I need about 30 feet of rack gear (2x12' + 1x6', or 5x6' which seems to
be what's actually available) and 3 spur gears for a project (upgrading
my old cable-drive CNC (woodoworking) router to gear drive). More
expensive, fancier options are relegated to fantasy, I think. I did
consider things like Acme screw drives, but the increased rotating mass
seems like a problem, as is the flopping length of the 12 foot longer
axis. Ballscrews are a post-lottery-winning option only, I think.

Of MSC and McMaster, McMaster is a bit cheaper for this item.

I suspect that for the limited purchase size they may be the only
sensible option, but thought I'd throw it out here to ask if there are
any other suppliers friendly to HSM/small orders that might have better
prices.

I suppose I might as well also get some input/commentary on design
parameters and/or stupid design choices I might be making. When (some
aeons ago) the makeer of my router made this change, they chose 14.5
degree 20 pitch racks. For a variety of reasons, mostly cash related, I
did not upgrade when they had an upgrade kit available. I think they
claimed less lash with 14.5 than with 20 degree - I don't know if that
has any basis in reality. The old drive system was steppers driving
cables, the first form of the gear upgrade was steppers driving belts
driving spur gears, the latest went to servo drive. Given that I would
be happy to get the admittedly slow/weak stepper system working pretty
much the same as it does with cables, but without the cable slip errors
it sometimes gets, I'm inclined to ride the hairy edge of rack gear
engagement and use the steppers to direct drive a 15 tooth spur gear on
a 20 tooth rack, producing the same power and resolution I get now - 3/4
inch per full revolution. The added complexity and opportunity for lash
in the step-down system was going to a seemingly not-much-bigger gear of
about 20 teeth. I could also direct dive 20 teeth with some loss of
resolution and power.

I understand that small spur gears are supposed to be evil (prone to
shearing teeth, etc). Loads are not stunningly high - stall force on the
unit as built now is something like 30-40 lbs. I really have no idea how
hard I would be pushing a 15 tooth 20 pitch spur (standard width seems
to be 3/8 inch for 14.5 degree) if I loaded it to 40 lbs...I gather that
brass or bronze gears running on the steel rack might have better wear
behavior than steel on steel, but be weaker, as would plastic gears.

--
Cats, coffee, chocolate...vices to live by

"Ecnerwal" wrote in message
...
I need about 30 feet of rack gear (2x12' + 1x6', or 5x6' which seems to
be what's actually available) and 3 spur gears for a project (upgrading
my old cable-drive CNC (woodoworking) router to gear drive). More
expensive, fancier options are relegated to fantasy, I think. I did
consider things like Acme screw drives, but the increased rotating mass
seems like a problem, as is the flopping length of the 12 foot longer
axis. Ballscrews are a post-lottery-winning option only, I think.

Of MSC and McMaster, McMaster is a bit cheaper for this item.

I suspect that for the limited purchase size they may be the only
sensible option, but thought I'd throw it out here to ask if there are
any other suppliers friendly to HSM/small orders that might have better
prices.

we use a place called union iron for building rack & pinion slide gates..
quite a bit cheaper than McMC
here's a number
800-333-5148

hth
Rob

In article ,
says...
I need about 30 feet of rack gear (2x12' + 1x6', or 5x6' which seems to
be what's actually available) and 3 spur gears for a project (upgrading
my old cable-drive CNC (woodoworking) router to gear drive). More
expensive, fancier options are relegated to fantasy, I think. I did
consider things like Acme screw drives, but the increased rotating mass
seems like a problem, as is the flopping length of the 12 foot longer
axis. Ballscrews are a post-lottery-winning option only, I think.

Of MSC and McMaster, McMaster is a bit cheaper for this item.

I suspect that for the limited purchase size they may be the only
sensible option, but thought I'd throw it out here to ask if there are
any other suppliers friendly to HSM/small orders that might have better
prices.

Check with Motion Industries or other power transmission distributors in
your area. They're usually a bit cheaper than McMaster, though often not
enough to bother on small orders.


I suppose I might as well also get some input/commentary on design
parameters and/or stupid design choices I might be making.

Have you considered replacing the cables with a timing belt drive?
That's generally the most cost effective choice where high speeds and
accelerations, and moderate accuracy and stiffness are required. Breco
has pretty good engineering info, it looks like it's available online.

http://www.brecoflex.com/?CATID=1

Ned Simmons

There is a cheapskate alternative to a rack. Weld a piece of bicycle chain layed
edgewise along your part (or braze, or whatever) and use a bicycle sprocket to
interface with it. Probably not good for your application but really cheap ..

GWE

Ecnerwal wrote:
I need about 30 feet of rack gear (2x12' + 1x6', or 5x6' which seems to
be what's actually available) and 3 spur gears for a project (upgrading
my old cable-drive CNC (woodoworking) router to gear drive). More
expensive, fancier options are relegated to fantasy, I think. I did
consider things like Acme screw drives, but the increased rotating mass
seems like a problem, as is the flopping length of the 12 foot longer
axis. Ballscrews are a post-lottery-winning option only, I think.

Of MSC and McMaster, McMaster is a bit cheaper for this item.

I suspect that for the limited purchase size they may be the only
sensible option, but thought I'd throw it out here to ask if there are
any other suppliers friendly to HSM/small orders that might have better
prices.

I suppose I might as well also get some input/commentary on design
parameters and/or stupid design choices I might be making. When (some
aeons ago) the makeer of my router made this change, they chose 14.5
degree 20 pitch racks. For a variety of reasons, mostly cash related, I
did not upgrade when they had an upgrade kit available. I think they
claimed less lash with 14.5 than with 20 degree - I don't know if that
has any basis in reality. The old drive system was steppers driving
cables, the first form of the gear upgrade was steppers driving belts
driving spur gears, the latest went to servo drive. Given that I would
be happy to get the admittedly slow/weak stepper system working pretty
much the same as it does with cables, but without the cable slip errors
it sometimes gets, I'm inclined to ride the hairy edge of rack gear
engagement and use the steppers to direct drive a 15 tooth spur gear on
a 20 tooth rack, producing the same power and resolution I get now - 3/4
inch per full revolution. The added complexity and opportunity for lash
in the step-down system was going to a seemingly not-much-bigger gear of
about 20 teeth. I could also direct dive 20 teeth with some loss of
resolution and power.

I understand that small spur gears are supposed to be evil (prone to
shearing teeth, etc). Loads are not stunningly high - stall force on the
unit as built now is something like 30-40 lbs. I really have no idea how
hard I would be pushing a 15 tooth 20 pitch spur (standard width seems
to be 3/8 inch for 14.5 degree) if I loaded it to 40 lbs...I gather that
brass or bronze gears running on the steel rack might have better wear
behavior than steel on steel, but be weaker, as would plastic gears.

Ned,

Timing belts would also be my choice for this application. They are
remakably strong....steel cable reinforced...and quite accurate. They
can be obtained in a variety of widths. Properly pre-tensioned they
work well with good life expectancy. Industrial application is in the
X,Y,Z axes drives for rectilinear robots and pick & place automated
devices.

Wolfgang

"Ned Simmons" wrote in message
...
In article ,
says...
I need about 30 feet of rack gear (2x12' + 1x6', or 5x6' which seems to
be what's actually available) and 3 spur gears for a project (upgrading
my old cable-drive CNC (woodoworking) router to gear drive). More
expensive, fancier options are relegated to fantasy, I think. I did
consider things like Acme screw drives, but the increased rotating mass
seems like a problem, as is the flopping length of the 12 foot longer
axis. Ballscrews are a post-lottery-winning option only, I think.

Of MSC and McMaster, McMaster is a bit cheaper for this item.

I suspect that for the limited purchase size they may be the only
sensible option, but thought I'd throw it out here to ask if there are
any other suppliers friendly to HSM/small orders that might have better
prices.

Check with Motion Industries or other power transmission distributors in
your area. They're usually a bit cheaper than McMaster, though often not
enough to bother on small orders.


I suppose I might as well also get some input/commentary on design
parameters and/or stupid design choices I might be making.

Have you considered replacing the cables with a timing belt drive?
That's generally the most cost effective choice where high speeds and
accelerations, and moderate accuracy and stiffness are required. Breco
has pretty good engineering info, it looks like it's available online.

http://www.brecoflex.com/?CATID=1

Ned Simmons


Ned - I'm not the OP, but thanks for posting that link. The design info
will come in quite handy for an upcoming project.

Mike


--
NewsGuy.Com 30Gb $9.95 Carry Forward and On Demand Bandwidth

In article ,
Ned Simmons wrote:

Have you considered replacing the cables with a timing belt drive?
That's generally the most cost effective choice where high speeds and
accelerations, and moderate accuracy and stiffness are required. Breco
has pretty good engineering info, it looks like it's available online.

http://www.brecoflex.com/?CATID=1

I have, though that also requires (due to belt pitches available and
sprocket-sizes) some additional reduction (thus complexity) of the drive
to maintain resolution and power at present levels. A complete re-drive
(which might resolve that another way) is probably not on the menu for
this machine at this time or ever - if it can be made to work better, it
might manage to buy itself a friend eventually, and get retired to doing
more 3D input scanning and less cutting. Thus far it's been strictly a
hobby-use money sink. Fun, but less fun when it goes off and starts
ruining wood by not being where it thinks it is.

I get a bit squirrelly when dealing with places that have no price
information available until you specify everything and get a quote (how
can I make cost dependent engineering choices to specify things without
some idea of costs ahead of time?), which was one vote in favor of gear
systems, but given lots of replies supporting/suggesting/preferring the
belt conversion, I'll give that more thought and try to figure out what
components a quote might include without having any cost info - if it
comes out way above gears, I guess I'll know something at that point.

--
Cats, coffee, chocolate...vices to live by

In article ,
says...


I get a bit squirrelly when dealing with places that have no price
information available until you specify everything and get a quote (how
can I make cost dependent engineering choices to specify things without
some idea of costs ahead of time?), which was one vote in favor of gear
systems, but given lots of replies supporting/suggesting/preferring the
belt conversion, I'll give that more thought and try to figure out what
components a quote might include without having any cost info - if it
comes out way above gears, I guess I'll know something at that point.

McMaster carries urethane belting on open lengths in a couple pitches
and widths as well as matching pulleys. Even if they don't stock exactly
what you need the prices should get you in the ballpark. The belts and
pulleys aren't too expensive, but accessories like idlers and clamping
profiles are pricey enough that making your own may make sense.

When comparing to a rack and pinion system don't forget to account for
the difficulty in mounting and aligning a long rack.

Ned Simmons

In article .com,
says...
Ned,

Timing belts would also be my choice for this application. They are
remakably strong....steel cable reinforced...and quite accurate. They
can be obtained in a variety of widths. Properly pre-tensioned they
work well with good life expectancy. Industrial application is in the
X,Y,Z axes drives for rectilinear robots and pick & place automated
devices.

Wolfgang


Here's a system I designed and built about a year ago. The linear slide
and timing belt drive, minus motor and controls, came as a preassembled
unit from Item.

http://www.suscom-maine.net/~nsimmons/news/Washer01.jpg

Ned