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Drill size for 5.5mm self tappers


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I'm putting some brass pole and plank racks on the roof. What's the correct metric drill size for 5.5mm stainless self tapping screws into 5mm thick steel? I looked it up on Google yesterday but I was getting a range of drill sizes. I've used a 4.5mm drill but it's not even big enough to get the screws started. Should I go up to 5mm?

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4 minutes ago, blackrose said:

I'm putting some brass pole and plank racks on the roof. What's the correct metric drill size for 5.5mm stainless self tapping screws into 5mm thick steel? I looked it up on Google yesterday but I was getting a range of drill sizes. I've used a 4.5mm drill but it's not even big enough to get the screws started. Should I go up to 5mm?

I think you would be better tapping a 5mm thread in it.

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5 minutes ago, blackrose said:

 

Ok I will do. But what are self tapping screws for then?

Thin metal & wood....and bodging. You might get away with them on 1-2mm steel..slightly thicker if alloy. Even if you get them through thicker steel they won’t be mechanically strong as the hole will be too big and they won’t cut a thread

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13 minutes ago, blackrose said:

 

Ok I will do. But what are self tapping screws for then?

They work well in metal a lot thinner than 5mm.

 

The dont tap or cut a thread as such but rather distort the hole edge to conform with the threads.

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55 minutes ago, frangar said:

Self tapping stainless into steel will end in tears...and snapped screws. As above tap a M5 thread..4.5mm hole will be ok and use a machine thread ss screw. 

IIRC the correct tapping drill size is 4.2mm, 4.5 won't leave a lot of thread.

 

Richard

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10 minutes ago, PaulJ said:

Or use M6 tap if the holes in the plank rack are 6mm

Fair point!! I guess we don’t know what size the holes in the plank holder are....An 5mm still bit should be fine for M6 bolt if you have a taper & plug taps. Use a lubricant and go steady...and HSS taps

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1 hour ago, blackrose said:

I'm putting some brass pole and plank racks on the roof. What's the correct metric drill size for 5.5mm stainless self tapping screws into 5mm thick steel? I looked it up on Google yesterday but I was getting a range of drill sizes. I've used a 4.5mm drill but it's not even big enough to get the screws started. Should I go up to 5mm?

5mm steel is too thick for self tappers , tap the hole and use a proper set screw. 

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Just now, frangar said:

Agreed as long as M6 fits the plank holder nicely...or it can be drilled out to M6

Yes that's not a problem. Thanks.

3 minutes ago, WotEver said:

Drill it out to 5mm and tap it 6mm.

Are we sure a M6 tap will go through a 5mm hole? I thought it needed to be drilled slightly larger?

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2 minutes ago, blackrose said:

Are we sure a M6 tap will go through a 5mm hole? I thought it needed to be drilled slightly larger?

Yup.  5mm is the correct size for an M6 tap.

 

Obviously you need a taper tap to get it started.  And don't forget to lube it.

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1 hour ago, frangar said:

Self tapping stainless into steel will end in tears...and snapped screws. As above tap a M5 thread..4.5mm hole will be ok and use a machine thread ss screw. 

 A 4.2mm drill is the correct size for tapping M5.

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6 minutes ago, Sir Nibble said:

Rule for tapping drill sizes for metric threads. Size minus pitch. Standard pitch for 5mm is 0.8 so 4.2 is correct. 4.5 will be fine, just run the tap into that.

You won't get much of a thread in 4mm plate if you use a 4.5mm tapping drill.

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