Chipbreakers: The No. 6 Way to Reduce Tear-out

If you follow the conventional wisdom for setting your chipbreaker, you will hate your handplane.

What’s the conventional wisdom? According to Charles Holtzapffel’s seminal work on the cutting action of tools, you should set your smooting plane’s chipbreaker .02” from the cutting edge of your iron (other respected sources say to set it even closer than that) and to have an extremely tight mouth. The illustration shown on page 478 of Vol. II of Holtzapffel’s “Construction, Action & Application of Cutting Tools” shows a plane with a mouth as tight as one could imagine.

This, Holtzapffel says, prevents tear-out.

This, says your neighborhood blogger, makes your plane choke like a starving man at the Chicken Bone Buffeteria.

Chipbreakers do more harm than good in a handplane. Whenever I’m having trouble with a plane (especially if the plane is choking or refuses to cut), the first place I look is the chipbreaker. Whenever I fettle a new or vintage handplane and the bugger won’t behave, the first thing I’ll do is swap out its chipbreaker with another plane that has a working chipbreaker. In almost all cases, this solves my problem.

So what is the purpose of the chipbreaker? My cynical view of the gizmo is that it became widely used so toolmakers could use a cheap, thin steel cutter and reinforce it with an inexpensive iron or soft-steel plate. This is supported by the odd names given to chipbreakers. Some early sources call them cap irons, double irons, break irons or top irons. In other words, not everyone agrees that they were designed to break chips.

Early planes had thick irons and didn’t have chipbreakers, even during the age of mahogany, which has irregular grain that tends to tear-out.

In my view, the chipbreaker’s only real purpose in a modern plane is to mate with the tool’s blade-adjustment mechanism and to aid in chip ejection. Oh, and it exists to frustrate you.

You don’t have to take my word for it. Professor Chutaro Kato at Yamagata University did an interesting study of chipbreakers and how their shape and their position on the iron reduces tear-out.

You can read the entire study here. But here’s the quick summary: The chipbreaker actually did its job when it was located .004” from the cutting edge. I have tried to set a chipbreaker on a smoothing plane to this position (using a feeler gauge as a guide), and it doesn’t work well if you have a tight mouth on the tool. My planes just clogged because there wasn’t enough room for the shaving to escape.

If you read Professor Kato’s study carefully, you’ll note that he had better luck with a chipbreaker that had a radical forward-leaning angle – 80°! This 80° breaker worked better even when positioned back a little on the cutting iron. I have yet to try this setup on a plane because the numbers don’t add up. Professor Kato is working with a bevel-down plane bedded at 40°. Do the math: Putting an 80° chipbreaker on an iron bedded at 45° with a tight mouthseems madness. (If anyone has tried this, let me know. I also used to think that $8 for a six pack of beer was madness.)

So in what position should you place your chipbreaker? I set mine back about 3/32” in a smoothing plane in most cases — sometimes even a little further back if the mouth is really tight. All I’m really trying to do is to prevent clogging.

Which begs the question: Why did I list a chipbreaker as one of the ways to reduce tear-out? Well, I did mention one use for the chipbreaker in a modern Bailey-style plane – it mates with the tool’s depth-adjustment mechanism. This mechanism allows you to easily set your tool to take the finest cut possible, which really will reduce tear-out.

— Christopher Schwarz

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9 Responses to Chipbreakers: The No. 6 Way to Reduce Tear-out

  1. Eric Paisley says:

    Amen. I was struggling with some curly Claro Walnut and my smoothing plane was about to go out the window. Moving the chipbreaker did the trick. I was also getting plane tracks which I couldn’t figure for the life of me where they were coming from. I spent an hour fiddling with the blade to get it sharp and applied your method of rounding off the corners. Finally I checked the bottom of the plane sole. Those LN bronze planes will get the least little nob on their toe or sides if they bump up against something. In this case it turned out to be a bump against my jointer plane. One more thing to be aware of.

  2. Paul Kierstead says:

    "Professor Kato is working with a bevel-down plane bedded at 10°."

    Just a small correction: the clearance angle is 10 degrees and the bevel angle is 30 degrees, indicating the bedding angle was 40 degrees.

  3. Geoff Irvine says:

    I was wondering that if you make use of the "ruler trick" then 3/32" would be the closest you could have the chipbreaker without either creating a chip trap or the need to acutely sharpen the leading edge of the chipbreaker – more than the 1 degree that you find on the LN chipbreaker. The ruler trick is , to me, just a logical extreme of the normal sharpening process. The difference is that in the ‘normal’ sharpening procedure, where you flatten the leading 1" of the blades back, the chipbreaker edge contacts in the flattened 1". In the "ruler trick" situation you are aiming at just sharpening the leading 1/16" of the back of the blade. If the chipbreaker contacts in that 1/16" area would you have a shavings trap? Surely you would have to make sure the back bevel, with the ruler trick, stays at around 1/16" – any wider and you would have to regrind the front bevel wouldn’t you? I think the ruler trick is very useful and saves a lot of time sharpening and I assume these niggles I have are part of the ‘re-learning’ process changing over to it. I just remember the frustration I had when I was young with the mouth choking up.

  4. Paul,

    Thanks. Good catch. Corrected in story above.


  5. Chuck Nickerson says:

    A cutting angle of 45 deg with a chip iron at 80 deg would seem to mimic the cutting geometry of a scraper. If I’m right, then he’s right; we use scrapers to eliminate tear-out.

  6. Some of my planes work great with the chip breaker 1/16" from the cutting edge while others require more clearance. I’ve never been able to figure out why and just chalk it up to different personalities in my planes: some are ornery and some are sweet.

  7. A 1/16" is HUGE clearance by some standards (but not mine).

    If that permits the free passage of shavings, then that’s a good setting.


  8. Tom Price says:

    At one point, the denizens of the Old Tools list, led by Don McConnell, challenged the usefulness of the chipbreaker. So, I started backing up the chipbreaker to ~1/8", flattening the bottoms of my smoothing planes to get a sharp arris on the forward edge of the mouth and moving the frog up to narrow the gap between the blade edge and mouth. And that was when I began to plane through jointer tear-out on curly maple with vintage Stanley plane. It was an epiphany.

  9. Chris,

    I have to disagree a little here.

    The 1970’s Stanley 5 1/2 which I demonstrate with has a 0.003 inch mouth, and I set the chipbreaker edge as close as possible to the blade edge for difficult-ish timber. This may be as little as 0.008 inch.

    This is only possible because I generate a camber on my c/b front edge ~;-)#

    This plane never chokes, in medium hardwoods. Choking is caused by imprecise control of chipbreaker front edge angle, (or sharpness), which must not exceed 45 degrees if the front edged of your throat is square.

    If the throat edge has been filed for a 15 degree forward lean, then the C/B front edge could be 15 degrees steeper.

    C/Bs do one thing which you have not mentioned which is to pre stress the steel of the blade near the cutting edge. I do not know what effect this really has.

    Karl’s 98 works because the effective pitch is steep and can be varied at will by choosing different honing angles. Once EP is suitable and thin type two shavings are being produced the mouth could be 1/2 inch wide without causing any tearout!

    So I agree about the probability of cynical manufacturing cost cutting, but not about the causes of choking.

    best wishes,

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