1" thread form tap drill size

[jprobst]

I have to tap some aluminum extrusions with a 1"-8 tap 3" deep. I wanted to use a thread form tap. I found the tap but cannot find a chart anywhere on the web telling me what size drill to use. Does anybody know where I can find a chart or what size drill to use. I was going to use 60% of thread. Any help would greatly be appreciated.

 

[epmtool]

TAP DRILL SIZE = Basic Tap O.D. —(.0068 x % of Thread Desired)/Threads Per Inch
.949

I found the formula here http://www.kar.ca/pdf/catalog/H4.pdf

 

[evansmachine]

Are you talking about roll forming? I just roll formed some 1/2-13s in 6061 and it seems like a 1-8 is gonna need some power.

 

[jprobst]

I have a 20 hp machine. Yes roll forming. It is an extrusion. 1/3" of the hole is missing. It looks almost like a glorified split pin.

 

[Joe788]

My chart says .9447 for 65% thread, with .9375 as the "generally accepted size".

 

[Limy Sami]

Out of interest I did a quick search for a roll form tap that size - nothing,......... unless someone knows better.


X 2 on the power requirement,........ and don't forget to consider whether the workpiece will take the neccersary clamping to overcome the torque forces.

 

[david n]

1/3 of the hole missing? Like a pie with a slice out of it? Basically an interupted cut? I'd hesitate tapping it if that is the case. Just thread mill it.

If you still plan on tapping, make sure you have a toolholder that's up for the job. 1" tap will want to spin in an ER collet and the like.

 

[Ox]

There is SO much wrong with this....

A) 60% on a form tap = hardly any profile.

B) A form tap requires a lot of structural integrity of the part. Your split part could not be further from requirement.

C) Even with a cut tap I can't imagine the part to withstand the forces of the tap w/o springing open and creating a small thread once finished. Unless you possibly have some sort of fixturing that will back up the free floating part of the exptrussion? And too much pressure there will create the opposite and leave an O/S thread when done.

D) Even the cut tap will want to walk out the open door.



I don't know what for mill you can find that would werk, but I can't imagine trying to tap such a part.


--------------

Think Snow Eh!
Ox

 

[voodoochld2]

tough to find a 3" long threadmill. i would use a cut tap, especially in this case

 

[Greg White]

I bought some single point(6 flutes, I dont know proper name) thread mills from accra pro or somting like that to do 1 1/8 -7 in s.s. pipe flanges.

They have a very long reduced neck/shank,got em done,took forever,would love to try it in some alum.

anyway they may be 3" in length,and they would create very little resistance as you fellows are sweating over.
Gw

 

[scojen]

Sounds as if you would be better off using a thread mill especially if the hole is split through the length it would probably be as quick as try to drive a tap and use less horse power to boot.

Scott

 

[Metalcutter]

Will the video be on YouTube?

That's BIG! Better use some GREAT! "slippery."

Regards, Honestly, good luck on that one.

We used to use STP and black sulfured oil. About 1/3 STP Pretty good for form taps.

Stan-

 

[jprobst]

I want to thank all of you for your responses. There is alot of different opinions I need to work though. I have tapped extrusions like this before but there is a big difference between a 1/4-20 and a 1-8. Bottom line is I'm going to have to get the extrusion and see what works better. This part is for window moulding anchors.