Cutting a Centered Slot

[Alan S]

First, a correction. It is a 1/16th” slot cutter I am using. (I will also be using a ¼” slot cutter for another project involving the use of splines). But the present project using the 1/16” slot cutter is so I can add T-molding to the edges. I know the router is small, but I can keep it from tilting. The problem is after painstakingly getting the depth correct through trail and error on scrap, there is drift anyway. I’ll have to look into making an offset base as mentioned. Are there any online instructions? (With pictures).    :-) 

Thanks everyone.

Darren Harris
Staten Island, New York.

If the distance of the cut from the centerline varies, there are two kinds of possible problems.  I was assuming a wobble of the entire router that could move the cut up or down, but that would not change the distance of the cutter from the router base.  The other type of problem would be a change in position of the cutter during the cut, likely making it further from the router base.  If the router bit is moving out of the collet, that is a serious problem that needs attention.  This could be caused by dirt keeping mating surfaces from proper contact, or from improper tightening of the collet.  Make sure the bit is not bottomed out before tightening, since that can prevent proper tightening of the bit.

Here is a link to Pat Warner's website with pictures of offset bases: http://patwarner.com/offset_subbase.html  They are actually very simple: just a flat and rigid material cut to shape and bolted to your router casting, with a handle.  Use very stiff material, but make sure the thickness does not reduce the depth of router bit below the surface too much.  Pat's work met spectacular standards of precision, but you don't need that.

[Steve N]

I’ll have to look into making an offset base as mentioned. Are there any online instructions? (With pictures).    :-) 

Thanks everyone.

Darren Harris
Staten Island, New York.

Center the hole for the bit to project through.

The pins can be arbitrarily placed, you just need to assure they are both the same exact distance from your center hole.

So if you are going to center wide beams you may want a base 6 to 8" wide, with pins spread enough to go over your largest beam. Conversely if you are just working on edges of ply, or boards around 3/4" thick, you would want your base closer to that size, maybe 4" square at the most, and pins no more that 1 1/2" apart, but again equal distances from the center of your hole. That way when you twist the jig to close the pins tight, being equal distances away, they make the bit centered. Don't overthink it, it's so basic you can zoom right past it.

Becase it is so basic I am not sure if there are more detailed instructions

[KyleD]

Center the hole for the bit to project through.

The pins can be arbitrarily placed, you just need to assure they are both the same exact distance from your center hole.

So if you are going to center wide beams you may want a base 6 to 8" wide, with pins spread enough to go over your largest beam. Conversely if you are just working on edges of ply, or boards around 3/4" thick, you would want your base closer to that size, maybe 4" square at the most, and pins no more that 1 1/2" apart, but again equal distances from the center of your hole. That way when you twist the jig to close the pins tight, being equal distances away, they make the bit centered. Don't overthink it, it's so basic you can zoom right past it.

Becase it is so basic I am not sure if there are more detailed instructions

What Steve N is talking about does not use a slot cutter with the router 90 degrees to the ply edge but rather a straight cutter with the router in line with the ply edge. A 1/16 straight bit is not intended to cut to the depth you need for t molding. the off-set base you need just keeps the router from tipping while being guided along the edge. Make sure there is no sawdust or edge splinters getting between the interface between router base and the work. This could cause a small amount of wander as well.

[Steve N]

Correct, and in my first post I added a link to a spiral cut bit to use. Pretty sure OP understood that, just had asked for a plan to make a jig like I described, that was the second post.

[D_Harris]

If the distance of the cut from the centerline varies, there are two kinds of possible problems.  I was assuming a wobble of the entire router that could move the cut up or down, but that would not change the distance of the cutter from the router base.  The other type of problem would be a change in position of the cutter during the cut, likely making it further from the router base.  If the router bit is moving out of the collet, that is a serious problem that needs attention.  This could be caused by dirt keeping mating surfaces from proper contact, or from improper tightening of the collet.  Make sure the bit is not bottomed out before tightening, since that can prevent proper tightening of the bit.

Here is a link to Pat Warner's website with pictures of offset bases: http://patwarner.com/offset_subbase.html  They are actually very simple: just a flat and rigid material cut to shape and bolted to your router casting, with a handle.  Use very stiff material, but make sure the thickness does not reduce the depth of router bit below the surface too much.  Pat's work met spectacular standards of precision, but you don't need that.

Here's the weird thing. As I said I have no problem keeping the router from tilting. It is just that the cut even when starting out accurately will start to move from center. If I pull out and start the cut again a inch along it will be at the center again. This suggests to me something is flexing. 


I'll look into the plausibility of making an offset base, but I don't know if it will correct my problem.

Thanks.

Darren Harris
Staten Island, New York.

[Steve N]

Darren are you trying to start the cut full depth? Or are you using a plunge router and easing the bit into the cut part way, moving forward, and then starting again, a little deeper, until you are at full depth? I would suggest the second way, and apologies for not earlier suggesting it. A full depth of 3/4" or so with any width will be trying to cut a lot of wood, and sometimes wood resists being cut. That for sure can cause movement like you describe.

If you don't have a plunge router, just set your depth a fraction of the cut, and make it in several stages lowering the bit a little each pass. With the exception of really needing to aim the start it will be same as a plunge router. That also is the beauty of a plunge router, it allows you to set a depth, and basically insert the bit into the wood. Only moving forward once you are deep enough.

[D_Harris]

Darren are you trying to start the cut full depth? Or are you using a plunge router and easing the bit into the cut part way, moving forward, and then starting again, a little deeper, until you are at full depth? I would suggest the second way, and apologies for not earlier suggesting it. A full depth of 3/4" or so with any width will be trying to cut a lot of wood, and sometimes wood resists being cut. That for sure can cause movement like you describe.

If you don't have a plunge router, just set your depth a fraction of the cut, and make it in several stages lowering the bit a little each pass. With the exception of really needing to aim the start it will be same as a plunge router. That also is the beauty of a plunge router, it allows you to set a depth, and basically insert the bit into the wood. Only moving forward once you are deep enough.

I still think you are misunderstanding.

I'm not trying to create a 3/4" deep cut. I have a Bosch Colt router and I'm using a slot cutter to make a groove for T-molding.

Darren Harris
Staten Island, New York.

[Ray Newman]

Sounds like a job best done on a router table with the work laying flat.

If you do not have one, a router "table" can be quickly and easily made from a scrap piece of plyw'd: drill a hole for the bit to protrude, attach the router to the bottom of the plyw'd, a fence can be a piece of straight wood held in place by C- clamps, and clamp the whole thing to a table or workbench edge.

Cut to the desired depth in a series of shallow passes.

[Willyou]

I'm reading this thread a little late. Have you resolved your problem?

[Alan S]

You say "the cut even when starting out accurately will start to move from center. If I pull out and start the cut again a inch along it will be at the center again. This suggests to me something is flexing."

I agree it sounds as if something is deflecting, but the router bit is not moving in the collet. The problem with a 1/16" cut is that the cutter is of necessity quite thin. I suppose if the plane of the cutter were not precisely parallel to the motion of the router, there might be a problem. Does the cut move away or toward the router base? If it moves away, it would tighten up and might need to be restarted to move again. If it's moving toward the router base, it could be the whole router is riding up. In either case, the problem could be rotation of the bit around an axis that is not perfectly perpendicular to the router baseplate, combined with a very thin slot cutter that can't easily correct the problem by cutting sideways. If you can make the cut in a series of shallow passes, that might help. How deep is the needed cut, and how deep are the carbide cutters on the router bit?

If you make a mark on your router baseplate, you should keep that mark facing toward the edge as you slide it along, and making sure to hold the baseplate down tightly to the work. (That last point is what the offset base helps with.) If your problem is a slightly off axis rotation, you can fix it by trying a series of passes on the edge of scrap plywood. Between each pass, rotate the base of the router 45 degrees, and try to keep the same orientation for the entire pass. If the axis is the problem, once you find a good orientation, the router should continue to behave itself if you keep that orientation. That's because a very slight tilt won't matter in the perpendicular angle. It is usual however, that router axes are very perpendicular to their bases.

Another possibility is what Charles Neil calls "bump cutting". If your problem is that the router starts at the right position and then drifts off as the router moves along, you can simply start the cut in very many places along the edge, "bumping" the router against the edge as you go along. Then go back to the start and make a pass to connect all the bump cuts. The router should not be able to drift far off before falling into a correctly positioned cut.