Wonky threads using die

Hi all

Stupid question probably,.,. I have been trying to put threads on 3/16” rods using a 3/16” BSW die (this is to try to fix the half nut engagement mechanism on my lathe).

But they often come out wonky - see pic.

Does anyone have any tips for making sure they are straight?

Thanks

Steve

Cut the threads while the piece is still in the lathe chuck.....put the die holder against the end of the tailstock barrel and turn the chuck by hand while gently advancing the tailstock crank......

..and put a generous chamfer on the end of the stock to help start the thread..

I use a die-holder in the tailstock, something like this.

Thor

Fully agree with doing it in the lathe if you can. Alternatively, I have done it with the rod held in a bench drill chuck and the dieholder flat on the table to keep things true, turning by hand of course.

To my eyes the threaded section of the rod pictured seems to be actually bent, which is easy to do when hand threading something that small without a guide.

Mike.

I also use a tailstock die holder, like this.

Could you screwcut and then finish with the die? (I realise that the half nut problem may prevent this).

Or better a length turned down to the minor diameter of the thread.

Use a split dies and start with it fully open.

Don’t. Buy in a bolt and cut off the head.

Does the half-nut engagement work at all?  If so cut a rough thread with a single pont tool before going over wit the die.

Edited By not done it yet on 20/11/2021 14:12:01

What's the rod made of? I've had similar problems with DIY store metal - it doesn't machine well.

The rod may be on the soft side, deforming and twisting rather than cutting. Possibly a hard outer shell on a soft core, or it work-hardens like some stainless steels. A sharp new die with plenty of thread cutting lubricant like CT90 would help.

Dave

Just had to redo some studs for similar reason, was making 22x M3 so trying to rush it and made a mess of half of them. Hold material in a collet if small, chamfer end of material, apply lubrication cutting oil, ensure split die is fully open, push die holder against end of part squarely, rotate to cut thread manually back off every couple of turns to free die of swarf string. The swarf can cause the die to move off centre if pushed too hard.

In order to cut threads straight on bars, I made a die holder with interchangleable guide bushes, along the lines of a GB pipe threader. The bush keeps the thread straight, Works well.

When using a die making the first thread or so produces seriously unbalanced forces because there is nothing supporting the inside of the die opposite the cutting edges. This force imbalance tends to tip the die off line during the formation of the first thread so a drunken thread results as the die stabilises following the line of the first thread.

The coarser the thread the more pronounced the effect. Coarsness being defined by helix angle not absolute pitch or TPI. High TPI or small pitch on a small diameter can be coarser than a much lower TPI or larger pitch on a larger diameter.

3/16 BSW is very coarse. Small diameter doesn't help because the slender shaft and even slenderer core diameter isn't terribly stiff.

Hence the need for firm die support. Either a die holder system with a decent size, nicely stiff, rod up the middle or something cruder like a flat ended pusher in the tailstock chuck. Pusher has to be just small enough to push directly on the die when its held in the diestock. Need to figure out something to stop the diestock turning yet still permit it to slide as one hand is occupied turning the chuck whilst the other operates the tailstock feed. As methods go that is maybe a bit on the crude and primitive size but I did many threads that way before being able to afford better equipment.

Objectively the best answer is a Coventry die head but its hard for the normally impecunious Model Engineer or Home Shop Worker to justify the cost.

D'oH moment. Make sure the die is in the holder the right way round. Put it in backwards and the clearances will be wrong way round so it won't want to cut. Which really doesn't help with going straight.

Clive

Edited By Clive Foster on 20/11/2021 18:04:11

Hi

thanks for the replies.

I like the idea of the lathe tailstock die holder. My problem would be holding the rod.

I only have a 4 jaw chuck, dialling in a 3/16 rod may not be easy.

I could hold it in a collet but my headstock only has a 1mt spindle, I have a 2mt collet Chuck (ER25) but obviously that won’t fit in the spindle. The lathe actually has a 2mt tailstock.

When threading using a die in the tailstock, does one use the correct gear setting for 3/16 BSW? Or does one just let the die do the work? Powered or unpowered? Etc

cheers

Collet Chuck in the Tailstock and the die in the 4 Jaw maybe?

regards Martin

It is crucial that the die is started true, as the handles are usually used to check the die is square to the rod then it is important that the die stock is well made and the seating for the die is true with the handles and the die is seated properly in the holder. Once the die has started then check it is running true in at least two positions as you turn it. Once started correctly take care not to tilt the die holder as it will wander if uneven pressure is applied. I would review the equipment you are using and take great care to start the die true. Up to a point you can correct the start but if you are a couple of threads in and not true then start again.

Mike

Steve

When threading with a die in a tailstock die holder its probably easiest to just turn the chuck by hand with the drive in neutral. Use the tailstock handwheel to apply feed pressure. Once the die is cutting it will feed by itself. Best to back off the tailstock once the die is self feeding.

Spin the chuck backwards to run the die off the completed thread. If you hadn't backed off the tailstock feed earlier once the die began to self feed you will need to now to make room for it to come off.

I'd do it under power running at low speed but I've had plenty of practice. Turning by hand is less committed.

Do lubricate the material to be threaded, preferably with a proper threading lubricant, but grease of heavy oil will work adequately. Much better finish.

As I said in my previous posts a simple pusher in the tailstock chuck pressing on the die to keep it straight works well enough. More refined ways are just faster and need less setting up. No realistic difference in job quality.

Clive

Just a thought because simple mistakes happen, but are you certain it is 3/16" and not 5mm rod? 5mm is a too tight tight fit in the die, but this can be fixed by taking 0.2mm off the diameter with the lathe. Taking a little more off reduces the amount of work the die has to do without significantly reducing the thread's strength. If it is 3/16" same trick applies - reduce the diameter by taking a few thou off.

Also helps to pre-cut a groove of about the minor diameter ( 0.1342" ) where the thread ends on the rod. If the metal is squeezing as the die turns, the gap provides space for the overflow. It also lets the die cut the full depth without wedging up at the end. Another trick is to reverse the die to do the finishing cut because the chamfered input end can't reach all the way up to a face.

Dave

I am considerably better at it than I was when I started, but I still find it a pain in the neck TBH. The 3/16 rods I need are only about 1 inch long. I can imagine the dial indicator face facing the chuck to get the probe in the part etc….