Threading and threading

Hi

I'm finishing setting up my Sieg SC3 lathe and have a couple of questions about threading. Beofre that I had a smaller (non-Chinese) lathe and never threaded using a single cut, only with dies, but without a lathe die holder.

1) I see the compound has only 40 degrees. What is the best way to set 30/60 degrees for threading? I'm imaging some long process of aligning the tool holder with the compound, then align the 60 degree tool with the chuck, or some other cumbersome method... maybe there's a better way?

2) I use only adjustable dies. It seems like a tailstock die holder has to be precise to the diameter of the die and won't work well with an adjustable die. Am I missing something?

Thank you in advance

I use adjustable dies in tailstock die holder with out any problem.

Bob

Dies come in several sizes but they are standard. If your dieholder is the wring size it is just that it has been made wrong. Sometimes far eastern manufacturers interpret specs too literally without thinking. So they may strive to make the hole exactly to size +/- a thou thinking they will be criticised if not precise and fail to understand the reason for the hole.

Make a tin or plastic template to align the topslide eg side of it wrt the faceplate and don't be afraid of making a new alignment mark on the machine for future use.

You don't have to set the topslide over to cut a thread, its just a method some people use.

Easiest way is to get a cheap 60/30 set square, hold one edge against the headstock and the other against the side of the topslide. Then move it a fracrtion more as you really want 29.5 degrees not 30deg

There is an easy but not universally appreciated way and that's to keep the topslide at 90 degrees to the cross-slide (parallel to the lathe bed) and to advance the topslide each time by half the advance on the cross-slide.

That works out as being at tan^-1(0.5) = 26.565 degrees and so works well for both Whitworth and metric thread forms.

It means that the thread depth is the direct reading on the cross-slide and also allows a thread to be widened more easily once full depth is achieved by just advancing the topslide.

Martin Cleave's book **LINK** lists this as METHOD 3 in Section 8 and is the method he said he usually adopted.

HTH

Jon

Thanks.

Marking the lathe for the 30 degrees (or any extra) angle is what I thought of. So I guess I just have to spend more time doing it once.

Re the die holders, I'm not sure I understand. I don't have a lathe die holder yet but I thought of getting or making one. I thought the idea of a die holder is that the die has a very good fit inside the holder. Assuming the hole in the holder is big enough to fit the die at its largest opening, how would it be centered in the holder when closed to a smaller diameter?

The difference in an expanded and contracted die is minimal and the small amount of play in the die hoilder allows it to almost self centre. You can get "floating" holders but I've never felt the need for one.

.

No you are not "missing something" ... you are observant !!

Some better designs of tailstock die-holder allow the die to float radially, so that it self-aligns.

MichaelG.

My dieholders are self centering, ie. they are a bit loose on the spindle. It is the die that aligns with the rod to be screwed. The holders are made over size to allow the die to expand, but not much, difficult to be precise because it depends on the O/D and the hole in the die.

George Thomas made a holder for each of his dies Each one was centred by the use of multiple securing screws. he ground the dimples into grooves so that the screws would no misalign the dies.

Of course, by the time you have done this for your die collection you will have forgotten what you wanted to thread in the first place...

As Michael and Jason imply, what matters is keeping the die at 90 degrees to the axis of the lathe, it will centre itself if allowed to do so.

Neil

Although the ceoss slide parallel to the bed set-up for threading works pretty well its a major PIA getting things set up specially for threading. Especially on a small machine where there isn't room to leave the topslide permanently at 90°. Actually its a bit crowded on pretty much any home shop capable lathe. A technique I only use for ACME and similar feed screw/nut threads which have to be cut for minimal clearance.

I leave my topslide parked at around 25 and bit degrees off perpendicular to the work and use the Zero to Zero screwcutting method described in my post on this **LINK** forum thread, 8 th post down. Any top slide offset angle reasonably close to, but a bit under, the thread half angle will work just fine. No complicated maths, the lathe does all the calculations for you. When you finish, with both dials on zero, you have exaclty what you set-up in the beginning so if all isn't right you have a chance of figuring it out. The usualy offset at half angle method involves maths (Yikes!) which multiplies the chance of error for the inexperienced and when working solely from the cross slide the temptation is to start and finish on funny numbers depending on where things ended up on the previous part of the job.

If you don't have zero setting dials an adequate work around is to set the topslide on zero then move the cross slide past zero to the requisite thread depth and bring the threading tool up to the work before clamping it down. This requires your toolholder have some sort of register to slide the tool along and to be set perpendicular to the work. Both of which I regard as essential preconditions for any attempt at accurate craftsman like work.

Using 25 and a bit degrees offset angle as the normal cross slide position also keeps the cross and top slide handles out of each others way making the machine easier to drive.

Clive.

Thanks everyone.

That kids triangle ruler is a great idea I didn't think of... and I still have mine from 20 years ago, haven't touched it since, but I know where it is That's exactly the kind of suggestion I was hoping to get

Re the the die holders, I see how the slightly loose fit would work for adjutable dies, but I guess that just helps against gross misalignments. So I don't think I would have an advtange over my current method.

I'm guessing that old 29.5 or 30 degrees is so the angle is not more than the angle of the thread, creating a bump when it doesn't continue. So basically it just has to be at an angle no more than the cutting tool itself.

I can agree with that although on the Myford ML7 there is a pretty annoying design flaw in that the topslide is not free to rotate completely - You have to choose the quadrant by positioning the Tee-nuts to either left or right of the cross slide slots.

So, to save keep mucking about taking the topslide off and on I tend to keep my topslide mostly at 6 degrees from parallel to the lathe bed to give a bit of clearance with the tailstock but also to allow a roughly 10:1 ratio for top-slide versus cross-slide movement. If I find myself wanting to take 0.001" off the diameter, moving the topslide in 0.005" has the effect of advancing the tool in about 0.0005".

This is another trick from Harold Hall - See bottom of this page... **LINK**

It's probably of no help to you, but I HTH someone.

Jon

Tailstock die holders, its easy to make your own, why buy them , Bore the body to suit the die plus a couple of thou for clearance , drill and ream the body any convenient size ,say half inch and then get a short length of silver steel which will slide smoothly up the hole, the silver steel is held in the tailstock chuck,cross drill a tommy bar hole,and drill three holes for the grub screws,I use 2BA as I have a supply of socket screws, grind a point free hand on the screws.-The body of the die holder is mild steel,aluminium alloy is also OK ,I have about ten alloy ones which have lasted twenty five years.There is no need to worry about small amounts of eccentrity,after all if a piece of rod is held in the vice and cut with a die in a dieholder there is no control on the the die running true. It is a good idea to keep your commonly used dies in their own holder to save keep setting the dies. Screwcutting, when cutting threads on free cuting mild steel ,aliminium and brass, leave the top slide parallel to the bed ,set up the taxis of the threading tool at 90 degrees to the work,and plunge straight in,it works ok and depth is directly measured, if an accurate form is required then finish in the traditional way with a machine chaser. For tougher or very precise work set the top slide to half the thread angle, Use a screw cutting gauge to grind the correct angle and set the tool up true to the work,( rdg tools moore and wright no 200 screw cutting gauge) .

I have only cut a few threads on the lathe, but used the set-over 29.5deg method when cutting the thread for the ER collet chuck for my tool post mounted spindle. I made a useful little tool to set the top slide to the required angle - it can be seen in my album "top slide angle setter". It also came in handy for setting the angle for grinding the taper of the collet holder - see picture, above. I didn't want to rely on the graduations marked on the cross slide to accurately align the top slide.

John

Edited By John Hinkley on 20/05/2015 13:45:25