"Normal" CNC mill and CNC lathe tolerances in aluminum 2

[pceller]

Hi All,

I am looking for a "normal" or "average" tolerance which can be expected when designing for CNC milling or turning. I don't want to know what the best tolerance I can get is, just if I give a CAD file to a CNC machinist what tolerance I am looking at as his basic result when the machine finishes, and before he goes back and does any . I am designing parts with maximum dimensions of about 4" and want them made of aluminum. I don't have a shop picked yet that's why I am asking here. Any help would be appreciated.

Thanks,
pceller

 

[steveh]

A good average tolerance range on most lathe or mill work would be around 0.1mm or 0.005in.
Most types of Al are readily machineable and stable - it is no problem holding any reasonable tolerance.
If you don't have any specific tolerance requirements then remember that the closer the tolerance, the more you will pay for the part. If your part will work fine at 4.00 +/- 0.005, it will be much cheaper than a part that is 4.00 +/-0.0005.

 

[pceller]

Thanks steveh. Yeah, I realize that tighter tolerances mean hihger prices. I just wanted to know what kind of tolerance I could expect to put on unimportant CNC dimensions without significantly raising prices.

Cheers,
Pceller

 

[RRBD]

This is a poor way to go about a design IMO. Tolerance the part according to necessity of form, fit and function. Why tolerance it +/-.005 when +/-.125 will work fine,regardless of what a machine is capable of. If a part can be sawed to length, and the machining done with a roughing endmill then tolerance it that way with appropriate requirements for surface finishes as well.

Scott

 

[anderslr]

Is there a standard depth the taps are designed to cut? I keep hearing 1.5 x diameter. I want complete the tapping on a CNC machining center. Hand tapping in not a option.

Thanks in Advance

 

[psychomill]

pceller, the "general" consesus for "standard" tolerance would be this...

.X = .03
.XX = .01
.XXX = .005
Finish = 63
Angle = 1 deg

But keep in mind, none of this is set in stone. I agree with Scott (RRBD) though, tolerance the part as according to your needs. Don't try to "cheat" the price or machinability of a part just because you heard that "someone told me .001 is standard tolerance". That's why you have parts quoted at several sources and not just one.

 

[Tunalover]

pceller-
It looks like you are falling into the "how tight can they hold this" trap. Ask only for what you need not for "what they can give you." When I dimension and tolerance hole patterns in fabrications (note that I don't say if they are machined, punched, waterjet, etc.) I often call out a .25 diameter pattern-locating tolerance because that's what I can live with, not because that's what the mfgr can provide.

Of course the issue is not black and white. There are times when the manufacturing method must be taken into account but one should still keep functionality in the foregrounbd!




Tunalover

 

[pceller]

Thank you to everyone who responded but unfortunately what I din't want is exactly what happened. I knew that if I asked this question I would get a bunch of people telling me to figure out how much tolerances I needed and then "tolerance as necessary not as possible". I am well aware of this principle but that wasn't my question. I have a part where most of the features have critical tolerances and and those I have calculated and toleranced but I also have some purely cosmetic CNC'd features which are next to impossible to dimension and therefore tolerance. I just wanted to know how close a CNC mill is going to get to this feature given an electronic file.

Thank you to SteveH who actually answered my question.

Cheers everyone else. Its nice to see the principles of good engineering being upheld so adamantly but sometimes there's a good reason for asking a stupid question and wanting a just as stupid answer.

Thnaks guys,
pceller

 

[RRBD]

Cosmetic features don't need to be held within .005 in my opinion. You still have a guy trying to measure it with a caliper when all he really needs is a scale or tape. Dimensioning according to form,fit, and function applies to everything, even cosmetic features. If you want something cut to a Cad profile and not inspected then state exactly that.

 

[pceller]

RRBD - That is exactly why I asked what a "tolerance I am looking at as his basic result when the machine finishes, and before he goes back and does any [work]". I just wanted to know what I could expect, and based on the answer I got from steveh (and from a machine shop I recently selected) I can am not putting any tolerances on the cosmetic profile because the machine should do an adequate job.

BUT in order to as you say "want something cut to a Cad profile and not inspected" and to "state exactly that" I needed an idea of what a machine will give me if I don't state a tolerance.

Anyway, I have the information I need, thank you guys. lets not waste any more time on this,
pceller

 

[RRBD]

Sorry for offending you...
If I had a Quarter for each time I answered the question of CNC capabilities an then recieved a print that was way over-toleranced, I would have quite a collection of Pop money. The sad fact is, that most Engineering courses don't spend adequate time teaching proper ways of Dimensioning and Tolerancing, especially Geo. Tolerancing, and the result is, a Company spending alot of money on unnecessary operations. So, you see, I have been conditioned. BUT, in your case, you should see your parts come in very close to "nominal", considering the parameters of part size and material, realistically within a couple of Thousandths.
Sorry for wasting your time....BUT, one of the reasons, we (Americans)lose our jobs is due to high Manufacturing costs and Engineers play an important role in all of this. We can't control the exchange rates, but we can work to Design and Machine parts that are cost effective, by working together and having "tough skin". I'm tired of seeing Jobs lost, and will continue to "waste" both Machinst's time and Engineering time trying to get them to work together to produce a cost effective and usable product.Our future depends on it..............

Scott