Controlling Thermal Growth

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
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:
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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!
Reply to
Joe AutoDrill
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Let me add that one of the greatest challenges is keeping the guide rods from binding up in situations such as I've mentioned above. I may sell a 36" wide head with 1" guide rods at the ends... When the head grows a few thousandths of an inch, the guide rods bind. Roller ball bushings help, but still disintegrate quickly under such loads.
Reply to
Joe AutoDrill
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
Reply to
See, that's the kind of stuff I would never think of on my own... Keep 'em coming!
Reply to
Joe AutoDrill
Didn't Monarch put a thermostatically controlled heater in some lathes that stabilized them at at a constant operating temperature? jsw
Reply to
Jim Wilkins
In optical design, design for temperature independence is called athermalization. There is a huge literature, part of which could be relevant in that it involves using materials with different temperature coefficients of linear expansion to passively adjust critical dimensions. (The non-relevant part involves picking optical glasses that have contrary variations in optical properties with temperature.)
In a linear array of axes, one would use aluminum and steel in mechanical combination such that the inter-axis spacing doesn't change even as the temperature of the assembly changes. One way to do this is to have a steel frame with the spindle bearings attached to one long wall by flexures, pushed via a second set of flexures from two aluminum plates attached to the two short walls and parallel to the other long wall. If one dimensions things correctly, the two expansion effects will just cancel.
Joe Gwinn
Reply to
Joseph Gwinn
Are you interested in two Dumore No. 24 automatic drilling heads?
Reply to
Interesting idea. Rather than trying to keep the work at room temp, keep the work predictably above room temp? Makes a lot of sense.
I was thinking some kind of cooling bath with circulator. Water has a high heat capacity, compared to metals.
Christopher A. Young Learn more about Jesus
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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
Reply to
Stormin Mormon
I considered mentioning something like that, but you'd need to make sure that everything heats up the same, or at least in the same way.
Contrary scenario: You mount your spindles in aluminum, in a steel frame. The dimensions stay the same over temperature as long as everything is the same temperature. Now run the thing: The spindles heat up, the aluminum expands, the spindle-to-spindle dimensions actually _shrink_ before everything comes up to thermal equilibrium.
So -- your athermalization suggestion isn't to be dismissed, but it should be approached with care.
Reply to
Tim Wescott
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.
Reply to
Tim Wescott
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Well -- one thing which comes to mind is the way the higher quality grandfather clocks were made. The sensitive part is the length of the pendulum rod.
The trick which they used (at least one trick -- I don't think a vial of mercury would help much here) is to have a harp of rods sort of like this:
| | | | | | _______ =X==============================================X= / \ | | | | | | / \ =X=========================================X= | | | | | | | | | | ================================================X= =X========| | | | | | | | | | =X=========================================X= | | | | | | | | \ / =X==============================================X= \_______/ | | | | | |
The left hand end goes up into the clock, and the right hand end is the pendulum bob. (Sort of off scale.)
The intersections marked with 'X' are pinned or soldered to the crossbar
The middle rod, and the two outer ones are a material with a low thermal coefficient of expansion, and the 2nd and 4th rods are a high coefficient of expansion.
The spacing between the two main crossbars is selected so the total expansion of the center and outer rods will be cancelled by the expansion of the 2nd and 4th rods.
Now -- rods are not going to be rigid enough for your task, but possibly alternate layers of differing metals could cause the distance between the ends to stay pretty close to position. But this would *only* correct the spacing between the center and the ends -- and not any intermediate spindles. (I guess that you could mount two more spindles halfway between the center and the end ones, and mesh the quills with rack gears on either side, so they would travel half the total expansion of the others. But with your 40-spindle heads, unless you have a set of thermal sensors, and CNC to move each head as appropriate, I just don't know. (Hmmm ... what is the TC for quartz? fit optical scales on the quartz, floating the ends, and use CNC to move each spindle to the right spacing based on the reading from the quartz scales.) Way too expensive to be reasonable, but still a thought.
And the same to you, DoN.
Reply to
DoN. Nichols
Greetings DoN, My first thought when reading Joe's post was what you describe above. But I couldn't think of a simple, rigid and cheap way to implement it. Then I though about how the real problem is one of variable temperature and how to keep the temp constant. There are all kinds of cooling schemes and some were mentioned. They all require power. Then it hit me that a constant temperature didn't need to be 68F. It could be whatever is convenient, which led me to think of cartridge heaters and thermostats. They require power too but I bet the power consumed would be pretty close to the power consumed by a cooling unit. Of course having the drilling unit hot may contribute to an uncomfortable environment during hot weather. I don't know how much heat would actually need to be pumped into a particular unit. I can imagine a large drilling unit mounted on a Bridgeport being operated manually all day and the poor person using it sweating up a storm. On the other hand it might be a bonus in the winter. Three points in the cartridge heater favor: simple, pretty cheap, and widely available. Two points against: waiting time to heat up and having a power cord. I'm sure others can find more legitimate negatives. On the other hand Joe could make the whole thing out of ZeroDur. That stuff hardly moves at all. Kinda brittle though. Eric
Reply to
Nice thread, So there are a bunch of gears in there, turning all the spindels? I can't add much, but the obvious. I assume you've done your best to reduce the heat source.
Could you flow lubricating fluid through the gear body? Reduce friction and carry the heat away, maybe?
George H.
Reply to
George Herold
of course, your solution can't raise the cost much :(
Can you drill in water passages? Then sell a temp control unit to those customers that need it. You could go from simple to sophisticated here. Simple=solenoid+thermostat to city water
Water has 50X more cooling than fins to air.
Reply to
Karl Townsend
Yes. The devil is in the details. DoN's discussion of athermal pendulum rods is a parallel example of mechanical athermalization.
To keep temperature uniform, I would fill the assembly with hydraulic oil like Mobil DTE24 (thin enough that the operation of the mechanism will keep the oil well stirred) such as used in the headstocks of Clausing 5900-series lathes.
I also like the idea of a thermostatic heater to keep the assembly at a constant higher temperature. Simple enough, but does require electrical power.
Joe Gwinn
Reply to
Joseph Gwinn
Unfortunately, not many people are. I get calls from folks looking to replace them almost daily now because people can't get service, parts, etc.
If one calls with a #24 need, I'll shoot them in your direction.
Reply to
Joe AutoDrill
Reply to
Thanks! Any idea what they are worth?
Reply to
Honestly? Depends on the customer... One guy may need it and will be willing to pay $1000+ for each one if they work well. The next guy (more likely) is looking for parts and will pay $50 for them and complain about the shipping...
It's like that with used self-feeders. They simply don't hold value because they are often configured for a specific job, etc. That and most of my competitors haven't a clue when it comes to post-sale customer support on parts, etc. As their customer, you may have the perfect machine, but if it's missing a widget, they can't figure out how to sell it to you.
Reply to
Joe AutoDrill

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