A Blade Profiling and Cutting Geometry Question

[Droogs]

When shaping a blank plane iron or a blade for a scratch stock etc, does one have to angle the blank? What I'm trying to work out is, if i wanted to make my own reeding, beading or ovolo shaped irons, would I in order to get a true arc need to grind/file the shape of the arc with the blank iron held at the same angle it will sit in the tool?
As i think that if it's ground while held verticaly in a vice etc, once the iron is in position and used in the tool, the resulting shaped piece would actualy be an eliptical chord rather than a true arc or semicircle. Am I right in thinking this or am I missing something?

Although been using tools for years never realy made the components before and I'm looking at doing a few experiments and just want to get my head round this concept in a plain english kind of way.

cheers
droogs

edits for dire spelling :roll:

 

[dickm]

When shaping a blank plane iron or a blade for a scratch stock etc, does one have to angle the blank? What I'm trying to work out is, if i wanted to make my own reeding, beading or ovolo shaped irons, would I in order to get a true arc need to grind/file the shape of the arc with the blank iron held at the same angle it will sit in the tool?
As i think that if it's ground while held verticaly in a vice etc, once the iron is in position and used in the tool, the resulting shaped piece would actualy be an eliptical chord rather than a true arc or semiciircle

Yes!

 

[Droogs]

@ dickm
Many thanks, very plain indeed

 

[AndyT]

Like Dick said - but in case you want a slightly longer answer...

Most home made scratch stocks have the cutter at 90° to the work, so the shape of the cut in the metal is the same as the shape of the moulding produced.

This is my favourite scratch stock, a present from an old friend

One nice detail of this design is is that the underside of the crosspiece is a double bevel, not flat. This lets you adjust the angle and work pushing or pulling. The 'average' angle will be 90° so the shape is constant.

In contrast, the commercially made Stanley 66 has the cutter leaning back a few degrees, which gives it a nice scraping action but means it will only work in one direction, and (theoretically speaking) the shape of the moulding will be subtly different. In practice, I think it makes no difference, not on the scale of work that would be done with one of these.

(The big white thing is a home made fence, from a scrap of holly wood.)