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Machining what am I doing wrong?

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Stephen Rowley26/05/2011 11:27:14
57 forum posts
14 photos
Oops have I started something!
 
All is running well now thanks to all the good advice but, would it be a good idea to get a quick change tool post?
 
James
NJH26/05/2011 11:40:26
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2314 forum posts
139 photos
Hi James
 
In my experience - yes a very good idea. Not cheap but very useful.
 
regards
 
Norman
The Merry Miller26/05/2011 11:43:09
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484 forum posts
97 photos
If your getting on a bit and are running out of time then yes but if you are a youngster then whats the rush.
mgj26/05/2011 18:05:26
1017 forum posts
14 photos
My experience is mostly in 220M07/ EN1A and with that you can take shaving cuts with carbide as easily as with HSS. So you can on the Myford.I don't like to, but you can.
 
Commonly used inserts - no idea. Chronos were supplying tools with SECO inserts, which is how I met them. Greenwoods were supplying Sandvik tips, and DHS my local toolshop is a Sandvik agent, but gets me SECO. I also have Stellram non ISO tips. you can't tell hte difference between any of htem in free cutting mild, apart from hte Sanviks which don't like so much feed. (Taht doesn't apply to .8mmradius tips) Take your pick.
 
At the end of the day, the material has no idea wheter its being cut with carbide, HSS or a hacksaw. The difference is to do with rake angles.
Andrew Johnston26/05/2011 19:09:09
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7061 forum posts
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I suspect that EN1A is more forgiving than EN3 as regards finish versus speed, certainly within my limited experience. We'll find out when I do some tests this weekend.
 
Contrary to popular opinion I don't recall having a problem taking small depth of cut with carbide tools in various materials, including stainless, albeit free machining 303. I'll add a shallow depth of cut test to my trials at the weekend.
 
Regards,
 
Andrew
Steve Withnell26/05/2011 20:36:27
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858 forum posts
215 photos
For me, I prefer to use super sharp HSS for finishing cuts and Carbide for roughing cuts. The radius on standard tips is quite large and have been told it's useful to sharpen the tips on a green grit wheel for use on small lathes. I haven't tried this yet.
 
I have a QCTP on my small lathe and I'm a bit ambivalent to be honest, yes it makes tool changing faster, but it also reduces rigidity at the tool point due to increased leverage on the tool post. So these things are probably great on a "real lathe" rather than my chinese 5 x 22.
 
Call me old fashioned, but I got my bench grinder before I got a lathe and practised grinding HSS tool forms. I'm not sure how far you can go without this basic skill.
 
I'm trying(!) to make the crank for the Whittle V8 at the minute and given that the center main is just .180 inch wide, you have to start grinding a piece of HSS to get in and do the do.
 
I haven't found speeds and feeds in the books to be much use to be honest, other than as a starter for ten. I needed a super fine finish for a cast iron cylinder bore (my first model) and the quality advice from UKRME was super sharp HSS and as slow as possible (45rpm in my case), that gave a mirror like finish and it's certainly not whats in the published tables I have
 
The other thing I'd say as a novice with two models on the shelf now, is to stick to known spec, new materials. It sorts out one key variable which is the nature of the workpiece itself. I scrounged a length of steel round off my Dad, he'd found it in the scrap and I could make no sense of it at all on the machine. I even annealed the thing and it made no difference. Put it back in the scrap.
 
Jus' Sayin'
ady26/05/2011 21:36:02
612 forum posts
50 photos
Got shown the door at my local scrapyard today as I stood there with a big bar of steel in my paw.
They no longer let you root about or buy stock.
 
"health and safety" issues mean no more sales.
mgj26/05/2011 22:54:03
1017 forum posts
14 photos
There is a very good reason for that tip radius - its all about crack propagation from sharp corners in things subject to oscillating stresses like long thin crankshafts?
 
A radius also dramatically increases strength , in highly loaded shoulders like screws etc. You are talking of a factor of around x3 (don't quote me on that since memory fades, but it is by a masive margin.)
 
Why would one want a mirror finish in a cylinder bore? Seems a little unusual when its normal to go for a slightly rough finish to allow for bedding of the rings, a means of retaining oil and to ensure it glazes properly. Perhaps I have it wrong, but the normal way of finishing a cylinder is to go slightly undersize (by say .0002") and then fine hone to exact diameter - at least for ringed engines anyway. However, I am not party to the drawings, so I could well have it wrong.
 
Also all this super sharp HSS that I have been shown, and again I may have this wrong, has generally simply been high rake angles. You can have super sharp highly polished lower rake angles which also give a superb finish, but it doesn't look so sharp. The only advantage of a high rake angle generally is that a forwards vector is applied to the tool pulling it into the work. Too much is disastrous, but enough does one a favour by unloading the feed when taking big cuts.You don't need super sharp HSS when finishing - rather you want a precise nose radius. (see The model Engineers Handbook amongst others)
 
For example, fresh grind a tangential tool (relatively high rake) in its jig on a fine stone, and use it on free cutting mild, and the chances are you will get a finish like a badgers bum. Break the tip with a stone and apply a nose radius, and the finish should be superb. Start using it in tougher steels of course and one needs to reduce rake anyway, and a tool ground for such a purpose will look pretty blunt. It will also take more power to drive it, and it should give a good finish, - but it will appear pretty blunt.
 
Im afraid I rather disagree too about freehand grinding. I'm sure some can do it, and when one wants a say a form toool with a given radius and relief, can whip one up freehand on an offhand grinder, with all the angles correct and the whole cutting surface in one plane. I'm quite incapable (and the photos of those who say they are not are hardly impressive in that direction either) - so I use a free hand grinder for rough shaping only. After that it goes on the Quorn, and it comes out right first time, with a mirror ground finish, which is, in general repeated on the work.
 
I don't have a photo of a lathe tool, but there is one of a decently ground drill in my albums, just to make the point, though being a drill it wasn't finished on 100 fine grit finishing stone used for lathe tools.
 
So when it comes to sharp, and reliable shaving cuts, it comes down, so the experts like Tubal Cain et all say, down to correct rake angles and a high polish and nose radius on the tool.
 
Talking of which - were I cutting a .180" wide but  deep  slot which is what a main jounal is, I would simply use a 2mm wide tipped parting tool. They are so deep that one has enormous rigidity, and being in a single plane you don't have to worry about the holder hitting anything. There is a built in tip radius, and with the right tips, ( since there are roughing tips and fine finish parting tips, and of course it will all set up square very easily) it wil lcome out all shiny in one. And with that shallow rake angle and looking blunt, ones chances of a ruinous dig in are very small. 
 

Edited By mgj on 26/05/2011 23:01:14

Thor 🇳🇴27/05/2011 09:36:26
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1766 forum posts
46 photos
Hi James
I agree with Norman that a quick change toolpost is a very good idea. If you find it too expensive to buy one, you can make one yourself. Have a look at :
 
Thor
Richard Parsons27/05/2011 12:34:32
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645 forum posts
33 photos

JSK Hi


I have a rule of thumb which helps me to get the right speed for turning. The speed is given by the formula 288/D where D is the bar diameter in inches. If you want metric it is 7315/D (D is the diameter in Milimeters)


This formula is for mild steel. For harder steel you reduce the speed, softer materials I use a higher speed.


Try it and all the best.


Dick

chris stephens29/05/2011 00:21:54
1049 forum posts
1 photos
Hi Guys,
I thought I would add my 2 pounds worth, ain't inflation a bitch, for me there is a way to tell if your tools and lathe are working properly and that is to stop the traverse and not leave a witness mark on the surface of your work. Naturally I'm only referring to finishing cuts, not heavy roughing ones.
chriStephens
JDEng29/05/2011 10:45:28
27 forum posts
A lot of the problems with TCT tooling are due to a lack of rigidity in the work or machine set-up and also a lack of speed. If the set up is rigid, then poor finish can often be bettered by increasing the speed and feed - the exact opposite of HSS.
 
I think that there is a tendency to underestimate just how fast TCT needs to be worked to get a good finish, particularly on some of the smaller diameters. Sometimes machines are just not physically capable of going fast enough and it's actually more effective and not much slower to use HSS.
 
I can post some examples of how to work out speed calculations if anyone is interested.
 
John.
NJH29/05/2011 11:10:05
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2314 forum posts
139 photos
Yes please John
 
Regards
 
Norman
David Clark 129/05/2011 11:26:51
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3357 forum posts
112 photos
10 articles
Hi There
I accidently deleted a post on this thread.
Not sure whose it was.
 
regards david
 
Ian S C29/05/2011 11:55:53
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7468 forum posts
230 photos
Another thing that most of our lathes lack is HP. To get the speed, feed, and depth of cut often takes considerable power, a good way to finish off a carbide tip is to dig in and stall the lathe, or slip the belton a belt head lathe. Ian S C
Andrew Johnston29/05/2011 12:12:14
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7061 forum posts
719 photos
Posted by Ian S C on 29/05/2011 11:55:53:
Another thing that most of our lathes lack is HP. To get the speed, feed, and depth of cut often takes considerable power, a good way to finish off a carbide tip is to dig in and stall the lathe, or slip the belton a belt head lathe. Ian S C
 
I agree that a certain amount of horsepower is needed to achieve the higher speeds and feeds, but it is interesting to note that at the higher speeds and feeds the power needed to remove a given volume of metal per unit time is lower than at low speeds and feeds.
 
Regards,
 
Andrew
Steve Withnell29/05/2011 17:49:10
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858 forum posts
215 photos
I'm currently unwrapping a piece of EN8 to reveal the V8 crank that's in there somewhere...what I've noticed is that the finish with an HSS knife tool is noticeably better when traversing towards the chuck, than when traversing away (for the avoidance of doubt, I'm using matching LH and RH knife tools as appropriate to the direction of travel). I assumed this was minor differences in my hand ground tools. Having followed this thread, I decided to use my carbide equivalents (indexable type not brazed) and have the same result - better finish when machining towards the chuck and worse when machining away from it.
 
Is this just a signal to say I need to check the gib adjustment on the saddle? or is something else astray?
 
TIA
 
Steve
JDEng29/05/2011 19:11:32
27 forum posts
Posted by NJH on 29/05/2011 11:10:05:
Yes please John
 
Regards
 
Norman
 
 
Hi Norman;
 
Cutting speeds are usually expressed in either feet/minute or metres/minute. This is an indication of how fast the tool tip should move over the work piece and is dependant upon the material being machined as well as the material which the tool is made from. To convert the linear speed to RPM we use the following formula:
 
                                        RPM = Cutting Speed x 12
                                                      3.142 x diameter
 
where the diameter is expressed in inches and the cutting speed in feet/minute.
 
The formula is used for turning, milling or drilling however, if used for drills, the speed should be reduced to about 75% of the calculated figure.
 
Mild steel has a cutting speed of 100 feet/minute for HSS however, for TCT the speed is recommended as between 150 and 400 metres/minute (490 to 1310 feet/minute) - the lower figure for light roughing and the higher for general finishing. If we assume a piece of 2" dia bar then it will readily be seen that, using the formula, the RPM work out at 190 for HSS and between 935 and 2500 for TCT - a massive increase in speed.
 
For a piece of bar say 1/4" dia the speeds would be: 1527 for HSS and between 7485 and 20012 for TCT and this is where the problem lies. Most lathes cannot achieve such high speeds and therefore, in the smaller diameters, the finish suffers. In such a situation it makes more sense to use HSS.
 
TCT performs best when "worked" and that means using it as near as possible to the designed speeds and feeds and they are very fast. It is quite intimidating when you are not used to seeing the swarf coming off blue! Just as an example I was machining some EN24 yesterday, the bar was 2" diameter and I was running at 1850 RPM with a 0.010" feed and taking between 0.040" and 0.080" off the diameter on each cut (this was within the spec laid down for the tips); the finish, although slightly grooved because I was roughing the job for grinding, was like chrome.
 
The other things to be aware of are making sure that the whole set up is rigid with adequate support for both work and tooling. Coolant should be a flood or nothing; halfway house leads to premature failure due to stress cracks in the tool tip. I usually run it dry and then cool the job down with coolant before measuring if there is any concern over expansion.
 
TCT tips are very specific and it is important to select the correct grade for the work in hand and sometimes that means whether you are roughing or finishing as well as the type of material. Manufacturers do produce tables which detail the various feeds and depths of cut which specific tips cater for and again, it's important to work within these parameters if you want a good finish. With some tips it is not practical to take 0.005" finishing cuts; they are not designed to do it and the finish suffers.
 
TCT is a brilliant material but it is not a complete solution in itself and sometimes it is more practical and cost effective to use HSS.
 
I am not sure about the horsepower issue; I suspect that it would not be a problem provided the tips which are in use are designed for taking light cuts.
 
Hope this helps and I haven't gone on too much!
 
Regards,
 
John.
 
 

Edited By JDEng on 29/05/2011 19:12:38

Andrew Johnston29/05/2011 20:48:40
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7061 forum posts
719 photos
Hi Steve,
 
What type of lathe? Can you describe the differences in finish? For instance is the surface texture different over a small length, indicating that the cutting process is different, or is it inconsistent over larger lengths, indicating that the cutting process is flipping between different regimes?
 
It might be something to do with the headstock bearings being loaded differently?
 
Regards,
 
Andrew
NJH29/05/2011 21:00:19
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2314 forum posts
139 photos
Hi John
 
That's a very comprehensive posting. Thank you.
I have sent you a pm.
 
Rergards
 
Norman
 
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