Bad machining marks

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martin54
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Bad machining marks

Post by martin54 »

Can someone tell me what causes this sort of thing & if there is any way to prevent or at least reduce its effects. This is Beech, I have machined 2 of these & this has only happened on one of them & it is only the one area, the 2 pictures are the outside & inside of the part at the same point.
The other question that follows on from the first is can I repair this in any way without it being to obvious? These marks are to deep to just sand out without altering the shape :lol: :lol:
vase rough1.jpg
vase rough2.jpg

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TReischl
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Re: Bad machining marks

Post by TReischl »

Hello Martin,

The cause is cutting against the grain. Or think cutting uphill on the grain. Common on two places on a circle. Think of the wood fibers like hairs on a dog, pet him one way and those hairs lay all nice and flat, pet him the wrong way and those hairs all want to lift up.

Hand plane users are very familiar with this issue.

About the only cure I know is to take a light finish cut. If you are cutting full width of the cutter there is no cure because at some point the cutter will be trying to lift those fibers up and rip out a chunk of wood.

It is also common when turning bowls on a lathe, that is why turners always cut down hill on something like a bowl, as soon as they try to go uphill it is a disaster.

Ok, I know this is anathema, but believe it or not HSS cutters produce way less of this sort of thing. That is because they cut rather than plow.
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Re: Bad machining marks

Post by WNC_Ed »

I have gotten a similar look on soft wood on a router table cutting against the grain with a flush-cut bit.
Did your first part have the same grain orientation?
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martin54
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Re: Bad machining marks

Post by martin54 »

Both were cut from the one board at the same time & exactly the same orientation but this has only happened on the one. Not had any problems machining Beech before, this is the first time I have seen this sort of thing.
Is there any way to repair this or tidy it up ? I was wondering if it had anything to do with the fact that this was a thicker bit of stock. The Vectric project has 2 bits at 3/4" thick & I have made a couple of these using 19mm Beech without any problems. I recently picked up some timber from a Building Supply Company that was shutting down due to retirement & the wood I bought was mainly Ash & Beech, some of it is 40mm thick so I cut the vase centres as one piece rather than two :lol: :lol:

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TReischl
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Re: Bad machining marks

Post by TReischl »

It gets worse as a cutter dulls.
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Adrian
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Re: Bad machining marks

Post by Adrian »

TReischl wrote:It gets worse as a cutter dulls.
My thoughts as well. I've only seen it that bad with a tool that has lost its edge and/or is being pushed too hard. I adjust the feed rate of my tools based on the distance and material type they have cut in their lives to compensate for the wear to a degree but on something like that I would always go for the newest, best quality tool I had.

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Re: Bad machining marks

Post by LittleGreyMan »

I agree with Adrian and Ted.

But the culprit could simply be the wood itself: I used to cut serial parts (2D and 3D) which were strongly hold by a specific jig and sometimes some parts simply didn't cut well. They looked like cut against the grain or with a dull bit (often partially), but the next part with the same cutter and the same toolpath was perfect.

That's the problem machining solid wood: unlike manual cutting, it won't self-adapt to a particular piece of wood. I never found cutting parameters allowing to perfectly cut all pieces of wood of the same shape.

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Re: Bad machining marks

Post by ger21 »

Both were cut from the one board at the same time & exactly the same orientation but this has only happened on the one.
That points to the wood (more specifically the grain direction, or hardness) as the culprit. Softer woods are more prone to tearout. I get it with poplar a lot.
Sometimes climb cutting can help, but not always. Larger diameter bits can help, as well.

We used to make some parts where we used a right hand bit for one half, and a left hand bit for the other half, to guarantee we were never cutting against the grain. That's the only way to totally eliminate it 100% of the time.
Gerry - http://www.thecncwoodworker.com

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martin54
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Re: Bad machining marks

Post by martin54 »

Maybe it was just the wood then, it is a brand new cutter & this happened on the first of the 2 parts machined. 85-90% of the first part is good & the second part is 100% good. I did wonder if it might be the wood, this is the middle part of the Inlay Vase project & it's a sort of U shape, if you look at the 2 pictures one is the outside of the U shape & the other is the inside. The marks on both the outside & the inside are in exactly the same place. :lol: :lol:

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Re: Bad machining marks

Post by WNC_Ed »

One thought would be to cut it out a couple/few MM larger than the finished part and give edges a coat of sanding sealer or similar to firm up the grain.
When it drys run your finish path. Certainly not a production friendly solution but might help utilize the wood you have.
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Re: Bad machining marks

Post by woody53 »

I agree that a dull cutter, speeds and feeds all could be the culprit, but some pieces of wood will just do that. I can't imagine how many thousands of feet of lumber I have routed and planed, in 46 years , and seen the same board be perfect on one end and a disaster of rips and tears on the other. Its Wood !! Just my observations.

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Re: Bad machining marks

Post by Xxray »

Most of it can easily be blended in with some fine grit [120+] sandpaper if you are willing to put in the time and effort required.
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Re: Bad machining marks

Post by Leo »

Personally, I have never found a way to make that NOT happen. It's the nature of the beast with wood.

BUT

Knowing that it DOES and WILL happen can at least make you feel a "little" better and help a "little" to plan the next cuts.

Reversing the direction of the spin of the cutter in that grain orientation would help - but ONLY in that spot. If you reversed the cutter rotation - ie, climb mill vs conventional mill for the entire toolpath will nothing at all more than move that tearout to someplace else in the cut. So - that is not a solution.

impregnating the wood with something to hold the grain together, like gallons of CA under intense pressure might help. Not practical at all.

What I do------

Try like crazy to NOT use those woods that tear out.

Hard Maple is good. Cherry is good, but it is prone. High Density woods are best. The harder the wood generally is better, but not always. Some exotic woods splinter like crazy even though they are really hard.

Sometimes these cuts are better to leave material and use a spindle sander.
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Re: Bad machining marks

Post by ger21 »

If you reversed the cutter rotation - ie, climb mill vs conventional mill
Changing the direction of cutter rotation is not the same thing as changing from climb to conventional.
Gerry - http://www.thecncwoodworker.com

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Re: Bad machining marks

Post by Leo »

ger21 wrote:
If you reversed the cutter rotation - ie, climb mill vs conventional mill
Changing the direction of cutter rotation is not the same thing as changing from climb to conventional.
correct = my bad
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