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

For beginners, 20 TPI doesn't sound especially fine to me. Perhaps
32 TPI?? As a comment, your assembly sounds very similar to various
35 mm camera lens mounts. Perhaps you could check same and
sort of adopt whatever thread system used for them???

A final comment: The size and length of engagement sound as if this is
a one of a kind project. Therefore pick whatever is easiest for you!

Good luck!



"LPEL98" wrote in message
om...
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

Should be able to do this with very little math. If you have one of those
little, arrow shaped threading gages; look on the side and it will tell you
the "double depth of threads". In case you don't have one, I will tell you
it says you can use any thread finer than about 13 tpi for your parts. So
you can use 20 or 32 or any that you like the looks of.
Start with part A. Chuck it up in the lathe. Set your threading gearbox
to the number you want. Start cutting the internal thread. Take light cuts
until you have a nice thread with a slight flat on the tips of each thread.
Now with the tool sitting at the depth of the last pass you made for the
thread, move the tool into the thread like you were going to bore out the
entire thread. Only go about .010 deep. This will give you a counterbore
that is the OD of the thread. The purpose of this counterbore is so that
you can measure the OD. Now subtract .010 to .015" from this diameter and
you have the OD of part B.
Turn part B to this OD and start threading. Take light cuts. You should
end up going IN ABOUT the same amount you came OUT for the ID thread. Keep
checking fit with part A. Use oil. And DO NOT screw them together very
tight until you have the fit you want. If the fit is too tight the aluminum
will stick to itself and you will never get them unscrewed.
Good Luck



"LPEL98" wrote in message
om...
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

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

Hey LP,

Without commenting on anything else, using a thread of 1.75 X 20 will
give a pitch of 0.050. If the thread is a standard 60 degree, all 3
sides of the triangle formed will be the same 0.050", and the depth of
thread will be 0.050 X 0.866 = 0.0433", or a "diameter change" at the
roots of 0.0866". In your case, this is a "raise" from the 1.75 ID
of Part A to 1.8366, so you make the OD of the "screw-in" piece to be
that size (1.8366).

Taps and dies in "unusual" threads are expensive. For instance,
Wholesale Tool wants $115 for a 1.75 X 20 tap, but only $33 for a 1.75
X 10, while the norm is NC 1.75 X 5. Of course, you can't do a 1.75 X
10 with the material sizes you give, but cost wise you might like to
reconsider.

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXXXXX
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

I guess that I had "assumed" that both parts would be single point
threaded. No expensive taps involved.

Brian your depth of thread calculation is for a sharp V thread. The
formula for a standard thread with a 1/8P flat at the crest and root
is .6495/N. This gives a depth of thread of .0325 for 20 TPI and that
is what I had based my thinking on.

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/






On Tue, 20 Apr 2004 21:46:47 -0400, Brian Lawson
wrote:

Hey LP,

Without commenting on anything else, using a thread of 1.75 X 20 will
give a pitch of 0.050. If the thread is a standard 60 degree, all 3
sides of the triangle formed will be the same 0.050", and the depth of
thread will be 0.050 X 0.866 = 0.0433", or a "diameter change" at the
roots of 0.0866". In your case, this is a "raise" from the 1.75 ID
of Part A to 1.8366, so you make the OD of the "screw-in" piece to be
that size (1.8366).

Taps and dies in "unusual" threads are expensive. For instance,
Wholesale Tool wants $115 for a 1.75 X 20 tap, but only $33 for a 1.75
X 10, while the norm is NC 1.75 X 5. Of course, you can't do a 1.75 X
10 with the material sizes you give, but cost wise you might like to
reconsider.

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXXXXX
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