[PSUBS-MAILIST] hemi heads

[Sean T. Stevenson via Personal_Submersibles]

Head specifications are always to the inside dimension, as that conforms to the die, and in the case of elliptical (by design) heads, is the only surface that is true 2:1 shape, and thus must be the reference.

Thus, when supplying a drawing or specification, you dimension the inside radii, and then specify the required wall thickness (with tolerance). As the manufacturer, knowing how a part thins during forming, they would then select a plate of sufficient thickness to meet your specification (minimum thickness at apex), resulting in a head which is thicker at the flange. If you drew or specified a maximum thickness tolerance, they would then have to turn it down to meet it (expensive), or you could just accept the extra weight imposed by the thicker flange.

As for roundness, whether you measure inside or out is of little consequence. The thinning should not be so profound as to affect the deviation of the neutral axis within the shell wall carrying the hoop stress, but of course it will be at its worst on the outside, so that is the most conservative option, and is also the easiest to measure.  They'd have to really mess up to be grossly out on roundness, however apex thinning is always a possible problem, so specifying that minimum thickness is always a good idea.

As for me, I figured I should finally step away from the drawing board and get something built, so I will be embarking on a project soon.  Stay tuned.

Sean



On November 6, 2015 1:51:37 PM MST, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,There is a problem with EE  after I paid and was sent the work
>order I noticed they measure the head height from the inside when in
>fact the apex is the thinnest.  That makes the sphere elliptical.  I
>contacted them and it turns out the template tolerance they said they
>could meet is measure on the inside.  So I am modifying the order so
>that the heads are measured from the outside, this way they are 1\2 as
>tall as wide.  I will true them up from there.
>Why are you contacting them?  are you starting a project?  Do tellHank 
>
>
>On Friday, November 6, 2015 11:49 AM, Sean T. Stevenson via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>   
>
>Glad that you're making headway.  I might be contacting EE myself in
>short order.Sean
>
>
>On November 2, 2015 5:16:03 PM MST, hank pronk via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,I did just as you recommended,  I have been talking with EE and
>they say now they can produce the heads within my tolerance.  I told
>them 1\2 of one percent of dia  measured with a 120 degree template.
> They say they will  measure the heads with a template before they ship
>them.   They have been extremely friendly and accommodating .  I even
>told them what it was for.Thanks' for your inputHank
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