Thanks, Andrew.
Your point is a very good one, and when I started running into dimensions that were going to four or five digits, I realized I had to rethink how I was setting up the piece to start with...
....in other words, by carefully thinking about starting dimensions (especially for diameters) and what's going to happen when you cut those dimensions into halves or fourths or fifths (as so often happens in design), one can control how many digits dimensions go to and still retain good precision.
As a rule, I try to design to no more precision (accuracy?) than +/- .001", and will gladly go to +/- .005 whenever I can; your advice is very well given and I post this reply partly so that others who may not be as experienced may benefit from your advice.
I don't fault eMachineShop.com for being expensive; it's the way it is in the U.S.
This is why I'm in the Philipines right now.
A part that I designed in eMachineShop, keeping tolerances as loose as I reasonably could, came in at about $250...which is actually not at all bad for a cam in aluminum with several turnings offset from each other, even though the part was smaller than a golf ball overall....
...but here in Metro Manilla I traveled a few miles to a little hole in the wall machine shop and got the same cam made for....$1 USD. Yup, one buck.
And they normally work to tolerances of +/- 0.0005 or even +/- 0.0001 and were frankly puzzled at the huge tolerances I initially gave them; +/- 0.005 made them think that perhaps it didn't matter WHAT the dimension was....
I also learned here not to bother specifying chamfers and things like that; it's much easier to just give them the drawing, point out where edges or intersections need to be "broken" or "rounded", and let them choose the most convenient method that will automatically result in a reasonable edge or corner treatment.
Very Best, Charlie
PS: one thing that puzzles me about eMachineShop that you or someone may be able to shed light on...in their menu's, they keep trying to force grid spacings on me like 0.098", etc....when I prefer to work with 0.1", 0.050", 0.025", etc. Why the odd numbers? Is it some sort of machining convention worked out over the years to take into account normal tolerances (i.e. +/- .001", etc.), or are these dimensions most easily convertible back and forth between inches and metric????
--- In Homebrew_PCBs@yahoogroups.com, "Andrew" <a_wake@...> wrote:
>
>
>
> --- In Homebrew_PCBs@yahoogroups.com, "gandolfreefer" <synchronousmosfet@> wrote:
> >
> > emachineshop does also enable you to get online quotes. Of course, the prices are for idiot entrepreneurs who have millions but no brains, but it's interesting to see...
> >
> > ...and, actually, emachineshop has a lot of nice built-in "real world" machining features that help you design something a machinest could actually make without too much trouble...things like menus for corner rounding, chamfering, etc., help.
>
> Charles, I am not familiar with emachineshop, and certainly wouldn't doubt that a company of this sort (with their own software) might be over-priced. However, I wanted to mention something that could be a factor (I ask forgiveness if I am telling you something you already know):
>
> CAD software makes it really easy to specify things using as many digits of precision as you want to use. As a result, many engineers (and amateurs) are tempted to give, say, 4 digits of precision (down to .0001" or even finer. The problem with that, when it comes to machining, is that you pay -- a LOT -- for extra precision. Most machine shops can machine something within +/-.001" without much trouble, but specifying it to within +/-.005" makes it a piece of cake, and therefore a good bit cheaper. On the other hand, machining to within +/-.0001" requires a lot of extra care -- and actually is starting to be meaningless unless you specify the temperature at which this must be measured. So a simple part might cost you $1 per part to have made to +/-.005", but it would go up to $5 per part for +/-.001", and up to $500 per part for +/-.0001".
>
> Again, I apologize if I am telling you something you already know!
>