[BlueTwo]

Hi, newbie question i guess.

We are looking at buying a prototyping/low volume production vmc at around £40k (60k$). Mainly for working aluminium and plastics.

We have had samples cut on different machines from different manufacturers.

The samples have vertical ridges/lines in the side walls. These are visible on interpolated internal and external circular surfaces, and to a lesser extent on straight x and y axis cuts.
On some samples these are visual, an on others they can be felt with a fingernail as well.

The three attached images show this. The images of the metal parts are taken from elsewhere in this forum, but illustrate the effect. the plastic part is one of the sample pieces we have done, note the ridges on the flat as well as the curved surfaces (ignore the dust!)(scale on left is mm).

What causes this? servo step? ball screw pitch? backlash?
Is this typical for this level of machine?
Is it usually possible to tune it out/reduce it?

Thanks.

[txcncman]

It seems you are viewing what is called "chatter". Chatter is a result of harmonics in the cutting tool, the part, the work holding, or the machine, or a combination of these things. It can happen on any machine. The higher price the machine starts eliminating the machine as the problem. Trial and error and experience will help you to adjust programming so this does not happen.

[BlueTwo]

Thanks txcncman,

It does not look like chatter i have encountered previously (obvious harmonic variation along cuts), but i am not a pro. Presumably slowing the feed rate would reduce this?

Would you expect this level of chatter on a finishing cut run at modest speeds? We asked the machine tool vendors to optimise for surface finish rather than cycle time.

[LUCKY13]

Basically this can come from backlash and just simply fex and deflextion in a machine. If you will notice this surface finish will change between using conventional milling or climb milling. Most believe climb miling will give best finish, and you will always need to mack a extra pass with higher spindle speeds to get that extra fine finish on the part.


There is nothing wrong with the machine its just a learned skill to fine what it takes to get the smooth finishes with yoru mill.


Here is a great thread over at PM on the subject that might help you.


Climp vs conventional cutting: speed, feed, finish, life - Practical Machinist - Largest Manufacturing Technology Forum on the Web


So with each type stock you will have to learn what your machine needs to give the good finish. But it will always take a extra finishing pass to achive it.

Jess

[LUCKY13]

BTW this could be veiwed as chatter, its a natural frequency of the combined structure of the machine, part, work holding, and the tool bit. All though I know its not extreme chatter in the case of it getting out of control and causing big problems. I guess its kinda, still in control chatter.


Jess

[txcncman]

Thanks txcncman,

It does not look like chatter i have encountered previously (obvious harmonic variation along cuts), but i am not a pro. Presumably slowing the feed rate would reduce this?

Would you expect this level of chatter on a finishing cut run at modest speeds? We asked the machine tool vendors to optimise for surface finish rather than cycle time.

Machine tool vendors are not usually experienced machinists. Although I am sure they did their best to show their product in the best light possible. I am also sure they did not take the time or expense to try a variety of cutting tools and speeds and feeds and depths of cut to strive for perfection.

I think you will have extreme difficulty getting a totally smooth "mirror" finish on aluminum or plastics. External finishing processes will be required. The surface finish in the photos can almost always be overcome.

[BlueTwo]

Thanks for the comments. Really i'm just trying to get a handle on what is good, bad or normal for this category/price bracket of machine. Don't want to plunk down a load of cash for brand X and then find brand Y gives a better result. Most of our parts are not cosmetic, but a few are.

I have spent a lot of time going through the 'X vs Y vs Z' threads to try and get an idea, and followed that up with parts cut on the different machines.

So would you say that any new machine in this price bracket should be able to do better than those example images if set up with the correct adjustments/tool path/tool/fixtures etc.? -or are these images typical of what to expect?

And to clarify, these steps/ridges are not to do with the minimum size steps the servos can increment in?

[txcncman]

Everything is relative. Sure, a $60,000.00 mill can be set up to do better than pictured. So can a $5000.00 mill. It will be easier to get "good" results on the pricer machine - in most cases.

I understand you are wanting to reduce your risk and increase your chances of success. Nothing in life is 100%. It probably would serve you better to spend less on a machine and then send your employees and yourself to school to learn better machining skills. That will have the bigger payoff in the longer term than the machine tool investment. Good quality employees can and will work wonders. Button pushers will be just that, button pushers.

[Geof]

.....So would you say that any new machine in this price bracket should be able to do better than those example images if set up with the correct adjustments/tool path/tool/fixtures etc.? -or are these images typical of what to expect?....

The left hand image is pretty typical of what you can expect with a medium priced machine taking a reasonable cut at a reasonable feed rate.

So you need to know what is reasonable.

In 6061 alloy a new two flute cutter 5/8" diameter running at 8000 rpm with a 0.2" radial depth of cut radial depth of cut and a feed of about 0.020" per tooth (320 inches per minute, ipm) will give a finish where the vertical striations are more pronounced than in your image. This is a fast cut that you would use for removing material quickly.

The same diameter running at the same speed but with four flutes, a radial depth of cut of 0.01" and a feed of 40 ipm will give a finish where the striations might be difficult to see. This would be a finish cut for best appearance.

I think these striations are not really chatter but they are related. Chatter is a situation where the tool deflects due to the cuttung forces then springs back, digs in deeper and deflects again at a natural resonant frequency. Chatter can often be avoided by using more rigid tooling or fixturing.

I think these striations reflect two things; the first is minuscule tool and machine deflection, when the cutting edge strikes the workpiece; the second is the fact that the tool rotates and advances so it is creating minuscule circular scallops in the surface as it cuts. Reducing the radial depth of cut reduces the deflection and reducing the feed rate makes the scallops smaller and shallower so they almost blend into a smooth surface.

There is another source of surface imperfections that you may find more annoying with a machine in your price bracket. I have attached an image showing parts machined on a Haas VF2 which I think is a fair bit above your range but it is certainly not a top of the line machine.

If you look at the flat surfaces you will see they are fairly good but you can see large and small swirl marks. The large ones are the interpolated tool path as it faces of the top of the part in two circles and a straight pass. The tool is a 5/8" two flute running at 10,000 rpm at a feed of 100 ipm and a cut depth of 0.01 for the final pass. The smaller swirl marks are from the tool itself.

These surface imperfections are very difficult to avoid unless you have a machine that is much heavier and stiffer than a VF2. Again they arise from deflection but in this case I think it is predominatly the machine deflecting under the accelerations loads when the table changes directions. Obviously the deflections is very small but it results in a visible difference in the surface. It is easy to polish this type of surface to an even matt finish or even a mirror finish but getting a surface that looks perfectly even straight off the machine is practically impossible

[majmudar]

If you are using too small a diameter of end mill, or low RPM or higher feed, other things being equal you will get this type of finish.
Try using a larger diameter of end mill, at about 4000 or more Spindle RPM
Take a finishing cut with a low depth feed.