Best method for concave recess in Z direction?

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Blackhawk
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Best method for concave recess in Z direction?

Post by Blackhawk »

Hello All,

I need to make a concave recess in a wooden case. It will be roughly 1/2" wide 7/8" long and 1/4" deep. It is basically a little pocket to place small items and then scoop them out with your finger. This is a job where I will be making 20-30 cases at a time, so I want to reduce my machining time as much as possible. As a test, I was able to do this with a fluting toolpath. I made a line and then copied it many times with a .010" gap. The preview came out exactly how I wanted it. I just want to see if others have a better idea that would be more efficient based on machining times. I attached the preview image that I created.

Thanks!
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half_barrel.jpg
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ohiolyons
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Re: Best method for concave recess in Z direction?

Post by ohiolyons »

It could be done as 3d model. Possibly a moulding toolpath too.

You did not specify how long the fluting toolpath took. You didn't say whether the time was acceptable/reasonable. If it took 1 minute then there aren't many other toolpaths that would be faster..

It looks rather small so a 3d model maybe quicker than a fluting toolpath. Typically a fluting toolpath is faster, but since you put a bunch of lines close together and are cutting numerous flutes, 3d/moulding maybe faster in this case.

Please post the file for downloading.
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Blackhawk
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Re: Best method for concave recess in Z direction?

Post by Blackhawk »

Here is the file. It is about 4.5 minutes at a 2.5 factor. This file isn't the exact dimensions that I need. I was just trying this quick to see if the fluting toolpath will work. 4.5 minutes isn't terrible, but if I am making 30 at one time, knocking out 1 minute per piece is a lot of time.
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barrel recess.crv3d
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FixitMike
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Re: Best method for concave recess in Z direction?

Post by FixitMike »

You can try a wider spacing of the vectors. A flat end mill will give smoother results than a ball nose when you use the fluting toolpath. I found that vectors spaced at 1/4 of the end mill diameter produces acceptable results in most cases.

One thing about using an end mill, however, is that there will be a flat the width of the end mill diameter across the bottom of the recess. It will be not be noticeable in most cases.

Also, note that since there is a Z component to the path the tool follows, the effective feed rate is the lesser of the Feed rate or the Plunge rate. You may want to increase your Plunge rate setting since the tool is effectively ramping into the cut, and as a result will be cutting at the Plunge rate for the entire cut.
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Blackhawk
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Re: Best method for concave recess in Z direction?

Post by Blackhawk »

I did a 2 rail sweep and that is 17 seconds faster in the estimation. But, I think that the fluting time calculation may be a little slower than real life since it rapid travels to the starting side after each pass. The 3D toolpath doesn't rapid travel since it cuts left to right then right to left. The only thing is that the 3D toolpath creates a .023" deep divot at each end of the shape. Not sure if this is just a computer graphics issue or not. I have tried several boundary offsets and changing the depth below the zero plane but nothing gets rid of those divots, see attached screenshot. Screenshot shows 3D toolpath at top and fluting toolpath below for comparison

I attached my Aspire file also if someone can take a look to see how to remove the divots at each end.

Thanks
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barrel recess.crv3d
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half_barrel2.jpg
Brad

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ohiolyons
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Re: Best method for concave recess in Z direction?

Post by ohiolyons »

You can get rid of the divots with a zero plane.
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adze_cnc
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Re: Best method for concave recess in Z direction?

Post by adze_cnc »

Is the gap above the model in the material setup set to zero?

Also, divots are the blobs of grass and dirt dislodged on an errant golf swing. Hence the phrase "Gentlemen, replace your divots" found on signs in olden-days movies and TV.

I'd reluctantly call these "dimples".

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Re: Best method for concave recess in Z direction?

Post by SteveNelson46 »

For the 2 rail sweep, add a zero plane, extend the horizontal sweep setup lines to the vertical lines. Draw a box around all of the the setup lines and select it. Then use "Selected Vectors" when calculating the toolpath instead of the "Model Boundary". Add a boundary offset equal to at least half of the bit diameter.
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Smooth recess.png
2-Rail Sweep Rails.png
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Re: Best method for concave recess in Z direction?

Post by Blackhawk »

John - Thanks, the zero plane fixed the problem.

I attached my new file. What I notice with the 3D tool path is that it descends at a much steeper rate than the fluting toolpath. The fluting toolpath may not be a true arc like half of a barrel. I'll machine both as a test to see which looks better in actual wood. The machining times are really close either way. Not sure if the fluting toolpath could be manipulated to create a more true arc.
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barrel recess.crv3d
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ohiolyons
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Re: Best method for concave recess in Z direction?

Post by ohiolyons »

The resultant shape is dependent on several things. Warning math is involved and which options you checked or didn't check when you created your 2 rail sweep.

Math:

If the rails are farther apart than your profile vector width then the software stretches your profile to fit.
If the rails are closer together than your profile vector width then the software shrinks your profile to fit.

So play with you profile vector. Of course the material thickness has something to say about it too.

2 rail sweep options:

Scale cross sections with width Sweep between spans Scale to exact height

Playing with these 3 settings will change the shape too.
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Re: Best method for concave recess in Z direction?

Post by mtylerfl »

adze_cnc wrote:
Sun Jun 06, 2021 4:15 pm
...Also, divots are the blobs of grass and dirt dislodged on an errant golf swing. Hence the phrase "Gentlemen, replace your divots" found on signs in olden-days movies and TV.

I'd reluctantly call these "dimples".
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