Tapping Aluminum?

[Pump2005]

I have to retap new holes in an aluminum frame to attach an aluminum cover to seal a clearwell. Using 3/8 bolts, what size tap is required? Also, are there any good books on tapping?

Thanks.

 

[ajack1]

It is always worth considering using inserts when tapping aluminium. Do a google search for helicoil or start with the link below.

http://www.emhart.com/products/helicoil.html

 

[ProEDesigner00]

I am not sure what your asking for. If you are using 3/8 bolts then you use a 3/8's tap. determine if the thread are fine or course. As Ajack said inserts might be needed but that determination should be made due to application.

If your going to drill for a course thread a 5/16 drill is required for 70% thread.

 

[Barry1961]

Make sure the tapping fluid you use is made for use on aluminum.

Barry1961

 

[CoryPad]

A good book for tapping is the Machinery's Handbook.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pump2005]

What I should have said was if I am using 1/4" bolts, what size drill bit should I use to make the hole? Thinking that a 1/4" bit would make the hole too large to thread.

 

[pennpoint]

Pump2005
Your post 21 Feb 06,
What!! Seek information from Machinery's Handbook as CoryPad said. Simple stuff... I would use thread inserts as indicated by "ajack1". All data is given and available for the required
thread size you want or need.

Wish you well


Best regards
pennpoint

 

[Eltron]

The drill size that you will use depends on the pitch of your threads. You said your bolts are 1/4". Are they 1/4-20, 1/4-28, or 1/4-32? You would use a #7, #3, or a 7/32" inch drill depending on what type of bolts you have. Do a web search for "tap hole chart," and you should be able to find what you're looking for. You can also use the formula for 75% threads which is: Major Diameter - One Pitch Length = Tap Hole Size. Incidentally, if you are using bolts you would need through holes with nuts on the back sides. If you want to tap a hole they magically turn into screws.

Hope that helps.

 

[ozchevy]

There is a product better than Helicoils, and I've used these many times - they are could Keyserts, they are far superior in locking in, and don't tear out like a Helicoil does.

On the subject of tapping threads in aluminium, the best cutting fluid I've used is metholated spirits.

 

[ProEDesigner00]

A 1/4-20 thread requires a #7 DRILL.

 

[Pump2005]

Thanks ProEDesigner00. Hopefully I can get away with just tapping the frame instead of using an insert.

 

[pennpoint]

Pump2005
Well good on you. I too hope you can get away with just tapping the aluminum material. But your question eluded to retaping and the best way and thats how many of us answered. If you want to get away with something why ask? Just do it!


Best regards
pennpoint

 

[kenvlach]

Noticeably absent is any description of what went wrong with the original threads.

Perhaps a matter of corrosion or galling. Use a corrosion-resistant anti-seize compound (e.g., a Loctite or Permatex product) on the bolt threads. Avoid those containing graphite or copper in wet applications (galvanic problem).

 

[Pump2005]

I was just planning on tapping the aluminum frame and bolting the cover on with stainless bolts. Every one was recommending inserts. The original bolts weren't stainless and over time most had broken off in the frame. The cover was not put back on the way it was taken off and had a cutout made in it. With that done the original holes were unuseable. If all goes well I will not need the inserts.

 

[kenvlach]

"The original bolts weren't stainless and over time most had broken off in the frame."

If the original bolts didn't gall (cold weld), then corrosion products in the threaded area seized up the bolts. The advice to use corrosion-resistant, anti-seize compound (non-Cu, non-graphite) also applies for SS bolts in aluminum. Tefgel & blue Loctite are recommended in The Boatowner's Guide to Corrosion for SS/Al.

 

[Pump2005]

Thanks kenvlach. The bolts rusted and probably snapped when they were removed. The water in the clearwell is also chlorinated and the cover has to be periodically removed.