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

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Forum topic by DrTebi posted 10-11-2014 07:51 AM 2055 views 0 times favorited 29 replies Add to Favorites Watch
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DrTebi

256 posts in 2734 days


10-11-2014 07:51 AM

Topic tags/keywords: question jig tablesaw sawblade

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.


29 replies so far

View Ger21's profile

Ger21

1047 posts in 2598 days


#1 posted 10-11-2014 11:23 AM

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

-- Gerry, http://www.thecncwoodworker.com/index.html http://www.jointcam.com

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TheFridge

5765 posts in 954 days


#2 posted 10-11-2014 11:29 AM

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

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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Minorhero

372 posts in 2072 days


#3 posted 10-11-2014 11:52 AM

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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Logan Windram

303 posts in 1929 days


#4 posted 10-11-2014 11:54 AM

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.

View William's profile

William

9906 posts in 2310 days


#5 posted 10-11-2014 12:25 PM

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.

-- http://wddsrfinewoodworks.blogspot.com/

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knotscott

7225 posts in 2843 days


#6 posted 10-11-2014 01:07 PM

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.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

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DrTebi

256 posts in 2734 days


#7 posted 10-11-2014 07:25 PM

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?

View bigblockyeti's profile

bigblockyeti

3668 posts in 1188 days


#8 posted 10-11-2014 07:32 PM

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.

View gfadvm's profile

gfadvm

14940 posts in 2158 days


#9 posted 10-11-2014 07:49 PM

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”???

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View Redoak49's profile

Redoak49

1964 posts in 1456 days


#10 posted 10-11-2014 09:46 PM

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

256 posts in 2734 days


#11 posted 10-11-2014 11:28 PM


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…

View OggieOglethorpe's profile

OggieOglethorpe

1213 posts in 1578 days


#12 posted 10-12-2014 12:50 AM

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

256 posts in 2734 days


#13 posted 10-12-2014 06:06 PM



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…

- CessnaPilotBarry


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.

View sras's profile

sras

4392 posts in 2597 days


#14 posted 10-12-2014 08:25 PM

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…

-- Steve - Impatience is Expensive

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DrTebi

256 posts in 2734 days


#15 posted 10-12-2014 08:51 PM


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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