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  1. Sep 21, 2009 #1
    Probably a stupid question but...

    I have a device with a bolt that is designated #6-32 UNC. I need to drill a hole in a surface to for this bolt to fit into. How big should this hole be? If the material I am going to be threading the bolt into is plexiglass do I need to thread the hole I drill? I plan to put a nut on the other side.
  2. jcsd
  3. Sep 21, 2009 #2
    Is this what you want?
    This is a more complete table:
    http://www.marfas.com/machinescrewtapping.shtml [Broken]
    Last edited by a moderator: May 4, 2017
  4. Sep 21, 2009 #3


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    Because he's using a nut on the other side, he needs a clearance hole not a drill size for tapping.

    I use my Machinery's Handbook (27th edition) for all such things. In clearance holes on page 1621, a "normal fit" clearance hole for a 6-32 screw is a #18 drill, or .170". A "close fit" is a #23 drill or .154".

    I found this table of tap and clearance hole values: http://www.stanford.edu/~jwodin/holes.html [Broken]

    It has somewhat smaller sizes quoted than in my Machinery's Handbook, but it looks like they would work in a pinch.
    Last edited by a moderator: May 4, 2017
  5. Sep 21, 2009 #4
    So does that mean that if I drill a clean hole that .154", my bolt will fit nice and tight in there and then when I put the nut on everything will be nice and sturdy?
  6. Sep 21, 2009 #5


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    Is it just one hole or do you have to worry about multiple hole being aligned? If it is just one hole and you want it pretty tight, you can get away with going the next drill size up from the .138 (I think). That's going to be pretty darned tight. I would recommend not going below the .154 though.

    If you have multiple holes to align, you need to look at the tolerances of the bolt hole positions and what your machining can achieve to establish the hole size.
  7. Sep 21, 2009 #6
    Bolting Plexiglas is not recommended, especially if used outdoors, with a large difference in temperature. The reason is it has a large Coefficient of Thermal Expansion, about four times as much as aluminium. If it is not allowed to expand and contract it will crack.

    If you must use bolts: cut the panel with 1/8 to 1/4 inch clearance and holes should be drilled about 1/8 inch oversize. Screws should be tightened down and then backed off one full turn, using elastic stop nuts and large washers.

    It works better if you use a drill bit with an included angle of 150 degrees or a unibit drill.
    Better method available here:
    http://www.altuglas.com/expert/data/116 [Broken]
    Last edited by a moderator: May 4, 2017
  8. Sep 21, 2009 #7
    This is going to be inside. I am not worried about temperature variation.

    Actually what I am going to need to do is have two layers of plexiglass with three Honeywell Model 31 Load cells in between and attached to each layer.
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