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Question on cutting tapers on a table saw

4K views 100 replies 25 participants last post by  bandit571 
#1 · (Edited by Moderator)
I've noticed that when I use a tapering jig on a table saw, the taper is uneven. More is taken off at the "bottom" of the cut than at the top. This is due to the blade being round. It's alleviated to some extent when the blade is fully raised, but it's still there on thick stock, such as a table leg.

So then how is any miter not affected in the same way to a much lesser extent?

It's so pronounced on a taper jig that I started cutting tapers on my bandsaw. Has anyone else encountered this?

Thanks!
 
#7 ·
+1 a round blade has nothing to do with it.

- pottz
It sort of does if the fence is diverging from the blade. If so, the front of the blade (bottom of stock) would be closer to the fence than the top edge of the blade (top of stock). Raising the blade all the way up would reduce the problem by bringing the top and bottom points closer together in the fore-aft direction. Of course, this would be a problem with all rip cuts, and not just tapers.
 
#8 ·
If your blade is not vertical it will do that. Also, check to make sure your fence is parallel to the blade.

- bilyo
Make sure the blade is actually 90* to the tables surface. Check that the blade is parallel to the miter slot, and or fence, depending on what you are following against. Something isn't 90, and for a good taper things need to be 90. as in 89 starts making differences. If I had to guess I would say blade tilt.
 
#9 ·
If your blade is not vertical it will do that. Also, check to make sure your fence is parallel to the blade.

- bilyo

Make sure the blade is actually 90* to the tables surface. Check that the blade is parallel to the miter slot, and or fence, depending on what you are following against. Something isn t 90, and for a good taper things need to be 90. as in 89 starts making differences. If I had to guess I would say blade tilt.

- therealSteveN
+1 i agree
 
#10 · (Edited by Moderator)
I'm wondering if you are using the taper jig in a different way than other people are.

The cut should always be parallel with the blade.

The piece being cut should be fixed in relation to the taper jig.

The taper jig and workpiece should slide as one.

The side of the taper jig should slide along the fence.
 
#12 ·
I am trying to balance what's being said here with what I'm observing. I have a saw that is well set-up, sharp blade, etc. The taper jig itself is straight.

To me, it's as simple as this… and maybe I just need to be "straightened out," but imagine you're tapering a 3" leg… something with some heft. You're moving into your blade. Contact is made first with the part of the leg that's against the table. How much further do you have to push your piece until the top of the blade makes contact? That's where the unevenness comes in. The taper begins at "blade zero" at the bottom of the cut and at "blade zenith" at the top of the cut. In between initial contact with the bottom of the blade and initial contact with the top of the blade, the piece has moved a couple of inches. Rub a little finish on a piece before tapering it. Make sure the piece is thick. Then tell me if you're seeing even removal of the finish at the beginning of the cut.

I'm not arguing with anyone here… it may be me that's wrong. Maybe there's a problem in the setup between my own ears… my wife says there is… but I'm just explaining what I think are the physics of the blade on a taper.

Thanks!
 
#14 · (Edited by Moderator)
Only takes a minute to recheck the 90 of the blade with a combination square, and then run it in the miter slot to check against the blade, if the blade is good scoot the fence over to the close miter slot. Miter slots don't move, so if the blade side is good, the fence side will be too.

You have gotten several responses saying check this, no kidding here, your TS is not 90* square, some direction it's off.

If you find this to be a fact, it may take a little bit to make it right. Don't sweat that, you will instantly become safer, and your work will improve once you either shift your table, or if you are lucky all you will need to do is fix that indicator which is off saying you don't have any tilt. I think you do. Maybe realign your fence. It's one of them, or you would note the bend in the taper jig.
 
#17 ·
Everything is square, guys. And no, I'm not moving the piece along the taper jig.

Here are some photos. First, here's "proof of square."



Then a picture of my taper jig on my square-checked fence.



Some finish rubbed onto my thick piece of scrap stock:



And the resultant cut:



Notice how that curvature roughly looks like the curvature of the blade? Now I'm no physicist, but the bottom of the cut makes contact sooner and your workpiece is moving. This isn't to do with my saw, it's to do with the blade being round. This doesn't happen on a bandsaw. Someone try this in your own test and tell me if you get different results, please… on a thick piece of stock where the curvature of the blade comes into play.

Again, I very much appreciate the opinions of everyone who has responded, and conflict is not my objective. Believe me, I'm used to admitting when I'm wrong, and would love the opportunity to do so here in this case as well!

Thanks!

Thanks!
Jeff
 

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#19 · (Edited by Moderator)
Your explanation is incorrect. Sight the blade and it is a straight verticle line. This is the cut line and it matters not that the bottom of the blade starts first. The piece is moving parallel to the fence and as long as the piece is pushed all the way thru the start point matters not.

Now if anything isn't square (the edges of the workpiece, the fence, blade, etc. ) then you get that result. In the example pic the end wasn't smoothly cut and the taper followed the uneven edge. Square cut first and the variance will vanish. Always take a thin square trim cut. Lots of store bought S4S ain't square.

You can prove that the top is trimmed at the same point as the bottom by drawing a line, setting the leading edge of the blade to the line and seeing the cut isn't curved. If you can't get that result then you have something wrong in your gear, set up or technique.

M
 
#20 ·
Okay. Maybe I'm incorrect, and I'm just going to drop it. I know the part of the cut that is a through cut is straight… it's the very beginning of the cut that suffers. You can watch almost any tapering jig video on YT and see the shape of the offcuts have that little arc at their thinnest part. Not one video, but many. The first contact is at the bottom. The through cut doesn't begin until another inch or three. Once the through cut begins, everything is fine.

Maybe it was a bad question… or maybe it's a little thing that just doesn't show up. Still… next time you're making one of those cuts… check me out. Anyone that can show me a straight line at the start of the taper, I'll buy 'em a coffee. :)

Thanks for all the feedback, guys!

Jeff
 
#23 ·
I think what you are seeing is indeed caused by blade curvature but it's not the curvature you think. Side forces can cause the blade to take on a dish shape. This effect is usually called blade deflection. The shallow angle of the taper causes this side force to be the only force on the blade when the material first touches the it. Once the blade is fully surrounded by wood the side force imbalance changes. The result is a bit of wandering of the cut in the first part of the taper as the amount of deflection varies over the length of the cut.

The same thing can also happen in a skimming cut but is not nearly as obvious without the magnifying effect of the taper.

The deflection will be worse with a thin kerf blade as well as with dull teeth.

Actually, what you are getting is not too bad. The shallow taper angle amplifies the visual difference. If you put a straightedge along the cut, top and bottom, I think you'll only see a few mils if curvature.

Pardon me for asking the obvious question, but why are you using a tapering jig on the rip fence? When I got my slider the tapering jig was the second thing to give up it's storage space, right after the sled.
 
#25 ·
Is the pictured blade the one you use for ripping your tapers? It looks like a combination blade used for rough framing purposes. You might want to invest in a quality ripping blade, and definitely get rid of that tapering jig. You can build a tapering sled for not a lot of money and also use it to cut a straight edge on crooked boards.
I also know what you're talking about, and unless the arc of your blade is high enough to come in contact with the center of the thickness of your material, you will have a slightly offset cut at the very beginning of the taper. Obviously as you push the piece forward, you would be engaging more of the blade, thus creating full surface contact and an even cut. A simple hand plane would remove that bothersome issue of yours in one or two passes at the most. : )
 
#26 ·
You are putting far too much emphasis on the fact that your blade is circular and that your taper cut begins with the teeth closest to the table. Of course it does. But this has nothing to do with the results of the cut. As long as your blade is vertical and your fence, blade, and miter slot are parallel, your results will be a straight vertical cut.
Looking at your last photo, I assume the the brown part on the left is the original surface and the light color on the right is the fresh cut. The resulting curved line between them has nothing to do with the blade. It is the result of the original surface not being flat and square to the bottom surface of the work piece. Check it with a square to see if that is true. If you make sure that the original surface is flat and square before starting and then make your taper cut, you will then end up with a vertical line where they meet. If it will help you visualize this, start out with a profiled edge (use your router and most any profile bit). Then cut your taper through the profile. You will not get a straight line where they meet, but a crooked line that shows where the profile surface and the flat surface meet.
Hope this helps.
 
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