Bar bending when threading

[Stephen FollowsParticipant @stephenfollows82099]

What causes bar to bend when threading using a die?

I’ve just threaded two x three inch long pieces of EN8 steel to make a toolmaker’s clamp. First one perfect, second one threaded but bent in two places. Same bar, same size, same die.

Any ideas?

[JasonBModerator @jasonb]

Is the bar actually bending of the die wandering which can make things look bent? Hand held or using a tailstock die holder?

[Stephen FollowsParticipant @stephenfollows82099]

Hand held die, bar definitely bent. Unusable result.

[bernard towersParticipant @bernardtowers37738]

As Jason says its most likely Die wander.

[Howard LewisParticipant @howardlewis46836]

Wherever possible, particularly with small diametr stock, I use a Tailstock sliding Die or Tap holder.

The small amount of [play between the arbor and the holders allow the Tap ot Die to self align, so that the Tap or Die follow the work, to avoid drunken threads, from trying to align it.

Sliding Tap holder?  In its simplest form, a drill chuck with adequate sliding clearnce on the arbor.  A more sophisticated version has a body to take ER collets.

In both cases the grip is sufficiently weak that when the going gets tough, the tap slips in the chuck or collet, avoiding breakage, so that the hole can be finished with a Tap Wrench, by hand to “feel” what is going on.

The lathe should already have provided enough alignment to kep things on the straight and narrow.

In many cases, a centre in the Tailstock will support and guide the Tap, and reduce the risk mof bending, or a broken Tap in a scrap workpoece.

For Taps with pointed ends, a recessed centre (Centre drilling) in a bit of bar,  (Ideally a MT arbor -The one carrying the holder?) can provide the supprt and guidance needed.

Howard

[JasonBModerator @jasonb]

have you got a photo? I’d still say it is wander particularly as you say it goes one way and then the other.

 

If the die does not start true then it wanders to one side until the resistance from the side cutting the most gets too high and that starts to pull the die back the other way but too much and you end up snaking your way along the bar. Mor elikely with handheld.

[HollowpointParticipant @hollowpoint]

What type of die are you using? If its a split die make sure it isn’t being squeezed and open it up a little in the holder so it takes a smaller cut.

[MacolmParticipant @macolm]

I have found it possible to reduce die wander by keeping it axially aligned using a suitable tube fitted to the tailstock. Use the tailstock hand wheel to apply just enough light pressure to the die or the die holder to keep it running square, following up as the threading progresses. The end of the tube, of course, must itself be accurately true. In most cases the tailstock itself with the taper clear, works fine.

This usually works, but there is no guarantee it will fix a drunken thread due to a blunt or poorly made die.

[HopperParticipant @hopper]

Turn the diameter to be threaded down to five thou under the nominal diameter before attempting to thread it. This provides much needed clearance at the thread tips and makes life much easier.

And does not weaken the thread significantly. You can go down to 65 per cent thread depth of engagement and still have 95 per cent of thread strength, according to Tubal Cain’s ME workshop manual.

Then thread it in the lathe with tailstock guidance for the die as others have described above. Be sure to use a back and forth motion when threading, half a turn forward, quarter of a turn backwards etc to break up the chips and stop the die getting clogged.

And look closely at your die and make sure you are leading with the side of the die that has the lead-in taper ground on the first couple of teeth.

[John HaineParticipant @johnhaine32865]

Use some good quality stainless threaded rod.  Life is too short to make long screws with a die.

[Nigel Graham 2Participant @nigelgraham2]

I’m a bit puzzled by the bar bending. I can understand the wandering die (had it happen!). I wonder if in order to try to correct that you were pushing the die-holder over too hard and that was distorting the bar itself.

As others say, use the lathe as a die-guide, but John’s suggestion of using good stainless-steel studding is perhaps the best method.

Turn the knob as a separate item, tap the thread through it almost but not quite full height, secure the thread with locking compound. Cross-drill the assembly for the tommy-bar, or if not using one, a pin; for added security.

Stainless-steel studding is not only of higher tensile strength than mild-steel, perhaps comparable to EN8 strength, but usually has a much better surface finish.

[Stephen FollowsParticipant @stephenfollows82099]

Thankyou for your replies. I had thought about single point threading on the lathe but such a thin bar would need tail stock support leaving tool access difficult. maybe the threaded rod idea is a runner next time.

 

[bernard towersParticipant @bernardtowers37738]

Not a tailstock support but a travelling steady! If  you look up Small dia turning these supports can be handy for this type of work.

[Nigel Graham 2Participant @nigelgraham2]

I carried out a fair amount of such work recently, and used both a tailstock centre and a travelling-steady together.

For both diameter reduction and screw-cutting (single-point then a chaser), I solved the access problem by setting the top-slide round by about 30º and feeding the tool in with the cross-slide; using a half-centre to reach the end of the bar.

[HopperParticipant @hopper]

What diameter is this errant bar?

A “normal” sized toolmaker’s clamp will have 5/16″ diameter screws of about the length of 3 inches you mention. They are easily screwcut with just a tailstock centre. Anything smaller than that diameter definitely should not need screwcutting before using a die. A die alone used correctly will do the job.

Or are you making tiny clamps with smaller screws than that?

Last time I made toolmakers clamps was in 1974 as a first year apprentice. Still have them hanging on the workshop wall. So my memory may be a bit cloudy but I am pretty sure we turned the whole screw and head out of 1/2″ bar then screwcut the 5/16″ threaded section with a tailstock centre.  (With the diameter turned to 5 thou under the nominal size.) No need for moving steady if you take light cuts and use a sharp tool bit.

Another possible cause of your woes could be if you are trying to cut a thread on a piece of stock mild steel bar of the same diameter as the thread. Raw bar can be out of round, oversized, work hardened in patches, bent and all sorts of dodgy things. Far better to start with larger diameter bar and turn it down to size for threading.

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