Depth of a hole for a screw

[davidrdguez]

Hi there!

Here I bring an easy one, but I'm a beginner.

I'm connecting several parts to a sheet with screws.

The screws are ISO 4762 M8x25; the supported parts thickness is 11.6 mm.

I need to specify the depth of both holes and thread in the sheet. A V-Bottom will be included (120º).

In principle this sheet has a thickness of 20 mm. I would like to stick tot he value but it could be modified.

Attached a drawing to explain this better. (Distances A and B are related)

Thank you very much

PD: Sorry for the bungling thread representation!

Your file's link is:

http://files.engineering.com/getfil...13-430b-92b1-b18a4805c2aa&file=hole_depth.jpg

 

[drawoh]

davidrdguez,

You have not specified any materials, or what you are trying to accomplish. Are you after maximum strength? Are you trying to control clamping forces? What control do you have over assembly process?

Most of this stuff is covered in machine design and fastener textbooks.

JHG

 

[hydtools]

Specify B. Two to three full threads plus the lead of the tap.

Ted

 

[hydtools]

I should have said 13.4mm plus two to three thread plus the lead of the tap.

Ted

 

[davidrdguez]

Hi,

Thank you for the reply; here my answer.

We will use steel screws on an aluminium sheet. We control (and do!) all the process of screwing. Also we have a workshop that will machine everything.

No after maximum strength.

I attach a picture to make it more understandable. The light green platform is the aluminium sheet. And then, on top of it, these other parts where one can see the screws (4 per piece).

Thanks again

Cheers,

Your file's link is:

http://files.engineering.com/getfile.aspx?folder=1ecda918-3603-4c28-98aa-80b952031eca&file=03.jpg

 

[davidrdguez]

I haven't read your message hydtools when I answered.

What do you mean with the "lead of the tap"?

Thanks!

 

[hydtools]

The thread tap has a short portion of the tip tapered to make starting the tap easier. It creates imperfect threads for which you must make allowance in a blind tapped hole.

Ted

 

[PeterStock]

I would take the 13.4 nominal plus the tolerance of each of the thicknesses round it up to the nearest 5mm and call that the min full thread depth. Then I would add about 12mm (1.5 * 8mm) and call that the max hole depth.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

http://www.linkedin.com/profile/view?id=30064526&trk=tab_pro

 

[BrianPetersen]

It's in a 20mm plate. Is there a reason for this not to be a through hole?

 

[KENAT]

I prefer to say 'x min full thread do not break thru' for blind holes and just make sure I have enough depth that I don't need a bottom tap. Generally, at least in US & UK you specify your end product not how to get there so you don't normally specify tap drill etc.

However, in this case, I think Brian may be nearer what you want as a thru hole may be better - if you can handle it.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[chicopee]

The Machinery Handbook has a section on thread depth for similar and disimilar materials.

 

[davidrdguez]

Yes. I think making through holes would be the best.

I write down all or your comments and I will cross check with the workshop.

Thank you very much

PS: chicopee (or anyone else). I was thinking about getting a mechanical handbook, any recommendation??

Cheers,
David

 

[Cockroach]

Generally speaking, the minimum distance between the depth of the drill hole to that being tapped is about three (3) threads. Since you have M8 which is either 1.25 or 1.0 mod, you're looking about 3.0 mm standoff. I would make C = 28mm.

B is simply the drill depth that would comfortably give you full thread engagement. I like something like 3 threads beyond C, so that would put B = 31mm.

Dimension A does not matter. It is a natural consequence of full shoulder drilling to point B. Only B and C are important relative to your 25 mm screw shank length.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada