Widen End Mill Cut?

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Widen End Mill Cut?

Postby Jim@HeirloomTable » Sat Jun 23, 2018 9:31 am

Have 1/4" end mills with 3/4" cut length, and want to cut through a 1-1/4" thick piece. Tried, but the non-cutting part of the bit is binding up on the slot when the cutter edge gets below top of wood. I've ordered a couple end mills with 1-1/2" cutting length to use, but is there any way to program the machine to offset passes just slightly so that the finished cut is slightly wider than the bit? Thx! Jim
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Re: Widen End Mill Cut?

Postby Rando » Sat Jun 23, 2018 12:36 pm

If the inside of the pocket has a slope to is, a couple degrees, and you use an appropriate 3D surfacing (not pocketing) toolpath, it should mirror that. Works in some situations. The other is to use a "reduced-shank" extended-reach end mill. In those, the flutes are only about 1.5D (1.5 times the bit diameter) long, and then behind that they reduce the diameter by 0.010" or so, with no cutting edges. This helps by reducing the ever-growing torque and deflection as the total engagement length grows with each deeper cut.

Then, if you can adjust-out the sag so you spindle is as true as it can be, you'll have good luck.

But, remember, a lot of whether you're successful with this depends on how much cutting-force induced bit/spindle/z-axis/gantry deflection are generated by the cutting parameters you choose. If you're trying to get a deep pocket, you'd better cut gently, or you'll end up with this:


That's one of those extended-reach bits, trying to clean out the radius of a piece of aluminum angle. That was caused by the extended reach shank hitting the uunprepared material above, because the cut started with more radial engagement than the shark's deflection could handle. The trick was to prep the top surface for in-tolerance but in-the-way material and take far-thinner (0.080" SO vs 0.010") but faster speed (20-25IPM vs 50-60). Much safer, and in many cases as fast overall, with less bit wear. Sadly, in this case, because ruining the bit and part weren't enough, it then picked up the part, started spinning it, and proceeded to move across the Shark bed and hit the OTHER vise and knock it out of alignment too. Nothing a good 4 hours of adjustment won't fix. :(

With practice and careful alignment, using a 1/4" diameter 2.25" extended-reach bit, and having them add an 0.030" radius on the tips so they don't turn it into a drill, allows me to cut concentric 9 and 12mm holes through a 2" block of aluminum...on a Shark HD2+. Unfortunately, that STARTS with a $78 bit and adds another $19 for the radius grinding. Best to buy them in sets of five for the 15% discount! :shock: But, my buds over at Swift tool get me what I need to get the job done.

Hope that helps: use a slightly sloped walls and 3D toolpaths when feasible, adjust your machine to as close as possible (the process is called "tramming") for that bit to be exactly vertical, keep your cutting parameters gentle, and if feasible, go with reduced-shank extended-reach bits. But, avoid the full-fluted bits. They are not your friend for deep pockets. Or deep slots for that matter. Well, except maybe for deep finishing passes, but that's a whole 'nother topic.


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