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Thread: Roubo Bench Issues and Questions
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31st March 2012, 11:32 AM #241
Hi Basil
With the BU plane one only needs a single secondary bevel to create the final cutting angle. With a BD plane, two bevels are needed - one at the face (as usual) and another as a backbevel to increase the cutting angle.
How can two planes with the same cutting angle feel different when pushed along a board? I recognise that does not make sense. In fact I approached BobL some time ago with this question (since he is a professor of physics and maths). Bob did not do any calculations but suggested that it was due to the different angles of the tote. I believe that is a part of it, but there is more and that this has to do with one plane having a lower centre of gravity than the other. With the BU plane one thrusts from lower down, and (in my logic) this is more efficient that pushing from higher up at an angle. This difference becomes apparent when using the two planes alongside one another.
Regards from Perth
DerekVisit www.inthewoodshop.com for tutorials on constructing handtools, handtool reviews, and my trials and tribulations with furniture builds.
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31st March 2012, 02:13 PM #242
Derek I don't see why it is necessary to have a secondary bevel on the primary bevel, when using a back bevel. Just treat the primary bevel as you would the back of the blade under normal circumstances keeping it flat & polished. Then have a back bevel which is where the wear on the edge is removed and this replaces the secondary bevel.
If the forces applied are at different angles then yes BobL is correct that a Vector diagram may verify that for the same effective force a greater applied force may be required. I guess I will have to borrow a friends BU plane and conduct a trial to see how noticeable this is.
Regards
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31st March 2012, 03:31 PM #243
Hi Basil
Perhaps I was not clear enough. The BU plane has two bevels: the primary and the secondary. The primary is there for the BD plane as well. However, when the BU plane is honed, the primary bevel is ignored since attention is given only to the secondary bevel. Hence the BU plane has a single bevel angle for honing.
The BD plane requires that the primary bevel is honed (or a secondary in its place), but then requires a backbevel to create the the higher cutting angle. In other words, with the BD plane one must hone an additional bevel compared to a BU plane to achieve the same cutting angle (assuming that the BD plane starts with a lower bed than the desired cutting angle).
Regards from Perth
DerekVisit www.inthewoodshop.com for tutorials on constructing handtools, handtool reviews, and my trials and tribulations with furniture builds.
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31st March 2012, 10:36 PM #244
Hi Derek,
in my mind I can see the geometry Basil is talking about.
For the moment ignore the probably that the primary bevel on a BD blade is hollow ground and take it as being flat off a belt sander or bench stone.
If you polish all of this bevel you will have a polished plane similar to that sought on the back of the blade.
Now when you create the back bevel at 10° say (to give a 55° cutting angle) your cutting edge will be the intersection of two planes.
When the blade is blunt you will only renew the 10° back bevel. (I'm not sure how you would treat the wire edge, but my experience is that with some steels and fine stones the wire edge just falls off)
You can repeat the honing of the back bevel until the relief angle represented by the primary bevel on the other side of the blade needs renewing at which time the back bevel can be left alone, much as you leave alone the flat back of a normal blade.
So instead of staying in line with the back of the blade, your cutting edge migrates back and forth across the thickness of the blade. BUT you only need hone one flat bevel at each sharpening.
I'll try and post a diagram or three laterregards from Alberta, Canada
ian
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31st March 2012, 11:30 PM #245... If you polish all of this bevel you will have a polished plane similar to that sought on the back of the blade.
Now when you create the back bevel at 10° say (to give a 55° cutting angle) your cutting edge will be the intersection of two planes. ....
This idea ignores the fact that the "polished plane" is still a bevel face and that it will wear at a far greater rate than the back of the blade (where the back bevel has been ground).
You will need to do more than polish the "polished plane" to keep it this way. You will need to treat it as a bevel (which it is), and run through all the grits.
Am I missing something?
Regards from Perth
DerekVisit www.inthewoodshop.com for tutorials on constructing handtools, handtool reviews, and my trials and tribulations with furniture builds.
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