Threading 4mm silver steel using the lathe tailstock

[david gregg 1Participant @davidgregg1]

Can members any advice on using a die on the lathe tail stock to cut 4mm silver steel bar I have threaded hundreds of these but the thread is not as good as I would like it to be I have tried Dormer ,Osborn ,The tap and die company and Presto dies ,the Presto dies probably gave the best results.I have tried a range of speeds ,from 180 ,to 570 RPM ,I have also tried various tapping compounds the 4mm thread is to be 9mm long and I do these in various batch numbers of 100 to 300 to any advice how to improve the quality of the thread would be appreciated

[VicParticipant @vic]

I’d be interested to see what folks say as I sometimes get slightly torn threads when using a die on the lathe. I always use the slowest speed on my lathe.

[HuubParticipant @huub]

I assume you are threading M4. For a normal M4 thread, tolerance g6 (hardware store threads) the outer diameter of the bar should be 3.92 mm. If you use a 4 mm bar, the die will thread the rod but get blunt pretty fast. The high torque will “torque”  the bar and the thread will not be of high accuracy.

Silver steel is, as far as i know, available in 2 steel grades (1.1541,1.2210) with different hardening properties and probably will behave different when threaded. Changing the silver steel grade may be an option.

4 mm at 560 RPM still is only 7 m/min, pretty slow for steel. I do most threading on the (CNC) lathe using a single point threading tool at 300 RPM max. Under these conditions, most of the steel I am using don’t give a nice and shiny thread. A sharp HSS tool give the best results but most of the time I use an ER11A60 (half profile) insert for steel.

 

 

[HopperParticipant @hopper]

Silver steel seems to be a bit nasty to machine at times, especially cutting threads with dies. It seems to benefit from the traditional hand-turned back and forth motion to break off and clear the chips as it goes. Which may not be practical when doing hundreds of threads as you are of course. Is it possible to change material to something more easy machining?

The problem maybe comes at least in part from the limited chip clearance space in small dies like that. A Coventry die head might work better as it has way more clearance between the cutting inserts for the chips to fall out through.

Also, as Huub said, dies do not like cutting threads on full sized material. It works best if the stock is a few thou undersized so the die is not cutting all the way up to the thread crests. Presumeably this helps with chip clearance. But not practical in a production situation I guess.

Another possibility might be to use one insert out of a Coventry die head held in the toolpost as a screwcutting tool bit and set the gearbox to the desired pitch and screwcut the thread that way with the multiple teeth on the die head insert allowing you to do it in one pass, or maybe two. With thin material like 6mm though you may need to rig some kind of small travelling steady attached to the toolpost too, something like a capstan lathe roller box.

Another might be have a go at sharpening up your dies by running a Dremel tool down the leading cutting face, ie inside the four holes drilled around the periphery to provide the “teeth”. You could experiment with providing more or even less rake angle and see if that helps.

 

[HopperParticipant @hopper]

Or have you considered a rolled thread? Motorcycle spoke thread rolling machines are inexpensive to buy (hand cranked) but I think may only go up to about 3.2mm. But if you google them you might  be able to make your own version with three silver steel hardened rollers held in a suitable body in the tailstock. Their pivot pins would need to be located pretty carefully to get the correct thread depth, I would think.

Motorcycle spokes are some tough high tensile material but the rollers work well on them. The rolling may increase the OD though, not sure if that is always the case.

[DiogenesParticipant @diogenes]

Is changing the material spec. an option..?

[JasonBModerator @jasonb]

Can you change to M4 fine which will need less effort to cut. Although Silevr Steel can be a bit of a problem I get on OK with fine threads and the diameter is not reduced, this is M8 x 0.75 cut at about 100rpm with CT90

[HopperParticipant @hopper]

One more stray thought: Are you using a tailstock die holder that slides separate from the arbor that fits into the tailstock, so you are not trying to drag the whole weight of the tailstock along with that tiny 4mm thread being cut?

If you are just using the cheap fixed die holder that holds the die solid to the tailstock, the unlocked tailstock can kick over under torque and jam up and stop those little threads from pulling that big lump of cast iron along the bed, which causes poor thread cutting.

[bernard towersParticipant @bernardtowers37738]

Silver steel nasty stuff there are materials out there that are more machinable and just as strong and if you are doing that many it would better to buy full lengths. How about EN8M?

[Charles LamontParticipant @charleslamont71117]

If you are being paid to make them, then a Coventry type die head might be a worthwile investment.

[JohnFParticipant @johnf59703]

Making batches of that size I would also go for a Coventry Die head, remember they do a roughing cut and finishing cut, both are preset.  Silver steel is possibly to worst material to thread, I use rocol cutting compound and sometime mix a Little cutting oil with it.

You can probably pick up a used die head, either 1/4″ or 5/16″ size would suffice and yo may find these less expensive than the 1/2″ size ??

John

[Neil LickfoldParticipant @neillickfold44316]

Rocol or similar sulphurised  cutting oil is a big help.

One option is to cut them twice, use an expandable die for the first pass, and then another die in another holder that changes onto the tailstock mandrel.

Just check that you have the die the correct way. Sometimes it is not obvious for which side is at the start of the thread.

Neil

[Roger WoollettParticipant @rogerwoollett53105]

I think silver steel is supplied half hard. It might be worth annealing the end you want to thread.

As others have said – reduce the diameter a little – I would go for 3.9mm.

Use a cutting oil – I find Trefolex helpful.

Open the die as far as it will go for a first pass.

 

 

[Howard LewisParticipant @howardlewis46836]

Definitely, hold the Die in a holder that slides along an arbor in the Tailstock.

Asking a M4 thread to drag a Tailstock along the lathe bed is asking for stripped or damaged threads.

Definitely lubricate before starting the cut, Trefolex or ROCOL RTD.

And put a chamfer on the end of the work, to help the Die centre itself, and start to cut.

Since this is Silver steel, it would be worth taking the cut in two stages. Open up the Die, as far as possible, without cracking it, and take the first cut.

Then for the finishing cut, reset the die, using an M4 fastener as a gauge.

This will reduce the cutting loads, and the risk of problems.

Ideally, with hard material, rotate the chuck by hand (Mandrel handle, or using the chuck key; minimal risk of danage to chuck key, since this is a small thread)

HTH

Howard

[noel shelleyParticipant @noelshelley55608]

For numbers like that a Coventry Die Head is what you need and good lube. Either flood or Rocol Etc. Noel.

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