Chasing threads not on the gearbox

stannp368 · Nov 27, 2009

A shortcut to chasing the tough ones not on the QC gearbox especially the 9" SB lathe. EX-required to chase 38 TPI. Try the formula--
req pitch for 38 tpi/trial pitch for 36 tpi=.0263/.0278=.946
multiply this by N teeth on stud/N teeth on screw which is 20/56
.946x20/56=.3379 which is close to 20/60 (our new ratio)
NOW-set the QC gearbox to 36 tpi & place a 60 tooth gear as the screw gear
this will give you 38 tpi with just a few thou error per inch
This has worked in the past & is very useful when chasing thds which are not shown on the QC gearbox

Forrest Addy · Nov 28, 2009

A couple of thou error s as you say OK but: provided you reverse back for successive cuts. Or unless you pick up the threads for every pass. Otherwise of you open the half-nut for each pass the error will accumulate until you wind up splitting the lead.

You're right, threads don't have to be perfect for short engagements but the more accurate the lead match the better and more durable the thread pair.

stannp368 · Nov 28, 2009

Great idea, I never thought of the conflicting settings

oldbrock · Nov 28, 2009

for odd and metric pitches I have often calculated the pitch and see if there is a feed that comes close. Tighten the feed clutch and pitch in the threading tool and thread with the feed. of course you can't disengage the feed but if it's a one off and a short thread you can fudge it. You can make a gear to change the ratios of the QCGB, takes a little calculating but works for me. Peter

Clive603 · Nov 28, 2009

An adequate rule of thumb for assessing the acceptable maximum cumulative error over the length of the thread for ordinary, non critical, fastener duties is up to 1/10 of the thread pitch providing the result is less than 1/5 th pitch per inch. Obviously you avoid this sort of thing for critical jobs, high tensile fasteners and the like.

It helps if you have the mating component and can adjust the depth of cut to be the minimum which allows that fastener to screw on. This is a good way to find out just how much 'economy range" fasteners vary!

For ruff jobs taking a simple 1 tpi error is frequently acceptable.

Clive

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Chasing threads not on the gearbox

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