Help identify this screw

[rick]

Had a new item arrive today .. including for fixing a pack of stainless Steel self tappers - but due to material backing they are passing 'through' they are not long enough ... need to obtain longer versions. These are 5/8" and I will need 1"

As there are pre-drilled holes in the body .. I don't want to go up a size and risk cracking the body.

Went along to my local fixings shop - they had nothing to match .... bigger than a No. 6 and smaller than a No.8 ....

When I looked up on line the data .. does not seem to fit anything exactly

Major Diam 3.76mm 0.148" Minor Diam 3.24mm 0.1275" ~18 tpi (which points to a No.7)

Anybody have a better guess than a No.7 Self tapper

Then the 2nd Q ... anybody know where I'm likely to get stainless No.7's

[Bob Minchin]

Could you tap it m4? tapping size 3.3 which is pretty close to you minor diameter and possibly the bore of the holes in the body.

[rick]

Don't know these are blind holes in moulded nylon item ... currently have less than a 1mm hole ... basically just enough to guide the screws. Worried about cracking the plastic. Guess I could tap.... but in nylon maybe the coarse and deep pitch of self tappers holdd better ?

[Alan Dawes]

What about something like a 4mm diameter, 25mm long stainless steel pozi drive wood screw? Lkke this:

Alan

[rick]

I have loads of No.8 stainless self tappers (~4mm diam) ....but when you put them alongside these 'No 7' they seem a hell of a lot bigger - afraid of cracking the moulding.

[newshound]

Nylon should not crack if you go up slightly in size.

[rick]

The unit has no ?pilot? holes just ?shallow dimples? to allow screws to start.

The unit has same feel & weight as Polypenco I have used in the past - but I have no knowledge of its characteristics other than it is injection moulded, and therefore likely to be thermoplastic

I am fitting the unit through a plywood base which will be reinforcing a lid to collecting drum ? overall thickness of about 12mm

The screws supplied ? No.7 self tappers are not long enough, so I need to use longer screws. I could try and source some longer No. 7 screws, although the No.7 seem quite small for the task ???. Or take 2 different approaches.

#1 as suggested - drill out the moulding and tap to M4 ? is the fairly fine thread form of M4 suited to holding in a such a material.

#2 slightly simpler ? I have plenty of No8 stainless self tappers - but would not like to simply screw these in, in case they crack the moulding. From what I find in my ref books the hole for a No.8 self tapping screw is 3.5mm if drilled clean. Any issues with this approach ? the coarser thread may be of benefit in holding in the plastic.

For both options is there a recommended drill speed & lubricant for such material?

[newshound]

I'd definitely go with self tappers rather than threads designed for metal. I can't see any risk of cracking with No. 8's and 3.5 mm pilot.

Drill speed low, no lubricant. Assuming you have a decent cordless drill/driver, just take it gently, rather as you would when inserting a screw into wood. The trouble with high speed is that the plastic will tend to melt, although that is not as bad when drilling as when sawing.

The other thing with thermoplastics, they may get more brittle around zero centigrade so if it is an outdoor job, do the drilling in a warm room (with the plastic at room temperature). Although if it is actually designed to be fixed with self tappers, I think you are probably worrying unnecessarily.

If the instructions say use No 7 with no pilot hole, I can't believe that you would need the full 3.5 mm with No. 8. That drill size is probably recommended for sheet metal.

[rick]

I can use my pillar drill to drill it ... I guess the issue is, if I don't go with 3.5 .. what size would I go with. Maybe I'll try 2.5mm and see how first screw goes in.