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 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 2
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!
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On 12/28/2012 1:44 PM, Joe AutoDrill wrote:

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.
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Joe,
Machine tools are nothing but a big pile of compromises. What you need to do is look at all the available options and decide which compromise or combination of compromises best suits the purpose economically. It may very well be that none do and that your product simply has limitations.
If I'm understanding correctly you have to control how thermal displacement affects part tolerance. There are a limited number of options: remove heat to keep the drill head as close to ambient temperature as possible, design the head to be accurate at operating temperature, or use a material for the head that is minimally affected by heat.
You can remove heat in a variety of ways; all of which will comlicate the set up and add cost. If the heat is small using air/oil mist lube system might provide enough cooling to the bearings and any gears.
You might have some success with external coolaing with a cold gun. Or run chilled air through journals within the body of the unit.
Both of these methods will produce mist which is now governed by OSHA regulations. Plus they have the additional drawback of consuming compressed air which is probably the most expensive utility in the shop.
You could also cool the unit with a recirculating oil like Velocite that you run through a heat exchanger that is sized to be adequate to remove the heat down near room temperature. Again this comlicates the design and machining of the unit as well as complicates the set up.
Simlar to a recirculating cooling system, you could use a recirculating shower lube system in the head, where lube oil is run through a heat exchanger or chiller and is sprayed or dripped internally at the top of the unit and collected at the bottom and directed back to the tank on the heat exchanger or chiller.
You could calculate the temperature rise and build the unit so that it is accurate at operating temperature. The drawback here is that the calculations are complicated and if the machining operations changes, the load changes. In addition there must be a warm up eberytime you use the unit from a cold start, which isn't very productive.
The final option would be to make the body out of something like Invar, which doen't expand nearly as much as other metals. The downside would be the cost of the raw material and the added expense of machining it on your end.
On our spindles we use either a combination of recirculating cooling oil and air oil mist, or recirculating oil run through a chiller. But it's a very different application and that edditional equipment is designed to be mounted on the machine itself and doesn't ever have to be removed during a changeover.
--

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On 12/28/2012 3:23 PM, Dan Murphy wrote:

Lots of similar ideas there and much appreciated. Every little bit of info triggers a potential solution for a customer so it's all very helpful.
Someone in another forum said something I'd never heard of before. Engineer for the growth and put a heat source near the head that shuts down when the head is running at operating temperature. In other words, maintain the growth rather than trying to fight it. ...Interesting. Not sure how often I'll get to use that, but it's the truly unique ideas like some of yours and his that get me thinking.
Thank you.
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I've used a similar idea in the past, we measured the temperature rise in the cutting oil and put a tank heater in the coolant tank to keep the oil at that temp. It cut the warm up time way down. We went from chasing size for two hours to about 15 minutes on a 0.0003" total tolerance on diameter.
I would imagine that if you were building heads for automotive production that would see millions of cycles, paying more for a better system would pay off. Air/oil mist lube extends bearing life considerably and would probably pay for itself in that type of application.
--

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On 12/28/2012 3:54 PM, Dan Murphy wrote:
CLIP

Absolutely. An oil fog set-up helps quite a bit. It generally will double and sometimes triple the maximum RPM rate for the multi-spindle product we sell.
The downside to it is that it involves a liquid coolant VS a no-sling grease so the environmental side of it is significant for some customers. The other downside is the cost for some people. A good fog set-up costs as much as some of my off-the-shelf heads.
If you need it, you pay for it. Otherwise, you pay for it later, right?
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On Friday, 28 December 2012 18:44:18 UTC, Joe AutoDrill wrote:

I see that as your customers problem more than yours. You cannot be held responsible for something over you have no control. Unless you deliver a device to keep that multisplindle at the same temperature.
All you can do is to warn them of thermal expansion and say is not under your control. You already have some solutions like you said. All I can add is try to make that mutisplindle out of a material with low thermal expansion if cooling does not work.
If it was me using it I'd run a warm up program at same spindle speed as production, run a batch continuously and mark their production number and inspect them to try and see a trend. I think all that is to do is decide the time for the warm up program.
DanP
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r,

y
responsible for something over you have no control. Unless you deliver a de vice to keep that multisplindle at the same temperature.

your control. You already have some solutions like you said. All I can add is try to make that mutisplindle out of a material with low thermal expansi on if cooling does not work.

production, run a batch continuously and mark their production number and i nspect them to try and see a trend. I think all that is to do is decide the time for the warm up program.

I agree with the Dan(s). It is a trade off as far as design is concerned and responsibilities are shared between user and supplier. May be best for you to determine what the normal operating temp and tolerance(s) are for your standard units (design), document and guarantee them.
Provide documentation which clearly show operating temp and the effects of thermal expansion and contraction as they vary from the mean. Let them decide how they can meet what they need to internally (with you providing helpful advice). Of course there will always be Banquer's out there that don't read or comprehend technical data (his mini-mill spindle HP rating rant as just one example).
If tighter tolerance is a requirement then you can offer alternatives (extra cost options).
Tom
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