How to countersink a flat bottomed hole

[Darren Vass]

I am drilling holes in my router table base plate for my router. The router baseplate has countersinks that are almost 1/2 inch in diameter but with a flat bottom for a flat head screw. How do I countersink such a hole? Do I use a forstner bit? If so, would you drill the countersink first or the smaller hole and then the countersink?

Thanks,

Darren

[Tim Self]

The forstner bit sounds like the way to go. And I would drill the large hole first. To large a small hole could lead to the bit jumping or walking. Or possibly using a plunge router. Just another idea.

[John Coloccia]

Do you know any machinists? The easiest way to do this is with a piloted counterbore. Ever see pen blank trimmers? It has a shaft that's guided by the tube, and a larger cutter to flush the end of the blank with the tube. Well, that's a piloted counterbore. Any machine shop should have a nice collection of counterbores and pilots.

If I didn't have access to a counterbore, I would drill the counterbores first with a brad point bit. Don't go to final depth. The brad point will leave a nice little dimple in the center of the counterbore that you can use to align the bit for the through hole. Drill the through hole. Then you can come back and flatten the bottom with a forsterner. You'll have most of the counterbore done at that point so the sides will support the bit and it won't walk.

For anyone who saw my thread on aligning the drill press table: this is exactly the kind of stuff I'm talking about! :D

[Lee Schierer]

The tip angle on most twist type drill bits is close enough to the countersink angle for woodworking applications. Put a stop on the bit so you don't drill the hole deeper, just enough to bevel the edges.

[Cliff Holmes]

The tip angle on most twist type drill bits is close enough

I think he's saying that the bolt heads are flat, not angled. He essentially needs a rabbeted hole.

Edit: "rabbet hole" ... heh ... I crack me up ...

[Lee Schierer]

I think he's saying that the bolt heads are flat, not angled. He essentially needs a rabbeted hole.

Edit: "rabbet hole" ... heh ... I crack me up ...

No I believe he said his holes were flat bottomed counter bores and he wants to use countersunk screws not pan heads. And upon further review, I would fill the counter bore with PC-7 epoxy puttyhttp://ecx.images-amazon.com/images/I/41oSOuCQdwL._SL500_AA280_.jpg, let it set and then countersink them with a countersink bit on a drill press. I fixed an oops on my dovetail jig fingers with this material and it has held up extremely well.

[Ellen Benkin]

Take the drill bit out of the countersink and just use the part that cuts the countersink.

[Tom Esh]

Any particular reason they have to be flathead / countersunk? (If you're thinking of re-using the router's original sub-base screws, chances are high they won't be long enough anyway.) Since the plate is already counterbored I'd be inclined to just use round/oval head machine screws.

[Stephen Edwards]

I get the impression that he's asking how to drill a flat bottom countersink. If so, another option would be an end mill, similar to a solid carbide spiral router bit. That's what I used in my DP to get the flat bottom counter sinks for my Rockler router table plate. It's about the same length as my drill bits. So, I didn't have to adjust the table height for any of the bores.

[Zach England]

I have a bit that is a number of increasingly larger concentric circles--I think it is called a stepped drill. I believe it is for this type of operation.

To me the obvious solution seems to be to get different screws.

[Stephen Edwards]

The one advantage that I can think of for using flat bottom holes and the same type of screws that holds his router base plate to the router is that he'll have a bit of adjustment room to perfectly center the router bit in the router table plate, the same as we can adjust the baseplate on our router itself.

Maybe I'm reading his post wrong. I get the impression he's drilling new holes from scratch into the router table plate.

[Roger Newby]

Just for the record, the point on a standard drill bit is 118 degrees and the taper for flat heads is usually 82 or sometimes 90 degrees

[Vince Shriver]

I have a bit that is a number of increasingly larger concentric circles--I think it is called a stepped drill. I believe it is for this type of operation.

To me the obvious solution seems to be to get different screws.

I'm with Zach. Or how about using the bit that comes with the Kreg Jig? (using different screws sure does seem the simplest way, tho)

[Tom Veatch]

I think terminology is causing some confusion. Counterbore is a flat bottomed hole, countersink is a hole with a conical shaped bottom - typically an 82° included angle for commercial hardware most often encountered in woodworking; 100° included angle if you're countersinking rivet holes or other Mil.Std. hardware.

Twist drills are usually 118° or 135°, but in wood at the bottom of a counterbore where the mismatch isn't visible, would work if you had to use a flathead screw. But for counterbored holes, I'd recommend leaving it flat and using something like a round or pan head screw

[Alan Schwabacher]

Those counterbored holes have a function that will be destroyed if you countersink. The reason for the flat is so you can fine adjust the position of the router so that the bit is at the center of the base.

If what you are asking is how to drill such counterbored holes, note that the flat is larger than the minimum size, so as to give you adjustment room. So is the through hole, for the same reason. A consequence of that is that the exact position is not as critical as it would be with countersunk holes. That meanse you simply drill the counterbore first, with whatever drill bit you have that leaves a flat bottom (forstner, Kreg pocket hole bit, end mill, twist bit modified on a grinder, plunge router bit in a drill press...). Then drill the through hole close to the center of the counterbore, and don't worry if it's a little off.

[Cody Colston]

I am drilling holes in my router table base plate for my router. The router baseplate has countersinks that are almost 1/2 inch in diameter but with a flat bottom for a flat head screw. How do I countersink such a hole? Do I use a forstner bit? If so, would you drill the countersink first or the smaller hole and then the countersink?

Thanks,

Darren

A Forstner bit will do it. Always drill the big hole first. There's a little pilot point on the bottom of a Forstner bit that will mark the center for drilling the smaller hole.

[Darren Vass]

Again, thanks for all the replies. Yes, I'm trying to counterbore, that is to get a larger flat bottomed hole on which a flat head (the bottom of the head) will sit. The smaller hole is also slightly larger than the screw so there is room for the screw to move. This is necessary so that the router can be centered in the plate with a centering jig device that came with the Milwaukee router.

Thanks to all.

Darren

[Harry Hagan]

You want it to end up looking like this . . .

[Bruce Wrenn]

I am drilling holes in my router table base plate for my router. The router baseplate has countersinks that are almost 1/2 inch in diameter but with a flat bottom for a flat head screw. How do I countersink such a hole? Do I use a forstner bit? If so, would you drill the countersink first or the smaller hole and then the countersink?

Thanks,

DarrenThe counter sinks in the router base plate are to allow centering of guide bushings. PC used this style for a while. With the table plate, you most likely aren't going to be using guide bushings.

[Stephen Edwards]

I think that what the OP is saying is that he wants flat bottom holes with straight sides, with the smaller hole a bit larger than the diameter of the bolt/screw, so that he has adjustment room to center his router bit in the center of the plate hole.

I agree that he may not ever need to use bushings on the RT. But, if he does ever anticipate using them, now is the time to get the router mounted properly onto the plate.

Darren, this centering jig that you mention, is it the one that's 1/4" shaft on one end and 1/2" on the other and sloping to cone ship in the center? Does your router plate have an insert that accepts PC style bushings? Does the centering jig come with a bushing?

You're on the right track.