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  1. #1
    Join Date
    Jul 2008
    Location
    Bondoola
    Age
    81
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    182

    Default HSS Tool Angles, - Angles? What angles??

    HSS tool Angles, - Angles? What Angles!

    One of the most difficult parts of a lathe tool to understand, or any other cutting tool, are the clearance angles and the rake angles. To many people, this is where the black arts of machining hide. To understand them is very easy and requires only elementary geometry. So let’s jump right in at the deep end shall we? Everything you have read in books or charts regarding the “best angles” for tools, FORGET IT. (“Oooh, Joe is sure sticking his neck out” says Kody). At this point, let me say that if you are looking for tools for high production, this may not be for you. If you want info on the “how and why” as a beginner or newbie, keep on reading.

    To help understand these black arts, we will be looking at a HSS lathe tool that’s mounted directly in the tool post. The top of the HSS is parallel to the top surface of the Cross-Slide. (C/Slide). At this point, the HSS has not been ground in any way, shape or form. It is a brand new piece straight from the packet. We will make a slight change to the end of the HSS for this exercise. When bought, a new “stick” of HSS has the end shaped to an angle. This angle is ground off and the end is square. The HSS tool bit used in this example is 5/16” square. The top of the tool is exactly at the center height of the spindle, or, as we say, “it’s on center”. When we have finished grinding and shaping the tool, it will be known as a “Right Hand Knife” tool.

    If we present the tool to a rotating job (lets assume it’s a short MS bar, about 40 or 50mm in dia.) the tool will of course simply rub on the surface of the rotating bar. With the end of the tool exactly square, the tool cannot penetrate into the bar. However, if the tool were lowered about 1mm below the center height, the tool would actually begin to cut in some fashion if pushed directly into the steel. By lowering the tool, the HSS now has a small amount of clearance. This ability is due to the circular nature of the bar. It’s not much, but it’s enough to show that very little “Front Clearance” is needed for the tool to begin cutting. This is usually the first clearance angle most people grind on the lathe tool. Most beginners to machining (and many more skilled) grind far too much “Front Clearance” on the end of the tool. Remember, the tool is horizontal in the tool post for this exercise. Only about 3 or 4 degrees of Front Clearance is required, not the 15 to 20 that I have seen so often. You could put this tool in the tool post and it would cut quite nicely if it were plunged straight in if positioned about 3mm in from the end, like as if you were going to face off (machine) the end of the bar. The lathe tool would start to complain because there is another clearance required to be formed. This is called “Side Clearance” and is formed on the left side of the tool. Again, most people grind this to an angle of 15 degs. or even more (as a beginner). All that is required is just enough for the tool to penetrate sideways. However, with NO side clearance angle, the tool will face off the bar if it is moved away from 90 degs. to the axis of the lathe. Yes, it will cut but some side clearance is needed. The amount of side clearance needed is still only about 3 or 4 degs. To produce a good finish and increase the “life” of the tool, the point that is cutting must be rounded off to a small radius, about 0.5mm is a good start.

    To machine our MS bar, two more angles are needed. Looking directly down on the tool, the front is dragging along the machined surface. The tool must now be ground so the right hand corner is cleared away from the job. This angle is called “Front Relief”. It is very important to shape this angle to suit the material and the finish required. It also denotes how long the tool will continue to cut even though it only forms clearance and has almost nothing to do with the actual cutting. To obtain an increase in tool “life”, i.e., a longer time span between resharpening, a cutting tool needs as much bulk as possible. This bulk acts as a HEATSINK. The worst enemy of any cutting tool, is heat. By having too much front clearance, too much side clearance and front relief, the heat build-up will destroy the life of the tool very quickly. The steep angles also weaken the cutting edge and the tool will break down very quickly. The amount of front relief also has an enormous bearing on the quality of the finish of the machined surface. I will describe more about this another day.

    We now have the front clearance, side clearance, radius on the cutting point and the front relief ground to about 8 degs. This tool is perfect for machining Brass. No other angles are required and must never be ground on the tool for machining brass. With extra angles so formed on the TOP of the tool, the tool will “dig in” with such force you can be severely injured (as the job is ripped out of the chuck!) or the lathe can be badly damaged.

    The MS bar can now be machined down to any diameter with this tool. However, steel wont cut easily without at least one more essential angle. This angle is called “Side Rake”. This angle is perhaps the most misunderstood angle of them all. The top of the tool is now ground to form a shallow angled slope from the cutting edge on the left side across and down to the right hand side. This angle, more than any other, affects the efficiency of the tools cutting action. If the angle is too little, the motor has to work harder to cut the steel. If the angle is too steep, the tool will break down much faster than it should. A good starting point is to grind the angle to about 4 degrees. This angle will vary when cutting very ductile metals like aluminium, copper, leaded steel etc. If we were to do all our machining only on the diameter, no “Top Rake” would be needed. This is because all the cutting is being performed on the side edge of the tool. Top rake is generally needed only when facing off the steel bar, i.e. machining the end of the bar, cutting from the outside of the bar to the center. The first problem usually encountered is, “how do we measure such a small angle?”

    It was simple really; I drew the tool with the CAD program and measured the angles and edges that would be ground away. The drawing will give you all the info. The dimensions show you how much to grind away at the edge to produce the desired angle. Keep in mind that these clearance angles are the absolute minimum required. I only suggest these angles to give you some idea of how little is actually needed to make a highly efficient cutting tool. Depending on what kind of metal you are machining, you will find that increasing some angles will make a big difference but it will come at a cost. The greater the rake angles the easier the tool will cut but it will break down or “burn out” very quickly. The “rule of thumb” is, keep the clearance angles as small as possible and adjust the rake angle a little at a time. When the tool faces are ground, the faces are actually hollow ground. It is very important to grind each face so it has just one facet. (Ooooh, Kody, that’s hard!) The most common fault is grinding the faces so they look like the facets on a diamond. It is because of this fault that the tools don’t work as they should do. It’s easy to correct with the right technique. It’s called practise and more practise!

    If you are using a standard type of angled tool holder, you don’t need to add “Top Rake” by grinding. The upward slope of the tool in the tool holder forms the required Top Rake. You need to know the angle of this slope that is built into the tool holder. If the angle is, say, 8 degrees, you need to grind this angle on the front of the HSS tool PLUS the required front clearance angle. This is where most errors are made. Many beginners grind far too much angle on the front without realizing just how much clearance angle is needed. Top rake is only ground into the tool bit when the HSS is clamped horizontally in the tool post. However, grinding top rake into the tool will weaken it to the point that the tool snaps.

    DANGER – DANGER
    There is great danger in using HSS and working it so hard that the tool snaps and /or shatters. The small fragments instantly become like shrapnel from a grenade. These small pieces will penetrate the body like a bullet and go in just as deeply. A friend of mine dropped a “Shell Milling” cutter and a sheared flute sliced thru his ankle, severing the Achilles tendon. He had no idea it had happened until he tried to take a step and crashed down onto the floor. Make sure you wear full and proper safety glasses and gear when working and NEVER hit the end of a stick of HSS with a hammer

    After the final shaping and grinding, the cutting edges should be honed. This is best done with a fine diamond hone and WD40 or similar. The honing produces razor sharp cutting edges and enables the tool to machine beautiful surface finishes. As this post is now extra long, I will discuss the art of honing later.
    I hope this info will help all the beginners and even some of the more experienced to understand the art of grinding HSS.


    The above drawing is of a 5/16 inch square piece of HSS. The amount to remove is shown in inches at the top of the drawing. This is where a little makes a big difference in the result of grinding. All dimensions are in inches. I hope you are able to understand the drawing.

    Kody

  2. #2
    Join Date
    Sep 2008
    Location
    Adelaide
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    66
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    151

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    Wow Kody Thanx for the info, I am getting there with the grinding of hss but like you say it is practice. Sometimes I grind a bit and it cuts good and leaves a fine finish other times the exact opposite. I look forward to the honing bit as it is something I havent done yet. I think I need to buy a diamond hone (dam more gear) 1 thing you never mentioned is what grit I should use on the grinding wheel? Watching the teachers at trade school sharpen bits shows me that practise makes perfect as they just go up to the grinding wheel and viola a perfectly ground bit emerges. I am sure if I got a protractor out the angles would be exact.
    Cheers
    Gene

    Holden Hill Crash Repairs
    607 North East Road
    Gilles Plains South Australia 5086
    (08) 8261-3979
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