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krems
02-01-2008, 02:21 PM
I'm debating on using either 10-24 or 10-32 threads on a stainless steel bolt for a project (low stress bolt) that will be screwed into 2024 aluminum. I only have room for .380" - .400" total thread length. I'm woried about the threads wearing out over time in the 2024 aluminum. Do you think I should use the 10-32 or 10-24 unf threads for this project.....Also - should I use conventional taps or a roll forming taps.......Are roll form threads really that much stronger??......inquiring minds wants to know the truth from someone w/ real experience.

Thanks..............krems

smiller6912
02-01-2008, 02:30 PM
Just a thought, have you considered threaded inserts. We have used them for MIL airframe work and they do a great job.
Ref Keensert: http://alcoafastener.thomasnet.com/Category/keenserts-reg-

Mcgyver
02-01-2008, 03:02 PM
wear out, as in from repeated assembly? generally fine threads are superior - stronger, lower helix angle, less wall thickness required. .4" is longer than it need be; the bolt cross sectional area would be less than than the total thread area in shear.

I've not heard roll formed female threads are stronger (but I've not heard everything) , seems like it might a challenge in 2024 at that size.....generally when you hear roll formed threads are stronger they're referring to the bolts, and a rolled thread on a bolt is stronger than cut because cutting creates stress risers (sharp internal corners where the localized stress will be higher and where failure can start)

SGW
02-01-2008, 03:07 PM
You've got over 2X fastener diameter for thread length, which is plenty. I wouldn't worry about it.

Can't help you on the rolled/cut thread question.

How often do you plan to assemble/disassemble this thing? If it's "a lot," the threaded insert idea might be worth looking into.

bigbill04
02-01-2008, 03:56 PM
I'm debating on using either 10-24 or 10-32 threads on a stainless steel bolt for a project (low stress bolt) that will be screwed into 2024 aluminum. I'm worried about the threads wearing out over time in the 2024 aluminum. Do you think I should use the 10-32 or 10-24 unf threads for this project. Thanks..............krems

The first time I had to take apart my Ingersoll-Rand 1/2 inch air impact gun, I noticed that the three steel allen bolts that held the air inlet cover to the aluminum pistol frame threaded not directly into the aluminum but into steel heli coils that were installed in the aluminum frame holes. At first I thought that maybe the gun was repaired by the company and sold to me as new, but after taking apart my other air gun of the same model, I realized that they had installed the heli coils. I think that this is a good idea if you have to remove steel screws from an aluminum part. Also, wouldn't this prevent the corrosion that happens between steel and aluminum? I think that Ingersoll locktited the heli coils in place to prevent them from backing out when the screw was removed. As far as using fine thread screws, I would as they are stronger than coarse thread screws. My humble opinion anyway.

fasto
02-01-2008, 04:05 PM
As far as using fine thread screws, I would as they are stronger than coarse thread screws. My humble opinion anyway.

Coarse thread screws are stronger than fine thread screws, as there is more "meat" left in the female thread helix.
Roll-formed threads are stronger still, as the metal isn't torn out to make the thread, it's pushed out of the way, a form of work-hardening (don't know how much that'll help in 20xx Al, though).

Quote from Machinery's Handbook:
"Coarse-Thread Series: This series, UNC/UNRC, is the one most commonly used in the bulk production of bolts, screws, nuts and other general engineering applications. It is also used for threading into lower tensile strength materials such as cast iron, mild steel and softer materials (bronze, brass, aluminum, magnesium and plastics) to obtain the optimum resistance to stripping of the internal thread."

Evan
02-01-2008, 05:27 PM
I don't recommend using stainless steel against aluminum if it must be repeatedly disassembled. Both metals are prone to galling and will gall against each other. That's why helicoils are often used, not so much because of strength issues.


Also - should I use conventional taps or a roll forming taps.......Are roll form threads really that much stronger??......inquiring minds wants to know the truth from someone w/ real experience.

I wouldn't think of trying to roll form 2024 aluminum. In the T-3 or T-4 temper it has about the same strength as mild steel (60,000 psi yield) but has a much greater tendency to grab the tap. It isn't easy to tap and you will want to go to perhaps less than 75% thread as well as using the best available taps. Use either spiral point or spiral flute taps depending on whether you are tapping a through hole or not.. Spiral point is for through holes and spiral flute is for blind holes. Also, don't use reduced shank taps but stick with full diameter shanks.

ptjw7uk
02-01-2008, 06:22 PM
Interesting Evan about galling, I was cutting some fine threads in SS nuts the other day and the tap got stuck and didnt want to go in or out anymore when I eventually managed to get the tap out there were 3 areas where SS had galled in the tap. Had the devil of a job to get it out, so what can I use to stop the galling in the first place!!
Peter

Evan
02-01-2008, 06:50 PM
Tap with lard oil.

Mcostello
02-01-2008, 08:43 PM
Can you install a stud and avoid wear on part entirely?

J Tiers
02-01-2008, 10:04 PM
The fine threads Do have more strength due to the larger core area of the screw. For aluminum female threads, that isn't going to be a lot of problem. Although the stronger grades are similar to low carbon steel, they are nothing like as strong as heat treated steels.

The bigger problem even with stronger grades of aluminum is cross-threading. Coarse threads are much easier to get lined up and mated properly.

wirewrkr
02-01-2008, 10:17 PM
I'm with MCostelo on the stud idea. Use them anytime I can get away with it for something that's gonna be coming apart often enough for you to wonder about thread integrity. Second choice would be high quality thread inserts.
I have been using Time-sert for years (20 or more) in the motorcycle and German auto business, love em.
Helicoil is a joke in comparison imho.
Robert

agrip
02-01-2008, 10:32 PM
IIRC Helicoils were one of the first really practical thread inserts used un aluminum. Dating from WWII.

The stretch problem in bolting causes highly stressed top-end of hole threads.
The fine-thread end-threads are subject to higher percentage of damage and then higher loading without the meat to carry the load and failure.

Helicoils had a load distribution feature, they greatly increased the shear out strength because of greater dia of the stressed Al, required the least use of rationed alloy metals, diamond formed stainless wire was quite smooth and strong, and all in all coarse threads in aluminum took command particularly with helicoils.
One of the ways we kept em flying, with strength and reliability of the threaded joint.

Since WWII many designs of inserts have happened.
There are solid inserts that take up the same space as helicoils, and are much easier to install.

dfw5914
02-02-2008, 02:21 AM
If you use HeliCoils, the pitch wont matter as long as you install the HeliCoils correctly. Be very particular about keeping the tap perpendicular to the work so as to arrive at a very accurate, straight, thread. Slightly countersink the entry and seat the insert past flush at least 1/4 turn into a full thread form. 2024 is corrosion prone, so use some zinc chromate primer (wet on the insert during installation) or even better would be cadmium plated HeliCoils. If you do use the cad plated coils, use cad plated fasteners as well. Standard HeliCoils are stainless so a stainless fastener is fine in that case.

For more info check out the HeliCoil web site or Google MS33537

If you choose to skip the inserts and thread directly into the aluminum, then use the coarse thread and by all means use a thread forming tap (done right of course, correct size hole and all) they are the obvious choice in malleable materials. A little wax on the fastener will go a long way to help prevent galling.

Evan
02-02-2008, 06:25 AM
Good point on the zinc chromate application. That is SOP in aircraft work with 2024. If using a thread forming tap in 2024 it will locally anneal the metal during the threading operation and it will be necessary for the work to age for at least several days before assembly in order to regain some hardness in the thread form. Thread cutting taps don't pose this problem as they don't generate the heat of plastic deformation during tapping. 2024 is a standard aircraft alloy and I don't recall ever seeing tapped holes used in aircraft applications. For thin material a clinch nut fastener is normally used and for deep holes an insert is standard.

krems
02-02-2008, 10:44 AM
Great information on the helicoil and other inserts. I can see why they are popular in aluminum. I don't think they are necessary in my application. I'm making a bullet mould and just looking for the strongest thread type to use. The stainless screw/bolt will be used to hold the sprue plate in place. It's not a part that will removed very often. What about the heat factor (750 degrees) for a sustained time......Is stainless steel a good choice or should I go w/ a hardened tool steel of some kind ??



Krems

Evan
02-02-2008, 11:01 AM
Why use 2024 for a bullet mold? Any strength advantage will be lost the first time it is heated. Any aluminum alloy will do since any alloy will wind up in the dead soft condition after exposure to that temperature for long. Use 6061. It's easy to machine and doesn't have the same susceptibility to corrosion that 2024 has.

krems
02-02-2008, 11:45 AM
Good point Evan....I decided to use 2024 aluminum only because a couple manufactures (LBT, NEI..?? Rapine) use this alloy. I figured they did all the research and settled on what they thought worked best.

6061 does machine nice however........I'll make one of each then decide for myself

Thanks......Krems