Up or Down Cut Spiral Bit For Through Slot Cutting?

[scott vroom]

I'll be making a couple of jigs that require through slots in 3/4" ply. I haven't decided yet on whether I'll cut the slots on my router table or with a hand held router and clamped on fence. The first priority is safety, followed by appearance. Would you recommend an up cut or down cut spiral bit?

A second question is the diameter of the slot vs the bolt that will pass through it. e.g., for a 1/4" bolt should I cut a 1/4" slot or a 5/16" slot etc?

Many thanks...

[Bill Huber]

I would think it would make no difference, the up cut would be good for one edge but the down cut would be good for the other.

I have made some jigs with slots like you are talking about. I do it on the router table. If I am using a 1/4" bolt then I will use a 1/4" bit and make a second pass with a spacer between the fence and the board, like two sheets of paper. If I need the slot exactly in the center then I will make a cut with one sheet then flip the board and use one sheet again.

I set stops on the router table fence for the start and finish of the slot, if the slot is to long for the router table I have a MDF board that I cut a slot in for a bushing and will use the router hand held with clamped on stops.

My MDF board is 1/2" x 8" x 24" MDF with a slot about 20 inches long for a bushing, I can use it with the clamped on stops to make any length slot I need.

[Judson Green]

Why not have both? Compresses all to the center. You'd have to rough out your opening first. If that's not possible then I'd use a up cut.

301624

[Brian Tymchak]

I think I would use a down cut bit but add a backer board under your finished piece to keep the bottom edge from tearing out badly. The backer board could be thin enough that you could mill all the way through it or if it's thicker, just mill 1/4" or so into it and that should give you a decent bottom edge.

[John Coloccia]

If I want clean edges, I'll often times make an initial, lighter cut with a down spiral, and then finish with an up spiral.

[Myk Rian]

You want a compression bit for ply.

[Peter Quinn]

They run compression bits on the CNC at work for through cuts in plywood, its basically an up shear on the bottom and a down shear on the top, so both faces of the plywood come out clean. I'd measure the bolts with calipers and shoot for .020" OD difference, depending on application, maybe even .030" for something that slides. Might be worth a test piece, though, often the threads of a 1/4" bolt are less than .250", so you might be fine with a 1/4" slot.

[Mick Simon]

I second the compression bit - up from the bottom and down from the top. No chipping on either surface.

[glenn bradley]

Bill Huber and Judson have your answer. That being said I cut slots all the time with upcut bits that I primarily use for mortising (since I already have them) and have not trouble.

[scott vroom]

I purchased a Whiteside compression bit. Thanks for the inputs :)

[Mike Heidrick]

If cut on a router table, I would go up-cut as the table's surface supports the work's face closest to the router. If you are cutting above the work like with a handheld router jig setup use a down-cut. On my cnc where the spindle is above the work, down-cut works best.

Are you compression bit users recommending these from experience? Are you using a flush trim version only? If you are using a compression endmill and you only plunge in .25" on a 1/2" shank compression bit, that first .75 to 1" (depending on CL) is going to be up cut and the chipout damage will already be done. With a CNC machine I can set up a final clearence pass and take care of the roughing and final pass automatically. Even then the chipout on ply might be too severe on cheap ply. Do you plan for that using one manually?

[Myk Rian]

Like everyone has a CNC machine?

[scott vroom]

If cut on a router table, I would go up-cut as the table's surface supports the work's face closest to the router. If you are cutting above the work like with a handheld router jig setup use a down-cut. On my cnc where the spindle is above the work, down-cut works best.

Are you compression bit users recommending these from experience? Are you using a flush trim version only? If you are using a compression endmill and you only plunge in .25" on a 1/2" shank compression bit, that first .75 to 1" (depending on CL) is going to be up cut and the chipout damage will already be done. With a CNC machine I can set up a final clearence pass and take care of the roughing and final pass automatically. Even then the chipout on ply might be too severe on cheap ply. Do you plan for that using one manually?

I don't have a CNC machine.

I had planned to first drill holes at both ends of the slot and then use the compression spiral bit to complete the cut. My understanding is that the compression bit used this way will avoid top and bottom tear out.

[pat warner]

Solid carbide would be my choice for single diameter pathway slots, 2 flutes and straight.
Would not bother with a spiral; it only adds complexity to an already simple process.
Slot slop depends on the application. But + .005"-.010" is plenty sloppy.

[scott vroom]

Solid carbide would be my choice for single diameter pathway slots, 2 flutes and straight.
Would not bother with a spiral; it only adds complexity to an already simple process.
Slot slop depends on the application. But + .005"-.010" is plenty sloppy.

I'm trying to figure out if what you're suggesting falls into the category of "more than one way to skin a cat", or if you believe the spiral bit is inferior to a straight router bit?

[pat warner]

Spirals have their place, especially in production with 5/8" shanks and bigger.
Constant feed rates, materials match etc, spirals are the high speed cutters of choice.
For kitchen routers, they're way over rated and over priced.
They always booger one face of ply (through cuttings), can pull themselves from collets, lift the work unexpectedly, and shoot chips into your socks. Moreover, their webs, for small routers/cutters, are so thin that you lose their stiffness (modulus) advantage. There's more, but I would not blow good straight bit money for spirals in your limited application.