Best way to cut a slot in hardboard

[Rolly]

I've got a 12-inch square 1/4-inch hardboard and need to cut a 6-inch long slot 3/8 inch wide through the hardboard. I plan to use my plunge router. What is the best router bit to use for the cleanest cut? Any tips would be appreciated. Thanks for your response!

[Parkingmeter]

I would use a 1/4" panel bit that has a plunge point in two passes, the plunge point and the 1/4" (as opposed to a 3/8" bit) will keep any tear out to a minimum with the soft, thin hardboard.

Follow the link below to see what the bit looks like, even for a Whiteside bit, it's inexpensive and can be purchased from many suppliers.

http://www.routerbits.com/cgi-routerbits/sr.cgi?1198888700_26918+23

[John Hunter]

I would use a spiral bit there is a good artice (free download) at http://www.plansnow.com:80/071228download.html

[cabinetman]

If you want to use a router, you could use a flat bottom cutting mortising bit. Or, you could just do the cut with a few passes on your table saw.
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[RayintheUK]

I'd sandwich the workpiece between two other hardboard squares (to eliminate any "furring" on the internal edges), then use a 3/8" straight cutter.


Ray.

[LCHIEN]

I'd drill a 3/8" hole first, on the line where you want the slot.

Then I'd take a 1/4" or 3/8" (if you have it) spiral upcut bit in the router table.
Put the bit about 1/4" proud of the table top and set the hardboard withthe hole directly over it so the bit is in the hole. Now put the fence agasint the hardboard edge parallel to the slot. pressing the bit to the closest edge. Now you can start the router and run the bit back and forth against the fence cutting a slot. If the slot is to be 3/8 and the bit is 1/4, you will need to stop and move the bit to the other side of the pilot hole (by moving the fence over) and route again to finish the width.

if the slot needs to be exactly 3/8" very closely, then you can use a 3/8 bit to begin with or use a 1/8 shim material between the fence and the hardboard to move it over exactly 1/8".

If you put the smooth hardboard side faceup then the ragged side of the cut will be on the rough side of the hardboard. (The upcut bit will pull any loose fibers up towards the router.) You should have a clean edge on the smooth side of the board.

[JoeyGee]

Ray beat me to it. Sandwhiching between two other pieces is how I would do it.

[cabinetman]

The length of the slot just slipped by me. Somehow I put "through the hardboard" to mean all the way. The 6" didn't register...must have been another CRAFT incident.

You said it's 12" square and 1/4" thick. You didn't say how deep a slot, or how a 6" slot lays out on the 12" inches. But in any case, a straight cutter can be used and set up stops for the start and finish if the slot lays within the 12", or at one end if the slot starts at the edge. If the start/end of the slot needs to be square, just chisel it out where it's rounded.

As for a clean cut, sandwiching may provide a fairly clean cut. A sharp carbide bit is always best to start with. Absolutely clean cuts are not always achieved, and the slot can be dressed carefully with fine sandpaper, or lightly dragging a very sharp chisel as a scraper, to remove any "fur".
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