First time cutting a thread ?(Novice)

This is the first time I have attempted to cut a thread although I have done some simple lathe work in the past on a Super7 some 30yrs ago.
An inset ceramic sink I had ordered came with an overflow device which consists of a moulded plastic elbow, one side of which fits into a corrugated pipe which connects on to the drain, the other branch is internally threaded. The tubular matching male threaded bit is a moulded bit of plastic with a bright finish which locates in a hole on the inside of the sink but unfortunately it's too short to engage the threads in the elbow. I thought originally that the thread was 3/4BSP, the thread is certainly 14TPI, but on measuring the major dia it's 1.084" and the minor dia is 1.008" which differs from the dimensions in my source of info for 3/4BSP which states 1.041" and 0.9496" respectively. I propose to use the measured major and minor diameters.
My initial reaction was to send it back, but on reflection I thought it was a nice little project to start using my recently purchased ML7. I had some white nylon bar which I've turned to the correct dimensions, I've used a parting off tool to create a groove at the end of the potential threaded part to give me time to disengage the clasp nut. I'm using a tail stock centre and the work piece projects approx 3/4" from the chuck jaws, the threaded portion will be approx 1/2"
I've set up the correct gear train to give me 14TPI, and engaged the back gear and slowest speed and tried the to cut a shallow thread in a smaller dia scrap bit of nylon and the thread seems satisfactory.
Reading Ian Bradley's book he suggests setting over the top slide to half the Whitworth 55 deg angle i.e to 27.5deg. I find this process somewhat complicated and wondered if it was really necessary with a soft material like nylon? Could I in fact just use the cross slide and cut directly in at 90deg to the work piece, 5 thou (or less?) at a time. The threading tool is a new and sharp HSS.
Any comments gratefully received.
Regards Don
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On Tue, 20 Apr 2010 10:08:09 +0100, Donwill

Don     The purpose of setting over the top-slide when thread-cutting is to reduce the cutting contact area between tool and workpiece and to cut only with one side of the tool tip, rather than with both, the case if you were to plunge straight in at 90 degrees.          Although it would not be good practice, I can't imagine you would have any particular problem with a relatively soft material and a sharp tool of the right shape. Is it really that complicated to learn the 'proper' way, after all you only have to set the top-slide once, a minutes work - my own top slide rarely moves from this position, regardless of the work I'm doing.
     Practice makes prefect - good luck!
     --
Chris Edwards (in deepest Dorset) "There *must* be an easier way!"
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Hi Chris, Shame on you for implying to a novice that there is only one "proper" way, ie the set over top slide method. The straight in method is just as "good practice" as any other one that works. T.W.
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<..>

I'd second that, and add that you can get the same result as rotating the compound slide if you instead feed in and advance the compound (parallel to the bed) by about half the in feed, you can check the trig if you want it exact, Tan 27.5 = 0.521.
Just another option, not right or wrong, but personally I'd go straight in, especially for a fine-ish thread in something soft.
Richard
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I'm inclined to agree. However, I would add (in response to the OP's words) that 5 thou is OK for the first pass, but then you need to go down as you get deeper. I tend to go 5, 4, 3, 2, 2 until it sounds to be stressed, then 1 for the last few passes.
I would also add that, if you don't have a 3-phase VFD (so you can control the speed on the fly) it's worth using a mandrel handle and turning over by hand, especially if you are relatively new to screwcutting.
I have also become quite attached to the replaceable screwcutting tips, see e.g.
http://www.greenwood-tools.co.uk/ishop/728/shopscr6.html
though other suppliers do them cheaper.
David
--
David Littlewood

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David Littlewood wrote:

I'd third that - anything softer than brass, and going straight in is fine - even for brass it's okay unless you want a very fine finish or very accurate thread.
However, I would add (in response to the OP's

I second that, though what's a mandrel handle? Afaik it's just a handle which turns the lathe spindle (but don't forget to remove the handle ... treat it like a chuck key, never leave it on the lathe),
I'd also add that to begin with (and probably everafter as well) it's better not to use a thread dial indicator. At the end of a pass withdraw the tool, wind the carriage back with the ?mandrel? handle, advance the tool, then cut forward again.
With this method the worst thing which will go wrong is that you miscount the turns on the handle when adjusting the tool and try to stick the tool in a turn too far, which you will immediately notice - with a dial indicator all sorts of things can go wrong.
-- Peter Fairbrother

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Actually, I find Al alloy is worse for tearing than F/C mild steel.

Yes - there are several published designs, and Hemingway do a kit.

Never had a problem with a tdi on an imperial lathe cutting an even tpi thread, just drop it in at any whole number and you can't go wrong. Can't remember the last time I cut an odd-tpi thread.
By contrast, a thread dial indicator on a metric lathe is a PITA.

David
--
David Littlewood

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David Littlewood wrote: ... snipped

... snipped

Forgive the (ahem) thred drift, but why is that?
Dave
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Dave,
With an imperial tdi, one gearwheel serves for all threads; with a metric one, you need several different ones and have to change the setup for different threads. This is because metric threads are defined in mm pitch instead of threads per unit length.
Since my metric lathe (an M300) has a 3-phase motor and VFD, it's much easier to leave the leadscrew half-nuts engaged and reverse the motor to back out.
David
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David Littlewood

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wrote:

Hi David, Second that, just fitted a VFD to my Bantam, I can now flick from forward to reverse at will. Pondering the possibility of arranging an automatic back and forth switching arrangement via limit switches, which combined with Bogstandard's/John S's lift up screw cutting device, should make screw cutting a complete doddle. T.W.
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Don
Another reason for setting over the topslide is it makes reversing easier as ther is no contact betwwen the tool and the job when reversing.
Before starting to screwcut bring the tool up to the job and set the crosslide dial to zero. Put the cut on with the topslide. Take the cut either manually or under power. At the end of the cut back the crosslide off enough to clear the thread. Reverse manually or under power. Return crosslide to zero. Put on next cut with topslide and repeat. At no time do you need to undo the clasp nuts. The cut being put on with a topslide set at an angle is less than the dial indicates, about 80% of the figure indicated. If you do decide to set over the topslide make sure that you the set the angle from the right starting point.
Good luck - once you have done one successful screwcutting job you will never look back.
John H
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Hi John H, If I might add, making and fitting a cross slide stop helps speed things up if using the set-over-top-slide method. It saves having to even look at the cross slide dial. T.W.
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I've wired my single-phase Myford to run in reverse when required - means I can leave the half nuts engaged so I don't lose the thread - doesn't always work, mind, if you're using indexable tipped tools, make sure they are attached tight enough as my cheap ones can move quite a bit - a problem on a fine thread. The Screwcutting in the Lathe book from the workshop series is a great reference guide. I set my topslide to 29.5 when cutting ISO metric threads - there are table available to convert the micrometer readings into actual depth of cut
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On 20/04/2010 21:49, David Littlewood wrote:

Thanks to all for the advice, however, I find it difficult to visualise the difference between cutting the thread with the top slide at an angle, and going straight in. As far as the work-piece is concerned the tool is presented at the same angle and the cutting effect is controlled by the leadscrew. The only difference that I can see is that the final thread is moved to the left of the initial cut. Dooh, having written that down and reading it I can see that to have achieved the moving of the thread to the left there must have been more cutting action on the left side of the tool. But it's still difficult to visualise the actual cutting action. Ah well, old age and to much wine taking it's toll I guess. :-) Cheers Don
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Don,
You have it. Since the tool will cut better if only one edge is working (and since some tools have a slight side rake, making the LH edge better at cutting) this is reckoned to give a better finish.
Best of all, say various experts, if the topslide angle is set to slightly less than half the thread angle (27.5 degrees for Whitworth form, 30 degrees for metric). That way the RH side of the tool takes a very slight shaving cut, improving the finish of that side. Don't know whether this really works, or whether it is simply to ensure that it isn't set too far and leaves a series of steps on the RH flank.
Personally I have never found it makes much difference, straight in works OK provided you don't try to hog it (see my previous post).
David
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David Littlewood

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wrote:

One very specific issue with the ML7 is that the calibrations on the topslide mount only go to +-45 degrees from the lathe axis. The 27.5 degrees is actually 62.5 from the lathe axis.
The way I got around this was to make a new datum mark on the cross slide at +-30 degrees with a sharp prick punch, using the calibrations as a guide. This allows you to set the topslide to the required angles.
Eventually, when swinging the topslide over to these sorts of angles, you may discover that you need to reposition the two clamping T bolts to get the swing you need.
Mark Rand RTFM
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