Thread Milling

[Bob La Londe]

I have been using tension compression tap holders for most of my tapping in the Tormach mill.  They have worked very well until just recently I snapped a 1/4-20 spiral flute.  I then swapped it to a 1/4-20 spiral point and snapped one on the first hole.  I double checked everything and changed it for another spiral point which snapped after about 10 holes.  Everything looked fine, it was only over running about 1 thread (well within the limits for the holder).  It just snapped.  I thought maybe it was cheap taps (MSCs Insterstate brand).  I checked the hole diameter and the drill diameter.  Its a #7 drill that measures pretty close to .201 and the hole as expected measures .203.   Better than average for jobber drills, and about average for a screw machine drill.  I honestly can't believe its just a cheap tap.  The previous tap had tapped thousands of holes, and the only reason I broke it is because I over drove it. (programming mistake)  Somehow I feel something must have changed, but I don't know what to look for.  I am using the same "style" I have been using for ages. 

On the little high speed mills I use single form thread mills and they crank out tens of thousands of holes.  At 24000 rpm they will thread mill a hole faster than the Tormach can tap a hole.  I considered thread milling the holes, but single form thread mills will take a very long time at only 5120 RPM. 

So... I started looking for multi form thread mills.  I found two things that surprised me or rather that I didn't know.  In the smaller sizes the total reach is much less than I expected or than I can thread with the single form thread mills I have.  I often tap or thread mill through 1/2 inch pate, but the multi form thread mills look like they can barely thread 5/16 to 3/8 in my most common size.  10-32.  There seem to be two different styles of multi form thread mills.  Those that can ONLY thread ONE hole size, and those that can thread a bunch of hole sizes.  The first one I found for 1/4 -20 claims to be able to thread just about every 20TPI hole size from 1/4 inch to 1 inch.  Others specifically are useful only for one size.  They are expensive compared to other tools, but I already knew that. 

[Dragonfly]

I've seen those multi form mills but never used one. The 'Tool flute length' in the Thread milling MOP is for such mills IMO. I only use fixed pitch single form mills and frankly can't imagine how an universal one can be used for standard (metric) threads as the hole diameter, cutting head diameter and pitch are strictly related. If it is a case of finer thread like M8x1 then I can use the M6x1 mill.

[lloydsp]

I've always done large-hole mill threading with a single-point tool.  It'll handle any pitch and diameter larger than twice the tool's own width.

I (very early-on; years ago) wrote a 'bottom-up' threading script for right-hand threads, so that I could minimize the effects of chip accumulation in deep holes with closed bottoms.

Oh!  And lest I forget -- Merry Christmas to all of you who celebrate it!!!

Lloyd

[Bob La Londe]

I've always done large-hole mill threading with a single-point tool.  It'll handle any pitch and diameter larger than twice the tool's own width.

I (very early-on; years ago) wrote a 'bottom-up' threading script for right-hand threads, so that I could minimize the effects of chip accumulation in deep holes with closed bottoms.

Oh!  And lest I forget -- Merry Christmas to all of you who celebrate it!!!

Lloyd

That's fine if you have one or two holes, but if you have 20 or more and multiple parts with 20 it takes a lot of time.  That's why I used TC tapping and that's why I started looking at multi form.  Time is money.  If I can do a good job faster I should. 

Merry Christmas

I've seen those multi form mills but never used one. The 'Tool flute length' in the Thread milling MOP is for such mills IMO. I only use fixed pitch single form mills and frankly can't imagine how an universal one can be used for standard (metric) threads as the hole diameter, cutting head diameter and pitch are strictly related. If it is a case of finer thread like M8x1 then I can use the M6x1 mill.

Single form isn't fixed pitch.  I think something is lost in translation there.  I use single form form for a range of pitches.  Usually a tiny thread single form can not cut much large pitches, and a large one obvious can't fit in small holes.  As relates to multi form I didn't say the odd ones I found where universal.  Just that they claimed they could cut the same pitch in multiple sizes.  For example 1/4 inch NC is 20 TPI and 1/2 inch NF is also 20 TPI. 

HOWEVER, when the thread mill arrived the package was labeled specifically 1/4-20 so I think whoever made the catalog entry was trying to be creative or overly optimistic.  I didn't really see how it would work either, but I might try it in a piece of scrap just to see. 

The main goal of multi form thread mills is increased productivity.  Thread an entire hole in just one interpolations ideally.  Of course you need the torque and rigidity to be able to make that cut. 


Merry Christmas

[newlinuxuser]

A couple of links.

I sometimes use a little tool from Vargus for tapping 1/8" GAS, very nice and fast.


https://www.vargususa.com/Vardex-Products/Thread-Milling
https://www.dormerpramet.com/en-us/productssite/pages/new-carbide-thread-milling-cutters.aspx?country=us

Merry Christmas.

Andrea

[kvom]

Both single and multi-tooth endmills have limitations.  Both are limited as to depth of cut (tooth size) and ramp angle based on relief ground into the tool. 

Whether you can cut a thread in one pass depends on hardness of the material, but a spring pass is usually a good idea.

[Bob La Londe]

Well, after some testing I was able to get the TC tapping to work again with the tap I needed.  As near as I can tell I set nearly every parameter the same as the original style (which unfortunately I deleted).  All I can figure is that I got some out of spec taps and/or new taps that were not as sharp as they should have been or probably both.  I pulled an old gun tap out of the turret I sometimes use in the lathe tailstock and it worked fine. 

In the mean time I had been using a multi form thread mill.  It works fine, but on this machine is no faster (a little slower) than the tap.  I went back to the TC tapper and ordered some better quality name brand gun taps.  I'm not pitching the thread mills though.  I have a couple operations I do on the end or edge of parts that do not leave enough room under the head to use the TC tapper.  Since they can just go directly in a much shorter collet type tool holder there is plenty of room for the operation if I thread mill it. 

Frack!  I'm going to have to make another tool rack shelf for that machine.  LOL.