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Question: How to properly do a Slope Side Profile  (Voting closed: November 15, 2019, 01:27:33 am)
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Author Topic: Side profile with Slope = wrong measure  (Read 1062 times)
Samuel
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« on: October 06, 2019, 02:27:33 am »

Hi there CamBamers,
I am simply trying to cut a 22.5° angle Side Profile with either a ball nose or a 45° v-bit, but the measure on the resulting piece of wood have a great difference between what it is and what it should be.

The piece of wood is 3/8" thick, so because I want a 22.5° angle, I drew a line 3/16" away from the sides of the piece of wood (toward the inside). I set the Side Profile to Slope, Value at 22.5 and Milling Direction at conventional. I took care of setting Tool Profile at Ball nose (I also tried with a 45° V-cutter, measure difference is exactly the same), did not machined too fast and used tiny increments for the ball nose to do a great slope job.

The measure I should get is a 1.767" wide piece of wood at it's widest, with both sides sloped, and the result is 1.690". It is a 0.077" difference! it's huge! I have repeated the cut many times, and the result is always the same, so it's most certainly not from a loose part on my cnc.

What did I not set up properly? I could probably come to the measure I need by trials and errors, but I would really like to know how to do it properly instead.

Thanks a lot in advance for your help!
Samuel
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EddyCurrent
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« Reply #1 on: October 06, 2019, 11:21:04 am »

This is the number I get for the offset and it's not 3/16 (0.1875)

offset = tan 22.5 * 0.375 = 0.155330085
« Last Edit: October 06, 2019, 14:47:31 pm by EddyCurrent » Logged

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kvom
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« Reply #2 on: October 06, 2019, 14:44:33 pm »

If you measured the space between the two lines you've drawn for the profile you'd know something was wrong.  Just draw them with the desired final width and compare to the edge of the wood.

If you compare Eddy's offset to yours the difference doubled still leave it off a bit.

As always, posting the CB file is best.
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EddyCurrent
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« Reply #3 on: October 06, 2019, 14:48:28 pm »

As always, posting the CB file is best.

Yes, this is key.
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Samuel
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« Reply #4 on: October 15, 2019, 15:27:32 pm »

Ok, finally catching a little time to answer back, sorry for the delay

EddyCurrent: Yes, you are totally right, I mistaked on the offset measure by dividing the height in two, regarding 22.5° angle was half of 45°. Fool me! I was very enthusiastic about the result, but even by fixing this, the final measures are still wrong...

I went through many tries and ran out of 0.375" thick material.... so I've been going with 0.35" thick instead. Same thing here: tan 22.5 * 0.35=0.1449"

So I drew a rectangle the size I need in the end ­(1.7677" x 2") and a smaller one inside, offset by 0.1449" on each side so 1.4778" x 1.71". Gave the inside rectangle the 22.5° angle slope and ended up with 1.734" x 1.97". A 0.03 difference on x and on y....

Odd enough, with the v-cutter, the tool path in CamBam shows the tool to be going right on the 1.767"x2" lines, but on the table, the difference is present. My v-cutter is pointy at the end, it has no flat end.

I zero my Z axis on the wasteboard itself, and after machining, I see no trace left on the wasteboard from the bit. It could've been a case of unproperly setting my Z axis, but it doesn't seem to be so.

I did checked for any loose screws on my cnc, and everything seems to be just tight. I also cut the rectangles with an end mill and the result is near perfect.

The difference is pretty much the same again, weather I use the ball nose or the v-cutter, so I'm fairly confused to where is my mistake here.

I am adding the .cb file of both ball nose and v-cutter paths.

Thanks again!

Samuel

* Table a thé 14-30 test ball.cb (15.39 KB - downloaded 36 times.)
* Table a thé 14-31 test 45DEG.cb (15.37 KB - downloaded 34 times.)
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kvom
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« Reply #5 on: October 15, 2019, 21:07:13 pm »

In your first CB the inner rectangle is inset .125   Huh
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kvom
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« Reply #6 on: October 16, 2019, 12:28:35 pm »

Another thing to consider is that with a ball mill the cut point is tangent to the ball, so when the tip is at the target depth there is an uncut portion of the slope.
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Samuel
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« Reply #7 on: October 17, 2019, 15:05:05 pm »

In your first CB the inner rectangle is inset .125   Huh
I don't think so, in both rectangles, the inset is 0.1449. Here I drew a square in the corner to make sure it's the right inset.


* inset 0.1449.JPG (61.95 KB, 943x756 - viewed 74 times.)
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Samuel
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« Reply #8 on: October 17, 2019, 15:19:38 pm »

Another thing to consider is that with a ball mill the cut point is tangent to the ball, so when the tip is at the target depth there is an uncut portion of the slope.
Well, I did choose Ball Nose as Tool Profile, so I beleive CamBam should consider this and adjust to it.

Even then, On the second .cb I posted (45DEG) I choosed V-cutter as Tool Profile, gave it 0.001 Tool Diameter and the measure of my final part is around 0.03 smaller than it should.. At first I tought it was a matter of ballnose flunk in CamBam or something, but the problem is still there with a v-cutter, so I'm even more confused as to where it comes from
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10bulls
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« Reply #9 on: October 18, 2019, 14:20:55 pm »

Hello Samual,

CamBam currently does not take the Vee angle of a V cutter into account when calculating side profiles.
V cutters currently use the same offset as calculated for ball nose cutters.
I will look into adding support for V cutter side profiles.

For ball cutters, the attached example may help visualize things.

The image shows points along the toolpaths (in an XZ side view).
I have drawn circles at each point denoting the tool radius.  The center/tip of the tool is on the toolpath.

Drawing a tangent along these circles shows what the cut stock should approach.
Measuring this tangent line and doing some math gets a gradient (from vertical) of 22.5 degrees.

Note that at the bottom of the cut, the stock will flair out due to the radius of the cutter.
The only way to avoid this is to overcut the depth (going up to one tool radius deeper in your target depth).

Also note that the top toolpath results in the cutter's tangent point being level with the stock surface at a point along the source shape outline.  This means you will end up with a distinctive 'rounding over' with ball nose side profiles at the stock surface (even though the resulting cut profile will be a straight slope).  Another way to imagine this is to think of a ball rolling over the side of a slope and plotting the path of the lower point of the ball.



* 22.5-deg-side-profile-ball.jpg (76.12 KB, 947x632 - viewed 110 times.)
* Table a thé 14-30 test ball-10B.cb (39.76 KB - downloaded 30 times.)
« Last Edit: October 18, 2019, 14:24:08 pm by 10bulls » Logged
dh42
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« Reply #10 on: October 19, 2019, 21:48:26 pm »

Hello

Another interesting thing to know is that you can use the external or the internal side of the slope with the mop. The result is not the same in both cases on the corners ; rounded if you use the internal "limit" of the slope or straight if you use the external one.

++
David


* slope.jpg (130.67 KB, 1280x765 - viewed 119 times.)
« Last Edit: October 19, 2019, 21:50:54 pm by dh42 » Logged
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