On Fri, 28 Dec 2012 12:29:24 -0800, etpm wrote:
On Fri, 28 Dec 2012 13:44:00 -0500, Joe AutoDrill
wrote:
I sell to customers all the time who need to hold tolerances of 0.001"
or so while drilling, milling, reaming, etc. This is not extremely
difficult when you are talking about a single spindle device. However,
I sell multi drill spindle devices. ...sometimes with as many as 40
spindles.
One of the challenges I run across on a weekly basis is controlling
tolerances that closely on a head that has either a wide spread or many
spindles. The thermal growth of the head housing ranges significantly
from room temperature to operating temperature.
We have a few tricks to compensate for this including everything from
cooling fins to calculating the growth and specifying a warm up period
on the machine.
Knowing that there is an abundance of information contained in the
brains here, I thought I'd throw this out there and see if I got any new
ideas for us to try.
Here is a page showing a typical head for a Bridgeport:
http://www.drill-hq.com/?page_id=832
We never really know how the customer is going to use the device. Here
are some example scenarios:
Some folks put them on a simple drill press and spin them at low speed
because cycle time really doesn't matter. They turn off the drill press
between parts so the head never warms up.
Others will put them on a drill press, turn it on and run it at maximum
RPM for the duration of a shift. After 10 minutes, the head is at a
relatively stable operating temperature as long as they keep working
parts. If it spins freely, it cools down and shrinks.
The next guy may stick the head onto a Bridgeport or other style mill
and use it to drill holes one day and then pseudo-manually tap holes the
next. Drilling builds up more heat because of the RPMs involved and
thus on a wide head, the pattern can change a bit. Normally, this isn't
a problem, but on a really wide head, the end taps tend to bind a bit
and put side load on my device's spindles. Floating tap holders are not
really an option for industry standard ER style spindles - and that is
what I strive to use most of the time.
We may put a head on a self feeding drill unit. The customer may run the
head for one hour a day or 24 hours a day... We often don't know which
will take place as demand for the parts made by our units may change
overnight for some customers.
...The same basic challenges exist for CNC mounted heads or even direct
motor driven heads. We simply can't predict the growth of the head if
we can't figure out just how the head will actually be used.
So... Our goal is to simply take the thermal growth out of the
equation. Sometimes that is as simple as asking the customer to use
drill bushings or guides. Sometimes, we add cooling fins to the head
housing and ask them to run a fan on the housing whenever the head is
spinning. We have even gone so far as to bore holes through multiple
solid portions of the aluminum housing so the customer can pump liquid
coolant through the head at all times.
Some heads run in a shower or flood coolant atmosphere to control the
heat build-up...
we have tried steel housings, steel plates bolted to aluminum housings,
thin wall housings, thick wall housings, over sized housings, etc.
Anything we can think of to create a better atmosphere for either
physical control of the growth or management of heat build-up.
So... With all of that having been said, does anyone have any creative
ideas on how we might be able to solve this issue for some of our
customers?
...Happy 2013 to everyone and much success in the new year!
Greetings Joe,
At first thought, at least to me, the following idea seemed kind of
hokey, but maybe it will work. How about using a heater? Instead of
waiting for the head to heat up or having variable results in different
shops why not use a thermostat controlled heater? Then the thing stays
at some pre-determined temperature and tight tolerances can be held.
Eric
That's a pretty common thing to do in high-precision oscillators for
electronics: the oscillator lives in an oven, which is maintained at
about five degrees C higher than the specified temperature of the
oscillator package. Any time the outside is colder than the inside, the
oven works to keep the inside temperature constant.
An alternative, that would use less power but more parts (and add design
grief), would be to equip the head with both fans and heat: when power is
applied, heat the head up to a bit higher than you ever expect with
maximum load and maximum ambient temperature. Then as the thing runs and
starts generating its own heat, first turn off the heaters, then turn on
the fans.
The advantage to this is that with a heat-only solution you've basically
got to design the sucker for August in Pheonix, which means its gonna be
HOT all the time. With the fan cooling you can at least design for fan-
cooled August in Pheonix.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com