Dados - Spiral Down Cut or Standard Straight Bit?

Bill Satko

Member
Messages
3,209
Location
Methow Valley
I need to buy a router bit to make dados in prefinished sheet goods. Normally I would get a standard two flute straight bit but am wondering if I should use a spiral down cut bit instead. It will be used in a Festool router running on the track. The bit diameter is 7/32", which may force me toward the spiral down bit. Which one would use?
 
It will be a shallow dado and I will take a couple of passes if I have to. If I was doing a mortise, I would go with the upcut. I thought that the consensus would have been for a downcut in this case and did not offer that up in my original choices.:) OK, I should have known better! Get any three woodworkers together and you will get three different answers. So is everyone leaning toward spiral bit (down or up in question) and not a normal straight bit?
 
Spiral down for dadoes and spiral up for mortises. Sharp in any case.

Quarter inch deep dadoes are virtually self-clearing, so choose the downcut to get crisp edges on the thin (very!) veneer pf the the ply.

Deeper mortises need all the help we can give then to clear out the swarf.

Heat kills sharp edges. Clogged cuts generate heat.

Hope that helps.

RouterLady (retired)
 
Will it not also pull up on the edges of the top ply and leave a rough edge, possibly chipping it? :huh::dunno:

I use an upcut on mortices to remove waste where there is no worry about leaving a rough edge.

That's a good point. I've used an upcut in my router table with good results on a variety of materials. I've used a downcut for edge trimming veneered panels for the reason you state. I can see where the downcut bit might make for cleaner edges on dados but I haven't had an issue - yet.

Incidentally, for those who have to stop and think about the up and down terms (like me!) the direction is related to a handheld router. An upcut bit will pull waste down into a router table which can be confusing. :huh:
 
I say... upcut, toss the track, but use a shopbuilt underlay straightedge/zero clearance plate to prevent tearout & guarantee a match to your layout line. You know, the kinda' deal you make up from a stripe of Masonite with a hardwood straightedge attached to it, the kind you trim with the router & bit in question on the first use, and that is forevermore dedicated to that router & bit. You get real live zero clearance from one, and they do a VERY GOOD job of placing your cut exactly where you want it, AND holding down all the fibers around the cut in the bargain.

A straight bit will pack up chopped fibers in the dado/groove something awful, and so will a downcut. I mean PACK.
 
Thanks everyone for your input. I have decided to get the spiral bit over the standard straight bit, even if it costs a little more. I also am going to use a downcut for the reasons stated by Rennie and Carol. You just have to take the advice of someone with the title of Router Lady (Carol, not Rennie).:) Besides it matches up with the advice I read from Pat Warner (Router Man?) in this Woodcraft article.

<< Link>>

It was reading this article that made me ask everyone about spiral vs standard straight cut.

By the way, normally I would use my table saw for dado cuts, but these are stopped dados and I find it more accurate using my router.
 
+1 spiral downcut on veneered surfaces, spiral upcut on mortises, straight bits when its not really important although in many materials, they do well for me. I use them to save wear on my spirals when its a no-show cut.
 
Top