# Metal lathe - how to get a good finish?



## Monkey Mark (4 Jul 2016)

I can't seem to get a nice finish on mild steel and wondered if anyone could give any advice? I've had a look round the net but everything I've come across assumes you have a reasonable amount of experience but I'm completely new to this and learning as i go. The only bit that i could understand was to have a high speed. 

I have both hss and carbide cutters. I've tried running it at 1800rpm both with auto feed and manually feeding very slow and steady. I always end up with a pretty rough finish, certainly not what I'd be happy with. 

Any pointers please?


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## adidat (4 Jul 2016)

Are you using coolant?

Adidat


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## DTR (4 Jul 2016)

What shape is the tip of your cutting tool? If it's an acute angle, try sharpening a very slight radius on it. 

Also do you know the grade of the steel? Certain grades machine better than others; I try to use EN1A wherever possible, or failing that EN3B.


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## DTR (4 Jul 2016)

Forgot to ask, what diameter is the steel? 1800 rpm is probably a bit fast, try slowing it down


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## marku (4 Jul 2016)

Do you know what grade of steel you are using, plain mild steel does not turn well compared to a more suitable type of free machining steel.

I am sure other people will be experts in steel types and advise accordingly.

Mild steel can have a rough finish that appears like tear out you would get with a wood plane, it is possible to overcome this but it is easier to get better steel to start with.

Mark


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## woodpig (4 Jul 2016)

Poor finish can be down to quite a number of reasons. High speed and low feed is a good start. Make sure the tool is sharp and on centre height. As said a slight radius when using HSS can help. Make sure all the gib strips are properly adjusted as a lack of rigidity won't help. Mild steel should machine nicely if all the above are ok. How much steel do you have sticking out of the chuck and is the end supported with a centre? What Lathe are you using, a quick pic of the lathe with the workpiece in situ might help diagnose the problem further.


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## Rorschach (4 Jul 2016)

Low quality mild steel will pretty much always leave a poor finish with standard tools because it is so soft.

Best tool for tricky metals is a shear tool, be aware though it will only take a very fine shaving, so you need to rough down to almost final dimensions before using the shear tool. Here is a good video showing it in action.

https://www.youtube.com/watch?v=WUcyHenjkew


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## woodfarmer (4 Jul 2016)

try some free cutting mild steel grade EN1A

cheap mild steel is often soft and tends to tear when cut.
the maximum speed on my lathe is 400 rpm, but it will cut beautifully, use a slight radius on your cutting tip and make sure you have tool at the correct height. too high will rub, too low will tear.


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## Monkey Mark (5 Jul 2016)

Thanks for the replies guys.
I've tried to answer some of the questions and added some pics in the hope that I can get something a little nicer.


adidat":59xmwlsr said:


> Are you using coolant?
> 
> Adidat



I've tried both with and without.



DTR":59xmwlsr said:


> What shape is the tip of your cutting tool? If it's an acute angle, try sharpening a very slight radius on it.
> 
> Also do you know the grade of the steel? Certain grades machine better than others; I try to use EN1A wherever possible, or failing that EN3B.


The carbide bit had a radius.



DTR":59xmwlsr said:


> Forgot to ask, what diameter is the steel? 1800 rpm is probably a bit fast, try slowing it down


It started as a 100mm bar.



woodpig":59xmwlsr said:


> Poor finish can be down to quite a number of reasons. High speed and low feed is a good start. Make sure the tool is sharp and on centre height. As said a slight radius when using HSS can help. Make sure all the gib strips are properly adjusted as a lack of rigidity won't help. Mild steel should machine nicely if all the above are ok. How much steel do you have sticking out of the chuck and is the end supported with a centre? What Lathe are you using, a quick pic of the lathe with the workpiece in situ might help diagnose the problem further.


The carbide tip was a nice fresh one.
I have no idea what a "gib strip" is. There was no support required.



woodfarmer":59xmwlsr said:


> try some free cutting mild steel grade EN1A
> 
> cheap mild steel is often soft and tends to tear when cut.


I don't know what grade the steel is as I didn't order it however I have seen another employee machining a bit cut from the same bar and he had a super smooth finish on it (he's not here to ask how though).


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## DTR (5 Jul 2016)

1800rpm is definitely too fast for a 100mm diameter. A few hundred would be more appropriate. There's a formula that calculates the ideal rpm from the size of the material and the optimum cutting speed. I can't remember it off hand but I'll post it up when I get home. 

The gib strip is a strip that sits between the fixed and moving parts of a slide (i.e. the saddle, cross slide and topslide). The gob should be adjusted so that the slide still moves freely but without any sideways play.


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## DTR (5 Jul 2016)

As promised....

rpm = CS / 0.26D

where CS is the cutting speed in surface feet per minute, and D = diameter in inches. Sorry it's in imperial, but I took it from an American book written in 1936! There is of course a metric equivalent that I learnt as an apprentice, but I've long since forgotten that one....

The cutting speed for mild steel using an HSS tool is around 100sfm. The cutting speed for carbide will be higher, but I'm not sure by how much (carbide wasn't in common use in 1936!). So, for your original diameter of 4" (100mm), and using HSS:

rpm = 100 / (0.26 x 4) = *96 rpm*

This should only be taken as an approximation, the nearest speed setting will do. Naturally, as the diameter gets smaller the optimum rpm gets higher.


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## AES (5 Jul 2016)

Monkey Mark, sorry, I'm a bit late into this thread. I'd agree with all the excellent advice and tips given above, but would add a couple of items I've not seen mentioned so far (sorry, No. 1 is completely obvious, but still sometimes overlooked):

1. Are the bolts holding the tool into the lathe tool holder tight?

2. Your set up looks OK to me, but just to be on the safe side, especially if the steel you're turning is a bit hard, if possible I'd shift the work piece a bit deeper into the chuck - yes, I can see the shoulder you're machining, but provided you rotate the job by hand once you've re set the work piece in the chuck (to ensure nothing fouls), AND if using self-act, watch it like a hawk at the end of each saddle run, you'll be fine with the work piece a lot deeper into the chuck. Makes it much more rigid.

3. That looks like a carbide tipped tool to me. Remember that they were originally made for industrial efficiency, so can take a lot bigger (deeper) cut than an HSS tool cutting the same size & type of material can. The converse of that is that the tipped tool will often produce a lousy finish (worse than in your pic) if made to take a cut which is too fine. The trouble with a deep cut if you're trying for a nice finish is that it will expose any lack of rigidity in your whole set up (hence my comments 1 & 2 above, and someone talking about gib strips above).

If you're not sure about the gib strips (there should be 2 under each of the top slide, cross slide, and carriage/saddle), first of all grab hold of each slide/saddle in turn and see if you can move it in any direction. If you can see any movement at all, even very slight, then the relevant gib strip needs adjusting. You're looking for a couple of small bolts, probably cap head screws, each together with a lock nut. Once you've found them, slacken off the lock nuts and tighten each bolt EQUALLY until the handle which moves the relevant saddle/slide becomes very stiff to turn. Then slacken each off about a quarter turn and try the "grab test" and the "stiff handle test" again until the saddle/slide doesn't move one little jot when you grab it with all your strength and try to move it - whilst at the same time you can still turn the relevant handle comfortably without feeling any stiffness anywhere within several full rotations. This will most probably take several attempts - damhikt! (But you do get quicker with practice, I promise)!

In short Mark, you've almost certainly got some chatter somewhere in your set up and that is the arch enemy of a fine finish. All the ideas and suggestions above will enable you to get rid of all (or at least the vast majority) of the chatter. As you're not used to turning metal but do turn wood, please remember that the forces we're dealing with here are generally speaking MUCH greater than when turning wood, and even the smallest unwanted movement between the tip of the cutting tool and the work piece (much less than a thou) can cause great problems with getting the fine finish that you're looking for.

HTH, good luck.

AES


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## Monkey Mark (6 Jul 2016)

DTR":3r62d5gz said:


> As promised....
> 
> rpm = CS / 0.26D
> 
> ...


Thanks for that. I knocked the speed down but still didn't get a particularly good finish. The finish was not important for this, but I figured I'd try as it's all a learning experience for me.



AES":3r62d5gz said:


> Monkey Mark, sorry, I'm a bit late into this thread. I'd agree with all the excellent advice and tips given above, but would add a couple of items I've not seen mentioned so far (sorry, No. 1 is completely obvious, but still sometimes overlooked):
> 
> 1. Are the bolts holding the tool into the lathe tool holder tight?


Yep


> 2. Your set up looks OK to me, but just to be on the safe side, especially if the steel you're turning is a bit hard, if possible I'd shift the work piece a bit deeper into the chuck - yes, I can see the shoulder you're machining, but provided you rotate the job by hand once you've re set the work piece in the chuck (to ensure nothing fouls), AND if using self-act, watch it like a hawk at the end of each saddle run, you'll be fine with the work piece a lot deeper into the chuck. Makes it much more rigid.


I did think that myself. As it's all new I was a little nervous of getting too close but as time went on I was feeling mor comfortable with this and ended up getting much much closer to the chuck.


> 3. That looks like a carbide tipped tool to me. Remember that they were originally made for industrial efficiency, so can take a lot bigger (deeper) cut than an HSS tool cutting the same size & type of material can. The converse of that is that the tipped tool will often produce a lousy finish (worse than in your pic) if made to take a cut which is too fine. The trouble with a deep cut if you're trying for a nice finish is that it will expose any lack of rigidity in your whole set up (hence my comments 1 & 2 above, and someone talking about gib strips above).
> 
> If you're not sure about the gib strips (there should be 2 under each of the top slide, cross slide, and carriage/saddle), first of all grab hold of each slide/saddle in turn and see if you can move it in any direction. If you can see any movement at all, even very slight, then the relevant gib strip needs adjusting. You're looking for a couple of small bolts, probably cap head screws, each together with a lock nut. Once you've found them, slacken off the lock nuts and tighten each bolt EQUALLY until the handle which moves the relevant saddle/slide becomes very stiff to turn. Then slacken each off about a quarter turn and try the "grab test" and the "stiff handle test" again until the saddle/slide doesn't move one little jot when you grab it with all your strength and try to move it - whilst at the same time you can still turn the relevant handle comfortably without feeling any stiffness anywhere within several full rotations. This will most probably take several attempts - damhikt! (But you do get quicker with practice, I promise)!


Now I know what gib strips are! :mrgreen: 
The machine was professionally serviced only two weeks ago and all seems to be in good order. Nice and smooth with no noticeable movement.


> In short Mark, you've almost certainly got some chatter somewhere in your set up and that is the arch enemy of a fine finish. All the ideas and suggestions above will enable you to get rid of all (or at least the vast majority) of the chatter. As you're not used to turning metal but do turn wood, please remember that the forces we're dealing with here are generally speaking MUCH greater than when turning wood, and even the smallest unwanted movement between the tip of the cutting tool and the work piece (much less than a thou) can cause great problems with getting the fine finish that you're looking for.
> 
> HTH, good luck.
> 
> AES




Well. I thoroughly enjoyed my first foray into the metal lathe. I'll certainly be finding excuses to make more things (hammer) 

As it is, here is my finished item. Not as nice a finish as I would have liked but that was more to do with me learning how do achieve it rather than it being a necessity.

And in those famous, and now rather creepy words, can you tell what it is yet?


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## DTR (6 Jul 2016)

To me it looks a bit like the bottom of a trophy, but you said the finish isn't important.....


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## Monkey Mark (6 Jul 2016)

I'll be posting more details later today in another thread as I'm just going to get ready to leave work now.
But, to cut a long story short, it's to fine my Axi Thicknesser.


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## AES (6 Jul 2016)

Monkey Mark, if that's a first ever attempt at metal lathe working then you've got no reason at all to 'ang yer 'ead in shame mate. I wish my first attempts were anything like so good, very well done.

If the machine's been serviced recently and all the wheels feel silky smooth throughout the range, then my bet is that having ensured your work piece is as well set up (supported) as possible, that the tool is well clamped in the holder, the tool is the right shape (a titchy little radius right at the tip), and the tip of the tool is exactly on centre height then it's just down to experimenting with a combination of work piece rotation speed, feed rate (along the job) and depth of cut. You're looking for a nice "fizzing" sound as the swarf comes off the job (sorry, listening!) and at the end of the day it's just a question of practicising until you get it - just like planning or sawing a piece of wood square for example. Can't be learnt from a book - books will only start you off in roughly the right place - "Lady, you gotta practice" to quote the very old ad. Good fun, and very satisfying when you get there.

But as said above, for a first job, it ain't arf bad mate.

AES


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## Harbo (6 Jul 2016)

It looks good to me - well done

I work a lot with Silver Steel (making clocks) and the quality varies enormously especially as they changed the content spec.
I managed to get hold of some "old" stock and what a difference that made.
Also on small items I get much better results with my hand shaped HSS cutters than the inserts.

Rod


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## woodpig (6 Jul 2016)

I've found there's no "right" tool for every material unless you have a large lathe and a comprehensive selection of different grade carbide inserts. I use HSS when I can in a Tangential tool holder and normally get a good finish. Sometimes though I need to use a carbide insert tool and the finish is just as good. Small hobby lathes are often more of a challenge as they are less rigid but my little 8" x 14" works well most of the time and I'm not ashamed to get a file out if I need to.

My first metal lathe was a real Heath Robinson affair that I picked up cheaply and the only way I could get a decent finish on it was with a heavily radiused HSS tool.


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## AES (6 Jul 2016)

+1 for Harbo's and woodpig's comments re HSS & tangential tool holder.

AES


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## Monkey Mark (6 Jul 2016)

Thanks for the kind words and advice.

I've added a few more pics to the thread below to give an explanation as to why I did this.

Again, thanks for the help everyone.

https://www.ukworkshop.co.uk/forums/post1071787.html#p1071787


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