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Harmonic Variation

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Peter Cook 623/09/2020 23:19:19
18 forum posts
2 photos

I was struggling with backlash on my new SXlLP mill, and much of it seemed to come from end play in the leadscrew bearings. Tighten them to eliminate play and they were hard to turn. Slack and there was too much backlash. So I followed advice elsewhere on the forum and fitted 16 x 8 x 5mm thrust bearings.

In doing so I needed to bore the bearing holders to 16mm. First experience of machining cast iron! I got (as I expected) some chatter but was fascinated by the regularity of the patterns produced.

The bearing block was held in the 4 jaw chuck (image 1) not balanced, so there as some vibration. However the pattern produced was very regular (image2), suggesting some sort of harmonic vibration at a higher frequency than the out of balance rotation.

Speed was 450rpm ( as slow as the lathe will go) with a carbide insert boring bar. The result was not a problem - the bearings are a great improvement - but I wonder if the experts can suggest the source off such an interesting vibration pattern.chatter setup.jpgchatter.jpg

Jeff Dayman23/09/2020 23:28:13
1896 forum posts
45 photos

The work is a long way out from the headstock, and the boring bar has a long stickout as well. Suggest clamping the work on carriage and putting the shortest boring head possible in the chuck, to clean it up. Or do the boring in a friend's bigger lathe.

Michael Gilligan23/09/2020 23:34:18
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16420 forum posts
715 photos

Until the real experts come along, I will say it’s most likely spring in the boring bar [or indeed the whole structure including the slide and saddle] ...

MichaelG.

.

No disrespect intended by my opening comment, Jeff

... I hadn’t seen your post !! 

Edited By Michael Gilligan on 23/09/2020 23:36:45

I.M. OUTAHERE24/09/2020 00:12:36
1468 forum posts
3 photos

It may be an optical illusion but the toolpost doesn’t look like it is sitting down on the top of the cross slide .

I would ditch the insert tooling on a lathe that small and grind up a HSS tool then hone it so it is razor sharp , keep the tool stick out as short as possible , also check the carriage and cross slide gib adjustments to eliminate any free play . I think on a Taig you can lock the cross slide with thumb screw and i would also lock this each pass .

oldvelo24/09/2020 01:04:05
243 forum posts
53 photos

Xd 351 points out the gap under the toolpost as the most probable cause. High speed steel ground for cast iron needs negative rake and the tip set not lower than centre hieght. Honing a high speed steel insert with an oil stone for better finish on almost all metals.

Eric

Martin W24/09/2020 01:17:32
854 forum posts
29 photos

The tool post looks to have chamfered edges which could explain the apparent gap between it and the cross slide. Looking at the reflection of the tool post on the cross slide in the enlarged picture it appears there are chamfered edges on the bottom of the tool post which could explain the gap but worth checking anyway.

Martin

Bandersnatch24/09/2020 01:51:47
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1719 forum posts
60 photos
Posted by Martin W on 24/09/2020 01:17:32:

The tool post looks to have chamfered edges which could explain the apparent gap

 

Enlarging and zooming in, it looks to me more like radiused edges and not nearly enought to give that much illusion of gap.

If I had to guess, it would be that there's a shim (washer or washers?) under the toolpost to get the tool height right.

 

Edited By Bandersnatch on 24/09/2020 01:52:32

Speedy Builder524/09/2020 06:51:21
2112 forum posts
146 photos

Again, may be an illusion, but is the tool bit horizontal, or is it at a negative angle.

Or is the boring bar too big for the diameter it is cutting ?

Edited By Speedy Builder5 on 24/09/2020 06:53:13

JasonB24/09/2020 07:13:21
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The is no gap under the tool post, just look at the large chamfers on all the other corners.

Tool looks to be sitting correctly as it has flats on the 6mm shank so can't easily be mounted wrongly and should have a min diameter of say 10mm so OK for the 16mm

Less overhand of the tool would help and fitting a CCGT tip will lighten the load on the small lathe. With irregular items if they are causing in balance then turn at a slower than usual speed.

Also looks a bit of a bright finish for cast iron, more like steel or possibly SG iron

Edited By JasonB on 24/09/2020 08:55:58

not done it yet24/09/2020 07:50:16
5038 forum posts
20 photos

I would also question the cutting tip height, as well as the above. Just looks low.

I was wondering if the tool post was simply sitting on the T-nut, not the cross slide!

Michael Gilligan24/09/2020 08:29:40
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16420 forum posts
715 photos
Posted by Bandersnatch on 24/09/2020 01:51:47:
Posted by Martin W on 24/09/2020 01:17:32:

The tool post looks to have chamfered edges which could explain the apparent gap

 

Enlarging and zooming in, it looks to me more like radiused edges and not nearly enought to give that much illusion of gap.

If I had to guess, it would be that there's a shim (washer or washers?) under the toolpost to get the tool height right.

.

This page is mostly about modifications and accessories, but includes an excellent photo of the apparently unmodified clamping face.

http://www.micro-machine-shop.com/rocker_tool_post.htm

MichaelG.

Edited By Michael Gilligan on 24/09/2020 08:30:18

Martin Kyte24/09/2020 09:43:53
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2067 forum posts
37 photos

Don't just think about the buissness end of the boring bar. The back end is long enough to act as a tuning fork. Either swap to a very much shorter bar, cut the end off, clamp a mass to it. and abosutely ensure that all the clamp bolts are screwed down tight. If that packing piece goes the whole length of the boring bar add some toolmakers clamps.regards Martin

Richard S224/09/2020 10:25:12
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184 forum posts
110 photos

+1 for HSS tooling on this job, as well as minimal tool overhang. Backlash and gibs to check.

I bought this lathe model back in 1985 to learn with and the first thing I did was to incorporate a countershaft.

Reduced the speed options by 3:1. This allowed me to turn larger diameters of 80mm in Ali and 55 mm in cast iron & steel with very few 'teething' issues. Feed, speed & DOC were reduced, but never had any issues with finish, even using carbon steel tooling.

In comparison, my tool post is flat and square to the cross slide surface, so the chamfers are a later addition to the product.

So super sharp tooling, finer feed at your minimum speed after eliminating backlash etc. Otherwise look to lower the speed option.

Hope you get your machines sorted and running well soon.

Regards

SillyOldDuffer24/09/2020 10:27:26
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6344 forum posts
1395 photos
Posted by Peter Cook 6 on 23/09/2020 23:19:19:

... I needed to bore the bearing holders to 16mm. First experience of machining cast iron! I got (as I expected) some chatter but was fascinated by the regularity of the patterns produced.

The bearing block was held in the 4 jaw chuck (image 1) not balanced, so there as some vibration. However the pattern produced was very regular (image2), suggesting some sort of harmonic vibration at a higher frequency than the out of balance rotation.

 

This is related to one of my many stalled projects, an attempt to identify faults by analysing vibration.

Causes and mitigations have been well covered already, but the pattern is related to some mix of vibrations produced by the set up. The unbalanced bearing block will vibrate the whole machine at a frequency related to spindle speed, while the boring bar vibrates at a higher frequency related to the insert being energised by bouncing and resonating at a frequency determined mostly by the bar, but also by tool-holder, saddle and lathe. Bit like holding a rule flat on the edge of a table and twanging the free end - boing.

grc radio companion is a useful tool for experimenting with waves and mixing. I knocked this up in a few minutes:

grcmixer.jpg

It makes two streams of complex numbers from Signal Sources representing a 600Hz sine wave and a 6000Hz sine wave and adds then together. The result is converted to floating point and sent to an Audio Sink top right: this is my computer's loud-speaker so I can hear the noise - it's horrible, nothing like either of the pure tones it's generated from. Bottom right, the GUI Time Sink graphs the two waves mixing together:

mix.jpg

Now, although the output's a spiky mess, it still has a regular structure based on the two clean inputs. Although the Taig mixes vibrations in the same way, it's not working with two clean sine waves of equal strength. So the pattern produced on the work by chatter is probably dominated by the two main components - off balance and boring bar vibration - but modified by the rest of the machine vibrating. Vibration being related to rigidity means it's much easier to avoid chatter on big heavy machines bolted down on a concrete floor than small ones on a bench. (Testing stepper motors on my dining table proves it makes a good sounding board!)

Dave

Edited By SillyOldDuffer on 24/09/2020 10:29:40

Dave Halford24/09/2020 12:08:59
942 forum posts
9 photos

If those tips came with a 4 piece under £20 CCMT set you can get CCGT from the same source. They make a huge difference.

JasonB24/09/2020 12:15:53
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It's an ARC boring bar, they do suitable CCGT inserts, Though a 0.2mm tip radius would be even better on the small machine

Neil Wyatt24/09/2020 12:33:04
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18250 forum posts
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I agree with Dave(!) the low-frequency chatter in probably created as a 'beat frequency' of two higher frequency resonances.

Some people have found that something as simple as a bit of lead sheet, plasticine or blu tac on the boring bar can break the resonance. Equally changing speed or overhang or try any variable, really.

Neil

Peter Cook 624/09/2020 13:03:42
18 forum posts
2 photos

Thanks to everyone for their replies. Much helpful information to help a beginner up the (ever steeper) learning curve.

The toolpost is sitting down on the slide as flat (and tight) as I can get it. The gap is an optical illusion created by the chamfered edges of the toolpost. You can see the same illusion on the cut-off tool image on the website that Michael G references. As Jason says the tool is sitting on the flats on it's shank, and was set to centre height with the packing piece. I will file away Martin's suggestion of adding damping mass to the back end for future use.

I am not absolutely sure what the material is. It is a standard Seig SX1L bearing block bought from ARC (I am not yet brave enough to modify the ones that came on the mill without first getting spares!). It certainly cut like cast iron is reputed to. Lots of very fine black granular chips that made everything VERY dirty. When I initially opened out the holes on the mill I assumed it was steel and used a cutting oil - and that made a real mess. So I looked up cast iron machining (on here) and cut dry on the lathe; which worked well.

I chose the insert tooling because I could not get the lathe slow enough to use the correct speeds for HSS on cast iron. The tip is a CCMT - I will research the different tips - another task for the list . I have a project under way to replace the motor with a speed controlled DC one that will let me get down to 100 rpm or less. The problem (laugh) is that the lathe has now become the project rather than the enabler of the clock maintenance - the reason it was bought. The clocks languish unserviced!

The bearing modifications were a success, the mill is far smoother to use and with far less backlash (0.1mm on both axes - I haven't yet tried adjusting the split nuts). I was just fascinated by the regularity of the chatter. Thank you to SillyOldDuffer, that explanation seems a good match for the evidence. If I take the bearing blocks off again I will measure the chatter and see if I can get the same pattern on that software.

Thanks again to everyone for their time - one more inch up the cliff face of learning!

Edited By Peter Cook 6 on 24/09/2020 13:07:33

Edited By Peter Cook 6 on 24/09/2020 13:14:06

Mick B124/09/2020 13:21:36
1736 forum posts
91 photos

Why is there no chatter on the bottom face of the bore? It looks as if it's been faced across very smoothly and if so, and with the same tool, might simply argue greedy depth of cut.

For correcting, I'm another of those pretend experts who'd use HSS tooling. I wouldn't even use an 'official' boring bar, just a square section blank ground with plenty of curved side clearance, a small tip radius, and set with a few degrees plan trail angle.

JasonB24/09/2020 13:25:14
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On a facing cut the cutting speed is constantly changing so any harmonics are far less likely, just like using the trick of modulation a variable speed knob during a cut to reduce chances of chatter.

Insert boring tools are more likely to chatter with too small a DOC particularly if **MT

Edited By JasonB on 24/09/2020 13:28:19

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