Machinery's Handbook shows that an internal 1 and 13/16 - 20 thread
has a minor diameter of 1.7584 so that might be a good choice. You
will only have to open the ID of the tube by .0084 so you won;t lose
hardly any wall thichness.

Therefore your OD of part B will be 1.8125. The minor diameter of the
outside thread will be 1.753. If you do not have thread wires with
which to check the thread you could turn a short length of the part to
the minor diameter and then thread until the tool just kisses the
1.753 pilot diameter.

If it were me I would make the part B first and use it as a thread
plug to fit up part A. Your fit will depend on the care you take in
the final few cuts. Remember, threading is an exercise in patience.
As you get close to finish size advane the tool only .001 at a time,
take several spring cuts and check, check, check. Rushing will not
accomplish anything.

You might try using mineral spirits (as in paint thinner) as a
lubricant when cutting aluminum. Just a suggestion.

Good luck and let us know how it turns

Errol Groff
Instructor, Machine Tool Department
H.H. Ellis Tech
613 Upper Maple Street
Danielson, CT 06239

860 774 8511 x1811

http://pages.cthome.net/errol.groff/

http://newenglandmodelengineeringsociety.org/ out.








On 20 Apr 2004 17:05:17 -0700, (LPEL98)
wrote:

I have a piece of 6061 AL tube, 2"OD, 0.125" wall, 1.75" ID, 1.25"
long. Lets call it Part A. I want to machine a second piece from
1.875" OD solid round 6061AL rod that will screw into the first piece.
Call that second piece Part B. The rod, Part B will have a 1" hole
through it and will be 1" long, and will screw into the tube, Part A
to a distance of 0.25".

Two questions.

First, since Part B will be around 1.75" diameter but only screw into
Part A for 1/4", does that suggest any specific thread pitch? I
arbitratily chose a 1.75-20, thinking that a fine thread will hold it
more snugly (less movement side to side) because of the higher thread
count. I want the joint to be as secure as possible with as little
slop and play as possible.

Second, since Part A is already 1.75" ID, the threads will be cut into
that. This means the OD of my Part B will need to be 1.75" plus
whatever the thread depth of Part A is, plus/minus a tolerance value.
I haven't done threading before. Is there a standard thread depth for
a given pitch? What is it? And how much extra (if any) should I
allow for to be sure that when the parts are made, machining
tolerances don't cause them not to mate correctly?

In other words, given Part A is 1.75"ID, what should I machine the
diameter of Part B to, and what thread depth should I machine to get
them to mate cleanly without a ton of slop and side-to-side play?

Thanks!
LP