How to center router bit on a workpiece in router table?

[Dan Karachio]

Is this is a dumb question? I often struggle to center a bit (dovetail, dado...) on a workpiece on my router table. Of course with dado cuts on the table saw I can just flip the piece and get it centered every time (by virtue of making two cuts), but making a reverse cut on the router table is not smart or safe (right? - I won't try it). Is there some trick to all this? Is there any way at all to center a bit on a piece? For the life of me, other than endless test cuts, I can't seem to figure this out.

[Jon Toebbe]

If you're trying to cut dados or sliding dovetails, I'd recommend taking the router to the workpiece, rather than the other way around. Woodworking Magazine had a nifty, dead-simple router jig in one of their early issues... ahh, here (http://www.popularwoodworking.com/article/Router_Dado_Jig1/) it is.

I tarted up something along the same lines, trying to make it adjustable for "not quite 3/4" plywood." And it worked, more or less. But it was fiddly to adjust since I made it out of (fairly slippery) MDF. It didn't hold itself square too well. Keep it simple and use their non-adjustable jig.

Jon

[Dan Karachio]

Thanks John, I appreciate it, but some of my pieces are too small for this and I am really interested in tips on lining up bits on the router table. For example, I have had some luck with using spacers to set the bit distance from the fence.

[Salem Ganzhorn]

If the width of the cut is not that important you could simply flip the piece like on a table saw. Depending on which way you were off you either run the piece in the "normal" direction or in the opposite direction and it won't be a climb cut. Just think about the direction the router will be pushing the wood. You don't want it pushing in the same direction that you are pushing.

If you need it to be dead on you can use a centering pin (http://www.leevalley.com/US/wood/page.aspx?p=32316&cat=1,180,42311) to get your initial "guess" much closer. Switch to the real bit and try it with a piece of scrap. Should not require much fiddling.

If you need a dead centered (or dead aligned period) cut then I still think it is prudent to have some extra stock for a test cut.

Good luck!
Salem

[Chip Lindley]

You could use a "center-finding" ruler to find equidistant from the center of the router bit. Here's one"

http://www.ptreeusa.com/layout_products.htm#1295

[Neil Brooks]

I do a fair amount of this.

Once you know the distance from the edge of the board to the center of the cut, you measure so that the distance FROM the fence TO the center of the bit, spacing the fence accordingly.

Did that make sense ?

In other words, if you need the CENTER of your cut to be 3" from the edge of the board -- the edge that's going to be against the fence -- then you set the fence exactly 3" FROM the center of the bit.

If you're using, say, a 3/4" straight bit to cut a dado, then what you'll end up with is a dado that runs from 2-5/8' to 3-3/8" away from the fence.

The center will be at 3" from the edge of the board/the fence.

I use either a good steel rule OR my Wixey height gauge to get that distance precise.

[Will Overton]

I prefer to measure from the edge of the bit. If I need a 1/2" groove in a 1 x 6 (5 1/2" board) I know the edge of the groove will be 2 1/2" from each edge of the board. I cut a 2 1/2" spacer and place it between the fence and the bit. With the router unplugged I turn the bit by hand and move the fence until the spacer just touches the bit, then lock it down.

For me, this is easier than trying to measure from the exact center of a bit.

[Alan Schaffter]

I use any of a couple of techniques-

1. I use a centering bit - it piece of round stock, pointed at both ends, half of its length has a 1/2" diameter, the other 1/4", so will fit both collets. Then I position the fence so the point is on the line (for edge work I use one of those marking gauges with a pencil between two pins).
2. Another way is I use my centering bit and measure the gap between the tip or shank and the fence with a Wixey or inside calipers, then do the math.
3. The safest way to avoid ruining good stock is to make up some test pieces and run them through the router in each direction. When centered the dado will be as narrow as it can get. Not centered will result in a wider dado.
4. If the size of the dado doesn't matter (make the dadoes first and adjust the tongue or stub tenon to fit it). Set the fence by eye so it looks centered. After making a pass with each edge against the fence you will have a centered dado of no particular width- then cut the tenons to fit it.

[Salem Ganzhorn]

Note on #4: You don't want to trap material between the fence and the bit. If after you make the first pass you reverse it and the bit is going to engage the material between the fence and the bit instead of the outside edge shouldn't you also reverse the feed direction (and use a feather board to keep the stock against the fence)?

[Alan Schaffter]

Note on #4: You don't want to trap material between the fence and the bit. If after you make the first pass you reverse it and the bit is going to engage the material between the fence and the bit instead of the outside edge shouldn't you also reverse the feed direction (and use a feather board to keep the stock against the fence)?

Yup. I left out some details because I was mainly talking about centering. You should always use proper router table technique. Also a lot depends on the bit size and depth of the dado (and how much of a hurry I am in :confused: ). I make deep dadoes in multiple passes, and use a feather board for most edge work. I like spiral bits over straight bits, too.

[Noah Katz]

If your router table is on your TS with a digital readout in its fence, use it.

[glenn bradley]

For example, I have had some luck with using spacers to set the bit distance from the fence.

Funny you should say that/ My knee-jerk response is "setup blocks". This includes commercial ones, accurate shop made blocks and even ones that will only be used once. I will often take the time to make a jig for use on one project. Later if it has parts, I will strip them for the next jig. If it has no parts it gets tossed or burned depending on what it is made of.

My router plate has graduations on it. I find it helpful. I will also clamp a block in front of or behind the fence and then use spacers to move the fence 'N' distance away for the next cut. If you do a lot of this sort of work, maybe an Incra style fence would be a good investment. The 1/32" tooth-strips or thread version allow very repeatable setting from all reports.

Shop made gauges (http://www.sawmillcreek.org/attachment.php?attachmentid=130859&d=1256312662) can help with the original setting and then setup blocks help measure off from there.

[Alan Schaffter]

Funny you should say that/ My knee-jerk response is "setup blocks".

I find "setup blocks" only marginally helpful in woodworking and if I use them it is mainly to set the height . As an example, say you want to put a 1/4" dado or groove centered on the edge of a 3/4" thick board for doing a tongue and groove. Conventional wisdom says to put a 1/4" gauge block between the cutting tips of the bit and the fence to set the fence. However, assuming you had the bit in the right orientation in the first place, all that guarantees is the groove will be 1/4" from one edge. It won't be centered unless the board is perfectly 3/4" wide and the bit makes a perfect 1/4" wide groove. With woodworking and wood, that may not always be the case. Making two passes, each face against the fence is one of the few ways to truly guarantee a centered dado.

[Dan Karachio]

Thanks everyone. Minus the digital fence (I wish!), I will explore all this. I think I could use a centering bit since I often fail to see the center of the actual bit (eyes are very tired).

Here is an actual example - cutting dados in a drawer that will use runners. If the drawer is already made, I could set the bit at a certain distance, then make two cuts, but in two directions so the distance will be exactly the same on both sides. However, I'm worried one of those directions (the opposite of how I usually cut) could cause problems, chatter, safety...

If I try and center the bit on the piece, and make the cuts from the same direction (right to left) I will more than likely fail to get it perfect without a lot of set up.

Now I can see why these Incra, Jessem and Kreg fences are so sought after.

[Glen Gunderson]

I'm not sure I see the dilemma. If you cut a dado on a router table and want to flip it to center the cut, you just have to ensure that the next pass has wood being taken from the side of the bit that's away from the fence. To do this, you just make sure your first cut takes wood from the other side.

I'll use a simple example. Let's say you wanted a 1/2" groove in a 1" thick piece of wood. For simplicity, let's refer to the fence side as the right side of the cut and the operator's side as the left. First you'd set up so the bit would cut on the left side of center (leaving 1/4" of wood between the left side of the bit and the operator's side of the workpiece). Then when you flip the workpiece you'd again be cutting on the left of center which is the correct method for a router table. Again the cut would leave 1/4" of of wood between the bit and the left side of the piece. Doing that, you'd end up with a perfectly centered 1/2" groove.

It won't give you 100% accurately sized grooves without some test cuts, but neither will flipping wood end for end with a dado set on a table saw. Either way you need to set the fence, do both cuts and measure the width of the groove and make any adjustments. If you do the above though, you will never be climb cutting.

[Will Overton]

Making two passes, each face against the fence is one of the few ways to truly guarantee a centered dado.

Alan,

I agree that this guarantees that you hit the center. What do you do that guarantees a 1/4" groove?

[Alan Schaffter]

Alan,

I agree that this guarantees that you hit the center. What do you do that guarantees a 1/4" groove?

I use a bit I "know" cuts a 1/4" groove then use a centering bit to line up the fence or scrap wood to sneak up on the correct fence position- INCRA and other micro adjusting router fences have earned their keep IMHO.

Which reminds me, and not to hijack the thread, but I hope nobody seriously thinks the sum of the published widths for the two trimmers and chippers in a dado blade will equal the width of a dado- if it does you are EXTREMELY lucky. That has been the reason for much hair pulling when people try to cut specific size dadoes without doing test cuts and one of the causes for poor box joints. And even with the exact same setup, the size can change session to session, because you torqued the arbor nut differently, got a plastic shim caught in the arbor threads, or chip of wood, etc. caught between blades.

[glenn bradley]

Here is an actual example - cutting dados in a drawer that will use runners. If the drawer is already made, I could set the bit at a certain distance, then make two cuts, but in two directions so the distance will be exactly the same on both sides. However, I'm worried one of those directions (the opposite of how I usually cut) could cause problems, chatter, safety...

In this example I have no problem with direction. I clamp a secondary fence parallel to the primary fence. The bit is pressing the material towards the primary fence in the "normal" feed direction and against the secondary fence in the opposite direction. Use the same side of your object and your fence as the reference surface.

[Will Overton]

I use a bit I "know" cuts a 1/4" groove then use a centering bit to line up the fence or scrap wood to sneak up on the correct fence position- INCRA and other micro adjusting router fences have earned their keep IMHO.


This is where I'm confused. If you use a 1/4" bit, and it takes anything on the second pass, the groove is wider than a 1/4". If it doesn't take anything on the second pass, it's unnecessary.

What am I missing, or do you just use the two passes for the setup?

[Alan Schaffter]

This is where I'm confused. If you use a 1/4" bit, and it takes anything on the second pass, the groove is wider than a 1/4". If it doesn't take anything on the second pass, it's unnecessary.

What am I missing, or do you just use the two passes for the setup?

I think that is what I said?

[Dan Karachio]

In this example I have no problem with direction. I clamp a secondary fence parallel to the primary fence. The bit is pressing the material towards the primary fence in the "normal" feed direction and against the secondary fence in the opposite direction. Use the same side of your object and your fence as the reference surface.

Thanks Glen, that is exactly what I was trying to ask!

[Will Overton]

I think that is what I said?

I told you I'm confused. :o

I just reread your original post ... I now feel enlightened. :cool:

Thanks for your patience.