Rabbeting bits sized for baltic birch plywood

[Bill Fleming]

I gave up trying to find bits to cut the exact size dados years ago and now use a smaller bit and a jig built much like this one https://www.google.com/search?q=adju...fRYKG7MPenL-M:

You put the board that you want the dado to fit in this jig and close the jig against the board surfaces. Then remove the board and use the jig and a smaller bit to cut an exact width dado to fit the board. It will fit perfectly every time no matter what the board thickness is.

Charley

Charley - I have made several fixed jigs similar to this for specific applications. I’m not sure how the one you linked to works, I.e. if the ears of the angle are used to set the width to the shelf say, what is the guide collar, bit. To get the exact fit?

[Glen Gunderson]

Charley - I have made several fixed jigs similar to this for specific applications. I’m not sure how the one you linked to works, I.e. if the ears of the angle are used to set the width to the shelf say, what is the guide collar, bit. To get the exact fit?

The size of the guide collar and the bit wouldn't matter as long as you used the same setup each time. You just need to attach the upper pieces to the lower ones far enough from the edge so that you'll cut the edge of lower piece the first time you run the router along it. That way the offset between the pieces will match the size difference between the guide and router bit perfectly. Then you set the width of the dado with the gap between the lower pieces but the router is guided by the upper ones.

[Bill Dufour]

I doubt you can buy a precision ball bearing in non metric sizes. They have pretty much all been metric since they were invented and mass marketed in France. There were some inch ID bearings made but not much after 1942 or so. And I think they had something like 3/4' or bigger inner bore. I believe the outer races have always been metric. Kind of like light bulbs and spark plugs, except the ford NPT ones of old.

That said I suppose you could wrap tape around the outer race to make it a slightly larger diameter. It might last long enough to make a rabbit or two if you get lucky.
Bill D.

[Bill Fleming]

I see, clever. You end up with a jig tuned for a specific collar and bit...I can’t forget to mark that on the jig! Thx

[Charles Lent]

No collar/bushing is used. The shoulders of the top layer of the jig guide the router by it's base. Just use the same router bit every time so the distance between the router bit and the shoulders of the jig are always the same with the router bit always being smaller in diameter than the dado that you will be making. It will always take at least two basses to widen the dado to the width that you need. Down the left side shoulder and back the right side, but the resulting dado will match the thickness of the board that you set the jig width to.

When you make the first cut on the new jig with the router you are trimming the lower parts of the jig so that their edge to the upper shoulder of the jig is the distance from the router bit to the outer edge of the router base. Do both sides of the jig this way. Now, when you insert the shelf board in the gap between the two lower boards of the jig you are widening the jig by the width of the desired dado. Lock the jig width to the thickness of the shelf board and the space between the upper shoulders of the jig will be the diameter of your router base, plus the thickness of the shelf board. Run the router base against the shoulders of the jig and the router bit will cut the left side of the dado when you go down the left shoulder of the jig and the right side of the dado when you come back along the right shoulder.

If you put a straight edge on a board and run your router along it, the board being cut will be cut to 1/2 the diameter of your router base minus 1/2 the diameter of your router bit when measured from the straight edge to the edge of the cut. With this jig you are doing the same thing, but from both sides, with two straight edges. Since you trimmed off the excess wood in the lower level of the jig it's width from the shoulder (straight edge) to the edge of the cut will also be the diameter of the router base minus 1/2 of the diameter of the router bit and pushing both sides of the jig together until the bottom pieces touch should result in the space between the two shoulders being the diameter of your router base. When you insert the shelf board between these two freshly cut jig edges you are moving the two sides of the jig apart by the thickness of the shelf board so the dado that you cut by riding the router base against both shoulders (straignt edges) of the jig will only be this added space (shelf board thickness) that you have added to the width of the jig.

(I'm wishing that I had a working video camera as it would be easier to demonstrate this than it is to explain).

Charley