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What am i doing wrong

Making a tapered cone

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Steviegtr10/03/2020 19:05:41
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Today I have made a 6 degree internal tapered cone in mild steel. 1.4" at wide end of throat. The finish is rubbish. The boring bar is only a small 12mm one that came with the lathe, but I have fitted a new tip & that is in good nick. Not sure if I set it right.

The tool tilts down to the left. At the tip point, is this the centre of my work or not. I tried various speeds ranging from around 250 right up to 850. At about 400 it chip broke pretty good so left it at that. I tried taking it in very slow & must have been too slow at one point because it started to rub.

I took 10 thou per cut. Tried 5 but same. Anyway see pictures & if anyone can help. Much appreciated.

Steve. boring bar 12mm.jpg6 degree cone.jpg

old mart10/03/2020 19:38:06
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I f you don't have the luxury of a taper turning attachment, you have to advance the compound by hand. This makes an even speed very nearly impossible. The depth of cut may be too deep for the diameter of the boring bar and the insert may not be sharp enough for finish cuts. The tip needs to be at or just above the centre line (+0.005". I would use the biggest boring bar that would fit the hole. You can probably get a larger diameter bar to fit your lathe. Measure the height of your bar above the bottom of the toolpost , it may be possible to get a 16mm one if the 12mm one has 4mm of packing shims under it.

Steviegtr10/03/2020 19:45:44
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Yes I can get up to a 16mm in there. I was at Rufforth autojumble at the weekend, I picked one up & put it back down. Should have bought it. Next month I will. Thanks for the tips. The insert was a new one Tried to show in picture.

Steve.

old mart10/03/2020 19:57:29
1906 forum posts
151 photos

You don't have to pay much to get a reasonable 16mm boring bar. And CCGT09 inserts for aluminium will also fit it.

**LINK**

DC31k10/03/2020 20:29:26
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Posted by old mart on 10/03/2020 19:38:06:

I f you don't have the luxury of a taper turning attachment, you have to advance the compound by hand. This makes an even speed very nearly impossible.

You can jury-rig a power compound slide using a cordless drill on the handwheel nut. That may improve the finish.

SillyOldDuffer10/03/2020 20:35:41
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Posted by Steviegtr on 10/03/2020 19:05:41:

... The finish is rubbish. ... Not sure if I set it right.

The tool tilts down to the left. At the tip point, is this the centre of my work or not. ...

Good to eliminate problems one at a time!

I set the tool height using either a length of rod faced carefully to centre height as a gauge or the steel strip method as per photo. Sorry it's sideways!

dsc04505.jpg

The steel strip is pinched between the tool tip and a rod. If the tool is too high, the top of the strip moves away from the operator. If too low, it tilts towards the operator. The tool is centred correctly when the strip is vertical.

When facing, adjust tool height until there's no pip, or deliberately make one so the point can be compared with a centre in the tailstock:

dsc04503.jpg

If still not cutting reasonably with the centre-height correct, it might be the material: ordinary mild-steel isn't wonderful. En1a-Pb machines much better.

Other possibles, 850 rpm is too slow, the bar tends to chatter because it's on the slim side, and/or a sharper insert is needed.

I think I mentioned in another post that running before one can walk isn't the best way for a beginner to start! In this case, boring an internal taper is harder than boring parallel. Once boring parallel is mastered, it's easier to spot what might be going wrong with a taper. And one shouldn't be boring until plain turning is mastered.  When still at the unsure about tool-height stage, keep it simple. Only move to trickier jobs once reasonable results are achieved with the basics.

Dave

Edited By SillyOldDuffer on 10/03/2020 20:38:28

Mick B110/03/2020 20:41:26
1656 forum posts
88 photos

The wall thickness at the mouth is low, so it won't support much depth of cut without chatter. If the insert can do it, I'd try a very shallow cut of only a couple of thou or so. When you hand-feed with the compound slide, use the other hand to brake the handwheel so as as to keep up an even feed, and swop hands when you have to without actually stopping.

Me, I'd use a dead-sharp 1/8" or 3/16" HSS toollbit in a 1/2" boring bar, but I guess we all have our favourite ways of doing things.

Ah, one other thing - you've probably checked this, but is the compound slide gib properly adjusted to minimise play without binding?

Edited By Mick B1 on 10/03/2020 20:46:21

Dave Halford10/03/2020 20:59:02
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Tip radius looks big, if the last two numbers of the six on the box that begins TPMR or TNMG etc are 08 it's a roughing tool. You need 04 or 02 for finishing

Andrew Johnston10/03/2020 21:39:13
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Three things:

1. The insert may be new, but where did it actually come from?

2. The screw holdng the insert looks odd; they're normally Torx rather than slotted, and the countersink is not standard so an ordinary screw won't work

3. I adjust the gib on the compound to be slightly loose if anything, so there is no hesitation when feeding

Andrew

Ian P10/03/2020 22:33:32
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2411 forum posts
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My introduction to lathe tools with carbide inserts was about 15 years ago when I bought a toolholder with 10 tips at a local steam rally, I was instantly converted!

I then bought a set of five tools with inserts from a well know supplier. The tip were pure rubbish, if the purchase had been my first introduction to indexable tooling then without doubt I would have been put off for life.

Point of me mentioning this is that the tip in Steve's picture looks to be identical to the ones supplied in the set I mentioned, they have the same dull grey look and the same parallel 'gutter' around the perimeter.

Ian P

PS, I did not know much about tips when I made my steam rally purchase but I now know they were the fine grained polished type for machining aluminium and plastic. (which I now use for 90% of all my turning)

Steviegtr10/03/2020 23:31:08
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1358 forum posts
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Thanks for all the replies. I will check that insert tomorrow. It came with the lathe???. I have some more that a friend got from an engineering company when it closed down, so they may be better. They are gold coloured.

I did adjust the gib when I started, as I was going to do a 3" pass so needed full travel. It was a tad loose, but I tightened it up a bit. I do need to make a variable speed feed drive for it. List of many things to do. + 1 for the Y axis on the milling machine once I get it home. Strangely 2 weeks ago I made a coin ring self centering punch, which entailed turning an inside taper in stainless. See picture. It worked out great. Unfortunately as advised, most of the projects I have on the go are turning internal tapers. Mainly in stainless steel.

The hand wheel smooth operation I have sort of got but by the end my fingers were aching. So from feedback some of the things I need to do is speed up the machine as 800 is too slow.

Check the insert or get a bigger boring bar asap.

Check I am on centre.

The tool height bit I always check against a centre in the tail stock. But the reason I asked was the boring bar has a tilt on the end, confused me as to which part had to be centre.

Slow down, try harder.

Thanks again for all the help. I will go through everything again tomorrow. Here are the ones I have if anyone can identify them for me. Regards. Steve.new punch round.jpgnew punch.jpginserts.jpginserts 2.jpg

Hollowpoint10/03/2020 23:51:24
331 forum posts
31 photos

The one in the middle is a **GT tip and will give the best finish. 

But If you have a HSS tool use that instead as its much sharper, set the tool just above centre height, ramp up the speed, small depth of cut, feed in as smooth and slow as possible, use cutting fluid. You're welcome.

Edited By Hollowpoint on 11/03/2020 00:01:51

Steviegtr11/03/2020 01:06:00
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The more I think about the problem. Mr Holmes sort of reasoning is that I am pretty trashy with the smoothness of the handwheel. + the wrong cutter. + I need a stronger bar. This all worked out from the comments from you guys in the know. Also just a mention the steel was marked up EN1. I bought from a guy at Rufforth who sells nothing but bar stock. I have spoke to him at length so I do trust him. Ex Coded welder. So he knows his metals.

Steve.

JasonB11/03/2020 06:59:12
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When turning the handwheel don't just use one hand on the handle, you should use both hands on the rim of the wheel which helps keep a constant feed but more importantly helps eliminate the risk of up/down movement as you crank the handle. Locking th eother slides that are not being used will also help.

Corner of the tip that first makes contact is what you want on ctr height.

I also saw that screw last night, if the tip is not held firmly down and back against the edges of the pocket it could be moving about get a proper sized replacement with the right CSk angle.

SillyOldDuffer11/03/2020 09:32:05
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Posted by Steviegtr on 11/03/2020 01:06:00:

...

Also just a mention the steel was marked up EN1. I bought from a guy at Rufforth who sells nothing but bar stock. I have spoke to him at length so I do trust him. Ex Coded welder. So he knows his metals.

Steve.

EN1 is 'ordinary' mild-steel of the type I warned about. It doesn't machine particularly well. (Nor is it avoid at all costs horrible.)

EN1A is markedly better on a lathe because it's formulated for machinability. The specification includes the magic words 'free-cutting', and it's roughly twice as machinable as EN1.

Best of all in the family of mild-steels is EN1A-Pb. It contains Lead, and is about 50% more machinable than EN1A.

Machinability isn't an obvious concept, but the techniques needed to cut anything vary tremendously. At a pinch a wood-saw can cut butter, but don't use it on steel. Likewise Lathes and Milling machines don't cut all metals equally well, or at all! Try turning a square file into a round spike.

Beware who you take advice from, including me. A Coded Welder will certainly know about metals, but his focus is welding not machining. Different skill set. For example, a Welder should avoid EN1A and EN1A-Pb because they don't weld well! Your colleague may be completely trustworthy in the ordinary sense, but this is engineering. It's fact based, look it up in a book, confirm by experiment knowledge. Judgement calls influenced by a firm-handshake, old-school tie, convincing patter, or solid friendship aren't a good way of making workshop decisions.

Other way round when dealing with people because most of us are an emotional mess! Then personal trust matters big time.

Dave

Neil Wyatt11/03/2020 10:37:12
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With that kind of bar it's usually mounted so the insert is angled downwards towards the tip to stop it rubbing. The flats on the body usually force correct orientation so I'm guessing this isn't your problem, but it may help someone else.

Neil

Neil Wyatt11/03/2020 10:47:18
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Posted by SillyOldDuffer on 11/03/2020 09:32:05:

EN1 is 'ordinary' mild-steel of the type I warned about. It doesn't machine particularly well. (Nor is it avoid at all costs horrible.)

Are you sure? I thought EN1 was always supplied in the A state.

Neil

JasonB11/03/2020 10:55:34
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EN1A should also be OK to weld as there is no lead in it to give problems.

Ketan Swali11/03/2020 11:02:26
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Steve,

Before you consider buying a new boring bar, you may wish to consider some of the suggestions already mentioned earlier, in particular about increasing the lathe speed, and cut smaller depths of cut, keeping a positive steady feed.

Also, you need to be aware that:

  • the cheaper boring bars, regardless of from where you buy, are subject to flexing, resulting in a tapper in boring operations involving a deeper hole. The more expensive bars – such as those made by Sumitomo – used for more regular industrial repeat operations are less likely to flex. We have a musical instrument restorer working near buy, who has tested both types for us, on his Myford Super 7, and these comments are based on his and others findings. He uses the Sumitomo 10mm and 12mm boring bars.
  • The more the bar is held outside the body of the holder (for deeper hole boring), the greater the chance of chatter, based on a variety of factors, so locking down non-moving components/axis, can help – as explained by Jason.
  • If you are considering the 16mm diameter bar, on the one hand it may reduce the chance of flexing, but you will need to check/figure out if it can be held securely in your tool holder/tool-post, with the cutting tip height locating centre to the work, as suggested by others. Do consider the type which are flat on the top and bottom of the bar. Is the bar you are using ‘fully round’ or is it flat on the top and bottom?

Looking at the picture, it appears to be flat on the top and bottom. So the round area of the bar does not locate in the locking area of the tool post/holder. That round area would make the whole diameter of the 12mm boring bar you are using to be between 15~16mm - at the cutting head.

If you are considering a 16mm boring bar, where the flats may be 16mm, if it fits in your tool holder/tool post, the cutting tip of the insert may not be able to be adjusted in height to the center line of the material you are boring. Have a look at this snip it picture below from this page link , which may help you to understand:

sumitomo boring.jpg

Ketan at ARC.

Edited By Ketan Swali on 11/03/2020 11:10:48

Andrew Johnston11/03/2020 11:42:33
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Posted by Neil Wyatt on 11/03/2020 10:47:18:

Are you sure? I thought EN1 was always supplied in the A state.

That's what I thought as well.

It would be interesting to know what grade of stainless is being used. Of the austenitic stainless steels 303 and 316 are easy to turn, whereas 304 is 'orrid stuff. In my experience 303/316 are fairly insensitive to cutting speed and depth of cut, although I'd regard the depths mentioned as rather shallow. The only golden rule is never let the cutting tool rub, as the steel will work harden, 316 especially so.

I don't understand why a flexible boring bar would cut a tapered hole. There might be some bell mouthing as the cut comes on. But once the cut is established, and the bar has deflected by whatever it wants, then the cutting process is stable and the diameter of the work won't change. The diameter might not be what you expect, but it won't be tapered. That's definitely my experience. It's counter-intuitive but the best way to kill chatter when boring is increase the depth of cut and/or the feedrate. The boring bar may deflect a little more, but if the cutting process is stable then the bar will not chatter.

Andrew

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