Thread: Tensioning a backsaw plate
5th Nov 2014, 05:57 AM #1
Tensioning a backsaw plate
I did a quick search of this site and I didn't find (at least over the past couple of years that is) a posting specifically discussing saw-plate tensioning so here goes.
I've read and heard a lot of conflicting opinion on the subject of saw-plate tensioning. Some consider it a vital and often overlooked part of the processes of making and maintaining saws, others call it hokum.
In my experience making my own saws I have come to find that saw plate tensioning, for want of a better term, is vital to producing a saw with a straight tooth line. I only make backsaws using folded backs because I like the traditional looks as well as the servicability afforded because they can easily be removed and replaced without resort to heat-guns and/or solvents to remove the plates of saws made with slotted backs.
I make my backs by driving the back down the length of the saw plate, some call this the 'side-entry' technique. My backs have zero-clearance between the lips of the back when I'm done folding them which prevents driving the plate down on to the plate perpendicular to the long axis of the back.
Here's an illustration.
Saw plate stretching during back installation.jpg
As the back is driven from left to right down the saw plate the friction between the lips of the back and the top of the saw plate tends to stretch the plate ever so slightly. The tooth line however is not stretched and the differential stresses applied to the saw plate tend to cause the toot line side of the plate to wrinkle. This is a problem when using thinner saw plates and it is particularly acute when I put backs on my taper-ground 0.015" saw plates. These saw plates generally taper between 0.002" and 0.003" from the tooth line to the bottom of the back and unless properly tensioned they are impossible to get straight.
In the process of changing the position of the back on a saw I thought to document the process of tensioning on a saw build I have described on another thread in this section (https://www.woodworkforums.com/showth...=181862&page=2).
First here's the saw.
Here it is all fitted up and ready to test.jpg
It's my interpretation of a Disston #4, the saw plate is 0.0155" tapering to 0.0135" measured just below the back. When I was setting up this saw I originally fitted a stainless back but found that it was too light for my liking and switched to one of my 0.125" thick heavy brass backs. I got the saw back together and found that the projection of the back above it's mortise in the stock was higher than I like.
Some is okay but this is too much.jpg
This is how the tooth line looked before I started.
Starting out nice and straight.jpg
So I need to move it down and deepen the mortise to suit. First I took the plate/back assembly out of the stock and clamped the toe end of the plate gently but firmly in the vise.
Step 1, clamp the saw plate firmly but not enough to deform it..jpg
You want the vise tight enough to hold the plate without slipping but not so tight as to dent the plate with the vise jaw serrations.
Now to induce some flex to the plate I use my patent-pending saw-back remover to lever the back up.
Using the patent-pending saw back remover we apply a little flex..jpg
Now that's not very pretty is it?
Um, now that's a bit of a problem..jpg
So how to fix it? First clamp the plate up in the vise and then, with a soft faced hammer tap the back back down onto the plate VERY GENTLY.
Tap lightly on the toe end of the back.jpg
As you tap you will notice two effects. First the tooth-line will visibly straighten and second the ringing sound of the plate will decrease in frequency until it reaches a minimum.
better Getting straighter..jpg better still Straighter still..jpg good Just about done, still a little weave near the heel..jpg
Then, if there is any residual ripple in the plate toward the heel swap the plate around in the vise and tap lightly on the heel end of the back.
a little tap while holding the other end.jpg
And finally we're straight again.
Back where we started.jpg
And the back is down where I want it to be.
If the back is already stuffed all the way down on the saw plate you won't get anywhere tapping it down, you must first pry it up a bit using the patent-pending saw back remover.
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5th Nov 2014, 11:12 AM #2GOLD MEMBER
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- May 2008
Excellent tutorial Rob. Its the 1st I have come across that explains the topic extremely well. Kudos.
5th Nov 2014, 12:14 PM #3
Ahh... slit backs or folded backs ... You might open a can of worms with this discussion,
Just a query, as you tension the blade, wouldn't the ringing frequency increase rather than decrease?
Nice description of the process
5th Nov 2014, 12:20 PM #4
Let's hope the worms stay in their cans - don't need battles over trivialities.
Frequency wise, I can only say what I hear. May be that the vise damps the higher frequencies or it may be that I've lost hearing in that range, or both.
10th Apr 2016, 09:36 AM #5
I thought this wasn't supposed to happen?
Browsing on eBay today I happened on this posting. Lie Nielsen Tappered Carcass Saw 14 PPI 14 inch Saw Plate Needs Repair | eBay
Wherein the seller writes: "THE SAW FELL OF A PEG RACK I HAVE IN THE SHOP ABOUT 4 FEET OFF THE GROUND, THE SAW PLATE HAS A SIGNIFICANT WAVE IN IT AND I DO NOT CARE TO REPAIR IT"
Here's a picture of the wave.
I thought that the bond between the blade and the back in this type of saw was sufficiently strong to prevent this kind of damage. Has anybody else had similar happen?
The fix here should be pretty straightforward.Innovations are those useful things that, by dint of chance, manage to survive the stupidity and destructive tendencies inherent in human nature.
10th Apr 2016, 11:33 AM #6GOLD MEMBER
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- May 2008
Rob. This may best explain why the saw plate dislodged after being dropped.
We slit the brass backs to receive the saw plates, then rather than just relying on friction and adhesive to hold them in place, we compress the brass back onto the plate, which also increases the tension in the plate.
10th Apr 2016, 11:46 AM #7
Interesting, kind of a hybrid between the glue-in slotted and gripping method of folded backs. Should be very easy to fix in light of the construction method. I have a number of LN and Wenzloff slotted saws and have never had a blade slip.Innovations are those useful things that, by dint of chance, manage to survive the stupidity and destructive tendencies inherent in human nature.
10th Apr 2016, 12:11 PM #8GOLD MEMBER
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10th Apr 2016, 12:32 PM #9Innovations are those useful things that, by dint of chance, manage to survive the stupidity and destructive tendencies inherent in human nature.
10th Apr 2016, 04:17 PM #10
This is a really good topic and very well thought out by you. I am going to have to study it a good deal more carefully than I already have. Thank you for the time and effort.
On the subject of civility I would like to think that this forum is better than most: In fact I think it is, but when we step out of line, which does happen, sometimes deliberately and sometimes inadvertently, either the mods or our peers step in fairly smartly.
I particularly like the fact that there are many Forum members with a huge store of expertise, some have already contributed to your thread, and yet they converse with mere mortals as if they are just the same.
The human quality of humility I prize above almost all others.
"Power tends to corrupt. Absolute power corrupts, absolutely!"
10th Apr 2016, 11:07 PM #11
I've learned this as I've gone along. Nothing profound in this area but there's a lot of myth, myth-making, misinformation and active disinformation involved.
Many woodworking communities on-line appear to operate as clique's composed of two groups: Cognoscenti and plebs. I've looked into the pattern of behavior and to me it sounds a lot like tribalism and or feudalism which is, as I'm prone to say interesting.
I do this kind of work because, for me, it's fun. I strongly dislike like the acrimony that is displayed far too often because that takes all of the fun out of it.
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