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Surface finish when turning.

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Robin Graham26/06/2021 23:44:01
865 forum posts
258 photos

I'm at my wit's end trying to diagnose the underlying cause of this problem. I was turning some 3.5 inch EN1A today and was unable to get a decent finish, despite trying a number of tools, speeds and feeds. An example:

surfacefinish.jpg

On the larger diameter part of the work you can perhaps see something resembling a fine screw thread. An immediate reaction would be that the feed is wrong for the tool tip radius, and it is indeed a fine screw thread. But it's more complicated than that - the 'pitch' of the pattern has no simple relationship to turning speed and feed. In fact certain speed/feed combinations give a sort of cross-hatched pattern, like right and left hand threads intersecting.

I've spent an afternoon experimenting with various tools - both carbide and HSS, and different speed and feeds, but always see some variation on this theme. Can anyone shed light?

Ancillary info:

- The lathe is a single-phase 1.5kW gearhead - fairly rigid, about 600kg.

- I don't notice the effect on work below ~2.5 inches in diameter.

- The same thing happens with or without tailstock support.

Robin

 

 

Edited By Robin Graham on 26/06/2021 23:48:07

Edited By Robin Graham on 27/06/2021 00:26:26

Zan26/06/2021 23:47:18
280 forum posts
19 photos

What speeds and feeds are you using?

Martin W27/06/2021 01:10:00
894 forum posts
29 photos

All the gibs tightened/snugged up, half nuts clean and locking firmly, gear train driving the lead screw clean with no swarf/muck in the gears? Can't think of anything else other than obvious problems like tool chatter/spring, spindle bearings, belts etc.

Ady127/06/2021 01:38:38
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4681 forum posts
713 photos

Carbide should give a fabulous finish on moderate speed if your headstock is good and your shears are clean

I've just got a barely used 60KG Drummond M, modded the headstock and picked out the wood shavings

I don't even need to use the leadscrew, the carriage handwheel does a fabulous job scooting up and down the lathe bed to do a job, its just like using a lever operated cross slide, if there are no vibration errors and microscopic muck you will cruise effortlessly through the work using the saddle handwheel with the leadscrew disengaged

This new one has been a bit of a revelation after using old clunky for 10 years

Check the headstock adjustments and thoroughly clean the bed and saddle

If the saddle and shears are really good you can tighten the shears gib and still use the handwheel to move effortlessly up and down the bed

JasonB27/06/2021 07:04:58
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Moderator
21300 forum posts
2419 photos
1 articles

Cross hatch pattern suggests some form of fine chatter, looking at how shallow the cut is compared with OD of stock the tool is not really engaging in the cut. If you want to just skim it then get a sharper insert, if wanting to take more off then take a decent depth of cut as it sounds like the lathe can take it.

Luker27/06/2021 08:15:27
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65 forum posts
79 photos

Hi Robin, I normally don't get involved with online prognosis because there could be any number of reasons for what you getting there, but in this case I feel your pain! I had something similar when I bought my lathe and it was actually a design issue on the machine itself, and very specific to the machine, so may not be applicable to you. Most of the modern Eastern import gear head lathes sold with CQ numbers suffer from this. If you drop us a picture of your lathe and makers code, and its the same as mine I'll take some pictures with a short write-up and post...

David Noble27/06/2021 10:02:22
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268 forum posts
11 photos

When I worked for a living! there was a Triumph 2000 in the workshop and it started to mark the work oddly. I traced it to a chipped headstock bearing.

David

Howard Lewis27/06/2021 10:37:54
5228 forum posts
13 photos

You are effectively cutting a screw thread by moving a cutting tool along rotating work at a constant rate.

The finer the feed the finer the pitch of the thread. The radius on the nose of the tool, and a small depth of cut, disguise this.

A deep cut with a radius nose tool makes chatter more likely.

Howard

Oldiron27/06/2021 11:35:12
829 forum posts
23 photos
Posted by Zan on 26/06/2021 23:47:18:

What speeds and feeds are you using?

Stated in the post.

regards

Andrew Johnston27/06/2021 12:07:56
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6222 forum posts
676 photos

I'd suspect the material, what are its antecedents?

Andrew

Tony Pratt 127/06/2021 12:13:30
1643 forum posts
8 photos
Posted by Oldiron on 27/06/2021 11:35:12:
Posted by Zan on 26/06/2021 23:47:18:

What speeds and feeds are you using?

Stated in the post.

regards

Where?

Tony

Oily Rag27/06/2021 12:40:42
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460 forum posts
147 photos

My questions would be:-

1. Tool overhang - is it excessive for the shank size?

2. Is there a 'ringing' noise coming from the feedshaft gearing - Overtight engagement of the drop gears can cause a high frequency vibration which affects the work finish (especially if taking light cuts)

3. Finally, I am never convinced that single phase drive is conducive to getting a good finish in certain circumstances. Usually this occurs (a bad finish) when a combination of factors amplify and create aliasing of the torsional vibration inherent in an AC singe phase drive.

Martin

Edited By Oily Rag on 27/06/2021 12:41:30

Martin Johnson 127/06/2021 12:53:57
131 forum posts
1 photos

I regularly get similar marks on tough materials. By counting the cycles (number of starts and pitch) I inferred that it was due to the rather hummy single phase motor. Different motors improved things a bit, as did rubber motor mounts. I have just ordered an inverter and 3 phase motor.

It is no accident that Myfords used to come with resilint mount motors.....

HTH

Martin

Robin Graham29/06/2021 01:07:10
865 forum posts
258 photos

Thanks for replies.

I didn't give numbers for all the speeds/feeds I tried because I didn't document all my experiments - that would have taken me a very long time!

The photo I took was 'posed' just to give an idea of the sort of thing I'm seeing - it was a light skim (perhaps 2 thou) on a 3.5" EN1A bar using a fresh 0.4mm tip radius CCGT insert. Turning speed was 550rpm, feed about 0.05mm per rev - obviously not the sort of thing you'd normally do, but I was trying to understand what was going on, and searching (unsuccessfully) for some sort of pattern to the patterns I was seeing. Reducing the spindle speed to 65rpm (the lowest I can go) , but leaving everything else the same, the cross-hatch pattern appears.

In my opening post I mentioned that it's a single-phase machine because I wondered if that might be relevant. After reading Martin's reply I listened diligently to the machine and felt its various parts while it was spinning under no load. There is a definite hum and I can feel 50Hz-ish vibration just by putting my hand on the tailstock. I'd not noticed the hum before because being a gear-head machine it's quite noisy, and I wasn't listening for it. It's most noticeable at low spindle speeds - something to do with resonant frequencies I suspect.

If this is the cause I guess I have to live with it or stump up for a 3-phase motor/inverter.

Robin.

Edited By Robin Graham on 29/06/2021 01:52:04

DMB29/06/2021 07:42:50
1153 forum posts
1 photos

Robin,

Give this a try, round HSS bar held in toolholder square to the work, high - ish speed, slow hand feed. Cutter, grind flat 1/2 dia depth on the end and square to sides. Bottom 1/2 that now projects forward, grind 5° receding angle down, away from just touching the flat edge. Set new sharp edge at 45° to the vertical in toolholder.

Experience has shown mild steel swarf comes off in fine feathery stream and looks like an ultra fine version of the kitchen pan scouring "wire wool"pads. I found the finish on the work just has to be seen to be believed, shiny, smoooooth! Didn't use any lube, just dry turning.

Do try it and good luck!

John

DiogenesII29/06/2021 08:45:00
299 forum posts
149 photos

Robin (given that the finish on the already-reduced portion looks okay), the questionable finish on the larger diameter looks very similar to the effect I get if I forgetfully engage the half-nut handle instead of the much-less-obvious fine-feed lever..

Tony Pratt 129/06/2021 08:56:05
1643 forum posts
8 photos

You are also only just 'skimming' the cold rolled bar stock. I have seen this effect on 3 phase lathes as well so not sure it's the motor?

Tony

Mark Simpson 129/06/2021 10:19:44
103 forum posts
30 photos

Hi Robin

I'm sure you will try all of the above things (first) and hopefully find the culprit.

I recently had a similar problem, which eventually was the motor having lost a winding or two (but still running).
1.5Kw Single phase cap start / cap run chinese motor.
Once Cap was blown, but replacing it and the motor bearings didn't fix the problem

New motor (I went 3 phase and VFD) and it's much quieter, more grunt at low sped and silky smooth

The motor was loud/grumbly since new, but gradually got a lot worse....

Good Luck
Mark

Chris Mate29/06/2021 20:20:15
8 forum posts

Interested to know if your V-Belt is toothed or smooth-?

Norfolk Boy29/06/2021 21:04:31
57 forum posts
19 photos

Quote:-Finally, I am never convinced that single phase drive is conducive to getting a good finish in certain circumstances. Usually this occurs (a bad finish) when a combination of factors amplify and create aliasing of the torsional vibration inherent in an AC singe phase drive.

I don't wish to cloud the problem with my lack of experience, however I will say that I have a Harrison M250 factory single phase resilient mounted originally, and I converted it to 3 phase vfd hard mount (as per factory spec) and the difference in cut finish was certainly extremely noticable (in the positive). I would not go back to single phase. As said there may be other issues causing ths, but felt it worth reinforcing the above quote from Oily Rag. There are so many variables in this pursuit.

Alan

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