Thread cutting problem

I am trying to replicate a small part from an antique mechanism, circa 1900. The part is made from 10mm x 2mm mild steel bar and one end is narrowed to 5mm for about 1cm length. This section, now 5mm x 2mm, has a 5mm thread formed on its narrow edges and I am having great difficulty replicating this section. I am trying to "screw-cut" in the normal way in a Myford, using a 60deg HSS tool and very slow cutting speed, and a depth of cut of 0.0025 inches at a time. I get so far but then the steel just twists and bends. The cutter is sharp. Any ideas on improving the method, or is there a completely different process for doing such a job? I have spoiled 4 pieces so far.

John

Try soft soldering a bit more of the 10x2 on each sidesay 25mm long, turn down10mm to 5mm dia, thread and then melt off the bits you don't need. Very small ctr drilled hole and tailstock support will also help.

J

Edited By JasonB on 01/04/2014 17:59:55

Maybe it is the shock of the interrupted cut which causes the steel to twist. Barmy suggestion follows.

Why not make the part from round bar, to maximise strength and maintain a continuous cutting load, and cut the thread and then mill the workpiece back to 10 x 2mm (5 x 2mm for the section concerned).

This method may seem wasteful of material, but might save filling the scrap box, or a broken tool.

Hope that it works out O.K.

Howard

I assume from you description, you are plunge cutting. Why don't you set the topslide over to 60 degrees and come in so you only cut on one side of the tool

Loctite it between two 10mm wide bars and hold in 4-jaw chuck. Turn a 5mm spigot on one end, then thread it. Now use heat to get the Loctite to let go.

Neil

Thanks for all the advice so far, all much appreciated. For old Al, yes I am already using the 60 degree topslide method. For Howard, I did think about turning down 10mm round, but shied away at the difficulty of then milling it down to 2mm (how do you hold the last bit!. So, I will be trying either the solder or the Loctite method which seem the easiest to try. It will be the end of the week before I am next in the workshop. I will try to remember to report back.

John

John, A method I have used to make very slender pieces is in this case use a piece of 10mm bar round or 10mm x 5mm say 4 or 5 inches long, machine your thread then machine away the surplus material while still attached to the original material then cut off the component.

The component shown is only .040" x 0.150" section on the thin leg you can see, it was made from EN45 then heat treated after cutting off with a slitting saw. 

John.

Edited By JohnF on 01/04/2014 22:45:57

Edited By JohnF on 01/04/2014 22:50:16

Hi John,
As the section of thread is so thin it may not matter if the groove was at right angles to the axis of the bar rather than part of a helix. So if you clamped the end of the bar between toolmakers clamps leaving the thread depth sticking out on one side and facing the front of the lathe you could rock the chuck backwards and forwards and plane each thread. To index along you could disengage the first quadrant gear from the spindle and arrange the gearing so that the first gear rotated revolution for each thread. Put a mark on the gear and line it up with some reference point. Rock the spindle to cut one groove of the thread to its full depth. Rotate this gear one revolution and cut the next groove. repeat until that side is complete. Now move the clamp to expose the uncut side and rotate the chuck 180 degrees. Put a mark on the gear at 180 degrees from the original mark making sure it can be distinguished from the first one. rotate the gear to take the tool back to the start on the uncut edge and repeat the procedure but using the new mark. There are many variations to this idea. For instance if you have a DRO you could just move the tool a measured amount on the DRO.

Les.

John,

It is quite possible that the original part was made from Cast Steel [ Crucible Steel ]

If so, it may be that your Mild Steel simply isn't good enough to make the piece.

It may be wise to use Gauge Plate instead, or maybe even [Shock/Horror] cut a strip from something like an old Plane Iron.

[Sorry, I forgot to ask the length of the piece]

MichaelG.

.

P.S. ... Alternative "method" might be to use a milling spindle [or a Dremel tool?] in the toolpost.

hi les,

what is the part for ?

it sounds an interesting project.

i should imagine that circa 1900 a far more rudimentary proceedure was adopted, knowing how microscope parts were threaded with hand chasers on piece work etc.

cheers,

julian

.

John,

I've been thinking again [heck, that's two days running] ...

The diagonal of a 5mm x 2mm section is 5.385mm, which is somewhat oversize for a 5mm thread.

MichaelG.

.

Edit: ... and I note that Michael Williams turned his test-piece ... which would make its diagonal 5mm.

Edited By Michael Gilligan on 02/04/2014 18:14:59

By the way John , topslide setover angle is 1/2 the thread angle ie 30 degrees in this case.