Ferrous, facing, HSS tool geometry

[SillyOldDufferModerator @sillyoldduffer]

Posted by choochoo_baloo on 25/10/2021 02:16:43:

…

All of this, is to ask: is the generic HSS to blame (cheap bundle from RDG) ie should I upgrade to a better quality HSS blank? Admittedly I'd assumed that on hobby machines at least, HSS was much of a muchness.

…

The HSS might be to blame, but there's a long, long list of other possibilities. Rather than jump to conclusions about 'quality', I prefer to isolate causes methodically.

When things go wrong, it could be due to the Tool, Material or Operator. Perhaps all three.

Operators are major suspects. Beginners often have hazy ideas about tools and materials, and are quite likely to leap into the deep end by tackling jobs an expert would approach cautiously. They might believe a clapped-out second-hand lathe is in perfect working order or have stripped and re-assembled a good one incorrectly. Might have fitted a QCTP before confirming the lathe cuts OK with a plain tool-post, and not realised the QCTP is wonky. Possibly, the HSS got too hot during grinding and was cracked by cooling too abruptly in water.

Not all materials machine well. Some are easy, others all but impossible. It pays the beginner to start with straightforward materials, and tackle awkward customers later. Silver-steel is a little difficult.

Tool problems are another complication. Choochoo's HSS is only one part of the system. If everything else about the lathe set-up is OK, and the HSS fails again, then maybe the HSS is unsuitable. Or perhaps not: often assumed there are two sorts of HSS, 'quality' and 'rubbish'. Actually there are seven different types of HSS, and not understanding which is which opens the door to trouble. Though ordinary M2 cuts metal OK for me, it's not as suitable for lathe tooling as M42. Possibly Choochoo's HSS is for wood-working… )

Be nice if all my workshop problems were due to poor quality tools and materials. It's not true. Starting out as a self-taught learner I had failures galore, but get much better results today. No magic or special gear – the big difference is I've learned a lot! (And still have a long way to go.)

If I was Choochoo, I'd start by practising on leaded Mild-steel with the HSS tools mounted in a conventional tool-post. If there's trouble, check the lathe for untoward movements.

Having made sure the lathe is OK, grind several more HSS tools. Be careful not to overheat the tip – if HSS goes blue, that section is ruined and has to be ground off to get to a undamaged bit. Plunge cooling over hot HSS is likely to crack it.

If the suspect HSS cuts leaded mild-steel OK from the ordinary tool-post, try the Silver Steel. If silver-steel cuts OK switch to the QCTP and confirm that's good too. It will be necessary to experiment with feed-rate, RPM, and depth-of-cut for best results. Dump the HSS only when confident it's been nailed as the cause – guilty beyond reasonable doubt. The investigation has extra value if it flushes out other problems. Persist, and all will be well.

Dave

[Ramon WilsonParticipant @ramonwilson3]

Posted by SillyOldDuffer on 25/10/2021 11:50:31:

Having made sure the lathe is OK, grind several more HSS tools. Be careful not to overheat the tip – if HSS goes blue, that section is ruined and has to be ground off to get to a undamaged bit. Plunge cooling over hot HSS is likely to crack it.

Dave

Have to disagree with you there Dave – HSS will hold its hardness at much higher temperatures than that. Carbon steel yes but not HSS. Choo's example tool looks to me as not enough top rake, definitely not enough side rake and almost non existing front rake – for use on silver steel and similar that is (for some reason the image of that was not there before)

You can push HSS well beyond 'blue' before any degradation of the edge will occur – something that regularly happens when machining En24t for example.

As you say though – this can be a complex (to the beginner) but is actually a relatively simple skill to acquire – the main thing it takes is patience and experience, the latter only coming with time elapsed trying and practicing

Tug

[Howard LewisParticipant @howardlewis46836]

It is important that the tool has sufficient clearances. Tool, little and it rubs and overheats. Too much and the tool ,is weak and wears quickly, if it does not break.

Maybe the good surface finish resulted from too little clearance so that the tool rubbed and burnished the work, until the edge failed, giving the poor finish towards the centre.

5 – 10 degrees will suffice for most jobs, it is rarely necessary to be exact, unless dealing with something more exotic that mild steel.

Sometimes a lower speed and some lubrication will give an improved surface finish.

A steady, fairly slow feed rate will provide a good finish in most cases. Without a power cross feed, that entails learning how to use both hands to turn the handwheel mat a reasonably steady rate. (One of the first things that we were taught in the Training School ) No more than 0.004" (0.10 mm ) per rev is what to aim for.

For the next try, regrind the tool to give clearances, and try again, on mild steel.

HTH

Howard

If that works, then have a go at the silver steel.

[choochoo_balooParticipant @choochoo_baloo]

This is precisely what's keeping me going on this long path, Lee

Posted by Lee Rogers on 24/10/2021 15:59:33:

Above all enjoy the day when you turn out a perfect finish with a tool that you made .

[SillyOldDufferModerator @sillyoldduffer]

Posted by Ramon Wilson on 25/10/2021 12:05:14:

Posted by SillyOldDuffer on 25/10/2021 11:50:31:

Having made sure the lathe is OK, grind several more HSS tools. Be careful not to overheat the tip – if HSS goes blue, that section is ruined and has to be ground off to get to a undamaged bit. Plunge cooling over hot HSS is likely to crack it.

Dave

Have to disagree with you there Dave – HSS will hold its hardness at much higher temperatures than that. Carbon steel yes but not HSS. …

You can push HSS well beyond 'blue' before any degradation of the edge will occur – something that regularly happens when machining En24t for example.

…

Tug

I didn't explain at all well! Whether blue heat matters or not isn't clear cut, and I suggest there's a reason.

Blue indicates HSS got to within one or two hundred degrees C of the point where the steel is damaged and it's true that's not too hot. However, if an HSS blank is heated to blue heat by grinding there's a good chance the sharp tip got considerably hotter than the shank body. If so the edge and metal immediately behind it are done for.

I tend to give advice based on my own cack-handedness! Watching a guy sharpen HSS at an exhibition I noticed he was much faster and cooler than me. In comparison, it can take me several attempts to get the right shape on HSS and in the process I heat the blank up considerably. If I turn the end blue, there's a good chance it got too hot!

Chaps with a steady hand and good eye find grinding easy. It's beginners and clumsy SODs who mess up! The cure is practice, practice, practice…

Dave

[Martin KyteParticipant @martinkyte99762]

Well dressed wheels of the appropriate grit help enormously to keep the tool cool.

regards Martin

[John ReeseParticipant @johnreese12848]

I made up a couple tangential tool holders. The tool shank slopes 12* in both directions and the end of the bit is sharpened at about 30*. It gives an excellent finish either turning or facing.

[Howard LewisParticipant @howardlewis46836]

+1 for J R's Tangential Turning Tools recommendation.

You can choose the size of toolbit that best suits you and your machine.

If you decide to make one, making the shank of the tool holder trapezoidal is so much the easier way.

Make the slot for the toolbit first, then drill the hole for the clamp screw; and then put the angle on the top and bottom of the shank

Otherwise you have to mill at compound angles. Not impossible, but SO easy to go wrong .DON'T ask!

It is a good tool, easy to sharpen the toolbit, and will turn or face without any change.

Since you have to set the bit on centre height after every grin, life will be much easier if you make a Centre Height Gauge of some form.

(The one in my albums has to have two blades because of the parting and chamfering tools in in the rear toolpost.

For the Front post you only need one, and set the cutting edge to the underside of the blade.

Freshly ground, you can take minute cuts, of the order of 0.0005", and fine feed rate gives a good finish.

Howard

Edited By Howard Lewis on 28/10/2021 11:27:03

[VicParticipant @vic]

Posted by John Reese on 27/10/2021 22:25:06:

I made up a couple tangential tool holders. The tool shank slopes 12* in both directions and the end of the bit is sharpened at about 30*. It gives an excellent finish either turning or facing.

Yes, same here. Easy to make a cheap to use.

[VicParticipant @vic]

This thread reminds me, have any of you made a shear tool?

**LINK**

[bernard towersParticipant @bernardtowers37738]

Vic, I use them on my shaper but they are a revelation on a lathe turning 316 or 318

[Howard LewisParticipant @howardlewis46836]

I tried a shear tool, but my uncalibrated finger nail cannot distinguish between the surface finish produced and that from a Diamond tool at the same speeds and feed rates, with fine (0.001" ) cuts.

Quite impressive though!

Howard

[John BaronParticipant @johnbaron31275]

Hi Guys,

I use a "Shear" tool particularly when I want a good finish for a bearing fit !

The only problem is that you can only take about a thou DOC without destroying the tool edge. The swarf comes off like a very fine spider web.

[Author]

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