Thread: Threading...Aaarrghh!!!

Here's Pete's explanation graphically (no attempt has been made to represent in-feed as the correct amount)

Bottom left is a straight plunge cut - removing the same amount of material both sides of the tool.
Top left is a plunge but moving the tool back and forth a little. While material is still being removed from both sides of the tool, it is less on one side.
Top right. The compound is set to 29.5 degrees. Most of the material is coming off the left side of the tool. The right is just shaving the thread flank
Bottom right is the same thing but with the compound at 20 degrees. Again, most of the material is coming off the left of the tool but more is removed by the right as it feeds in.
feed.jpeg

Michael

Originally Posted by Steamwhisperer

Hi Pete,
I think I actually get it now. Of course it doesn't matter what the angle is, it just means that the closer you get to 30 deg. the less work the trailing cutting edge does.
Going over 30 deg and as you have said, would be a huge mistake. If I have this right, the closer you go to 30 deg, the less load on the tool (and machine) but at the end of the day, the angle won't matter as long as it is kept under 30 deg. and 'always' make sure the cutting tool is square to the job.

Phil

Yes Phil, spot on. To introduce a slight complication, well more a consideration I guess, if you hand grind the tool (and no reason you shouldn't) the two ground sides may not be at the same angle with respect to the back of the tool. So even though the total included angle between them is correct at 60 degrees (or 55 degrees for a Whitworth), that angle may be "pointing" left or right. In which case there's no point in setting the tool itself perpendicular to the work, as the business end of it will be at the wrong angle. Fortunately our predecessors solved that problem for us and came up with what I refer to as a fishtail gauge Center gauge - Wikipedia, the free encyclopedia I see however Wikipedia call them centre gauges, I can't say I've ever heard them called that before, so I guess I've learnt something new already today! They're cheap and come in different flavours depending on what type of thread you're cutting. I only have 55 and 60 degree gauges, but need to get some Acme and Trapezoidal ones too. Anyway, when you're grinding the tool, grind and offer it up to the fishtail gauge and look at the light coming through the edges. You can actually grind the tool remarkably accurately this way, even freehand (luckily I have a T&C grinder so grind tools like this on the Clarkson, thus the angles will be correct and symmetrical on the tool, but will still always check them exactly the same way). When you mount the tool, again use the fishtail gauge, but this time put it on the work with one of the notches facing out. Put the tool in one of the notches and then clamp the tool down, that way the actual cutting edges are at the correct angle to the work. Very easy, and takes much longer to explain than to do, but it's important to get this part correct.

Michael, thanks for those diagrams, they're excellent and explain it very well. About the only thing that's missing is if you go beyond 30 degrees, but it should be clear that the flank on the right hand side will be cut to the compound angle, whereas the flank on the LH side will be cut to the correct thread angle, that's why it looks like a funky buttress thread. Anything less than 30 degrees, and it will cut to the thread angle on both flanks, as it's just a form tool. Hopefully it should also be clear from the bottom right diagram that the dashed line (ie the tool path) can go from anywhere in that thread angle range, just not beyond it. So I'm now confident that next time guys here are watching a Youtube video and hear the guy stating that the compound must be set to precisely 29.5 degrees there will be a collective sigh of "That's BS!"

I must admit I don't get how anyone can cut decent threads by "wiggling" the compound backward and forward as they plunge, as there's always some backlash in the top slide. Somewhat ironically however, that's exactly the way some CNC machines are programmed to cut threads, but they are of course very different animals in this regard. Winding the top slide a specific amount forward with each successive plunge works, but must be a PIA to calculate the amounts for each amount you plunge, and will change for each different thread angle. I figure the compound is not just there for decoration, and why guys leave it as if it's welded in position is simply beyond me. Hopefully it can now be seen that the compound can be swung around and set just roughly right with no particular care as to precise angles, and with a dial indicator no calculators need be harmed in the making of these threads!

Pete

Originally Posted by Steamwhisperer

Hi Pete, I think I actually get it now. Of course it doesn't matter what the angle is, it just means that the closer you get to 30 deg. the less work the trailing cutting edge does.

Spot on. Thread cutting is quite simple as a rule. The worst one I did recently was a 4" OD 4 start LH square 2 TPI in Aquamet. Had to fit the mating nut which fortunately I didn't have to machine..... the shaft was 1.2m long out of a fishing boat. Times like that you're grateful for a Monarch with single tooth dog clutch and apron mounted leadscrew reverse. PDW

Hi

For what its worth,I find that straight plunge threading works fine onAluminium . On harder materials, I use the 30 degree compound set over method.

Originally Posted by Michael G

Top right. The compound is set to 29.5 degrees. Most of the material is coming off the left side of the tool. The right is just shaving the thread flank

Hi Michael,
Top right is how I plunge cut, no need to "wiggle" as such. If the thread is large enough to require anything other than a straight plunge that is.

Stuart

Top right for me too!

Cheers,

Simon

I wouldn't suggest the method shown in the top right, and I think Michael only put it up as a theoretical example. Theoretically there's no issue at all, and in theory the method works well. However in comes practice, and in practice our lathes just aren't set up to set angles that accurately off the compound, so you set "30 degrees" but is it really that much or just a little over 30 degrees? For many who adopt the rough and ready approach to machining it's possibly "good enough", but you're flirting with danger so why take the chance? I guess you could set up a sinebar or similar, but for this application anyone who did would be a complete idiot in my opinion. We've now seen what happens to the thread flank when you go over the thread angle. This is where the 29.5 degree rule came from, as it's just a poofteenth under 30 degrees to ensure we don't go over the 30 degrees, it just gives a little margin for error. The trouble is people who did this realised they were getting good results but didn't really understand properly the geometry behind it and this "29.5" figure got set in stone. They started up Youtube accounts and signed up to forums, and so the stone got bigger. In reality there's nothing special about 29.5 degrees.

About the only advantage setting the compound to precisely half the thread angle is that you can then use a single sided tool, as it's no longer acting as a form tool. I'm not sure why you may want to do that, too lazy to grind a proper tool or something, I have no idea. In practice if just a whisker under the 30 degrees the trailing edge is just skimming the material off anyway so that little margin for error is of no practical consequence and it will cut almost identically.

Pete

Originally Posted by Stustoys

Hi Michael,
Top right is how I plunge cut, no need to "wiggle" as such. If the thread is large enough to require anything other than a straight plunge that is.

Stuart

The figure bottom left is a plunge cut. How are you moving the tool down the thread angle if you're using the top right diagram?

[QUOTE=Techo1;1794382]

Originally Posted by Pete F

Again, don't worry too much about geometry, helix angles or any rubbish like that, it's completely unnecessary and is clearly just confusing many people. With a HSS tool you've ground yourself, helix angle is completely immaterial for regular pitched threads, and the normal side clearance will take care of that.the same as per a V thread.


That's a big statement to make Pete, even though you did qualify it by referring to "regular pitched threads". Leading edge geometry is very important and is the reason why insert manufacturers make anvils to go under threading inserts in 1 degree increments.

Having insufficient clearance under the leading edge is one reason that a lot of people have trouble cutting threads because the tool rubs on the flank instead of cutting cleanly at the cutting edge.

Insert threading tools are far too expensive for the amount of threading I do. But I remembered there was not too long ago a good article about this in Model Engineer's Workshop. I just found it, issue 134 page 18 to 21, the article is from Harold Hall. Well worth reading.


One more thing to be aware of before spending money on indexable threading inserts and toolholders, is that there are two very different types of inserts (but almost indistinguishable from a distance):

- There are those inserts that cut a "full profile". They will cut out of the box the proper thread profile with proper radiusses on crest and valleys. The downside is that you need a huge collection of different inserts. For example, only to cover the metric 60 degree right hand thread external thread range from 0.5 to 2mm pitch, one needs to buy six different inserts. One for each pitch. And another six inserts if you want to cut left hand threads........ here a link to a SECO threading insert catalog: http://www.secotools.com/CorpWeb/Pro...re_english.pdf

- There are those inserts that cut a "partial profile". They leave you with pointed crests and valleys. With these, one single insert will cover a range of say 1 to 2mm pitch plus 24 to 12TPI as well. That sounds a lot more reasonable for home use. You still need two inserts to cover from 0.5 to 2mmm, you still need a different insert for R and L threads, .... but its a lot more affordable.

Originally Posted by cba_melbourne

- There are those inserts that cut a "partial profile". They leave you with pointed crests and valleys. With these, one single insert will cover a range of say 1 to 2mm pitch plus 24 to 12TPI as well. That sounds a lot more reasonable for home use. You still need two inserts to cover from 0.5 to 2mmm, you still need a different insert for R and L threads, .... but its a lot more affordable.

I think I have a couple of these, they cut 60 deg metric 1 - 2mm pitch. They were "reasonable" price from memory but they are still brand spanking new! I use HSS for everything now ….. so far anyway.

Cheers.

Simon

Ben, you could do multi point screwcutting with the cutters from a Coventry die head like this one.

Phil
DSC_0739.JPG

Originally Posted by bwal74

Hi,

yep confused there

yep..me too

Hi,

I'm backing to banging my head any hard place I can find. I managed to do a couple of good threads on my lathe, then tried doing some on the Harrison at work (a couple of years old) won't go into the correct gear to cut threads. Going to have to get it serviced.

On my lathe, I wanted to cut a M12 x 1.75 thread for a bolt I need to make up. Forgot I was practicing cutting a 2.0 & 1.5 threads (you have to change your gears around on the hercus to cut different pitches every now and then). So I wasted a couple of blanks. Finally figured out what I was doing wrong and corrected myself. Did some cutting, more wasted blanks. Checked my inser, chipped my insert. And now I've used up my allocated shed time....

Breathing deep and taking panadol.

Ben.