Cutting gears on a vertical mill...
Pete C.
there is some metalworking on other parts of this project.
I have a project where I need to cut two pairs of gears out of .220"
acrylic sheet stock. Each pair consists of a roughly 24" diameter and a
roughly 3" diameter gear. All four gears are not exactly the same size
and this is a one time project.
The speed and load on these gears will be quite low, ~4 rpm on the large
gears and <1 lb-ft of torque. This is part of a set of display fixtures
I'm building.
I have available a Bridgeport vertical mill, a 10" tilting rotary table
and a dividing head with about a 4" chuck. I also have an Atlas/Clausing
lathe although that is not likely to be useful for this operation.
From what I've read gear cutting is more typically done on a horizontal
mill or a specialty machine. I don't have a proper cutter and arbor to
cut the gear profile either.
At this point I'm think the best approach would be to use the rotary
table and a suitably sized end mill to make the bulk of the cuts and
then hand finish them to a reasonable profile with a file. On .220"
acrylic sheet this should not be too unreasonable.
Before I start trying to make them this way I thought I'd see if the
good folks in this group had a better way to to this.
Thanks,
Pete C.
Randal O'Brian
news:41A4F203...@snet.net...
Since you have a lathe, you can make a hob out of drill rod that will cut
your plastic gears with ease. It will run in the mill with the gear blank
on a horizontal arbor driven by the rotary table or the dividing head.
See www.strictlyic.com and go to Special Interest Articles. Scroll down
to "Making Your Own Spur Gears the Easy Way".
I used the method to make several gears in brass and it worked perfectly.
It makes true involute gears that can be as good as any you can buy if you
are careful and patient.
Randy
Pete C.
Thanks for the information, I made notes and found some other good web
info for future use, however I don't think I'm going to be able to use
that technique for this application for a few reasons.
1. The center height of the rotary table in a vertical position (for a
horizontal arbor) is only 7.25" so I'd have to block up the mounting
considerably to fit a 24" dia blank. The dividing head is even lower.
2. It's a manual rotary table with no provisions for any sort of spindle
synchronization and with the soft/brittle acrylic I think the "self
feed" method would just chew up the edge.
3. To make the cutter and rig everything up would take more time than I
have to complete the project (about a week).
I might be able to rig up a router or dremel on the compound borrowed
from the lathe to allow me to have a horizontal spindle to mount a small
non helical cutter in and then take an individual cut on the edge of the
disk for each tooth.
I'll have to think about it a bit more. However I do it, I have to start
making the gears on Friday.
Thanks,
Pete C.
DoN. Nichols
>Ok, so it's not quite metalworking but I think it's close enough and
>there is some metalworking on other parts of this project.
>
>I have a project where I need to cut two pairs of gears out of .220"
>acrylic sheet stock. Each pair consists of a roughly 24" diameter and a
>roughly 3" diameter gear. All four gears are not exactly the same size
>and this is a one time project.
Hmm ... those 24" ones will be a bit of a problem with the
tooling which you have listed below.
>The speed and load on these gears will be quite low, ~4 rpm on the large
>gears and <1 lb-ft of torque. This is part of a set of display fixtures
>I'm building.
>
>I have available a Bridgeport vertical mill, a 10" tilting rotary table
>and a dividing head with about a 4" chuck. I also have an Atlas/Clausing
>lathe although that is not likely to be useful for this operation.
The lathe would be useful for turning the stock for the smaller
gears to a round shape before going on with the rest of it.
How big a center hole will the gears have? If it is not too
big, you could mount the dividing head (or the rotary table) on a block
to space it far enough above the table to allow you to swing the
workpiece. Given the flexibility of acrylics, I would suggest that you
include a plate of 1/2" thick aluminum or whatever else you have on hand
to support the workpiece close to where the cut is being made. That
0.220" thick acrylic will flex *way* too much, otherwise.
>From what I've read gear cutting is more typically done on a horizontal
>mill or a specialty machine. I don't have a proper cutter and arbor to
>cut the gear profile either.
Do you know the tooth profile you want? I would suggest going
for as coarse a pitch as you can manage -- if only to minimize the
number of cuts you will need to make on those large gears.
>At this point I'm think the best approach would be to use the rotary
>table and a suitably sized end mill to make the bulk of the cuts and
>then hand finish them to a reasonable profile with a file. On .220"
>acrylic sheet this should not be too unreasonable.
This is a function of how small the teeth are to be. Small ones
would need tiny end mills. and those tend to get buried in melted
acrylic at speeds which would get things finished in any reasonable
period.
Considering gear tooth milling cutters, and looking at eBay, the
coarsest that I see is a 10 Diametrical pitch.
A bit of calculation suggests that a 3" gear would have 28
teeth, and a 24" gear would have 238 teeth. This would be a Number 4
cutter (26-34 teeth), and a Number 1 cutter (135 teeth to a rack gear).
With the ratio of sizes I can't see fewer than two cutters doing
it.
And with the number of teeth on the large one, I would go with
the indexing head, not the rotary table -- unless the table has a set of
indexing plates and arms as part of its set.)
Anyway -- I would put the index head up on a rigid block of
aluminum or steel to enable you to swing the full 24" clear of the table
with the axis of the index head horizontal (along the X-axis).
I would use the lathe to make an arbor for the gear tooth mills
(probably 1" arbor), to adapt it to the mill's spindle, and to hold it
perhaps 8" below the spindle nose.
I would slide the ram far enough to allow the arbor and gear
tooth mill to come down beside the edge of the gear, and use the X-axis
feed to cut each tooth -- cutting towards the support from the 1/2"
thick aluminum backplate mentioned above.
The 3" diameter gears will require a less extreme setup.
O.K. -- back to the gear cutters. I've dug into the MSC
catalog, and it looks as though a 5 DP tooth form is your best bet.
Coarser than that results in too few teeth for the smaller gears.
For the 24" gear, I calculate 118 teeth.
For the 3" gear, I calculate 13 teeth.
You have a choice of 14-1/2 degree pressure angle or 20 degree
pressure angle. Of the two, the 14-1/2 degree pressure angle will cost
you less. At these coarse tooth sizes, you need thicker cutters.
So -- you would need a #2 cutter (55-134 tooth) for the 24"
gear, and a #8 cutter (12 to 13 teeth) for the 3" gear. Each will cost
you $59.39, based on the MSC catalog, page 423. Part numbers are
03190527 for cutter #2, and 03190584 for the #8 cutter. These fit on a
1" arbor, while the 2 DP through 4 DP ones take a 1-1/4" or 1-1/2"
arbor.
>Before I start trying to make them this way I thought I'd see if the
>good folks in this group had a better way to to this.
Well ... you see how *I* would attempt the task with the tools
which you have described as being available.
Consider hand filing 118 teeth each on the 24" gears -- and how
your chances of screwing it up increase with each tooth done.
Of course, your final diameter will have to be tweaked a bit
based on the tooth pitch selected. The 24" gear works out right with 5
DP, as does the 13 tooth gear at 5 DP -- but if you change the number of
teeth desired, you will have to adjust the diameter,, and if you make
them too much smaller than 3" you will need to change to a finer DP (6
or 7 depending), to keep the minimum count of 12 teeth. (You can cut
fewer teeth with a real gear hobbing machine, but the undercut of the
teeth is too great for milling them.
Now -- early *large* gears were made by laying out the tooth
profile and sawing and filing, and the old books tell you how to do
this. Perhaps download the Rose _Modern Machine Shop Practice_ (far
from modern these days), or _A Catechism of Steam_. Both have been
scanned and are available for download -- I'm not sure where, but
someone here could fill you in on that.
Good Luck,
DoN.
P.S. I would suggest going to a thicker acrylic for this project,
perhaps 1/2" thick or more. It would look more like a real
gear.
--
Email: <dnic...@d-and-d.com> | 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 ---
Pete C.
Yikes, thanks for all the info. It looks like I need to add to my
tooling collection. It seems that a horizontal mill would be a lot
easier for this stuff. The right angle heads for the Bridgeport are
nice, but I've got a round ram 1J so I wouldn't be able to use the end
support which is designed to mount on the dovetail rams.
I kind of like the idea of building a "horizontal mill" add on that I
could mount on the Bridgeport next to the rotary table or index head
specifically for cutting gears like this. For the level of precision I'd
need I could probably build it fairly simply, although not in time for
this project.
I'd already thought about the edge stability of the acrylic and was
planning on sandwiching it between 1/2" MDF disks. The center hole of
the gears isn't critical as I also need to make the shafts to mount them
on.
The basic idea of this mechanism is that there are two large disks that
are stacked together. The top disk will have 5 radial slots that 5
"carriers" will fit into so that they can slide in and out about 2"-4".
The top disk will rotate this carrier array around at about 4 rpm. The
bottom disk will rotate at perhaps 3.5 rpm and will have a "zig-zag"
track milled into it that a follower on the bottom of each carrier will
ride in. This should give me an array that rotates at 4 rpm in a circle
that expands and contracts. The carriers will probably be milled out of
some UHMW stock and will hold magnets to couple to items on the top side
of an 1/8" plex. mirror.
I was planning on an edge drive setup although it's sounding like it
might be a bit easier to make a coaxial drive shaft setup and use
smaller gears mounted on the shaft below the main disks.
I guess I'll have to think about this some more over turkey and see what
I can come up with.
Thanks,
Pete C.
Bernd
> there is some metalworking on other parts of this project.
>
> I have a project where I need to cut two pairs of gears out of .220"
> acrylic sheet stock. Each pair consists of a roughly 24" diameter and
> a
> roughly 3" diameter gear. All four gears are not exactly the same size
> and this is a one time project.
Pete,
Do they have to be gears. Seems like a lot of complex work to just turn
a display. Could you use pulleys instead? Cut a groove in the plastic
and use a round belt.
Bernd
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Doppler Effect: The tendency for stupid ideas to seem smarter when they
come at you rapidly. -- unknown
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Charles Morrill
Are you using cell cast acrylic?
Charles Morrill
Pete C.
for belts (I just moved a few months ago). I'd have to do MSC or
Grainger.
Pete C.
Pete C.
moved a few months ago and haven't found a good plastic supplier in the
area yet. The stuff seems to machine nicely on the mill. I rough cut a
small test disk, mounted it on the rotary table and milled it to a nice
round without any issues.
Pete C.
Brian In Hampton
Tim Williams
where's your post?
Tim
--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms
"Pete C." <au...@snet.net> wrote in message
news:41A5F359...@snet.net...
<Snip>
John.Llo...@cwcom.net
>Pete C.
How about glueing a narrow "cambelt type" of belt inside out around the
circumference of the large gear, then you'd just have to worry about a
small gear to mesh with the belt? Possibly buy the small gear from the
belt supplier?
-----------------------------------------------------------------------------------------------------
John.Llo...@boltblue.com John Lloyd - Cymru/Wales
-----------------------------------------------------------------------------------------------------
Ecnerwal
wrote:
> ride in. This should give me an array that rotates at 4 rpm in a circle
> that expands and contracts. The carriers will probably be milled out of
> some UHMW stock and will hold magnets to couple to items on the top side
> of an 1/8" plex. mirror.
If your display is sliding things back and forth via magnets on top of a
mirror, plex is probably a bad choice for the mirror - no matter what
the base of the sliding magnetically-coupled object is, I think you'll
find the plex badly scratched in short order.
--
Cats, Coffee, Chocolate...vices to live by
Pete C.
you skate on it.
Pete C.
Pete C.
the other items in the display set does use a sync. timing belt and the
two pullies were $36.
Pete C.
Bernd
>I could probably do that although I haven't found a good local source
> for belts (I just moved a few months ago). I'd have to do MSC or
> Grainger.
Pete,
Small Parts in Florida has a material that is round called polycord. It
is a green synthetic round belting. You make up your own lengths. Very
easy to do. The belting comes in 1/8", 3/16", and 1/4" dia. Take a look.
Bernd
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Pete C.
After some turkey and other delays I got out to the shop to fiddle
around with the gear cutting issue this morning.
After coming up with a rather arbitrary tooth profile that I would be
able to cut on the mill, I made two smaller test gears. The tooth
profile is a simple 1/2 cut with a 3/16" end mill with a 1/8" land
between teeth. This profile seems to mesh well and looks like it should
be sufficient for the low speeds and loads involved. It's certainly not
a "legitimate" tooth profile, but it's easy to cut without having to
make any special tooling.
I've posted a picture of the test gears in the dropbox if anyone is
interested:
http://www.metalworking.com/DropBox/Acrylic_gear_test.jpg
Unless anyone has a reason I shouldn't use this simple tooth profile for
this project I'll probably cut the large gears later today.
Thanks,
Pete C.
Nob...@nowhere.noose
>
>Unless anyone has a reason I shouldn't use this simple tooth profile for
>this project I'll probably cut the large gears later today.
>
>Thanks,
>
the only thing I'd suggest is rounding the corners of the tips
slightly, file, sandpaper, anything to just take the sharp corners
down a little.
Brian In Hampton
can cut on the laser..Made from wood but can do acrylic also.......Brian
Pete C.
half my shop and put me in bankrupcy with the expense of laser gas mix
and maintenence.
I'm trying to stick to things I can do for this project since time is
short and I'm designing on the fly so I need to be able to make changes
quickly. Are you anywhere near the DFW area for future projects?
Thanks,
Pete C.
Pete C.
I've now cut the big gear, a 200 tooth. Tomorrow I'll get some more
stock and cut the other large one at 190T. Kind of a pain to cut, but
they seem to work fine. Pic:
http://www.metalworking.com/DropBox/Acrylic_gear_big_one.jpg
Pete C.
Brian in Hampton
Pete C.
Pete C.
Pete Bergstrom
Topposters, sheesh.
"Tim Williams" <tmor...@charter.net> wrote in message
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