need help cutting a centered grove - Router Forums
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post #1 of 18 (permalink) Old 01-31-2020, 11:53 AM Thread Starter
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Default need help cutting a centered grove

Trying to cut a centered grove for 1/4 tempered hard board on some 2 1/2 wide pine boards . To make a frame to fit over a table to put puzzles together.
I'm using the method where you set the table saw fence so saw blade is slightly passed the center of board width and move fence until you get the width want for dado .
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post #2 of 18 (permalink) Old 01-31-2020, 12:21 PM
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That's the easiest way to make sure it's perfectly centered. Once you find the spot that leaves you the grove you want on a test piece, you can just run all your pieces to get the outsides cut, then one pass to clean up the middle.

Hope that makes sense...

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post #3 of 18 (permalink) Old 01-31-2020, 01:53 PM
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If you have enough test pieces, you can line up the fence so the outside edge of the blade is about a little less than 1/8" to left or right of exact center...make a cut, then turn the board end to end and run it again...bump the fence as necessary. Once you adjust the fence so that you get the right size groove you don't need to move the fence at all for all the pieces...this will produce a groove in the center of the board.


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post #4 of 18 (permalink) Old 01-31-2020, 02:42 PM
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I try to get the first cut as close to dead center as possible on narrow grooves then make the first cut and rotate 180* and make the second cut. That gives you a baseline where you can measure the width and make a second adjustment that is just shy of the finished groove and then sneak up on the final cut if the fit needs to be tight. Remember that every fence adjustment is times two because you are making that cut on either side of the groove.

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post #5 of 18 (permalink) Old 01-31-2020, 02:58 PM
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All of the above, but why is it critical that the groove is precisely centered for this application?
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post #6 of 18 (permalink) Old 01-31-2020, 08:24 PM
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Originally Posted by DaninVan View Post
All of the above, but why is it critical that the groove is precisely centered for this application?
I assume you're trying to keep puzzle pieces from falling off the edge into jigsaw puzzle oblivion, from which it will never emerge. Therefore it is unnecessary to have it perfectly centered, in fact, having one side slightly deeper than the other is desirable.

My method would be similar, but I would get the test piece cut as well centered as possible, which would give the starting width, centered. Then I would cut all four pieces to that width. After that, make the adjustment require for a perfect fit on the test piece and once found, run all four pieces through. A perfectly centered groove would satisfy the picky, fussy part of me, but it would be unnecessariy perfectionist part of me.

Not any difference in technique, just in the sequence of cuts. Think about it, if you're using a full kerf, 1/8th inch wide blade, and you line up to the centerline of the frame pieces, you're going to be incredibly close, so the third pass, you are going to be taking off thousandths. That's pretty hard to do by bumping a fence to keep it perfectly centered. You would be making two passes, so your adjustment would have to be half the thousandths. Maybe for that you could blow on the fence to get it to move that miniscule amount.

A question, are the corners mitered? if so, you might want to use a spline to reinforce the corners, especially if the center board floatsk, is not glued down.
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post #7 of 18 (permalink) Old 02-01-2020, 03:32 AM Thread Starter
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I have done this before but I have gone through a lot of scrape . Keep missing and over shooting. Dan your right it doesn't have to be perfect . I think your right make the first cut as close to the center. I think get my calipers measure the thickness of the hard board . Do the 2 passes measure the width of the initial half the difference of the thickness of select and make 2 more passes.
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post #8 of 18 (permalink) Old 02-01-2020, 06:32 AM
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skip the calipers..
they're your Albatross...
rough center measure to the center of the blade..

eyeball the rest and test fit to completion...
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post #9 of 18 (permalink) Old 02-01-2020, 07:06 AM
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I would make the board a little wider, cut the groove as close as possible then cut the board down to size so that the groove is in the center.
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post #10 of 18 (permalink) Old 02-01-2020, 07:33 AM
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I may be confused on what is being done here but it seems it a groove, not a dado that is being made. If so, here’s a few things I do to get both good fit and control the position.

1) Confirm with a test cut that the bit makes the right size groove/slot for the inserted material. Get the right bit or change your insert material for the correct thickness dimension. Materials are measured In wonky ways now so this step takes a lot of guesswork out of the process.

2) Assuming a centering into the frame, position the bit as close to center as possible. Since it will not likely be perfectly centered, move the first test piece end for end will reveal if the slot is correct or too wide. Adjust until the correct width is achieved. Not too tight if glueing is being done - just easily inserted with space for the adhesive. A dry fit can be more snug but should not forced. The bit or fence do not need to be moved for the second pass.

3) If the groove/slot is being cut out of center, you can control its position just as accurately. Position the first cut where desired and make the test cut for location accuracy. Make all workpiece cuts. Then add some masking tape along the fence and retest for the second cut. Use the tape to arrive at a correct fit for dry or glue assembly. This will also prevent any need for moving the bit or fence for the second cut.

You could also add the tape to the fence first to maintain the slot’s location/dimension to the frame face and remove it as needed to add a larger gap to the interior of the slot if needed. Bit size (Step 1) will determine if this step will be needed.

Repeatable technique is the best way to approach new issues to solve. A good router table and a reputable book on milling/jointing techniques is worth its weight in time and materials. Routers make this kind of work very successful once learned. It’s a good skill to practice on scrap wood before you start a project.

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Last edited by Bstrom; 02-01-2020 at 07:41 AM.
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