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Accurate rips about a centerline?

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Forum topic by pkunkel5 posted 01-19-2010 08:47 AM 1082 views 1 time favorited 23 replies Add to Favorites Watch
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pkunkel5

18 posts in 2221 days


01-19-2010 08:47 AM

Topic tags/keywords: jig tip question tablesaw

I can’t be the first person to have to do this… How do I make a table saw jig to accurately cut stock to width about a centerline? I’m ripping roughly 1in x 1in x 30in boards, and I need to size the board width to within a 100th of an inch. The sizing has to be symetrical around a predetermined centerline. In case this isn’t clear, imagine a 1in (25mm) board with a pre-existing centerline. Now assume I need to cut it to 22mm with 11mm on either side of the centerline.

Obviously, I could gradually reduce each side using many very thin alternating cuts, but I’m looking for something more efficient as I will be doing this a lot.

Thanks!
Paul


23 replies so far

View Gene Howe's profile

Gene Howe

6032 posts in 2172 days


#1 posted 01-19-2010 01:01 PM

Sounds like a lob for a planer or a router table with a 1mm offset on the out feed.

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

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Bob #2

3808 posts in 2765 days


#2 posted 01-19-2010 02:56 PM

I can’t tell what you are trying to do from your description.
Perhaps you could tell us why you are starting with a 1×30” piece?
Also what is the intended end use of the finished pieces?
What is a pre exisiting centerline and why is it important to center it?

Bob

-- A mind, like a home, is furnished by its owner

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dusty2

319 posts in 2173 days


#3 posted 01-19-2010 03:05 PM

I think Gene has the right answer – use your thickness planer to get down to that thickness in increments. But it might be too late if you already have stock cut to 30” 1”x1”s and need pieces that are 11mm thick.

If this is the case, the first question has to be how much do you loose to the kerf. I would assume .300” (7.62mm). If you successfully rip that 1” (25.4mm) stock exactly down the centerline (splitting the kerf precisely), you will have two identical pieces that are too small (8.89mm for the intended project if the target dimension was 11mm. The resulting pieces will be .0831” (2.11mm) too thin.

That was fun. I don’t work with mm very often. Hmmm, maybe the results have already shown that? Somebody check my arithmetic please.

-- Making Sawdust Safely

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Kindlingmaker

2654 posts in 2270 days


#4 posted 01-19-2010 04:19 PM

Without using any others tools besides your table saw… Set up feather boards so to hold your stock against the fence and cut all your stock with that setting then readjust to the other side of the center line and cut all your stock to that measurement. Feather boards can be used on both front and back of the saw blade but make sure they are pointing in the right direction. ...The other shop tools as the other LJ’s have said such as the thickness planner or a drum sander might be better.

-- Never board, always knotty, lots of growth rings

View Dave Owen's profile

Dave Owen

234 posts in 1817 days


#5 posted 01-19-2010 05:13 PM

Your problem sounds rather unique. I think my approach would be to build an accurate sled with a marked centerline locater and a good clamping mechanism to hold the board straight along its length (like some taper-cutting sleds have). You would have to reverse and reclamp the board after each cut, but with a lever-clamping setup that shouldn’t take too long. If you need better cut edges than the table saw provides, you could make your sled so that it would allow two quick, equal passes through the thickness planer.

PS: Glad I don’t have the problem you describe!

-- Dave O.

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CharlieM1958

15777 posts in 2962 days


#6 posted 01-19-2010 05:19 PM

It sounds to me like a job gor a thin rip jig (which I think is basically what Kindlingmaker is suggesting with the use of featherboards). The one below is from Rockler, but you can make your own. Once you set it the proper distance from the blade, you move the fence for every cut until you get to your centerline. Then flip the board over and repeat the process.

-- Charlie M. "Woodworking - patience = firewood"

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Bob #2

3808 posts in 2765 days


#7 posted 01-19-2010 06:36 PM

As I am not at all sure what the poster is looking for I will also direct you to a thin stirp jig I built a while back.

http://lumberjocks.com/projects/19940

The advantage that this jig provides is a simple repositioning of the tablesaw fence is all that is required to cut each strip to identicle width like a meat slicer.

The sketch above requires resetting the jig opening after each cut plus sacrificing the jig to make cuts narrower than the first setting.

-- A mind, like a home, is furnished by its owner

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pkunkel5

18 posts in 2221 days


#8 posted 01-19-2010 06:44 PM

Thanks guys! The sled idea has me thinking (thx Dave O.). My challenge is actually more involved than I wrote-up. Maybe I’ll elaborate and post some drawings when I have time.

View GFYS's profile

GFYS

711 posts in 2214 days


#9 posted 01-19-2010 06:44 PM

Obviously, I could gradually reduce each side using many very thin alternating cuts, but I’m looking for something more efficient as I will be doing this a lot.

I would say you need a gang saw of some type.

View stefang's profile

stefang

13623 posts in 2078 days


#10 posted 01-19-2010 07:28 PM

I have to agree with Dave Owen and Bentlyj on this one. The whole premise is based on the predetermined center line. The jig described by Dave and Bentlyj cater to that premise. The work piece is placed in the jig to match the line marked on the jig and the cut is then carried out according to the design of the jig. If that center line isn’t perfectly centered to start with, you would still get an accurate result. The solution with the thin strip jig would not cater to that possiblity. All of the solutions are good, it’s just how you interpret the problem. Maybe my interpretation is wrong.

-- Mike, an American living in Norway.

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GFYS

711 posts in 2214 days


#11 posted 01-19-2010 07:31 PM

I need to size the board width to within a 100th of an inch.

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pkunkel5

18 posts in 2221 days


#12 posted 01-19-2010 08:38 PM

I think a hybrid idea would work where the first cut is done on the sled jig eyeballing to the centerline mark and the second (final) cut is done on the same sled with some kind of spacer block to ensure the final total width is within the 100th of an inch tolerance. The sides individually may not pass the tolerance test, but this would work for my requirements. Again, THANKS!

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Bob #2

3808 posts in 2765 days


#13 posted 01-19-2010 09:14 PM

I’m just curious but how you plan to measure 100th of an inch?
That’s roughly .2 mm and a human hair averages .1 mm.

Bob

-- A mind, like a home, is furnished by its owner

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stefang

13623 posts in 2078 days


#14 posted 01-19-2010 09:17 PM

That is a real good question Bob. Is it bound for the space station? Also moisture content could change that dimension pretty quick.

-- Mike, an American living in Norway.

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pkunkel5

18 posts in 2221 days


#15 posted 01-19-2010 10:33 PM

Regarding curiosity on the .2mm accuracy requirement. The end product has to mate seamlesly with a second half without further finishing. Also, the end has to fit snugly into a fixed dimension attachment (sorry for being vague here). The wood is pretty stabalized in the end product, since it’s a hollowed out lamination with epoxy/composit coating.

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