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Thread: Two-Sided Job, Bleeder and Spoil Board

  1. #1
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    Default Two-Sided Job, Bleeder and Spoil Board

    I'm a newbie who's PRS Alpha is not even finished construction yet. I'm planning my first job for when it's done and may have started with something over complicated for my meager experience.

    I've watched the excellent tutorial on two-sided machining http://support.vectric.com/tutorials...hp?video_id=92 and laid out a two-sided job in VCarve 9.0.

    My job has two holes for dowel pins that are drilled all the way through into the spoilboard for alignment when flipped. Since I have a vacuum system, the spoilboard is the bleeder board and I don't like the idea of drilling into it and risking breaking the vacuum if I go too deep.

    The tutorial assumes you are drilling new holes into the spoilboard each time, but I don't want to do that. I only want to drill into the bleeder once.

    Suggestions? How do I maintain alignment when flipped without risking damage to my bleeder-spoilboard?

  2. #2
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    You can add a sacrificial spoil board to your vacuum spoil board using the vacuum to hold it down and then drill into that for your dowel pins. Just make it big enough for that particular job each time...joe

  3. #3
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    You don't need dowel holes...it's kind of a kludge for flip ops & as you are discovering in your mind's eye, counterproductive for vacuum hold down setups.

    The *most important* thing with 2-sided work is properly dimensioning and measuring the raw material block before it goes on the machine. Assuming you'll flip the part towards/away from you using the long X axis as an axle of sorts (flip in Y direction), the Z thickness of the part AND the Y front to back dimension is critical. Any deviation in the Y dim will result in 2X the error and the part will be shifted. In the Z, you could either gouge the part or leave a thin plenum of material in the middle.

    For a new tool, I find it most helpful to machine the X and Y edges of your spoilboard/bleeder after you have made sure the machine is square with a 3,4,5 method. Simply drilling a few holes on the spoilboard say 1/16" deep at 0,0 (make sure it's not on the edge of the table...manually zero XY at 2,2 after your C3...Just do a M2,2,2 and then Z2); another hole @ 30,0 and 0,40. The hypotenuse should measure 50"...3,4,5. Adjust your machine as necessary. Kiss the edge of the sheet with a 1/2" straight/end mill and you're all set. The table is now true to the table. I found it helpful to put the YZ car and spindle on first, then slap the support board on the table and align it to the spindle and clamp it...then drill your carriage bolt holes and do the glue up. It will get you really close to where you need to be.

    Just set your material up on the table, aligning it as you would anything else to the edge of the machine and pull vacuum. Then take a fine mechanical pencil and scribe the left edge and back edge of the material. There's your alignment marks. All done. No dowels or complicated jigs required.

    Be sure to pick a spot on the table to do your Z zero. Use that EXACT same spot for all tools for the project. I pick a round number off the work....like 12,12. Move the tool there and use the Zzero plate. If Zzero reference is to the bed, you're all set. If it is to the top of the block, just Zzero to the bed, then MZ, {your material thickness} and then ZZ - which effectively zeroes the Z @ the top of the block that you measured accurately. If you mic'd it out at 1.257", then you'd C2 on the bed with the plate, MZ,1.257, then immediately ZZ.

    This may sound a little confusing now, but it will get clear to you when you do it. Welcome to the forum, congratulations on the new tool - if you have questions just ask. There are a lot of knowledgeable kids on here eager to help.

    Just a side note - make sure your machine is level, adjusting the 4 outside legs first as low as they can go, then drop the center legs down by hand...then turn the adjusters half to one full turn to get the sag out of the rail. A straightedge or your good eye will tell you when it's right ...and check that those lower YZ car roller bearings are tight, but not too tight + Y motor gear fully engaged in the rack...Remove the stop block on the gantry closest to Y0 for an extra 1.5" of travel...

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  4. #4
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    Thanks, Joe. I like this in that it does not require me to drill into the bleeder. It will require me to add tabs to my parts and somewhat defeats the purpose of the vacuum hold-down. Is this what you do?

  5. #5
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    Thanks for the detailed response, Brady. If I understand you correctly -- be patient with the newbie -- you are aligning the machine and dimensioning the stock precisely, but then only relying on pencil marks for the flipping. Do I understand correctly? Is this sufficient alignment for the reverse? I suppose what you can get away with depends on the requirements of the job.

  6. #6
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    Unless I am doing several of the same part, it doesn't warrant making a custom jig for it - so pencil is the way to go. I even use the pencil marks on the spoilboard for production depending on the part. Sometimes you can't use fences, dowels etc because they'll be in the way. Once you're doing a dozen or so, then fancy pockets, pins/dowels, fences etc provide easy alignment.

    I often use carpet tape for parts of a reasonable size for 2-sided jobs. Align to your trued table edge, stick that puppy down, scribe your lines with a fine/mechanical pencil on the back and left sides & do it. On the flip, align to your marks. Bob's you're uncle as they say in Canuckistan.

    Dimensioning the material is absolutely key to an accurate result. Imagine for a moment that the software expects the material to be 4 x 10" long and 1" thick. But in reality it is 5 x 10 x 1" thick. When you flip the part over for the 2nd side and machine it, it will be shifted from one side to the next because the centerline 'axle' is shifted in the Y. To get a good visual for yourself just draw a 4x10 and 5x10 rectangle, with aligned bottoms. Put a 2" circle in the center of the 4" tall one. Then group them all together. Mirror the group vertically about center (not top) to emulate flipping the part over. It is easy to see how they are not aligned on the flip.

    The same thing goes for the Z/thickness of the material. Let's say you tell the software it is 1.00" thick, but in reality it is only .75" thick. The first side (if zeroed at the top of the block) will cut fine, but not if it is zeroed at the bed/bottom of the block. The design will be truncated because it is expecting another .25" to machine. With thicker than specified in CAM material, you'll be left with a web/plane of material that neither side will machine. It isn't totally out of the ordinary to get some 'paper' between the 2 sides (due to material deflecting on the 2nd side in Z), but this is often eliminated if you use carpet tape because it has a thickness of about .008-.01", which effectively pushes the material into the cutter by that amount per side. I mean unless you're carving trinkets out for NASA...there's no need to get crazy about it. If you are, bump the Z up or down to compensate for the tape thickness.

    So...not to over-think things here...the main thing is to make sure the physical world matches the digital world. Otherwise - there will be a deviation and things will not machine as expected. There is no fudge factor with this work...so be specific with measurements and don't lie to CAM about the material. You have to see what method you are most happy with - some people require elaborate setups for doing 2-sided machining. It really isn't necessary - all you are doing is manually doing a table rotation as if you had it setup in a rotary axis. As long as the centerline/axle matches on both sides, the center of the design will match too.

    I'd encourage you to just cut a simple example to see what works for you. Cut a rectangle out of a piece of scrap (let's say 1/2" thick - or whatever) and pocket out 2 holes say 1" dia, .25" deep. Then flip it using your pencil marks...then drill 2 holes through the center of where the 1" holes were supposed to be and look at it. Are they on center? Skewed? Adjust as necessary. Once you get it, you got it...that's it!

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  7. #7
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    I do use tabs, I don't have a vacuum hold down. I would like to use vacuum, but my machine (Buddy) is not a conducive to a simple vacuum table set up. But, I do a lot of two sided machining and a few dowel holes here and there should not ruin your spoil board. Just don't drill all the way through, say like .5" is sufficient. And, like Brady says, a well defined guide or set of marks will do just fine.....joe

  8. #8
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    Quote Originally Posted by Joe Porter View Post
    ...but my machine (Buddy) is not a conducive to a simple vacuum table set up...
    Oh yes it is! ---> BT48 Vac System Circa 2008 (post #6)

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  9. #9
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    A vacuum table for my Buddy is something I would really like to have and I know it can be done, it's just my sorryness(?) that is preventing me from following through. There are several good examples on the forum and maybe I can add to these some day. I do have a really nice commercial grade vacuum pump that I intend to make a Joe Wood Worker type set up some day, Someday........joe

  10. #10
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    Joe,
    Many of us have way too many projects going on, and many aspirations...believe me, I am no exception!

    I don't think a veneer press vacuum will be right for universal vacuum hold down unless it is very high CFM. Instead, a regular old shopvac will get you going or even better, a $100 Lighthouse motor.

    The thing about vacuum hold down to someone that's never used it before is...it's so simple that quite often people have a hard time believing that it will even work - and that stops people from trying; myself included long ago.

    Try this: Grab a piece of scrap in the shop say 12" square - can be any wood based sheet product. Machine some lines into it (with machine on vector) 1/4" deep to make a grid. Then cut a thru-hole the diameter of your vac hose. Plug that puppy in & turn on the shopvac. Now stick it to something (preferably NOT the dog)...pretty easy, right?

    Gotta go plow snow now...I am overjoyed

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

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