Mirror finish - what's going on?

Thank you all.

I have very poor spatial ability (seriously) and I struggle with grinding HSS because I cannot get my head around the geometry - one of the reasons that I tend to use carbide insert tooling. I know that you can buy HSS inserts in the US, but I haven't found anyone over here who can supply them.

I'll see if I have a diamond Dremmel wheel to trim the brazed carbide. Or simply whip out a better quality tool.

The deflection theory would explain why I got far better results with a deeper cut.

This stuff is so hard that I think I'll struggle to cut a thread in it with an HSS tool. I'm wondering if it might not be sensible to put the stock back on to my shelf, now that it looks so good, and see if the next "lucky dip" specimen might be a little easier to machine.

Kind wishes,

Nick
 
With a shallow cut in hard material, the tool oscillates up and down by maybe a thou as it loads up, cuts, then springs back (forward.)

With a deeper cut, the tool is cutting constantly, and remains bent at the same deflection for the whole cut.
 
Nick Hacking said:
Thank you all.

I have very poor spatial ability (seriously) and I struggle with grinding HSS because I cannot get my head around the geometry - one of the reasons that I tend to use carbide insert tooling. I know that you can buy HSS inserts in the US, but I haven't found anyone over here who can supply them.

I'll see if I have a diamond Dremmel wheel to trim the brazed carbide. Or simply whip out a better quality tool.

The deflection theory would explain why I got far better results with a deeper cut.

This stuff is so hard that I think I'll struggle to cut a thread in it with an HSS tool. I'm wondering if it might not be sensible to put the stock back on to my shelf, now that it looks so good, and see if the next "lucky dip" specimen might be a little easier to machine.

Kind wishes,

Nick
Click to expand...

Nick, if you can cut it with a carbide insert then you can thread it with a carbide insert. Many of the tool steels and higher carbon steels cut best with carbide tooling so why not go with that? The only kicker is that you have to know how to use the insert, meaning you have to know what the acceptable roughing and finishing depths of cut are and plan your cuts accordingly. This is not difficult to do but it is also not forgiving.

All insert manufacturers will tell you the recommended depth of cut for their inserts; typically, this will be a few thou over the size of the nose radius and in harder materials it is wise to heed their advice. Also pay close attention to their recommended speeds; hopefully, your lathe will be able to turn at that speed. Feeds are also recommended but that will depend greatly on the power and rigidity of your lathe. The best you can do is the best you can do so run as close to their speed as you can and play with feed until your results are acceptable; harder materials tend to like faster feeds to finish well. For roughing, try to stay close to their recommended depths of cut. For finishing, you can usually get away with a depth of cut of about 1/2 the nose radius. This depth will cut but it will not be the depth you dial in because there will be some deflection. However, it will be consistent. You need to take a cut, measure it, sort out what that dialed in depth gave you and record it, then modify it if needed. You should be able to modify it deeper but you probably won't be able to go shallower. In any case, once you sort out the minimum finishing and workable roughing depths of cut are, then you need to rough until you can dial in that finishing cut and you should be able to come in on size.

It is wise to keep the nose radius on your inserts on the small side when working with harder steels. The bigger the radius, the greater the deflection and the bigger your finishing depths of cut will have to be.

Again, if you can cut it then you can thread it with a carbide threading insert. Carmex makes them for a reasonable price and the maker is good about communication. Ask what they recommend re tool holders and inserts. I use the lay down style and it seems to work just fine.

HSS is the right stuff for many materials but not all. When the hardness and carbon content go up, switch to carbide. If you need some help learning to grind HSS tools, come on over to our Model Tools thread and we'll give you a hand.
 
Nick Hacking said:
I tried again at .002" - rubbish - and then .005" - perfection again.
Click to expand...

This is not unusual and not because of the steel being cut. It has more to do with how the tool is ground and how it is presented to the work.
It is important to understand the question of nose radius and lead angle. The above posts have very good info. There are some excellent guides out there also to help. For example:

Sandvik Coromant

www.sandvik.coromant.com www.sandvik.coromant.com

You did get a perfect finish at .005'' d.o.c. so you have a lot going for you. But getting to the final dimension with a good finish requires some experience with your equipment. Even then you might need to use a different tool for finishing than for roughing.
You do need to grind budget brazed carbide even when new.

You are off to a good start so hang in there. It is so sweet to eventually get on top of an operation after pushing through some frustration.
To allow us to work around your poor spacial ability could you explain for us what type of information means the most to you.
You can count on us to accompany you up the learning curve and cheer you on.
 
Janderso said:
In addition, those chinesium brazed inserts are not designed to cut right out of the box, they need to be ground for the application.
The geometry is usually off as well. Ask me how I know.
Click to expand...

I picked up one of those cheapo brazed carbide tool collections when I got the 7x16 benchtop mill. I thought they were just garbage - didn't realize they could be salvaged. For the curious, there's a thread here about doing so with diamond grinding discs from HF. I'm gonna give that a go, along with some of the green stones for the Dremel. Once I get the 7x16 rebuilt of course; it's been in pieces since Nov.
 
Nick Hacking said:
I have very poor spatial ability (seriously) and I struggle with grinding HSS because I cannot get my head around the geometry - one of the reasons that I tend to use carbide insert tooling.
Click to expand...

What I do to help with this is blue the tool bit, and scribe lines with an awl. There are probably thousands of drawings on the internet of "good" grinds for hss. Remove your spatial ability from the equation, and exercise your mimicry ability. We're primates...It what we are best at. Anyways, you can scribe lines on every side of the bit, according to the drawings you can find on the internet, and grind to the lines.
 
Not to mention jigs. All it takes is a flat piece of wood or weldable steel, with square-cross-section piece bolted to it at an angle to serve as a fence. Clamp that to the grinder toolrest. Make one for each angle that you will grind, mark them A, B, C, etc, then sharpie the toolbit (if it's large enough) so that side A is facing up when you use the A jig, B when you use the B, etc.

I admit I haven't made these for lathe tools yet, just a couple of simple ones for 90 etc, 45 deg etc. Got plans (well, intentions anyways - "plans" implies drawings) for a more sophisticated grinding toolrest for the belt-sander.
 
Thanks,

I'll try jigs, I think.

It's hard to describe what it's like to have poor spatial ability to a "normal" person: my wife has put up with me for more than 30 years and she still doesn't really "get it". I can cope with the idea of a 2-d shape having an angle on the tip; I can image stretching that to a 3-d shape; I can just about cope with the idea of adding a second angle in a different plane... after that I'm stumped.

Digression: when I was at university I had mycoplasma meningitis and was a bit off-colour for a while. The Dean allowed me to rejoin the course on the condition that I had an EEG and a batch of psychometric tests. The EEG was fine and I did pretty well with all of the cognitive skills except for spatial ability. IIRC I scored little more than 10% on the tests.

The Dean was all for throwing me out but, by good fortune, I'd sat a series of aptitude tests at school as part of a careers guidance exercise. All of my scores were respectable except for spatial awareness which was down at the "That bad?" end of the scale. I argued that they had admitted me with poor spatial ability, this wasn't new damage caused by the meningitis, they could hardly throw me out for being no-less capable than when they admitted me. Anyway, I was readmitted, I graduated eventually, and I've survived to late middle age somehow.

Another thing that I'm bad at is facial recognition. I can meet someone I saw a couple of weeks ago and be totally stumped as to who they are or what we said. If I wrote a letter about the first encounter , as I often do in my line of work, the letter brings it all flooding back and I'm fully up-to-speed.

Anyway: those of you who haven't fallen asleep on reading this might like to know that I got my threads cut in the end. I was wrong about the speed: I get about 20 rpm per Hz on my VFD which goes up to 60 Hz, so top speed is around 1,200 rpm. Obviously, I used a much lower speed for cutting the threads and found that back-gear is a pig to engage on my old Myford-Drummond.

I used an carbide insert on a threading tool, made a lot of passes at about .004" each, ran the thread through a 14 x 1.5 die at the end (HSS) - minimal effort required - and it's turned out really well. I have noticed that this steel simply will not cut on my bandsaw: I have to use either an angle grinder with a thin cutting blade or do it by hand and then throw away the hacksaw blade. I think it must be a type of chromoly. It does look very pretty, but I wish I'd chosen something a bit softer from my random pile.

I do appreciate all of the advice that I get on these fora (forums?) I am learning, slowly, but I'm not quite the useless numpty that I was when I first started.

Kind wishes,

Nick
 
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