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Thread: Need Help Designing Fire Piston Cylinder and Cap

  1. #11


    Next step is to make a half-cylinder pattern.

    Here is a height map of a half-cylinder:

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    Download it to your computer (click on the thumbnail image to enlarge it, then right click and select Save Image As...) and put it somewhere handy. I have a folder for height maps.

    Open Designer and click on File – Import – Import Image File. Under Source, click on From File. Browse to the half-cylinder height map and open it:

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    Click on Next. Click on the red lock icon to unlock (blue). Set the width to 1, the depth to .5, and the length to 5:

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    Deselect the Floor Edges checkbox. Click on Finish.

    Pick a folder, or use the New Category button to make one. (I often create a category for a specific project.) Enter a name such as half cylinder and click Save.

    Before we use this pattern, we need to cover the topics of Bit Optimization and Floor Feather. Those will come next.

    Meantime, go through these steps. Play around with the options if you like to get familiar with them.

    Extra credit: Google height map, click on Images, and find some grayscale graphics. (Careful – not everything that you see is a true height map!) Here is a good example:
    Make a pattern out of it. (Hint: for this particular pattern, click Invert, then click the down arrow under Raise/Lower to lower it a bit. What happens?)

    And you thought that you had to pay money to get a penny pattern! You might be surprised at what interesting height maps you can find and convert into patterns with Designer Basic only.
    Attached Thumbnails Attached Thumbnails half-cylinder.png  
    Last edited by DickB; 06-15-2018 at 05:54 PM. Reason: Replaced the half-cylinder height map with the correct one.

  2. #12
    Join Date
    Nov 2008
    Vancouver Island


    I have yet to make a fire piston. On my list. I cannot, however, see how the CW can carve the cylinder to anywhere near the precision required. Is there a plan to "hone" out the crude cylinder that the CW makes?

    How about inserting a brass tube when gluing the two halves together?

  3. #13


    Yes, we discussed making the cylinder slightly undersized and finishing with a 3/8" drill bit. You may recall I drill pilot holes in my clock gears with Carvewright and finish with a drill press to get the precision that I need.

    I saw that some DIY instructors on the web use metal tubing. That would be an option, but this one is to be all wood.

  4. #14

    Default Bit Optmization and Floor Feather

    By default, when carving a pattern the center of the bit will follow the contour of the pattern. I’ve used the 3/16” ballnose carving bit to emphasize the result with our half-cylinder pattern:

    Click image for larger version. 

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    While the tip of bit follows the pattern, because of the width of the bit the resulting carving is not a perfect semicircle like we want.

    Fortunately there is a solution: Bit Optimization. With our pattern selected, click on Bit Optimization and select Best. Now the software will take into account the width of the tip of the bit, and carve the semicircle that we are looking for:

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    In Designer Pro, there is a feature called Floor Feather. Without going into a lot of detail, this feature was implemented to make feathering more pleasing when the pattern’s periphery does not have a uniform depth. It forces the entire periphery of the pattern to be equal to the lowest part. This is nice in some cases, but for our pattern it is not. Here is an illustration of our pattern in Designer Pro with Floor Feather disabled (left) and enabled (right):

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    We do not want to use Floor Feather! Unfortunately, in the latest version of Designer Basic, Floor Feather is always enabled, and there is no way to turn it off!

    However, as it happens, when Bit Optimization is enabled for a pattern, it virtually eliminates the Floor Feather result. Not entirely – can you see the small “lip” at the edges of our half-cylinder in Designer Basic and in the resulting carving?

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    This can be a problem in some cases. For us it is not going to be a problem, because we are going to carve our cylinder slightly undersized, then finish with a 3/8” drill bit. So this small defect will not matter.

    Next, we will work on creating a pattern to make the hexagon-shaped outside of our cylinder.

  5. #15


    To design the outside of the cylinder in a hexagon shape, we’re going to need two sloped sides, or wedges. Here is a height map for a wedge:

    Click image for larger version. 

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    Follow the same steps as you did for the half-cylinder to make this into a pattern. Set the depth to .75”, the width to .75”, and the length to 4”.

    (BTW these dimensions are somewhat arbitrary, because we’re going to change them after we place the pattern on the project board. I like to generate patterns as large as or larger than necessary. My thinking is you get better resolution by shrinking a larger pattern than stretching a smaller one. I could be wrong. Also, these patterns can be used for a variety of projects by simply altering the dimensions, depth, and height.)

    We’re going to make the two halves of the cylinder from ˝” stock. I used an online right triangle calculator to compute the following dimensions:

    Click image for larger version. 

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    (bergerud is the mathematician, so if you have questions on how to do this he is your man!)

    Finally, we are ready to start laying out our project!

  6. #16

    Default Start designing!

    Start by creating a new project board with dimensions 6” x 3” x .5”.

    Use the Pattern Tool to select your wedge pattern and place it near the bottom of the board, below the board’s centerline. Click on Pierce. Center the wedge horizontally with the Center Horizontally tool. Do not change Bit Optimization for the wedges – leave it at None.

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    (If you can’t find any of the tools to which I refer, you can add them using the View – Toolbars checkboxes and Customize, or just click on Tools to find them.)

    Place the wedge exactly half the width of the top surface,.577/2 or .289”, away from the centerline. To do this, click on the green number at the centerline (your number is likely different from mine) and enter.289. The number should turn yellow, indicating that it is a constraint:

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    In the toolbar under Size: , set the width of the wedge to .289 (the second number). Designer Pro lets you enter width and length numbers independently if you wish; Designer Basic preserves the original pattern’s aspect ratio, so you can’t change the length directly. But you can change it by dragging the edge of the pattern. Select the yellow dot to the right of the pattern and drag until the length is 5”. Zooming in on the dot will help with this. No worries if you can’t get it exact.

    Change the feather from the default ˝” to 1/8”. Mirror the pattern vertically with the Mirror Vertical tool:

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    Draw a rectangle. Center it horizontally and vertically. Set the dimensions to 5” and .577”. Click on the Carve Region tool. Change the default depth from .250 to .001. (We actually want this to be zero, but Designer complains when you try to upload a zero-depth region to the card. It treats .001 the same as zero without complaining.)

    Click image for larger version. 

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    If you had done the above steps in Designer Pro, and unchecked Floor Feather on the wedge patterns, life would be good. Not so for us. Click on Rear View. What do you see? Our wedges are disconnected! This is Floor Feather rearing its ugly head. Fortunately, there is a work-around: making the top surface of our hexagon a bit wider than it should need to be. Go back to the front of the board and increase the width of the top rectangle slightly, and then go back to the rear to see if it has been fixed. Repeat as necessary. Make the width as small as possible while still fixing the problem.

    Click image for larger version. 

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    Next we’re going to clean up the pierced part of the carving by making the pierced region a bit larger. If this is not done, the pierced area could be very ragged, or not completely pierce at all. Draw a rectangle, center it horizontally and vertically, and set the dimensions to 5” x 1.25”. Click on Carve Region and then click Pierced. Set Feather to 1/8” and flip feather.

    We could be done at this point, but notice that the carving bit is going to “cut” the ends of the wedges. That’s OK, but unnecessary. Because it is the tapered carving bit, the cut is not going to be square – we would need to square it off in a subsequent step. Also, we are carving more material at either end of the wedges than necessary.

    We could simply eliminate feathering of the wedges and pierced rectangle, but it turns out that feathering is actually less hard on the machine and actually carves a bit faster. Why? Without feathering of the pierced rectangle, the bit has to stop and plunge at the rectangle edge. With feathering, it instead starts at the surface and smoothly lowers as it travels in the y direction.

    To eliminate the unnecessary carving, draw a rectangle at the left side of the board. Dimensions are not critical as long as you span the excess carved areas. Click on Carve Region and set the depth to .001. Right click on the right yellow dot and select Attach. Move the cursor over to the vertical centerline and click. Enter 2.45 (it should be 2.5, but there’s that darn Floor Feather again! If you want the cylinder to be exactly 5" long and not 4.9", do you know what to do?). Mirror the rectangle horizontally.

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    I’m going to assume that you have a table saw, miter saw, band saw, or some other saw to cut off the ends of the cylinder after it has been machined on the Carvewright. If not, let me know, and I’ll show how to do this on the Carvewright.

    Extra credit: In the Carvings list, select the top rectangle and the two wedges. Click on the Group button at the bottom of the Carvings list. Select the Group that you just created, then click on Make Pattern, and save the pattern in your choice of folder. Start a new project and select and place the pattern that you just made. What do you think?
    Last edited by DickB; 06-16-2018 at 08:57 AM.

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