Re: Single Point Left Hand Threading

Steve Wells

Saving subject posts for the FTE webpage...:)

Correct, the SBL automatic feed screw machine threads
at 0 degrees, straight in, in both directions, reversing the
spindle and leadscrew direction on front and rear tool bit passes.
Another little strange fact, at the minor flat of the acme thread,
the SBL print has a 0.010 depth square clearance cut. This is cut
by the front "rougher bit" flank minor, which has this square flank extension profile.


-----Original Message-----
From: <> On Behalf Of m. allan noah
Sent: Sunday, March 7, 2021 9:06 AM
Subject: Re: [SouthBendLathe] Single Point Left Hand Threading

On Sun, Mar 7, 2021 at 8:24 AM Guenther Paul <> wrote:

You can through that book away American and metric thread is 60 degree not 58 you set the compound on 30 degree.
The British withworth thread is 55 degree

Sigh. Think about it. The angle of the compound in-feed does not set the angle of the thread- the shape of the tool does. The 29 degree thing is so you always cut a little on the back side, leaving a clean flank, but do the bulk of the cutting in a direction that keeps the backlash taken up in the compound. So, the angle does not matter at all. I thread with the cross feed going straight in, and don't touch the compound at all, regardless of handedness. I certainly don't make a 0 degree thread :)


"well, I stand up next to a mountain- and I chop it down with the edge of my hand"

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