Troup:
To answer your question, let's start by comparing cutting a long lead on a milling
machine versus a planer. Suppose you want to cut a thread with a six foot lead on
a milling machine equipped with a long lead attachment, the limiting factor will be the
length of table travel. On a #3 or #4 miller, this will be limited to 34 inches or 42 inches.
To cut the full length thread, the job will have to be reset. There is also the problem
of supporting the outboard end of the shaft. So far we are only talking about cutting
a single start thread. For multi-start threads you will still need some form of independent
indexing in addition to the long lead attachment. The simplicity of the double disc mount
ing plate for either the chuck or a drive plate can be used on a lathe, miller, shaper or
planer.
If a planer with a stroke length equal to/or greater than the length of the lead is
available, then the job can easily be done on the planer. When a number of long leads
are to be cut, a special drive unit can be built that is usually powered by a rack mounted
along side the planer bed. When you are dealing with a one-off, an ordinary dividing
head off the miller will work just fine. The dividing head is set at the end of the platen
and usually off set from the centerline by one tee slot, this allows for mounting the
drive system. With the rack drive system, a series of gears is used to connect the rack
with the dividing head. With the one-off method a wire rope is used to supply the drive.
An upright anchor post, mount at the end of the planer bed, is connected to one end
of the wire rope, which is usually about 1/8 to 3/16 inch in diameter. The other end of
the wire rope is wound around a drum that has a partition in the middle of its length.
The drive rope is wound on one side of the partition and a counterweight rope is wound
on the other side, but in the opposite direction. With this arrangement, one rope is
unwinding while the other rope is winding, maintaining a constant pressure on the drum.
The drum shaft, which is independently supported by its own bearings, is connected to
the index plate shaft via a coupling. If necessary, a gear on the drum shaft can be
connected to a gear mounted on the index plate shaft, when this arrangement, an outboard bearing is used to support the gear on the index plate shaft. By choosing the
correct drum diameter and gear ratio, the dividing head will rotate one turn when the
platen has travelled the length of the lead. Once you have the various bits and pieces
made up, the job can usually be setup in about an hour.
With the set up checked to make sure you are getting a complete rotation, the cut
is started about two or three inches from the end of the shaft to allow for the removal
of all slop. After each pass, the tool is fed in deeper until the proper depth is reached.
On a multi-start thread, you can either cut each thread full depth and then index or
cut and index all threads before changing the depth for the next cut, thereby spreading
any errors among all the threads. While this set up and procedure may sound com-
plicated, it is really very simply and produces very good results. One more thing, I
arrange for the tool to be lifted on the return stroke just in case something goes out of
synch, I don't want to tear up a thread or break a tool.
If I get a chance later today, I will post the set-up for screw cutting in a shaper.
Hendeyman