Edges tear off in 3D wood working during finishing

[cptX]

Hi friends! I have a serious problem that could lead to a cancellation of 2 months of work!

I'm trying to mill in walnut and/or pine wood a 3D shape and edges tear off during finishing!

I'm using a 6.6mm 2-flute straight end mill for roughing. Roughing runs in 2mm depth per plane letting an offset of 0.5mm from the final part for the finishing to take out.

Then I'm using a 3.5 ball end upcut spiral mill (carbide) for finishing and during that process some edges tear off (see attached pictures).

The problem is very significant because it leaves the item useless!

Is it an issue with the wood? It has happened both in african walnut and in pine so probably is not only wood related. The edges that tear off, tear off along the wood lines (no idea how they are called in english)...

What chances do I have? Should I leave thicker offset of the roughing or thinner than 0.5mm? Should I change the roughing depth? Change feed rate? Change rotation speed? Change the size of the finishing end mill?

Any ideas and knowledge would be much appreciated!

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[awerby]

Is the problem happening on the roughing or the finishing pass? If it's on the roughing pass, think about leaving more of a finishing margin. If it's just on the finishing pass, your tool may be dull, or you're taking too much at once. You can always divide the finishing pass into two, taking half as deep a cut at a time. A down-cut tool will be less likely to chew up those top edges.

The sharp edges on that volute are going to be difficult no matter what you do. If nothing else works, leave them rounder and fatter than the final part, and finish them up by hand.

[cptX]

Is the problem happening on the roughing or the finishing pass? If it's on the roughing pass, think about leaving more of a finishing margin. If it's just on the finishing pass, your tool may be dull, or you're taking too much at once. You can always divide the finishing pass into two, taking half as deep a cut at a time. A down-cut tool will be less likely to chew up those top edges.

The sharp edges on that volute are going to be difficult no matter what you do. If nothing else works, leave them rounder and fatter than the final part, and finish them up by hand.

Hi awerby, it is happening in the finishing pass. The tool was brand new and a good quality carbide. I'm using advanced finishing in catia with 0.5mm step both in horizontal and vertical areas. So I consider the step to be small enough...
Unfortunately I don't think upcut or downcut would make any difference in that volute because there actually only the tip of the mill is touching.

[ger21]

If you can, climb cut in a circular pattern, from outside to in, with a .1mm stepover.

[bostosh]

Conventional cut outward on the fins retract and pass from top to bottom.
Finish the opposing side first
The neck pocket got snagged and ripped from a climb cut.

[cptX]

Hi ger21, thanks for your reply, actually I was thinking to increase the stepover to 1mm instead of 0.5mm in order to make fewer passes from the problematic point and reduce the chances for these pieces to tear out (although 1mm will require more sanding).
Do you think that making 5 times smaller stepover will actually make it better?
I agree with you about running with climb cut but bostosh suggests a conventional cut. What would make more sense?

Bostosh, doesn't a conventional cut make more tear out usually? What do you mean by top to bottom, on the Z axis I suppose, right? That is a good idea as it will always press it a bit downwards although my tool is upcutting, so it pulls the chips up... and regarding the algorithm I'm not sure if I can change the operation on that specific position...

Also, what do you mean opposing side?

If I could work on that area in a spiral method (so with horizontal passes instead of vertical ones) I believe I would minimize the chances of tear out. My algorithm though both in spiral milling and in advanced finishing in Catia, at that point start moving also along the z axis making a lot of passes like sweeping. At that phase is where the chips tear out. I don't know how I could make it passing more horizontally at that point...

I also got an idea now to film the whole process in order to notice in the future the movement of the tool that caused the tear outs!

[tinmancarving]

I have been going through a steep learning curve with my programming and tool selection for wood here myself lately.

First, conventional cuts are your friend.

Down cut end mills are your friend. I haven't used an up cut end mill in the months now, with the exception of ball end mills for 3D work.

End mills ground for wood, by a company that specialize in wood cutting ARE different than metal cutting end mills. Take a look at Amana brand of tools. Buy some before proceeding.

Feed rate (IPT) for wood needs to be far more aggressive than for metal. Way past the comfort level for metal with one exception. Most woods do not have the strength to support fine/thin edge geometry. In those areas I locally slow the feed rate down aggressively, and/or make more, much finer cuts in those areas only. Leaving stock and hand finishing is a great option as well.

Tools in wood really want a deep Axial Depth of Cut. There are many cases where I use 7° tapered end mills with .5mm tip radius right to finish shape with no roughing passes. The tools will be buried 13mm deep at most points of the geometry with a .25mm step over.

Those are just my thoughts.

You will get there.

Rob

Rob

[bostosh]

Finish one side of a thin edge then conventional cut (push away) opposing edge, cut top to bottom -Z

[benjamint76]

So, those species of wood are not "tough" or perhaps I should say they are weak.
Walnut is brittle and can frequently chip out(did you know walnut sawdust is toxic/caustic) and pine is also normally just weak in general...(I know there is some nice pine out there but it is slightly exotic...)
These two species normally have wide grains. Perhaps you could get these species as quarter sawn? This results in a tighter grain pattern.

If you machined your part in something like birch(tighter grain) I think you would be pleased...

Here is what I would do:
1) try to find quarter sawn walnut
2) rough with a ball mill
3) finish with a ball mill
4) start at the inside of the spiral, leaving the tallest/deepest portion of the spiral for the end
5) in my opinion conventional vs climb is trivial because it really depends on the direction of the grain. I personally prefer climb
and finally for consideration, try HSS end mills instead of carbide. I was told years ago by a gentleman that shaped cutters, that HSS can be sharpened more than carbide. This can be extremely beneficial when machining softer materials.

[KH0UJ]

Have you tried lowering the resolution of your autoCAM program? in our case in the shop some wood will tear off or break when the cutter is getting dull, lowering your resolution of your autoCAM will consume time but with a finer spacing maybe it`ll do

[RCaffin]

You are using carbide - but carbide is never as sharp as good HSS. And you need SHARP. Never ever use your HSS wood cutters on metal.

Also, cutting against the grain will always make problems. Cut with the grain so the fibers are flattened down, not lifted up.

Cheers
Roger