This is a proposal to purchase a “fancy” surfacing bit that the CNC could use to clean up the spoil board but also surface materials on the CNC (kind of like a huge planer).
This cutter has 4 blades, two provide the side cutting and two provide the actual surfacing. It is ~$180.
So the proposal is for $220 for the cutter along with one set of replacement blades.
The idea is that users would buy their own blades for them to use for personal projects.
I vote Yes.
Yes
yes
Yes
Yes!
Bill
I think Brad has the right idea, but I’m wondering about this bit:
https://www.amazon.com/Amana-Spoilboard-Surfacing-Rabbeting-Flycutter/dp/B00O4THVNC/?_encoding=UTF8&psc=1
Slight more expensive (additional $75), but nearly 2x the cutting space (1.5" for Brad’s suggestion, 2.5" for this bit)
People who have actually done the surfacing can comment more intelligently than I can, but it seems like the additional expense is more than made up for by the increase in speed in what’s a very long process.
That is a big bit.
FWIW the bit Tim was using was about the size of the one you’re talking about, and it seemed like a half day operation.
Not sure how much he was taking off in a pass but when I talked to him he was using a much slower speed than what everyone used in the Joe’s CNC forum. When I did it I think I was close to or above 400 inches per minute.
I was going at around 100 inches per minute for a majority of the cut. It really had issues around the sides (those damn nails) and I slowed it way down for them. The bright side though, I didn't see any definitive damage to the blades even though it hit tons of nails.
There should not be any nails in the board. The spoil board should be held down by glue only. The bottom layer is bolted to the frame. I believe John said he was adding it to the curriculum but the nails should only be a quarter to three eighths inch longer then the workpiece. That way they don’t get broken off in the soil board.
Well there’s your problem. You are using a soil board. . . . .
Dirt is pretty tough stuff. Stops planes and cars all the time. 
I’ll join in. Brad is on the right track in what he cautioned up above.
There is a limit and a bigger tool is not necessarily the be-all-end-all solution. The tool must be sized to the spindle. A larger tool will have more inertia and a spindle motor can burn out trying to spin it. Larger diameter cutters are generally run at lower RPMs because of cutter perimeter surface speed concerns. Motorized spindles typically generate maximum horsepower at higher RPMs. Spindles remain at higher risk of burn-out spinning heavy tools at slower speeds with added cutting loads.
There is a SAFETY issue with larger tools and higher speeds. Larger diameter cutters with inserts and piece parts (may or may not apply?) are an added risk if/when they break. The extra radius and cutter perimeter surface speed has a larger moment arm that will throw broken pieces further with more force.
CNC machining can get dangerous. That is why appropriate guarding is included on commercial machines. Hobby-use machines with smaller cutters are perhaps less of a risk, but a risk just the same. It is a particular concern for the Hive when our discussion threads talk about exposed clamping choices in and around the cutter and toolpath. Mistakes will happen. Tools can break while cutting. It is a VERY COMMON failure situation to drive the cutter into a clamp, even in production shops with experienced users. I’ve used small form-factor aluminum hold-down screws myself (rather than steel screws) for this very reason. It is safer when you accidentially drive the cutter through a softer aluminum screw than a steel screw. Believe me. We have an especially high risk with newbie operators running first-time programs. Dry-running programs (with no cutter in the spindle) and running pre-machining toolpath verification software are appropriate precautions, This is not failsafe either. Excessive cutting forces loosen and pull a tool down in the spindle, unexpectedly and rapidly changing what would normally be a safe cut. The typical CNC is much more dangerous than the typical 3D printer or the laser because of the physics involved in CNC cutting.
Finally there can be performance issues using larger diameter cutters to surface the spoil board. Think about the vertical alignment of the spindle rotary axis relative to the table. Everything has a level of perfection/tolerance. On a CNC machine, static alignment is one thing and dynamic alignment can be another. The ultimate machine is perfectly aligned and has stiffness and rigidity under load to maintain the alignment. A lesser machine can be statically miss-aligned and will demonstrate compliance under load that further degrades the alignment dynamically.
In this case, think of two orthogonal angular orientations of the spindle axis perpendicular to the table surface. Assume you are standing at the front of the machine and the tool is making passes traveling front-to-back or back-to-front.
One angle is in the direction of the cut. The tool axis either leans forward or backward in the direction of the cut. Now think about a large diameter cutter. If it’s circular bottom surface is not perfectly parallel to the table, the tool axis angular lean (static and/or dynamic) will plow a semi-circular scallop into the table. Successive passes will look like ocean wave scallops side-by-side.
The other angle is perpendicular to the direction of the cut. The tool axis either leans left or right relative to the direction of the cut. Now think about a large diameter cutter. If it’s circular bottom surface is not perfectly parallel to the table, the tool axis angular lean (static and/or dynamic) side-to-side will make a flat surface at a slight angle to the table. Successive passes will look like clapboard siding on the side of a house.
Such effects are always present. You will see less effects using a smaller diameter cutter. There is always a tolerance which may or may not be noticable for what you are trying to do. There are always trade-offs. Smaller cutters will produce smaller scallops and clapboards but take more passes and time to surface the table. Larger cutters will produce larger scallops and clapboards but take less passes and time to surface the table. Like layers of an onion, each time you learn and see one layer there is a next layer to see and understand as you learn. It is always complicated, but that is the joy of learning. Know what will happen before you push the button and have a plan to react when it goes wrong. Have fun and be safe my friends.
JimD
So it sounds like Jim is leaning for the 6" diameter multi head cutter from China . . . .
Seriously we may need to think about safety around exploding bits. 24,000 rpm is smoking fast, the loads on the tooling are tremendous. Most tooling has a max RPM rating for that reason. I wonder if there is any kind of guarding arrangement that is not to intrusive. The dust collector would probably catch some of the smaller shrapnel, but if a large cutter exploded I would not want to be nearby.
Brad, lets not forget that last week the CNC machine was wildly spinning up to 24000 rpm when set at a spindle speed of 10000...
So it sounds like Jim is leaning for the 6" diameter multi head cutter from China . . . .
Could we get the 12" bit? I know 6" seems like a lot, but 12" is obviously better. Also I’m thinking I can probably make one out of angle iron, jb weld, and hot glue. Maybe we really want to save some money here? Safety is SO over rated!
All joking aside, if Jim is right, and the 2.5" bit is a safety problem, let’s not use it. However, if it’s not a problem, I think the added expense is worth it.
Seriously we may need to think about safety around exploding bits. 24,000 rpm is smoking fast, the loads on the tooling are tremendous. Most tooling has a max RPM rating for that reason. I wonder if there is any kind of guarding arrangement that is not to intrusive. The dust collector would probably catch some of the smaller shrapnel, but if a large cutter exploded I would not want to be nearby.
Putting it in the corner would partially address this by reducing the amount of open area to just two sides. Further one of those sides would be right next to a very long table, which removes another 4’ from being accessible to people. So that would leave 1 short side totally open, and one long side partially open. Finally maybe a plexiglass shield for the remaining sides isn’t a bad idea?
Another possible solution would be to put it into the current lounge area, and operate it from the other side of the glass window that’s currently covered up.
1 1/2" sounds good to me.
as a heads up - there are pallets of mdf in an upcoming auction. is this something worth bidding on?
http://bid.bidfta.com/cgi-bin/mnlist.cgi?mcmann987/category/ALL
This is hard to say mdf is easily damaged by water. However getting an entire pallet at the price of one or two sheets might be worth that risk (imo). We would cetainly not have to feel bad about replacing the spoilboard often if the purchase went through.