bench planes – tapered or parallel blades?

This article is about blades used in wooden planes(I originally submitted most of this research in a discussion hosted on woodcentral, with thanks to very helpful observations and constructive criticisms from Steve Voigt – you can see the full discussion here: woodcentral.com/parallel-vs-tapered

)), but there is an interesting parallel between the blade choices available to wooden plane users at the start of the 20th century and the choices facing metal plane users at the start of the 21st (the contemporary choice is between thick vs thin irons, rather than tapered vs parallel, and we’ll take a look at that in a future post).

Old wooden planes were generally equipped with thick irons, and as a rule, these were tapered from tip to the toe, so that the fat part is at the cutting end.

This is a convenient arrangement for a wooden plane since the blades are held in place by a wedge and the taper of the blade increases the wedging effect.   This means you need to tap less hard on the wedge to hold the blade firmly in place – a good thing, since if you clout the wedge too hard it can damage the body of the plane.

two of my blades, both attached to their respective cap irons. The tapered version is on the top, and the parallel on the bottom.

Tapered vs Parallel blades

Parallel blades for use in wooden planes can be found advertised in several catalogues from the turn of the 20th century:

The Howarth catalogue has a parallel iron as an option on all the bench planes, but the Nurse catalogue offers it for smoothers only. You can see similar options in the early 20C Marples catalogues too. 1)these catalogues are available from http://taths.org.uk/shop 

As you can see in the catalogues, by the end of the 19th century parallel irons were being offered as a more expensive alternative to the standard tapered offering.

It is not clear exactly when parallel blades were first introduced for wooden planes, but here is a snippet from Turning and Mechanical Manipulation Volume II  – published in 1846 by Charles Holtzapfell – which adds some (inconclusive) support for the idea that it was an option widely available only towards the end of the 19th century, in as far as it implies that  “ordinary” (bench) planes only used tapered blades:

Parallel irons were certainly popular with users of the high-end metal bodied planes made by Spiers, Slater and Norris from the 1840s onwards

here is my Slater plane with a (probably original) parallel blade

 

Many models of these infill planes – so called because of the wooden components that fill the metal body – have a lever cap that is screwed tight to hold the blade, and this that the additional wedging effect created by the shape of tapered blades is of no advantage.

But why would makers try to sell the same parallel blades to wooden plane users, where the shape requires more wedge pressure to hold in place, and is thus a disadvantage in relatively fragile wooden planes?   I suppose it may simply have been marketing, the parallel blades being more attractive to some simply by association with prestigious inflill planes, but there is another, perhaps more plausible theory, which relates to an inherent limitation of tapered blades.

Like all blades, tapered blades get shorter as they are sharpened and – because metal is removed from the fat end  – this means they get gradually get thinner too.   A consequence of this is that when set up in the plane, the blade tip ends up being slightly further away from the leading edge of the mouth each time it is sharpened.  The effect is to create a mouth gap that gradually widens over time (compare parallel blades where the mouth gap stays the same no matter how much they are sharpened).

Quite likely this fact would be of interest to people who like to keep a tight mouth on their planes to control tear-out 2)one of several options to reduce tear-out, c.f previous posts on this blog for an explanation.

This topic is discussed in Modern Practical Joinery, by George Ellis (1902):

Bench Planes.—The Jack Plane, f. 1, is the first plane used in preparing stuff, its purpose being to remove irregularities left by the saw and produce a fairly smooth surface. It is also used generally for reducing scantlings quickly. It consists of a beechwood stock 17 in. long by 2 3/4 by 3 in., with a 2 1/4-in. cutting iron and similar back iron. The cutter is better parallel or gauged, as once fitted, the wedge will then always sit properly, and the size of the mouth remain the same throughout. This applies to all planes whose cutters are fixed by wedges.

and in William Fairham’s book :

It is clearly a fact that as tapered irons are used up they get thinner and thus the mouth opens, but is this a compelling reason to switch to parallel blades?

At this stage we should note another feature of wooden planes, namely that their soles need periodic flattening because of movement caused by changes in moisture levels or due to wear and tear.  If you look closely at wooden bench planes you will see that leading edge of the opening in the plane in which the iron sits is angled forward to allow shavings to more easily escape, and this means that as the plane is trued up and material is removed from the sole, the mouth widens.

In the spirit of scientific research I bought a parallel blade to replace the worn out tapered iron in my wooden jointer.

The used up blade on the right is from Thos Wales – the last date I can find for this maker is the Midland Tool works, Sheffield in Whites Directory of Sheffield & Rotherham – 1911.

The previous owner had created a small leather packer to fit under the blade in an attempt to close the mouth of the plane somewhat.

The plane itself was made by Thomas Turner, a maker that apparently traded up until 1912. Here is an example of their trademark (from another plane, the trademark on the jointer is a littler hard to make out)

 


Although not relevant to this little experiment, the replacement iron is made by I Sorby (coincidentally it came with a cap iron made by T. Turner).

Extrapolating from the worn out Thos Wales blades’ taper I calculated that the it would have been 3/16” (4.7 mm) at its widest point when new, assuming there was around 2 1/2” inches of useable iron originally. In it’s reduced state the iron is 5/32” (3.9 mm).  The effect of a lifetime of sharpening was to widen the gap between the tip of the blade and the leading edge of the mouth by by about 1/32” (0.8 mm).

wide mouth on my ~100 year old jointer plane

 

How does this compare to the inevitable widening of the mouth that comes from the wearing and flattening of the sole?  I worked out that, had my plane lost a 1/4” of an inch from the sole over the length of its life 3)as is plausible, since this would have meant it was originally 3 1/4” square, rather than the 3 1/4” x 3” that it is now then the distance between the blade edge and the front of the mouth with a full length tapered blade would have been half what it is now.  This means the mouth gap was originally  1/16” (1.6mm) and this has been increased by around 1/16th of an inch through a century of flattening.

sums

So there you have it, on a not very scientific survey of one plane, over around 100 years the mouth opened up 3/32” (2.4 mm) – a fair old gap – and about a 1/3rd of this was caused by sharpening the tapered blade and the rest from flattening the sole.

Considering that plane mouths inevitably widen due to periodic flattening, and seemly at a faster rate than could be offset by using a parallel blade, it is hard to disagree with Steve Voigt in the aforementioned woodcentral discussion:

it seems like parallel irons were something that was offered at the twilight of the woodie era, based on some misguided notions about blade life and mouth opening. Interesting, but mostly a historical curiosity, I think

References   [ + ]

1. these catalogues are available from http://taths.org.uk/shop 
2. one of several options to reduce tear-out, c.f previous posts on this blog for an explanation
3. as is plausible, since this would have meant it was originally 3 1/4” square, rather than the 3 1/4” x 3” that it is now

One Response to “bench planes – tapered or parallel blades?

  • John Cadd
    1 month ago

    I have a splendid chisel made by Thos Wales and Sons Sheffield 1918.
    It needed a new handle .1 1/4 inch wide with 4 inch blade left. Something quite special about a ninety nine year old chisel .

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