Starret ACME gauge - root vs crest
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  1. #1
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    Default Starret ACME gauge - root vs crest

    Hi all,

    Looking to cut some acme threads - not for the SB 10L as yet, but it'll be some good practice for compound screw and tailstock screw which are both due for a refresh. I've not cut acme threads before.

    Machinery's Handbook gives the root width of an acme thread as 'crest-0.0052". With the crest being '1/P * .3707' where P is the pitch.

    I have a starrett acme thread gauge and if I offer up the calculated root width to each slot it is slightly too small - i can move it about in the slot. How small? About 0.005 small. If I offer up the calculated crest width then it is a nice fit in each slot. I have tried all to about 8tpi with the same result.

    I am using a digital caliper that has proven to be pretty accurate so far.

    So, just checking my sanity here. The gauge slots *are* meant to be root width right? It is the only thing that makes sense to me.

    Why did I check? Because I ground an 8tpi HSS acme tool and it was dead nuts in angle and in slot width according to my acme gauge .... but when offering the tool to an unused portion of (what I believe to be) an 8tpi acme screw it didn't reach the thread root.

    All help and advice appreciated.

    Greg.

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    Manufacted Acmes often have slightly rounded roots so as to avoid stress risers.

    as far as the 284 gage don't know but better to have a slightly small tool to let you sneak up on final fit so maybe that is why?

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    Thanks 10k,

    I'll get the thread under a good magnifying glass to see if it is rounded. Thanks.

    Re the gauge thing, I am at a bit of a loss there so I might just have to use a micrometer to get the right root width on the tool. I still figure I have got something wrong somewhere but not sure what. The root widths come from Machinery's and I reckon the starrett is unlikely to be inaccurate so it has to be my measuring.

    As we say in the software world .. "PEBCAK", .. or "Problem Exists Between Chair And Keyboard".

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    Don't forget that as in the case of a standard 60 deg thread, you don't want to be cutting material on both sides of the tool as you advance it into the work piece. Thus, I would expect a correctly ground tool to be slightly narrower than the actual thread being cut. At least, that's how it works when I cut ACME threads---rarely!

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    Thanks Doberman. In this case, I measure the the slot to be *wider* than the specification root width, so that actually makes the tool wider than it should be if I used that gauge. That is, cutting the thread to a spec thread depth will create a broader root and a narrower crest.

    It seems sometimes starrett tools do have issues:

    Bad Starrett Acme Thread Gage No. 284

    Who knows.


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