Tear Out Prevention

[esteeme1]

Let me start with my disclaimer. I'm not educated in the realm of CNC operations other than my own trials, errors and observations with in the past 5 year. I only have my portfolio to substantiate my concepts. (Vectric tutorials have helped tremendously but mainly for modeling purposes). The technique I am providing may not work/solve every problem but I have used them effectively on several occasions.

There are two techniques I will discuss. One is conventional and climb cut strategies and the other is relief pocketing. Both are used in different situations.

End Grain Cutting
The obvious technique is the use of conventional and climb cut on the end grain. (I use this when cutting scallops out of the corners) This has been discussed in many strings. The only thing I would like to add to this technique is to position your start point on/along the long grain. When generating your toolpath select the "Use vector start points" option. This will prevent the bit from abruptly hitting the edge of the material and the bit will plunge along the edge to cut the material in the same spiral as the bit. The other part of this idea is to use the conventional and climb options to drive the bit out of the end grain and not across it. No consideration was given to using an up cut, down cut, or compression bit (my opinion is that only effects the surface of the cut and has no bearing on the edges).

Perimeter Cutting
The next technique that I have found works great is the use of relief pocketing. (I have not heard of anyone using this technique and find it note worthy. Thus.....) I learned this by cutting the scallops for my cutlass racks. As described above if you plunge into the edge of the material the bit will not tear out the opposite edge of the cut. Thus if you remove the material away from the corners the bit can cut around the edge with out ripping it out. This will require that you have a will tuned machine free of vibration and minimal backlash. A sharp bit is most helpful.
Follow these steps:
1. Create a arched vector approximately .01" larger than the diameter of the bit you will be using. example .25"EM = .26"d vector
2. Position the vector on the corner of the profile and slightly overlap the corner so it will not be noticeable on the end result place it as close to the apex of the circle as possible.
3. Generate a pocket toolpath and cut as deep as you would for the final profile cut. Using the pocketing strategy will also help to clear the hole as you drill because the bit will raise with each pass.

I hope that you find this helpful.

[Pete Cyr]

I understand the first tip but have not had the issue you describe a solution for.

The second tip has me a bit confused as to what you are doing ....I'm a bit slow so I will study a bit more.

[esteeme1]

Thanks for getting back with me pete. i will try to clarify. I think I missed writhing the last steps. The pictures are in order though

I use this technique when cutting a perimeter. I'm sure you have experienced a blown out corner when the bit comes around a corner against the end grain???? Most of the time it occurs on the second pass. It is not practical to to set up several vectors to cut out the end grain, so what I do is drill relief holes at the end of each corner. This works especially well when cutting out stars. The important part of doing this is to clip just about a .01 of an inch from the corner. So when cutting the final profile the bit doesn't hit the tip and rip out the corner.

I have found this very effective and it doesn't take a lot of time to set up and execute.

Step 4. Generate your profile toolpath
Step 5 Make sure to cut the relief holes before cutting the profile.

The result is a clean corner without tear out.

Let me know if this helped my explanation. I got pulled away from it yesterday and didn't realize I didn't complete my thought.

R/Jim

[Pete Cyr]

Ok, I get it now. Makes perfect sense.
Thanks

Pete

[armbrusterco]

Jim,

Now I understand the second tip and really appreciate your sharing it. Thanks,

[esteeme1]

Thanks for getting back with me and I'm glad it will help.

[ChrisInEstes]

I think Jim's idea of putting relief pockets in at the transition corners of edge-grain/end-grain is really good thinking. You can't tear out what's not there.

I recently ever-so-slightly changed corner geometry on some parts I make and it started chipping out on some of them. I'm about to route 80 more of these, and will put relief pockets in and see how it works. I should know how the first 3 go in an hour or so.

Thanks, Jim!
Chris

[esteeme1]

Thanks for getting back Chris, the most important part is to step the pocket relief in about .001 so when the bet travels around for a final cut it doesn't hit the corner. I'm sure you will see better results for the effort. Please let me know how this works out.

[ChrisInEstes]

I'm back from the depths of the woodshop with several routed wood signs and 100 tap handles. Whew... Now I have to catch up on everything else.

The relief holes worked great. I ended up peck drilling the sharp corners 0.005" into finished part. Using 0.005" instead of 0.001" gave the wood room to move a bit since I was cutting many smaller pieces from single slabs of wood. I didn't have a single tear out and the finish sanding left no trace of the tiny inside radius. This will now be standard procedure for me on sharp corners with wood.

Thanks for the tip!

Chris

[esteeme1]

Thanks for getting back Chris,

Glad it helped.