need help cutting a centered grove

DaninVan said:

All of the above, but why is it critical that the groove is precisely centered for this application?

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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.

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.

While the original question was about using a table saw, this is also an issue for using a router table and it's about fence adjustment. I have an Incra fence on my RT (Incra LS) and it allows micro adjustment down to better than 1/1000". As was said earlier, when you get down to micro adjusting, bumping a traditional fence accurately is pretty close to impossible. And, if you bump too far, it's impossible to get back to where you were before bumping. However, the Incra fence can do much much better. Once you have used that level of control, anything else seems stone age crude.

Yesterday I was using my router table to cut a centered slot in the ends of some frame pieces and, after a bit of setup and test, I got it spot on. Cut using the flip once method - get the bit super close to centered, cut, flip, cut. Then I cut the matching tenon using the flip method described in several previous posts - cut, flip, cut, test, tweak, repeat. Dead centered and perfect degree of tightness. Tweaking was done in multiples of 1/32ths of 1/32" (ie, multiples of 1/1024").

While 1/1024" is in the noise level, fractions of 1/32" aren't. Especially because, as was pointed out above, you are doubling the movement with the flip. If the Incra table saw fence wasn't so huge I would have it in a heartbeat. Not because of the need for 0.001" accuracy but for that level of precision "tweakability" and 1/32" repeatability.

By the way a good router lift will also give you this level of tweakability for the depth of cut.

measure the width of your technically uniform boards in several places along it's length... (all of the boards)
average these measures...
divide by two...
use this measure to set the fence to the center of your blade...
set the blade to depth of cut...
cut your kerf using a TFG blade... (a wide kerf blade is the easy way to go)
test fit your hardboard to the kerf...
need a wider kerf???
measure for what you need...
divide by two...
move the fence that much plus a fuzzy skosh...
recut your kerf...
flip your board end for end w/ the kerf down...
recut the kerf again...
test fit again...
adjust the fence for what you need to meet your happiness...
you'll find that if you leave things alone after you get happy you can make your fitted kerf in two passes...

We trust you did this on a test piece to start...

@rmark This is a nice starter project for someone who's not real experienced with woodworking. Teaches some very basic skills. The one thing that's a pain is getting the miters in the corners to be exactly 45 degrees. If you miss, you'll get a gap. If the gap is small enough, or the cut isn't perfectly straight, you can use a filler. I do picture frames for my artist wife and even with specialty tools, still find I have to fill. I found the best filler for this purpose on Amazon called Timber Mate. https://www.amazon.com/Timbermate-A...1580586255&sprefix=Timber+Mate,aps,238&sr=8-8

You can purchase it in the type of wood you're using, so it takes stains that match your project's finish. I just rub it into the crevice with my finger until it's well filled, then use my fingers to "sand" it down flat before I stant final sanding. It sands very well too. An Aussie product.