Thread: External thead cutting tool

With all this talk about ACME threads reminds me of the difficulty that I have getting a HSS tool properly shaped (primarily not rounding the corners)
I have seen pictures of disc style cutters made of silver steel where the profile is cut on the lathe and the cutting edge is obtained after hardening by grinding the top surface.
For those that can't picture this, imagine a disc say 25mm in diameter. The edge is profiled with two 14.5 degree chamfers, such that it matches the groove cut into bar stock to form an ACME thread. There is a hole in the centre of the disc and a notch cut into it such that to sharpen you just grind/ hone to top surface.
It is mounted on a piece of square steel. I've tried to illustrate this in the sketch below.
I'd be interested to know if anyone has tried this; what the finish is like and how best to hold the disc to the shank.
Any thoughts?
Michael
Attachment 190386

G'day Michael, that looks quite interesting. My only thought would be in regard to the helix angle of the thread. From what I can see there is the same clearance on both sides of the tool, would you mount the disk in the holder at the appropriate helix angle for the thread being cut. I'm just trying to imagine it in use, however am I speaking out my clacker?

Pete

Once upon a time you could buy commercial items similar to that.... I have some that I aquired from an old turner, in fact I took a photo of them ready t post here, then left the camera in the shed..... Maybe tomorrow...

Like these, except acme of course. These are Whitworth.

Phil

hi Michael A company called TOOLFLO makes carbide ACME threading inserts http://www.google.com.au/url?sa=t&rc...pxB2Mg&cad=rja

I know that carbide thread inserts are available but I really haven't considered them for home use as carbide needs speed to cut properly - I'd use them if I had a CNC lathe threading at 600 rpm, but at the 60rpm I normally use I would not be cutting as much as ploughing. Also, while my lathe is big enough and rigid enough (one of the reasons I bought it) the average small lathe used in a home situation really doesn't cope well with the forces (and hence deflections) that carbide causes. A previous lathe used to snap carbide parting inserts like crazy, probably for that very reason - my thoughts on carbide for home use anyway.

RC, Phil, do you use them at all? Are they worth the trouble - it seems a good way of getting an accurate form (whitworth/ ACME/ whatever)

Pete, off hand I don't know. The natural relief on the cutter may be enough for fine pitch threads but if not, the holder would need to be canted over. That would need some compensation on the form to preserve correct thread dimensions, and may also require more complicated sharpening to avoid negative rake on one side of the tool. All reasons for asking the question...

Michael

Hi Michael,
They work very well, but then anything sitting in an Armstrong tool holder is going to work well

Phil

Michael
With a bit of practice its not that hard to grind up a single point threading tool. What I'd suggest is try to get a bit of square mild steel about the size of the HSS that you use and practice grinding on that, rather than waste your HSS. Do you have a thread tool grinding guage? if not get one, they are invaluable.


Make sure your grinding wheel is dressed and the front is flat. For an acme, just grind up a V tool to the required included angle and once you have the sides right , just grind a flat on the front back to the correct width.
For a V thread tool I normally grind the sides first, then once they are correct I grind the top/back rake on. just use fine oil stone to break the sharp edges

Sometimes its easier to hold your HSS bit in the tool holder for grinding, other times just hold the bit in your hands. You will soon develop a method that works for you.

hope this helps

bollie7

This tip may help you . Do the cuts in two stages with two tools . First,use a thin ( say 1/16" ) parting off blade and cut a square groove to close to the required depth. Change to the ACME cutting tool and then complete the process. It should result in a cleaner finish as the ACME tool is only cutting lightly MIKE

Mike,
I haven't tried that method, so next thread I might try it.

Bollie,
I have a thread gauge as pictured. For me the problem is more an off shoot of the grinding process. I can grind up a tool to shape, but the corners easily overheat so that when being used they soon round off. This method appealed to me as a way of getting an accurate form without running the risk of a mis-grind. (I should also make up a grinding fixture so that I'm less reliant on steady hands - it's on the list)

Michael

Here are mine

Originally Posted by .RC.

Here are mine

Hi RC,
Do you know if they are still available in acme form.

Phil