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Pricing tolerance - not sure this is the right forum

vettedude

Stainless
Joined
Nov 25, 2009
Location
Texas
How does someone go about doing this, what gets justified as a tight tolerance and a loose tolerance.

If per say you had 3 different parts 1/3/12" ID hole to be turned in a 24" long part 1.5" wall thickness with the following unilateral ID cylindrocity and ID dimensional tolerance for the sake of discussion lets say these parts are 4140 @ 80 ksi heat treat or 6061 T6
0.0001
0.0005
0.001
0.005
0.010

My experience says that the 0.0001 is very difficult to achieve on 4140 I would have the part roughed, heat treated and final ground to size, even then the cylindrocity would be very difficult without a knowledgable grind shop.

I am not asking for pricing, but is there some sort of rules of thumb, or is this all experience based?
 
It depends on the number of parts too. If you are trying to make one complicated part and not blow any .001 tolerances, it will be much more difficult than if you were setting up a 200 piece run and you could scrap one trying to get it right.

Lots of things can add to the cost. Things like grooves and back counterbores often can't be checked while in the machine. If you have to un-clamp the part, your chances of scrapping it are higher.

IMO, .0005 and lower tolerances require climate control and specialized measuring equipment. Usually that would be grind territory, but some folks can hit that in a lathe.

Experience is important. This is why you often see that shops have a specialty. Large parts, small parts, aluminum, stainless, grinding, welding, etc. They know how to bid on work that they specialize in. If they start taking in work that is outside their wheelhouse, they are more likely to misquote.
 
^ Very well spoken.

I can't comment on bidding and quoting, but my experience making parts and trying to hold those tolerances suggest that once the tolerance on bores fall below the +/- .002" range, the quality of work and equipment required to hold those tolerances grows exponentially. And since time, skill, and equipment = money, the pricing of these tight tolerances must reflect that.

I've done quite a bit of boring in CNC lathes, CNC HBM's, as well as some bore grinding on different hardened steels. There's so many variables that play into it, it's hard to come up with a formula, so this is where experience is crucial.

Since your example asks about boring, I would consider these points...
-Dimensional Tolerances? The tighter it is, the more expensive
-Length x Diameter? The longer the bore, the more expensive
-Material? The more difficult to machine, the more expensive
-Equipment involved? The more machine time / processes required, the more expensive.
 
Do I understand correctly that this is a bore diameter you're talking about? If so, hone it. The temperature control, the machine and the measuring equipment will cost a fair bit but you can hone practically anything to tenths while making it round and straight with a low-microinch finish. That's how hydraulic cylinders are made.
 
Experience, knowing your equipment and its limitations. But the worse I find is material stresses, darn hard to predict it. Made some parts like week that took almost twice as long as usual cause its was from a different mill/batch and full of stress, chased it around like a rabbit on crack, of course that's after lowering the price since I had the time down on them...

Anyhow...

Complicated work that really pushes for tight tolerances(sometimes the impossible..) goes 2 ways, there's the shop that is very well equipped and has the knowledge and experience to do it well and charges accordingly.

Then there's the shop that hasn't a effing clue what they're getting into that quotes it way too low, then through any amount of delays and messing around the thing finally gets accepted cause its close enough and seems to work.



I solve it rather simply, If I'm pretty sure I can't do it, someone else can have fun doing it.
 
I believe that pricing for tolerance is tied to the surface finish requirements. If your machining method is such that the typical roughness of the finished surface is comparable to the tolerance, then you've got to charge plenty. For example, lathe turning and boring operations might typically have a surface roughness of .0005", so across a diameter, this could provide a .001" tolerance on the part. So when you are working to .0005" or less, you're working beyond the scope of the natural ability of the equipment, and somebody's got to pay for the risk potential of a screw-up.

And if you have to resort to a process that makes surface finish roughness easy to hold within a tight parameter range, chances are, that process is slow as hell, and you've still got to charge for the extra time that takes, even though the risk of a screw-up is probably a lot lower. For example, if honing a bore, you're very unlikely to suddenly go .001" oversize trying to remove hone marks, because it takes forever to remove that much.
 








 
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