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Jerry In Maine
05-13-2013, 08:19 AM
does a standard router mortising bit work ok for evening the surface of a table's spoilboard? my spoilboard is made of rather soft doug fir. ive seen some that look like fly cutters used in metal working and others that look like mortising bits. any recommendations? my table is 48" x 70" so i'd need something of a fairly large diameter so it wouldnt take all day.

ger21
05-13-2013, 06:49 PM
Either buy the largest 2 flute straight router bit you can find, or get a dedicated surfacing bit like this.
Insert Spoilboard Surfacing CNC Router Bits - Spoilboard, Planing & Hogging -ToolsToday.com - CNC Insert Tooling Router Bits (http://www.toolstoday.com/c-440-spoilboard-insert-cnc-flycutter-router-bits.aspx)

Grant Nicholson
07-12-2013, 12:58 PM
I like to use a spoilboard bit for planing hardwood as well. My 2.5" dia Amana with two wings (uses .5" planer blades) is ok, but I find it leaves a hump on entry that is hard on the gantry. I have tried arcing into the cut, but still no success because the two blades are so far apart.
It also causes a bit of tearout, regardless of feed/rpm configuration.
I'm wondering whether a 3-wing cutter would solve the problem, or if there is a bit that is designed for planing and spoilboard surfacing that has blades that are longer (closer together).

mmoe
07-12-2013, 04:21 PM
Spoilboard planers are made to cut short passes, usually around .2mm. The problem is that usually there is not much clearance between the bit center structure and the total dept of the blade, so there isn't much blade exposed at the backside of the cut. If the depth of cut is greater than the amount of blade exposed for the backside of the cut, it will have to force it's way through until it gets to the material already cleared by the leading cutters. That's where you may experience some binding on the gantry.

I have a 2 1/2" custom bit that will surface 1/8" at a time easily, and as much as 1/4" if you could be sure the part won't move, but with a 1/2" shank I doubt I'd ever push it that hard. If you really use something a lot, custom made bits are not that much more of an investment than stock bits when you're talking about a $300 indexible surface planer. I've had quite a few custom bits and saw blades made by Leitz, and they've always been excellent.

Otherwise, you'd have to make a helical or ramp entry shallow enough that there is never any more than perhaps .5mm - 1mm taken off at a time. Once you've reached the bottom of the helix, you should not have any binding for the rest of the planing motion. Helical would be more desirable, since you'd end in the same place you started and could run the tool path from there just like you simply plunged the bit. It's also a bit smoother motion than ramping and reversing the ramp to land back at the origin.

Grant Nicholson
07-12-2013, 04:33 PM
Thanks mmoe, that makes sense - I've been trying to take up to 1/8" off during some planing operations, and that's where the trouble begins. I think your suggestion regarding a custom insert tool for planing makes a lot of sense.

The link in this thread from Ger shows an interesting option too - the two extra blades could make a real difference, although I'm not sure that tool could take a deeper pass. It may reduce tearout on hardwoods though!

mmoe
07-12-2013, 04:53 PM
They are fantastic bits (the ones Gerry linked to), but I don't think you are going to be able to plunge with them. You would still need to do a helical entry because those middle inserts are for cutting horizontal motions, not vertical motions (they have a slight angle, but you'd have to plunge slowly I'd think). Here's some code that you can see a mild helical motion into the material. Depending on your machine, it would take a few extra seconds to run this extra motion. then you'd just run your standard back and forth planing motion (I,J are incremental in this case, XYZ are absolute):
Z-70.
G1 Z-75. F750.
G17 G2 X191.82 Y200. Z-75.5 I-71.82 J0. F5000.
G2 X191.82 Y200. Z-76 I-71.82 J0.
G2 X191.82 Y200. Z-76.5 I-71.82 J0.
G2 X191.82 Y200. Z-77 I-71.82 J0.
G2 X191.82 Y200. Z-77.5 I-71.82 J0.
G2 X191.82 Y200. Z-78 I-71.82 J0.
G2 X191.82 Y200. Z-78.5 I-71.82 J0.
G2 X191.82 Y200. Z-79 I-71.82 J0.
G2 X191.82 Y200. Z-79.5 I-71.82 J0.
G2 X191.82 Y200. Z-80 I-71.82 J0.
G0 Z-70.


Adding a Z value (different from the current Z position) to a standard circular motion will generate a helical motion unless your controller doesn't have that functionality (most do for the last 20 years).

All I do when I program a custom helical entry that perhaps my software won't quite produce, is this:

1: draw circle with diameter slightly smaller than bit diameter
2: generate g-code for a profile of the circle, but don't offset just follow the circle
3: determine the top of the material and the final depth
4: drop z to .5mm above surface
5: use circle g-code generated earlier, but add Z value .5mm-1mm deeper
6: copy previous line, but edit Z value to another .5mm - 1mm deeper
7: continue until final depth is achieved

example:
G01 Z-74.5 (.5mm above surface)
G02 (XY coordinates from generated code) Z-75 (I,J coordinates from generated code)
G02 (XY coordinates from generated code) Z-75.5 (I,J coordinates from generated code)
G02 (XY coordinates from generated code) Z-76 (I,J coordinates from generated code)
etc.

ger21
07-12-2013, 05:31 PM
Have you tried using a 6-8 inch ramp into the cut?