Wood Stabilised wood CNC machining

[ong.kelvin]

This is my first time sending my design out for machining. I got a call from my machine shop saying my stab wood has cracked and a piece has broken off. The shape of body was machined first and it cracked when a hole for the battery was being machined.
So I decided to drive there with another piece. This time we decided to try drilling the hole for battery first then work the body. It didn't crack! So work continued the next day, machining out the area for the chip and wiring. Get a message from the shop, "bad news" followed by a photo of a hairline crack which wasn't even at the thinnest section. The thinnest area of the wood is 4mm.
Anyone else encountered this? Is stab wood just so brittle or do I have a bad batch of stab wood?

 

[ong.kelvin]

For the first image(red) it's probable that the grain and gaps in that area were brittle hence the crack.
In the 2nd pic it has no reason to crack. Machine shop has used a slow speed on its spindle to avoid heavy vibrations.

 

[Fyla]

I think that on first block (red one) wood is not stabilized good. To much open grains and pores. Thats why it cracked.
Also, slow speed of spindle isn't that good solution, that way you tear wood apart instead of cut it. Ofc, tool needs to be really sharp and well balanced, that way there will not be vibrations.
And yes, its better first to drill hole for batts and after shape it outside.
I would suggest to do final external shape with sanding. That way tension to the wood is not big, and you minimize potential cracks

 

[ong.kelvin]

So drill the holes for batt and electronics compartment then shape the body?
I will do all the corner and edge radius by hand but I will need the machine to shape the body for me, not confident to sand the body to its shape as the rear is curved.

 

[zoiDman]

This is my first time sending my design out for machining. I got a call from my machine shop saying my stab wood has cracked and a piece has broken off. The shape of body was machined first and it cracked when a hole for the battery was being machined.
So I decided to drive there with another piece. This time we decided to try drilling the hole for battery first then work the body. It didn't crack! So work continued the next day, machining out the area for the chip and wiring. Get a message from the shop, "bad news" followed by a photo of a hairline crack which wasn't even at the thinnest section. The thinnest area of the wood is 4mm.
Anyone else encountered this? Is stab wood just so brittle or do I have a bad batch of stab wood?

Machining Stabilized Wood can be a Test of Patience. And a test of one's Pocketbook sometimes also. It's Very Hard to say what the Best Approach is. Because each piece of wood can have Unique Grain Directions.

It is Also very Hard to say what went Wrong by looking at a Picture. Because there are So Many parameter that are involved in Machining.

Sometimes when doing a piece that is going to have Thin Sections and or Hollows it can help to do a Cut Out, Apply Wood Stabilizer, let it Fully Cure and then Fill the Cavity with something like Clay or REM Wood to give it some support.

When doing a Pocket/Cut Out, I like to start from the Outside and work in. This is kinda Counterintuitive to conventional Machining. But it can give the Walls more support. And I will take Many Depth cuts verses doing 2 or 3 Deep Plunge Cuts.

And sometimes using 2 or even 4 Flute regular Endmills works better than using Traditional Wood Cutting Bits.

Whatever tools you use to Mill with, they need to be Very Sharp. And Smaller Diameter Tools will tend to put less load on the Work Piece than Larger Diameter tools. And taking Many Light Passes is Almost Always better than taking a smaller amount of Heavy Passes.

Higher RMP is Usually better than Lower RPM. And of course, use the Shortest Length of Cut tools possible to increase Tool Stability.

Setting up a Shop Air spray on the Cutting Tool can also Help Greatly. But also make a Huge Mess in the Process. LOL

 

[Fyla]

So drill the holes for batt and electronics compartment then shape the body?
I will do all the corner and edge radius by hand but I will need the machine to shape the body for me, not confident to sand the body to its shape as the rear is curved.

Yes, first drill inner compartments. That way you have thicker walls that prevent cracking. Never drill whole hole with one pass. Use multiple passes and every time go out with tool to move away drill dust and lower tensions.
Any shape can be sanded. With something like this: