Helical Milling On A Small Mill Anyone?

I have been reading a lovely book about milling machines and the uses thereof (published by Browne & Sharpe in about 1911 I think) and I was particularly interested in the explanations of helical milling to produce spiral flutes in drill bits etc. Coincidentally a friend asked me if I thought I could make a whopping great tap to cut threads in wood - 2.5" x 2tpi or thereabouts. Seems like an interesting project I thought and went to look at the change gears on my Boxford. To achieve 2tpi I would need 4 turns of the leadscrew per rev of the spindle which I think is achievable but is it sensible? So I'm wondering if it would be better to try to mill it. A bit of googling has thrown up some videos of helical milling on manual machines using manufacturer or homebrewed geartrains driving dividing heads but all on hefty industrial sized mills. Has anyone tried it on a hobby sized machine like my 626? I can imagine making a geartrain to connect the x axis leadscrew to a rotary table say any advice or experience from someone who has done it would be very gladly received.

William.

Probably easier to arrange a milling spindle on the lathe and rotate the leadscrew by hand. That's how I made the helical gears for my Wyvern engine.

On the mill you will need to connect a gear on the X axis leadscrew to something like a dividing head. An alternative might be a timing belt. One way or another a bit of construction will probably be involved.

Good luck,

Rod

Long lead helical milling is simple enough using a universal mill and dividing head:

Or on a vertical mill with right angle head for short leads:

For the short lead gear the table feed couldn't drive the table via the speed increasing gear train. So the gear was cut by turning the dividing head and using that to drive the table. Some universal dividing head manufacturers sold short lead attachments. They're nothing fancy. It just means the table leadscrew drives the dividing head spindle directly rather than via the worm and worm wheel. The end of the dividing head spindle is in the middle of the picture above. For short leads one simply drives an expanding arbor in the hole via a gear train directly from the table. Some of the resulting gears:

Although my lathe (4 tpi leadscrew) will cut a 2 tpi thread I chose to use a CNC mill when I needed a 2 tpi worm:

The worms:

In the absence of smartypants equipment I'd be with Rod; use the lathe but using the leadscrew to drive the spindle. Whether one uses a single point lathe tool to cut the thread or a milling spindle would depend upon the thread form. A square thread could use either method but Acme, V or buttress might be simpler with a single point tool.

The problem with Rod's method for cutting a short lead tap is that the milling cutter needs to be larger diameter than the milling spindle so the tap can pass under the milling spindle. Again milling machine manufacturers used to advertise short lead or rack milling heads which had a small diameter to pass over the work while still being able to cut the thread, or rack.

Andrew