How to understand the orientation of wood fibers for the direction of best cut.

[Odie]

OK, newbies. listen up!......here is something you NEED to know:

(You will need to click on the photo illustration below.)

I went for a few years of woodturning before I fully understood this......and, it wasn't until I saw a similar illustration before it was cemented in my mind. I first saw this illustration in a John Jordan VHS video.....thanks John for coming up with the original illustration. It's probably the most important thing that stood out in my mind in the past 15yrs about that video......and, an exceptional way of seeing the significance of how to use the wood fibers to best advantage for a clean cut.

The blue lines in the bowl's interior represent the grain of the wood. If you were to slice through the bowl along the long grain, this is representational of how the grain would be orientated.....and exposing the end grain at it's toughest angle to make a clean cut.

This is to visualize, or demonstrate how to make a cut where the fibers of the grain support the fibers that are cut before it. If you were to reverse the direction of cut over that shown, the fibers do not support the cut.......THIS IS A MAJOR CAUSE OF TEAR-OUT. It can't be stressed strongly enough just how critical it is to understand this principle....it is an EXTREMELY important point, and until you "get it", your efforts in bowl turning will be plagued by not applying this knowledge.....until such time as you DO......"get it"!

The outline of the bowl profile is in black

The required direction of cut for maximum fiber support is shown in red.

The blue lines show the orientation of the grain fibers within the interior of the bowl walls.

Rules:

1.....For exterior cutting, cut from smallest to largest diameter.

2.....For interior cutting, cut from largest to smallest diameter.

The rules are simple, and I hope the reasons why they exist are obvious to you in the illustration.

Note: There will be times when the rules will need to be broken.....but to get the best, most cleanest cut, the rules will need to be adhered to.......

ooc

 

[Clifton C]

Odie,
I use a similar drawing when I’m explaining direction of cut. And usually get “but I thought you were supposed to cut down hill.†If I don’t get that response I add it, knowing someone is thinking it. I believe this is bleed over from the spindle turning “rule of thumb†to cut “downhill†i.e. largest to smallest. But as you state in your note, there are times when rules need to be broken. (Or just bent a little.)
cc

 

[MichaelMouse]

Probably the best net resource on this subject of cutting is this one. http://www.turningtools.co.uk/wtintro/wtintro.html It uses standard edge/wood terminology in explaining "with" and "against" the grain as well as sharpness angles and slicing. Chapter 7 has the basics.

You can be cutting from larger to lesser diameter and find yourself against the grain, but by and large you want to keep the edge cutting across and move the tool down grain. If you find yourself cutting against the grain, do what the flat folks do and increase your sharpness angle to curl and break the shaving rather than sliver and plunge. Might find it necessary to go to an angle we normally regard as scraping.

 

[Odie]

Odie,
I use a similar drawing when I’m explaining direction of cut. And usually get “but I thought you were supposed to cut down hill.” If I don’t get that response I add it, knowing someone is thinking it. I believe this is bleed over from the spindle turning “rule of thumb” to cut “downhill” i.e. largest to smallest. But as you state in your note, there are times when rules need to be broken. (Or just bent a little.)
cc

Very true, Clifton......

I'll bet you are right about the origins of "downhill" coming from spindle turning.

I've never really quite understood how the "downhill" theorem applies to bowl turning........mainly because the best direction for the cleanest cut reverses, depending if the interior or exterior is being discussed. I've never really embraced the "downhill theorem" because long before I heard it for the first time, I was already mentally conditioned to thinking of it like John Jordan explains it in his video of the mid 90's. John Jordan's drawing is the inspiration for the drawing I gave in this thread, but I had already known the principle rules by which the drawing illustrates prior to that........The drawing just makes the principle more understandable......

ooc

 

[Odie]

If you find yourself cutting against the grain, do what the flat folks do and increase your sharpness angle to curl and break the shaving rather than sliver and plunge. Might find it necessary to go to an angle we normally regard as scraping.

Also, very true, MM......

Sometimes, if a gouge is preferred, it's necessary to go against the grain......but, a scraper, or scraping-like cut done with a gouge can cut in places it would be difficult, or sometimes impossible to maneuver a gouge in the traditional fashion.

It should be understood that the two basic rules of cleanest cut given above, still apply, whether using a gouge or scraper.

ooc