Tenon saw is not sawing coplanarly

[tibi]

Hello gentlemen,

I am now making a saw bench with hand tools only. I have found an issue with my tenon saw. It is not sawing coplanarly. I have started to saw big dovetails for the bench. The top cut is square to the face and the front cut follows the line. The angle is the same on both front and back tape. The back cut wanders off to the side.

I had the same issue when cutting these boards to length. The top and front edges were square and the back and bottom edges wandered off. The board is 120x40 mm. Is it possible to get such big cross section right out of the proper tenon saw or the shooting board is always necessary? I just cannot get it coplanar, so I always err to the waste side and then finish off with the shooting board for square cuts. For dovetails, I would need to pare off the back side and it would save me a lot of time, if I could get it right from the saw.

Interestingly, when cutting square small cross sections, like 20x50mm, I can get it right off the saw without the shooting board.

Thank you for any advice.
Front view

Top View (does not look square, but it is - growth rings and distortion make it look out of square)

Back view

Tenon saw is straight

 

[rogxwhit]

Forget 'co-planar', one of those recently popularised terms and not the right one here. It might be your technique that's in error. Make the first cut with the saw at an angle to the wood, following the mark across the top of the piece and down one face of the tail both at once. Turn the wood round in the vice & do the same from the other side. The cuts will meet (overlap) along the top - that's ok. Now put the wood upright and saw down with the saw straight across. The previous cuts will be a physical guide. Continue the cut to the bottom. !!!

 

[tibi]

Forget 'co-planar', one of those recently popularised terms and not the right one here. It might be your technique that's in error. Make the first cut with the saw at an angle to the wood, following the mark across the top of the piece and down one face of the tail both at once. Turn the wood round in the vice & do the same from the other side. The cuts will meet (overlap) along the top - that's ok. Now put the wood upright and saw down with the saw straight across. The previous cuts will be a physical guide. Continue the cut to the bottom. !!!

Thank you very much. I did exactly that, square and down the front side, but I did not turn the wood around, but continued from the same side, once I had the guideline on the top and front. I will try to turn the wood around on the other end of the board tomorrow an I hope the results will be better.

 

[rogxwhit]

Yes, to attack from both sides is a must. You can't really saw to a line that's out of sight. A sharp saw of course, with a decent set to the teeth.

 

[tibi]

Yes, to attack from both sides is a must. You can't really saw to a line that's out of sight. A sharp saw of course, with a decent set to the teeth.

The saw has been sharpened just two weeks ago and I used it maybe a couple of times. Set also seems right. I know the technique you have described, but it did not dawn on me that I should use it, as most people do not even mark the opposite side of the dovetails. They only mark the front and top.

But for such thick dovetails, it is probably a must, as you say.

I am a newbie, so even though I know that the technique exists, It does not come to mind naturally, as if I had used it regularly for 20 years. It will come with experience

Thank you again.

 

[rogxwhit]

I always mark dovetails right round, even little ones! And I expect them to fit most of the time straight off the saw.

 

[Sgian Dubh]

Coplanarly? Now there's a word straining for relevancy, ha ha!

I'd just say it's not cutting, well, er... 'straight'.

There is the slight, but unlikely, possibility that the teeth pointing to one side of the plate don't project as much as the teeth pointing to the other side of it that could cause a saw to do that. As I said though, probably unlikely. Apart from that it is, as rogxwhit said, most likely down to technique. He provided a pretty good description for cutting joints in thick or wide material, such as wide tenon faces and, in your case, dovetails in thick material.

With enough practice and the right saw* I suspect you'd be able to cut those particular dovetails from just one face, basically following the first part of rogxwhit's guidance, i.e., establish the kerf across the end grain and down to the shoulder line facing you followed by using that kerf to guide the saw and, on each stroke, raise the saw's heel two or 3 mm until the cut is complete. I'm guessing you may not get enough of the necessary practice to be able to do that with confidence, so you're probably best following rogxwhit's tip through all the stages described.

* The right saw would be a rip tenon saw, or possibly dovetail saw, with an appropriate number of teeth per inch (TPI) for the thickness of the wood. Slainte.

 

[tibi]

Coplanarly? Now there's a word straining for relevancy, ha ha!

I'd just say it's not cutting, well, er... 'straight'.

There is the slight, but unlikely, possibility that the teeth pointing to one side of the plate don't project as much as the teeth pointing to the other side of it that could cause a saw to do that. As I said though, probably unlikely. Apart from that it is, as rogxwhit said, most likely down to technique. He provided a pretty good description for cutting joints in thick or wide material, such as wide tenon faces and, in your case, dovetails in thick material.

With enough practice and the right saw* I suspect you'd be able to cut those particular dovetails from just one face, basically following the first part of rogxwhit's guidance, i.e., establish the kerf across the end grain and down to the shoulder line facing you followed by using that kerf to guide the saw and, on each stroke, raise the saw's heel two or 3 mm until the cut is complete. I'm guessing you may not get enough of the necessary practice to be able to do that with confidence, so you're probably best following rogxwhit's tip through all the stages described.

* The right saw would be a rip tenon saw, or possibly dovetail saw, with an appropriate number of teeth per inch (TPI) for the thickness of the wood. Slainte.

Hi Richard,

coplanarly was meant to mean that all 4 edges of the cut should be in a single plane, which in my case were not.

In my original attempt, I did exactly the first part of rogxwhit advice, i.e. establish the kerf on the top and then proceed with the cut on the front face only, forming a diagonal triangle. both horizontal and vertical lines were straight. Then I continued cutting diagonally, but lifted the back of the saw a little, so that I do not cut over my bottom line and finishing the cut from the front face. The result was as shown.

My saw is a vintage tenon saw filed rip with 11 - 12 TPI (forgot the exact number). My gents dovetail saw is just deep enough to touch the bottom of the dovetail when cutting straight down. It would not finish the diagonal cut needed for the dovetail.

I have already cut the outer shoulders of the tailboard with flipping the board and I got better results. So I have to do it this way until I learn better.

 

[deema]

It looks to me like a classic occurrence where the saw gullet’s (space between the teeth) are getting full causing it to wander. I’m not sure how thick the material is, or what tooth count the saw has, but I suspect that the tooth count is too high for the stuff. Try sawing something that’s around 5 or 6 teeth thick, see if it wanders, if it doesn’t, you need a saw with a lower teeth per inch (TPI)

 

[tibi]

It looks to me like a classic occurrence where the saw cullet’s are getting full causing it to wander. I’m not sure how thick the material is, or what tooth count the saw has, but I suspect that the tooth count is too high for the stuff. Try sawing something that’s around 5 or 6 teeth thick, see if it wanders, if it doesn’t, you need a saw with a lower teeth per inch (TPI)

Thanks. The material is 40 mm thick (1 5/8), tenon saw is around 11 TPI. I do not believe myself so much as to cut dovetails with a full-length hand saw and expect them to fit straight from the saw.

 

[FaecesForBrains]

If you follow the lines on top and front, but are off on the back, the saw blade is being twisted ever so slightly in the cut. Relax your hand. Don't put a death grip on the saw. Let it cut at its own speed. Don't try to steer it. Get it started right and just move it in the kerf and let it cuts its own way. Don't press it. Stand in front of the work-piece in a way that your arm can move straight forward and back. Don't work in a cramped space or in a cramped, tense position.

 

[tibi]

If you follow the lines on top and front, but are off on the back, the saw blade is being twisted ever so slightly in the cut. Relax your hand. Don't put a death grip on the saw. Let it cut at its own speed. Don't try to steer it. Get it started right and just move it in the kerf and let it cuts its own way. Don't press it. Stand in front of the work-piece in a way that your arm can move straight forward and back. Don't work in a cramped space or in a cramped, tense position.

Thank you. That was my initial idea, that as the kerf is wider than the blade by the set distance, I have the ability to steer the saw by improper technique. Thus I can skew it and the blade will be twisted.

 

[Bod]

Try standing with your shoulder in line for the first cut.
Second cut, shuffle your feet across by the distance to the cut.
Shoulder in line for each cut.
The last photo shows what happens when you do two cuts from the same position, slight difference for two cuts close together. Imagine the two cuts being a metre/yard apart but you didn't move from the same spot. V

Bod

 

[ajs]

I've read your initial post but must admit I've only skimmed through the replies. Forget coplanar, that just confused me since I start thinking in a mathematical sense (coplanar to what?). As I understand it your cut edges are simply not flat.

If your saw is straight and flat then it has to be an issue with technique. I suspect it's likely caused by rocking the saw during the stroke, e.g. angling it up at the start of the stroke and down at the end. Keep it at a consistent angle, ideally horizontal, and I suspect the problem will go away.

Correcting bad habits takes time of course so in the interim I'd simply suggest cutting well to the waste side and cleaning up with good sharp chisels.

 

[--Tom--]

Easily done by twisting the plate due to the wiggle room created by the kerf

 

[Jacob]

Mark all round and follow the marks!
Takes practice to freehand a straight cut - and practice in covering your tracks when they go a bit off!
Mainly to take long strokes as full as possible but without forcing it - that's what sends them off. Also to "sight" down the back of the saw, like a rifle perhaps. Keeps it straighter

 

[rogxwhit]

Also to "sight" down the back of the saw, like a rifle perhaps.

He's using a back saw (I hope) to cut the sides of some dovetails, so he wouldn't want to be peering round the back of the saw to do that.

The tpi of the saw sounds about right, not to leave too coarse a cut. The technique of sawing to a line is to focus on both the teeth and the line where they interface. You are micro-attentive to the situation at that point. Almost as if you were to pre-empt any wander, and correct it before it happened. Which is a kind of nonsense but to me it expresses the experience and how control is exerted. Maybe to say it better, you are constantly micro-correcting because by the time any macro correction became necessary it would be too late.

It doesn't matter where your feet are or your shoulders or elbows or anything else (that'll all follow naturally). It's not a ballet class. Or even whether the wood is upside down (ok, I exaggerated there). But as I said, it's all in the nature of one's attention.

Years and years ago I remember reading somewhere that one was advised to set a piece in the vise so that the line to be sawn to was vertical. Yet another propagated myth. It makes no difference at all.

 

[tibi]

Thank you all for your advice. Here is an update from today's session.

1. Cleaned up tails from yesterday

2. Pins cut by applying rogxwhit's advice:

I cut them with a dovetail saw that did not reach the bottom, as the board with tails is thicker than the pin board. I will clean it after the waste is cut out.

Today I did not have enough time to cut out the waste and test the joint together. If I write no message tomorrow, it means that I transferred the tails to the pin with some alignment error and I am ashamed of it

 

[mikej460]

Much better but it looks like you are sawing on the pencil line instead of the waste side of the pencil line, or you have chiselled away too much during the clean-up?

 

[tibi]

I raised the eyebrows of many by using the word coplanar. As English is not my native language, that word was the first that came to mind to describe my problem. I have read the description on all-knowing Wikipedia which says:

In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all. For example, three points are always coplanar, and if the points are distinct and non-collinear, the plane they determine is unique.

I have realized that that is exactly what I wanted to say. We have 4 points that determine the cut area and those 4 points were not in a single plane in my case. But saying that my cut is not straight might have induced that it is not following a line, which it was on the top and front, but not on the back and bottom. So the plane of cut was twisted-like.