pocketing type engraving - best bit?

[vassilm]

Hi fellow shopbotters,
I got my Bot a couple of months ago and I am slowly progressing to taking control of this beast. I need to say that I've learned a lot from this forum, so a big thank you to everyone involved in it.
I need to do something like this:

The material is MDF, which after engraving is finished with poliurethane laquer.
The engraving is basically pocketing. I need to have a very good smooth level finish on the bottom and the edges need to be perfect too. The depth will be 1/8" (3mm).
What would be the best type of bit - straight, spiral, ball nose, upcut, downcut...?
My guess is downcut spiral endmill, but I will really appreciate any advice.
Also, I will be very grateful for any guidance about feed rates and RPM.
Thanks in advance.
Vassil

[Brady Watson]

2-flute straight would be what I would use. Downcut spirals leave swirl marks on pocket bottom.

-B

[srwtlc]

I'd recommend a 2 flute downshear FEM. I've been using the ones from Centurian Tools with very good results. The FEM tools leave a close to swirl free finish on the bottom of the cut. I recently ran 100 knife boxes in walnut with a 0.25" downcut FEM and the pocket cuts were perfect. Any tool marks should sand out quickly with MDF.

http://www.centuriontools.com/router...ECTIONSword=ww

[vassilm]

Thanks a lot, guys.
How about if the material is Corian? Same type of bits?

[Gary Campbell]

Vassil...
If material is Corian or one of the acrylic clones, use a single O flute

[vassilm]

Gary,

Straight or Spiral O? If spiral, up or downcut? (I guess the downcut is not an option due to possible welding of the chips). Also would the O flute leave a good surface on the bottom during this pocketing operation?
Also I went through ONSRUD's catalogue but there are so many O flute bits. Could you suggest which exact bits you meant?

Also, would the O be good for MDF, or is it good only for plastic and aluminium?

Another thing I was wondering about, when it comes to cutting or engraving and choosing bit type, feed rates and RPMs, how similar/different are acrylic solid surface materials like Corian from other acrylic sheet materials like Plexiglass? I am sorry if this is a super lame question.

Thanks a lot.
Vassil

[Gary Campbell]

Vassil...
I am actually using Belin Single O upspiral. Dont know the number as I bought a dozen of them from another Botter. The bottom finish is not always the best, but I have "touched" the bottom point on a diamond hone and made it a little better.

I dont cut MDF, so I cant help you there.

The Corian clones are not as brittle as the clear types and cut much easier.

[vassilm]

Thanks a lot for all the advice. This forum is such a great place to learn.

[vassilm]

I started doing it with straight 2 flute bits and I am happy with the result, except for a few hiccups. In order to cut down on production time I combine two or three different bit sizes. In the example below I do a first pass with a 20mm bit, followed by a 6mm (1.8") bit. I generate the pocketing files in ArtCam and I use the arc post for that. I get two problems:

1. Even though I zero both bits at the same spot on top of the material I get different levels of the bottom surface in some areas. The smaller bit goes deeper than the first larger bit.
I have surfaced the spoilboard so that should not be the problem.
I guess one solution is to do the whole area with the smallest bit, but I thought that would be madness.
On the attached image the center area is a leftover of the first pass done with the bigger bit and then the smaller bit went deeper around it.


2. The second problem is that at several places I have a uneven edge, which is highly repeatable and now I am sure it is in the file and not the result of vibration. See the first attached image and the following attached image:


V

[joe]

VM,

Hope someone has a good answer why smaller bits, zero'd at the same level as larger one go deeper. It happens to me all the time.

J.