Iain Downs | 06/02/2023 16:07:01 |
976 forum posts 805 photos | As an example of my issues, here are some pictures of a very fine tipped toll I ground, put to work to cut 3 grooves. Sadly I don't have any before pictures, but the profile was reasonably square from the top and of course there was a point from the side. The end flat was originally about 0.35mm - the images are considerably enlarged!
In this case this end is the opposite end from the pointy end of a parting off blade which I'm reasonably sure was sourced from ARC and thus should be of decent quality. All three cuts were in stainless. The first in 304 the last (when I noticed the quality is of uncertain consistency. I expect that the tip got to blue temperature at some point in the grinding process. Any idea what I can do to fix this? Many thanks
Iain |
SillyOldDuffer | 06/02/2023 16:18:26 |
10668 forum posts 2415 photos | Posted by Dave S on 06/02/2023 15:08:37:
... I have heard the micro crack theory, but not seen it happen. Dunking a piece of red hot silver steel to harden it doesn't tend to cause issues, so I don't see why a much cooler piece of tougher steel would fail. I suspect that the micro crack 'myth' (going to call it a myth for now) propagates from the cemented carbide world - where certainly older carbides were prone to premature failure if the coolant supply was intermittent. Dave Difficult to see microcracks because they're too small. The effect they have is rapid wear, not catastrophic failure. Plenty of cracking to be seen on TV's Forged In Fire programme. Not so sure overheating whilst grinding is a myth either. Although the body of an HSS blank is unlikely to get red hot, the point, edges and surfaces in contact with the grinder have much less volume and could easily get much hotter. Notice any sparks? The fine fragments of HSS cut off by a grinding wheel can get hot enough to catch fire. Overheating and microcracking probably only damage HSS near the edge at the hot end, so gently removing a couple of millimetres should get back to safe metal. Wear on cutters can't realistically be assessed in home workshops because the conditions are so variable. Noting tool life in hours at a fixed cutting rate isn't what we do, but when tested properly it's found that cool sharpened tools last longer than overheated and temperature shocked tools. Unlikely to matter in a home workshop, because our mixed work load makes it difficult to optimise tool life, and we re-sharpen as necessary. Nonetheless I feel it's hardly worth taking the risk of burning when backing off slightly and frequent dunking is so easy to do. Dave |
Howard Lewis | 07/02/2023 15:52:50 |
7227 forum posts 21 photos | IME , HSS tools do not need to ground absolutely accurately with regard to clearance angles , unless you nare machining something quite different from mild steel. I tend to use Zero top rake , especially for parting tools, to minimise the risk of dig ins, just front and side clearance of about 5 - 10 degrees. Avoid excessive angles, they weaken the tool and reduce the amount of metal to conduct away heat from the cutting edge.. You are not chasinbg seconds of arc; scarcely whole degrees even! If the toolbit gets too hot to hold, take a rest and let it cool. Even, make a holder to make holding the tool safer and easier. Quite easy, may need asome filing and and a tapping, but a useful accessory. Grinding a tool should not take hours. I am now fortunate to have made a cutter grindeer, so can grind very small amounts at a time, but usually it only takes a few minites from a blank toolbit to one ready for use. Off bhand grinding tends to be quicker and less maccurate, but stoll, does the job. Buy and study the WPS book on tool grinding, also L H Sparey, Ian Bradley or any of the other authors of books on basic lathe work (Stan Bray, Harold Hall, Dave Fenner, David Clarke - all former MEW editors, and the current one, Neil Wyatt. Get the basics right, a suitably ground tool, set at centre height and with the speeds and feeds, somewhere near right, things should work out OK. If the machine is not set up correctly, it doesn't matter how you grind the tool, the resu;ts will be less thatn good.. You can do an awful lot of work with a basic knife tool, possibly with a small radius on the cutting edge (But be careful that the radius extends far enough down that the tip contacts the work rather than rubbing below the tip. I tend to use a Tangential tool for a lot of work. That needs a holder / jig for grinding, and only one face needs to be ground.. HTH Howard |
Roger Woollett | 07/02/2023 17:34:52 |
148 forum posts 6 photos | You may be interested to know that SMEE do a one day course that covers this. The offhand grinder is covered with safety, choice of wheel and dressing to preserve the cutting surface discussed. Students then get to finish a knife tool having been shown how to get the desired clearance angles accurately. The next course is on the 23rd September. |
bernard towers | 07/02/2023 18:34:17 |
1221 forum posts 161 photos | The one thing I can recommend is that you have a suitable jig to hold the hss bit so when you return to your grinder (whatever sort) that the bit is at the same angles, meaning that touching up the edges does not mean large amounts have to removed. Especially important if you have hit on a set of angles that work for you. |
Andy Stopford | 07/02/2023 19:38:36 |
241 forum posts 35 photos | You can disprove the overheating myth and make holding small bits of HSS for grinding easier by brazing the HSS onto a mild steel shank, suitably sized to fit your toolpost. You can also file a seating on the shank for your toolbit to sit in for brazing, thus taking care of at least one of the rake angles. |
Andy Stopford | 07/02/2023 19:43:07 |
241 forum posts 35 photos | re. your photo, Iain, I would hazard a guess that the tool was allowed to rub on the stainless, which work hardened and wiped the tip off the tool. Try regrinding with no top rake and be reasonably aggressive in feeding it into the work - don't let it rub. |
Iain Downs | 08/02/2023 15:39:35 |
976 forum posts 805 photos | Thanks, Andy. I will be regrinding and will try various approaches on some test parts - rather than the tinal item. As far as agressive - the top is 0.3mm and I need to cut down to 0.3mm. I probably won't manage that agressively! Do you think I should be running as fast as possible (for this, we have a 1.8mm dia target), or run slower and more controlled?
Iain |
Andy Stopford | 08/02/2023 19:19:18 |
241 forum posts 35 photos | I'd try as fast as possible since the diameter is small (so you need a high speed to get a decent surface speed), and it'll be easier to maintain a consistent feed per rev, keeping it cutting but not causing it to dig in and break. You may get through a few test parts/regrindings! |
Dave S | 08/02/2023 20:40:17 |
433 forum posts 95 photos | Iain, you might do better brazing a small carbide tip to a piece of steel. I have an old circular saw blade I use for this sort of thing. It’s bstard tough steel to cut mind… The tips can be sharpened using one of the now ubiquitous diamond plates. Dave |
Iain Downs | 09/02/2023 10:14:17 |
976 forum posts 805 photos | Thanks Andy and Steve. I will probably have another go with an HSS tip and if that crashes and burns (probably literally), I will try and repurpose one of the carbide tipped tools I bought at the start of my journey and never really used (it was years before I found they are provided blunt!).
Iain |
Hopper | 09/02/2023 10:41:17 |
![]() 7881 forum posts 397 photos | I remember sometime the past five years or so there was a two-part article about HSS in MEW written by a metallurgist. His advice was to never dunk your HSS in water when sharpening it but instead put the toolbit down and let it cool off slowly in the air. He was quite adamant about that. On the shop floor of course, there is not time to stand around for five or ten minutes waiting for the tool bit to cool down every time it gets a bit warm to hold. So the workaround is to keep dunking the HSS toolbit in water frequently, BEFORE it starts to turn colour on the edges, in this way stopping the HSS from getting hot in the first place and doing away with the need to let it stand and cool off in the air for extended periods. I don't remember if the MEW metallurgist's concern was micro-cracking or heat treatment issues, but the don't-let-it-get- too-hot-and-dunk-it-in-water regime is in line with what I was taught as an apprentice both on the shop floor by crusty old craftsmen who could machine new hinges for the gates of Hell if need be, and at tech college out of a book written by experts in classes taught by crusty old craftsmen who had gone on to get engineering degrees. Both agreed on dunking the HSS in water frequently before it turned colour on the edges so it never got overly hot. Edited By Hopper on 09/02/2023 10:43:21 Edited By Hopper on 09/02/2023 10:47:21 |
samuel heywood | 09/02/2023 21:31:00 |
125 forum posts 14 photos | Appreciate all the input. eg: Use something to hold the toolbit~ certainly got got hot fingers a few times! Also, not to dunk the tool bit, but let it air cool. Have been dunking in water thus far. Still would like someone to chip in with roughly how long it should take to grind an average 3/8" toolbit? Because it does seem to take ages. |
Grindstone Cowboy | 09/02/2023 21:43:51 |
1160 forum posts 73 photos | For me, a little under ten minutes, depending how much care you are taking to get specific angles, maybe fifteen if you include honing with an oilstone or diamond plate. And I'd consider myself slow. Rob |
samuel heywood | 09/02/2023 22:14:58 |
125 forum posts 14 photos | Posted by Grindstone Cowboy on 09/02/2023 21:43:51:
For me, a little under ten minutes, depending how much care you are taking to get specific angles, maybe fifteen if you include honing with an oilstone or diamond plate. And I'd consider myself slow. Rob Thanks~ Definitely must be doing something wrong then! Wheels are rebalanced & dressed, so no idea as to what though. |
Hopper | 09/02/2023 22:49:40 |
![]() 7881 forum posts 397 photos | Posted by samuel heywood on 09/02/2023 21:31:00:
Appreciate all the input. eg: Use something to hold the toolbit~ certainly got got hot fingers a few times! Also, not to dunk the tool bit, but let it air cool. Have been dunking in water thus far. Still would like someone to chip in with roughly how long it should take to grind an average 3/8" toolbit? Because it does seem to take ages. You seem to have missed the point. Dunk the tool bit in water to cool it off. Just don't let the tool bit get hot enough to turn blue before you dunk it. So frequent dunkings and keep the tool bit cool. That is the quickest way to do it, the way it is done on professional shop floors. If you want confirmation of this common and well proven method, see LH Sparey's book The Amateur's Lathe, page 75. How long should it take to grind a 3/8" tool bit? For a standard turning tool shape such as a knife tool, five minutes or so. Maybe 10 at the most. The end of the blank comes already angled, so you only need to about 15 degrees angle to the top and leading side surfaces, plus about 10 degrees clearance on the end surface. It should not take too long at all. This is assuming a six inch bench grinder with 1" wide wheels, well dressed and a good tin of water for frequent dunking. Rough it out on the coarse wheel first then finish on the fine wheel. Then spend a few extra minutes rubbing it on a bench oil stone to get a nice smooth cutting edge and to put a small radius on the tip.
|
Hopper | 09/02/2023 22:59:53 |
![]() 7881 forum posts 397 photos | If it is still taking you forever to grind a tool bit, it may be substandard cheap grinding wheels. They are about on budget grinders. Or you may have lucked out and got a piece of extremely high quality HSS with extra additives. I have some old pieces of Swedish HSS that take forever to grind, like trying to grind hardened glass or something. But once done they hold an edge beautifully. Try different bits of HSS and if that does not help, try some new good quality grinding wheels. But it is much much quicker to use 1/4" HSS and sit it on a piece of 1/8' packing in the tool holder. That is what I have gone over to doing on my Myford when I use HSS, purely because of the quicker sharpening. |
Hopper | 09/02/2023 23:02:51 |
![]() 7881 forum posts 397 photos | Posted by Andy Stopford on 07/02/2023 19:38:36:
You can disprove the overheating myth and make holding small bits of HSS for grinding easier by brazing the HSS onto a mild steel shank, suitably sized to fit your toolpost. You can also file a seating on the shank for your toolbit to sit in for brazing, thus taking care of at least one of the rake angles. It;s not the overheating that is the problem, it is the dunking it in cold water while it is overheated that causes microcracks and a brittle cutting edge that does not last well. Braze away and let it cool naturally and all will be well. |
Ramon Wilson | 09/02/2023 23:09:04 |
![]() 1655 forum posts 617 photos | Just read this through - a few thoughts to go with the others. I've passed this on before I think - I've found the best thing for holding tool bits to grind is a hole that will accept the diagonal dimension across the tool bit drilled in the end of a short length of round bar. A simple grub screw in the side will hold the bit secure for grinding, the round bar giving a good hand grip and allowing infinite angular positioning on all faces. I've confessed before that when it comes to tool grinding I'm very tardy once the tool has had it's major shaping but after a life time of not doing anything but cooling the bit in water I do not share the fear of damaging the tool that some have implied. Just simply never had an issue with it and I've seen a few glow hot a few times for sure. An HSS tool will colour, as any steel, if too hot but, unlike carbon steel tools, this will not affect the cutting edge performance. Try this to see - grind a piece of HSS until you do get it blue then cool. Very lightly regrind the faces to remove any trace of blueing. The tool will still cut perfectly well. Do the same with one of carbon steel and you will soon see the difference as the edge deforms under cutting pressure. As has already been stated HSS will still cut at near red heat though obviously not for long. For what we do in the home workshop and what we used to do at work you'd have to work really hard to spoil HSS hardness - it's hardening temperature is way in excess of carbon steel - it comes out of the oven quite yellow at the correct temperature. I rough grind tools on an 8" grinder but finish grind them on a small 5" one. There is no need to grind text book angles - just near enough will do - remember it's only the part that's at the depth of cut that does the work so that can be tweaked to get optimum performance - the rest of the tool behind is just there to hold it in position!
Best - Tug |
Mick B1 | 10/02/2023 09:52:32 |
2444 forum posts 139 photos | There'll be loads of other way of doing this in the picture, but the value of learning the bench grinder/HSS tool blank game is that you only have to visualise the tool you want - and you can have it ! 'Course I missed a trick there - should've made it a bit wider and ground the 60° back chamfer on the M6 flank... |
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