Boxford metric screw cutting chart

[Andrew SchofieldParticipant @andrewschofield]

Hi

I have not yet cut a thread on my metric Boxford lathe even though it has a built in gearbox at the end of the lead screw.

I believe that I do not need a thread dial indicator if I keep the nuts on the saddle engaged as I stop and return the cutting tool back to the beginning.

What I would like help with is an explanation of what are the various settings in each of the 2 lists relate to for the attached chart on the gearbox to engage the the correct levers.

 

[bernard towersParticipant @bernardtowers37738]

I am ready to be shot down in flames but here we go. The lower chart is your fine/surfacing feeds for normal machining operations with the 20 tooth gear in the g position. The top chart is screwcutting using the 50 tooth gear for the top 3 lines giving you on the top line D setting threads from 7.5 to 4mm pitch, next one down C setting gives 3.75mm to 2mm pitch and the next line B setting gives 1.875 to 1mm pitch.Then if you change the 50 tooth gear for the 20 then the next 4 lines come into play.

Dont forget that the surfacing lever is a different one to the screwcutting one .Best of luck.

[Andrew SchofieldParticipant @andrewschofield]

Thanks for answering my question.

If I wanted to cut an M12 thread with a coarse pitch of 1.75 would I be correct with the 50 tooth gear at g in setting the first lever on the left at the 2nd position from the left and setting the second lever on the right at position B

[SillyOldDufferModerator @sillyoldduffer]

The way to find out is by putting a rod of any reasonable diameter in the chuck and setting the gearbox up as you understand the instructions.  Not clear are they, so could go wrong!

First paint the rod with engineers blue or felt-tip pen.  Move the cutter so it just scratches the blue, and then run the lathe.  It will scribe a helix that you can measure to confirm the pitch.  The diameter doesn’t matter.

As an inch is a convenient distance to measure with a steel-rule, I’d convert the sum into turns per inch:  TPI = 25.4/pitch

25.4 / 1.75p = 14.5 tpi

If the gearbox is right you should count 14 and a bit turns over an inch.  If the count is different, the gearbox setting is wrong,  so think again.  Informed by how many turns a wrong setting produced may make it possible to work the trick backwards: this thread was produced by this gearbox setting, so does that line up with anything on the front panel diagram?

Repeat with a few different threads until the gearbox instructions are decoded and understood.

If scratching the rod goes against the grain, a fine felt tipped pen can be substituted for the cutter.  It paints an easily removed heliix but not as well defined as a scratch on blue.

You mentioned a thread dial indicator in the opening post, and, yes they’re not particularly useful on a metric lathe.  As you said the preferred method is to retract the cutter and reverse out without disengaging the half-nut.

By the way, if the lathe cuts in reverse without unscrewing the chuck, it’s safer to cut threads away from the headstock towards the tailstock.   Safer, because there’s no danger of crashing under power into the chuck, and fast because you can wind up the RPM.

Dave

 

[bernard towersParticipant @bernardtowers37738]

Yes absolutely right Andrew, but dont forget that you can use any pitch on any dia if you want to stray from standard ( although I dont know why you would)

[FulmenParticipant @fulmen]

Honestly, why wouldn’t you? Standards are GREAT when you need off-the-shelf parts to fit. But what we do is often bespoke parts interacting with other bespoke parts. Granted there are more arguments for adhering to standards, but never forget that they are general purpose and might not be the best fit everywhere.

For smaller threads you’re usually limited by the availability of taps, but once you can single point a whole new world of designs opens up.

[bernard towersParticipant @bernardtowers37738]

For smaller threads its easier to thread mill and this is easily done with manual lathe and toolpost spindle, especially internals

 

[SillyOldDuffer]

On SillyOldDuffer Said:

The way to find out is by putting a rod of any reasonable diameter in the chuck and setting the gearbox up as you understand the instructions.  Not clear are they, so could go wrong!

First paint the rod with engineers blue or felt-tip pen.  Move the cutter so it just scratches the blue, and then run the lathe.  It will scribe a helix that you can measure to confirm the pitch.  The diameter doesn’t matter.

As an inch is a convenient distance to measure with a steel-rule, I’d convert the sum into turns per inch:  TPI = 25.4/pitch

25.4 / 1.75p = 14.5 tpi

If the gearbox is right you should count 14 and a bit turns over an inch.  If the count is different, the gearbox setting is wrong,  so think again.  Informed by how many turns a wrong setting produced may make it possible to work the trick backwards: this thread was produced by this gearbox setting, so does that line up with anything on the front panel diagram?

Repeat with a few different threads until the gearbox instructions are decoded and understood.

If scratching the rod goes against the grain, a fine felt tipped pen can be substituted for the cutter.  It paints an easily removed heliix but not as well defined as a scratch on blue.

You mentioned a thread dial indicator in the opening post, and, yes they’re not particularly useful on a metric lathe.  As you said the preferred method is to retract the cutter and reverse out without disengaging the half-nut.

By the way, if the lathe cuts in reverse without unscrewing the chuck, it’s safer to cut threads away from the headstock towards the tailstock.   Safer, because there’s no danger of crashing under power into the chuck, and fast because you can wind up the RPM.

Dave

 

Why would you even mention inches when he’s trying to cut metric threads with a metric leadscrew?

Andrew: Using the chart is very simple. Find the pitch of thread you want to cut. Move the tumbler lever to the hole directly below that column which shows your pitch. Now move the ABCD lever to the correct letter for your pitch. Your lathe is now set up for cutting.

So in the image below for cutting a 1.5mm pitch you would move the left selector lever to the 4th hole (directly below the column containing 1.5mm) and the right lever to the hole under B.

You are correct that keeping the threading lever engaged and backing up the motor means you don’t need to use the threading dial, but you should also know that because you’re using a 3mm leadscrew you can cut 3mm, 1.5mm and 0.75mm threads, possibly even 1mm pitch threads as well, without using the dial at all just by closing the half-nuts anywhere you like.

[Author]

Posts