Spindle Thread form specs ???

Hi

I have an English Viceroy lathe (like a Boxford) but my question is probably applicable to anyone that wants to cut a spindle thread.

I want to make a morse taper spindle. It will have a male No.3 morse taper on one end and a spindle nose on the other. I will allow me to hold attachments like 4-jaw chucks in the tail stock.

This is not an original idea and I thought it would be easy to find info to make one. I have found the info I need to make the morse taper. I have been unable to find full specifications on the spindle thread form. I know it has a major diameter of 1.5". It has a 8TPI pitch and 60 degree angle. I have measured the diameter of the spindle on my lathe using the 3-wire method and then used a cad program to calculate the dimensions.

I don't know the correct radius at the thread peaks and valleys and I don't know how far away the thread is from the limits.

To maintain accuracy, I need to leave the work mounted in the lathe and just cut the thread to the correct size. I won't be able to trial fit the thread.

So what I'd like to know is the correct specifications for the following:

-the major diameter if the thread peaks were sharp and not rounded. This will be greater than 1.5". I will be able to work backwards to calculate the 3-wire diameter measurement I should be aiming for.

-the correct radius for the peaks and the troughs. I will use this info to help grind the tool to the right shape.

With this information, I will be able to cut a spindle thread that just works. Without the thread form specs, I will just have to make a close copy of the present spindle thread.

Dazz, 1.5" x 8 tpi sounds like a BSF (British Standard Fine)thread to me.
If it is, and quite possible as it's an english lathe, then the thread angle will be 55 degrees not 60.
BSF threads use the same profile as Whitworth (BSW) threads, in which case the radius and thread depth can be calculated from the following:

p = pitch of the thread
d = depth of the thread
r = radius at the top and bottom of the threads
then:
d = 0.640327 p
r = 0.137329 p

To get the major diameter without the radii you just need to do a quick sketch or use a bit of trig.

More information here:
http://shopswarf.orcon.net.nz/bsf.html

Peter

Dazz, I've made a few of these, and found by painful error, the best way to make them is machining between centres, yes you can do the roughing in the chuck, it is more rigid, but do the finishing between centres, as long as you don't move the driving dog/carrier, they can be taken out and tried.

In my opinion, I'd follow the standard thread specs, as Peter's post but ultimately you've got to make the most accurate copy of your existing spindle possible.... theorey and reality don't always coincide.
Tip:- Get the thread fit right first, (IMO, not too tight) then go for the register.

Take care. Sami.

Check the thread flank angle with a "fishtail" or center gage.

There is a way of checking the flank angle using two different sets of thread wires and running a lot of math. The formulae and the process is grist for most entry level machinist and metrology texts.

The root flat or radius may be taken from your spindle. The US standard for 60 degree thread esternal root flat is P/4 Max. I prefer P/5 to ensure ready engagement if the option is open to me.

Before you make any internal thread have a means of gaging it handy. It's easy to check the PD of a male thread with wires so if a mating part is unsuitable or inaccessible for use as a gage, make a gage having the same PD as the part thread.

What ever the thread on your spindle, it complies to a standard that was published someplace. Here in the US the default reference is "Machinery's Handbook" but there are many others including the original standards published by our NIST (National Institute of Standards and Technology formerly NBS - National Bureau of Standards). I'm sure you in the UK sphere have any number of similar resouces. You merely have to find them and use them.

I frequently advise new machine tool owners to make spindle gages and replacement lead screws and nuts as early projects and keep them for later use. When you need replacement lead screws and/or wish to make spindle tooling, you need to do so quickly without fuss or bother. Having replacement parts and gages on hand makes the re-commissioning or the making so much quicker.

Tony Griffiths site says the Viceroy lathes had an odd mixture of thread standards "BSW, BSF, UNC and BA fasteners." It seems the Viceroy is sort of an improved copy of the Boxford copy of the South Bend 9 inch lathe. The South Bend had a 1 1/2-8 UNS spindle thread, also used on other American lathes. The Unified thread form has 60 degree flanks and does not have rounded crests and roots. In the USA, chucks and backplates with this 1 1/2-8 thread are very common. Some of these chucks are made outside the USA. Some chucks with the Unified thread have probably found their way to NZ.

So before you make your tailstock arbor to fit your four-jaw chuck, ask yourself if the chuck has the same thread form as the lathe spindle. If your lathe has rounded threads, it may also have 55 degree flanks. But 80 years ago, American 60 degree threads frequently had rounded crests and roots, so anything is possible here.

Just remember your goal is to fit the arbor to the chuck, not to exactly copy the lathe spindle. If you have sets of thread pitch gages in both 55 and 60 degrees, try to determine which fits the chuck and the spindle.

Larry

Hi

I used my CAD program to convert the 3 wire measurements to diameters. Easier than doing the trig. I like theory, it always works. Reality is uncertain, ambiguous, contradictory and just hard work.

This lathe was made about 1980, and it is based on a Boxford. I expect the thread is the same, but I don't know how this compares to a Southbend. The thread crests are small flats and the roots have a small radius or flat.

I have a couple of chucks plus plates, so I'd like to be able to cut a thread that will fit all of them plus anything else I might buy in the future. I'd like to be able to compare my spindle thread to the spec to see how closely they compare.


When I first looked at making a taper spindle, I thought the specs would be easy to obtain, but it appears I was wrong.

Hi

After taking some more measurements, I found that the spindle thread is 55 degrees, not 60. I then did a CAD drawing of both my measured thread and a Whitworth spec thread.

I found that my thread does not match a Whitworth but it will fit in the same thread. The diameter is slightly less than a Whitworth. My thread would be a loose fit in a Whitworth nut.

So it would appear that the thread is a hybrid. It's like a UNC with almost flat crests and roots but with a 55 degree Whitworth angle.

I made up a bitmap to show the results, but this forum won't let me insert an image. I don't have a website to point a URL to.