If you can move your tool along a tad before the required depth is reached then you can widen the root

so you cut on a single flank to widen the root area

i.e. the line which intersects the c in "pitch". Move the tool along so the line intersect/cuts at t

(only a suggestion)

The usual route I presume would be to finish with a thread chaser

Edited By Ady1 on 23/05/2014 09:21:56

When cutting threads I normally off set the compound slide to just over the thread angle so that the tool is cutting most of the material on one flank and only a minimal cut on the other flank, takes a bit of trig to calculate depth but easy enough to do.

The OD is in this case 1.125" the only reason to truncate the thread form is speed of production, after all there was a war on.

The root is normally taken care of by the tool by just add a radius to the tool tip, crest can be taken care of with a chaser or just carefully rounded off with a file.

Or you could either rough it out on the lathe and then run a die down it to clean up the root and crest, RDG sell both taps and dies.

If you were going to truncate then it should only be about 0.017" off the diameter for this thread

Bob

Edited By Bob Brown 1 on 23/05/2014 09:55:36

Edited By Bob Brown 1 on 23/05/2014 10:06:24

I would be more worried about the sharp angle at the bottom of the thread. Stress raiser. Non myford flat earther.

On my S7 the spindle thread diameter is 1.124" which suggests that it is not a true Whitworth thread form, although it is 55 degrees. The crests are fairly sharp. I think Myford relied on the register to locate the chuck so the backplate threads can be cut fairly loosely with a pointed tool.

cheers

Rod

Methinks the method given by Ady has far to risk of operator confusion leading to major mess-up.

Best to use what was taught to me as the "Zero to Zero" method for screwcutting using the top-side set over at an angle which will allow you to do such broadening cuts at the base of the thread with minimal risk of getting lost. The major advantage of the Zero to Zero method is that it not only does it avoid all the tedious trigonometry to calculate thread depths and sidesteps any errors due to the top-slide angle being a touch off it also gives you an accurately measured account of what you ahve done so if things don't fit you have a fighting chance of figuring out what the gremlins did this time.

Basic method is to turn or bore the workpiece to the correct size then mount the screw cutting tool and bring it up to just touch the workpiece. Set both to cross and topslide dials to zero. Pull the tool back clear of the workpiece and crank the saddle along until the tool point is in clear air. Using the cross slide move the tool back past zero until the dial reads the depth of thread to be cut. Set the dial back to zero and use the topslide to bing the tool back far enough to make the first cut. Use the cross slide to pull the tool into and out of cut for saddle travers, cutting passes being made with slide on zero. Cuts are put on with the top-slide. Job done when top slide at zero and last spring cut is clear.

Obvoiusly the lathe does all the trigonometry to calculate the top-slide feed needed to achieve the desired depth of thread for whatever angle its set at. Which only needs to be close to half the angle of the thread. My topslide lives at 25° off which works just fine for both 55° and 60° threads.

If you need to widen the base of the thread make a cutting pass with the cross slide set a little short of zero with a compensatory extra in-feed on the top-slide to keep the depth the same. For all practical purposes the extra width at the base of the thread will be half the cross slide shift when a compensatory top-side feed of 1.15 times the shift is applied to keep the depth constant. The trig is relatively simple if you want more accurate values but whether your lathe can deliver them is a whole n'other matter.

Frankly is far easier to truncate the threading tool to the proper width tip, maybe stoning a radius on the corners too for a perfect job, than mess around with threading techniques. I've seen some right difficult ways of measuring tool truncation. The easy way is to grind it sharp, measure the overall length, figure out how much shorter it needs to be to give the right width at the end and trim it back by the required amount. If you are confident in your trig the amount to trim is easily calculated otherwise refer to thread data book which will tell you how much the real thread is truncated by when compared to a sharp triangle.

Clive

Hello all

When single-point cutting threads, I find that the crests can be rounded off using one of my thread files.

They are designed to be used manually to clean up bruised threads, but I use them in the lathe with the lathe rotating under power at slowest back gear speed.

The file is applied to the threads and not moved, but with the work located on one of the cross-grooves in the file.

The file then acts just like a chaser. (I am a grown-up so please don't anyone tell me its dangerous. I know it's frowned upon, but many time-served machinists used files in the lathe, they just didn't boast about it. If you are not sure about it don't do it.)

The advantage is that each file has 8 different pitches. I have three files, BSF,Whitworth and BSP so 24 chasers although there is overlapping of pitches, all being 55 degree thread angle.

Works for me.

Jim

I have made several back-plates for my Myford, both from noggins of mild steel and un-machined cast iron blanks. I have cut the threads on my Colchester. I can't say that, apart from the major and minor diameters of the thread and a slight rounding of the point of the screw-cutting tool that I have paid much attention to the precise details of the thread, except obviously for depth, but I have paid a little more attention to the registers. The back-plates screw on and off the lathe freely and the chucks appear to remain concentric. I always cut internal threads on the rear of the hole with the screw-cutting tool inverted in the tool-post and the top-slide set over to half the included angle. Using this method means you don't have to grind a tool to the precise angle because the rear flank is formed by its advance into the work and you don't have to think about which way to withdraw the tool at the end of the cut as it is withdrawn, as of habit, in the same direction as cutting an external thread, you also have a better view of the thread forming as the tool enters the work.