The Shop > Tools
What was the thinking behind BA threads
Will_D:
--- Quote from: Muzzerboy on August 10, 2019, 12:53:30 PM ---but most BA pitches are impossible to make on a lathe without a bucket of changewheels,
--- End quote ---
But why would you want to screw cut a 6 ba thread? You use dies!
Dies when adjusted correctly cut a perfectly formed thread (including the roundy tops and bottoms) by a process of shear and plastic deformation.
Screw cutting on a lathe really comes into it own when you haven't the physical grip on the work piece or the die to resist the torque.
Just look at a pipe fitter cutting 2" x 11 tpi pipe threads - it certainly leaves its mark on the pipe from the pipe vice
Or when you want to cut a unique thread like 40 mm by 1.5 or 1" by 50!!
Simple screw cutting involves pushing a pointed tool (with no accurate tip radius) into the work, the crests of the thread are left raggy and are usually rounded of with afile!
djc:
--- Quote from: Will_D on August 10, 2019, 03:55:19 PM ---But why would you want to screw cut a 6 ba thread? You use dies!
--- End quote ---
OK, I will bite...
10 How was the 6BA die made?
20 With a 6BA tap.
30 How was the 6BA tap made?
40 With a 6BA die.
50 Goto 10
Screwcutting for BA could be made very logical if you approach it with the right mindset. Start with a 1mm pitch leadscrew. Gives you 0BA. Gear it 9/10 for 1BA. Gear it 9/10 x 9/10 for 2BA. You only need two sizes of change gear (and a very big banjo!).
timby:
--- Quote from: djc on August 11, 2019, 05:14:42 AM ---
--- Quote from: Will_D on August 10, 2019, 03:55:19 PM ---But why would you want to screw cut a 6 ba thread? You use dies!
--- End quote ---
OK, I will bite...
10 How was the 6BA die made?
20 With a 6BA tap.
30 How was the 6BA tap made?
40 With a 6BA die.
50 Goto 10
--- End quote ---
Not if you have one of these,
djc:
--- Quote from: timby on August 11, 2019, 06:44:56 AM ---Not if you have one of these
--- End quote ---
Looked a bit big for 6BA :-)
And what, pray tell, do you think was moving the workhead when it was grinding the threads?
A ballscrew, which is a fancy leadscrew, with electric cogs for pitch. So it was screwcut.
You need to find an example that is not screwcutting. Maybe that lost episode of the A Team where they were locked in a barn and they filed a 6BA thread on a pitchfork handle.
DavidA:
One could say, tongue in cheek, that it was a far - sighted plot to make money out of model makers.
'I say, that man is using Metric screws on his British loco models. Fella should be horse whipped'.
Dave.
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