Depth of cut

[Eric ArthrellParticipant @ericarthrell78468]

Hello Can anyone advice on how to work out the depth of cut required for cutting a 1-7/8" x 8TPI whit form thread please.

I get a value of 0.100" "not confident that this is correct " not the brightest bulb on the tree

Regards Eric

[Eric ArthrellParticipant @ericarthrell78468]

[HopperParticipant @hopper]

Whitworth thread depth = .6403 x pitch. Pitch of an 8tpi thread is one eighth of an inch, or .125.

.6403 x .125 = .080"

So thread depth is .080"

But don't trust my math, I majored in English Lit at college so check it yourself to make sure.

[Eric ArthrellParticipant @ericarthrell78468]

Thank you for your reply Hopper .

confirmed my calculation, the extra .020" is for crest and root addition using a pointed cutting tool profile.

ie P x 0.160083 = 0.020".

Kind Regards Eric

[BazyleParticipant @bazyle]

There are various tables to save you the maths for depth but the tricky bit is deciding on the max diameter of the blank to start with to avoid the pointed crests fouling when you try the nut.

I can confidently suggest you make it out of metal or something not liquid or gas as I majored in Metallurgy & Materials Science.

[Howard LewisParticipant @howardlewis46836]

If anything, machine the blank very slightle undersize, (in this case say 0.010&quot so as to truncate the thread to prevent root / crest intereference. This is assuming that the thread to be cut is a male one. If female, machine slightly oversize, for the same reason.

Somewhere in my Apprentice Notes there is a formula to to calculate by how much the thread should be truncated.

Found it!

For WHITFORM threads truncation should remove (0.160083 x Pitch) from the crest of the thread, which looks like 0.020" for 8 tpi. So I was erring on the side of caution, above.

Truncated dia of SCREW = Nominal Dia – 0.1478 / TPI (1.835" in this case)

Truncated dia of NUT = Nominal (Core) Dia + 0.1478 / TPI

BUT allowance needs to be made for the truncation when setting depth of cut, so maybe the depth of cut on a 1.875 x 8 TPI thread would then become 0.060" ( If the blank were not truncated, the tool would make contact at 1.875", but the truncation has removed 0.020" already, so the tool should only be fed in 0.060" to achieve a full depth of thread.

Howard

[richardandtracyParticipant @richardandtracy]

For info, on metric external threads, with a sharp point cutter, DOC = 0.7578 x P where P= pitch.

Source, Kempes Engineers Handbook, 1988, page E6/3 Fig 3, calculated from thread height H = 0.866025404P x 7/8 to allow squaring off of the ends.

For metric internal threads, it's DOC = 0.6495 x P.

Regards,

Richard.

[Eric ArthrellParticipant @ericarthrell78468]

Thank you for your replies now feel more confident to have a few trial runs before machining back plate for my four jaw chuck.

Regards Eric

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