Hi Neil,
[snip] Frames like that, and Like the Knew Concept saw, clamp the
blade only between a screw end and an anvil, as opposed to the
'German style' frame that clamps the blade between greater areas of
metal. With such a small contact area between screw end and anvil
there is less friction to hold a blade in. I don't know if you
could call this a design flaw, but it certainly isn't optimal. Use
of a much thicker screw would have been a better choice with this
style of clamping.
Err… no.
The use of smallish screws was entirely deliberate. Believe it or
not, they actually clamp much harder than the traditional wide pad
clamps. Allow me to explain.
Picture an anvil. Better yet, picture an Indian mystic, fast asleep
on a bed of nails. We all know how this trick works: one nail, and
his entire body weight is on that one nail, (briefly) and he has a
bad night’s sleep. Spread the weight out across a whole bed of nails,
and no one nail has enough pressure on it to puncture the skin.
To bring that back to clamps, go back to picturing an anvil. Imagine
the anvil sitting on its square base. The weight of the 100 pound
anvil is spread across the 8x8 base, giving a pressure of 1.56
pounds per square inch. Not much, really. Put a string under the
anvil’s base, and you can pull it out pretty easily.
Now picture standing the anvil on the point of the horn, so that all
100 pounds is concentrated on one little point. No way you’re
pulling that string out. (assuming you find a way to keep the anvil
from falling over.)
So, going back to clamps: you’ll only get so much pressure from a
hand tightened screw. The question then becomes how much surface
area do you spread the load over? With the wide pad clamps, the load
is spread over nearly a square centimeter (100 sq/mm), and with the
KC screw clamps, it’s spread across a little more than 10 square
millimeters. So for every pound of pressure you get from the screw,
the smaller clamp gives 10 times the clamping force. (very roughly)
So, you’re correct in that there’s less area for friction to act
across, but incorrect, in that the smaller clamp generates ten times
the clamping force of a clamp with larger contact area. It nets out
that the smaller clamps hold better. There was actually a fair bit
of testing and optimizing involved in where we ended up with all
this.
As far as roughing them up, I wouldn’t. It won’t help. The screws are
grade 8, which means that they’re harder than the sawblades. If you
use a large blade, and clamp it hard, you can sometimes see dents in
the tangs of the blades. If you’re denting the blade, surface texture
won’t help.
We built testing rigs at one point to test the relative deflections
of various sawframes. Looks like we’ll have to build a rig to
demonstrate clamping power too.
By way of example, I’ve got one of the absurdly rigid 3" titanium
saws. I’ve pulled #6 blades apart with it. The clamps held on, but
the blade tore in half.
(You’ve never heard a #6 ping in high-E before either…) I’m not
sure what more we could do beyond building clamps that can hold on
for that sort of abuse.
The clamps really are a very simple mechanism. There are only three
ways they can fail.
(A) if the anvil screw gets out of position,
(B) if the blade isn’t fully seated. (Far enough back in the hole)
(C) if there’s a bit of broken blade or other debris in the hole,
interfering with the clamping action.
The most common problem we see with them is that people break a
blade, and then forget to unwind the tension knob to allow the blade
clamp to come down so that it can grab the full length of the blade.
(A subspecies of problem B) When the clamp’s far enough down, the
blade will have a slight bow to it as it sits in the loose clamps.
To troubleshoot them, in order of complexity:
(C) use a bit of compressed air to blow out the blade hole, or
perhaps unscrew the knob, and peek down into the hole with a
flashlight, to see if there isn’t some bit of crud down there
interfering with things. (This isn’t unique to KC style clamps. I’ve
had broken bits of blade hide out in traditional clamps too. It’s
just easier to see them and knock them out in open style clamps.)
(B) My trick to make sure I’m seated all the way into the back of
the blade socket (If I’m unsure) is to pull the blade out of the
socket entirely, and then close the clamp screw, so that it’s solidly
against the anvil screw. Re-insert the blade until it stops, having
bumped into the screw of the closed clamp. Then open the clamp
slightly, and you’ll feel the blade slide farther back into the clamp
(now that it can). Shove it down in there until it stops again.
You’ll have felt it go in another.170". (4.3mm) Then tighten the
clamp. Repeat with the other side if necessary. The blade stops
against the back wall of the threaded hole for the clamps.
(A) To check to see if the anvil screw’s out of position. (Rare, but
it does happen.)
Step 1: insert a large-ish sawblade into one of the clamps. (but not
the other one)
1A: tighten the clamp until you feel it bite. (not tight, just enough
to feel it biting.)
Now look at the blade. Is the loose end tilted off to one side or
the other? Or is it pretty much in line with the axis of the
sawframe?
If it’s in-line, that clamp’s fine. Check the other one.
If it’s angled off to the side, the anvil screw’s out of position.
You’ll need a 5/64 hex wrench to adjust it. (The anvil screw is the
little set screw directly opposite the normal thumb screw that you
clamp with.)
When the blade angles in the clamp like that, it normally angles
away from where the anvil screw is.
So if the loose end of the blade is angling towards the thumbscrew,
it means the anvil screw is backed off too far, and needs to be
adjusted “in” towards the thumbscrew. If it’s pointing away from the
thumbscrew, it’s in to far, and needs to be backed out.
The goal is to get it to the point where the thumbscrew locks the
blade up solidly while the blade is centered in the blade hole. When
we put them together, we end up with a wrench in the anvil screw, a
blade in the clamp, and our third hand on the thumbscrew, playing
them off against each other to get the blade centered in the hole
with the arms of the knobs in the right positions. For your purposes,
don’t worry about the knobs, just try to get the blade centered.
There are pictures of this whole process on page 3 of the sawframe
instructions, which can be downloaded heRe:
http://www.ganoksin.com/gnkurl/16a [PDF file]
Once you get the first clamp re-adjusted, test and adjust the other
one if necessary.
Hope this helps,
Regards,
Brian Meek.