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Tailstock Die Holder

Ever since I first got a lathe, i've been making do with improvised tooling until I hit a crisis point. It happened with a fixed steady, it happened with a 4 jaw chuck, it happened with a boring bar, and now it is happening with a tailstock die holder. Currently, if I want to put a thread on something on the lathe (other than using the change wheels and single-point cutting it), I have to break out the die holder, steady it with the nose of the chuck in the tailstock, and painstakingly wind it on. Over the last few days, i've done a fair bit of screwcutting and i'm fed up of this method. So, time to make myself a die holder!
The first thing I will need is an arbour to mount in the tailstock. Being a Myford ML7, the tailstock ram has a 2 Morse taper cut into it, so I shall make an arbour to fit that. Which brings me to the first hurdle:

Accurately Cutting a Morse Taper

I don't have a fancy taper attachment for my lathe, just the standard topslide. The divisions on this are nowhere near accurate enough for me to set the required angle, so I need some other method. A quick search came up with this page, that explained how an accurate setting could be made for the topslide using a humble Dial Test Indicator. The trick is to set up a dead centre in both the tailstock and headstock of your lathe, and support a reference taper between the two in its centre marks. I used the arbour from a live centre that I had. Set it up like this:


As you can see, I have roughly set the angle of my topslide by eye, and placed my DTI into the toolpost. The DTI has to run along the centerline of the taper, otherwise any readings it gives will be false! Once you have set this up, run the topslide back and forth along the taper. You will see the needle on the DTI rise and fall as it travels, and you can use this information to gently adjust the topslide. What you are aiming for is for the needle to remain stationary as it travels along. This will indicate that the topslide is running perfectly parallel with the taper, meaning that when you replace the DTI with a cutting tool, you can cut that taper easily!

It is worth mentioning that unless you have a VERY sharp tool and a VERY rigid lathe, the chances of you getting the fit absolutely perfect using just this method are slim. It will be pretty darned good, but there will be high and low spots on the surface. To correct these, use engineers blue (or a sharpie will do!) to colour the taper, then place in the tailstock. Turn it a few times, then withdraw. The areas where the colour has been rubbed off indicates high spots, which you can tackle with a very fine emery cloth. Be patient, repeat the colour test frequently until the whole surface rubs evenly. Then, you are done and can be confident you have a true Morse Taper!

The next step is to
Cut The Arbour

This step is pretty easy, and just requires some accurate turning. Remove whatever chuck you have from the headstock of your lathe, and pop the taper you have just made into the matching taper in the the headstock spindle. (My Myford ML7 has a number 2 Morse Taper cut into both the headstock and tailstock, so this is easy. If you happen to have a lathe that has a different taper in the head and tailstock, you may need to track down or make an adaptor for this step). Once you have mounted it, give a gentle tap in with a soft mallet to ensure it is seated properly, start the lathe and take a surface cut off the part of the arbour that is protruding. You want a reasonable length of arbour, I would recommend a good inch or so. It should look something like this:


This part will form the shaft that the die holder itself will slide and rotate along. It needs to be a smaller diameter than the OD of the smallest die you are planning the tailstock to hold. In my case, I chose to turn it down to 5/8" as that seemed as good a size as any! This dimension needs to be cut as accurately and with as good a finish as possible, so finish with light cuts and emery cloth, and use that micrometer frequently!

Making The Die Holder

Next, you will need to get a chunk of mild steel bar that is a bit bigger than the diameter of your largest die, and about 1 1/2" long. It's the kind of convenient chunk you will find in your scrap bin, if you've been tinkering for a few years! Mount your 3 Jaw chuck on the lathe and stick the bar into it as squarely as possible. Take a facing cut to neaten it up, and skim the surface as close to the chuck as you can get to tidy THAT up too:


This makes for a nice, clean and round surface that will look a lot nicer than the rusty skin my offcut had to start with! It also means that when we reverse it in the chuck, as we will do now...


...the facing cut and surface skim done on the other end will be concentric with this end.

The next trick will be to bore a hole straight through the centre of the bar. Start with a centre drill (as you should always do if you want a concentric hole on a lathe!), then work your way up through a few sizes, until you get a hole large enough to get a boring bar through. Mount the aforementioned boring bar in your toolpost, and use it to turn the hole to the same dimension as the arbour you turned on the taper earlier. It is adviseable to take this very carefully, as you want a nice sliding fit. What I did, in order to help this, is to bore the hole ever so slightly undersize, so that the arbour would just about go in tightly, then use grinding paste on the arbour itself to bring the hole out to the final diameter. It worked beautifully! Just remember to clean ALL of the grinding paste off when youve finished, it is insidious stuff...



Cutting The Die Slots

So, we now have a thick cylinder that slides freely on a Morse Taper arbour. The next step is to make the recesses for the dies themselves to locate in, so that it can actually do some work! The design of this holder allows for 2 different sizes of die, in my case a 1" die and a <need to check...> die. Pick one of the sizes, and use your boring bar to enlarge the hole at one end of the die holder to that dimension. The depth of the enlarged section should match the thickness of the die it is going to hold, so that the face of the die will lie flush with the face of the holder. Repeat this for the other size die at the other end of the holder:


Retaining Screws and Tommy Bar Holes

We now have a holder that slides freely along an arbour, but at the moment if we were to place a die in one of the recesses and turn the holder to cut a thread, the die would most likely just rotate inside the holder. Most dies have a slot and two little recesses cut into the outside edge, and the purpose of these is to receive set screws (also to adjust the cutting depth of the die, but that is for another time). We will need to place screws in the outside edge of the holder in line with these recesses to allow the holder to 'grip' the die. This is most easily achieved by lining up the die in the holder and scribing lines in line with the grooves. Measure how far from the face of the die the centres of the recesses are, and use this information to centre punch where the holes will need to be drilled.

To be continued...

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