5 Axis Centres Thermal Movement

[yowzman]

We've recently had our first 5 axis delivered, Mazak CV5, 730mm X stroke, reasonably small machine.
We're seeing the centres of rotation move around with temperature about 20-40 microns (about a thou) depending on temperature change machine warm up etc, which is really f**king me up as I'm trying to set up for a part where I've got to hold 5 microns on a blend between two toolpaths (2 tenths).
Obviously there will be some thermal movement but I was expecting it to be microns (from the machine's perspective) with all the thermal comp software etc.
If you have a 5 axis, how much do you see the centres move? Or what kind of tolerances are you able to hold day to day with the rotary axis' moving?
Thanks in advance.

 

[rootsandstones]

I'd say that's to be expected, what's the temperature like during a workday in your shop? Our centres move around 80 microns with a room temperature going from 18 degrees celcius in the morning to 26 degrees celcius late afternoon. A lot of touch probing to keep everything precise but we mostly do prototypes.

 

[yowzman]

At the moment it's going from about 12-15 celcius morning to evening, which to my mind was plenty stable but upon reflection a meter of steel grows 10 microns per degree I think, which is a lot when you get to these tolerances. We're similar, mold making, so it's all 1 off and got to be right first time, no pressure ha.
So that's the best strategy you've found, probe the crap out of everything?

 

[Matt_W]

I have used various, from a Roeders that would move less than 0.002mm week to week. That was the nuts. It had full cooling circuit running through the casting, and also on the drives it's self along with spindle growth comp. I've also used stuff that's moved the same as your seeing. The first 5x I worked on was a Mikron UPC600, when we first got it we found the same as yourself if not more. We had major issues trying to blend toolpaths in from different tool vectors. We initially thought it to be a machine problem, then the supplier paid to have it put in a temperature controlled tent for 2 weeks as a trial, and that helped the issue straightaway. It still wasn't perfect but it helped massively. If you're mould making and trying to blend 2 tool paths in on an impression it's always going to be a battle. I also found that varying tool diameters caused blending issues as well.

Its a difficult one as it's not really feasible to reset your spin centres numerous times a day.

 

[yowzman]

Wow, 2mic is awesome. I've seen Roeders machines before but didn't realise they were that end of the spectrum.
Yeah, we're in the same position right now, there's literally an engineer here.
Probing centres repeatedly is indeed infeasible, and the automated probing cycle for centres on the machine still had an error in it as I see it using G43.4 and a dial test indicator on the reference ball.
I'm loathe to take a tool to steel until I'm fairly confident they're where they should be.
Perhaps temperature control is the way to go. I've been working on a test program which we're going to run shortly and compare the automatic cycle to my method with the clocks. Reassuring to know that it's more than likely to a machine issue.

 

[Richard King]

Buy an Infared temp gun and point it at various locations around the machine. If you get different temps then your AC unit could be causing the issue. Or you have sunshine going thru a window and it is warming one side of the machine. To get a machine stable you have to have a temperature controlled room at 68 F degree's or a constant temp. Where I am from where it gets down to 30 below F I have seen heaters blow warm air on the sides of machines and change the structure and the spec's change hour on hour.

 

[yowzman]

Got a temp gun and will check.
It's away from the sun and doors and shutters and heaters/AC (which don't exist) but the temp in here does change with the weather. Looking like temp control is gonna be answer. Might have a chat with our rep because we should probably have had temp control ball screws as well the numbers I'm seeing Vs what I was expecting. Unfortunately it is my first rodeo so I guess that's the price of ignorance.

 

[PROBE]

What really has impact on any machine, and 5 axes rotation centers position in particular, while the ambient temperature is changing is the temperature gradient. Machines are built from massive pieces of metal. Temperature measured on the surface of any machine part is not the same as the temperature 1,2,3 inches inside. This of course, beside expansion or compression, causes the distortion of the part, and known results.
Unfortunately there is no other way, but to keep ambient temperature constant, doesn't really matter if it is 68 degrees or any other, but constant. I suggest to build nylon sheets tent around this particular machine with dedicated air conditioner. I live in Israel, the temperature difference between day and night is frequently more then 15 degrees centigrade. Such tents solved the problem in many cases during my professional career.

 

[Richard King]

I would suggest coolant tank temperature control radiator, Many super precision machines have a temperature control spindle that uses a radiator / coolant pump to circulate to keep the spindle bearings the same temp all the time.. I have had to build a room around machines inside a building and install a AC unit to keep the machine at 68 degree's. I would also leave the control turned on all the time. 24/7. Make a program to dry run (no parts) cycle the 5 axis to spread the lubrication and heat the machine for1 hour before machining your part.

 

[yowzman]

The machine has a refrigerant spindle chiller so hopefully it doesn't require a coolant radiator, but if temp control will nail it down then I will build a room around it. We own the building so can do what we like internally.
Thanks for you help guys.

 

[5 axis Fidia guy]

Roeders is no doubt an awesome and accurate machine, one of the best out there. The Hermle I am running is dead nuts as well. The mineral casting and the way the machine is set up, the things stays extremely accurate day to day. I have experimented with probing a preset after doing some crazy 5x simultanious cuts, shutting the machine completely off over the weekend so not temp comp is working, and first thing monday morning going on to probe the preset again and seeing less than .0001" in change. That is on the outside, worst case instance on an 80mm pallet. I have cut medical molds and held less than .0002" on 3d contoured molding surfaces with some extra caution. The one and only thing that messes things up is COOLANT. If I set up a pallet of a bunch of cores or cavities and pick up all the presets I cannot use coolant or the presets will change, unless of course I flood the shit out of everything with coolant and keep a constant temp in the entire machine, but for hardmilling, it's a bad idea.

 

[hanermo]

I disagree with all the industry expectations and "wisdom".
Simply warm the room, the tools, the materials, to about 24-26 C, and most of the issues go away.

Trying to "cool" a spindle and machine tool seems to me to be less than optimal.
The problem is mostly delta-T, aka thermal change.
Make the machine and spindle loose heat rather than gaining it.

Theres a given amount of heat involved in cutting a workpiece.

 

[Matt_W]

Roeders is no doubt an awesome and accurate machine, one of the best out there. The Hermle I am running is dead nuts as well. The mineral casting and the way the machine is set up, the things stays extremely accurate day to day. I have experimented with probing a preset after doing some crazy 5x simultanious cuts, shutting the machine completely off over the weekend so not temp comp is working, and first thing monday morning going on to probe the preset again and seeing less than .0001" in change. That is on the outside, worst case instance on an 80mm pallet. I have cut medical molds and held less than .0002" on 3d contoured molding surfaces with some extra caution. The one and only thing that messes things up is COOLANT. If I set up a pallet of a bunch of cores or cavities and pick up all the presets I cannot use coolant or the presets will change, unless of course I flood the shit out of everything with coolant and keep a constant temp in the entire machine, but for hardmilling, it's a bad idea.

We had a production job that had 2 off 8mm dowel holes over 300mm in an ali part with true position of Ø0.05. Holding that size was ok during proving off the part. Once in production they were struggling to hold it. Turned out they hadn't spec'd a coolant chiller with the machine and over the day the coolant was warming up, which was transferring in to the part in machining. The part would then shrink in room temperature and the holes would fall out of spec.

 

[Matt_W]

The machine has a refrigerant spindle chiller so hopefully it doesn't require a coolant radiator, but if temp control will nail it down then I will build a room around it. We own the building so can do what we like internally.
Thanks for you help guys.

Another thing, Always warm up your tools before measuring by running them in the spindle for a few minutes to get them to operating temp. If you measure your tools clap cold then they will grow as they are used and your tool length will be incorrect.

Also is you are programming at different tool vectors with different tools and trying to match tool paths. The accuracy of the radius of the ball nose and or diameter of tools comes into play as well.

 

[yowzman]

All excellent points that I will do my best to execute on.
Thankfully (I think) all my work is one and twos, the machine is normally cold so perhaps getting it stabilized and keeping it at 10% rapid to avoid heating the screws up is the way to control that. Getting the blend perfect I'm happy comping the toolpath out in CAM and sneaking up on it, but I'm getting a missmatch side to side of 40microns. One side a perfect blend, the other 40 too deep. They've thrown kit at it, we've probed the centres a bunch and got things pretty stable so just about to run the test program for the real part. It'll probably be all out again tomorrow when the weather changes.

 

[Matt_W]

That sounds to me like your centrelines are out if one side is perfect and the other is out. Not sure they they are set on a mazak, but on a roeders there is a probing cycle that probes a tooling ball at all different c and a angles and updates the centreline coordinates.

 

[hanermo]

Another thing, Always warm up your tools before measuring by running them in the spindle for a few minutes to get them to operating temp. If you measure your tools clap cold then they will grow as they are used and your tool length will be incorrect.

Also is you are programming at different tool vectors with different tools and trying to match tool paths. The accuracy of the radius of the ball nose and or diameter of tools comes into play as well.

Yup.
Anytime you want to make anything better than 0.01 mm, everything comes into play.
It´s not necessarily hard, more technique.

Clockmakers et al have been making parts to about 2 microns or 0.002 mm for more than a hundred years.
So did victorian-era engineers.

A std Haas mill will make large parts of 1000 mm to 0.005 mm hole tolerances.
I know, because I sold it, and my customer was very pleased.
First Haas I sold with glass scales.

Amateurs routinely lap glass lenses to better than 0.1 microns, making telescope mirrors.

Using probing/testing, it´s quite easy to automate cnc machining to very high accuracies.

 

[empower]

I have used various, from a Roeders that would move less than 0.002mm week to week. That was the nuts. It had full cooling circuit running through the casting, and also on the drives it's self along with spindle growth comp. I've also used stuff that's moved the same as your seeing. The first 5x I worked on was a Mikron UPC600, when we first got it we found the same as yourself if not more. We had major issues trying to blend toolpaths in from different tool vectors. We initially thought it to be a machine problem, then the supplier paid to have it put in a temperature controlled tent for 2 weeks as a trial, and that helped the issue straightaway. It still wasn't perfect but it helped massively. If you're mould making and trying to blend 2 tool paths in on an impression it's always going to be a battle. I also found that varying tool diameters caused blending issues as well.

Its a difficult one as it's not really feasible to reset your spin centres numerous times a day.

the tool length/diameter is something a lot of people dismiss as well...
on the Mikron XSM600 i worked with a few years back, if you set it to high accuracy setting - it would automatically run a tool warm up cycle for a few minutes so that the metal would get to its consistent temperature. IIRC all machines capable of low single digit micron tolerances should have something like that.

 

[empower]

I disagree with all the industry expectations and "wisdom".
Simply warm the room, the tools, the materials, to about 24-26 C, and most of the issues go away.

Trying to "cool" a spindle and machine tool seems to me to be less than optimal.
The problem is mostly delta-T, aka thermal change.
Make the machine and spindle loose heat rather than gaining it.

Theres a given amount of heat involved in cutting a workpiece.

wether you cool or warm - doesnt matter. as long as the temperature stays constant. most say cool because its better to cool the machine so that it matches a comfortable work environment, rather than warm up the environment to match machine temperature.

 

[yowzman]

Would you say that glass scales are necessity for these kind of accuracies? We were advised they were unecessary.

Yup.
Anytime you want to make anything better than 0.01 mm, everything comes into play.
It´s not necessarily hard, more technique.

Clockmakers et al have been making parts to about 2 microns or 0.002 mm for more than a hundred years.
So did victorian-era engineers.

A std Haas mill will make large parts of 1000 mm to 0.005 mm hole tolerances.
I know, because I sold it, and my customer was very pleased.
First Haas I sold with glass scales.

Amateurs routinely lap glass lenses to better than 0.1 microns, making telescope mirrors.

Using probing/testing, it´s quite easy to automate cnc machining to very high accuracies.