Cutting a Centered Slot

[D_Harris]

I have a Bosch Colt hand router, and I’ve been having a difficult time with depth adjustment. To cut a 1/8” wide slot directly in the center of 3/4” thick plywood requires a lot of trial and error. I am not able to rely only on the router’s built-in depth adjustment. And even if I get an accurate start the cut tends to drift to the side as I’m making it.
 
I’d appreciate any advice on how to get perfectly centered slot.
 
Thanks.

Darren Harris
Staten Island, New York.

[JGrout]

I have a Bosch Colt hand router, and I’ve been having a difficult time with depth adjustment. To cut a 1/8” wide slot directly in the center of 3/4” thick plywood requires a lot of trial and error. I am not able to rely only on the router’s built-in depth adjustment. And even if I get an accurate start the cut tends to drift to the side as I’m making it.
 
I’d appreciate any advice on how to get perfectly centered slot.
 
Thanks.

Darren Harris
Staten Island, New York.

if you are getting drift make a closed slot fence and use a bushing to guide the cut. 

As for the depth I make a stick the same as the depth I want + the template and use it to set the router bit

[Teak]

I have a Bosch Colt hand router, and I’ve been having a difficult time with depth adjustment. To cut a 1/8” wide slot directly in the center of 3/4” thick plywood requires a lot of trial and error. I am not able to rely only on the router’s built-in depth adjustment. And even if I get an accurate start the cut tends to drift to the side as I’m making it.
 
I’d appreciate any advice on how to get perfectly centered slot.
 
Thanks.

Darren Harris
Staten Island, New York.

Are you using a slot cutting bit? An example http://www.leevalley.com/en/wood/page.as...9435,46176
Assuming a piloted bit, depth of cut will be trial and error however, once done, should be a cinch.

[mad_planter]

Are you getting drift because of bit deflection?  1/8" is pretty thin, I imagine the laminations of the plywood would be more than enough to deflect the bit one way or the other as its cutting.

Someone else mentioned using a slot cutter bit if you're not already.  I wonder if making multiple shallow passes to get the full depth you need might minimize the deflection.

[mike4244]

I have a Bosch Colt hand router, and I’ve been having a difficult time with depth adjustment. To cut a 1/8” wide slot directly in the center of 3/4” thick plywood requires a lot of trial and error. I am not able to rely only on the router’s built-in depth adjustment. And even if I get an accurate start the cut tends to drift to the side as I’m making it.
 
I’d appreciate any advice on how to get perfectly centered slot.
 
Thanks.

Darren Harris
Staten Island, New York.

Because the edge is narrow, 3/4" the laminate trimmer tends to tilt. Best way has been suggested, a slot cutter. Adjust the base depth, try on scrap plywood. When cut is centered you can safely make your slots. 
Using a slot cutter lets you ride on the face instead of on an edge. One example is Freud #63-108 which will give you a 1/8" wide slot and 9/16" deep. If the slot needs to be shallower then a larger bearing is needed

mike

[Alan S]

... To cut a 1/8” wide slot directly in the center of 3/4” thick plywood requires a lot of trial and error. I am not able to rely only on the router’s built-in depth adjustment. And even if I get an accurate start the cut tends to drift to the side as I’m making it.

It sounds to me as if you are likely already using a slot cutting bit because you refer to the router depth adjustment in regard to positioning the cut in the center of the stock.  You just need to find a way to set that depth with sufficient accuracy.  Then as you cut you need to make sure the router rides smoothly on the surface of the plywood and does not tip.  It has a tendency to tip because the router base is so small: a larger base gives you more guide surface.

This is exactly the problem that Pat Warner's offset bases were designed to address.  Unfortunately you can't buy from him: he is no longer with us, and he made them all himself.  But you can look up his offset bases and make a similar one.  One hand holding the base down flat at a distance from the cut, while the other hand guides the router, should help give a consistent cut.

With regard to centering the cut, in some applications that may not be needed.  If you cut such a groove at the same distance from the face in two pieces and then join those with a spline, the two reference faces should align as long as you are careful to work from the proper face.

[MichaelMouse]

It sounds to me as if you are likely already using a slot cutting bit because you refer to the router depth adjustment in regard to positioning the cut in the center of the stock.  You just need to find a way to set that depth with sufficient accuracy.  Then as you cut you need to make sure the router rides smoothly on the surface of the plywood and does not tip.  It has a tendency to tip because the router base is so small: a larger base gives you more guide surface.

This is exactly the problem that Pat Warner's offset bases were designed to address.  Unfortunately you can't buy from him: he is no longer with us, and he made them all himself.  But you can look up his offset bases and make a similar one.  One hand holding the base down flat at a distance from the cut, while the other hand guides the router, should help give a consistent cut.

With regard to centering the cut, in some applications that may not be needed.  If you cut such a groove at the same distance from the face in two pieces and then join those with a spline, the two reference faces should align as long as you are careful to work from the proper face.

What he said, with a thought or two.  For instance, you're using a mini-router when sturdy and thicker shank would be easier.  You can make offset bases out of plexi or even Masonite if you like.  Or try clamping a board of similar thickness just out of reach of the cutter, parallel to the board being modified.  It will support the other side of the base, helping control or eliminate tip.

[ez-duzit]

Make an auxiliary router base with 2 equidistant guide pins on either side of a 1/8" carbide bit--self centering. And works on any thickness. Long pins provide stability.

[Steve N]

Make an auxiliary router base with 2 equidistant guide pins on either side of a 1/8" carbide bit--self centering. And works on any thickness. Long pins provide stability.

I agree here. Plus I hate slot cutters.

A jig like this one will find center every time and, as long as you keep the pins in firm contact with the outside edges of the stock, you are dead set in the middle, and if you keep the pressure twisted to remain in contact you will just go zippity right up the 3/4" edge of your sheet goods. Use the router bit right here




If you really really want to TILT even though you are using a trim router, set 3 edges together, if you can put solid blocks between 3 sheets of ply so the outer two sheets are out near the far edge of you base, think of them as outriggers to support your base, as you concentrate on keeping the twist to remain centered. Just make sure the blocks are mounted lower than your pins, so the router can move freely along the cut. Zippity zip............

[D_Harris]

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.