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Old 10-22-2006, 01:08 AM
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general machining tolerance question (not just automotive based machining)

ok, the reason im asking IS because of an automotive project, but im sure theres plenty of aplications thats are similar, but not automotive based, hence the title

lets say you took a throttle body off a fuel injected car and overbored it 5mm

whats an acceptable tolerance for maching the hole "on center" ?

if the hole is .0045" off center (in reference to teh shaft) is this considered "acceptable" tolerance for a simple butterfly valve?

if not, what is ?

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Old 10-22-2006, 04:23 AM
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That specification is set by the designer of the part as to what he thinks is acceptable..An example is that some tolerance may be there to provide idle air or he may want to get a good seal when the buttrefly is shut..One of those "it depends" sort of deals..

Good enough is good enough..I know this is not much of an answer to your question..but laets look at say overboring an engine block..we may spec out a clearance of say +/- 0.0001 or 0.0002 on the bore diameter..in the throttle body we may spec out +/- 0.0020 t0 0.0030 tolerance on the bore..we are still boring holes but it depends on what we are after as to how much tolerance we can tolerate in a particular piece..

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Old 10-22-2006, 06:19 AM
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Holding perpendicular bore axes within 0.005 is very difficult when you're chucking up a part in a single axis milling machine. The original part, however, was machined in a computer controlled multiple axis machine and I wouldn't be surprised if the print tolerance was plus or minus 0.002!

Now, as to whether the part you have will be satisfactory: We've all experienced intake leaks...usually associated with carburetor or manifold gaskets...which were far in excess of a couple of thousandths. I would go ahead and install it. I don't think you'll have any problems.
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Old 10-22-2006, 08:56 AM
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Quote:
if the hole is .0045" off center (in reference to teh shaft) is this considered "acceptable" tolerance for a simple butterfly valve?
.0045 is quite a bit off center. Becuase you figure that with .0045 off iit is shoving the butterfly that much one way and that much less the other. I would want to hold center to at least .001-.0015 max.
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Old 10-22-2006, 11:30 AM
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best i can figure, a stock unit is .008" off-center
my modified unit is .0045 off-center

guess im within factory spec
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Old 12-27-2007, 01:10 PM
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Whatever x may be - so long as flow, performance and operation/function are not affected what difference would it make if it were .010 or .001? What is factory spec?
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Old 12-27-2007, 05:56 PM
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In the aerospace world, regular machining tolerances are +/- 0.030". We use +/- 0.010" only when necessary due to cost. +/- 0.001" is reserved for special needs due to the cost of achieving it. The throttle bore in question is more than close enough at +/- 0.0045". So long as there is adequate adjustment in the screws that hold the throttle blade to the shaft to allow it to be centered in the bore when the throttle is closed, you'll be fine. As you point out, the OEM tolerance is even larger.
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Old 12-28-2007, 07:40 AM
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Originally Posted by joe_padavano
In the aerospace world, regular machining tolerances are +/- 0.030". We use +/- 0.010" only when necessary due to cost. +/- 0.001" is reserved for special needs due to the cost of achieving it. The throttle bore in question is more than close enough at +/- 0.0045". So long as there is adequate adjustment in the screws that hold the throttle blade to the shaft to allow it to be centered in the bore when the throttle is closed, you'll be fine. As you point out, the OEM tolerance is even larger.

.030???? I can do that with my bench vice and a drill press. You must mean .003 and .0001 when needed. Most decent machines can hold .003.
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Old 12-28-2007, 10:50 AM
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Originally Posted by TurboS10
.030???? I can do that with my bench vice and a drill press. You must mean .003 and .0001 when needed. Most decent machines can hold .003.
No. I mean 0.030". This is for things like machined brackets. Obviously if the tolerances are critical you tighten them up, but there's no reason to pay for the extra machining costs if you don't need them. The reality is that the default tolerance on engineering drawings is +/- 0.030" and you only use tighter tolerances by exception.
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Old 12-28-2007, 11:33 AM
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Originally Posted by joe_padavano
No. I mean 0.030". This is for things like machined brackets. Obviously if the tolerances are critical you tighten them up, but there's no reason to pay for the extra machining costs if you don't need them. The reality is that the default tolerance on engineering drawings is +/- 0.030" and you only use tighter tolerances by exception.
Gotcha....brackets makes sense now.
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