Advice wanted: Flush trim bit ... tear-out?

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Forum topic by Marco_N posted 10-19-2021 11:50 AM 550 views 0 times favorited 18 replies Add to Favorites Watch
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1 post in 42 days

10-19-2021 11:50 AM

Topic tags/keywords: question

Hello Lumberjocks gang,

This weekend, my son and I made a circle cutting jig, and traced out a circle on some heat-treated ash. Yay!

We were following a method of doing one shallow cut using the jig, then cutting out the rough circle using a jigsaw, and finally making the edge neat using a flush trim bit.

After many years at the router table, something unusual happened! The flush trim bit tore out large-ish chunks (3” long) where the grain lined up with the cutters. It actually threw one almost 10 feet into the driveway. Check out these two pictures.

I think I understand what happened. Would this have happened on any wood, or is it because of the heat treatment that made this ash especially vulnerable? Not sure how to prevent it? We plan to go back over later with a roundover bit, but pretty hesitant to do so, for fear of more tear-out!

Any advice?

-- San Francisco Bay Area, hobbyist woodoworker, some pics at

18 replies so far

View Fred Hargis's profile (online now)

Fred Hargis

7292 posts in 3775 days

#1 posted 10-19-2021 11:53 AM

No idea what effect the heat treatment had, but I’d climb cut the troublesome area.

-- Our village hasn't lost it's idiot, he was elected to congress.

View Tony1212's profile


617 posts in 3017 days

#2 posted 10-19-2021 01:19 PM

... Would this have happened on any wood…
- Marco_N

Maybe? Wood is funny like that. No two pieces are the same. I’ve had that happen on a lot of hardwood and I’ve not had it happen on hardwood.

It’s all a matter of the wood’s grain (hence plywood is fairly immune to this kind of thing). Using a straight bit, it will grab under any wood grain and rip it off.

This is what happened:

The solution is to look at the grain and figure out where the bit’s cutting edge is likely to dig into the grain and climb cut those areas.

Using a spiral router bit will have the cutting edge do more of a shearing cut, which may help but it may not.

-- Tony, SW Chicago Suburbs

View sunnybob's profile


51 posts in 48 days

#3 posted 10-19-2021 01:30 PM

If you used a flush trim bit, what template did you use? Is it thinner in one spot? causing the bearing to slip over the top and the cutter to dig in?

Or, make the template smaller, start again, and take the finest cuts you can.
I have had similar tear out in oak and had to reduce the cut to 0.5 mm (20 thou”) at a time to get where I want to be.

View Ocelot's profile


3561 posts in 3920 days

#4 posted 10-19-2021 01:37 PM

I”m watching with interest since I still have a round table in work. I would certainly be disheartened if that happened to me. I would think that a new, sharp, high-quality bit would be the thing to do. Maybe this one was not so sharp?

I would not use a guided bit but would use a router trammel with a pivot hole drilled in the center of the bottom of the table. I have not done this, but have seen it done. That way, you get a true circile without having to have a smooth template or guiding edge.

Nice looking table, by the way.

Just adding a noten to say that I’m sure you have much more experience with routers than I do.

-- I intended to be a woodworker, but turned into a tool and lumber collector.

View waho6o9's profile


9096 posts in 3859 days

#5 posted 10-19-2021 02:00 PM

Up and down compression bits may help.

View Axis39's profile


565 posts in 879 days

#6 posted 10-19-2021 02:08 PM

This is why I bought a Ridgid Oscillating Sander for building guitars (and since, many other items). I use it all the time to sneak up on the finish edges of items I build.

It is also why i really need to buy some compression bits like Waho6o9 posted.

Heck, I was running some styles and rails for a cabinet the other day and had to really be careful about grain direction and feed speed to make sure I got nice clean edges on the profile. It was good, soft maples, with relatively straight grain…. That was all straight stuff, and even going the correct direction, it was all about the feed speed.

That’s definitely a scenario for flipping the work over and coming at it from the other direction to make sure you are cutting downhill!

-- John F. SoCal transplant, chewer uppper of good wood

View SMP's profile


4951 posts in 1188 days

#7 posted 10-19-2021 02:16 PM

Well even kiln dried ash can be a bear to work sometimes. I’m not sure what the heat treatment does.

View splintergroup's profile


6016 posts in 2504 days

#8 posted 10-19-2021 03:41 PM

I agree with the climb cut option, but when dealing with any significant amount of excess to trim off (> 1/16”) I worry about the bit grabbing the piece and tossing it.

What I do is use a top/bottom bearing pilot bit so I can flip the part when a climb cut is indicated.

Still, in these areas where the chances are high for a chunk to go missing, I’ll take very light passes to shave down the excess material before committing to a final pass.

View MPython's profile


398 posts in 1094 days

#9 posted 10-19-2021 06:39 PM

Another vote for the Whiteside compression flush trim bit. Pricy, for sure, but it works great. It handled this tricky cut without a flaw:

View therealSteveN's profile


9208 posts in 1856 days

#10 posted 10-19-2021 06:51 PM

I agree with the climb cut option, but when dealing with any significant amount of excess to trim off (> 1/16”) I worry about the bit grabbing the piece and tossing it.

What I do is use a top/bottom bearing pilot bit so I can flip the part when a climb cut is indicated.

Still, in these areas where the chances are high for a chunk to go missing, I ll take very light passes to shave down the excess material before committing to a final pass.

- splintergroup

I also like the top bottom bearings of either the Whiteside flush trimmer, or the Infinity flush trimmer, which both come with a larger diameter cutting area, and are both upcut/downcut compression bits, so they leave a pretty flawless edge, where a standard 2 wing cutter flush bit can get grabby. Especially as it passes the rougher end grain.

Heck even on a straight rectangular board it’s the end grain edges going to straight grain where it usually comes apart. You solve that by doing end grain first, so even if it does blow out, when you do straight grain it smooths it down. I’d probably start on the end grain portions first.

Climb cuts do tend to smooth down rough spots, just know going the wrong way can cause a board to get snatched from your grasp if you aren’t ready. I’ve found using these new compression trimmers that they are very smooth going the right way, and if you meet resistance all you need to do is flip your board so the grain is agreeable. I think the secret to them is allow the bit to work, so a slower feed, if you got burning (not likely on the heat treated Ash) just do another pass, and it will groom it off. Only caveat is router table only with them, NO handheld use.

-- Think safe, be safe

View CaptainKlutz's profile


5020 posts in 2776 days

#11 posted 10-20-2021 08:10 AM

Did you check moisture content?

Suggest part of problem is low moisture content due heat treatment.

Here in AZ, RH is often in single digits. Deal with sub-6% MC lumber all time. Older lumber will often measure below 3% limit of my meter. Dry wood forces me to be extra careful about grain direction when using a router on edges, particularly with large grain species.

Local supplier recently added some roasted Oak, Ash, and Maple to his inventory. Not an expert, only used it for single shelve and a box project.
IME – The heat treated wood was bone dry and does not re-absorb moisture as easily as it un-treated wood. Found it had more tear-out than same un-treated wood. Even the roasted maple gave me chip out issues, instead of normal burning.

FWIW – One trick taught to me by a seasoned desert wood worker was:
- Wet the ‘too dry’ wood before every fine machining operation.
- Use several passes, taking small cuts each time.

Use a spray bottle with distilled water, or a damp rag; to wet the edges of boards till I get a color difference. Most of the tear out issues are prevented with damp edge, unless I try to cut too much at one time.

Still have to be mindful of grain direction. As posted above, if you attempt take a large cut, and work against the grain; bad things happen.

Luckily, in AZ the surface moisture added to the wood is immeasurable 48 hours later. Couple times I checked, a small board machined on all 4 sides measured 6-7% on side using pin probes the next day; while larger boards measured lower.

Water treatment may not be a solution for folks in humid areas, but even during monsoon season when RH is 30-40% outside; I still wipe a damp rag along edges on well seasoned dry lumber before using a router bit on it.


-- If it wasn't for bad luck, I wouldn't have no luck at all, Doom, despair, agony on me… - Albert King - Born Under a Bad Sign released 1967

View DevinT's profile


2095 posts in 249 days

#12 posted 10-20-2021 12:28 PM

I had this happen to me on some dry African Teak. I was using a spiral cutter, taking shallow passes, not working the edge, and it still happened. It can strike you any time with any bit in any place of the wood in my opinion.

I would imagine paying attention to MC might have helped but also, maybe not. My climate is not that dry. I just happened to hit a vacuous space in the wood where the lignin failed, and a large chunk ripped off.

I grabbed a spokeshave and in less than 60 seconds it looked intentional with a nice stopped chamfer on the edge blending through the missing chunk. I measured the resulting chamfer and made an identical one on the other side of the sign to make it symmetrical, and we were off to the races. The customer had no idea and to this very day compliments me on it.

-- Devin, SF, CA

View Ocelot's profile


3561 posts in 3920 days

#13 posted 10-20-2021 01:16 PM

Thanks for the tip on wetting the wood. Also thanks to Devin for a reminder to adapt to the circumstances and make lemonade.

-- I intended to be a woodworker, but turned into a tool and lumber collector.

View Robert's profile


4783 posts in 2763 days

#14 posted 10-20-2021 02:10 PM

Compression bits are quite expensive if you’re not using them much.

A technique I use is to pad out a template with several layers of tape so the final pass is extremely shallow. That way I can do climb cuts safely.

In your case, routing a slightly larger circle first and several shallow passes.

Marking out the danger areas with tape is a good idea, too.

-- Everything is a prototype thats why its one of a kind!!

View DS's profile


3985 posts in 3702 days

#15 posted 10-20-2021 02:58 PM

Spiral bits in general will cut cleaner.

If you had a CNC you could climb cut with a straight cutter.

I wouldn’t try to climb cut by hand as it will want to run away on you.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS

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