Screwcutting in the Lathe – for Home Machinists

Screwcutting in the Lathe – for Home Machinists
Reference Handbook for Both Imperial and Metric Projects
Martin Cleeve
Contents
Acknowledgment
SECTION 1
INTRODUCTION
Introductory notes – Conversion notes –
Quick reference thread sizing formulas
SECTION 2
PRINCIPLES OF LATHE SCREWCUTTING
Altering the pitch – Calculations – Simple and
compound gear trains – Schematic gear train
presentation – Calculations for mixed number
TPI – Equal ratio setting – Threads
designated by lead – Proving gear trains –
Self-act feeds from leadscrews – Diametral
pitch worms – Formula for worms by DP –
The DP formula – how evolved – Proving
worm thread gearing.
SECTION 3
GEARING AN ENGLISH LEADSCREW FOR
METRIC THREADS
50-127 translation ratio explained – Basic
conversion formula – Checking metric gear
trains – Modified gearing systems for metric
pitches with an English leadscrew – Reduced
pitch translators – Alternative translationgearing – The 2-21 (63-160) method –
Checking the 2-21 gearing – 13-33 translation
gearing and 15-38 – Worms sized by module
– Gearing an English leadscrew, practical
examples.
SECTION 4
LATHES WITH METRIC LEADSCREWS
Probable standard pitches for metric
leadscrews – Screwcutting calculations –
Gearing for English threads with a metric
leadscrew – Disadvantages of 127-50 step up
– Disadvantages overcome by alternative
translation ratios – Very small pitch errors –
Proving metric – English gear trains – Finding
exact pitch given – Quick checking for
nominal pitch – Worms for gears sized by
module: gearing with metric leadscrew –
Worms sized by DP: gearing with metric
leadscrew – Change gear calculations by
approximation.
SECTION 5
PROBLEMS AND ANALYSIS OF REPEAT
PICK-UP
Meaning of pick-up – Examples – The thread
dial indicator or leadscrew indicator – Action
of English indicator – Metric threads: pick-up –
Geared leadscrew indicators – Special
application of leadscrew indicator: How to
pick-up for short metric threads being cut from
an English leadscrew – Leadscrew indicator
for metric leadscrews – Pick-up when gearing
is approximate – Repeat pick-up from chalkmarks – Repeat pick-up: electrical indication –
Pick-up with dog-clutch control – Advantages
and theory – A little-known method for
obtaining even faster screwcutting with dogclutch control – Calculating metric pick-up.
SECTION 6
MULTIPLE-START THREADS
Automatic start indexing (by dog-clutch
control) – Designation of multiple start threads
– Feasibility test (to ascertain whether or not
any given leadscrew is suitable for indexing a
required number of starts automatically) –
Pick-up for automatic multiple start indexing –
Metric multiple start threads from an English
leadscrew – Automatic start indexing
possibilities by dog-clutch control explained –
Automatic start indexing of metric multiplestart threads from metric leadscrews – Special
leadscrews for auto-start indexing – Special
leadscrew design formula: For metric working,
for English working – Example of lathe with
dog-clutch control to the leadscrew drive.
SECTION 7
SINGLE POINT LATHE THREADING TOOLS
Various single point threading tools discussed
– High speed steel – Stellite – Blackalloy –
Tungsten Carbide – Carbon steel – Cutting
angles – Internal threading tools – Inserted bit
tools – Internal thread tool bit fitted by brazing,
a tested design – Thread tool sharpening and
grinding – A simple jig for the production of
accurate angles and a new design retractable,adjustable height, swing-clear, general
purpose and threading toolholder – The
thread tool grinding jig described –
Sharpening external and internal threading
tools of various types: solid-with-shank and
inserted bit.
SECTION 8
PRACTICAL ASPECTS OF LATHE
SCREWCUTTING
Four ways of depthing a screw thread – Five
ways of depthing a nut thread – Square
thread cutting – Acme thread cutting – Nut
threads: notes on bore sizing – The
percentage approach to bore sizing for nuts –
Tap finishing – Special tap making – Multiplestart threads – How cut in lathe without dogclutch control – Multiple-start nut threading –
How cut in lathe without dog-clutch control –
Multiple start thread indexing by use of
leadscrew indicator – Multiple-start worm
thread notes – General observations: Thread
crest radii – Taper threads – Improvised
cutting method – Screwcutting speeds –
Lubrication – Effect of coolant on light cuts –
Screwcutting troubles, possible causes.
SECTION 9
PRACTICAL THREAD SIZING
MEASUREMENT
Definitions of screw thread terms – The 3-
Wire method of thread checking – Wire
diameters and thread depth – Three wire
formula for 55 deg. screws – Checking Whit,thread by metric measure – One-wire
checking – Pitch diameter calculations –
Acme thread checking – Summary of formula
for 1-wire checking – Helix angle of screw
threads – Formula – Gauging nut threads –
Thread classes – Threads designated by class.
APPENDIX
LIST OF TABLES
Appendix
1
LIST OF TABLES
General conversion formulas
Quick reference thread information summary
Table T1 – Gearing for threads/inch from a
leadscrew of 8 T.P.I.
Table T2 – Gearing for threads/inch from a
leadscrew of 10 T.P.I.
Table T3 – List of approximations for Pi
Table T3A – Gearing for worm threads sized by
diametral pitch
Table T3B – A selection of close ratios factorised into
two or three element ratios
Table T4 – Gearing an 8 T.P.I. leadscrew for metric
thread pitches
Table T5 – List of approximations for 50/127
Table T6 – Metric pitches from a leadscrew of 8 T.P.I.
gears 20-20-75 by fives (plus one 38)
Table T7 – Approximations for Pi x 50/127
Table T8 – Gearing for threads/inch from a
leadscrew of 3mm pitch, gears 20-20-75
(plus one 38)
Table T9 – In-line metric/imperial conversion forleadscrews of from 3.0 to 7.0 mm pitch or
lead
Table T10 – Basic gear ratios for metric pitches from a
leadscrew of 4 threads/inch
Table T11 – Basic gear ratios for threads/inch from a
leadscrew of 6.0 mm pitch
Table T12 – Secant factors for oblique top-slide
depthing
Table T13 – Maximum root widths, Acme threads

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