Screwcutting

[Allen Smithee]

So this afternoon I got away from work early, and had nothing on SWMBO's "little job list". I'd also received the final bits I needed in the post, so I thought I'd quickly knock off a job I've been failing to get around to for a while - knocking up some drawbars so I can use MT2 collets in my Myford's spindle and tailstock. Simple enough, although slightly complicated by the fact that I'd decided to fit them with "machine wheel" handles (the cheap 80mm chromed ones in ArcEuro's machine spares section) and a brass "thumping button" to tap with a hammer to release the collets.

The design is simple enough - some 10mm steel bar with a 3/8Whit thread on one end and an M10 thread on the other. I started by making a nylon bush that I could slip over the bar and into the spindle bore so that I could run a ~300mm length of bar through a collet at 2,000rpm without the free end whirling around. I then skimmed the end of the bar down to 3/8", turned a 4mm wide rebate where I wanted the end of the thread, dug out a 55deg tool and set up to screwcut the 16tpi thread at my lowest possible speed of 25rpm (because it's over 25 years since I last did this, and I'm chicken. Other than stupidly trying to do this with a carbide insert tool (which hung on gamely for the first couple of passes, but then cracked so I switched to a toolsteel one) the exercise went well enough with some tapping fluid, and the resulting thread just needed a light clean-up with the only 3/8W die I have - a "hexnut" die. I was on a roll. If anyone's interested I used the "plunge in square" technique rather than the "set the compound over 22.5 degrees and cut one side only" one, and it worked fine.

So I then swapped the bar around to cut the M10 thread on the other end, and that's when the fun started. I can't screwcut metric threads without fitting the metric conversion set, which takes longer than I wanted to spend on it. So I skimmed the bar down to 9.8mm just to ensure it was round, turned a 10 degree taper on the first 6mm or so to allow an easy start and placed my M10 die in a die holder on the end of the work, pushed square with the tailstock. Using a manual handle on the spindle, for the first couple of turns (on the taper) it seemed to cut well enough. But then no matter how much I tightened the collet it would only cut about an eighth of a turn before grabbing and twisting in the collet. After half an hour I had about six turns of thread and it was getting very tedious so I took the job out of the lathe and grabbed it in the vice (grooved brass soft jaws). This was a little better, but again I could only cut about an eighth of a turn before the bar rotated in the vice. After another 30-40 mins I had about 20mm of thread, but I need about 40mm and it's getting seriously tedious! I've considered cross-drilling the bar and putting a small bolt or pin through it to stop it rotating, but I'd really rather not.

Two thoughts occur to me - one is that my die isn't a spilt die, so I'm trying to cut what is a fairly deep thread in one go. Another is that possibly I need to anneal the bar a bit. But when I cut it to length it hacksawed and filed easily enough, so that would surprise me.

This is such a simple, mickey-mouse job. I don't remember it being this hard! What am I doing wrong?   

AS

[philjoe5]

Allen,
I just went through the same exercise cutting 1/4" - 20 threads on a piece of 1144 steel held in an ER-32 collet chuck on my lathe.  I used a tailstock dieholder to start the threads but after a few turns the work turned in the collet.  I had turned the work down to 0.245" and thought I had allowed enough, but eventually I found that by turning the work to 0.240" I was rewarded with some nice threads which are not at all sloppy in the tapped piece they screw into.  So I had a 4% reduction in diameter compared to your 2%.  I'd turn the work to 9.6 mm

Cheers,
Phil

[mklotz]

Genau!  Phil's nailed it.  I went through the same exercise in the past.  After measuring a bunch of commercial bolts (no doubt roll-formed so on the large side), I deduced that a -5% or so allotment when die-cutting threads is perfectly acceptable.

[kvom]

Single-point is your friend, even if you only cut partial depth.

[Art K]

Allen,
I'm with Tim, Just don't do something stupid like putting a piece of fuel hose on your crankshaft throw to "protect" it from the lathe chuck jaw's
Art

[Jo]

So I then swapped the bar around to cut the M10 thread on the other end, and that's when the fun started. I can't screwcut metric threads without fitting the metric conversion set, which takes longer than I wanted to spend on it.

Yes it does put you off having to assemble the set. But once assembled it is easiest to leave it assembled, with a note on it for what thread it is set up for and sitting on its box ready for next time.

Yes I too should remember to turn down the spindle speed when I cut threads but its so easy to forget

Jo

[tangler]

AS,

I find die cutting any thread above 1/4" / 6mm in steel a pain so I do tend to single point cut most threads and just finish with a die.  Which is where this table comes in handy:



Just slip the 24 tooth gear below the tumbler gears off and replace with either a 33 or 34 (available from Myford).  Pitch accuracy is good enough for almost all purposes.

HTH

Rod

[jadge]

It shouldn't be a problem cutting an M10 thread on a 10mm diameter bar with a die. If the die is not split are you sure it actually a die, and not intended for just cleaning up existing threads? It would also be interesting to know the provenance of the die.

Andrew

[Don1966]

AS,

I find die cutting any thread above 1/4" / 6mm in steel a pain so I do tend to single point cut most threads and just finish with a die.  Which is where this table comes in handy:



Just slip the 24 tooth gear below the tumbler gears off and replace with either a 33 or 34 (available from Myford).  Pitch accuracy is good enough for almost all purposes.

HTH

Rod

Rod there is an error on the M fine threads the second calculation with 12 using the 33 stud gear and 20 box gear. It should be 1.746 value.
For any one interested I have attached a Myford gear box calculator. It will calculate TPI and Metric threads along with gear box ratios up to three gears sets along with the Norton QC gear box.

[Jasonb]

I'm with Andrew the dienut is the most likely cause of the problem.

You could lock a couple of 3/8" whit nuts on the other end and hold by those which should help stop the bar rotating

[tangler]

Rod there is an error on the M fine threads the second calculation with 12 using the 33 stud gear and 20 box gear. It should be 1.746 value.
For any one interested I have attached a Myford gear box calculator. It will calculate TPI and Metric threads along with gear box ratios up to three gears sets along with the Norton QC gear box.

Quite right Don, there is a typo in the line - it should be 33, 28 like all the other 1.25 pitch threads.

many thanks,

Rod

[Allen Smithee]

Many thanks peeps. Years ago I used to reduce the diameter by ~5% to make the die run easier, but I'd assumed I was bodging it and I'd sort of decided to stop bodging, so this time I didn't. It's reassuring to learn that people I respect don't consider it a bodge!

The die (not a die-nut - that was the whit one) is one from a cheap set I bought somewhere over the years, so it could be that.

I'd heard about the "using a 33/34 tooth gear to cut metric threads" but never found the details, so many thanks for that info Rod. I think my first action on Tuesday morning will be to place another £20 order with RDG!

I think I'll put the bar back into the collet, get it as centred as I can and skim it down a bit.

Thanks again - just the sort of thing that makes this place so good!

AS

[PStechPaul]

The 1mm thread uses 34/36 or 17/18, and if you multiply that by 25.4 (mm/inch) you get 23.989, or just 0.04% error, or less than 0.001" in 2" (or 26mm) of thread. I made a chart of metric pitch equivalents for my HF 9x20 lathe, although it's probably better to use the 100/127 tooth gear and do it properly:
 

I didn't try a 36/34 combination. It's a spreadsheet so maybe I'll try that.
 
Here it is:

[jadge]

The die (not a die-nut - that was the whit one) is one from a cheap set I bought somewhere over the years, so it could be that.

Very possibly; many years ago I bought a cheap metric tap and die set. It was absolutely hopeless so it went in the bin. More recently I had a need to cut 5/8" UNF threads using cheap taps and dies bought 15 years ago from the well known ME supplier in the UK. Again, absolutely hopeless. Two parts made with the taps and die were such a tight fit that they had the gall to gall. No way I could get them apart so had to scrap the parts. I ended up buying a proper HSS tap from a commercial supplier and screwcut the external threads. Here is what happens to cheap tooling, it gets Widlarised before being binned:



There are two rules for buying cutting tools:

1) Never buy cheap cutting tools - they're the source of much grief

2) See Rule 1

As for reducing the diameter before threading I've never had a problem not doing so. On the other hand, for internal threads, I select a tapping drill to give 60-70% thread engagement.

Andrew

[Allen Smithee]

Again, thanks for all the above advice. I've just ordered the 33 and 34 tooth gears.

Having stared at the geartrain for a few minutes this morning I'm guessing I remove the outer gear on the shaft below the tumblers (taking care not to lose the woodruff key!), slacken the banjo bolt, fit the selected alternative gear and then reposition the banjo to re-mesh the new gear, retighten the banjo bolt and Bob's my ankle. Is this correct?

How critical is the meshing - do I just push it into contact while retightening the bolt or should I be feeding a riszla paper in there to set clearance or something?

Finally - am I right in believing that I can't use the thread position indicator when running the 33 or 34 tooth gears unless U do the trick about setting a saddle stop position and re-engaging the half nuts with the saddle held against the stop or similar?

AS