Closing the slot: An addendum to “Saw build along – Back, Part III”

One of the posts in my saw build along series showed how I squeeze the slot closed so that the blade is held in the back by friction. Although this works well for me, it requires the use of a hydraulic press that is not likely to be found in most workshops. The method shown below requires nothing more than an ordinary machinist’s or mechanic’s vise and two short pieces of aluminum angle.

Before squeezing the slot closed, I now chamfer the edges of the blade slot slightly. I use an old needle file to cut the chamfer, but any file with a sharp corner will work. This chamfer, along with the roundover on the edge of the blade, makes it easier to install the blade into the back.

Chamfering the edges of the blade slot.

Chamfering the edges of the blade slot.

Chamfering the edges of the blade slot.

Chamfering the edges of the blade slot.

End view of back or spine.

End view of back or spine.

 

 

To squeeze the slot, I line the jaws of the vise with aluminum angle to prevent the steel jaws from marring the brass. I had some 3/4″ x 3/4″ x 1/8″ angle laying around, but the actual size is not too important.

To concentrate the force on a shorter section of the back, I cut one of the pieces to 1″, which means that you will need to squeeze the blade in 1″ increments along its length. I also trimmed one of the legs to a 3/8″ width so that I could keep the force at the edge of the back (first picture below).

Squeezing slot closed, end view.

Squeezing slot closed, end view.

Squeezing slot closed, top view.

Squeezing slot closed, top view.

 

There are a few things to keep in mind when doing this:

  • The force required to close the slot in a brass back is much greater than that for a bronze back.
  • The slots in brass backs will spring back open when the vise is opened, and may require multiple squeezes before it takes a set.
  • If you are unable to generate enough force to close the slot up, try a bigger vise. Or, cut the angle shorter; by halving the length of the aluminum angle, you effectively double the force applied to the back.
  • A set of feeler gauges is invaluable in measuring how wide the slot is. These have many uses around the shop, and can be had for short money at most auto parts or hardware store.
  • Aim for a slot that is about 0.002″ – 0.009″ narrower than the blade (the thicker the blade, the more you can close the slot up without getting in trouble). This difference generates enough friction to securely hold the blade without making it overly difficult to install the blade.  For the math averse, I have included a short table at the end showing the slot widths that I use for various blades.
  • Above all, take it slowly. It is far, far easier to squeeze the blade back (Corrected 28 Feb 2014. Does no one proofread these entries? – Ed.) a little more than it is to try installing a blade into a slot that is mashed shut.
Feeler gauge set.

Feeler gauge set.

Blade thickness and slot width table. Includes metric measurements for use in the civilized (civilised?) world.

Blade thickness and slot width table. Includes metric measurements for use in the civilized (civilised?) world.

 

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4 Responses to Closing the slot: An addendum to “Saw build along – Back, Part III”

  1. Stewart Simpson says:

    Well presented article Isaac. I am also interested to understand why you prefer this method over the alternate of using lock tite to hold the blade firmly within the back slot.

    regards; Stewie.

    • admin says:

      Stewie, there are a few reasons I chose to do it this way.

      First, it is easily reversible, so long as you don’t make the slot too tight.

      Second, I have no reservations about the permanence of friction.

      Third, it’s a little closer to the traditional way of folded backs.

      Fourth, friction is free. That’s not really a reason, but there is less mess to clean up when I’m done with it.

      Keep in mind that I have never done this myself with LocTite or epoxy, so make of it what you will.

      Now that I think of it, I don’t recall you ever talking about how you join your backs and blades. Do you mind sharing that?

      Isaac

  2. Peter Evans says:

    Hi Issac, looks doable until I find someone with a press. A question – with the different sized metal plates on each side, is the pressure the same on each side of the brass strip at the relevant spot?

    • admin says:

      Peter,

      Using a vise is actually easier in some ways than using the press, so I wouldn’t go out of the way to find one. Unless you just want to have one, which I can understand.

      I didn’t really think about it at the time, but the pressure on the short side should be greater than on the longer. The pressure on the longer side will decrease rapidly as you move away from the ends of the shorter plate. If you shorten the longer piece so that it is the same length as the shorter, then you should be able to use less force to close it up, but I would not expect it to make a major difference.

      Isaac

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