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straight vs. mortise?, and what bit for cleaning non-flat surface?

This is a discussion on straight vs. mortise?, and what bit for cleaning non-flat surface? within the Router Bits - Types and Usage forums, part of the Routers category; Having never used a mortising bit, and having seen the very angled bottom ends of ...


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Old 12-12-2012, 08:03 PM   #1 (permalink)
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Question straight vs. mortise?, and what bit for cleaning non-flat surface?
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Having never used a mortising bit, and having seen the very angled bottom ends of bits like this -
CMT 801.817.11 Mortising Router Bit 1/2-Inch Shank, 1-1/4-Inch Cutting Diameter, 1/2-Inch Cutting Length - Amazon.com
- I'm a bit confused as to the difference.
On one hand (ignoring non-plunge straight bits), what is the difference beyond straight bits being relatively narrow and deep vs. mortise bits being relative shallow and wide?
On the other hand, what does the blade angle on the bottom of a mortise bit like the one above do the the shape of the cut, and is that the difference?


Next, which of these bits to use:
I want to make some very large-radius (vertical edge) corners on a box. The box will be well taller then the longest straight or flush bits, and obviously with enough layers (of MDF) to have meat in the corner to cut away. I would cut these layers slightly above size, perhaps bevelled a bit with a chamfer bit so that after assembly, the corner just looks like a slightly pixellated radius. Then, the box would have to be suspended by pivots at the axis of the intended radius at the top and bottom surface, and aimed at the bottom of the router bit, and turned within a 90-degree range, and shifted over time so that the router eventually cover and planes the entire "height" and angle range of that corner - a bit like a limited special case of router lathing.

A bottom-cleaning bit, rather more expensive, would make for a flat cut on its entire surface, thereby smoothing a largish stripe of wood on each pass - but perhaps so much that the wood has to be moved quite slowly.
A plunge-worthy straight bit would be heavily used at its usually least-used point, though (particularly if small-diameter) it might get rid of more bulk quickly when moved up and down the corner rather then turning on the pivot.
While I'm not sure whether a mortise bit is primarily intended to cut down by plunging vs. cut sideways by moving around in the template, as compared to a straight bit, it seems that if the angled bottom edges do not affect a flat-bottom cut, it might be the best compromise for this task.

Any advice would be appreciated. Please correct me where I'm wrong.

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Old 12-12-2012, 08:21 PM   #2 (permalink)
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sorry, i'm not following your description of what you are trying to do. do you have any pictures of what you want to end up with?

a bottom cleaning bit is good for using a router to plane a piece that is not even.
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Old 12-12-2012, 09:41 PM   #3 (permalink)
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Hi Benedikt - I'm sorta in the same boat as Chris, not entirely sure what you are trying to do. Round over the corner of a box? Just how large of a radius are you wanting and how tall is the box?
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Old 12-13-2012, 08:00 AM   #4 (permalink)
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"Then, the box would have to be suspended by pivots at the axis of the intended radius at the top and bottom surface, and aimed at the bottom of the router bit, and turned within a 90-degree range, and shifted over time so that the router eventually cover and planes the entire "height" and angle range of that corner - a bit like a limited special case of router lathing."
Are you trying to spiral the box or just round off all the corners...top,bottom sides?
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Old 12-13-2012, 08:46 AM   #5 (permalink)
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Your description is confusing to me, as well. Fact: it doesn't matter on flatness of the bottom of a mortise - it is the vertical, parallel sides that adhere to the glue with the tenon. Mortising bits for router use have twin-axis directionality (x or y and z), whereas mortising bits for use with a drill press only work along line of pressure (usually vertical). Chamfering bits play no role that I can think of in the making of a mortise. The wood needs to stay steady when a mortise is being cut. It's kind of the reverse of a trapped board - a trapped bit will cause you a trip to the emergency room if you rotate the workpiece in a multi-axis (3d) diagonal manner. You have to consider all four positional relationships: x, y, z and time - all must be harmonious to be safe and effective. No piece of wood is worth a trip to the emergency room, PLEASE STAY SAFE!
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Old 12-13-2012, 10:59 AM   #6 (permalink)
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Your mortise will be slightly deeper than the tenon to allow for glue squeeze out, it doesn't have to be dead flat. The bit you linked to is larger than typical mortises for any projects I have done. I think the difference is semantics. I could say this is my small mortising chisel, but it would still be a chisel, if that makes any sense.

Rounding the corners can be done on the table with a round over bit with a bearing. Still wraping my head around that question, so I may be way off what you want to do.

Last edited by richtink; 12-13-2012 at 11:08 AM.
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Old 12-13-2012, 11:23 AM   #7 (permalink)
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Still thinking about the nonflat smoothing. I believe you would need a pattern to follow with a bit with a bearing. Sounds more like a job for the planer or the jointer.
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Old 12-13-2012, 04:05 PM   #8 (permalink)
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The box would be 10" tall (or more), and the corner radius would be 2" or more, so no roundover or flush trim bits (or unaffordable if they do exist).

For an easier frame of reference, picture a router lathe with a piece of 4x4 on it, and the router only moves up and down the length of the 4x4 (no depth changes), independently of the 4x4's rotation.
The 4x4 turns only within a 90-degree range, so the router bit goes from the center of one face of the 4x4 to the center of one adjacent face, thereby taking off the square corner, making a 2" radius corner.

Now for proper router lathe operation, you'd obviously have a very small and pointy bit, but since there's no depth changes, and max. evenness looking up and down the length of the 4x4 is desired, something more flat-faced is obviously appropriate.

What I'd actually do (I think) is have the router stationary, and the box, of which the 4x4 represents just one corner, would have pivots 2" inward from the square corner (analogous to the 4x4 centered on the lathe) around which to swing 90 degreees. And the box gets moved instead of the router for the router bit to move up and down the box height (analogous to the length of the 4x4).

Or the other way round, put the radius on the very top and bottom of the box the normal way with a circle jig, and then make a variation on a router ski set that lets the router skis stay raised up above the top and bottom corner curve...
That really sounds too difficult (since the wall that the skis are held up by aren't up any, and the blocks to raise the skis off the walls can slide with the skis).

For skis, it might perhaps make more sense to make 2 walls that are corners of a radius larger than the target by however much the bit will protrude out the bottom of the skis. Then these walls are clamped to the box, and the router skis all over this to plane the box corner to the wall radius less the bit depth.
(Of course the skis have to stay orthogonal to the walls, or be quite wide, so they don't dip down while descending down opposite sides of the walls.)

Actually, this last method does sound better than pivoting the box or riding on its own edges, and the template walls could be slightly more complex, like compound or changing-radius curves.

Of course, as mentioned before, I wouldn't start this with a fully solid square corner, but cut/shape the layers roughly to start with so there shouldn't be more than 1/4"-1/2" of material to remove at any point.
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Old 12-13-2012, 04:49 PM   #9 (permalink)
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Lay out the arc and you will find the pivot point to be more than 2" from the corner, it will be 2" from the arc.
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Old 12-17-2012, 07:53 AM   #10 (permalink)
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You could bandsaw the edges then finish them off with a sander down to the line.
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