Fire in the workshop! Do I need a spiral bit?

[Mark Grotenhuis]

I'm having an issue. I'm using an undersized straight router bit to cut dado's in plywood and the chips don't clear and they start to smolder and burn. Would a spiral up or down bit correct this issue? I'm worried that a spiral up bit would chip the face on thin veneer plywood and a spiral down bit would make the problem even worse but eliminate the chipping. Does anyone have any experience with this? Last time I did this I had my Festool router hooked up to my dust collector and some embers got sucked in the vac and started a fire in the collector. Thank God I noticed it before it got beyond the smoldering stage. I just had my router bits sharpened professionally and it did reduce the problem, but it did not eliminate it. Any help would be great!

[Frank Martin]

How deep a cut are you taking at each pass?

[Mark Grotenhuis]

How deep a cut are you taking at each pass?
No more than 1/4"

[Charles Lent]

You are probably running the bit speed too fast and/or moving too slowly for the cut you are trying to make.

Charley

[Matt Day]

Smoldering? I'm no expert but it sounds like something else is going on like a dull bit.

[Bruce Page]

A straight bit in a plunge cut will not eject the chips efficiently. Look into an up/down cnc compression bit. A compression bit is more expensive than a standard straight bit but they work very well. They are common in the cnc world for cutting plywood, mdf, and laminates and give a nice clean edge. I have never use one on a hand held router but it is my go to bit for plunge, dado like cuts on the cnc.


Here's some short videos:
https://www.youtube.com/watch?v=OlbxiKjiN_U
https://www.youtube.com/watch?v=ymwBdZsDP2Q
https://www.youtube.com/watch?v=OlbxiKjiN_U

[Frank Martin]

In addition to all the suggestions, I think your depth of cut could be a contributing factor as well. 1/4" depth in dado with a straight bit is probably pushing it too much.

[pat warner]

Dead cutter, cutting too deep. Down spiral is called for.
But I'd use a new solid or carbide faced cutter that is about the same diameter as the width of the cut.

[Matt Day]

What I don't understand is that I've done many types of cuts (like deep mortises on a breadboard end) where the chips are lodged in the cut and need a screwdriver or the like to get out, and none have ever been close to hot let alone smoldering. Something else is going on. Could you post pictures of your setup? Hand held or table? Any dust extraction?

[Jim Dwight]

It isn't really the depth of cut that has to be limited it is the amount of material you are removing in a single pass. 1/4 inch deep passes with a 1/4 diameter bit may be OK but it would be too much IMHO with a 3/4 bit and I would be leery of a 1/2 inch bit cutting this deep. My experience with burning chips has been me pushing the router too hard. Plunge cutting without a bit intended for that is another way to get burnt chips.

The advantage of spiral bits is chip removal. They should stop you from having to pry the chips out. That is mostly a problem with somewhat deep dados. For hand held use, you want an upcut spiral bit. It pulls the chips towards the motor out of the cut. A downcut will try to push the chips harder into the dado, doesn't seem indicated in this case.

[Art Mann]

I can't begin to count all the times I have successfully cut dadoes and grooves in plywood with a hand held router and a straight cut bit. Your bit might be dull or gummed with the plywood adhesive or it might be your technique but there is no reason you can't be successful with a straight bit. I prefer spiral bits but don't consider them necessary. If you are getting the burn while plunging, you need to develop a technique of moving the bit backwards and forwards as you plunge. By the way, not all spiral upcuts are suitable for vertical plunges. It depends on the design.

[Mark Grotenhuis]

I'm using a festool OF1400 router with the accessory that guides it along the track saw track. While I am cutting you can see the chips lodging in the groove and starting to smoke. I tried to increase the dust collection power to clear the chips but I suspect all that did was fuel the fire with more oxygen. All of my undersize plywood bits do this. Perhaps my rpms are too high? I may just purchase a spiral up bit and a spiral down bit and see what works. I am usually cutting baltic birch plywood. It doesn't seem to do it as much on lesser grade plywood, and again my bits were just sharpened by a professional carbide specialty business that does great work on all my blades and bits.

[Paul Girouard]

Once you go to spiral cutters, in 1/2" shank when ever possible , you'll never buy a non-spiral bit again , at least in a straight cutter.

Up/ down compression , with guide bearings either above or below the cutter. The cuts are just superior , there fore your work have every / the best chance of being the same.

[Jim Dwight]

Mark,

My most recent dados were with my PC 690, I think it is less powerful, using a 3/4 straight bit. I guided it with the router base for my DeWalt tracksaw track. I only cut about 1/8 deep in softwood plywood. It cut just fine with no burning. I recognize you are talking about baltic birch but I am still pretty sure a reduced cutting depth would solve your burning. An upcut spiral bit would remove the material from the dado. But if you reduce the depth, you may find the chips are pretty easy to remove.

Jim

[Robert Engel]

I don't use routers for dados, I find the set up a bit cumbersome compared to a TS and a dado.

I doubt its the speed. Most likely a dull bit.

Down spiral works much better in plywood. Be sure to scribe the lines to eliminate tear out.

I use 1/2" shank bits 90% of the time 1/4" only in my trim router.

[Al Launier]

As Robert intimated. Do you have a TS & set of dado blades & if so would you have "open" access with the blades to use them, or are the dados inside all edges of the plywood.?

[Mark Grotenhuis]

As Robert intimated. Do you have a TS & set of dado blades & if so would you have "open" access with the blades to use them, or are the dados inside all edges of the plywood.?
I only use the router to cut dado's when I deem it too dangerous to cut them on a table saw (example: I need a dado cut perpendicular in the middle of a long skinny piece). About 95% of all my dado's are done on the table saw. Not trying to be rude, but isn't a dado with "open access" considered a rabbet?

[Jim Dwight]

OK, I looked up the Festool router the OP is using and it draws 11.7 amps. That's a very nice but mid-sized router. My old PC 690s draw 11 amps. So I was right that they are smaller but they are closer than I thought. Knowing it's a mid sized router makes me think even more the burning is the result of trying to remove too much material in one pass.

I have several dado sets for my table saw but if you haven't tried a dado with a router on a track saw track you should. I needed them 6 feet long on pieces of plywood 4 feet by 6 feet (3/4). That would have been tough on my table saw. It was a piece of cake with my track guided router. The router attachment rides the rib of the track just like the saw so it can't go off course. I was making little 1/8 dados to locate the pieces during assembly. For deeper dados I would have needed to make multiple passes which takes a little more time but isn't otherwise difficult. When the wood gets big, it is a lot easier to move the cutter than the wood. Even on smaller pieces, it is nice to clamp the track to the wood and know the dado will be exactly where I set it up to go. With the table saw, you have to keep the wood tight to the fence or your cut will be off. There is no similar risk with a track guided router.

[Stan Calow]

I use straight bits for this as I think spiral bits chatter or want to do strange things because of the torque. Agree that 1/4" is too much for first pass.