I have carbide inserts fror thread cutting and hand wind the spindle on my Warco WMT300/1. Feed in 0.025mm every cut. Works on steel, acrylic and aluminium. Thread like this with the work smeared in Rocol RTD cutting paste. No complaints so far from the tip or work.

Regards

Richard.

I use HSS for screwcutting, and much else. Carbide seems like a waste of money for a lot of work, fine if you want big cuts at high speed, but then you need lots of power and rigidity, which most hobby lathes don't have.

Even using HSS I wouldn't take anything lie 20 passes to cut 1/2" BSF, it's only 60 thou deep

Full form carbide thread cutting inserts are just great. If turning the lathe by hand, like some do, it is essential, that at no point do you move it backwards while it is engaged in cutting the thread. I just have a small run out area and use now use a micro switch on the VFD to stop the spindle when the tool is in the relief area. I also cut the area to be threaded about 0.1mm bigger on diameter. Us a marker pen for colour. I keep cutting until the marker pen is gone, measure the OD of the threaded area, eg 10.05mm and then make another pass at 0.1mm so it makes the OD of the form 9.95mm for a M10X1.5 thread. Works perfect every time. On threads smaller than 6mm I use -0.03mm and on threads smaller than 3mm I make them -0.01mm, so M2 and M2.5 I make them -0.01mm , for M3 -0.02mm , M4-M6 -0.03mm, above M6 , make -0.05mm , all are on diameter references. I buy the full form ground inserts for home. They will cut as hard as 58Rc and will cut all materials I need to cut, The inserts I use for threading Titanium, I only use on Titanium and nothing else. Thread cutting at 90 to 300 rpm is just fine, on hobby lathes. Use a good cutting fluid suitable for the material you are cutting also makes a difference. I either cut slow and dry , or with a flood of fluid to wash away the chips and it gives a good thread form finish. There are charts for carbide inserts for the number of passes in thread cutting. It is like about 4 to 6 passes, depending on the thread depth and pitch etc.

There's no compulsion to use carbide at high speed if the circumstances prevent it. Thread as slow as you like, but beware you might suffer broken edges on the tip of the insert especially if you stop the spindle with the tool in the cut or don't have a groove to run into. I've been cutting 1.5mm threads today at 200rpm with a hand-ground carbide tool (ground from a busted end mill) and I've had to re-grind it several times after breaking the edge. HSS wouldn't stand up to it at any speed in this material. The task doesn't allow for a groove so I've been manually retracting the cross-slide and a couple of times I've missed-timed it then off came the tip.

I have many HSS bits ground for screw cutting. Some are 60* V and some are 29* Acme in several pitches. I also have holders for on edge carbide inserts as well holders for lay down inserts. I use the on edge inserts the most for external threads. It is a matter of convenience. I use the lay down inserts for internal threading if the hole size is large enough. I have form ground small boring bars for threading smaller holes. There are some instances where I have to go back to HSS . Examples are 3/4-4 Acme internal threads or 1 1/2-8 and 1 1/4-7 internal V threads.

Running in reverse is OK, as long as the chuck is not threaded on the spindle.

Vargus GENius software is available [ free ] for either thread turning or thread milling. Tells you all you need to know about your chosen profile. You can specify the number of passes you want to make and it gives you all the depths ect Choice of Flank infeed or Radial infeed, cutting speed, rpm, ect ect. Although it is aimed at CNC it is so versatile it can be used for manual machines. I use it constantly, wouldn't be without it.

Sandvik recommend using a normal full form tip then finishing with threading insert. This reduces the chance of chipping the tip, alternative is to take multiple small depth cuts. Initial material diameter should be a thou or so over size to finish the full form.

The problem you have is that the thread needs to be concentric to the taper seat in the head insert. If you make your heads, bore the inner diameter for the thread form, and turn the taper seat angle as well. Then use a small threading tool to screw cut the thread form. When it is mostly thread cut, you can use a tap to act as a thread chaser. If the tap is run just straight down the drilled hole, often the tap does not start concentric to the hole. I also found, that if the taper seat angle is made a very small amount, tighter on the front, over the nominal angle. I make the Nelson taper seat 34.75 deg instead of 35 deg. With the Turbo plugs, I make the taper 60.5 deg included angle. With bearing blue, the plugs will seal on the very front of the taper seat. Some glow plugs the seat is not concentric to the thread form. Those plugs will always have sealing issues. You can also correct a head, by making a precision thread form and use that to get the head insert concentric to the thread. Then rebore the taper seat.

SInce setting up the myford S7 with a VFD and a micro switch to stop the spindle, it has made doing internal threading quite easy, with confidence to internal thread cut.

A friend of mine used to work at the local Dormer factory but then it closed down. Anything that was not new, unused and boxed was thrown into a skip!

My friend rescued some master threadcutting equipment that had been used many years before to manufacture tap & dies and he gave them to me.

The 3 tool holders, including the round home made one, hold the 'inserts' which have a common spigot but different forms on the cutting end.

The boxes of chasers include UNF, MM, light bulb threads, bottle threads and something called GKN threads, which seem to be Whitworth form.

I don't use them very often but they are VERY useful at times. I dread to think how much they cost to make but they cannot have been cheap.

Brian

So I have an album here, https://www.model-engineer.co.uk/albums/member_album.asp?a=48822

I have the microswitch on S1 one connection. So when the switch (normal closed) is opened, it stops only the forward direction of the motor. Reverse direction still works . I set the VFD to stop in about 0.3 seconds I think, maybe 0.1 seconds. When the switch opens, the motor stops, then I switch to reverse, wind back.and can take another cut. The microswitch is set on a movable stop to set it in the Z distance.

Will add more to the album when I thread cut next.

Neil

Edited By Neil Lickfold on 29/07/2018 12:47:03