Lithophanes with uneven bottom border

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Lithophanes with uneven bottom border

Postby sifolk » Sat May 02, 2015 5:51 pm

Hi everyone. We have been experimenting with lithophanes recently and they are working really well except at the bottom right the picture (where the cnc starts). Even though the photo has an level bottom. It slopes up by 4mm or so in the right corner. We are using 5mm acrylic and carving max depth of 4mm with a. 5mm radius cutter.
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Re: Lithophanes with uneven bottom border

Postby roy_okc » Sat May 02, 2015 6:12 pm

A photo or few would probably help.
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Re: Lithophanes with uneven bottom border

Postby tonydude919 » Sun May 03, 2015 3:46 pm

Always post a photo of the problem your having, makes it a lot easier to help the person out.

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Re: Lithophanes with uneven bottom border

Postby sifolk » Sun May 03, 2015 6:05 pm

Very good point. Here is the picture with the problem in the bottom right corner. You can see the slope.
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Re: Lithophanes with uneven bottom border

Postby Xxray » Sun May 03, 2015 6:15 pm

What do you use for hold down ?
Might be a case of deflection on the 1st line, when stress on bit & material is the greatest.
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Re: Lithophanes with uneven bottom border

Postby sifolk » Sun May 03, 2015 9:47 pm

We use some very strong clamps. There is no chance of movement. What exactly do you mean by deflection. If it was happening on the first line would it not show the error in the corner. Thanks for your help Xxray.
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Re: Lithophanes with uneven bottom border

Postby Xxray » Sun May 03, 2015 11:57 pm

Are you talking about the slight unevenness of the bottom cut compared to the bottom edge ?

A bit strains its hardest on the 1st pass, as the tip is 100% cutting through the material - On all other passes, it is only cutting through part of the material and doesn't strain as hard. It is a battle between the material vs bit, spindle/router and machine [and also whatever hold down method you use], sometimes the material wins and when this happens, the bit can deflect or snap, the machine itself can lose steps, or the material will come out of position.
Not saying that is the case here, I'm not sure I'm grasping what the problem is.
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Re: Lithophanes with uneven bottom border

Postby Mike-S » Sat May 09, 2015 12:42 am

A bit strains its hardest on the 1st pass, as the tip is 100% cutting through the material

I've found it helpful to put a boarder vector around the picture and cut it first at 1/2 the material depth. That can be done with a profile "ON" the vector and can be made with a couple passes. Then run the litho file (with the same tool) and the bit has far less material to cut on the first pass. Also the boarder adds a little class to the litho.
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Re: Lithophanes with uneven bottom border

Postby davemartin88 » Sat May 09, 2015 11:37 am

What rastor angle are you using for cutting this litho? I usually use 315 which starts at the lower left hand edge and then cuts across diagonally. You only have the bit fully cutting in to the material on the first few, very short cuts that way.

With a small stepover value, that should minimize any deflection in the bit. I'm also assuming that you don't see this error on the preview within photocarve? It might be the angle of the photo but it looks like the bottom line of the litho carving is a bit higher on the left side, comes down in the middle and then slopes up on the right as you mention. Is it possible that the actual bottom of the photo you used to generate the picture isn't actually square along that edge?

May be way off track but just a couple of thoughts after looking at the photo you added.
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Re: Lithophanes with uneven bottom border

Postby dhellew2 » Sat May 09, 2015 3:20 pm

I always use a 315° angle which makes the first cut a hole. No strain on the bit, every cut is a step-over cut. My cut are usually with a 3/64" bit up to 0.20" deep single pass and haven't broken a bit yet while cutting.

Its difficult to evaluate without seeing the cnc in action, but, we can all make educated guesses :oops:
Did you create a border vector for your lithophane or just using the photo?

I always use a border, often not a rectangle, stars, hearts, etc.; just select the border vector then create the finish toolpath [and roughing if you use one].

lighted lithophanes.jpg


Good Luck! Dale
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