# 1/8" kerf does not cut 1/8" kerf



## DrTebi (Jun 16, 2009)

I am a bit puzzled. I built a jig, sort of like a screw-advance box-joint jig, everything works just fine, but the joints turn out too loose.

After checking any possible jig errors, I realized that my saw blade doesn't cut an 1/8" kerf. It's a Forrest Woodworker II, modified to FTG (to cut flat bottoms).

The blade itself is advertised to cut 1/8", and when measuring a tooth, or even when trying to measure two teeth with my calipers, it measures 1/8" as promised.

But when I make a single cut, it's at least 1/64th over 1/8". It's certainly too much for a glueable box-joint..

What's going on? My table saw is the SawStop Professional Cabinet Saw… Vibration? Wood contracts after cutting?

Puzzled.


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## Ger21 (Oct 29, 2009)

Make sure the blade is aligned with the miter slot, and that it doesn't have any runout.


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## TheFridge (May 1, 2014)

Sounds like you have runout or your fence is off a hair. You have blade marks on the wood when you rip?


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## Minorhero (Apr 8, 2011)

The three possibilities are.

1) Your blade has wobble in it. Most likely cause is the bearings either not being seated correctly or they have worn out. The test is simply to look at the blade edge on while running and look and see if it grows "bigger" or "fuzzier on the edge as compared to when it is off.

2) Your table is not aligned properly to the blade. You need to make sure your miter slots are parallel. Additionally, if you are using a sled of some kind to cut those joints (as I assume you are) that sled has to be perfectly aligned to the miter slots and the blade, being off a bit would make this problem.

3) If you are using a fence your fence might not be perfectly aligned.

My money is on number 2.


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## BLarge (Aug 29, 2011)

I'd agree, check the blade to the mitre slot and arbor runout. Also, no blade I've ever run into is actually what it's advertised to be. A dado blade at 1/4 is never 1/4. I have spent hours, put in tracing paper in the Dado set, shifted that fence many times just to get a finger joint right.

I think 1/8 is too small for that as well, but that is just me. Great joint, nightmare to glue up.


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## greasemonkeyredneck (Aug 14, 2010)

I agree about dado blades not being exact either. 
I have a screw advance jig that I cut anywhere from 1/4" to 3/4" with. 
I most often do 1/4" and, to get that accurately, is the two outside blades plus a .30" shim.


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## knotscott (Feb 27, 2009)

The runout of the arbor and the blade are cumulative, as is any deviation of the alignment. Add in any technique deviations, and it's nearly impossible for a blade that's exactly 1/8" to make a cut that's no wider than exactly 1/8". Make a test cut, and calculate the expected kerf from the measurement of the cut. Then you've still got wood expansion and contraction to deal with.


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## DrTebi (Jun 16, 2009)

Thank you for all the helpful replies.

Here is a picture of the actual jig:









Here a link to a bigger picture, if you want to see more detail:
http://drtebi.com/dump/lumberjocks/table-saw-x-joint-jig.jpg

The jig is basically a crosscut sled, with a box on top that can be moved with the linear stage. The sled has two miter bars, and the I did cut the kerf into the sled after everything was assembled. The fence is perfectly square to the blade (it is adjustable).

Correct me if I am wrong, but if the sled moves smoothly on two miter bars, then I should be able to rule out the possibility of a bad alignment between miter slots and sled.

I would then assume that either the saw blade is not parallel to the miter slots, and/or that the saw blade has some run-out caused by worn out bearings or the blade itself.

It would be quite sad if I could not use this jig for box joints, which was the main purpose. It still works great for tenons and as a cross-cut sled (I clamp an alignment-block to the box as a stop), or even small dadoes.
I must say I had much better success with a router-table based box-joint jig… but it was limited to 6" wide boards.

I assume the only way to get this right would be to invest into an adjustable dado blade, but I wanted to avoid this, as it would require a new sawstop break as well (hence close to a $200 investment).
Another thought I had would be a 1/4" groover saw blade, but if there is any run-out or misalignment in the saw blade arbor, that wouldn't help either.

Can I save this jig?


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## bigblockyeti (Sep 9, 2013)

Alignment is key, I only get at most 0.003 over nominal when everything is properly aligned. Expecting perfection is a recipe for disappointment too, that's why dado sets come with shims. I working with metal a drill can get you close, if you need perfection you have to move onto a ream. Unfortunately that's much harder trying to attain such precision with wood. Fortunately, you don't need to over 99% of the time, wood glue can fill a few thousandths and offer excellent bond strength.


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## gfadvm (Jan 13, 2011)

As an inexpensive experiment, put a Freud Diablo blade on your saw and see how that works. Mine cuts a 7/64 kerf.

It appears on your calipers that it is showing more than 1/8"???


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## Redoak49 (Dec 15, 2012)

It should be fairly easy to figure out what is wrong. A home made jig and dial indicator should be able to determine any blade wobble, fence alignment and alignment of the blade to the slot.

I have a SawStop PCS and have checked this on mine and find it is all within 0.005" which is good enough in my shop.


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## DrTebi (Jun 16, 2009)

> It appears on your calipers that it is showing more than 1/8"???
> 
> - gfadvm


Yes, that is exactly the problem-it should be exactly 1/8", or at least very close.



> It should be fairly easy to figure out what is wrong. A home made jig and dial indicator should be able to determine any blade wobble, fence alignment and alignment of the blade to the slot.
> 
> I have a SawStop PCS and have checked this on mine and find it is all within 0.005" which is good enough in my shop.
> 
> - Redoak49


I have just done this. It turns out the Forrest blade is within 0.007" when measured from one spot at the miter slot. This is a bit much… but *I forgot to mention one important thing*, which I should have said in my initial post:
The Forrest blade has been repaired-I cut into the aluminum part of my miter gauge and the sawstop break went off. So I must assume it is no surprise that the blade is off by 0.007", although I hoped that the repair had taken care of it 

When I checked with a Freud FTG Heavy Rip blade, that blade was within 0.002" at one spot, and when checking along the front and back of the blade vs. the miter slot, it was within 0.004" or less.

I get better results with the Freud blade, but it does (and that's "official") cut a bit wider than 1/8".

I think my best bet for now, without buying a dado blade, is to use the Freud blade and try to adjust for the slightly wider kerf on the linear stage. It's cumbersome though…


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## OggieOglethorpe (Aug 15, 2012)

All of my WWII's cut a ~ a 5/32" kerf.

I've never had a problem cutting box joints, or anything else, with a 5/32" kerf, as long as the narrowest cut was wider than 5/32". I use a dado set to match the finger width on fingers wider than a single kerf.

I think your jig design is suspect…


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## DrTebi (Jun 16, 2009)

> All of my WWII s cut a ~ a 5/32" kerf.
> 
> I ve never had a problem cutting box joints, or anything else, with a 5/32" kerf, as long as the narrowest cut was wider than 5/32". I use a dado set to match the finger width on fingers wider than a single kerf.
> 
> ...


The idea of this jig was that it would work with a regular blade. What I didn't account for was the fact that an 1/8" kerf blade doesn't necessarily cut 1/8", since one has to factor in run-out etc.

For now I will use my Freud blade and just don't do a full turn when getting to the last cut of each slot, and it will work fine. I know I don't need accuracy in the 0.0001", so a workaround will be fine until I invest into an adjustable dado blade.


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## sras (Oct 31, 2009)

If I understand correctly, you are looking for an error of 1/64th or 0.015 inches.

The blade had an error of 0.007 and the miter slot had an error of 0.004 (if I read your note right).

That totals 0.011 inches meaning you are missing 0.004 inches.

If you allow for a measurement error of 0.001 for each of three measurements (gap, blade & slot) you might have everything understood…


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## DrTebi (Jun 16, 2009)

> If you allow for a measurement error of 0.001 for each of three measurements (gap, blade & slot) you might have everything understood…
> - sras


I am not sure what you mean by that.
I assume you summed up the error numbers? 1/64" is pretty close. I think cutting a single kerf and measuring that should also give me a good idea of how far I have to "hold back" the last cut.


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## Dal300 (Aug 4, 2011)

It hasn't been mentioned that the error of your dial caliper is .01", that is a whole bunch of error possibilities, and as *sras*, mentioned for every measurement you take the the error will multiply.


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## DrTebi (Jun 16, 2009)

> It hasn t been mentioned that the error of your dial caliper is .01", that is a whole bunch of error possibilities, and as *sras*, mentioned for every measurement you take the the error will multiply.
> 
> - Dallas


The dial calipers itself have no errors?!?

The dial caliper's tick marks represent 1/64". The first picture shows that the kerf is almost 1/64" too wide, which is about 0.015".


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## Blackbear (Apr 28, 2012)

The dial caliper has an accuracy rating, as in "accurate to +-0.001". This means the measurement may be off as much as 0.001", which leads to error in your measurements.

I don't know which model caliper you have, but if you look at the specifications tab on this page you will see accuracy +-0.001": http://www.starrett.com/metrology/product-detail/1-Precision-Measuring-Tools/11-Precision-Hand-Tools/1102-Slide-Calipers/110202-Dial-Calipers/1202F-6

All that being said, my bet is also on your contraption not being perfectly perpendicular and not sliding perfectly parallel to the saw blade. I have had the same issue.


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## TheFridge (May 1, 2014)

.005 is about the max you want in joinery

From woodgears.ca


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## Dal300 (Aug 4, 2011)

On the bottom of the caliper, just above the .5, you will notice a listing of .01

Blackbear, Starrett makes cheap versions as well as expensive versions. I have a Starrett vernier caliper that is no closer than .0015, another one that is accurate to .0001, depending on how much was paid for accuracy. It also depends on when or if it was last calibrated.

Good luck with all this *DrTebi*. You asked a question but don't like the answers you got. I may be wrong in my assessment, I don't care, it's up to you to figure out what is wrong. 
All of these answers from everyone are just suggestions. How you use them is totally up to you.


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## AlaskaGuy (Jan 29, 2012)

It's been reported by many many woodworker that this will save a lot of time and frustration.

http://www.amazon.com/INCRA-I-BOX-Jig-Box-Joints/dp/B008LD4S5G/ref=sr_1_1?ie=UTF8&qid=1413170162&sr=8-1&keywords=incra+i-box+jig


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## oldnovice (Mar 7, 2009)

*AlaskaGuy*, that's the easy way but not the cheap way out!


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## Redoak49 (Dec 15, 2012)

I think that it is much easier to do box joints on a router table. I built one for both 1/4" and 3/8" fingers and it works perfectly. It has an adjustment so that you can dial in the exact finger width for the router bit and the wood you are using.

I tried doing the same thing on my table saw and it was more difficult for me and took longer to cut the joints.


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## verdesardog (Apr 2, 2011)

Any box joint jig needs to be fine tuned to whatever you are using to cut the joints. I have never had any luck just making the joint using the measurement of the saw blade….and any error will be cumulative as you make more cuts….


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## bLau (Oct 24, 2014)

If your jig doesn't have a registration pin you could layout the fingers by laying the first piece on the second and transferring the dimensions with a sharp pencil or knife and then sneak up on the line slowly. This will allow you to custom fit each finger separately.


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## DrTebi (Jun 16, 2009)

> If your jig doesn t have a registration pin you could layout the fingers by laying the first piece on the second and transferring the dimensions with a sharp pencil or knife and then sneak up on the line slowly. This will allow you to custom fit each finger separately.
> 
> - bLau


Thanks for the idea. It would be possible, but then I could only cut box joints where all fingers are the same size. My original idea was, that I would be able to cut variable width of finger joints, as I find this quite an interesting departure from the standard box joints…

However, I may have found a solution. I have just ordered a 12" digital distance scale from igaging. The scale should perfectly fit in front of the linear stage, and, once attached, would let me dial in quite precisely. 
It would then only be necessary to get an exact measurement of the kerf that my blade cuts, and then, with a bit of math, I should be able to figure out all kinds of different box joints… So far the theory. I will report back once I get it all together and do some test cuts


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## cutmantom (Feb 2, 2010)

an indexing type jig where you make a pin that fits the kerf that is cut would be simpler but not give you variable fingers, what if you use a pointer that is the same as the kerf instead of just a hairline


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## DrTebi (Jun 16, 2009)

I have done exactly that.
I cut a shallow kerf into a thick piece of acrylic, and attached that to the linear stage. The acrylic piece rides above a ruler, and it was working so-so.

I think the digital scale will return better results. It wasn't too expensive ($36), so I though it is worth a try. If it still doesn't work, I will go back to using the router table, and can still use the digital scale for my table saw fence…


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## DrTebi (Jun 16, 2009)

Problem solved.

As I mentioned, I purchased a digital read out scale. It was a bit tricky to install it, since I had not planned to implement this nifty device. I ended up installing the sliding bar directly underneath the linear stage, and made a custom bracket to connect to the "wagon" of the linear scale. It's hard to see in this picture, but you get the idea:










I still need to mount the read out itself properly.

However, I made a few test cuts, and it works really well. I measured the kerf that my Forrest blade cuts, which turned out to be 0.134". To make a simple box joint, I made a cut, reset the digital scale, moved the linear stage by 0.134", made a cut and so on:









Then I tried it with some different intervals, which worked also nicely (2-3-2-3…):









But I didn't like the resetting, as this can lead to possible incremental errors. So I wrote a small Python program that calculates all the distances for me. The output looks something like this:









Now I just need to dial in the numbers. It's slow, but otherwise, it works really well. And it allows me to make some interesting box joints, like the one you can see being cut here:









So far so good! I am glad I found a solution. And I will probably be able to use the whole apparatus in other configurations (everything is un-mountable).

DrTebi


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