[shobley]

After months of practicing on particle board and ply, I finally attempted some CNC milling of Mahogany (is this technically a hardwood? It actually feels kind of soft).

I was cutting this form out:

http://img.photobucket.com/albums/v5...6/6f588bbc.jpg

18KRPM and about 50ipm feed on a 1/8" endmill.

Interestingly, I was getting blowout on only two of the 4 sharp corners. My first assumption was that this was a grain/climb mill cut problem.

I've been thinking about drilling around the problem corners before doing the main outline, but are better ways of dealing with blowout problem?

[I've seen "fishtail" downcut end mills for sale, but I have no experience of working with them]

All constructive advice appreciated.

Steve

[ger21]

If there is material on both sides of the bit, then that helps a lot. The other thing, is if you start in a corner, make sure it cuts the endgrain first. Ideally, you'd want to start in the middle of a straight edge.

And Downcut's tend to tearout less, but can still do it.

It's all about grain direction, and where you start and stop.

[shobley]

Thanks for the reply.
Hmmm... so you think that drilling isn't going to help?

I thought it was interesting that while two of the corners where sharp, the opposite diagonal-pair were consistently blown out. Made me think that doing some kind of climb+conventional strategy might fix it.

[Mountaincraft]

I was cutting this form out:

http://img.photobucket.com/albums/v5...6/6f588bbc.jpg

Where'd you get the file for that project?
Hopefully it's free!

[shobley]

I drew it myself...

(in blood) ;-) *

It's pretty simple, just a bunch of circles, a square and some lines that bisect through the middle.
If I'm successful I might put together an article about building the complete prop.

Actually on the topic of the "star pattern"

I am cutting from a 0.75" plank of mahogony on my CNC, so I created 4 'slices' through the model and then glued all the parts together, but the internal faces are obviously not one piece of wood.

In the picture it looks like its been cut from a single block of wood. The only way I can think to achieve this to use a drill press and make some initial holes, then cut out the 'slices' with a band saw - *then* file like crazy to get the corners nicely flattened.

Or is there a better way...?

Steve


* [Just to explain that rather bizarre comment, the box in question "opens the doorway to Hell" it's a prop used in the movie Hellraiser. - Ed]

[ger21]

Hmmm... so you think that drilling isn't going to help?

I keep looking at the pic, but can't quite picture what you're routing, and where it's tearing out???

[michaelthomas]

I have a right hand and a left hand 1/4" bit in my tool changer (both downcuts). You can attack from both sides. It's very handy for eliminating chip out.

[CarbonKevin]

An aside, "Hardwood" has nothing to do with the actual hardness of the wood - Balsa is a Hardwood, for example. Fir is much harder than many hardwoods, despite being a softwood.

Hardwoods reproduce with covered seeds - fruit, nuts, etc. and have broad leaves which typically fall off during winter. Softwoods drop cones or release seeds into the wind, and most species stay green all winter.

[Mountaincraft]

An aside, "Hardwood" has nothing to do with the actual hardness of the wood - Balsa is a Hardwood, for example. Fir is much harder than many hardwoods, despite being a softwood.

Hardwoods reproduce with covered seeds - fruit, nuts, etc. and have broad leaves which typically fall off during winter. Softwoods drop cones or release seeds into the wind, and most species stay green all winter.

I did not know that...

Thanks for the information!
Mark
:cheers: