Maximum depth of cut

[mikejohn]

I want to remove excess wood prior to 3D carving on a 'blank'
I am thinking of using a 18mm(3/4") two flute bit.
The material is poplar or lime.
The cut can be rough, as I will be 3D carving into it later with a much smaller ball nosed bit.
What do you feel is the maximum depth of cut I should risk with a PRT96/router combination?
Thanks

..........Mike

[Brady Watson]

I would rough out between .25 & .35" per pass. If the router sounds labored and you are getting chip out, then back things off a bit. I usually do things on the conservative side at .25" deep for most woods. I leave between .02 & .04" allowance on the part for the finishing pass to clean up. If you don't have the option for allowance in your CAM program, you can cheat by nudging the Z up a little and zeroing it higher than the top of the block.

If you have a vector outline of the 3D part, it is also helpful to clear out the perimeter .125" out of the bounding box to avoid the bit slamming into the sides when it rasters back & forth.

-B

[mikejohn]

Thanks
Follow up
How much wood should I take off with the 6mm(1/4") ball bit?

.........Mike

[Brady Watson]

Well if you left .5 to 1mm of material on the part (left over from roughing), just run the toolpath zeroing at the top of the block...then you will only be taking off that little bit of allowance, plus whatever meat is left over from using a straight bit. This gives you a light chipload and a smooth finish.

If you are rastering back & forth in 3D and the ends of each pass are making a horrible noise...stop & run the bit around the outside of the part's boundary to clear out parts that the bit is hitting. This will be obvious when you run it...

-B

[mikejohn]

Maybe I should have said from the start I am only making a 'coarse' 3D to be hand finished.
Hand finished is what I want, it shouldnt look machine made.
So just a series of passes along the length, 3mm(1/8") apart with a 6mm(1/4") ball nosed bit

...........Mike

[Brady Watson]

Mike,
Run 40% stepover at the max...fine finishing is usually done @ 8 to 12% SO.

-Brady

[hespj]

Mike, it hardly needs saying, but the bigger the ball nosed bit the better. I like as big as the detail will allow. If you're hand finishing I guess detail of the coarse blank isn't the finest. 8% of 32mm (say) makes for a much bigger stepover than 8% of 6mm.

If you used a 6mm bit with 8% stepover (0.5mm) you'd be getting a very good finish. What about doing this and then distressing it? (The answer: If this a rocking horse body it will take a considerable amount of time to do that finishing cut. Sorry, i'm drifting here.)

[mikejohn]

Not the body John, it's the head.
But I am willing to have a coarse finish and finish by hand, so no two look exactly the same.
What the Shopbot does is put all the 'bits' (eyes,eyebrows, cheeks, nostrils, mouth,etc in the right place). Then it just needs cleaning up.

So you are saying keep the same 3mm spaced 'lines', which go from the top of the head to the chin, but use a large ball nose?

I wil try it, see what I get.

.............Mike

[hespj]

It depends. Certainly for the bits that you're going to finish by hand use 3mm stepover and a large ballnose. But if I understand you correctly the ShopBot will be cutting the more detailed areas to a finished level, so for here you'll need a small ballnose cutter (just smaller than the smallest radius on the surface unless you don't mind losing some detail). The stepover here might want to be much smaller i) to keep it in the 8 to 12% Brady mentions, and ii) at the low end of this for the best finish.

Personally I've ended up using ballnose cutters for roughing out too, but see how you get on.

I find these are suprisingly good for the price:

http://www.axminster.co.uk/recno/12/product-Perform-Round-Nose-Radius-Cutters-22879.htm