About screws
I have a small library which generates shapes that are holes for screws. So I have a table of numbers describing the screws I use. I took the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the diameter of the hole should be the nominal diameter - a function of the pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
-
specs - the screw measurements, what I want to get from BOSL2/screws.scad
-
gap - space between the bottom of the head and the nut
-
clearS - clearance on the screw end
-
clearH - clearance on the head end
-
pos - one of 6 positions to return the hole in, top of the head, middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using the same parameters. The slot can slide a square or hex nut from the side, or drop in a hex nut longitudinally. The use of the “pos” parameter makes it easy to position the screw hole properly.
I'm not sure I understand exactly what your parameters are. The BOSL2
screws library is machine screws, so it doesn't have dimensions for a
self-tapping wood (?) screw. So I'm not sure it's much help for that
application. I suspect wood screws are less standard than machine
screws, since they don't have to mate with anything, so I don't know
what kind of standards they adhere to. For machine screw clearance
holes I suppose the best way to do it would be to use the d_major that
you get for a nut and if you do that then the bolt should fit---if it
didn't it wouldn't be able to fit into the nut, so I don't see the
need for any adjustments assuming your fabrication is accurate.
I don't understand what the various clearance and gap parameters do.
Of course, the easiest way to make a clearance hole would just be
something like screw("#8", thread=0, length=25) and maybe a head if
you want a countersink. But that'll be undersized, so you'd need to
oversize it by some amount with the oversize parameter, so that's a
little less tidy than just starting with the right dimension.
If you're going to use BOSL2 then instead of returning "pos" you
should use the BOSL2 attachment capability to control positioning of
the hole.
On Tue, Nov 2, 2021 at 7:00 PM bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for screws. So I have a table of numbers describing the screws I use. I took the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the diameter of the hole should be the nominal diameter - a function of the pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get from BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top of the head, middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using the same parameters. The slot can slide a square or hex nut from the side, or drop in a hex nut longitudinally. The use of the “pos” parameter makes it easy to position the screw hole properly.
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For metric machine screws the pilot hole for tapping is just the outer
diameter minus the pitch. For self tappers and wood screws I think to some
extent it depends on the material it goes into and its thickness. I either
google it or measure the minor diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws in my library but I just
noticed No4 and No6 are both 2mm holes and I can't remember why. The
comments do say into soft wood and it must have been the drill I used
making my first Mendel90 out of MDF. It doesn't really matter much into
soft wood but it is more important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for screws.
So I have a table of numbers describing the screws I use. I took the
measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the diameter
of the hole should be the nominal diameter - a function of the pitch,
presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
-
specs - the screw measurements, what I want to get from
BOSL2/screws.scad
-
gap - space between the bottom of the head and the nut
-
clearS - clearance on the screw end
-
clearH - clearance on the head end
-
pos - one of 6 positions to return the hole in, top of the head,
middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using the
same parameters. The slot can slide a square or hex nut from the side, or
drop in a hex nut longitudinally. The use of the “pos” parameter makes it
easy to position the screw hole properly.
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He didn't say he wanted to do tapped machine screws. So using OD minus
pitch means you're making an assumption about the desired amount of
extra for tapping. The nominal ID will be the diameter mus
sqrt(3)*pitch so by subtracting only the pitch you are implicitly
making the hole larger than the screw ID by sqrt(3)-1 times the pitch,
or about .7 times the pitch. Is that always the right amount of
"extra" to leave for tapping? I don't know the answer. This rule of
thumb will work the same for UTS screws because they have the same
geometry.
On Wed, Nov 3, 2021 at 4:41 AM nop head nop.head@gmail.com wrote:
For metric machine screws the pilot hole for tapping is just the outer diameter minus the pitch. For self tappers and wood screws I think to some extent it depends on the material it goes into and its thickness. I either google it or measure the minor diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws in my library but I just noticed No4 and No6 are both 2mm holes and I can't remember why. The comments do say into soft wood and it must have been the drill I used making my first Mendel90 out of MDF. It doesn't really matter much into soft wood but it is more important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for screws. So I have a table of numbers describing the screws I use. I took the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the diameter of the hole should be the nominal diameter - a function of the pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get from BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top of the head, middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using the same parameters. The slot can slide a square or hex nut from the side, or drop in a hex nut longitudinally. The use of the “pos” parameter makes it easy to position the screw hole properly.
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For metric screws the minor diameter is not nominal minus sqrt(3) times the
pitch. That is the base of the triangle but it is fillited at 5/8 and 1/8.
According to Wikipedia it is the major diameter minus 1.082532 times the
pitch. Using standard drill sizes, that is close enough to 1 times the
pitch.
[image: image.png]
[image: image.png]
On Wed, 3 Nov 2021 at 10:53, Adrian Mariano avm4@cornell.edu wrote:
He didn't say he wanted to do tapped machine screws. So using OD minus
pitch means you're making an assumption about the desired amount of
extra for tapping. The nominal ID will be the diameter mus
sqrt(3)*pitch so by subtracting only the pitch you are implicitly
making the hole larger than the screw ID by sqrt(3)-1 times the pitch,
or about .7 times the pitch. Is that always the right amount of
"extra" to leave for tapping? I don't know the answer. This rule of
thumb will work the same for UTS screws because they have the same
geometry.
On Wed, Nov 3, 2021 at 4:41 AM nop head nop.head@gmail.com wrote:
For metric machine screws the pilot hole for tapping is just the outer
diameter minus the pitch. For self tappers and wood screws I think to some
extent it depends on the material it goes into and its thickness. I either
google it or measure the minor diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws in my library but I
just noticed No4 and No6 are both 2mm holes and I can't remember why. The
comments do say into soft wood and it must have been the drill I used
making my first Mendel90 out of MDF. It doesn't really matter much into
soft wood but it is more important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for
screws. So I have a table of numbers describing the screws I use. I took
the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the
diameter of the hole should be the nominal diameter - a function of the
pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get from
BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top of the head, middle
of gap, etc.
To the basic screw hole a slot for a captive nut can be added using the
same parameters. The slot can slide a square or hex nut from the side, or
drop in a hex nut longitudinally. The use of the “pos” parameter makes it
easy to position the screw hole properly.
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Yeah, I forgot about the clipping of the top of the screw profile. But
what's the ideal tap hole size? Should it be equal to the ID of the
threaded hole? If that's the case, the original poster can just use that
value from the screw data. Note that d_major is not equal to the nominal
screw size---or rather, it's a range. So for M6 the spec requires d_major
to be between 6mm and 6.3mm. But subtracting the pitch from the nominal
diameter does seem to put the result in the middle of the inner diameter
range, which is 4.9mm to 5.1mm. Of course, it didn't look to me like the
original poster was interested in tapping machine screws. Another
question: should tapping diameter be different for plastic than for metal?
I think for tapping holes in wood for machine screws one may use a hole
undersized compared to the hole for metal tapping.
On Wed, Nov 3, 2021 at 7:06 AM nop head nop.head@gmail.com wrote:
For metric screws the minor diameter is not nominal minus sqrt(3) times
the pitch. That is the base of the triangle but it is fillited at 5/8 and
1/8. According to Wikipedia it is the major diameter minus 1.082532 times
the pitch. Using standard drill sizes, that is close enough to 1 times the
pitch.
[image: image.png]
[image: image.png]
On Wed, 3 Nov 2021 at 10:53, Adrian Mariano avm4@cornell.edu wrote:
He didn't say he wanted to do tapped machine screws. So using OD minus
pitch means you're making an assumption about the desired amount of
extra for tapping. The nominal ID will be the diameter mus
sqrt(3)*pitch so by subtracting only the pitch you are implicitly
making the hole larger than the screw ID by sqrt(3)-1 times the pitch,
or about .7 times the pitch. Is that always the right amount of
"extra" to leave for tapping? I don't know the answer. This rule of
thumb will work the same for UTS screws because they have the same
geometry.
On Wed, Nov 3, 2021 at 4:41 AM nop head nop.head@gmail.com wrote:
For metric machine screws the pilot hole for tapping is just the outer
diameter minus the pitch. For self tappers and wood screws I think to some
extent it depends on the material it goes into and its thickness. I either
google it or measure the minor diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws in my library but I
just noticed No4 and No6 are both 2mm holes and I can't remember why. The
comments do say into soft wood and it must have been the drill I used
making my first Mendel90 out of MDF. It doesn't really matter much into
soft wood but it is more important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for
screws. So I have a table of numbers describing the screws I use. I took
the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the
diameter of the hole should be the nominal diameter - a function of the
pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get from
BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top of the head,
middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using
the same parameters. The slot can slide a square or hex nut from the side,
or drop in a hex nut longitudinally. The use of the “pos” parameter makes
it easy to position the screw hole properly.
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To unsubscribe send an email to discuss-leave@lists.openscad.org
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The ideal tap size for machine screws is the nominal OD minus the pitch *
sqrt(3) * 5 / 8. I.e. the ID of the internal thread. Rounded to the nearest
drill size it is just nominal minus pitch. If the hole was smaller the tap
would tend to bind as it has to widen the hole as well as cutting the
threads. If the hole is too big you get a weak thread because the OD is too
big.
For self tappers it is more complicated because they are often used in thin
sheets of metal and tables I found online have different values for
different thicknesses. To some extent you are forming the metal, rather
than just cutting it. But for thick materials I just use the thread ID,
which I often measure with calipers or eyeball the drill against the screw.
Wood screws are more forgiving because they have a tapered point and wood
is soft. So the hole can be considerably undersized or even none at all.
On Wed, 3 Nov 2021 at 23:36, Adrian Mariano avm4@cornell.edu wrote:
Yeah, I forgot about the clipping of the top of the screw profile. But
what's the ideal tap hole size? Should it be equal to the ID of the
threaded hole? If that's the case, the original poster can just use that
value from the screw data. Note that d_major is not equal to the nominal
screw size---or rather, it's a range. So for M6 the spec requires d_major
to be between 6mm and 6.3mm. But subtracting the pitch from the nominal
diameter does seem to put the result in the middle of the inner diameter
range, which is 4.9mm to 5.1mm. Of course, it didn't look to me like the
original poster was interested in tapping machine screws. Another
question: should tapping diameter be different for plastic than for metal?
I think for tapping holes in wood for machine screws one may use a hole
undersized compared to the hole for metal tapping.
On Wed, Nov 3, 2021 at 7:06 AM nop head nop.head@gmail.com wrote:
For metric screws the minor diameter is not nominal minus sqrt(3) times
the pitch. That is the base of the triangle but it is fillited at 5/8 and
1/8. According to Wikipedia it is the major diameter minus 1.082532 times
the pitch. Using standard drill sizes, that is close enough to 1 times the
pitch.
[image: image.png]
[image: image.png]
On Wed, 3 Nov 2021 at 10:53, Adrian Mariano avm4@cornell.edu wrote:
He didn't say he wanted to do tapped machine screws. So using OD minus
pitch means you're making an assumption about the desired amount of
extra for tapping. The nominal ID will be the diameter mus
sqrt(3)*pitch so by subtracting only the pitch you are implicitly
making the hole larger than the screw ID by sqrt(3)-1 times the pitch,
or about .7 times the pitch. Is that always the right amount of
"extra" to leave for tapping? I don't know the answer. This rule of
thumb will work the same for UTS screws because they have the same
geometry.
On Wed, Nov 3, 2021 at 4:41 AM nop head nop.head@gmail.com wrote:
For metric machine screws the pilot hole for tapping is just the outer
diameter minus the pitch. For self tappers and wood screws I think to some
extent it depends on the material it goes into and its thickness. I either
google it or measure the minor diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws in my library but I
just noticed No4 and No6 are both 2mm holes and I can't remember why. The
comments do say into soft wood and it must have been the drill I used
making my first Mendel90 out of MDF. It doesn't really matter much into
soft wood but it is more important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for
screws. So I have a table of numbers describing the screws I use. I took
the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the
diameter of the hole should be the nominal diameter - a function of the
pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get from
BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top of the head,
middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using
the same parameters. The slot can slide a square or hex nut from the side,
or drop in a hex nut longitudinally. The use of the “pos” parameter makes
it easy to position the screw hole properly.
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If the ideal tap size is the ID then that value can be used directly from
the screw parameters. Note that the ID is not just nominal OD minus
pitch*sqrt(3)*5/8 because that ignores the tolerance parameters. An actual
nut will have a larger ID than that value.
When I referred to wood I was talking about tapping wood for machine
screws, not using wood screws in wood.
I think self-tapping screws are basically like wood screws, a not very
well-specified screw with threads that are supposed to cut their way in.
On Thu, Nov 4, 2021 at 4:38 AM nop head nop.head@gmail.com wrote:
The ideal tap size for machine screws is the nominal OD minus the pitch *
sqrt(3) * 5 / 8. I.e. the ID of the internal thread. Rounded to the nearest
drill size it is just nominal minus pitch. If the hole was smaller the tap
would tend to bind as it has to widen the hole as well as cutting the
threads. If the hole is too big you get a weak thread because the OD is too
big.
For self tappers it is more complicated because they are often used in
thin sheets of metal and tables I found online have different values for
different thicknesses. To some extent you are forming the metal, rather
than just cutting it. But for thick materials I just use the thread ID,
which I often measure with calipers or eyeball the drill against the screw.
Wood screws are more forgiving because they have a tapered point and wood
is soft. So the hole can be considerably undersized or even none at all.
On Wed, 3 Nov 2021 at 23:36, Adrian Mariano avm4@cornell.edu wrote:
Yeah, I forgot about the clipping of the top of the screw profile. But
what's the ideal tap hole size? Should it be equal to the ID of the
threaded hole? If that's the case, the original poster can just use that
value from the screw data. Note that d_major is not equal to the nominal
screw size---or rather, it's a range. So for M6 the spec requires d_major
to be between 6mm and 6.3mm. But subtracting the pitch from the nominal
diameter does seem to put the result in the middle of the inner diameter
range, which is 4.9mm to 5.1mm. Of course, it didn't look to me like the
original poster was interested in tapping machine screws. Another
question: should tapping diameter be different for plastic than for metal?
I think for tapping holes in wood for machine screws one may use a hole
undersized compared to the hole for metal tapping.
On Wed, Nov 3, 2021 at 7:06 AM nop head nop.head@gmail.com wrote:
For metric screws the minor diameter is not nominal minus sqrt(3) times
the pitch. That is the base of the triangle but it is fillited at 5/8 and
1/8. According to Wikipedia it is the major diameter minus 1.082532 times
the pitch. Using standard drill sizes, that is close enough to 1 times the
pitch.
[image: image.png]
[image: image.png]
On Wed, 3 Nov 2021 at 10:53, Adrian Mariano avm4@cornell.edu wrote:
He didn't say he wanted to do tapped machine screws. So using OD minus
pitch means you're making an assumption about the desired amount of
extra for tapping. The nominal ID will be the diameter mus
sqrt(3)*pitch so by subtracting only the pitch you are implicitly
making the hole larger than the screw ID by sqrt(3)-1 times the pitch,
or about .7 times the pitch. Is that always the right amount of
"extra" to leave for tapping? I don't know the answer. This rule of
thumb will work the same for UTS screws because they have the same
geometry.
On Wed, Nov 3, 2021 at 4:41 AM nop head nop.head@gmail.com wrote:
For metric machine screws the pilot hole for tapping is just the
outer diameter minus the pitch. For self tappers and wood screws I think to
some extent it depends on the material it goes into and its thickness. I
either google it or measure the minor diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws in my library but
I just noticed No4 and No6 are both 2mm holes and I can't remember why. The
comments do say into soft wood and it must have been the drill I used
making my first Mendel90 out of MDF. It doesn't really matter much into
soft wood but it is more important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for
screws. So I have a table of numbers describing the screws I use. I took
the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the
diameter of the hole should be the nominal diameter - a function of the
pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get from
BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top of the head,
middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using
the same parameters. The slot can slide a square or hex nut from the side,
or drop in a hex nut longitudinally. The use of the “pos” parameter makes
it easy to position the screw hole properly.
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Well tolerance parameters are ignored when you drill holes for tapping, you
just use the table I posted. And your previous comment about M6 being
between 6.0 and 6.3 does not match my experience. All my machine screws
have an outer thread diameter slightly smaller than nominal. So my M6
screws measure 5.75mm.
On Thu, 4 Nov 2021 at 10:35, Adrian Mariano avm4@cornell.edu wrote:
If the ideal tap size is the ID then that value can be used directly from
the screw parameters. Note that the ID is not just nominal OD minus
pitch*sqrt(3)*5/8 because that ignores the tolerance parameters. An actual
nut will have a larger ID than that value.
When I referred to wood I was talking about tapping wood for machine
screws, not using wood screws in wood.
I think self-tapping screws are basically like wood screws, a not very
well-specified screw with threads that are supposed to cut their way in.
On Thu, Nov 4, 2021 at 4:38 AM nop head nop.head@gmail.com wrote:
The ideal tap size for machine screws is the nominal OD minus the pitch *
sqrt(3) * 5 / 8. I.e. the ID of the internal thread. Rounded to the nearest
drill size it is just nominal minus pitch. If the hole was smaller the tap
would tend to bind as it has to widen the hole as well as cutting the
threads. If the hole is too big you get a weak thread because the OD is too
big.
For self tappers it is more complicated because they are often used in
thin sheets of metal and tables I found online have different values for
different thicknesses. To some extent you are forming the metal, rather
than just cutting it. But for thick materials I just use the thread ID,
which I often measure with calipers or eyeball the drill against the screw.
Wood screws are more forgiving because they have a tapered point and wood
is soft. So the hole can be considerably undersized or even none at all.
On Wed, 3 Nov 2021 at 23:36, Adrian Mariano avm4@cornell.edu wrote:
Yeah, I forgot about the clipping of the top of the screw profile. But
what's the ideal tap hole size? Should it be equal to the ID of the
threaded hole? If that's the case, the original poster can just use that
value from the screw data. Note that d_major is not equal to the nominal
screw size---or rather, it's a range. So for M6 the spec requires d_major
to be between 6mm and 6.3mm. But subtracting the pitch from the nominal
diameter does seem to put the result in the middle of the inner diameter
range, which is 4.9mm to 5.1mm. Of course, it didn't look to me like the
original poster was interested in tapping machine screws. Another
question: should tapping diameter be different for plastic than for metal?
I think for tapping holes in wood for machine screws one may use a hole
undersized compared to the hole for metal tapping.
On Wed, Nov 3, 2021 at 7:06 AM nop head nop.head@gmail.com wrote:
For metric screws the minor diameter is not nominal minus sqrt(3) times
the pitch. That is the base of the triangle but it is fillited at 5/8 and
1/8. According to Wikipedia it is the major diameter minus 1.082532 times
the pitch. Using standard drill sizes, that is close enough to 1 times the
pitch.
[image: image.png]
[image: image.png]
On Wed, 3 Nov 2021 at 10:53, Adrian Mariano avm4@cornell.edu wrote:
He didn't say he wanted to do tapped machine screws. So using OD minus
pitch means you're making an assumption about the desired amount of
extra for tapping. The nominal ID will be the diameter mus
sqrt(3)*pitch so by subtracting only the pitch you are implicitly
making the hole larger than the screw ID by sqrt(3)-1 times the pitch,
or about .7 times the pitch. Is that always the right amount of
"extra" to leave for tapping? I don't know the answer. This rule of
thumb will work the same for UTS screws because they have the same
geometry.
On Wed, Nov 3, 2021 at 4:41 AM nop head nop.head@gmail.com wrote:
For metric machine screws the pilot hole for tapping is just the
outer diameter minus the pitch. For self tappers and wood screws I think to
some extent it depends on the material it goes into and its thickness. I
either google it or measure the minor diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws in my library but
I just noticed No4 and No6 are both 2mm holes and I can't remember why. The
comments do say into soft wood and it must have been the drill I used
making my first Mendel90 out of MDF. It doesn't really matter much into
soft wood but it is more important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that are holes for
screws. So I have a table of numbers describing the screws I use. I took
the measurements myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws. I assume the
diameter of the hole should be the nominal diameter - a function of the
pitch, presumably a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get from
BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top of the head,
middle of gap, etc.
To the basic screw hole a slot for a captive nut can be added using
the same parameters. The slot can slide a square or hex nut from the side,
or drop in a hex nut longitudinally. The use of the “pos” parameter makes
it easy to position the screw hole properly.
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Some years ago I made a chart for common tapping size's and put it on
one of my web sites http://yertiz.com/cnc/index.htm under the
'something for nothing' section. It was mainly aimed at amateur use in
metals such as aluminium, brass and copper. For harder materials, it is
usual to use a bigger tapping drill. Plenty of on-line charts for that.
In practice, the diameter will depend on the material, the tightening
torques, the machining method and many other parameters. Roll forming
taps, another ball game. If talking about 3d printing, try whatever
works for you, The freely available charts for tapping drill sizes are
only for guidance, in most instances. Self tapping and wood screws are
very tightly specified, it is just we don't know what the specifications
are. Tapping soft materials, (wood/plastic) you need a sharp tap, you
most likely Wil not be able to use all three, (taper, bottom, plug) and
you can't use normal coolant or lubrication in most instances either, so
chip evacuation can be a problem.
A number 8 wood screw, 1.5 turns into an oak beam will support the
weight of a man - my dad said, so it was right, and this guy proved it
for me https://www.youtube.com/watch?v=2KaJlDC0ZdM&t=159s. I don't think
a machine screw into wood will do that. The 60deg metric thread angle is
far from ideal for many materials, but it looks easy.
Best wishes,
Ray
On 04/11/2021 10:35, Adrian Mariano wrote:
If the ideal tap size is the ID then that value can be used directly
from the screw parameters. Note that the ID is not just nominal OD
minus pitch*sqrt(3)*5/8 because that ignores the tolerance
parameters. An actual nut will have a larger ID than that value.
When I referred to wood I was talking about tapping wood for machine
screws, not using wood screws in wood.
I think self-tapping screws are basically like wood screws, a not very
well-specified screw with threads that are supposed to cut their way in.
On Thu, Nov 4, 2021 at 4:38 AM nop head nop.head@gmail.com wrote:
The ideal tap size for machine screws is the nominal OD minus the
pitch * sqrt(3) * 5 / 8. I.e. the ID of the internal thread.
Rounded to the nearest drill size it is just nominal minus pitch.
If the hole was smaller the tap would tend to bind as it has to
widen the hole as well as cutting the threads. If the hole is too
big you get a weak thread because the OD is too big.
For self tappers it is more complicated because they are often
used in thin sheets of metal and tables I found online have
different values for different thicknesses. To some extent you are
forming the metal, rather than just cutting it. But for thick
materials I just use the thread ID, which I often measure with
calipers or eyeball the drill against the screw.
Wood screws are more forgiving because they have a tapered point
and wood is soft. So the hole can be considerably undersized or
even none at all.
On Wed, 3 Nov 2021 at 23:36, Adrian Mariano <avm4@cornell.edu> wrote:
Yeah, I forgot about the clipping of the top of the screw
profile. But what's the ideal tap hole size? Should it be
equal to the ID of the threaded hole? If that's the case, the
original poster can just use that value from the screw data.
Note that d_major is not equal to the nominal screw size---or
rather, it's a range. So for M6 the spec requires d_major to
be between 6mm and 6.3mm. But subtracting the pitch from the
nominal diameter does seem to put the result in the middle of
the inner diameter range, which is 4.9mm to 5.1mm. Of
course, it didn't look to me like the original poster was
interested in tapping machine screws. Another question:
should tapping diameter be different for plastic than for
metal? I think for tapping holes in wood for machine screws
one may use a hole undersized compared to the hole for metal
tapping.
On Wed, Nov 3, 2021 at 7:06 AM nop head <nop.head@gmail.com>
wrote:
For metric screws the minor diameter is not nominal minus
sqrt(3) times the pitch. That is the base of the triangle
but it is fillited at 5/8 and 1/8. According to Wikipedia
it is the major diameter minus 1.082532 times the pitch.
Using standard drill sizes, that is close enough to 1
times the pitch.
image.png
image.png
On Wed, 3 Nov 2021 at 10:53, Adrian Mariano
<avm4@cornell.edu> wrote:
He didn't say he wanted to do tapped machine screws.
So using OD minus
pitch means you're making an assumption about the
desired amount of
extra for tapping. The nominal ID will be the
diameter mus
sqrt(3)*pitch so by subtracting only the pitch you are
implicitly
making the hole larger than the screw ID by sqrt(3)-1
times the pitch,
or about .7 times the pitch. Is that always the right
amount of
"extra" to leave for tapping? I don't know the
answer. This rule of
thumb will work the same for UTS screws because they
have the same
geometry.
On Wed, Nov 3, 2021 at 4:41 AM nop head
<nop.head@gmail.com> wrote:
For metric machine screws the pilot hole for tapping
is just the outer diameter minus the pitch. For self
tappers and wood screws I think to some extent it
depends on the material it goes into and its
thickness. I either google it or measure the minor
diameter with calipers and use that.
I have some constants for No 2, 4, 6, and 8 screws
in my library but I just noticed No4 and No6 are both
2mm holes and I can't remember why. The comments do
say into soft wood and it must have been the drill I
used making my first Mendel90 out of MDF. It doesn't
really matter much into soft wood but it is more
important for printed holes.
On Tue, 2 Nov 2021 at 23:08, bob@rjcarlson.com wrote:
I have a small library which generates shapes that
are holes for screws. So I have a table of numbers
describing the screws I use. I took the measurements
myself. I want to convert it to use screw_into from BOSL2.
One type of screw hole is for self tapping screws.
I assume the diameter of the hole should be the
nominal diameter - a function of the pitch, presumably
a percentage. What should that function be?
FYI, the library specifies a screw hole with
specs - the screw measurements, what I want to get
from BOSL2/screws.scad
gap - space between the bottom of the head and the nut
clearS - clearance on the screw end
clearH - clearance on the head end
pos - one of 6 positions to return the hole in, top
of the head, middle of gap, etc.
To the basic screw hole a slot for a captive nut
can be added using the same parameters. The slot can
slide a square or hex nut from the side, or drop in a
hex nut longitudinally. The use of the “pos” parameter
makes it easy to position the screw hole properly.
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