Advertised kerf size accurate?

[PeteStaehling]

I am thinking about making a box joint sled for tiny boxes that used the 16 tpi pitch of 3/8" all thread rod to get accurate spacing on the cuts by counting turns. I think this should work fine if the blade makes an accurate 1/8 cut. I could use a new blade any way so I am wondering if I can count on accuracy in the advertised kerf of the blade. I am thinking of a Freud 10 In. 50 Tooth Combination Saw Blade with 5/8 In. Arbor (LU84R011). It is listed as having a 1/8" kerf and looks like it ought to have a flat bottom cut.

Comments?

 

[ehpoole]

A quality manufacturer will pretty accurately produce and specify a blades kerf and generally deliver on that specification to within a mil or so. However, advertised kerf width is only part of the story, any runout, or wobble, in your arbor or flanges will add to that kerf and that will vary on a saw by saw basis, and is beyond a blade manufacturer's control since it is an innate inaccuracy in your tablesaw rather than the blade itself. Actual kerf width will also vary if your fence and/or miter gauge are not perfectly aligned with the blade as any error there will be feeding the stock into the blade at an angle causing the stock to rub against the sides of the teeth as it is pushed past the blade, increasing the effective kerf. Regardless, you will want to look up the true specifications for the blade of interest as anything very close to a nominal 1/8" may be called a 1/8" blade (e.g. a 0.120" blade may be called 1/8" even though it is 5 mils undersized, but the actual specification sheet will typically make note of the true kerf width and not just the nominal 1/8" figure).

Which essentially means you will need to make some test cuts on your saw, take some measurements, and work from there.

That all said, it does not really matter how accurate the blade's kerf width is provided you accurately set the joint spacing to match your blade's effective kerf width (per your tablesaw), as long as they are both absolutely equal (no matter how far out from a true 1/8") then your box joints will be a matched set and fit together properly. You will always want to set joint spacing to match your blade kerf, no matter the what that value, and not just assume a true nominal figure, not the other way around. Whether or not it is a perfect 1/8" only really matters if there is some other overarching reason why the spacing must be precisely 1/8".

 

[TENdriver]

Pete, I just watched an episode of the WoodSmith Shop that was about making boxes with kerf sized finger joints. They get the fine adjustment from a screw. No complicated jigs to manufacture but the exact same adjustability.

I think it's still on my DVR if you're interested in the episode number.

 

[Joe Scharle]

Sounds like you're thinking Lynn jig. Here's a pic of mine making saw kerf box joints with the blade that came with the saw.

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[PeteStaehling]

Thanks, guys.

Whether or not it is a perfect 1/8" only really matters if there is some other overarching reason why the spacing must be precisely 1/8".

Yes true, but there is a reason. I want to use a 16 tpi thread and to be able to count full turns to get the spacing. Unless I am missing something that will require something pretty close to an exact 1/8" kerf or just a few thousands over. 0.005-0.006" over might actually be pretty close to an ideal fit so a small bit of wobble or misalignment would be tolerable if the blade were exactly to spec.

Tendriver, I will try to check that out. Do you have the episode number still handy?

Joe, yes that is very similar to what I was thinking of.

 

[Rick M]

The saws run-out will be enough for the joints to fit, as you say, you'll need a bit over .125. The Freud combo doesn't produce a true flat bottom cut but is close, as the raker is a hair lower than the other teeth. The Freud heavy duty RIP blade is .125 and flat. Forrest's WW2 #1 grind is flat.

Sent from my Moto G (4) using Tapatalk

 

[TENdriver]

Joe's box joint jig is a beautiful tool.

The WoodSmith process is good for kerf sized fingers but limited to just kerf size.


I think this is from the first part of season 10, episode 1. I'll update this if I'm in error.






Screen shot of jig fastened to Miter gauge. Stop block on right is actually the fine adjustment method. Note clamp on left holding part of jig to miter fence.


You've looked at finger joint jigs. There is a 1/8" brass bar to the left of pencil tip that is for indexing each cut. Cut slot, lift stock over index and cut the next slot etc.




To fine tune jig, loosen main clamp (out of shot to left) and adjust the screw in or out to fine adjust finger spacing. Clamp in view (right clamp) remains fixed while adjusting.

Episode goes into greater detail and offers an excellent explanation. Very simple to build and use at low cost to get your feet wet in kerf sized finger joining.

 

[ehpoole]

Thanks, guys.

Yes true, but there is a reason. I want to use a 16 tpi thread and to be able to count full turns to get the spacing. Unless I am missing something that will require something pretty close to an exact 1/8" kerf or just a few thousands over. 0.005-0.006" over might actually be pretty close to an ideal fit so a small bit of wobble or misalignment would be tolerable if the blade were exactly to spec.

Tendriver, I will try to check that out. Do you have the episode number still handy?

Joe, yes that is very similar to what I was thinking of.

There is another style of fence one can build for making box joints that is quite simple to make. It consist of a board to be used as the fence that you saw a single kerf width into, then insert a finger of wood equal to that kerf into the kerf you first cut in the fence, then offset that finger so that it is exactly one kerf width offset from the blade (you can use leftover stock used to create the finger to set the offset, or use trial and error), then after offsetting the finger by one kerf you saw a final clearance kerf into the blade for the blade to pass through, and bolt it all to your miter fence (my Incra miter fence makes attaching auxiliary fences very easy and allows for adjustment of position after attachment, other miter fences may vary). From there you simply cut a kerf in your stock, then move the stock over and insert the newly cut kerf in your stock over the finger in the fence, then just keep repeating -- the fence finger allows you to quickly and easily maintain the perfect offset quickly and consistently. The next section of stock is then aligned with the edge of the blade for the first pass (this is easy to do as the blade has cut a kerf in the fence that you simply align the edge of your stock to) then continue sawing fingers as before, this offsets the matching piece by a finger so everything meshes together during assembly. It is a very simple fence to make and requires no moving parts and can quickly be be remade in the future when needed and is not dependent upon a blade with an ideal kerf width, so that is another option for anyone who needs extra flexibility if their blade is not an ideal imperial width and it works very nicely for making box/finger joint projects (I have used such to build such projects).

Or, if you wish to do a lot of finger/box joint projects then one may also wish to consider Alan's iBox jig, available from Incra, which is a very flexible design. It is a bit pricey but the accuracy and flexibility it offers seems very impressive (I have not used one myself, but Alan's reputation is top notch).

I wish you all the best on your project and hope you might post some photos of your completed project for everyone to admire!