[Jim Wilkins]

On Jun 26, 3:18*pm, " wrote:
On Jun 26, 1:10*pm, "Ed Huntress" wrote:

It's also interesting that
Acme threads derived from a group of miscellaneous flat-bottom threads known
as "******* threads." The 29-degree angle, says Colvin, probably was chosen
because Brown & Sharpe published a simple method for laying out cutting
tools with 29-degree angles.

--
Ed Huntress

I think Colvin is wrong in this case. *My understanding is that the 29
degree angle is the strongest angle. *It is also the angle used for
gears *(14.5 degree pressure angle. *). *Brown and Sharpe came out
with involute gear cutters in 1858 so they are likely the source of
the angle. *Brown and Sharpe probably published a method for laying
out tools with 29 degree angles as that would be what is needed to cut
a rack using a shaper to work with 14.5 PA gears.

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Dan

The sine of 14.5 degrees is 1/4, so a patternmaker can accurately
construct the angle by linear measurement.

jsw

[Phil Kangas[_3_]]

"Wes" wrote in message
"Phil Kangas" wrote:

Wes
Actually I don't have a taper attachment, but I do have an
offset center device for the tailstock. Somewhere in the
past
I saw a pic of this thing so I made one. It is not the
most
secure
thing in the tailstock so means must be made to sprag it
in
place
to prevent movement. Shall I post a pic to the dropbox?
Maybe
I should as others may copy it.
Tomorrow.......maybe.....phil


Heck yes, post a picture.

Wes

Pic is in the dropbox under:
http://www.metalworking.com/DropBox/...ock_center.jpg

[Doug White]

"Phil Kangas" wrote in news:i0anrm$pih$1
@news.eternal-september.org:


"Wes" wrote in message
"Phil Kangas" wrote:

Wes
Actually I don't have a taper attachment, but I do have an
offset center device for the tailstock. Somewhere in the
past
I saw a pic of this thing so I made one. It is not the
most
secure
thing in the tailstock so means must be made to sprag it
in
place
to prevent movement. Shall I post a pic to the dropbox?
Maybe
I should as others may copy it.
Tomorrow.......maybe.....phil


Heck yes, post a picture.

Wes

Pic is in the dropbox under:
http://www.metalworking.com/DropBox/...ock_center.jpg

Someplace I saw a similar setup using a small boring head. Even gave
calibrated offsets on the dial. Not sure if the tang on the taper had
been cutoff, of carefully aligned so that the axis of the head was
horizontal.

I actually found a web site with a description of such a device, but he
doesn't address how you make sure it's horizontal:

http://homemetalshopclub.org/news/feb00/feb00.html

Doug White

[Phil Kangas[_3_]]

"Doug White" wrote in message
"Phil Kangas" wrote in

"Wes" wrote in message
"Phil Kangas" wrote:

Wes
Actually I don't have a taper attachment, but I do have
an
offset center device for the tailstock. Somewhere in the
past
I saw a pic of this thing so I made one. It is not the
most
secure
thing in the tailstock so means must be made to sprag it
in
place
to prevent movement. Shall I post a pic to the dropbox?
Maybe
I should as others may copy it.
Tomorrow.......maybe.....phil


Heck yes, post a picture.

Wes

Pic is in the dropbox under:
http://www.metalworking.com/DropBox/...ock_center.jpg

Someplace I saw a similar setup using a small boring head.
Even gave
calibrated offsets on the dial. Not sure if the tang on
the taper had
been cutoff, of carefully aligned so that the axis of the
head was
horizontal.

I actually found a web site with a description of such a
device, but he
doesn't address how you make sure it's horizontal:

http://homemetalshopclub.org/news/feb00/feb00.html

Doug White

I can see how that would work but it's a bit bulky for a 9"
SB. I have
two sizes of Flynn boring heads but they're on an R-8 mount
that I'd
rather not monkey with. I'd have room on my 13x40 turn-pro
but can't
justify the cost of another head, yet. A boring head
on a 3MT is kinda useless other than for a tailstock center,
eih?
Making this thing was a good schoolhouse lesson for me
....;))
And you get to use trig to set it up too!
phil

[Wes[_5_]]

Doug White wrote:

Someplace I saw a similar setup using a small boring head. Even gave
calibrated offsets on the dial. Not sure if the tang on the taper had
been cutoff, of carefully aligned so that the axis of the head was
horizontal.

There is usually two opposed parallel flat surfaces on a boring head. The part that holds
the boring tool. You indicate that. Insert the boring head in the tailstock loosely, set
the indicator on the cross slide, tram, tap, indicate, tap, then wack it in place for use
when you have it horizontal to the ways.

Wes

[Richard J Kinch]

Ed Huntress writes:

'Sounds reasonable, but why do they plunge straight in with *external*
Acme threads?

You're correct in your earlier posts that this doubles the chip load, but
the cutting forces oppose each other and cancel out, and moreso with the
narrow Acme angle versus the 60 degree thread form. This cancellation
means that the tool rigidity can handle a lot more pressure than if you
were making just a one-sided cut. It is like drilling a center hole, where
two cutting edges oppose each other and allow you to use a lot more
pressure and feed than if you were boring a center hole with one cutting
edge.

[Ed Huntress]

"Richard J Kinch" wrote in message
. ..
Ed Huntress writes:

'Sounds reasonable, but why do they plunge straight in with *external*
Acme threads?

You're correct in your earlier posts that this doubles the chip load, but
the cutting forces oppose each other and cancel out, and moreso with the
narrow Acme angle versus the 60 degree thread form. This cancellation
means that the tool rigidity can handle a lot more pressure than if you
were making just a one-sided cut. It is like drilling a center hole,
where
two cutting edges oppose each other and allow you to use a lot more
pressure and feed than if you were boring a center hole with one cutting
edge.

That could be it; the resistance of the tool and toolholding pieces are a
lot more rigid when resisting loads perpendicular to the bed.

I've never doubted that there must be some reason for it. The thing that's
always bugged me is that I don't recall ever reading or hearing an
explanation. The reason that's curious is that I immersed myself in old
machining books when I was an editor at _American Machinist_. And we had
what probably was the best collection of them in the world.

sigh It will have to remain one of those mysteries of the universe. g

--
Ed Huntress