Up cut or Down Cut spiral

[Steve Roxberg]

I need to use a flush trim spiral router bit.

The parts have an edge that will be very visible and so it needs to be straight and chip free.

I'm going to use a patern bit with a template on the top.

Facts:

1. Router will be table mounted.
2. Template and bearing will be on the top of the work piece.
3. The show edge I'm trying to protect will be the top edge.
4. the bottom edge will be hidden.

So do I want an upcut or downcut spiral bit?

Any other advice?

[Greg Funk]

What kind of wood are you cutting? I've always used a spiral upcut on hardwoods and have never had any chipout. You might be safer with a downcut bit if you are trimming melamine but then it won't work as well for other applications.

Greg

[David DeCristoforo]

The direction of "shear" should be based on which side of the stock you want to have a "guaranteed" chip free cut.

Variables:
Stock face up or face down?
Router mounted "under" or "over"?

With the router mounted "under" (as in a router table) an "upcut" bit will be a "downcut" bit (because the shear direction is based on the router being on top of the work) so if the stock is face up, the "cleanest" cut will be on the top of the edge. Properly confused now?

Truth be told, if you want a clean cut on both faces, you would be better to use a SHARP (sic new) three wing straight bit with no spiral at all.

[glenn bradley]

I would use a downcut. In your situation the bit will act like an upcut bit and your template will give you a ZCI like supported edge. Don't go cheap on the bit. I have an MLCS bit that was pretty poor and the rest of thier bits I have are pretty good. Although I have everything from Woodcraft $5 bits on up I really like my Freuds (don't think they make a flush trim spiral) and Whiteside (they do) bits the best.

[Mark Singer]

A straigt bit or could have a little shear

[Cliff Rohrabacher]

Just think about what you are trying to accomplish.

Will tear out (or chip out) be a serious issue or is it already? Maybe a down cut is the solution. Or an up cut depending on which direction the top of the wood where the chipping is happening.

Will Chip loading be a problem? An up cut will clear chips faster.

Most of the time the straight bit is the best all round bit. However there are places where the angle of the blade matters. It's all a function of the application and material

[jim mckee]

If you think you have a high chance of chip out, use a drill press mounted sander with a stylus to follow the pattern.

as far as up or down spiral or straight flute:
Show me the person that can predict the benefits of either against chip out (even if they are looking at the wood in question).
jim

[Tom Veatch]

...
1. Router will be table mounted.
2. Template and bearing will be on the top of the work piece.
3. The show edge I'm trying to protect will be the top edge.
4. the bottom edge will be hidden.

So do I want an upcut or downcut spiral bit?

...

As previously stated the "up" or "down" designation is based on use in a handheld router. To minimize chipout, you want shearing action to be directed from the surface into the stock. In your case, that translates to "toward the router" or an "upcut spiral". If you were doing it with a handheld router above the work, you would want a "downcut spiral" bit.

Another way to think of it - if the "good" surface is farthest from the router - "upcut" toward the router. If the "good" surface is closest to the router - "downcut" away from the router.

The point made about the template acting as a zero clearance insert is worth consideration. If the template has a good, sharp corner and is held in firm contact with the workpiece, it shouldn't make much difference whether the bit is straight or spiral cut.