scrub planes and camber - how does it work?

[Jacob]

Following a recent discussion I had some more idle thoughts on this topic

Anyone who has used a scrub plane knows it removes material fast and easily. That’s what it’s for.
At the same time it also removes material deep which is also what it’s for e.g. removing material from a dirty surface (paint, grit etc) by cutting mostly in the clean wood below (hence the name?).
It also does these things easily which at first glance would seem odd - more material for the same effort, how can this be?

Well the simple answer is that a deeper narrower scoop will remove more material than a wider shallower scoop, for approximately the same effort. The explanation is that effort is roughly proportional to the length of the cutting edge actually engaged and as it approaches a semi circle the volume of removed material is greatest. The scrub blade is heavily cambered, approaching a semi circle.
Hence all those rapid removal cutters such as roughing out gouges, adzes and others, roughly semi circular. It also explains the function of camber in general - faster - more material for the same effort.
It won’t be exactly the same effort of course, as the blade has to wedge itself through the material and prise it up. A scrub plane works along the grain quite well but somewhat faster across, I guess because the uplifting of waste is easiest across the grain.
Another factor with camber is that as it plunges through it also ploughs deep due to the planing angle. This reduces the need for downwards pressure as the plane pulls itself down, whereas fine shavings require more pressure (and a very sharp edge) to keep the blade engaged. One of the first things you notice about using a scrub is how little downwards pressure is need - once it's engaged it zips along anchored into the groove.

 

[Cheshirechappie]

Most of the roughing tools mentioned - including the scrub - work best across the grain. It's much less effort to plane cross-grain than long-grain, which may be a factor worth considering.

Thinking back to my college days, some work we did in the machine tools lab. looked at the energy required to remove a given amount of workpiece material by a single-point tool in a centre lathe. It turned out to be proportional to the cross-sectional area of the cut, rather than either depth of cut or length of cutting edge. I'm not sure how well this translates into hand-working of wood, though - metals tend to be more homogenous and less 'grained' than woods.

 

[Paul Chapman]

Most of the roughing tools mentioned - including the scrub - work best across the grain. It's much less effort to plane cross-grain than long-grain, which may be a factor worth considering.

While I think that's generally true, I frequently use my scrub plane along the grain. For example, when reducing a board in width, it can sometimes be quicker to take the material off with the scrub rather than a saw (depending on the amount of material to be removed, of course).

Cheers :wink:

Paul

 

[Random Orbital Bob]

Lets try again shall we. An interesting thread that was showing promise until the usual pot shots started coupled with the sarcasm followed by the personal abuse.

People that feel some sort of need to continually personally insult Jacob, would you stop please. Jacob when you use language to criticize alternative methods would you try and accept that there might just maybe possibly perhaps be other ways that suit other styles of working please. Lets have a bit of tolerance for goodness sake.

 

[Jacob]

Well yes it does work effectively along the grain but if you were to measure effort against material removed I think across (or diagonally) is probably slightly more efficient.
What I meant by "approaching a semi circle" - my ECE wooden scrubber has a 32mm wide blade with about a 26mm radius camber which is much closer to a semi circle than say a typical jack. It's all a compromise, no-one wants semi circular grooves anyway and different materials need different approaches and different depths of cut.

 

[Cheshirechappie]

Another factor that may be worth considering is the relative size and weight of different planes. A scrub (narrow iron, aggressive camber) is a small, light plane compared to a jack (wider iron, shallower camber); using the smaller, lighter scrub will be a lot less fatiguing than using the jack for the same duty on the same timber.

For an accurate assessment of the hypothesis that a more aggressively cambered, narrower, deeper-cutting iron uses less energy than a shallower cambered, wider, shallower-cutting iron, they'd both have to be fitted to planes of the same size and weight. How you then assess energy used for a given amount of stock removed I'm not sure - it's easy enough with a machine (measure it's electricity consumption), but less so with a human. How do you measure knackeredness?

 

[Paul Chapman]

How do you measure knackeredness?

The frequency I have to stop for a cup of coffee and a *** :lol:

Cheers :wink:

Paul

 

[Random Orbital Bob]

I've got a question for you plane experts. I doubt you'll have seen it (cos in woodturning) but I managed to score a stack of Yew recently from a churchyard amongst which was this:

I'm going to make a natural edge small table from it. Its too big to go through or over my planar thicknesser so I want to do it by hand. I've done plenty of planing before but never a very large flat surface. I rarely plane surfaces wider than the blade width of the plane or shoulders in mortises etc. So I have very little experience planing where stroke one needs to overlap stroke two without leaving a witness mark. In other words I'm used to planing edges and not faces so much.

Is it as simple as grinding off the corners of the blade and then using a regular smoothing or jack plane or is it more complicated?

 

[Paul Chapman]

Hi Bob,

Yes, you can simply take the corners off to prevent witness marks but I now tend to hone a very slight camber on my smoothing plane blades.

Looking at that piece of wood you have some "interesting" grain, which might cause you problems with tear out. If it does, ensure that your blades are super-sharp. If tear out is still a problem, you might want to consider using a #80 style scraper or scraper plane. If there's much material to remove, use of a toothed blade will help to remove it without tear out and you could then finish off with a scraper.

Hope this helps.

Cheers :wink:

Paul

 

[Cheshirechappie]

Oh - this could get complicated!

On mild, straight-grained timbers, it is that simple. However, the sort of wild-grained, interlocked lumps like that are the sort that can give cabinetmakers a lot of problems, and give rise to all sorts of solutions like expensive infill planes, high-angle frog planes, toothed-iron planes, scraper planes....

For starters, try the usual jack plane to get the surface level. It may well be somewhat torn and rough, but without lumps bumps and hollows. Then work on getting a finished surface by taking the corners off the smoothing plane iron, sharpening it as keen as you can get it, set the cap-iron as close as you can to the cutting edge, and taking very fine shavings. That should get you quite close to a finish, but on something like that you'll almost certainly get some tear-out. The pragmatic approach is to finish with a cabinet scraper, or (and I'll whisper this quietly so as not to enrage the planing demons) sand it to a finish with a suitable electrically-assisted device.

 

[Jacob]

..
For an accurate assessment of the hypothesis that a more aggressively cambered, narrower, deeper-cutting iron uses less energy than a shallower cambered, wider, shallower-cutting iron, ....

Not "less energy" but more material removed for a similar amount of energy.
It's not a hypothesis either - it's the very reason you'd use a scrub plane and is regularly proved by experience - it's easier to remove a lot of material with a scrub plane, steeply cambered jack etc (or a roughing out gouge, an adze etc). We know this for a fact, we are just looking for an explanation.

 

[Jacob]

....

Is it as simple as grinding off the corners of the blade and then using a regular smoothing or jack plane or is it more complicated?

You need more camber and narrower blades for rapid removal, less camber and wider blades for smoothing. Ideally progressing through several with reducing camber as you go.
Grinding the corners off leaves a trough with rounded corners - pointless, even though it is recommended in many texts.
Blade width is not simple either - with a cambered blade the effective width depends on the depth of cut.
The one configuration you don't want on your rough timber is a dead straight edge plane. This is only for board edges and is little use on faces.

 

[Jacob]

....sand it to a finish with a suitable electrically-assisted device.

Er, from start to finish IMHO. Bin there dunnit. Unless you fancy a hand tool challenge.
I've got some very similar pieces in my workshop - it's possible to plane them in parts, but without knowing quite where the tear-out will begin.
Belt sander 40 grit!

 

[Cheshirechappie]

..
For an accurate assessment of the hypothesis that a more aggressively cambered, narrower, deeper-cutting iron uses less energy than a shallower cambered, wider, shallower-cutting iron, ....

Not "less energy" but more material removed for a similar amount of energy.
It's not a hypothesis either - it's the very reason you'd use a scrub plane and is regularly proved by experience - it's easier to remove a lot of material with a scrub plane, steeply cambered jack etc (or a roughing out gouge, an adze etc). We know this for a fact, we are just looking for an explanation.

I'm suggesting that one reason that a scrub plane removes more material for a given amount of energy than (say) a jack plane is because it's a smaller, lighter plane, and is thus less fatiguing to use.

Hence the suggestion of trying different blade configurations in planes of the same size and weight, and seeing if either blade then makes a difference to material removed for a given amount of energy.

 

[Jacob]

....
I'm suggesting that one reason that a scrub plane removes more material for a given amount of energy than (say) a jack plane is because it's a smaller, lighter plane, and is thus less fatiguing to use.......

As it happens I've got three scrubby type wooden planes - similar weight but different widths/camber. It's the narrow steep cambered blade wot does the business without a doubt. You have to use one to appreciate it.

Video here. Philip C. Lowe's "scrub" is nowhere near a scrub in terms of camber. There's a lot of misunderstanding about it - I had no idea myself until I bought the ECE example. (No commercial interest here honest!)

 

[Random Orbital Bob]

Interesting feedback thanks chaps (bloomin fast too
I've made about 7 tables in my time and in all cases I've joined the boards with (respectively over the years) just glue, biscuits, pocket hole screws and dominos. Every time I've failed to get an absolutely perfect join though come close. So I really am used to belt sanding, finer grits, random orbit and then cabinet scraping by hand. Never before have I attempted something with such wild grain.

So.......wimp out and go electric is the advice. Excellent

 

[jimi43]

Good...we're back.

Ok...this is the video I simply love!

http://youtu.be/VYTVR0dIV_A

I think Giuliano did one in English but it's fairly obvious what's going on especially the diagonal action for the scrub which I think it the way it should be done initially and moving slowly to cross grain as the board flattens.

That is then followed by the jack and the jointer to a finished board.

Giuliano....I simply adore the way that your working dress typifies for me the chic that is Italy!

Great demonstration on all aspects of flattening a board.

Bob...I simply LOVE that yew log. Especially the second picture!

Any offcuts...shoot 'em my way!

Jimi

 

[Cheshirechappie]

You have to use one to appreciate it.

I have used a scrub plane. And a jack plane. Hence my suggestion.

 

[Jacob]

You have to use one to appreciate it.

I have used a scrub plane. And a jack plane. Hence my suggestion.

What sort of scrub and what radius camber?

Giuliano has got it but even his effective scrub looks a little shallow compared to the ECE 26mm radius. Nothing wrong with that there are no rules and he is planing a relatively flat board which wouldn't need the deeper cut.

PS I suppose that for effective planing in an ideal world one needs a set of planes with different cambers/widths not unlike woodcarvers sweeps. There's an excuse for buying more planes!

 

[Corneel]

I am sure I have posted this video before:

https://www.youtube.com/watch?v=LJKa9Czzy3Q

A year ago I finished my dining table, made of euro wallnut, and with handtools only. I needed to remove quite a bit of material to get all three boards for the top to the same dimensions and straight of course. The scrub removed the majority of the wood. Pretty quickly in fact, but it leaves a horrible surface, so the next step was a wooden foreplane to remove the scallops and make it really flat. That took a lot more time and energy then the initial gross removal with the scrub. The fore had a quite agresive camber too. Final work was done with jointer and smoother.

So I can't give in exact numbers, but I'd say that the scrub is a lot more effective in removing large amounts of wood the a jack or fore plane.