Help for selecting CNC machine

Salman Aziz · Jan 14, 2006

I am quite new to CNC machine. I want maximum machine operations for piston production by using a cnc machine and I don't have an idea which type of machine I should have?? please help me in this regard. Many many thanks

Dave K · Jan 14, 2006

You may need to give a little more information as to your operation before anyone could help you with an informed choice. Are we talking about turning or milling? What kind of sizes are we talking about? What kind of material are these pistons made of? How much volume do you expect to produce?

metlmunchr · Jan 14, 2006

Pistons are not round when at normal machining temperatures. Because of that, they're turned in custom built machines in most instances. On some of these machines, the cutting head spins and the piston stays stationary. Don't know many of the details past that, other than to know that normal CNC machine tools are not what you need to do this work on a volume basis.

SeymourDumore · Jan 15, 2006

Metl

I admittedly know a lot more about Nuclear Physics than piston details, but can you elaborate on this: :confused:

"""
Pistons are not round when at normal machining temperatures.
"""

Jeff · Jan 15, 2006

Like metlmunchr said, most pistons are oval in the skirt area at least, and tapered also...so as to be round when at operating temperature. Done on machinery purpose built for making pistons.
No idea what type of volume you are talking about, or what size, style, or application for your pistons. There is much more to making pistone than meets the eye.

Gary E · Jan 15, 2006

I want to know how to get a job selecting a machine of which I know NOTHING about for a process I know nothing about and either spend my own money or that of my employer, even if it is in Pakistan??

Anyone else smell something wrong here?

huskermcdoogle · Jan 16, 2006

I am not sure what we are smelling here. But I think it has to do with outsourcing....

metlmunchr · Jan 16, 2006

According to a friend who's from Pakistan, folks there buy mostly used Toyotas, due to their reliability. Then, they repair them as necessary to keep them running as long as possible. I'd imagine there's a good market there for rebuild parts for Toyota and other similar small car engines. Not everyone in the world views everything as being quite so disposable as we tend to here in the States. A lot of us could learn something from the less developed parts of the world, and maybe not be quite so broke and in debt all the time

SIM · Jan 16, 2006

I was asked to make some automotive pistons as per a supplied sample, with a ton of changes. Anyway, piston measured up round and was done on a 2-1/2 axis CNC mill. I know what machine because I called the manufacturer and we have same Haas VF-1.

Where abouts would it have been oval? More for my curiosity.

3t3d · Jan 16, 2006

Where abouts would it have been oval? More for my curiosity.

If you look at a piston, the wrist pin bosses have more mass than at a point 90 degrees to the wrist pin. As the piston gets hot, it -will- expand more in one direction than another. If it starts out non-round, it will fit the bore better when it is at operating temp. Also, the loads on the piston skirt are not symetrical around the bore. There is little side to side load on the piston, but in line with the connecting rod, the loads are higher, and since the loads change from the compression to power stroke, they are different from one side to another. Some pistons might have the wrist pin offset too. So, we have out of round, and offset, etc.
Now, by -how much- and in -what direction- and -where- are the million dollar questions.
A round piston will work, but not as well as one that is properly designed. The same as a 100,000 mile motor will run, even though a lot of parts no longer meet their original specs, but it will run not as well as new.

So, the answer is, you could make round pistons, but they will not be as good properly designed ones.
People might buy them, without knowing that they are not properly designed, and they might be cheap not being built on the proper (but expensive) equipment.

Lots of parts have a lot of really subtle engineering chnages and refinements to them, more so than meets the eye.

Pete

Dan Fritz · Jan 16, 2006

3t3d is right. Most pistons are not round. They are usually nearly round at the top, and become more "oval" in shape closer to the skirt. The temperature near the top of the piston gets quite high, and it expands to the desired clearance inside the bore. The skirt also expands, but less than the piston crown because the temperatures are much lower. The skirt is designed to run at a close fit to the bore at 90 degrees from the wrist pin, which is to keep the piston from "tilting" in the bore as it moves up and down.

There are so many different piston "grinds" that manufacturing them pretty much requires specialty equipment. Aluminum pistons can be machined at very high surface speeds, but the turning tools need to oscillate as the piston rotates to get the correct shape. What's more, the out-of-roundness of the piston changes as you machine from the top down to the skirt.

Go buy a piston for your favorite dirt bike from Wiseco and get out the mics ... You'll be amazed.

Jeff · Jan 17, 2006

VF-1? you can't be serious.

SeymourDumore · Jan 17, 2006

"""
VF-1? you can't be serious.
"""

What did I miss? :confused:

gbent · Jan 17, 2006

For those of you who don't know, the proper place to measure a piston for size is 90 degrees to the pin bore, at the pin bore location. Otherwise, just about any two places you measure will have different sizes.

toolmakerjim · Jan 17, 2006

this is a very intresting read for me since i have turned thousands of pistons for Federal~Mogul, mostly on OLD (1979) Cincinatti Milacron Acramatics these were diesel pistons for Catapillar Cummings and Detroit Diesel.
the tolerance was w/in microns and those Cinci's would perform day after day. the count was 325 per shift, 12 hrs, on the Cats and 175 per shift on the Cummings and DD. i likes the cat pistons better as they were 1/3rd the weight of the DD and Cummings.
anyway those pistons were round and gaged with .0001" indicators at 45* increments.

now i am aware that Harley Davidson pistons are oval but you dont get 200,000 miles on a set of them like you will a Cat...jim

Jeff · Jan 17, 2006

toolmakerjim, what sort of machine was that...? My understanding is that "acramatic" is C-M's name for the control?

It would be interesting (for me anyway) to see how automotive piston manufacturers are mass producing pistons on a vertical mill like the HAAS VF-1 (never too old to learn something new).

PMT · Jan 17, 2006

Perhaps its different in a low RPM diesel. And I've never measured a stock gasoline piston. but racing pistons are DEFINATLY oval and tapered (bigger at bottom and 90 Deg. perpandicular to the wrist pin). I probably shouldn't say this but I will any way, I'll wait until they are proven before installing Pakastani pistons in my race motor.

Donovan · Jan 17, 2006

Here you go. http://www.takisawa.co.jp/-e/e02_020102_4100.htm

toolmakerjim · Jan 19, 2006

Jeff;
these were plain late '70s Cincinnatti's and your right the Acramatic is the control name.
we ran the pistons in metric mode and adjusted the offsets in microns, very accurate...jim

wippin' boy · Jan 22, 2006

mazak

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