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Vertical sidewalls not vertical (2)...

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Gary Wheeler (Unregistered Guest)
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Posted on Sunday, July 28, 2002 - 12:51 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Pardon me if this is a duplicate, but I can't find my question on the board.

While trying to make architectural models, I have a problem with the sidewalls of the finished model not being vertical. It is as if the software generates toolpaths with the assumption that I was cutting something that was going to be used a master for a plaster mould. Instead of walls being vertical or 90 degrees, they are sloped. Any ideas?

web page: http://g.wheeler.home.att.net
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support (Support)
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Username: Support

Post Number: 98
Registered: 01-2002
Posted on Sunday, July 28, 2002 - 09:14 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

This is controlled by the step along setting. When you create toolpaths that are parallel to either the X or Y axis, you specify what I call the stepover distance. This is the distance between the parallel cuts. For illustration let's say you are cutting parallel to the X axis. The step over distance will be the Y distance.

At the same time you are given an opportunity to specify a "Step Along" increment. This is the setting in the same direction as the movement. In this case the X axis. If you have a steep side wall to cut and you have a step along distance of .25, that will mean that at every .25 of an inch(or mm, or whatever), an evaluation of the correct Z height at which to cut is made. The movement to the next evaluation point is always linear. That means that a large step along will result in angled sidewalls. Set this number relatively small to decrease the amount of error and hence the angle at the sidewalls. This will also increase the processing time, but you will see a dramatic improvement in the accuracy of the cuts. If you leave stock for roughing, you can also easily use a larger step along, then remove the stock and specify a tighter step along for the finish pass(es).
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Gary Wheeler (Unregistered Guest)
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Posted on Wednesday, September 11, 2002 - 01:11 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Thanks for the info.
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Mike Smithson
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Username: Cncorbust

Post Number: 1
Registered: 03-2004
Posted on Wednesday, March 10, 2004 - 04:14 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

I have been wrestling with STLwork2 demo. I have generated an *.stl file consisting of three cubes. The three cubes are 1 x 1 x 1, .6 x .6 x .1 and .4 x .4 x .2. The middle size is placed flat on the center of the top face of the big one and the two unioned together to make a single step pyramid. The third smallest one is placed with it's top flat face flush with and at the center of the top face of the step pyramid. The small cube is then subtracted from the step pyramid, leaving a square 0.2" square hole in the top of the pyramid. This is then a single solid saved as an *.stl file. I have tried x , y and water level strategies they all work to varying degrees but I can't seem to get vertical walls no m atter what I do with the step over figures. I do get tool paths, they do change when I change tool size etc but no matter what I do I cannot get vertical sides. I should have vertical sides on the inside of the hole in the top and vertical sides on the step from the tope of the base to the top of the pyramid. I have tried changing the step over to near zero and that improves the situation but does not get me to vertical. The water line strategy doen't seem to work as it does for other items (TeaPot) for example) but it tries so I suspect I may still not be doing all the right things. I don't get s series of tools describing the removal of material from the blank just an outline of the pyramid. Also I cannot seem to get a finishing cut to remove what was left by the "leave ##" value. Is there a more complete instruction book for using STL. STL looks like a fine tool but the help files don't help a neophyte like me? The individual functions of STL appear to be accurately described but how one combines these functions to produce a finished part seem to have been left out.

Mike
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Support
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Username: Support

Post Number: 4
Registered: 03-2004
Posted on Wednesday, March 10, 2004 - 05:17 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Mike, you are correct, the program assumes that you already know what cutting strategy you want to use and gives you the options to implement it.

1) The tool geometry. Are you specifying a flat bottomed tool? An angle tipped tool will never cut a vertical sidewall.

2) The X step over is the controlling distance if you are cutting back and forth along a path parallel to the X axis. The Y distance will have little effect on the vertical side walls in this case.

3) If you ever cut a vertical sidewall with a long side of a cutter, you will realize that it is nearly impossible to make a decent finish, unless you travel parallel to the vertical face. Hence the waterline toolpath. The waterline is a finishing only strategy and can be controlled by the angle of tangency which will show in the model as blue selections. It cannot be used to cut a horizontal surface.

The Vertical side walls are controlled by the contour of the model. If you tesselate more accurately(in your cad program), the smaller stepovers in X may more closely approximate the vertical side walls (which as I have just mentioned is not really a good cutting strategy for vertical side walls)

The max step in Z is set to a smaller value to get multiple waterline cuts instead of just one at the base of the part.

Fred Smith - IMService
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Mike Smithson
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Username: Cncorbust

Post Number: 2
Registered: 03-2004
Posted on Saturday, March 13, 2004 - 03:44 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

I have tried adjusting parameters but am still unable to approach vertical sides where vertical sides should be. The deviation from vertical is significant, can't measure it on the tool path presentation of STL but it is in the order of 15 degs from vertical! My Strategy is : Water line : No Flats ; margin 0.0 ; x1 -0.500 ; x2 0.500 ; y1 -0.500 ; y2 0.500 ; z1 -0.190 ; z2 0000; Tool# 2 ; dia 0.05 end mill ; Parameters x-y step 0.01 ; z 0.001 ; Stock to leave 0 ; Spindle 0 ; Feed 1.00 ; Plunge 0.1 ; Max Height 1000.000 ; Angle 90. I get only three steps on the outside edges I should have 10. Inside paths also come up short in step count i.e I get 13 steps I should have 20. The screem presentation of the raw stl files looks correct with the wire frame triangles showing in their expected positions.

There are 4 verticle sides with a height of 0.1, the hole in the middle penetrating from top to bottom is 0.190 deep with vertical sides. Is STL the proper tool to use to produce this piece. Using DesckCNC and applying pocketing I have no problem but that ofcourse that does not verify the STL file.

Mike
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Support
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Username: Support

Post Number: 9
Registered: 03-2004
Posted on Saturday, March 13, 2004 - 04:38 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Put a clearance margin (press enter) all around the model too. That way the cutter won't try to avoid plunging off the side and into unknown, but will be forced to find the bottom. Waterline won't create any cuts except all the way down to the base of the model. Set max height for the total depth of cut. This should be in the help file or else covered elsewhere in the forum.

Parameters x-y step 0.01

This is the number that needs to be adjusted to control the angled side walls. The cutter is moving .01 along the X axis, checking it's position, moving another .01, etc. When it comes to a vertical, it cannot get closer than .01, and with a .050 diameter cutter, it looks like an angle. If you have X selected for cutting direction, change X step to .001.

I don't understand why you set Z step to .001. This does not help cutting vertical sidewalls. If you have a smooth surface that can be cut with small stepover cuts, you should use the raster cutting pattern(X or Y). On vertical sidewalls, you should be using at lease a cutter diameter or some significant percentage for your Z step size.

StlWork will make this part, however, it is not the best tool. The best method is probably to draw the contour outline with a simple 2D cad program and use 2 1/2 axis cutting strategies like pocketing and contouring, instead of 3D surface milling. Then you can use a flat bottomed cutter and your flats will be flat, and the sides will be perpendicular, cut wih the straight side of the cutter.

Fred Smith - IMService

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