Strong 0-80??

[MickFlynn]

Hi all
I am currently using 0-80 3/16" stainless steel button head screws in an application to hold an inconel plate onto a stainless steel material. However I am finding that this type of screw yields when loads are applied to the assembly. These loads include axial forces and moments whose values I do not know (The moment on these screws could be anything up to 3Nm). What I really need is a stronger screw with the same physical attributes. I have looked into different types of steel but I am finding it difficult to find stronger materials in this size of screw. I am open to having the screws custom-made too. Any tips

Thanks
MF

 

[EnglishMuffin]

If it had the same physical characteristics it wouldn't be any stronger! (only kidding - just being a smart-ass).

If you don't mind going with a regular socket head instead of a button, you could try:

http://www.spstech.com/unbrako/htm/unbrako-1-21.html

(these are Unbrako - I would trust them)

The following site lists some as being alloy steel - I am guessing 12.9 - if so , thats as good as you'll get. But you have to be careful if they come from off-shore.

http://www.microfasteners.com/catalog/products/SSCSBA.cfm

 

[Metalguy]

Ask your machine shop if they can make such tiny screws from something like 17-4 or 17-7 PH. Easy to machine in the sol. ann. cond, and then HT for 4 hr. at 1100 deg. F. They'll discolor but shouldn't distort.

 

[EnglishMuffin]

A watchmaker could do it easy. Do they still have watchmakers ? And those little Sherline lathes that a lot of hobbyists have could handle it.

 

[EnglishMuffin]

One other thought - if you have them custom made you might have trouble putting a tiny internal hexagon into the head - which I assume is what you currently have, since you describe them as being button head. And you say you want "the same physical attributes". But, particularly in these very small sizes, that internal hexagon actually can lead to the head being weaker, so maybe using a different head style is what you might want to consider. How are they failing exactly - are the heads popping off ? A regular external hexagon head might be better - but since it sounds as though these are going to end up inside someone, maybe you don't want to do that.

 

[yates]

MickFlynn,
I somehow get the impression the screws you are looking for are for medical (prosthesis) applications. In that case, alloy steel screws are out of the question , and higher strength 4XX stainless steels are not really stainless in the human body. The best material would be Inconel, and since Inconel 718 is a commonly used material in aircraft fasteners, you may even find what you're looking for at an aerospace fastener distribution house. The '0-80 3/16'is not a familiar size with me, but standard aero screws for nearest fit would be 10-32 UNF (.190 diam. 32 tpi)
Try

http://www.fairchildfasteners.com

 

[TVP]

yates,

FYI, nickel is considered to have human compatibility problems, hence the use of titanium and low Ni/high N stainless steels vs. nickel-based alloys like Inconel for implants, bone screws, etc. More information can be obtained from standards such as ASTM F 543 and its references (F 1472, F1295, F1314, F1586, etc.).

 

[EnglishMuffin]

well , if thats the case, theres not a lot of wiggle room is there - looks like he needs bigger screws or more of 'em. I wonder why they have to be so small ? Must be a reason ....

 

[MickFlynn]

Thanks for all the info.

Metalguy
I have looked into getting 17-4 stainless steel 080 custom made but I cannot find anybody willing to make anything of this size. My machine shop does not have the capabilities to make something this small.

Yates
I have checked out Inconel screws but again I can not find anyone who could make something this size. 0-80 screws have a diameter of 0.06" approx and have 80 threads per inch.

EnglishMuffin
I am restricted with the diameter and height of the head of the screw. I have strain gauges on either side of these screws. At the moment, button heads seem to be the most suitable head I can find (small diameter = 0.11" and low head height = 0.04&quot. I am a little flexible with head height (+ 0.015&quot but can not spare much more room with the diameter.
I am also restricted by the diameter of the screw itself. I can't go any bigger.
Theres no room to add more screws either
The stainless steel screws I was using began to bend along the shaft of the screw during use. The heads didn't pop off.

Strength wise, the best 080 screw I could find was a 10.9 alloy steel with a black oxide coating.
FYI the instrument is not a permanent implant in the body. It comes in contact with the body for a short period of time and is enclosed (not completely) by stainless steel components.
I have contacted many companies who manufacture 'special' screws but many have shied away from making something this small with high strength and a high corrosion resistance.

Any further tips would be appreciated

Thanks
MF

 

[EnglishMuffin]

MickFlynn :
I'm sure there are a number of model engineers out there who would love to make these screws for you - they do stuff like that all the time and a lot of them are superb retired professional machinists. Try contacting the editor of Home Shop Machinist :

http://www.usubscribe.com/order.cfm?tid=11291&plky=21025>SE=GOTO>KW=The+Home+Shop+Machinist

 

[fastech]

Hello MickFlynn;

I have read your dilemma and teh comments with great interest. There is a company here in Kokomo that can help you I am sure. I am not affiliated with this company but do know of them and their work. They are very heavy into medical prosthesis and specialty screws. I have lost their web address but I can give you their snail mail and phone number.

He is a retired machinist that started his own business and has grown very rapidly.

WIS Sheet Sheet Metal
Attn Bill Summers, Prsident
1501 E Co Rd, 200 N
Kokomo, IN 46901
765-459-8804

Good Luck.
Bill P.


Bill P.
CSFT, Inc.