[jaguar36]

I'm working on a new frame design for my mill and have been doing some analysis to understand how stiff its going to be and to optimize the design. I've been looking to get a reference on how stiff some other designs are and recalled seeing a research paper where they had created an FEA model of a Haas mini-mill. Unfortunately I can't seem to find that anymore... or possibly I imagined it.

Has anyone come across it? Alternatively, does anyone have any data for the stiffness of other mills?

[peteeng]

Hi Jag - A modern CNC VMC has a stiffness in the order of 150um/N. This is loaded at the end of the toolholder and restrained at the bed somehow. Unfortunately there is not a standard method for the test. Small benchtop mills are <10um/N. Something like a taig is <2um/N. The stiffest quoted machine I have been able to find is 650um/N. Pick a number and see how you go.... Many people restrain the machine at its bed feet but this over constrains the machine (results in it being stiffer then practice) and does not represent the actual load loop or load path. Model a vice for instance in the centre of the bed and hold something in it like in practice or at least some dummy geometry. Whichever way you do it be consistent. Plus you need to figure how your going to deal with bearings and bolted connections., Does your FE model bolts? From me doing FE on many machines you can "bond" all the connections and assume the model is about twice as stiff as it will be in reality. So say you aim at a machine stiffness of 50um/N then build the model at 100um/N... Peter

[pippin88]

I don't think the Haas mini mill is that stiff Peter

[peteeng]

Hi Pippin - I don't think so either. If Jag models it we may find out. I sent a note to Haas many moons ago asking about their machine stiffness and the reply was we don't know. Seems strange but maybe was the easy way to say its proprietary info.... I think I asked via a white paper they published on machine design.

and jag I quoted the stiffness wrong way round should be N/um not um/N Peter

[Buoyen]

I think I've seen a CAD model of the mini mill at grabcad.
If the castings are drawn okey you could download the model and run simulation of the machine.

[jaguar36]

That's some good info Pete. What machines were the 150N/um and 650N/um?

My current machine is only about 16 lb/thou (3N/um), so I'm shooting for an order of magnitude improvement. That would still put it at like 1/5th of a 'real' VMC, but then it's going to weigh an order of magnitude less than a VMC too. I'm currently looking at about 250lb/thou (45N/um) but am ignoring a bunch of stuff at this point.

[peteeng]

Hi Jag - The 650N/um machine was a custom built high MRR for machining titanium. The quote is 2 million lbs per inch at the spindle face. Peter

https://www.mmsonline.com/articles/a...e-for-titanium

[peteeng]

Hi Jag- more info on stiffness I've just found. Plus a std way to test for compliance. Peter

[jaguar36]

Hi Jag- more info on stiffness I've just found. Plus a std way to test for compliance. Peter

That's a great standard. Section 7.2 is the applicable section if anyone is looking at it. Interesting that there were no machine tool makers on the committee, but alot of aerospace and automotive OEMs. I found that the equivalent ISO standard is ISO230. I wonder if anyone is actually using these standards. I would have thought that you would be able to find some test reports based on it for some of the popular VMCs.

[peteeng]

Hi Jag - Yes I have been digging in this area for years and this has just come to light. Machine makers specify everything else you'd expect the machine compliance/stiffness would be in their interest to spec as well. Maybe soon... Peter

[mactec54]

Hi Jag - Yes I have been digging in this area for years and this has just come to light. Machine makers specify everything else you'd expect the machine compliance/stiffness would be in their interest to spec as well. Maybe soon... Peter

This is well documented within any of these companies, it is all proprietary information, the likely hood of you finding any of their real design information, is slim to none, unless a company has been hacked you are not going to find it, do you know the recipe for Coca Cola a lot have tried to reverse engineer it and have not succeeded, even if you reverse engineer a machine it still will not be the same as the original manufacturers spec's, there are too many unknown variable's.

[jaguar36]

This is well documented within any of these companies, it is all proprietary information, the likely hood of you finding any of their real design information, is slim to none, unless a company has been hacked you are not going to find it, do you know the recipe for Coca Cola a lot have tried to reverse engineer it and have not succeeded, even if you reverse engineer a machine it still will not be the same as the original manufacturers spec's, there are too many unknown variable's.

How they get their stiffness/compliance may be properitary, but the actual data isn't. Its easily measured and I've found that some OEM's actually require their suppliers to provide results of the ASME spec tests on a regular basis. I would have thought that the top end machine manufacturers would be advertising these test results to show why their high-end VMCs are better than a low end one.

[mactec54]

How they get their stiffness/compliance may be properitary, but the actual data isn't. Its easily measured and I've found that some OEM's actually require their suppliers to provide results of the ASME spec tests on a regular basis. I would have thought that the top end machine manufacturers would be advertising these test results to show why their high-end VMCs are better than a low end one.

They have nothing to gain by doing so, so why use resources to do so, suppliers of parts are totally different and this would be expected

[jaguar36]

They have nothing to gain by doing so

Sure they do, they can show that their machines are more rigid than their competitors.

Interestingly, I did find that Langmuir advertises a stiffness for their MR-1, "40 millionths per pound of cutting force" (25 lb/in 4.4N/um). Which is actually pretty terrible.

[mactec54]

Sure they do, they can show that their machines are more rigid than their competitors.

Interestingly, I did find that Langmuir advertises a stiffness for their MR-1, "40 millionths per pound of cutting force" (25 lb/in 4.4N/um). Which is actually pretty terrible.

You could not call that any more than a hobby machine at best, and a poorly built one at that, it does not compare to any real industrial machines

[jaguar36]

You could not call that any more than a hobby machine at best, and a poorly built one at that, it does not compare to any real industrial machines

Right, that's why its odd that they are advertising their stiffness.

[mactec54]

Right, that's why its odd that they are advertising their stiffness.

They advertise a lot of specs for that machine which are all questionable, some people will say anything to make a sale, to unsavvy buyers

[peteeng]

Hi Jag - Thks for the MR-1 info. Good info as I'm nearly going down that path but my machine will be much stiffer. Peter

[Mogilogi]

Hi Jag - Thks for the MR-1 info. Good info as I'm nearly going down that path but my machine will be much stiffer. Peter

Same here

[jaguar36]

I was digging into this some more today and found a paper out of Spain that discusses designing a milling machine. In it, it has a table that gives the stiffness required for various cutting operation in 1018 steel. Unfortunately it does not give any details about the cut, beyond roughing/finishing. It also does not site any sources for the data. Regardless, it gives a reference value of 142lb/thou (20N/um) which seems reasonable.