Bolt thread experts...HELP! - Model Railroader Magazine

loathar wrote:

I picked up a Dremel drill chuck to hold micro drill bits. It won't fit my Chinese Dremel knock off. (imagine that!)Different thread pattern. All is not lost! (I hope) I can still use it as a pin vise if I get the correct thread bolt to screw in it. Dremel said the thread pattern is .28 by 40. I'm assuming this is metric but I've never heard metric measured this way. My hardware store only has things like 8mmx1 or 6mmx .5 Any advise on what they are talking about???

Since it's Chinese made, it's most likely metric. Since you're saying it's close to 5/16th I'm thinking 8mm.

There are a few different options for 8mm diameter screws. 

ISO (International Standards Organisation) Metric Coarse Threads

Nominal Dia[M x mm] Pitch [mm]  Tapping Drill Size[mm]
8,0                         1,25          6,80

Thread Profile Drawing

ISO (International Standards Organisation) Metric Coarse Thread Profile
r = Basic Radius = 0.1443 p
hn = Basic Height of Internal Thread and Depth of Thread Engagement = 0.54127 p
hs =Basic Height of External Thread = 0.61344 p
p = Pitch = 1/t.p.i.
Angle of Thread = 60o Degrees

ISO (International Standards Organisation) Metric Fine Threads

Thread profile drawing

ISO (International Standards Organisation) Metric Fine Thread Profile
r = Basic Radius = 0.1443 p
hn = Basic Height of Internal Thread and Depth of Thread Engagement = 0.54127 p
hs =Basic Height of External Thread = 0.61344 p
p = Pitch = 1/t.p.i.
Angle of Thread = 60o Degrees

Nominal Dia [M x mm] Tapping Drill Size [mm] Nominal Dia [M x mm] Tapping Drill Size [mm]
8,00 x 0,50               7,50                         30,00 x 2,00             28,00
8,00 x 0,75               7,20                         32,00 x 1,50             30,50
8,00 x 1,00               7,00                         33,00 x 1,50             31,50

ISO (International Standards Organisation) Metric Special Pitches Threads

Diameter mm Pitch mm Root Dia Internal Threads Root Dia External Threads Tapping Drill Size
8                 0.75       7.188                           7.08                              7.2 mm (0.2835")

Thread profile drawing

ISO (International Standards Organisation) Metric Special Pitches Thread Profile
r = Basic Radius = 0.1443 p
hn = Basic Height of Internal Thread and Depth of Thread Engagement = 0.54127 p
hs =Basic Height of External Thread = 0.61344 p
p = Pitch = 1/t.p.i.
Angle of Thread = 60o Degrees

Is the chuck thread too big or small? If it goes on a threaded shaft on the tool that's larger, then maybe it can be drilled out and retapped to fit the motor tool's shaft? If it's bigger,  you might try wrapping some plumber's teflon tape around the shaft and building it up a bit.

loathar wrote:

I took it to True Value. The closest thing I found was a 5/16x24, but thread was too coarse. Do they make a 5/16x28 or a 5/16x30? My True Value is run by an old coot that won't let you touch anything and hovers over you saying "that won't work!" "that won't fit!" "We don't have anything like that!" Drives me crazy!(only hardware store in town)

When dealing with this store, bring a friend with you to keep said coot busy by whatever means necessary while you try nuts on your drill.