available at 1/10 the price and the engineer's squares should stay in square but you have to verify they are actually square. I once checked dozens of cheaper squares in a hardware store and found only two that were acceptably square. I don't really trust the cheap removable head stuff because who knows how quality the material is and whether it'll actually stay accurate.
Lee Valley has house brand and Bridge City stuff for more reasonable prices than the Starret stuff that should be good out of the box. I've heard good things about both and Lee Valley at least has bullet proof returns.
Feeler gauges (the thin strips of precise thickness metal) may be of use when used with a straight edge but your starting to talk about a precision that the saw can't support. Especially with anything other than 90 degree cuts the saw itself is pulling the angle open/closed by a minute amount.
Step one is to make sure your table and outfeed tables are flat. There isn't much you can do about the saw itself not being flat besides taking it apart and making sure it is clean if the swing part is proud of the side pieces. A 4' machined or extruded aluminum level can help here and is a good trade off between price and accuracy.
Step two is to adjust your fence to be flat and square to the right angle cross cut. Your Bosch looks like it has seperate left and right fences which are easier to get flat than one peice designs but also will go out of flatness easier. Flatness should be checked regularly and the fences tend to go slightly acute with use. Your 4' level will be handy here too. You can clamp the fences to the square when loose to keep them flat relative to each other while you adjust for square 0 degree cuts.
Step three is to make sure your fence is at 90 degrees to your table in the verticle direction. Hopefully it is machined bang on but you can use brass shim stop at the front or back of the clamp surface to make adjustments if necessary. This is where a set of feeler gauges might be useful. Honestly though if this is a new saw and it wasn't square I'd return it.
I don't really have any hints on making adjustments but I will note I've drilled and tapped the moveable part to accept a small piece of angle and the fixed part to accept a bolt that I could use as a stop block when I was making a lot of 45 degree cuts with a slider. The bolt lets you dial in the angle and then it can be set with a jam nut. The base pieces are aluminum so you need a good cutting fluid when tapping them. The Bosch has that big arm sticking out so you probably don't need to modify the moveable part.
If you securely bolt the saw to a piece of plywood you can do a similar thing with a piece of angle that you bolt to the plywood. Tap the upright part of the angle for a bolt. Secure the angle to the plywood with T-Nuts and you can move it around for different angles but you'll have to re adjust it each time (but that should be easier because of the bolt).
Typing this out I don't see any reason a person couldn't just clamp a piece of angle to the plywood at the initial set point and then use the bolt to make the fine adjusts.
BTW a carriage bolt works good for this sort of thing because the head is already smooth and the mushroom shape ensures minimal contact area.
One of the saws I've used had detents for the common angles that could be adjusted with set screws from underneath so something to check.
posted by Mitheral at 7:24 PM on November 1, 2020
Not a direct answer to your question but you could always fine tune the end of the board with a hand plane and a try square. It's made easier by using a “shooting board”. Lee Valley sells a kit but you could just make a one out of wood.
posted by bonobothegreat at 5:59 AM on November 2, 2020
posted by bonobothegreat at 5:59 AM on November 2, 2020
something better, I hope, than nudging one direction, checking, then nudging the opposite direction,
Here is a technique to avoid nudge-and-check.
1. Do a 2 cut test: Take a piece of wood like 1/4" x 8" x 12". Put it on the saw and draw an x on it. Put it against the fence and carefully cut it. Make sure the blade is not cutting only one side of the wood. Then flip the wood so the x is down, do not flip it end over end. Draw a "f" near the fence so that if you took a 2" cut the "f" would be on the off cut. Take another cut, about 2" wide.
2. Figure out how much the blade is out of true. Measure the fence width of your offcut piece and the away from fence width. Measure how far these are from each other. Say you end up with:
fence: 1.98"
away: 2.03"
distance: 8"
You probably know that because the fence side is smaller than the away side the angle is smaller than 90. At this point in the nudge-and-check you would nudge and then check again.
Here is the other thing you can do: You need to open up the angle so the difference between the two is zero. Instead of nudge you can set a stop with that difference.
We want to pivot the fence away from the blade. I take a point on the back of the fence, 16" from the blade, put a .005 shim from a feeler gauge set between the point and a clamped down stop of some sort. Then I loosen the fence bolt far from the blade and just loosen a little the fence bolt close to the blade, then I take the shim out and pivot the fence so it touches the stop.
Then I check it again.
If you need to make the angle smaller, put the stop and shim in the front of the fence.
If you find this too tedious and go back to nudge and check that is totally fine.
(Why 16"? Because the two cut test produces a difference twice as big as the true error off of 90.)
posted by bdc34 at 6:11 AM on November 2, 2020
Here is a technique to avoid nudge-and-check.
1. Do a 2 cut test: Take a piece of wood like 1/4" x 8" x 12". Put it on the saw and draw an x on it. Put it against the fence and carefully cut it. Make sure the blade is not cutting only one side of the wood. Then flip the wood so the x is down, do not flip it end over end. Draw a "f" near the fence so that if you took a 2" cut the "f" would be on the off cut. Take another cut, about 2" wide.
2. Figure out how much the blade is out of true. Measure the fence width of your offcut piece and the away from fence width. Measure how far these are from each other. Say you end up with:
fence: 1.98"
away: 2.03"
distance: 8"
You probably know that because the fence side is smaller than the away side the angle is smaller than 90. At this point in the nudge-and-check you would nudge and then check again.
Here is the other thing you can do: You need to open up the angle so the difference between the two is zero. Instead of nudge you can set a stop with that difference.
We want to pivot the fence away from the blade. I take a point on the back of the fence, 16" from the blade, put a .005 shim from a feeler gauge set between the point and a clamped down stop of some sort. Then I loosen the fence bolt far from the blade and just loosen a little the fence bolt close to the blade, then I take the shim out and pivot the fence so it touches the stop.
Then I check it again.
If you need to make the angle smaller, put the stop and shim in the front of the fence.
If you find this too tedious and go back to nudge and check that is totally fine.
(Why 16"? Because the two cut test produces a difference twice as big as the true error off of 90.)
posted by bdc34 at 6:11 AM on November 2, 2020
Only set one fence to 90 at a time. Do the longer of the two fences. Then set the other shorter fence to be co-planer to the longer fence.
posted by bdc34 at 6:13 AM on November 2, 2020
posted by bdc34 at 6:13 AM on November 2, 2020
Also: if it is not working take a step back and ask why. I had a high quality brand saw and one of the flats the fence bolted down to was .01" lower than the others. I could never get this saw true until I found this and put in a shim.
posted by bdc34 at 6:17 AM on November 2, 2020
posted by bdc34 at 6:17 AM on November 2, 2020
But I want it to be super-precise in its angles -- at least as precise as it can be -- and I haven't achieved that yet.
I encourage you to get your saw precise enough for your work. The error we talked in the instructions about was .025" over 8" so that would be .25" over 80". You can do better than that, but think about it once you get it more trued. Don't obsess over your tool to the point that you don't get to building anything with it.
posted by bdc34 at 6:30 AM on November 2, 2020
I encourage you to get your saw precise enough for your work. The error we talked in the instructions about was .025" over 8" so that would be .25" over 80". You can do better than that, but think about it once you get it more trued. Don't obsess over your tool to the point that you don't get to building anything with it.
posted by bdc34 at 6:30 AM on November 2, 2020
This thread is closed to new comments.
I've also had better luck building my own zero clearance fence out of plywood rather than using the factory fence. You may have holes in the back of your factory fence that you can use to attach a plywood one. The trouble with doing that is the process of aligning it requires that you make a few cuts, which will widen the slot made by the saw kerf. So once you get it aligned you might want to make a new fence, make a single cut in it, and use that from there on out.
I will say this... once you get it as perfect as you can, cut a piece of scrap wood and mark it with the angle. Use that as your guide to get that angle from here on out. Depending on what you're doing you may use the same angles over and over again.
If you know someone with a laser cutter you can have them cut you some angle guides of the various angles you might run across. You can probably find some on-line to purchase as well.
I only try to get it super accurate if I'm doing something like making a picture frame or rings for a segmented bowl. With basic carpentry stuff, cutting 2x4s and such, I don't worry about precision too much.
posted by bondcliff at 7:03 PM on November 1, 2020