Maximum tool stick out on a fly cutter

Bob Korves said:
Sometimes it's called maimed or dead. Pushing the envelope is great macho fun until we find the limits, then it's game over -- sometimes permanently.
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It would be difficult to injure oneself with such a small machine, if it has a 40,000 RPM capable spindle this is a different kettle of fish, no one would spin an unbalanced tool that large at that speed. If you spin a large fly cutter start at the slowest possible spindle speed and increase it if possible.
If it does not work at all you will have to go a different route, rough mill then finish by grinding. There is almost certainly a shop near you that does Blanchard type grinding.
 
Art, you just need to flatten that piece of aluminum, right? What is the objection to making overlapping passes if that makes it flat? I ask because a flycutter normally uses nominal sized cutting tools, which means you will have about 1-1/2 to 2" of a 3" long, 3/8" tool bit sticking out. This gives you a cutting diameter of about 3-4", which is about the max you can expect. Using an 8" long tool might give you enough extension to span 12" but there may be enough resonance to make the finish/flatness a problem.

The cutting forces you experience depends greatly on how your tool is ground and I hope you're pretty good at tool grinding if you go for that long a tool.

Another option may be something like a Pinnacle Flycutter (https://www.pinnacleflycutter.com/) or maybe a B52 Flycutter (http://www.kristitool.com/shop/b-52-fly-cutter/). Both will do 12" at a go for a price.

Personally, I would do overlapping passes to get it flat and live with the pattern.
 
mikey said:
Using an 8" long tool might give you enough extension to span 12" but there may be enough resonance to make the finish/flatness a problem.
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Yea your probably much better to use narrower passes which are smaller as the resonance will probably give an un even surface unless you run at a ridiculously slow rpm.

you could use fine abrasive paper spray mounted to glass after machining to get the last thou or 2 of total flatness.

like that tom does sometimes.
(he's using a sacrificial surface plate but glass is also a good thing to use)

starts sanding about 2:40

stu
 
You can spin a fairly large fly cutter as long as you balance it. balance is always the key to spinning large diameter objects. It's also important that the cutter and its extensions are rigid. Your only problem then with a large diameter is to get the speed low enough. Small machines often won't go that slow.
 
stupoty said:
Yea your probably much better to use narrower passes which are smaller as the resonance will probably give an un even surface unless you run at a ridiculously slow rpm.

you could use fine abrasive paper spray mounted to glass after machining to get the last thou or 2 of total flatness.

like that tom does sometimes.
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Tom is generally going for "Optically flat" or better than 1 micron.
 
Mitch Alsup said:
Tom is generally going for "Optically flat" or better than 1 micron.
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Yeah , when he's says flat he means really really really flat. :)

But the sand paper thing he recons gets you to a couple of 10th's so for most purposes for a milling table thats probably flat enough for the majority of work.

Stu
 
Downunder Bob said:
You can spin a fairly large fly cutter as long as you balance it. balance is always the key to spinning large diameter objects. It's also important that the cutter and its extensions are rigid. Your only problem then with a large diameter is to get the speed low enough. Small machines often won't go that slow.
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Correct
100 FPM is 32 RPMs at 12", I suspect that the machine will not go that slowly.
 
This reply may not be necessary, but:

If the flycutter radius was, say, 4", at an rpm of 800, then the tip of the tool is moving approximately 20 mph. If things go **** up, you have a small but surprising problem that will be OK if you are wearing safety glasses. At an 8" radius, the tip would be moving at 38 mph. If the piece that is flying around has enough mass (say 100 grams), your safety glasses are probably in trouble.

Assuming my math is correct....
 
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