Basic Lathework: HELP!

[Antman]

Hi and sorry to be such a hyper Questions poster at the back of the class but something Bogs posted rattled my gate.

“If you don't do that, you will find that you will be using many half learned techniques on the same job, and you will end up with only a half OK result.”

I’ve had pretty good finish and accuracy (as close as I can tell) with diameters around 10mm. Much bigger or smaller, the finish is much worse and the diameter is not constant. I must avoid getting a little better at doing the same wrong things. I am fairly confident my HSS tooling is quite well made, 1 thing my lazy sonny boy does well. I have had no luck with any radius tools only sharp knife and squarish finishing tool, the finishing tool actually made from a text on shaper tools. ( Works ok in shaper too )

I don’t know close to enough, what to do to improve things. I’ve read a lot of stuff on the web, downloaded gigs, ( I really appreciate the amount of free info from ME mag. The Geometer articles are amazing, so much cleverness on one page… )

Feeds and Speeds? I think I try to turn everything too slow. I can get my lathe down to 60rpm. Should I go for slower? Long feed? Using my biggest and smallest gears I usually feed at about 0.1mm per spindle rev. Is it a dumb idea to want to add to the gear tree for smaller feeds and is it the only way to get feeds to the right? In Feed? I find if my infeed is small I get an intermittent cut?

Tightness of slide gibs? I know next to nothing about how tight or free the slides should be adjusted.

Facing? Sometimes ok sometimes not good and I don’t know why.

Chamfering? I’ve had no luck making a simple neat 45deg chamfer to finish things off, and I’ve tried everything I can think of.

Most of my tooling is HSS and I use the white coolant, and a simple gravity flow.

If anyone can help I’ll shut up for a while, be good and quiet and busy … Monday … promise. Maybe in a month I’ll annoy you all again with my progress.

Ant

 

[Blogwitch]

Ant,

It looks like you are getting frustrated because you are not getting the results you expect. It all takes time if you are trying to do it off your own back, with many failures and a few successes, and you are not the only one it happens to. I bugger up things quite regularly, usually by trying to rush a job.

You should have mentioned the problems when you called the other night.

Gather all your questions onto one piece of paper, and make a slow stroll or fast bike ride over to my place.

A quickie coffee to talk the probs over, then I will try to go over all your problems in my shop, just as I did last time.

It is OK reading about something, but much better if you can be shown how to do it. You learned a great deal that first time, and your machining expertise took one giant leap forwards.

Lets see if we can do it again.

John

 

[rake60]

It is no annoyance at all Ant!

Rigidity it most important.
I tighten the gibs as tight as they will go while still allowing free movement.

The finish on different size material and in facing may be a tool height problem.

We are taught that when turning an OD the tool should be slightly below center
and when boring the tool should be slightly above center.

Forget that.
A tool set perfectly ON center will work well in either application.

I am sure other input will follow.

Rick

 

[steamer]

Tool geometry, a keen edge and the right feed, speeds and cutter geometry for the material are key points. Check in on these too.

Dave

 

[New_Guy]

try higher speeds remember the smaller work your doing the higher the speed you need i would be doing most stuff over 500rpm up to 2000rpm on a mini lathe 60rpm is far to slow oh and there is a formula for speeds and feeds if you take a gander at that and give it a try it may help

 

[Blogwitch]

I am actually going senile.

I will try to explain.

I have a young man , who calls himself Ant, who I have taken under my wing and am helping by phone, emails and posting on websites, and because he lives not too far away from me, has actually been across for some machine training.

I seem to have had a bit of a mental blockage, and I am replying to this Ant's posts as though I was replying to the one I am assisting.

No wonder I have been getting weird responses to my attempted assistance to this Ant (Antman).

Antman, I am very sorry if I have confused you with my replies, it won't happen again.

But the offer is still there for coffee and training if you want to make your way from South Africa. Unfortunately, I can't provide accommodation.


Bogs

 

[Antman]

Hey Bogs,
No problemo. I’d love to visit, see what you do with your machines, learn something. I’m sure I will never be allowed to set foot on your misty isle, but thanks all the same. I’m one of those Anthonys who has never been called Tony. Even my Mom and Dad always called me Ant. My formal title is The Antman, bestowed upon me, honoris causae, by the wife’s big brothers.
Ant

 

[DickDastardly40]

Ant,

What material are you cutting?

For 13mm steel ballpark 500RPM is about right, adjust down for bigger sizes, up for smaller. Brass faster, Aluminium, faster still.

All tools on centre height, tightly held, lock up any slides not in use, ie, lock the saddle and compound slide for facing.

For finish cuts 0.1 mm/rev is too much IMHO, try somewhere nearer .0127mm/rev (.005") or even less.

Infeed for me depends on what I'm doing roughing etc and how powerful the lathe is, mine is only .75HP and stalls on heavy cuts, so I'm limited to about .025" in steel

As I go from roughing to on size, I reduce the amount of infeed so the surface finish improves with each pass, I aim to hit a size on every pass and measure so I always know how much is left, eg .02", .02", .01", 0.05", .025", .01", .005", .001" On size!

Tell us again about the intermittent cut on small infeeds. Power off, bring the tool to the work so it is just off, place a sheet of something white undeneath so you can see downwards and have good light, gently rotate the chuck by hand and see if the work moves towards or away from the tool. Power on and run at normal speed and look again, if you have wobble, your chuck may not be tight or your lathe may need some attention. Small infeed will show this up and may be why you get inconsistent sizes/poor finish.

Chamfering, use the compound at which angle you like and a normal tool whih clears the work.

Pictures are good, even camera phone pics, far easier to diagnose what we can see, than guess.

Good luck, Hope I've helped somehow.

 

[Antman]

Hey Guys thanks for all the info. I do a fair bit of searching in HMEM also. I am particular in centering the tool. ( The Face Without a Pimple ) As for frustration, forget it, I love spending time in the workshop. I’ve been round the block more than twice, so outputting so much less than expected is just the present state of what I’m doing, now it is a learnt reality, no longer an expectation. I still expect to do more per time spent trying to do more, better. I still expect to be able to build nice things and I still expect you guys to be helping me. ;D And now, I have some ok tooling (already) that I am chaffed to be able to say that I made, I don’t need a pep talk.

Thanks Dick D yours is the sort of information I am looking for.
The intermittent cut: I think I mean something else. The best example is with screw cutting. I have been cutting M6 , M8, and M10x1 . I have tried feeding straight in and at 29 and 30deg, all with a fishtail. I get the result that if I feed in by 1 thou after a cut no metal is removed. I feed in ½ thou still nothing, until the tool digs and there is a deep cut. Hand cranking the spindle. Same sometimes on a finish cut but not nearly as bad.
Speeds: I am finish cutting much slower than recommended speeds and find the surface is not good faster. Any suggestions? Roughing sort of comes naturally to me and I find I am roughing near enough to recommended speeds.
Feeds: I have the motivation for another branch to my gear system, a started and put aside project.
And I’m ok with inches. If .001” is a thou, what do you call .01mm?

Bye,
Ant

 

[steamer]

4 tenths!... ;D

.00039" (inches)

 

[kvom]

You don't say how far from the chuck the material extends, and if you are using a center to support the tailstock end. If you work extends more than 3x its diameter the tool pressure will deflect it.

When turning down, I use a faster feed at the early cuts and fine feed for the final. Surface finish counts only for the final pass after all.

There's a good speed&feed online calculator here you might find useful:

http://www.whitney-tool.com/html/calculatorSpeedFeed.html

For lathe work, use the work diameter in place of tool diameter.

 

[DickDastardly40]

Further to what KVOM says about holding the work in the chuck, always support with the tailstock and running centre if at all possible.

Screwcutting set up; try this:

Set up to cut the thread in the usual way, gears/job to diameter/tool sharp and aligned/supported with tailstocck etc etc.
Start lathe at 60RPM and advance the tool to the right hand end of the job about the width of your tool in from the end.
advance the tool gently so the tip just cuts the surface and peels a small strip all the way round, after one rev no more metal should be removed.
Zero your cross slide dial here, retract the tool at least one turn of the handwheel and move it to the right away from the end of the job.
Advance your tool again to zero (the one turn of handwheel was to take out any backlash), it should now be exactly the same depth as before.
Engage the leadscrew and you should get the faintest spiral of thread show as the tool moves across the job. If this doesn't happen something else is amiss.

If you have brass try this as it threads beautifully.

Different steels can be a bugger to cut nicely, I wonder if this is at the root of your problem and you have something akin to scaffold pole or at the other end of the scale some stainless.

PS. 0.01mm is one hundredth (of a milimeter).

 

[kvom]

One other point on threading. The first pass should be a scratch cut, just deep enough to be visible on the surface of the material. You can then measure the thread pitch with a gauge to verify that your lathe's gearing is set up correctly. if it isn't, you can easily correct it at that point.

 

[Deanofid]

I get the result that if I feed in by 1 thou after a cut no metal is removed. I feed in ½ thou still nothing, until the tool digs and there is a deep cut.

That sounds like your tool isn't as sharp as it could be. If slightly blunted, it will take some pressure for the tool edge to get under the chip, and when it does, the work springs back against it and takes a larger cut than you expected.

Maybe I should say this; most everyone thinks their tool is sharp... until they've been at this a while, then they know better.

Dean

 

[zeeprogrammer]

That sounds like your tool isn't as sharp as it could be. If slightly blunted, it will take some pressure for the tool edge to get under the chip, and when it does, the work springs back against it and takes a larger cut than you expected.

Maybe I should say this; most everyone thinks their tool is sharp... until they've been at this a while, then they know better.

Dean

Exactly what I've been experiencing the last couple of days. It's amazing to me how much just can't be learned by book...you just have to try, play, do, and learn. I see see this a lot at work when I teach (a completely different subject)...I'm always surprised by how much I know without knowing how I know it or learned it. Ah...you know what I mean.

 

[RichD]

I don't know what kind of lathe you have, but many people that have the 9x20 Chinese lathe complain about similar problems related to too much flex in the compound mount to the cross slide. The solution in this case involves a modification to the mount that reduces flex. A four bolt plate replaces the two bolt plate improving performance greatly. If this sounds like your problem visit http://bedair.org/9x20camlock/9x20project.html. He has plans for the upgrade along with pictures of how he did it.


If the above isn't the problem check the following:
Lathe OFF
Make sure the work is not extending from the chuck too far unsupported. If it is use a center or the steady rest
Set the tool in the holder so the minimum amount of tool is extending from the holder.
The tool must be sharp and ground with correct geometry.
Bring the tool close to the work and place a 6" scale between the work and the tool vertically.
Advance the cross feed until the scale is trapped. If the height is correct, the scale will be perfectly vertical.
Adjust the height until correct, then lock it all down and remove the scale.

LATHE ON
Take a test cut at .010". It shouldn't chatter, nor should the work try to climb over the tool.
If it does chatter, you should check the following:

LATHE OFF
Attach a dial indicator above the chuck touching the top of the chuck. Use a piece of lumber to lightly pry upwards below the chuck. Depending on the size and sturdiness of your lathe the results may vary. My lathe (a 9x20) shows almost no movement on the dial indicator unless I pry really hard. I believe that is just minor flexing from the somewhat thin and hollow spindle.

If you see something wrong here check/adjust the bearing preload on the spindle (if your lathe has tapered roller bearings)

If you have sleeve bearings, the bearings may be badly worn. Anything more than .002-.0025" you might have a problem. Or more simply any more than .001" per inch of journal diameter is somewhat sloppy.

Remove the chuck and repeat the test. The results should be the same. If not, the chuck mount is bad.

Another possibility is the headstock hold down bolts may be loose.

Grab the carraige front and rear and check if it has any rocking up/down, there should be none

Ok, I'm done now with ideas.
Good luck
Rich

 

[Richard1]

The screw cutting problem sounds like a blunt tool but it could also be problems with alignmen the half nuts. There was an article in one of the 1950 Model Engineers that I was reading only last week and can't find now about screw cutting problems that matched your description. It was finally traced to half nuts that didn't line up and never had from new so when they were closed they tried to bend the lead screw and drive would sometimes change from the top to bottom nut moving the saddle backwards or forwards by the amount of misalignment of the nuts or somewhere in between. This could easily cause the tool to not cut at all sometimes and dig in others. This has to be a rare problem but if nothing else is wrong then it is worth looking at.

Richard.

 

[Antman]

Thanks for all your replies, lots to think about, to put into practice.

Most of my steel comes from a small town agri co-op, where they call it “blink staal” shiny steel, in English. The manager assured me it is EN3B but I know that is an out of date spec. However it turns nicely (sometimes) and I think we did an ok job of colour case hardening some pieces, so it seems like the right low carbon stuff.

I have until now been only turning things with a max length around 60mm and no smaller diameters than 6mm, always using the tailstock centre and graphite grease. Short workpiece and tool overhang. ( Still a bit wary of working very close to the chuck and having the toolpost very close to the work. ) Getting tool right at centre.

The tool itself? By sharp I assume you mean the edges of the tool. After grinding on the wheel, I work the faces on 3 grades of diamond stones (with the plastic backing), then finally and often on a slipstone. If, by sharp, you mean clearance angles, top and side rake, I think my son is getting it near correct. He makes most of our tools and those I made, were under his instruction. What if side clearance and the rakes are increased? Does that cause a weakening of the edge, also the tool to dig? What happens with reduced clearance and rake?

Recommended cutting speeds? These speeds are best for what actually? Removing most metal in a given time? What? I am still using my slowest speeds on small diameters, so turning larger diameters is still a dark art from where I am at.

Thanks RichD, you have given me much to work through. My lathe is like the Africa version of the Have-a-Fright 9x20. Seems like a longer bed (about 6”) and zero bells or whistles. Feed changes by chart or calculation, ratios by manually swopping cogs. Speed changes by v-belts and pulleys. I have had my doubts about the topslide too, but I think I don’t know enough to be sure. Mine seems sturdy enough from the base and slide, upto where the toolpost bolts on. The way the toolpost fits seems puny. On the top of the toolpost is a short pillar, +/- 22mm dia x 15mm high that looks to be 1 piece with the slide and throught it a 10mm rod threaded M8 at the top. The toolpost has a square base of 68mm sides but most of the base is not flat, there is a circular recess of 52mm so the toolpost rests on quite a small area, less if it is angled to the topslide. The topslide feedscrew might also be questionable, seems wimpy at 1mm pitch 60deg V-thread 15mm dia. Or are these thing adequate for this size lathe? Apart from my doubts about the toolpost and fixings, the rest of the lathe seems A-ok.

And Richard1, you want to see me squirm chasing phantoms? At this stage of the game I have no way of getting near the halfnut but I believe it to good. The screwcutting is good and clean down to the first 10 or 20 thou only deeper I have problems.

Hey Zee, I know exactly what you mean. It’s like when you’re at work, running on Automatic.

I haven’t been at the lathe yet this week. I am trying to rig up a vertical slide so will be on the koki pens then shaper this week. But all the lathe info is churning around in there somewhere.
Sincere thanks, you guys are great.
Ant

 

[Huntinguy]

I really hate to chime in here, considering so much good information has been posted.

I am smelling three things to check. Lathe tool and shaper tools are close but not quite the same (well, some shaper tools are way different).

It sounds like the tools may have the wrong geometry. Something is causing the tool to push away and then pull in. This could be too steep of relief, too shallow of relief. One would pull the tool in and make it dig, the other would push it away until there was enough pressure to make it dig. After taking a few passes look at the tool under a strong magnifying glass and see if there are any bright spots. If so, the tool is rubbing and you need to change clearance angles. Keep in mind there is a balance between the relief angles and clearance angles too much (or too little) on either one will cause problems. Also, some people put a top clearance angle on the tool and this will change the center height across the tool. (I have rarely found this to be a problems...)

Are you sure the tool honing process is not rolling the edge? One old fellow told me that a HSS lathe tool should be able to take shavings off of your finger nail (obviously you check this as you would with a knife and not while the lathe is running). He was the best HSS tool guy I have ever seen, he could make HSS cut with the same removal rate as cemented carbide... an artist if there ever was one.)

Centerline could be an issue, there could be volumes written on this... our lathes at work are slant beds and at first you would think you couldn't center anything. But what you are looking at/for center and clearance angles is a relationship from the center of the spindle axis through the top cutting edge of the tool. It is possible to have the proper clearance on the tool but have it set so the clearances all go away. Much is written on tool sharpening that assumes you are using what is called a "lantern" post. I have found I have to grind my tools differently when I am using a square tool holder or one of the adjustable tool holders.

It could also be a condition of flexing... and that would mean something is loose or has a chip under it (tool post may have a chip or galling under it). I tighten the gibbs on my lathe until they just drag. Have you put an travel indicator on your cross slide and compound to see if they are moving in smoothly and not jumping? What is the backlash like? If there is backlash in the nut there is not too much you can do about that (though I am sure there are those here who know things about tightening them that I do not). Also check the backlash at the dial nut. I have seen those come loose and mess things up.

What are you using to shim your tools? The best method I have heard of is using feeler gauge blades. The tool needs to be as supported as possible. I have seen aluminum used... not a plan in my book.

EN3B is the same as our 1020, should be easy to machine... You also might try a different cutting fluid... I prefer black oil (high sulfur) it stinks but works well on steel (not on aluminum). Also red oil (cut max 570, I think). It is a decent all around cutting fluid.

As you can see I don't count very well so take my advise accordingly.

Best of luck.