Change gear pressure angle

[Bob Engelhardt]

I have a Jet 1024 lathe with a QC gear box. The box has a broken gear
that is really oddball in that its dimensions are inconsistent with
published standards. Such that I cannot find a replacement in any of
the usual places.

Anyhow, I intend to make a new gear on my horizontal mill and will have
to buy a gear cutter. That means that I need to know the pressure
angle. I found a website with a procedure to do that, but it requires
CAD and before I get into that, I'm trying here.
(http://mikesworkshop.weebly.com/change-gear-pressure-angle.html)

Here is a photo of a couple of teeth on my gear and profiles of the same
DP from Boston Gear. The Boston Gear profiles are a different diameter,
so I can't compare by overlaying. I can't tell which angle is closest
to mine. Anybody know gears well enough to tell?

http://imgur.com/a/7O4O3

Thanks,
Bob

BTW - the lathe is from 1980 or so - if that helps.

[David Billington]

There are some programs or javascripts on the net which will generate
gear profiles such as http://woodgears.ca/gear_cutting/template.html and
others so maybe try a few and print off the result to compare with. Have
you considered it may be a module gear size rather than DP.

[Terry Coombs]

AFAIK the 2 most common PA's are 20° and 14.5° . My Logan is all 14.5 °
...
--
Snag

[et...@whidbey.com]

I assume you tried Jet already to see if a new gear is available and
found out it is not. Since you are only making one gear you could
grind a fly cutter to fit the existing gear. I have done this and it
works well if only one gear is being made. Do you have any radius
gauges? It may be that one will fit your gear teeth exaclty and if so
you can use it as a gauge to grind a cutter. I know, the teeth have an
involute form but only the beginning of the curve is used and it is so
close to a true circle that a true circle shape is within the
manufacturing toleranceof the original gear. As to the picture shown
all the teeth of a particular DP are the same size so the gear
diameter shouldn't matter, you just need to see if your tooth matches
the shape of a sample tooth. You may be able to determine the pressure
angle by tracing your gear teeth carefully on a piece of paper. Now,
find the center of the tracing as close as you can and use this with a
compass to draw a circle that is midway between the O.D. of the gear
and the bottom of the space between the teeth, and another circle that
is the same diameter as the bottom of the space between the teeth.
Next, draw a line the bisects a tooth and pases through the center of
the gear tracing. Where this line intersects the larger of the
circles you drew make a point. From this point draw a line that is
tangent to the smaller of the two circles you drew. Make another point
at the tangent point. Finally, draw a line from this tangent point
through the center. The angle between the two lines you drew that pass
through the center of the tracing is the pressure angle. You should
get an angle that is close to either 14.5 degrees or 20 degrees if
everything is drawn right and the gear is made to one of the two most
common pressure angles. It will be off a little because the smaller
circle is a little smaller than the base circle where the pressure
angle is measured from but I don't think it will make much of a
difference, especially since you are working from a tracing.
Eric

[Ned Simmons]

On Sat, 21 Jan 2017 13:53:29 -0500, Bob Engelhardt
<BobEng...@comcast.net> wrote:

What's the biggest gear in the box? The greater the number of teeth,
the closer the tooth form gets to the trapezoidal shape of a rack
tooth. For a rack, the pressure angle is equal to half the included
angle of the tooth profile.

If there isn't a gear with enough teeth to give you confidence in that
method, you can measure the gear with pins, though this can get
confusing if you're dealing with a non-standard pitch or tooth form
(for example, stub teeth). Machinerys HB has info on measuring over
pins.

http://www.zakgear.com/Over_Pins.html

--
Ned Simmons

[Ned Simmons]

On Sat, 21 Jan 2017 18:28:32 -0500, Ned Simmons <ne...@nedsim.com>
wrote:

Forgot to mention: You can generate a rack by rolling your gear thru a
lump of clay. Measure the teeth of the clay rack.

--
Ned Simmons

[Bob Engelhardt]

On 1/21/2017 2:04 PM, David Billington wrote:
> There are some programs or javascripts on the net which will generate
> gear profiles such as http://woodgears.ca/gear_cutting/template.html and

> others so maybe try a few and print off the result to compare with. ...

Thanks - I tried that. It took about 6 attempts to get it printed at
full size, but it did. The problem was resolution: these teeth are only
3mm deep and 5mm pitch, so trying to match profiles is really hard. It
did look like 20 degree PA, though.

Bob

[Bob Engelhardt]

On 1/21/2017 3:06 PM, Terry Coombs wrote:
> AFAIK the 2 most common PA's are 20° and 14.5° . My Logan is all 14.5 °

Oh, yeah - I forgot to mention that. The 2 Boston Gear profiles are 20
& 14-1/2 degrees. Old gears are most likely 14-1/2 & new ones 20, but I
don't know if one made in 1980 in old or new.

Bob

[Bob Engelhardt]

On 1/21/2017 4:04 PM, et...@whidbey.com wrote:
> I assume you tried Jet already to see if a new gear is available and
> found out it is not.

Yes - Jet has a bit of a reputation for its customers being SOL for parts.

Since you are only making one gear you could
> grind a fly cutter to fit the existing gear.

But then I'd have to make a fly cutter holder & worry about angles &
stuff. A gear cutter is $19 (the 20 degree one) from Wholesale Tool.

> ... As to the picture shown

> all the teeth of a particular DP are the same size so the gear
> diameter shouldn't matter, you just need to see if your tooth matches
> the shape of a sample tooth.

You're right, of course - they use the same cutter! I tried to compare
them in Photoshop, but at a reasonable size the Boston Gear profiles are
pixelating and my photo's lack of precision shows. It looks closer to
14-1/2, though (the $52 cutter).

You may be able to determine the pressure
> angle by tracing your gear teeth carefully on a piece of paper. Now,
> find the center of the tracing as close as you can and use this with a

> compass to draw a circle ...

That sounds like what mikesworkshop.weebly.com does, in CAD. I don't
think that my photo precision would support it.

Thanks,
Bob

[Jim Wilkins]

<et...@whidbey.com> wrote in message
news:npg78cd08sjnd26hp...@4ax.com...

> works well if only one gear is being made. ....
> Eric

I cut two satisfactory (not perfect) gears with flycutter bits too.

The job required a jig that guided the bit blank into centered
alignment with the tooth space. Bluing didn't transfer or print well
on the smooth hardened lathe bit so I held the bit and old gear up to
the light to see the contact and marked the blank's top surface to
show where to grind.

Roughing out the tooth spaces first with a more efficient cutter
helped a lot. The tooth spaces can be cut nearly to size before the
bit has been ground to final fit so it will be freshly sharp for the
finish cut.

The grinding jig might serve as a gauge for depth of cut.

-jsw

[et...@whidbey.com]

On Sat, 21 Jan 2017 13:53:29 -0500, Bob Engelhardt
<BobEng...@comcast.net> wrote:

Greetings Bob,
I read my post after posting it and I though I would add a little
clarification, or maybe obfuscation, your pick. So, the pitch circle
is the point of contact of the meshed gear teeth. The base circle is
the theoretical base of the gear tooth. Any pair of gears must have
the same ratio between the base circle and pitch circle in order to
work together. This ratio is usually expressed as an angle, the
pressure angle, for simplicity. Gear teeth are cut a little below the
base circle diameter a little bit for clearance. So if you use the
method I described to find the pressure angle it may come out wrong
because you will be drawing a line tangent to a diameter that is
slightly smaller than the base circle diameter. This will have the
effect of making the apparent pressure angle larger. Not much
different but if I didn't point out my error somebody else would have,
and I don't want to look any worse than I already do. I should also
point out that gears close to the same size with the same pressure
angle and pitch will have teeth that closely resemble each other. But
gears with a large difference in diameter will have teeth with curves
that look much different. For example a rack, which is a gear of
infinite diameter, has teeth with straight sides, with the angle of
the sides being at the pressure angle, whereas a pinion with only a
few teeth that meshes perfectly with the rack will have markedly
curved teeth.
Eric

[et...@whidbey.com]

On Sat, 21 Jan 2017 18:31:22 -0500, Ned Simmons <ne...@nedsim.com>

Greetings Ned,
That's a great simple solution. Thanks for posting it.
Eric

[Jim Wilkins]

"Ned Simmons" <ne...@nedsim.com> wrote in message
news:lnr78cdni2rvrglrh...@4ax.com...

Crafts stores sell polymer clay that hardens when heated to 130C in an
oven or with a hot air gun.
-jsw

[DoN. Nichols]

Hmmm ... Given that the lathe was from Taiwan (or maybe mainland
China -- not sure at that date) are you *sure* that it is an involute
gear which matches an Imperial DP? Look for "Module" gears to see
what you can find which is close to it in the metric world. It does not
really look like the usual involute profile -- of either DP. I'm not
sure what the pressure angles are for Module gears -- or whether they
even offer two common angles.

If that is the case, I'm not sure where you would go to get gear
cutters of the right size.

Your two sample images are both labeled "16 DP", but I do not
see a pressure angle label on either. I would guess that the one on the
left is the 20 degree PA.

Good Luck,
DoN.

--
Remove oil spill source from e-mail
Email: <BPdnic...@d-and-d.com> | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

[Bob Engelhardt]

On 1/21/2017 6:28 PM, Ned Simmons wrote:
> What's the biggest gear in the box? The greater the number of teeth,
> the closer the tooth form gets to the trapezoidal shape of a rack
> tooth. For a rack, the pressure angle is equal to half the included
> angle of the tooth profile.

Oh - good idea! I thought there was a much larger one, but I just
checked and the broken one is the largest. 8-(

> If there isn't a gear with enough teeth to give you confidence in that
> method, you can measure the gear with pins, though this can get
> confusing if you're dealing with a non-standard pitch or tooth form
> (for example, stub teeth). Machinerys HB has info on measuring over
> pins.
>
> http://www.zakgear.com/Over_Pins.html

OK, but I hope that I don't have to go there.

About the rack having trapezoidal teeth: as a rack is "bent" to become a
gear, the teeth center lines go from parallel to lines intersecting at
the center (and the profile changes, too). Basically, the tooth-sides
included angle is bigger by the angle between the center lines. So, if
you could approximate where the trapezoidal side has morphed during the
transformation, measure the included angle, subtract the tooth-to-tooth
center line angle, and divide by 2 you would have the (approximate)
pressure angle. It only has to be close enough to resolve between 14.5
and 20 degrees.

http://imgur.com/a/DAT5P

The angle between the white arrows is 51 degrees. It's a 36 tooth gear,
so the tooth-tooth included angle is 10. (52 - 10) / 2 = 21 degrees.

The red arrows are 38 degrees apart. (38 - 10) / 2 = 14 degrees.

The zakgear page that you linked to has a drawing that suggests that the
red lines would be the rack sides (the rack tooth is wider the top than
the gear tooth).

Thanks,
Bob

[Bob Engelhardt]

On 1/21/2017 6:31 PM, Ned Simmons wrote:
> Forgot to mention: You can generate a rack by rolling your gear thru a
> lump of clay. Measure the teeth of the clay rack.

I had read that on some forum and had tried it - the clay stuck to the
gear and generally was very uncooperative. I thought I might try using
epoxy putty in its almost-set state, but never got around to it.

Also, the teeth are so small that I don't think that I could have
measured the angles very well.

[DoN. Nichols]

On 2017-01-21, et...@whidbey.com <et...@whidbey.com> wrote:

[ ... ]

> all the teeth of a particular DP are the same size so the gear
> diameter shouldn't matter, you just need to see if your tooth matches
> the shape of a sample tooth.

All of the teeth of a particular DP on a gear of a given number
of teeth are the same shape -- with manufacturing limits. However, as
the number of teeth changes, the shape *has* to change to maintain the
involute mating behavior.

Note that most production cutting of gears these days is done
with "hobs", which can cut any number of teeth -- as the gear is rotated
in sync with the hob. The hob has its own teeth the shape of a rack
gear -- that is, like a section through an ACME screw for the 14-1/2
degree PA, and a section through a different shaped ACME-like thread for
the 20 degree PA. The shape has to change as the number of teeth change
to clear the rack gear as the spur gear rotates. This is why gear
cutters are offered in sets -- 8 for a normal set, which care close
enough for most gears. One end of the range covers from 138 tooth to a
full rack, the other end of the set is something much smaller just three
sizes IIRC. For better precision -- there are supplemental sets which
fill in the half-way point between two adjacent cutters.

But a fly cutter ground to shape for your gear will be fine for
that number of teeth.

And -- as I posted quite recently (after this to which I am
replying) it is quite possible that the gears in the Jet QC box are
metric "Module" gears, not US gears. Dig into _Machinery's Handbook_ in
the gear section, and compare dimensions to the metric gears. Just
because the QC box is for making inch threads does not mean that the
gears must be Imperial. :-).

Enjoy,

[Bob Engelhardt]

On 1/21/2017 7:34 PM, DoN. Nichols wrote:
> Hmmm ... Given that the lathe was from Taiwan (or maybe mainland
> China -- not sure at that date) are you *sure* that it is an involute
> gear which matches an Imperial DP? Look for "Module" gears to see
> what you can find which is close to it in the metric world. It does not
> really look like the usual involute profile -- of either DP. I'm not
> sure what the pressure angles are for Module gears -- or whether they

> even offer two common angles. ...

I _think_ that are just 2 different measuring systems. That any gear
can be described as having a certain DP or a certain module. There is a
simple arithmetic relationship between them. The teeth are the same
shape and that's what matters.

[Larry Jaques]

On Sat, 21 Jan 2017 20:47:20 -0500, Bob Engelhardt
<BobEng...@comcast.net> wrote:

>On 1/21/2017 6:31 PM, Ned Simmons wrote:
>> Forgot to mention: You can generate a rack by rolling your gear thru a
>> lump of clay. Measure the teeth of the clay rack.
>
>I had read that on some forum and had tried it - the clay stuck to the
>gear and generally was very uncooperative. I thought I might try using
>epoxy putty in its almost-set state, but never got around to it.

Mold release spray is your friend.

>Also, the teeth are so small that I don't think that I could have
>measured the angles very well.

Know anyone with an optical comparator?

--
There is s no such thing as a hyphenated American who is
a good American.  The only man who is a good American is
the man who is an American and nothing else.  We are a
nation, not a hodge-podge of foreign nationalities.  We
are a people, and not a polyglot boarding house.
--Theodore Roosevelt

[DoN. Nichols]

On 2017-01-22, Larry Jaques <lja...@invalid.diversifycomm.com> wrote:
> On Sat, 21 Jan 2017 20:47:20 -0500, Bob Engelhardt
><BobEng...@comcast.net> wrote:

[ ... ]

>>Also, the teeth are so small that I don't think that I could have
>>measured the angles very well.
>
> Know anyone with an optical comparator?

Well ... I have one, but whether I am convenient is a different
matter. I'm too close to Washington DC (Northern Va, and I'm not sure
where the OP is located.

[rangerssuck]

Just a few suggestions:

1) Make a friend at a precision machine shop that has an optical comparator. You'l be able to look at the gear in excruciating detail and make all the measurements you need.

1a) Take a good, sharp close-up photo, making sure that the camera is square with the gear. Blow it up to several times actual size and take your measurements off the print. You can help this along by including a ruler in the photo for scale (or scribing some lines on the gear).

2) send the busted gear to Boston Gear and ask them to match it. Make sure to talk to someone first and to provide a return shipping label.

[Bob Engelhardt]

On 1/23/2017 8:29 AM, rangerssuck wrote:
> Just a few suggestions:
>
> 1) Make a friend at a precision machine shop that has an optical comparator. You'l be able to look at the gear in excruciating detail and make all the measurements you need.
>
> 1a) Take a good, sharp close-up photo, making sure that the camera is square with the gear. Blow it up to several times actual size and take your measurements off the print. You can help this along by including a ruler in the photo for scale (or scribing some lines on the gear).
>
> 2) send the busted gear to Boston Gear and ask them to match it. Make sure to talk to someone first and to provide a return shipping label.
>

Thanks - I'll keep those in mind.

[Bob Engelhardt]

I was looking at a picture of the lathe and the rack for the carriage
power feed caught my eye. That rack won't have the same DP as the
broken gear, but it will _probably_ have the same pressure angle. I
mean, what's the chance that a lathe maker would use different pressure
angles in his gears? The point being, of course, that the rack teeth
have straight sides and easily measured angles.

[Leon Fisk]

I hadn't seen it mentioned yet, but the book "Workshop Practice Series
No 17 - Gears and Gear Cutting by Ivan Law" may help. If your lucky you
might find it at the library. Otherwise its around $23:

https://www.amazon.com/Gears-Gear-Cutting-Workshop-Practice/dp/0852429118/

It used to get mentioned here when your kind of question came up...

--
Leon Fisk
Grand Rapids MI/Zone 5b
Remove no.spam for email

[Terry Coombs]

I think I got my copy on Amazon for 17 bucks . Definitely a must-have for
a beginner gear-cutter . BTW Bob , what dp is your gear ? I have a full set
of 16 dp and a few 20 dp's . Ping me offline , my reply-to is good .
--
Snag

[Larry Jaques]

Always check eBay, too. $14.73 delivered from the UK
http://tinyurl.com/jx6jzbe

[DoN. Nichols]

On 2017-01-23, Bob Engelhardt <BobEng...@comcast.net> wrote:
> On 1/23/2017 8:29 AM, rangerssuck wrote:
>> Just a few suggestions:

[ ... ]

>> 1a) Take a good, sharp close-up photo, making sure that the camera is
> square with the gear. Blow it up to several times actual size and take
> your measurements off the print. You can help this along by including a
> ruler in the photo for scale (or scribing some lines on the gear).

If you have a scanner, put the gear flat in the middle of the
scanner's plate, and scan it. That will probably be as good an image as
you can get, and will make sure that the "camera" is square with the
gear.

Use the highest resolution (maximum DPI setting) on the scanner.

You can possibl even use a sub-program in an image processing
software package (e.g. GIMP or PhotoShop -- whichever you have) to get
an outline of the gear -- better to work on importing that to a CAD
program.

Good Luck,

[Jim Wilkins]

"Bob Engelhardt" <BobEng...@comcast.net> wrote in message
news:o65hf...@news4.newsguy.com...

Bearing Specialty in Nashua NH might be able to help you. Their phone
is (603) 889-2222.
-jsw

[Jim Wilkins]

"Bob Engelhardt" <BobEng...@comcast.net> wrote in message

news:o65h2...@news4.newsguy.com...

http://www.ereplacementparts.com/search_result.php?q=jet+change+gear

[rangerssuck]

Nice! never heard of that place before, I'll file it away for the future.

[Bob Engelhardt]

On 1/23/2017 1:21 PM, Leon Fisk wrote:
> I hadn't seen it mentioned yet, but the book "Workshop Practice Series

> No 17 - Gears and Gear Cutting by Ivan Law" may help. ...

I have the book. Good background & it even describes how to make a tool
to make a cutter. I'm not good enough to do that.

[Bob Engelhardt]

On 1/23/2017 11:54 PM, DoN. Nichols wrote:
> If you have a scanner, put the gear flat in the middle of the
> scanner's plate, and scan it. That will probably be as good an image as
> you can get, and will make sure that the "camera" is square with the
> gear.
>
> Use the highest resolution (maximum DPI setting) on the scanner.

> ...

That is a good idea! Why didn't I think of that?

I did do a scan, at 1200dpi (not max) and got a great image:
http://imgur.com/Zi8ZueK

Not exactly straight-on, probably because I had it at one end of the
scanner. It blows-up nicely:
http://imgur.com/jrykApV

Thanks,
Bob

[Bob Engelhardt]

On 1/24/2017 12:41 PM, Jim Wilkins wrote:
> http://www.ereplacementparts.com/search_result.php?q=jet+change+gear
>
>

One of those might be my gear. But their part numbers do not match
mine. And their model list doesn't have mine, so I can't find the part
in a drawing.

Thanks anyhow,
Bob

[Bob Engelhardt]

I took a photo of the rack & measured the angles: 20 degrees

http://imgur.com/pRfDmj8

I haven't yet convinced my self that the rack HAS to have the same
pressure angle.

[Jim Wilkins]

"Bob Engelhardt" <BobEng...@comcast.net> wrote in message

news:o6ar2...@news4.newsguy.com...

The page gives their phone number.

[Bob Engelhardt]

I ordered a 20 degree cutter. 20 degrees seems the mostly likely angle.

Today I just happened to be using some epoxy putty for another project &
I had a little left over. So I rolled the gear in it, to get a rack &
measure it. Even though the putty was stiffer than plasticine and set
up hard, it was not ideal. It tended to deform some from the perfect
rack that it was supposed to be.

Here is a photo of the molded epoxy, ground flat on one side & with
straight lines fitted to the profile:
http://imgur.com/a/ixovG
Right-leaning lines are 18-19-19-20 degrees and left leaning ones 14
(all). Inconclusive, although closer to 14.5 on average.

Bob

[DoN. Nichols]

Right! That is why I suggested it be in the center of the
scanner's plate.

> It blows-up nicely:
> http://imgur.com/jrykApV

Yes -- enough to read into a CAD program and draw the tangent
lines to pick up the pressure angle.

[DoN. Nichols]

On 2017-01-25, Bob Engelhardt <BobEng...@comcast.net> wrote:

It only *has* to be the same pressure angle as the pinion in the
carriage -- but it is unlikely that a maker would use different pressure
angles in different parts of a lathe. The main difference is how much
force is generated trying to move the axles of the gears apart. And
probably a difference in how much rubbing is going on between the
surfaces of the teeth. They proably buy all the gears from one vendor,
or just buy the equipment to make the gears from one vendor.

The only time I might think that there would be different PAs
would be in an *old* company, which continues to use some parts from
earlier designs, and uses newer parts for things which have been
re-designed in later years.

Enjoy,

[Bob Engelhardt]

On 1/26/2017 6:39 PM, Bob Engelhardt wrote:
> I ordered a 20 degree cutter. 20 degrees seems the mostly likely angle.

>...

I've posted this question on the HSM forum and a fellow there (Paul
Alciatore) played with my high-res 4-tooth scan and showed that my gear
has 20 PA _stub_ form teeth. I never hear of stub form teeth, but
they're regular teeth with the root & crest shortened. Makes them
stronger, they say.

His post is on page 3 of the HSM thread:
http://bbs.homeshopmachinist.net/threads/72646-Jet-1024-lathe-change-gear-pressure-angle/page3

[edhun...@gmail.com]

It's been almost 40 years since I covered gear making for American Machinist, so I forget much of the technology, but, FWIW, all of the machine tool builders I visited in those days hobbed their own gears. As for using different pressure angles, one hob can cut a variety of gear diameters, but only one pressure angle. So they normally settled on one angle for their gears and stuck with it.

Some of the gears in a lathe have to handle a decent load, but the change gears are just timing gears. The load actually is very light. And, as long as any pair of gears are within a reasonable size range of each other, they don't encounter problems with undercutting or excessive sliding motion. All of the gear trains in a lathe, and in most machine tools, are pretty simple and standard -- although stub gear-tooth shapes were pretty common.

This was all pre-CNC.

--
Ed Huntress