Machining what am I doing wrong?

[Anonymous]

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 WithnellParticipant @stevewithnell34426]

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’

[adyParticipant @ady]

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.

[mgjParticipant @mgj]

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 🇳🇴Participant @thor]

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 :

http://home.kleppnett.no/thorhansen/m/Emco8QCTP.pdf

 

Thor

[Richard ParsonsParticipant @richardparsons61721]

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 stephensParticipant @chrisstephens63393]

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

[JDEngParticipant @jdeng]

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.

[NJHParticipant @njh]

Yes please John

 

Regards

 

Norman

[David Clark 13Participant @davidclark13]

Hi There

I accidently deleted a post on this thread.

Not sure whose it was.

 

regards david

 

[Ian S CParticipant @iansc]

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

[Anonymous]

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 WithnellParticipant @stevewithnell34426]

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

[JDEngParticipant @jdeng]

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

[Anonymous]

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

[NJHParticipant @njh]

Hi John

 

That’s a very comprehensive posting. Thank you.

I have sent you a pm.

 

Rergards

 

Norman

 

[Martin WParticipant @martinw]

Hi

 

The formula given by John can be simplified by approximation with the value of pi taken as 3. The formula then is

 

RPM = Cutting Speed (FPM) x 4

                     Dia in inches

 

or for metric diameters it can be approximated by:

 

RPM = Cutting Speed (FPM) x 100

                          Dia in mm

 

 

Admittedly the second formula is a mix of imperial and metric but still can prove useful at times.

 

Cheers

 

martin

Edited By Martin W on 29/05/2011 23:50:03

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

Before any newbie gets too carried away with getting the speed exactly right, according to the charts, they are the speeds that you can work up to for maximum production rates in industry. Most of us don’t need to go anywhere near as fast in our home shops environments.

Again for newbies, forget the speeds in the charts for drilling especially when drilling in the lathe, slow right down and give the drill time to do its job. Drilling too fast just makes things hot. Remember that, “back in the good old days” drills used to work even when the only power input was you winding a handle. Oh alright, that way might be a little too slow, but the principle still holds.

chriStephens

[Martin WParticipant @martinw]

Hi Chris

 

I totally agree with your comments re cutting speeds and getting too involved with the published data.

 

For me the ‘Listen & Look’ approach works very well.

 

Listen to how the machine sounds when its cutting. Is it loading the motor significantly or is there tool squeal or any other strange noise.

 

Look at the finish on the workpiece for roughness or any other finished surface problems/defects.

 

If you are hearing/seeing problems then change things like the spindle speed, depth of cut or tool feed rate until things settle down. This assumes that sharp tools are being used and they are aligned correctly

 

If you’re getting a good finish and it sounds smooth when you’re cutting then there is a fair chance that is what suits your set up best or is pretty close to being right.

 

I have, on my small lathe and mill, variable speed control which I find very useful as I can change the spindle speed easily which often overcomes the problem. I use it when parting off and slowly increase the spindle speed as the tool cuts deeper into the workpiece and the cut diameter is reduced.

 

Just a few jumbled thoughts that might help.

 

Cheers

 

Martin

[chris stephensParticipant @chrisstephens63393]

Hi Martin,

Your suggestions have merit.

What I think you are describing is a sense of “feel”, and is this not what differentiates skilled workers from rank beginners, no matter how many books they have read.

chriStephens

[JDEngParticipant @jdeng]

Posted by chris stephens on 30/05/2011 01:02:21:

Hi Guys,

Before any newbie gets too carried away with getting the speed exactly right, according to the charts, they are the speeds that you can work up to for maximum production rates in industry. Most of us don’t need to go anywhere near as fast in our home shops environments.

Again for newbies, forget the speeds in the charts for drilling especially when drilling in the lathe, slow right down and give the drill time to do its job. Drilling too fast just makes things hot. Remember that, “back in the good old days” drills used to work even when the only power input was you winding a handle. Oh alright, that way might be a little too slow, but the principle still holds.

chriStephens

 

 

Hi Chris and Martin,

 

Martin: You are quite right in what you say about the equation I posted and in actual fact the simplified version is the one I use in the workshop “in my head” to work out RPM. I posted the full version because I feel it’s important for people to be told the correct way of doing things; you can always take shortcuts later.

 

Chris: I don’t entirely agree with what you say. High Speed Steel is a lot more forgiving than Tungsten Carbide and will tolerate a far wider range of cutting speeds and feeds. The recommended cutting speeds for HSS are designed to give the best compromise between production speed and length of time between regrinds; in other words they produce the optimum production/down time ratio. I totally accept that HSS can be run a lot more slowly without adversly affecting its performance although what is gained by this I’m not entirely sure. Drilling in particular is far easier if the correct speeds are used, the drill cuts more readily and less pressure is needed to feed the drill; if there’s a problem with heat use a squeezy bottle full of coolant!

 

Tungsten Carbide on the other hand has been designed for use in a high speed production environment. It is far less tolerant of slow speed which is the point I was trying to make in my original post. If you use it below its designed cutting speed and feed the cutting edge will rub instead of cutting, this results in the cutting edge being worn away or failing prematurely which produces an extremely poor finish.

 

As a comparison it would probably be fair to say that HSS should be used at or below the RPM’s calculated whereas TCT should be used at or above if a reasonable finish and tool life is to be expected.

 

I accept that most people don’t need to work as fast in a home workshop as would be expected in industry however, if you are to achieve results with TCT then you have to work at the speeds it is designed for.

 

Regards,

 

John.

[Steve WithnellParticipant @stevewithnell34426]

Posted by Andrew Johnston on 29/05/2011 20:48:40:

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

Hi Andrew,

 

Here is a picture of the workpiece showing the different finishes. It’s 8mm diameter and the material is EN8. The chuck is to the left hand side of the image. When machining towards the chuck, the finish is better and different to when machining away from the chuck.

 

Regards, Steve

[mgjParticipant @mgj]

I’m sure its something to do with the headstock bearings being differently loaded – in one case they (it is)are in compression and in the other under tension. It might be worth adjusting them up, because , machining away will pull any clearance forwards and allow a touch of movement in the front bearing.

What type of centre are you using in the tailstock?

[Martin WParticipant @martinw]

Hi

 

Taking MGJ’s comments as a starting point. Center drill the workpiece then using a running center in the tailstock load the headstock bearings by winding the running center in toward the chuck until resistance is felt, tailstock locked down, then lock the tailstock spindle.

 

Repeat the turning process again and if the results ARE different from previous runs then it is, as mgj suggests, probably a headstock problem. If the result is the same as the previous tests then look at tooling or play in saddle, cross slide or top slide etc.

 

Hope this helps but if I am trying to teach my gran how to suck eggs then I apologise.

 

Regards

 

Martin

[Anonymous]

Well I dunno, I find it difficult to believe that the headstock bearings are being affected by a cut on an 8mm diameter workpiece? Unless it’s a very small lathe and, or, the bearings are shot. A question: do you get the same inconsistent results on other materials, say brass or aluminium?

 

The finish in the picture looks slightly dull and torn to me? If that was with the carbide tooling I’d say you were not running fast enough. For my own curiousity I’ll experiment with some EN8 later this week and report the results in my thread on turning trials.

 

Regards,

 

Andrew

[Author]

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