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I am building a CNC machine (don't need to know what that is), and I have chosen wood to be what I make the frame out of. One problem that I am facing is joining the pieces together. It may seem like a simple solution (a few nails or screws) but for this project it might have to be completely disassembled for modifications/expanding it/maintenance/cleaning/storage/etc.

I don't want to ruin the wood in the process with plain screws, since it will be a normal storage procedure to disassemble parts of it when not in use. (I have a very small apartment and space is limited for storage.) I've decided to use bolts to fix this problem. However, bolts you have to drill a hole, and I would think that it would be rare that there is zero wiggle room for the bolt to move around in the hole. When assembling/disassembling often, this can be a problem with alignment, especially since a CNC machine should have the greatest accuracy possible within your budget.

Is there some sort of thing where I can put like a "long nut" all the way through the board so it will easily be removed, yet won't wiggle (noticeably)? What methods do I have?

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3 Answers 3

up vote 7 down vote accepted

Specialty Bolts

  1. Some bolts have long smooth shanks and short threaded sections. If you are able to find appropriately sized bolts like these, and able to buy a drill bit that is the same diameter as the shank for a sung fit, then this is simplest, if not the best, solution. The key is a smooth shank of appropriate length. If the threaded section is too long, it can, of course, be cut off with a hack saw and groomed with a file. To straighten out threads at the tip, spin a nut all the up to the shank and leave it on until finished with the grooming.

    enter image description here




Joinery

  1. The craftsman approach would be to use shouldered joints.
    This is a link to the wikipedia page.

    enter image description here




Pins

  1. Solid Pins

    • assemble with bolts
    • choose a nail size (diameter) and similarly sized drill bit for a snug (but not too tight) fit. The match between the nail and the holes needs to be snug enough for good alignment, but not so tight as to prevent the joint from being disassembled by hand and the nails tapped out if so desired.
    • cut nails to length if necessary
    • drill two or more holes through one member and into the second member
    • tap the nails into their holes
    • Note: see @bib's answer for a "blind" variant of this

  2. Tube Pins

    • choose the bolts
    • choose metal tubing through which the bolts easily fit.
      Note: as suggested by @AnonomousPerson in a comment, PVC tubing such as 1/2" plumbing can be used with good results for large bolts
    • in both members, drill through-holes in which the tubing fits snugly
    • cut lengths of tubing matching the members combined thicknesses
    • tap in a section of tubing
    • bolt together through the tubes




Splines

  1. Blind Rings

    • select 1/16" walled metal tubing whose diameter that is somewhat smaller than the narrowest member. For example, if the narrowest member is 2" wide, then select some tubing that is about 1.5" in diameter.
    • select a hole saw that matches the size of the tubing
    • temporarily assemble the members using bolts that match the diameter of hole saw's drill bit, typically 1/4".
    • disassemble
    • on each of the mating surfaces, using the bolts holes for guides, use the hole saw to drill 1/4" deep circular recesses
    • cut 1/2" long rings from the tubing
    • assemble with the rings sandwiched in between
    • if larger bolts are desired, re-drill the bolt holes

      enter image description here
  2. Blind Flats

    • same idea as blind rings but use 1/2" x 1/16" metal stock and saw-kerfs

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Thanks! Number two looks like it will work for me. I think that I will try to glue/fasten the tube on one end so it doesn't slide or fall out when not completely assembled. –  Annonomus Penguin Oct 24 '13 at 21:33
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@AnnonomusPerson - for precise alignment, I'd choose a drill bit that would create a snug hole for the tubing. You might also want to take a look at the new methods that I added. –  mike Oct 24 '13 at 21:53
    
Joinery might also help on some joints to make it easier to assemble in some spots too. Also, PVC seems easier to cut than metal, and has a cleaner cut. That part will be covered through (maybe some washers?). I know it wouldn't look pretty or anything but it is more for function than look. –  Annonomus Penguin Oct 24 '13 at 23:42
    
@AnnonomusPerson - Good point on the PVC for large bolts. Tubing cutters make quick work, or the heavier pipe cutters. –  mike Oct 25 '13 at 0:06
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Mikes answers all have merit. You might also consider using alignment pins.

  • On one surface of the pieces that will mate, drive in two or more finish nails, leaving about 1/4 inch protruding.
  • Clip off the head of the nails.
  • Align the pieces and clamp down with bolts, driving the protruding nails into the mating piece.
  • When disassembling, wiggle the pieces slightly to disengage.
  • When reassembling, align the pins with the holes before bolting.

This approach is not as versatile as those offered by Mike if constant assembly and disassembly is contemplated. But if it is only occasional, this might do.

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Bolting it together will make it easier to disassemble, definitely.

You might also consider adding metal to the wear surfaces. This starts with washers over both the bolt head and the nut. This reduces the amount the bolts will draw into the wood over time.

You could face the mating surfaces with metal, e.g., permanently screwing a metal surface into the groove of your rabbet to mate with another metal surface screwed to the other piece of wood. Since these screws won't be disassembled, they shouldn't shift.

One easy way to do this is with hinges; then you can pop the hinge pins for disassembly. You'll need to find hinges with minimal slop, however.

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1+ for hinges. Sounds like a good idea for some parts, however, it probably won't work for every joint. –  Annonomus Penguin Oct 25 '13 at 21:08
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