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rp designs
10-22-2011, 03:00 PM
My lathe is out of commission for a while and I would like to try and cut some threads using the fourth axis on my mill. I have seen it done with a lathe tool fastened into a fixed holder, but I was thinking you should be able to cut them with a 60 degree end mill. I have not tried this and was wondering if anyone has tried cutting threads in this fashion before? Any pitfalls or specific tool recommendations?

PixMan
10-22-2011, 05:08 PM
Sounds like thread milling to me. You don't need fourth axis to do it, just three will do.

There are many different types of thread mill (cutting tools), but you can even do it with a tap in a pinch. What kind of mill do you have?

rp designs
10-22-2011, 07:17 PM
Thread milling would be with the part axis oriented along the z-axis. I think for that type of thread milling you need a spindle encoder. I am specifically talking about orienting the part along the x axis and dropping a cutter along the z. I have seen it done with lathe tools attached to the z but my type of quill won't allow that easily. The biggest problem is a lack of a spindle lock on my machine (an older, ca.1980's comet cnc mill).

uncle pete
10-22-2011, 07:18 PM
The second post answered my question.

Pete

Toolguy
10-22-2011, 07:28 PM
I do thread milling all the time on my 3 axis CNCs. You point the part to be threaded up towards the spindle and program a helical interpolation with X,Y, and Z all working together at the same time. The depth of the thread is controlled by the amount of tool offset. I have a lot of carbide thread mills, but many times have used a 60 degree double angle cutter that is like a 2 sided dovetail cutter.
I have also made tapered round form threads with a round thread form insert mounted on a homemade shank. You can cut right or left hand, internal and external with the same cutter.
To use a tap, you grind all the teeth off but leave one flute untouched. You can get twice the pitch of the tap on the part if a 4 flute tap and grind the teeth off every other flute, leaving 2 flutes opposite each other. Taps don't work that good for thread milling, not enough back clearance, but it can be done.

rp designs
10-22-2011, 07:46 PM
My need would be for long parts, making the set-up very difficult. What I am basically trying to do is emulate a lathe op, but without the lathe tool. Is there an end mill that would be good for cutting threads in this particular situation?

Toolguy
10-22-2011, 08:44 PM
Assuming the thread form is a 60 degree V shape, you could use a 60 degree engraving cutter or any 60 degree end mill or router bit that is pointed enough at the bottom.

adatesman
10-22-2011, 09:16 PM
While good in theory, Toolguy, I don't think the tool would last terribly long used like that. Way too much stress on the tip.

Toolguy
10-22-2011, 09:38 PM
Maybe a lathe type threading tool in the spindle with a stabilizing bracket clamped around the quill and threading tool to keep it from turning.

PixMan
10-22-2011, 09:47 PM
I have actually done a type of thread milling in the X axis of a CNC knee mill that was quipped with a horizontally-oriented rotary table.

The rotary table had a four-jaw independent chuck and a tailstock. The spindle was equipped with a specially-shaped diamond grinding wheel mounted on a 90 angle head. Spinning the 6" wheel at 4000 rpm, I was able to figure out how to "roll over" the 4th axis, so I could program an incremental move of 14760 degrees. That made 4" of a 10tpi round root thread with a one-turn lead-in. The material was an aluminum oxide ceramic tube.

If your machine has a similar 90 head and a roll-over B axis, you could fly-cut the thread you need with a 60 included angle milling cutter. Food for thought.

rp designs
10-23-2011, 12:17 AM
If I use a 90 degree head I can use a thread mill, but I want to avoid the cost of a 90 degree head. Burning through a bit may not be an issue as these would be one off parts. I just want to find aninexpensive cutter that would be up to the task.

wbleeker
10-23-2011, 01:32 AM
rp Do you have a CNC Mill? you might find what you want to do very difficult without one, most of the replies on here are talking CNC and I think you are talking manual mill, so probably a bit more detail is required before you go any further.
Will

John Stevenson
10-23-2011, 06:53 AM
Done it a couple of times for odd work, one piece was an acme thread done with a D bit cutter after blocking the bulk out with an end mill.
Worked but slow.

Checked for any pictures and the only one I can find is a setup picture to prove the concept.

http://www.stevenson-engineers.co.uk/files/threadmilling.jpg

Very crude but you get the idea.

External thread milling and large internal work I do with Coventry die inserts fitted to a simple holder.

http://www.stevenson-engineers.co.uk/files/threadmill.jpg

Very fast as it can do it in one pass as regards depth and only requires one rev for a short thread as it uses all the teeth of the cutter.

I used to do a lot of lens retaining rings for Laser cutters and it could thread each one in under a minute.