Machining tolerances- general info?

It's really about common sense. A bearing fit in a farm implement is no less critical than it is in an engine for an aircraft. There are optimum clearances or fits that perform satisfactorily----regardless of application. Those that know and understand machining are able to work to tight tolerances as needed, regardless of the application. Anyone that suggests that it's for a plow, so it doesn't matter, is telling you more about their ability and knowledge than they are about proper fits. The only charts I know of are charts for specific type fits------and they are *not* job category specific. A slip fit is always a slip fit.

There is no better way to improve one's ability than to work closely when learning and even later when making widgets.. It becomes routine, so when a tough job comes along, it's handled in stride.

Harold

I won't argue with you that the OP's question was oddly stated, but consider the following cost equation:

Case 1: Bearing fails on a jetliner, airplane crashes into a cornfield killing 300 people and ruining the corn crop.

Case 2: Bearing fails in a combine on the cornfield, farmer has a long walk home (or more likely calls someone on his cell phone to come get him), corn gets in a day or two late -- at worst he loses his crop.

So the worst-case scenario in the "farm bearing" case is that the farmer dies from exposure on the way home, and you lose a corn crop. In the "jetliner bearing" case it's 300 people _plus_ the lost crop.

Airplanes must be light, jet engines turn faster than diesels and run hotter, joints must be much tighter to function properly, etc., etc., etc. To get light weight, reliability and high performance you design expensive equipment that requires careful maintenance. Anyone who designs farm implements that must be maintained like aviation equipment should be made to purchase and maintain it for several years -- and if that seems cruel and unusual you can take pity on them and use tar and feathers.

I think it's more a matter of what's allowable (or prudent) to design into the equipment that drives what you're likely to find, but I do think you'll see a greater need for high precision work in aviation than farming.

The rule is quite simple: Do it as coarse as possible but as precise as necessary. But this rule doesn't help the OP.

For aviation: Most of the costs are coming from QC, the rest from QC. Yes, it is true that they have tighter tolerances, but they need to have very tight reproducable results. They get them from QC. :-)

Nick

"EdFielder" wrote in news:vSElf.558$ snipped-for-privacy@newsread2.news.atl.earthlink.net:

Ed, There are a lot of standards out there. Most have already, and the rest are being converted to ISO standards. You have a standard for fits, for bolts, nuts, screws, shafts, bearings, and the list goes on and on. These used to be, typically, one of three different standards, DIN (European), JIS (Japanese Industrial Standard), and SAE (Society of Automotive Engineers). The ISO organization is tasked with taking those three standards, plus any others that exists and combining them into one, world-wide useable standard. You can purchase the book of standards for metal working. It is not cheap though.

I don't have an argument with that, but I see no difference in failures when a little care can avoid them. We were not discussing the cost of the failure, only the fact that failures occur. That's my point, and always has been. I'm the guy that twists the handles and makes the product (or I should say I used to be. I'm retired now). Those that used my services in the past reaped huge benefits because of my work ethics. I've always said that if you are interested in used oats, they come cheaper than nice, fresh ones. I sold only new oats. I see no difference in machining and oats. You get what you pay for. How slop work on a combine would make it work more reliably is not something I see clearly. I still suggest to you that one should use good work practice, regardless of application.

Harold

Well my personal take on it works this way.

It depends on what part of the plow you're talking about. If you're talking about the spindle then it needs to be pretty close. Not to tight where you can't get the wheel on and off but tight enough that the bearing won't wear out the spindle. In other words I usually shoot for between 1/4 up to 1 thousand undersize. This is better than the factories are turning out now days (I've seen some factory spindles that are way to loose). The hub on the other definitely needs a press fit.

On the other hand when I thread a plow shaft I always make the threads loose. Usually I'll get a good fit and then got about 0.010" under that. The reason being that I've spent well over a hour more than once just trying to get the nut off a shaft before. After they run in the dirt and rust they won't be to loose for long.

Most everything else on a plow falls under welding type tolerances. Never try to make a precision fit on a product that's going to sit out in the dirt and rain. It'll freeze up in short order. I learned this early on in my life working on the farm and repairing equipment.

Common sense combined with a little experience will get you a long way to knowing what tolerance you need to work to. I get complaints from customers that I make some things to tight. Like pulley bores and bearing fits. They want it to just slide on but many of these need to be tight or they'll wear the shaft and/or bore out. Other things like gate latches need lots of clearance. Nothing worse than trying to open a gate to find the latch is froze up.

Wayne Cook Shamrock, TX

I think you may be thinking about the standards for limits and fits. Machinery's Handbook and most other handbooks have tables with standard ANSI preferred sizes and limits and fits. Most specs. include these standards for designing equipment. Bugs

"EdFielder" wrote in news:8SWlf.844$ snipped-for-privacy@newsread2.news.atl.earthlink.net:

Probably because I design and build stuff for a living :)

Tiny URL for above:

IIRC, DIN, ANSI, SAE, ASME, JIS are in the process of being depreciated, meaning they are being phased out. ISO is the new standard and is under active, continuous development. That page provides an index of published standards and committees. There should also be links to where you can obtain the standards.

Nick,

It seems you are right on!

However, from the little bit of aviation work I was exposed to, it is really one of the better toleranced fields. It seems they truly follow the 'form, fit, function' concept of geometric tolerancing.

As to QC costs, there is a saying that you cannot inspect quality into a part. What I observed is that the buyers of aviation products do indeed inspect their incoming purchased goods, often

100 percent inspection!

As to the OP's question about tolerances: I have observed that most job shops (contract manufacturing) have developed their own set of 'standards' covering the basic machining operations. These often are taken from 'Industry standards' such as the typical sizes resulting from a drilling operation. This procedure lends itself to a relatively uncluttered drawing, where only deviations from accepted practice is desired. Hence the standard tolerancing note on most drawings.

These standards are then incorporated into their contracts. Consequently, it is the buyer's responsibilty to obtain and review these standards to see if they satisfy his requirements. Then upon reciept of purchased goods, their is a basis upon which to base a valid inspection criteria.

To paraphrase what you said, it is all a matter of economics. More precision costs money. Essentially it boils down to return on investment. You can quote lower hourly rate using a standard drill press as opposed to a "Devleig jig grinder.

I guess the OP should be looking for some good literature on the economics of manufacturing methods.

Generaly, you have a note on the plan that sais something like "untoleranced sizes according to DIN/ISO blablabla".

Yes. That's what the phrase "only as exact as necessary" says. For every job, there are accepted tolerances. For weldings, they are easy to be within. If this ain't enough, add tolerances. And the prize goes up. For bearing-fittings, they are quite small, but that's what is required for them to work well.

Nick