# Turning a Small Diameter



## Antman (Feb 29, 2012)

Hi Guys,
  I know this must be really basic and second nature to most of you, but my slow progress has hit the wall with my present workpiece.  Im trying to make a 5mm diameter cotter pin about 25mm long, using 8mm mild steel stock (The manager at the agri co-op told me it is EN2B), lathe running at 1000rpm, HSS and soluble oil coolant. The smallest I have turned before this is 6mm and that not too successfully. Im quite wary of running the lathe so fast, but it sounds good and is not vibrating, so I think Im ok on that score. I read somewhere that small diameters give you a steeper front clearance, but I dont think I understand the implications.  Should I grind a tool with near vertical front clearance? What about top rake?
  5mm is gross big compared to some of the parts you guys are making, Im only starting to work small. What I do instead of roughing it to near finish size is to try with many finish cuts as practice but I dont get nearly as good a surface as if I were turning about 15mm. What am I doing wrong, or is it my tooling?
  Thanks,
   Ant


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## n4zou (Feb 29, 2012)

You make one heavy cut leaving your pin the size you need. Take a short cut and measure it. Make your fine adjustment and then make the cut. If required file off the short section you used to make sure your cut was going leave the pin the correct size after the cut.


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## Antman (Feb 29, 2012)

Thanks n4zou for your input, but it's not hitting size that's my problem. I'm trying for a decent finish. I can get a finish that is well acceptable to me on larger stuff, down to 10mm. I have difficulty as soon as I try to turn smaller than 7mm.

  Ant


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## MachineTom (Feb 29, 2012)

Can I assume that the work is being used with a live center on the end. As 5X D is too long for a small D material. Your speed is slow, 1500 rpm would be closer, on small D work on center is absolute, what are you using to set cutter height. A shallow DOC, a nose radius, oil and a feed of .005-008 per rev. should give the right results.


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## smfr (Feb 29, 2012)

Also, there's nothing wrong with using a bit of emery backed by something flat like a parallel to get it down to final diameter, with a nice finish.


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## lensman57 (Feb 29, 2012)

smfr  said:
			
		

> Also, there's nothing wrong with using a bit of emery backed by something flat like a parallel to get it down to final diameter, with a nice finish.



I agree with you there, I normally use a 600 wet and dry followed by 1500 and its almost a mirror finish. I have not been able to come even close to that kind of finish no matter how I have tried.

regards, 

A.G


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## tel (Feb 29, 2012)

Allow me to remind all yawl of my little gizmo. Here it is taking 1/8" manganese bronze down to 1/16"


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## petertha (Feb 29, 2012)

tel  said:
			
		

> Allow me to remind all yawl of my little gizmo. Here it is taking 1/8" manganese bronze down to 1/16"



Thats neat. Almost like a travelling steady rest with nil side force on the material. So I guess for a specific OD of stock (before reduction) your bushing has a matching ID sliding fit ?


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## Ken I (Feb 29, 2012)

A bit off topic but follows from prior rig.

If you've never seen a sliding head or "Swiss" type automatic lathe - the tools are arrayed about a guide bush and the headstock rotates the stock and the headstock is driven forward on a slide, the rotating stock is thus fed through the bush while the tools work on it.

That's how you turn those rediculously thin / long aspect ratio shafts used in clockwork mechanisms.

More to the point a travelling steady (follow steady) will work for long slender aspect parts.

Or make up a fixture like Tel's (nice one Tel).

Ken


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## tattoomike68 (Feb 29, 2012)

Tels box tool is sweet, thats the ticket. :bow:


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## Antman (Feb 29, 2012)

Thanks Tom. Even 1000 rpm is scary for me. Why dont I see much difference in surface finish at 300 or 600 or 1000 rpm? Is it the shape of my bits that they cut eqaully badly at any rpm? I still dont get radiused tools. Most of my tools are like 3 planes intersecting. What means shallow DOC  ?  I took my new live centre for a spin for the first time today. Ive been doing stuff on the shaper and short things in the 4Jaw. 
  Nice little boxtool, Tel, I reckon that comes long after a follow rest which is still somewhere on my to-do list. 
  And like Ive said before, Im of the opinion that I wont be using abrasives on my lathe until I think I know what Im doing. Im working on the very basics here, having never even watched this stuff being done before and plodding along by my own trial and error only. Thanks Ken, I think you must be an awesome engineer. Im very hung up about turret lathes if you like box tools and such, and all sizes of them (mostly over 2 tons) go for like scrap value.
  Ant


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## AussieJimG (Feb 29, 2012)

Nice tool Tel. Now the question is whether I stop what I am doing to make one for when I need it,
or hope I remember it when later when I need it. :-\

It's a worry

Jim


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## miner49r (Feb 29, 2012)

Tels tool looks like it could clamped in the DP vice and the work turned in the DP's chuck. I nice option for someone with a tight equipment budget.

Tel,
  I gotta know. Is that a wheel keeper you are using for a tool post?
Alan


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## terryd (Mar 1, 2012)

> I still dont get radiused tools. Most of my tools are like 3 planes intersecting. What means shallow DOC  ?  ............
> Ant



Hi Ant, for a finishing tool you should put a radius on the cutting tip. Just a small one with a slipstone or a diamond file/slipstone will do although I use a fine wheel on my grinder. The radius will mean you get overlapping cuts which smooth out the finish whereas a sharp point can leave a rough looking finish as under magnification you get what appears as a series of 'grooves' (actually one long helical groove rather like a thread).

Also make sure that your tool grinding angles are correct especially the top rake and that the tool is sharp, again hone the faces with a slipstone to ensure the best edge possible with no grinding marks.

DOC is an abbreviation for 'Depth Of Cut' so a 'shallow DOC' is another way of saying a 'finishing cut'.


Terry


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## tel (Mar 2, 2012)

Thanks blokes, it's a handy gadget, even if it doesn't get used a lot. And yes - you need a different bush for each size of stock.



> Tel,
> I gotta know. Is that a wheel keeper you are using for a tool post?
> Alan



Nope, that's just the standard ML7 toolpost clamp.


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## picclock (Mar 2, 2012)

Hi Antman
Just my two cents, but the tool height is very critical when you get to small diameters. I was turning some silver steel rod from 6 to 2 mm and the adjustment had to be precise (araond a thou or two). Using a fixed centre may be better as you have no bearing play (just centre drill and load with Molybdenum grease). Other point is what lubrication and tooling are you using ? For the best finish (IMHO) HSS tooling and I use lard as a cutting lubricant. Picked up the habit from an old book and it has got me the best finishes. Use a fine stone (india stone ?) on the tool edge and a small nose radius around 0.4mm. I made an electronic leadscrew control so that I can accurately set the feed rate - your mileage may vary ;D. The other point is that as stuff gets small the rigidity of everything really matters more so nice tight gibs will prevent any other untoward movement. Depending on how fine you want the finish, fine emery then toothpaste or similar will do the job.

Good luck and don't give up. It took the members of this forum a long time to acquire the expertise available, freely given, so make the most of it. No doubt when everyone here started they were in the same boat, so if in doubt persevere. 

Best Regards

picclock


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## mzetati (Mar 2, 2012)

Tel,

that little gizmo of Yours is neat! Thank You for the idea.

Marcello


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## Antman (Mar 3, 2012)

Thanks Terry and Picclock. Well, finally I got some acceptable results. The centre height was not spot on and also I was taking too deep cuts. I ended up using a sharp squareish tool with a chipbreaker and I found my best results came at about 400 rpm. Recommendations for a 5mm diameter are about 3 times faster. I still dont get decent results at the higher recommended cutting speeds and I still get lousy results with radiused tools. Another thing I had to come to terms with on this little job was clearance using a 5 3Jaw and the bearing housing of my new live centre is all of 45mm, so I had to overhang the tool out of the toolholder by nearly 20mm. I must modify a toolholder or make another to give me better clearance on the tailstock end. Thanks again guys for all your help.
  Ant


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## JohnLanark (Mar 3, 2012)

I have a commercially made tool on the same lines as Tel's one. I think it came from Chronos. They are in the catalogue near the bottom of this page but shown as out of stock. 
http://www.chronos.ltd.uk/acatalog/Myford_Lathe_Compatible_Acessories.html
John


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## zeeprogrammer (Mar 3, 2012)

Very useful thread. Thanks for asking the question Ant.


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## Lew Hartswick (Mar 3, 2012)

picclock  said:
			
		

> Other point is what lubrication and tooling are you using ? For the best finish (IMHO) HSS tooling and I use lard as a cutting lubricant.


Crisco (registered trademark) etc. is just as good as Lard and wont go rancid if it sets
around (like it will in a shop).  Of course bacon grease smells better. 
  ...lew...


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## moconnor (Mar 3, 2012)

Hello Antman,

In an earlier post in this thread you mentioned that you didn't understand why a tool nose radius was necessary on your lathe tools. It is a very important part of the tool and greatly influences the surface finish on the work as well as the strength of the tool point and the life of the cutting edges. It is also one of those things that once learned and put into practice will positively influence your work in the future.

First, consider on a screwcutting lathe that the feed for turning is most often driven by the spindle and the tool advances in inches or mm per revolution. There are lathes with independently driven feed mechanisms like the Hardinge tool room lathe or Firebird's adaptation to his Myford that are fed in inches or mm per minute, but most are like the former and regardless, the same principles apply to the tool. When the lathe is set-up for single point threading and the tool is fed along the work, we refer to this relationship as the lead of the thread or the pitch. The pitch of the thread is the distance between corresponding points on adjacent threads. The lead of the thread is the distance that a nut would advance along the axis of a thread in one revolution. For a single lead thread, the pitch equals the lead. The formula for Pitch (P)= 1/N, where N is the number of threads per inch. An example: for 20 Threads Per Inch (TPI), the Pitch would be 1/20= .050". So for every revolution of the lathe's spindle the tool will advance .050" or .050" per revolution. The tool bit's profile, ground to the angle of the thread required, forms the shape of the helical groove or thread as the tool advances. In the case of our example, the pitch of the groove is .050". I only mention the thread cutting operation here to illustrate the shape of the helical groove generated. When feeding the tool along the work during a turning operation we are essentially cutting a similar pattern into the surface of our work piece with the pitch of the helical groove equal to the feed in inches per revolution if we use a sharp pointed tool.

Because the mechanics of what is happening at the chip/ tool interface with regard to surface roughness is measured and compared in microns, it is helpful to magnify this relationship in an enlarged diagram. Consider *Figure 1* below. If we have a sharp pointed tool and set the lathe for a feedrate of .005"/rev. for a finish cut, what we would end up with would be a 'thread' or helical groove along our workpiece with the profile of our tool and a pitch of .005"/rev.. You would be able to see and feel the groove or lines in the surface of the work. This tool bit would also have a relatively short life because the point is very fragile and would likely break down rapidly if not chip off completely. Sorry for the long winded build-up, but the relationship to threading was important to illustrate this point.

Now consider *Figure 2*. This tool bit has a Tool Nose Radius (TNR) that is greater than the feed rate selected. The net effect of this is that all of the peaks and valleys that were left by our sharp pointed tool are now smoothed over, leaving a much improved surface finish. In addition, our tool point is much stronger and will wear much, much slower. The TNR, once honed on the tool, can usually be maintained by a touch-up with a stone for some time before the tool develops wear lands that must be removed by grinding. It is critical that the TNR be tangent to both the end and front cutting edge angles and that the TNR have the same relief below it as these cutting edges. It is THE principal cutting edge and must have relief to work properly.

So there is a *General Rule* that can be used here. *The Tool Nose Radius must be Greater Than the Feed Rate *  or *TNR > Feed Rate*

Now there are some things to consider beyond the general rule. On our relatively lightly constructed lathes, there are limits to the size of the TNR that can be used. In general, I use an .008"/ .015" TNR for finishing tools and a maximum of .030" TNR for roughing tools. When you start using a larger TNR, the contact area increases which can cause chatter on our small lathes. You will find this out the first time that you try to use a form tool. Another benefit that a TNR provides is it allows a greater feed rate to be used for finish turning, which gets the job done faster. BTW, I own a Myford S7 and a 9" South Bend.

For our smaller lathes the best tool material IMO, is High Speed Steel (HSS). Yes, off-hand grinding is another skill set to develop, but once mastered it is a powerful tool to have for the home workshop. The tool bits can be optimized for the job at hand, they are relatively inexpensive and they can be ground and honed to a very keen edge. When the work material allows (ductile materials), it is advantageous to know that positive rake angles increase machineabilty or the speed and ease with which a material can be cut. Positive rake also reduces the cutting forces required, which is very important for small diameter turning. With regard to carbide inserts, they are available with all sorts of TNR's and positive rake, but if you examine the inexpensive inserts that are often used in the home shop, their cutting edges are rather blunt. The less expensive inserts used as molded can be frustrating to the newcomer who had hoped they would be the answer to their poor finish problems. They have their place in the home shop, but finishing small diameters isn't one of their strong suits.

A good general purpose HSS tool bit configuration that I use for finish turning and facing is attached in *Figure 3*. It has a 35 degree End Cutting Edge Angle and is set with the side cutting edge of the tool at a -5 degree angle relative to the face of the work piece. This allows the tool to turn, face and get right in close to the center because the end cutting edge angle is parallel to the point of the center. This tool will also provide a beautiful finish on the face of the work when feeding away from the centerline. Because of the -5 degree angle, it is like having a very large TNR as you feed outwards as the increased contact of the cutting edge smooths over the feed lines and provides a great finish with a fairly fast feedrate. The tool can also be used to undercut a shoulder because of the acute angle of the point if the TNR is too large or a sharp corner is required. Try it, I think you will like it a lot if you haven't done this before.

Kind regards,
Mike 



View attachment Fig.1 TNR Less Than Feed Rate.pdf


View attachment Fig.2 TNR Greater Than Feed Rate.pdf


View attachment Fig.3 TNR Turning & Facing Tool Bit.pdf


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## zeeprogrammer (Mar 3, 2012)

Excellent post Mike. This really helps me a lot to understand.
I had noticed this effect before but hadn't taken the time to figure it out.
Seems obvious now


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## Mosey (Mar 3, 2012)

The valuable lesson learned today...small diameter turning tool!
Is this appropriate for making small poppet valves, like for a small IC engine? (To avoid tapering of the stem)


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## moconnor (Mar 3, 2012)

Hello Mosey,

The tool that I posted in Figure 3 above would work well if you used a small TNR (.008"). For small diameter turning a part like a valve stem or any similar type of part where deflection is an issue, set the left side of the tool bit or its side cutting edge angle at right angles to the lathe's axis so that most of the cutting force is parallel to the part's axis. The -5 degree set-up angle would work for light finishing cuts, but if you were trying to turn the diameter in one step like say .250" dia. down to .125" diameter, anything that will minimize tool deflection will help. Use positive side and back rake to reduce cutting forces as well.

Regards,
Mike


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## picclock (Mar 4, 2012)

Hi Antman

Glad to hear you sorted it. Last cut depth should really be about 5 thou or less, and 20mm overhang is quite a lot, depending on the size of HSS you are using. To get those really good finishes everything needs to be really tight and spot on, because any movement of the tool due to load or differing metal hardness is going to affect the finish. This is also the reason for the shallow final cut depth, to reduce the load on the tool so that any flexing is minimised. You can overdo that though if the tool ends up rubbing instead of cutting.

If you get a good finish, polishing is quite fast and rewarding as it really makes things look professional.

Best Regards

Picclock

@ Lew - I don't think we have Crisco on this side of the pond (UK side), although I have never had lard go off. I just have a piece in an old tin on top of the radiator to keep it soft, and top it up with a chunk from the fridge time to time. I wonder if lard is the same in the US ?.


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## Antman (Mar 4, 2012)

Hey Zee good to have you back. Did you also learn something from Mikes discourse. Mike, thanks, I see why I should keep plugging away at radiused tools. Difficult to judge what the radius looks like. I have some music wire and some stainless fishing trace somewhere, Ill measure it and maybe some of it will be that small. Those diagrams made a lot of sense ...  back to the grindstone. I have a half tub of nice commercial cutting grease, but in the lathe I prefer to use coolant, that I dilute a little less than when I started out with it. I'll give the grease a try. Thanks guys for your replies.
 Ant


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## Mosey (Mar 4, 2012)

Lard over here, is of course pig fat. You can buy it in the market and those of us who wish to die young of arteriosclerosis use it for cooking, for example put it under the skin of the turkey or capon you are roasting. The rest of us buy Crisco, which is hydrogenated or artificially thickened oil. Just as effective at clogging your arteries, and tastes slightly less wonderful. Takes a few years longer to die, but tastes almost as good. No one I know of uses lard for cutting here, we buy expensive artificial cutting fluids, that are less effective but come in a can. Since I use carbide inserts, there is not much need for cutting oil.
Anyway, the comments about tip radius are really good. Must pay attention to that.


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## tomol409 (Mar 4, 2012)

I do not use a roundnose tool for a fine finish, as the small work I do requires sharp shoulders. I have always used the method of reducing the plan trail angle of the tool to a bare minimum, ( that is the angle between th front edge of a knife tool and the workpiece, as seen looking down) Too much and chatter will result, but set right, with the correct cutting speed for the metal , (and this a constant related to diameter and speed). with as agreed by all, sharply honed edges. The first time I did this many moons ago on a piece of En1A free cutting mild steel, I could not believe the quality of the surface finish, and have mostly used this method ever since. Half the battle is to use free cutting metals as much as possible. 
  To move to another hoary old point, I have never used tools with no top rake on brass, and I always use the tools ground for steel. I have never had any problems with this, and I feel it gives a better finish anyway. It saves grinding two sets of tools anyway.  Recently I acquired a Pultra lathe on which I make diesel model engine parts, and I mostly use a tiny parting tool for both turning and parting. The tiny flat edge of the parting tool cannot create ridges or screw effect and the finish is usually superb. Also the 3000 rpm speed
helps. Of course with a bigger set up chatter could set in.


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## Antman (Mar 5, 2012)

Tomol I get where you are coming from except the speeds. Maybe I'm trying too fine of a feed which is why I can't get my surface speeds up. Thanks,
  Ant


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