Straight versus spiral cutters

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Straight versus spiral cutters

Post by jcatcmc »


I notice in the V Carve tool data base that the term end mill is used to describe a flat bottom bit ( I always associate end mill more with metal cutting ). A spiral cutter is shown , however I have always used a 2 flute straight sided 1/4" carbide /steel bit. This has always worked out until recently doing a profile cut around a job in hardwood
(7 steps .125" ) the cutter decided to work its way down, stopped it in time. still tight in the spindle, I don`t know if it was the slight vibration in the hardwood got it moving or what. My question is should i consider using spiral sided bits for this purpose , down cut/up cut whichever? I think up cut would put pressure on the cutter to work its way down ( I do use two wrenches to tighten the collet.)


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Re: Straight versus spiral cutters

Post by peterurasky »

I have the same issue with my tool pulling out or moving my piece while cutting a round profile.
I found it to happen in the same spot, so I slowed the feed rate down and watched.
Standing if front of the CNC, at about the 4 o'clock position the X axis is at an impass to go left/right.
My guess is the 2 flute up cut tool can run at this point.
Any feedback would be great.

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Re: Straight versus spiral cutters

Post by Rando »

The larger the "helix angle" on the endmill, the GREATER the amount of pull-out force. A straight-flute is considered zero helix angle. Others are typically 30 or 45 degrees.

One thing you can do if you suspect the bit is being pulled out is after you get it in there and tighten it down, use a sharpie or other permanent-style marker to mark the spot where the bit comes out of the chuck/collet. Get the marker up in there good, and you should be able to tell if it's getting pulled out. If it is, consider cleaning the inside of the spindle/router shaft, the collet insert, the collet/chuck nut, and the bit with a degreaser like isopropyl alcohol (the 99.9%-pure, anhydrous kind). Lubrication and dirt up in any of those areas will increase runout of the bit, and allow it to more easily slip.

That said, any bit will pull out more as it gets dull :(.

The term endmill doesn't refer to the bottom of the bit, but rather that the milling action is done to the of the MATERIAL being cut. That is, "milling at the end of the bar", it's really referring to the fact that it is designed to cut along the vertical length of the bit. This is as opposed to a drill-bit, which is intended to cut into the material only in the downward direction. Indeed, for some older-style of endmill where there is no central slot on the tip, they cannot plunge at pure vertical. Even modern endmills that can center-cut will have a maximum ramping angle for entry.

Now, you wood-carving guys probably will never even notice an issue (beside bit/system deflection) plunging right in. But trust me, in aluminum, that won't work ;-).

You should be able to successfully use 0, 30, 45 degree helix angle bits in wood. If the collet/chuck is clean, if it's tight, then it should not come out. But again, if you're concerned, mark the bit and collet so you can tell if it's REALLY coming out, and not something like losing steps because the entire Z-axis is being pulled down against the cutting forces. But, at 0.125", that's not very likely.

Hope that rambling bit of information helps


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