For those of you who like to watch, here is a three-page photo essay showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water and
your fingers hurt.

--
Jeff R.
(well, *I* had fun)

On Sun, 18 Dec 2005 17:27:22 +1100, "Jeff R"
wrote:

For those of you who like to watch, here is a three-page photo essay showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water and
your fingers hurt.


OOOOO!!!! Very very nicely done so far, and an excellent series of
photos.

Well done Sir! Well done indeed!

Gunner

"Pax Americana is a philosophy. Hardly an empire.
Making sure other people play nice and dont kill each other (and us)
off in job lots is hardly empire building, particularly when you give
them self determination under "play nice" rules.

Think of it as having your older brother knock the **** out of you
for torturing the cat." Gunner

In article , Gunner Asch says...

OOOOO!!!! Very very nicely done so far, and an excellent series of
photos.

Well done Sir! Well done indeed!

Quite amazing. Is it really that easy to hobb gears?

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

I like it! But, being a novice, I have to ask: Why do you appear to be
turning from the center to the OD for facing operations? Does the carbide
cutter "like" that better?
Thanks

"Jeff R" wrote in message
u...
For those of you who like to watch, here is a three-page photo essay
showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo
tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water
and
your fingers hurt.

--
Jeff R.
(well, *I* had fun)

"Jeff R" wrote in message
u...
For those of you who like to watch, here is a three-page photo essay
showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo
tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water
and
your fingers hurt.


Cool shots.

One thing - your feed rate for the stainless is probably too low. Your
insert seems to have chip-breaker geometry. If the chips are not breaking,
you are not feeding hard enough. What's your SFPM and IPR? Mind you, your
machine may not have the power to run that cutter correctly, and I'm not
sure that insert is capable of breaking stainless chips so YYMV. Just a
thought.

Further, unless you're trying to achieve a certain angle with the leading
edge of your cutter (bad idea IMHO), you shouldn't set the cutter to
anything but ~90º from the work. You're increasing the load because of the
longer cut length, but not increasing your material removal rate. You're
using a finishing insert which has a reduced capability to withstand cutting
forces. You're more liable to break it in a "decent" cut.

You should systematically increase your feed rate until you get fairly low
edge life or your insert just breaks off - then back it off a bit. You'll
usually find your surface finish will be best when the insert is cutting as
it was designed, and you don't have to deal with razor blades in your face
(I nearly cut my right index finger off with such chips, seriously).
Obviously you need a rigid and powerful machine.

HTH.

Regards,

Robin

On Sun, 18 Dec 2005 17:27:22 +1100, "Jeff R"
wrote:

For those of you who like to watch, here is a three-page photo essay showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water and
your fingers hurt.

Very cool. Thanks!

On Sun, 18 Dec 2005 12:40:02 -0500, "Robin S."
wrote:

Cool shots.

One thing - your feed rate for the stainless is probably too low. Your
insert seems to have chip-breaker geometry. If the chips are not breaking,
you are not feeding hard enough. What's your SFPM and IPR? Mind you, your
machine may not have the power to run that cutter correctly, and I'm not
sure that insert is capable of breaking stainless chips so YYMV. Just a
thought.

It depends on the SS grade. If a manufacture can develop a reliable
chip breaker for 316 for example..or 304...he can make a ****load of
money.

Gunner

"Pax Americana is a philosophy. Hardly an empire.
Making sure other people play nice and dont kill each other (and us)
off in job lots is hardly empire building, particularly when you give
them self determination under "play nice" rules.

Think of it as having your older brother knock the **** out of you
for torturing the cat." Gunner

Hey Jeff,

Pix and chips both look great. Do more when you can. You've got
style.

I doubt that the tap you used as the hob would be making any more or
different cuttings than in regular use. It is not a type that is very
common around here, or maybe it's for a specific job with which I am
not familiar. The shank has flutes like a drill bit. What are the
flutes supposed to do when it is used in its "normal" mode?

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Sun, 18 Dec 2005 17:27:22 +1100, "Jeff R" wrote:

For those of you who like to watch, here is a three-page photo essay showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water and
your fingers hurt.

On Sun, 18 Dec 2005 14:54:30 -0500, Brian Lawson wrote:

Hey Jeff,

Pix and chips both look great. Do more when you can. You've got
style.

I doubt that the tap you used as the hob would be making any more or
different cuttings than in regular use. It is not a type that is very
common around here, or maybe it's for a specific job with which I am
not familiar. The shank has flutes like a drill bit. What are the
flutes supposed to do when it is used in its "normal" mode?


Get the chips out of the hole :-)


Mark Rand
RTFM

On 18 Dec 2005 07:52:07 -0800, jim rozen
wrote:

Is it really that easy to hobb gears?

No, it's a damn nuisance. Jeff is just making it look easy!

Whenever I've tried to hob worm wheels they've started out beautifully,
then "jumped a tooth" halfway through and demolished themselves. It
always took several goes to get a good one.

Ideally you need a tap that's ground for hobbing, with minimal flutes.
You don't need the chip clearance but you do need as much rigidity as
you can get. I've not done one as big as M10 but in the small sizes I've
only ever succeeded by using a centre as an additional support to the
far end of the tap.

Andy Dingley wrote:

....
Whenever I've tried to hob worm wheels they've started out beautifully,
then "jumped a tooth" halfway through and demolished themselves. ...

Hobbing is tricky: the pitch of the hob has to be the pitch of the gear
at the *minor* diameter, which is less than the gear pitch at the major
diameter. But you start the cutting at the outside (major diameter), so
there is a conflict. To handle this, the blank can be "gashed" at the
correct OD pitch and the hob will follow the gashes, even though they
are further apart than the hob teeth. A gash is just a thin slit across
the edge of the blank, easily done with a slitting saw. When I tried to
hob a 32 tooth straight gear without gashing, I got 33 teeth because of
the pitch discrepancy.

Ideally you need a tap that's ground for hobbing, with minimal flutes.
...

Jeff's photo's are of a drill bit with ground/turned threads, yes? If
so, was it purchased or made? Hobs are easily made, especially for
cutting soft material. It's basically the worm gear, with flutes
cut/ground out. Technical Video Rental
http://technicalvideorental.com/about_us.html
has a video on hobbing, which is pretty good, if slow (how many times do
you have to see the blank advanced and a cut made?).

Bob

Gunner Asch wrote:

On Sun, 18 Dec 2005 12:40:02 -0500, "Robin S."
wrote:

Cool shots.

One thing - your feed rate for the stainless is probably too low. Your
insert seems to have chip-breaker geometry. If the chips are not breaking,
you are not feeding hard enough. What's your SFPM and IPR? Mind you, your
machine may not have the power to run that cutter correctly, and I'm not
sure that insert is capable of breaking stainless chips so YYMV. Just a
thought.

It depends on the SS grade. If a manufacture can develop a reliable
chip breaker for 316 for example..or 304...he can make a ****load of
money.

Gunner

"Pax Americana is a philosophy. Hardly an empire.
Making sure other people play nice and dont kill each other (and us)
off in job lots is hardly empire building, particularly when you give
them self determination under "play nice" rules.

Think of it as having your older brother knock the **** out of you
for torturing the cat." Gunner

If you can't break 316 chips you need a stronger more rigid machine. We
run 316 all day long and can pack a 55 gal drum with 500+ pounds of
chips. ( they got to be small to do that. .024 feed with .5 inch off
the diameter)

John

"jim rozen" wrote in message
...
In article , Gunner Asch
says...

OOOOO!!!! Very very nicely done so far, and an excellent series of
photos.

Well done Sir! Well done indeed!

Quite amazing. Is it really that easy to hobb gears?

Jim


Yup.
Sure is.
Either that, or I am blessed with more than my fair share of dumb luck.

(or both)

I've made seven or eight by the method shown, and all have worked fine.
Not *brilliant*, but fine. I ruined the aluminium one by lapping it with
too-coarse paste, and the nylon one just didn't cut cleanly. All the brass
ones, however, have been OK.

--
Jeff R.

"Gunner Asch" wrote in message
...
On Sun, 18 Dec 2005 17:27:22 +1100, "Jeff R"
wrote:

For those of you who like to watch, here is a three-page photo essay
showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo
tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water
and
your fingers hurt.


OOOOO!!!! Very very nicely done so far, and an excellent series of
photos.

Well done Sir! Well done indeed!

Gunner

Thanks Gunner.
Its fun, making chips again.

--
Jeff R.

"Andy Dingley" wrote in message
...
On 18 Dec 2005 07:52:07 -0800, jim rozen
wrote:

Is it really that easy to hobb gears?

No, it's a damn nuisance. Jeff is just making it look easy!

(blush)
Dumb fool luck, actually.


Whenever I've tried to hob worm wheels they've started out beautifully,
then "jumped a tooth" halfway through and demolished themselves. It
always took several goes to get a good one.

Mine have just been well-behaved.
Never attempted to calculate the minor diameter required, just turned the
blank to suit the space available and the stock size on hand. It seems
there's been enough slack in the system to "take up" the difference when the
first revolution is completed.


Ideally you need a tap that's ground for hobbing, with minimal flutes.
You don't need the chip clearance but you do need as much rigidity as
you can get. I've not done one as big as M10 but in the small sizes I've
only ever succeeded by using a centre as an additional support to the
far end of the tap.

I did that for my first few, using a conventional straight-fluted tap. It
had a countersink drilled into its base, so I managed to support it between
centres (very cramped for space). I didn't like the action, though, so I
went onto a spiral-fluted tap. No countersink, so not between centres, but
it seems to be rigid enough. The blank, however, is not supported well
enough. More on that later.

--
Jeff R.

"Bob Engelhardt" wrote in message
...
Andy Dingley wrote:

...
Whenever I've tried to hob worm wheels they've started out beautifully,
then "jumped a tooth" halfway through and demolished themselves. ...

Hobbing is tricky: the pitch of the hob has to be the pitch of the gear
at the *minor* diameter, which is less than the gear pitch at the major
diameter. But you start the cutting at the outside (major diameter), so
there is a conflict. To handle this, the blank can be "gashed" at the
correct OD pitch and the hob will follow the gashes, even though they
are further apart than the hob teeth. A gash is just a thin slit across
the edge of the blank, easily done with a slitting saw. When I tried to
hob a 32 tooth straight gear without gashing, I got 33 teeth because of
the pitch discrepancy.

Ideally you need a tap that's ground for hobbing, with minimal flutes.
...

Jeff's photo's are of a drill bit with ground/turned threads, yes? If
so, was it purchased or made?

No, I used a conventional off-the-shelf spiral fluted tap. Some folks might
call them machine taps. I'm given to understand they will happily cut a
thread without backing off to break the chips (but I bought this one for
hobbing gears).

Here's a bunch of them: http://www.nachinip.com.sg/CT_TapsSeries_1.html
(not where I got mine)

When I hob my gears, I don't fight the tap's inclination to cut where it
wants to. I just let it rip and hope for the best. Kind'a like my
marriage, actually....

--
Jeff R.
(but let's not go there)

"Kelly Jones" wrote in message
...
I like it! But, being a novice, I have to ask: Why do you appear to be
turning from the center to the OD for facing operations? Does the carbide
cutter "like" that better?
Thanks


Dunno 'bout the cutter, but *I* like it.
I prefer it when the cutter finishes a cut by feeding into clear air, rather
than the back of the job - but that's just me.

--
Jeff R.

"Robin S." wrote in message
...

"Jeff R" wrote in message
u...
For those of you who like to watch, here is a three-page photo essay
showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo
tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water
and
your fingers hurt.


Cool shots.

One thing - your feed rate for the stainless is probably too low. Your
insert seems to have chip-breaker geometry. If the chips are not breaking,
you are not feeding hard enough. What's your SFPM and IPR? Mind you, your
machine may not have the power to run that cutter correctly, and I'm not
sure that insert is capable of breaking stainless chips so YYMV. Just a
thought.

Thanks Robin.
That thought occurred to me, so I increased the speed until the swarf
dutifully broke up - but I don't like the sound of a motor crying and
begging for mercy, so I went back to slow feed.

I'm using a pretty small school lathe (forget the exact specs) which is not
really happy with much more than I am feeding it now.


Further, unless you're trying to achieve a certain angle with the leading
edge of your cutter (bad idea IMHO), you shouldn't set the cutter to
anything but ~90º from the work. You're increasing the load because of the
longer cut length, but not increasing your material removal rate. You're
using a finishing insert which has a reduced capability to withstand
cutting
forces. You're more liable to break it in a "decent" cut.

Quite right on all counts. My excuses:
1) I set up the tool geometry mainly to achieve clearance - of the cutter
and the tool post. This is not a huge shaft I'm turning, and things get a
bit tight towards the end of the cut. The tailstock gets in the way.
Anyways, aside from producing those murderous long ribbons of death, the cut
is proceeding very nicely. You know the sound a tool makes when its cutting
*happily*? No screeching, no chattering, just a "shhhhhhh" like a breeze
through the treetops. Thats what it sounds like, and I like that. :-)
2) Choice of cutter. I'm working at an impoverished government (now
*there's* a tautology) school. This year we had to choose between lacquer,
abrasives or cutting tips. Lacquer and abrasives won out. I'm lucky I have
*any* tips left!


You should systematically increase your feed rate until you get fairly low
edge life or your insert just breaks off - then back it off a bit. You'll
usually find your surface finish will be best when the insert is cutting
as
it was designed, and you don't have to deal with razor blades in your face
(I nearly cut my right index finger off with such chips, seriously).
Obviously you need a rigid and powerful machine.

Yup. Right again.
Our machines are good - and well set up, but it would be dreaming to call
them "powerful".


HTH.

It did, thanks.



Regards,

Robin


Cheers

--
Jeff R.

"Don Foreman" wrote in message
...
On Sun, 18 Dec 2005 17:27:22 +1100, "Jeff R"
wrote:

For those of you who like to watch, here is a three-page photo essay
showing
the construction (incomplete as yet) of a heavy-duty GEM-style photo
tripod
head.

http://faxmentis.org/html/tripod.html

Ribbons of razor-sharp stainless steel - enough to make your eyes water
and
your fingers hurt.

Very cool. Thanks!

Welcome. Its all fun...

--
Jeff R.

"Brian Lawson" wrote in message
...
Hey Jeff,

Pix and chips both look great. Do more when you can. You've got
style.

(blush again)
Thanks Brian.



I doubt that the tap you used as the hob would be making any more or
different cuttings than in regular use. It is not a type that is very
common around here, or maybe it's for a specific job with which I am
not familiar. The shank has flutes like a drill bit. What are the
flutes supposed to do when it is used in its "normal" mode?

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

As Mark said, get the chips out.

Aren't spiral fluted taps in common use in the US and Canada?
I thought CNC machines would use them pretty exclusively...


--
Jeff R.

"Jeff R" wrote in message
u...

... so I went back to slow feed.


Sigghhhh.

I mean "slow speed".

J.

"jim rozen" wrote in message
...

Quite amazing. Is it really that easy to hobb gears?

Forgot to give credit where its due.
I am indebted to this site for inspiration:
http://www.atmsite.org/contrib/JSAPP.../wormgear.html

--
Jeff R.

In article ,
Jeff R wrote:

"Bob Engelhardt" wrote in message
...
Andy Dingley wrote:

...
Whenever I've tried to hob worm wheels they've started out beautifully,
then "jumped a tooth" halfway through and demolished themselves. ...

Hobbing is tricky: the pitch of the hob has to be the pitch of the gear
at the *minor* diameter, which is less than the gear pitch at the major
diameter. But you start the cutting at the outside (major diameter), so
there is a conflict. To handle this, the blank can be "gashed" at the
correct OD pitch and the hob will follow the gashes, even though they
are further apart than the hob teeth. A gash is just a thin slit across
the edge of the blank, easily done with a slitting saw. When I tried to
hob a 32 tooth straight gear without gashing, I got 33 teeth because of
the pitch discrepancy.

Ideally you need a tap that's ground for hobbing, with minimal flutes.
...

Jeff's photo's are of a drill bit with ground/turned threads, yes? If
so, was it purchased or made?

No, I used a conventional off-the-shelf spiral fluted tap. Some folks might
call them machine taps. I'm given to understand they will happily cut a
thread without backing off to break the chips (but I bought this one for
hobbing gears).

Here's a bunch of them: http://www.nachinip.com.sg/CT_TapsSeries_1.html
(not where I got mine)

When I hob my gears, I don't fight the tap's inclination to cut where it
wants to. I just let it rip and hope for the best. Kind'a like my
marriage, actually....

As an armchair machinist, unforunatly,I have to jump in hear and ask a
question;

*How* do you calculate and cut the dimater of the gear accuratly
enough so that the diamater divided by pi is an integral number of
thread pitch intervals, compicated by the need to measure points on a
thread. Did I say that right?


--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

On Mon, 19 Dec 2005 17:31:10 +1100, "Jeff R" wrote:

I didn't like the action, though, so I
went onto a spiral-fluted tap.

I should have mentioned that - I'd always been told you _must_ use a
spiral fluted tap, otherwise there's a portion of the tap's rotation
when nothing is touching and the blank can rotate out of step.

"Jeff R" wrote in message
u...

As Mark said, get the chips out.

Aren't spiral fluted taps in common use in the US and Canada?
I thought CNC machines would use them pretty exclusively...


Jeff,

In my experience, the spiral-fluted taps are more expensive than
spiral-pointed taps and thus less common.

You're right though. Very common in CNC applications and certainly widely
available in North America.

Regards,

Robin

On Mon, 19 Dec 2005 21:26:15 -0500, "Robin S."
wrote:


"Jeff R" wrote in message
. au...

As Mark said, get the chips out.

Aren't spiral fluted taps in common use in the US and Canada?
I thought CNC machines would use them pretty exclusively...


Jeff,

In my experience, the spiral-fluted taps are more expensive than
spiral-pointed taps and thus less common.


And it's my experience that the spiral flute taps aren't as strong
and don't cut as well as spiral point taps. I know I was real unhappy
with the amount of flex I got with 1/2NC spiral flute taps when
tapping SS. I was sure that I was going to break a tap. In the same
situation a spiral point tap is a walk in the park.

On the other hand spiral flute taps will curl the chips out of the
hole nicely. But after the above experience I won't use any unless I'm
forced to do a bunch of blind holes that don't have room for drilling
deeper for chip clearance (fortunately not something I run across very
often).


Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm

"xray" wrote in message
...
On 19 Dec 2005 09:38:56 -0500, (Al Dykes) wrote:

*How* do you calculate and cut the dimater of the gear accuratly
enough so that the diamater divided by pi is an integral number of
thread pitch intervals, compicated by the need to measure points on a
thread. Did I say that right?


Machinery's Handbook has a good listing of the many formulas for gear
stuff.

Some terms:
PD - pitch diameter - the diameter where the teeth meet (about half way
down the tooth)
N - number of teeth on the gear
CP - circular pitch - distance between two teeth around the pitch
diameter.
OD - outside diameter of the gear
Pi - 3.14159...

If you want to use a standard bolt thread as your worm gear, then CP
should be the same as the pitch of the thread.

From the book,

OD = (N+2) CP / Pi

CP = 1/TPI [TPI = threads per inch = teeth per inch]

so, OD = (N+2) / (Pi * TPI)


Say we use an 18 TPI tap as a hob and want a gear with 50 teeth

OD = (50+2) / (Pi * 18)
= .920 inch

I think that's right. Someone correct me if I screwed up.

Wow. gulp
No calcs from me.
I just turned the blank to a convenient size, and let the tap do its thing.
Magically - or stupidly - its worked every time.

--
Jeff R.
(call me lucky)

"Wayne Cook" wrote in message
...
On Mon, 19 Dec 2005 21:26:15 -0500, "Robin S."
wrote:


"Jeff R" wrote in message
. au...

As Mark said, get the chips out.

Aren't spiral fluted taps in common use in the US and Canada?
I thought CNC machines would use them pretty exclusively...


Jeff,

In my experience, the spiral-fluted taps are more expensive than
spiral-pointed taps and thus less common.


Now I'm feeling silly.
I'm not at all familiar with spiral pointed taps - just plain straight ones,
and spiral fluted ones.
Thanks for the info, gents

--
Jeff R.

On Mon, 19 Dec 2005 22:48:18 -0600, Wayne Cook
wrote:

On Mon, 19 Dec 2005 21:26:15 -0500, "Robin S."
wrote:


"Jeff R" wrote in message
.au...

As Mark said, get the chips out.

Aren't spiral fluted taps in common use in the US and Canada?
I thought CNC machines would use them pretty exclusively...


Jeff,

In my experience, the spiral-fluted taps are more expensive than
spiral-pointed taps and thus less common.


And it's my experience that the spiral flute taps aren't as strong
and don't cut as well as spiral point taps. I know I was real unhappy
with the amount of flex I got with 1/2NC spiral flute taps when
tapping SS. I was sure that I was going to break a tap. In the same
situation a spiral point tap is a walk in the park.

On the other hand spiral flute taps will curl the chips out of the
hole nicely. But after the above experience I won't use any unless I'm
forced to do a bunch of blind holes that don't have room for drilling
deeper for chip clearance (fortunately not something I run across very
often).


Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm
Wayne,
You are absolutely correct. The spiral fluted taps are weaker. Also,
because the chips are directed back out of the hole they drag on the
way out. Depending on the material this can damage the thread, break
the tap, or both. If the material is ductile enough I like form taps
as a first choice. Especially for blind holes. If the material can't
be form tapped and has a through hole the spiral point tap is my
first choice. Only when forced will I use the spiral flute taps.
ERS

On Tue, 20 Dec 2005 07:03:45 -0800, Eric R Snow
wrote:

On Mon, 19 Dec 2005 22:48:18 -0600, Wayne Cook
wrote:

On Mon, 19 Dec 2005 21:26:15 -0500, "Robin S."
wrote:


"Jeff R" wrote in message
m.au...

As Mark said, get the chips out.

Aren't spiral fluted taps in common use in the US and Canada?
I thought CNC machines would use them pretty exclusively...


Jeff,

In my experience, the spiral-fluted taps are more expensive than
spiral-pointed taps and thus less common.


And it's my experience that the spiral flute taps aren't as strong
and don't cut as well as spiral point taps. I know I was real unhappy
with the amount of flex I got with 1/2NC spiral flute taps when
tapping SS. I was sure that I was going to break a tap. In the same
situation a spiral point tap is a walk in the park.

On the other hand spiral flute taps will curl the chips out of the
hole nicely. But after the above experience I won't use any unless I'm
forced to do a bunch of blind holes that don't have room for drilling
deeper for chip clearance (fortunately not something I run across very
often).


Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm
Wayne,
You are absolutely correct. The spiral fluted taps are weaker. Also,
because the chips are directed back out of the hole they drag on the
way out. Depending on the material this can damage the thread, break
the tap, or both. If the material is ductile enough I like form taps
as a first choice. Especially for blind holes. If the material can't
be form tapped and has a through hole the spiral point tap is my
first choice. Only when forced will I use the spiral flute taps.
ERS

Amen!

Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm