Reasons for Bad Cuts in Metal, With Examples

[SillyOldDufferModerator @sillyoldduffer]

[SillyOldDufferModerator @sillyoldduffer]

In a recent thread Curtis Rutter asked for help with a facing cut taken with his new lathe. Reading the thread It struck me that many beginners struggle with finish, not being clear if they have a faulty lathe, blunt tool, wrong speed, wrong type of tool or a difficult material.

I wonder if anyone else would care to contribute photographs of poor cuts either with an explanation of the cause or asking for one.

Here's my offering. It's a powered facing cut on a 3/8" diameter BMS rod taken with an 8mm 'Form No2' HSS knife.

Yuk! This was caused by a badly ground tool rubbing after the point has made the cut. You can see a spiral of smudged metal where the surface of the rod has been smeared by the HSS tool.

Sorry about the poor focus, but you can make out at least four facets where a wobbly Duffer messed up sharpening the tool on his grinder. There isn't enough relief across the whole of the tool face.

This was partly due to lack of practice but mostly because I was in a hurry. (Strange how often there's not enough time to do a job properly in the first place when there's no shortage of time to fix it later.)

Anyone else got examples they aren't too embarrassed to share?

Cheers,

Dave

[Nick_GParticipant @nick_g]

Posted by SillyOldDuffer on 23/03/2017 12:51:17:

Anyone else got examples they aren't too embarrassed to share?

Cheers,

Dave

.

Not normally the sort of thing I take a photo of. So no.!

But next time I mess up and I cut a bit of metal that ends up looking like a well chewed wine gum I will do so.

Nick

[MWParticipant @mw27036]

I'm afraid I haven't got any examples to hand in pictures (though I do like to jabber instead) but i'm sure if I have a little rummage through the odds/sods I've gathered i'll find a piece with a raw face left after parting off or something!

As you say a lot of it is down to poorly ground tooling, or not the right one for the right job! It looks a little embarrassing when we hiccup but we learn best through observing it and then turning ones attention to ways of overcoming it. The majority of my learning for this came from an old Colchester master and I made quite a few bin jobs i'm not entirely proud of

The piece of brass I saw of Curtis's looks the bizz and the genuine article, I think it's just a case of having a little too much keenness/pointiness on that 1/4" HSS and probably fed it a little quickly across the face for such a big diameter, I certainly wouldn't call the machine at fault for that and i'm sure a round nose tool, cranked across the face will clean it up nicely.

I had a personal struggle with parting off a while ago, I always used insert tooling to do so and the tips would occasionally break and the holder piece would bend and burr,

so after a lot of frustration I bought a compliment of 5 or 6 pieces of 3/8" M40 HSS for a small fee and ground my own tool and it's served me the bizz ever since, it's a little thick, at 4mm across but it cuts through large diameters with ease and rigidity,stainless steel up to 40mm in Dia in one cut, i'll load it up soon to show thee. Only needs a touch up on the face from time to time with a fine Ally oxide wheel.  

Like I said, I only had a basic apprenticeship and learnt all my sharpening from the oldies, but with no specific feat or talent to boot and I have found sharpening my own tools serves me better than chucking bought in stuff all the time.

Would also add that for our segment of the lathing community, it's best to stick to positive geometry to ensure a proper cutting action, whereas bigger machines might have a go at forcing large amounts of stock off using negative geometry.

Michael W

Edited By Michael-w on 23/03/2017 18:46:48

[Steven VineParticipant @stevenvine79904]

Dave, I can't help you out. All my cuts are perfect. You will have to take my word for that.

Steve

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nick_G on 23/03/2017 17:26:36:

Posted by SillyOldDuffer on 23/03/2017 12:51:17:

Anyone else got examples they aren't too embarrassed to share?

Cheers,

Dave

.

Not normally the sort of thing I take a photo of. So no.!

But next time I mess up and I cut a bit of metal that ends up looking like a well chewed wine gum I will do so.

Nick

Ah, but thanks to me, anyone who needs to turn a well chewed wine gum now knows how it's done.

I have no shame…

Dave

[SillyOldDufferModerator @sillyoldduffer]

This example is 'quite interesting'.

Courtesy of GIMP I combined two photos into one. As if by magic the picture shows opposite ends of the same BMS rod.

The ends were faced one after the other by the same sharp point 'Form 2' HSS tool. Both were cut at 500rpm with no lubricant.

Hardly exhibition standard but the finish of the left-hand face is markedly better than that achieved on the right. The difference between the two cuts is that the right-hand was faced with the tool exactly on centre height whilst the left-hand end was faced with the tool about 0.3mm lower.

I converted the picture to black and white because artificial light fooled my camera. In colour the BMS looks exactly like brass!

Dave

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