Thread: HSS Tools, using and shaping them

With so many references to HSS lathe tools, I thought I should offer some of my own knowledge about them. The following info applies to high quality HSS. High quality is usually very shiny and smooth in apearance. Some very cheap and nasty stuff I saw was encrusted with a thin layer of scale, dont bother with any cheap stuff for use as tools.

Contary to popular opinion, HSS is undamaged by heating to a blue colour or even higher. I have often had the material glowing red as I ground it on a standard bench grinder. The important thing to not do, when it is red hot, is to quench it in water. Just let it cool out sitting on the bench if the bench is made of wood. Be careful the hot tool doesn't ignite the bench top. You can water quench the steel if it is at a blue heat. This wont affect the temper at all. The danger of quenching when red hot or just under red heat, is inducing cracks into the steel. These cracks may not show up untill you use the tool or resharpen the tool on a grinder. If it is cracked, there is a different "grinding sound" that will be heard and a piece often falls/flies off. I usually shape (grind) the tool required and then sit it on the vice to cool down from a blue heat. The vice acts as a heat sink. When it's cold, I then finish grinding it to size/shape and then hone the cutting edges. I had to make a thread cutting tool to screwcut the nut for a part of my 5" gauge Simplex loco. The nut had a 1/4" Acme thread and was 1/2" long. The tool was made from a small piece of HSS that I silver soldered to the end of a 3/32 hex key. It was then ground to shape and set in the lathe. The silver soldering had no affect on the properties of the HSS. and it performed perfectly in cutting the (Left Hand) Acme thread.

Cutting Speeds
I hope I dont walk on anyone's toes here for I think I need to clarify someone's posted info about HSS cutting speeds. The statement was, "HSS will cut at any speed". This is true but only when kept within the peripheral cutting speed for the material as applied to the use of HSS. You can machine mild steel in the lathe at a speed of <10 feet per minute with no problem. A Carbide inserted point will break up if used at such a low speed. The difference between the carbide and HSS is that the Carbide "fractures" the steel when cutting whereas the HSS actually "cuts" the steel as it is working. As a general guide, the following is a recognised general standard for using HSS of high quality. The speed is given in "feet per minute". You may convert to metres if needed.
Mild Steel, 100 for finishing, 80 for roughing, (100/80)
HT steels (like 4320) <80 / 55 or less.
Aluminium, 300 / 250
Brass, 200 / 150
Bronze, 150 /100
The speeds are flexible and success depends on the "stiffness" of the lathe, the physical size and shape of the job, the shape and finish of the tool, the feed rate and the coolant being use if any. A "Free-cutting" leaded steel can be machined at 200/150 with ease.
Carbide is extremely brittle and the cutting edge is therefore modified to increase its resistance to fracturing. This is achieved by forming a small radius on the actual cutting edge and also by using negatve rake angles. Some carbide tips do have a razor sharp edge but this is used for special machining sequences and great care must be taken not to damage the cutting edge. It is this small radius on the cutting edge that makes carbide inserts difficult to use in the home workshop (lathe is usually not powerful enough or robust enough) as they break down very quickley. Carbide requires a heavy force to feed the tool into the workpiece compared to the pressure needed for HSS. If your lathe has the power to do it, you can take the same depth cut and feed with HSS as you can with a carbide tip but not at the same speed.

Parting-off Tools
Parting tools are the home engineers worst enemy but it is very easy to have 100% success all the time. The bigest mistake most people make is to have far too much front clearance. Only about 1 degree front clearance is needed. If you have already ground in 10 degs clearance, only grind the very edge back to form the clearance. You will finish up with an angled flat on the end about 1 1/2 mm deep/wide and this will work quite ok. You will know if there is insufficient clearance because the tool wont penetrate. If the clearance is ok at 1 degree, check the center height of the tool. Because the clearance is so close to zero, a tool just a little too high will simply rub and not cut. If your lathe has the ability or can be addapted to do it, position the parting off tool at the rear of the cross-slide. It will of course be mounted upside down but its shape will not change. My Myford has "T" slots across the cross-slide to bolt down the inverted parting tool and if you can do the same, do it at any cost. The advantages of the rear Parting tool are, the swarf falls away from the cut. You can see what is happening and the coolant can be flooded onto the tool. You can also part off at about 75% of the normal machining speed without chatter!
The chatter is caused by the dovetails of the cross-slide being too loose. Adjusting the slide makes it hard to move and creates even more wear and tear. By inverting the tool, the upward force on the tool, actually lifts the cross-slide up and the dovetails of the cross-slide are pulled firmly together. The result is, no chatter and higher speeds can be obtained. I can part off 25mm dia mild steel at 400 rpm with no chatter using the inverted parting tool. The cut is made within 25mm of the chuck or the tailstock when a center is used or if a three point steady is used. I made the parting tool from 1/2" square HSS and ground the width of the blade to 1/8" (3mm). Only a minimum of top rake is used, just enough to get the swarf to coil into a tight "disk" that looks like a solid disk, 1/8" thick. The coil will develop to a dia of 1 1/4" with no jambing. It is important to run the feed by hand. This way, any jambing or foul-ups can be released instantly. It was mentioned that chatter marks can be removed by switching off the motor and as the lathe slows down, the feed can be controled to cut the chatter marks away. This is a great technique but it may need the chuck to be rotated for the last few turns by hand. Having almost zero front clearance helps a lot in this situation. I always shape the cutting edge to be at 90 degs to the side of the tool, ie, straight across. Hope this helps you to understand the hows' and whys' of HSS.

Kody