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Poor surface finish using Myford

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Steve Morris 328/05/2010 10:15:59
5 forum posts
Help, I am pretty new to lathe work and have a ML7, when machining no matter how fine a cut I take the surface finish is very poor, ali is not too bad, but worse on steel. A mate suggested that the machine was not running fast enough but it is at the highest rpm that the belt system will allow. All cutting tools are new.
 
Anything I finish, looks like someone took a file to it.
 
Any thoughts?
 
Steve
chris stephens28/05/2010 12:04:24
1049 forum posts
1 photos
Hi Steve,
Where to start?  
First off, it is virtually  impossible to diagnose your problem via a forum, there are so many things that can effect to finish on turned work. Condition of the lathe , material you are turning, speeds and feeds, tooling, etc etc. An experienced friend could probably diagnose your problem in seconds, do you not have one to hand?
Having said the above, the first thing that stands out is you say, "All cutting tools are new" this implies you are using pre-ground HSS tools. the ones I have looked at are so shoddy that I would not recommend there use. If you are using carbide tooling, whether indexable or brazed, it could be you are taking too small a cut, some will not work at less than 1mm depth of cut. With carbide you have to get the right sort for your application, not assume that they work well under all conditions.Turning speed is dependant on material and size, going too fast for the conditions can ruin even the best of tooling in no time at all. A blunt tool will of cause give a poor finish. As a learner , without a local mentor, I would suggest that you look up the right speed for the metal you are turning and use that as a starting point. Next, there is no substitute for a SHARP tool, these are easier to make than some learners would think. Without a local Mentor, take a look at you-tube where you will find some videos on how to sharpen your own tooling. It is worth the effort to learn how, there will be times when only a home ground tool will do, so learn how sooner rather than later.I shall leave it there, and let the others have their input. Listen to everything they say, even when they contradict each other, and try things out for yourself. Learn from the tips that work, experience is the best teacher.
 chriStephens
 
 
 
 
 

Edited By chris stephens on 28/05/2010 12:04:56

Ian Abbott28/05/2010 12:33:01
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279 forum posts
21 photos
All of the above, plus...
 
Are you getting juddering or does the cut sound smooth.  If the tool hangs out too far from the holder, it could vibrate or distort.  See if you can feel any backlash in the feed screws, if so, it could be bouncing in and out.   What about tool height, too high/too low.
 
Ian 
JasonB28/05/2010 13:47:40
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23039 forum posts
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Also make sure that the cutting tip of the tool is exactly on centre height as this will affect the cut even if the tool is sharp.
 
You don't say what diameter material you are working on, again the larger the diameter then slower it needs to rotate to keep the periferal speed the same.
 
Are these facing cuts accross the dia of the work or along the length of the work and are you feeding by hand or the power feed?
 
Jason
Steve Morris 328/05/2010 14:36:49
5 forum posts
Thanks for the replies guys, when I got the machine second hand, a mate and I went through it taking out as much of the play as possible on the gib strips. The tools are HSS and it appears to cut smoothly, I will look at how to sharpen them as I assumed they would be sharp out of the box.
 
The max dia of material used so far is about 1/2 inch and the cut is along the length, the toolpost is the one that came with the machine and is Myford I assume, it seems rigid enough and I have the tool pretty close to it but will try again, I also ensure that the tool tip is centered on the work piece. Currently all cuts have been hand fed although I will try power feeding it too.
 
Steve
Mike28/05/2010 14:44:20
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713 forum posts
6 photos
Just a thought, but the headstock bearings could be out of adjustment. That's what the big "C" spanner you should have got with the machine is for.
Steve Morris 328/05/2010 14:47:15
5 forum posts
Mike, no spanner with the machine, I did look at bearing adjustment in the handbook I bought but it mentioned removing metal from the bearing shells, which frightened the life out of me as I didn't want to screw the machine up.
 
Steve
chris stephens28/05/2010 14:50:52
1049 forum posts
1 photos
Hi Steve,
It is a mistake to assume that any edge tool is "sharp" straight out of the packet, be it a wood chisel or a lathe tool. The only tools that are ready for use are indexable tips, any others will need a little work done. The commercial pre-ground HSS lathe tools should only be considered to be "shaped" not sharpened, also the ones I have seen are far too soft for even light use. Buy yourself some English or American made named brand HSS and grind your own, much less heart-ache in the end. Steer clear of HSS that only says "HSS" on it, you might get good stuff, but without a name how will you know you are getting the same good stuff next time? There is some dire quality stuff about, so be warned.
chriStephens 
Mike28/05/2010 15:18:49
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713 forum posts
6 photos
Steve: Do you have a dial gauge with a magnetic base? If so, place the base on the bed at the headstock end, with the tip of the gauge resting on top of the chuck. Then press upwards on the bottom of the chuck. Movement on the dial should be no more than about half a thou. If more, the bearings could be suspect.
Mike
KWIL28/05/2010 15:42:58
3562 forum posts
70 photos
Mike, Steve says it is a ML7, no C spanner involved, white metal bearings and shims between cap and lower housing.
Steve Morris 328/05/2010 15:51:31
5 forum posts
Mike, I do have a magnetic dial gauge and will check the chuck for movement.
Mike28/05/2010 15:53:16
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713 forum posts
6 photos
Thanks, KWIL - you are obviously much more familiar than I with the ML7. I used to have an ML7R and a Super 7B, and both had adjustable bearings at the rear end of the headstock, so I wrongly assumed Steve's lathe would be the same. Apologies for causing confusion.
Mike
Ray Lyons29/05/2010 08:58:16
200 forum posts
1 photos
Steve, I have owned a few ML7/Super7 lathes, all pre 70s. The problem I found with the older models is that the saddle bears on the front shear of the bed. This is a little unstable and causes a poor finish no matter how well the jibs etc are adjusted.
I know it is difficult for the beginner but the saddle needs to be removed and a small amount milled off the inner side  thus allowing the saddle to bear on the front and rear shears of the bed. After reassembling and adjusting the improvement should be substancial.
I know because it hit me recently when I bought  an old Super 7 and did not realise that  the problem existed on this model as on the straight ML7.
I got this tip from an article in Model Engineer in about 1972 by I think PJ Radford.
Don't forget to buy a new felt wiper and holder from Myford which covers rhe rear shear.
Ray
KWIL29/05/2010 09:52:34
3562 forum posts
70 photos
If properly adjusted on a bed with GOOD quality shears there really is not that much difference, I have both!  The theory says a single long narrow reference, look at a modern V bed, one V along the front.  This does not work quite as well on the early Myfords because of the shorter length of the saddle reference surface which runs on the rear of the front shear, much shorter to allow the tailstock to approach the saddle.  Users tend to over tighten the gib screws on the right hand of the saddle, pulling the saddle in a clockwise direction viewed from above, this can leave the saddle in an unstable condition. With the later modification using both shears, the saddle reference surface, now running on the rear face of the rear shear, now tightens up if you do the same over correction of the right hand gib screws.
Metalmuncher29/05/2010 10:18:42
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34 forum posts
Hi I have an old super 7 thats past its sell by date but it can still put a mirror finish on a piece of steel. No one has suggested using a slow feed and suds to get a good finish. My lathe will cut steel dry with carbide tools but the finish is crap and no amount of emery will make it better. Keith.
Ian S C29/05/2010 10:57:57
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7468 forum posts
230 photos
I thought i,d put something up last night,But I suggested lowering your speed, make sure your tool is sharp, and on center hight. Going by Stan Bray's book 'Introducing the Lathe', for the 1/2" piece you'v been trying, about 700rpm,1" should be 240rpm, 2" 150rpm. Get to work on your tools with an oilstone or a diamond lap, and get them sharp.Ian S C
Chris Trice29/05/2010 11:52:42
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1375 forum posts
10 photos
I'm surprised no one has asked how long the piece of steel is projecting from the chuck and what type of steel it is. A 1/2" diameter piece should project no more than about 2" without using the tailstock centre to support it. Is it free cutting mild steel or just a bit of cast fabrication steel? Steel also likes a cutting fluid. WD40 would do just for this initial testing phase. You'll also get a better finish by engaging the power feed rather than trying to advance the saddle by hand or at least use the leadscrew handle to turn the leadscrew. RPM for 1/2" mild steel on a home machine should be around the 500 number give or take.
Lawrie Alush-Jaggs29/05/2010 15:05:09
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118 forum posts
32 photos
Hi Steve
 
So far no-one has asked you  if you have the bed set up correctly.
When I first bought my lathe I was very disappointed in the quality of finish I got.  More than twenty years ago I bought a copy of the book by the South Bend Lathe Company "How to run a Lathe".  In it is describes how to test the bed for truth.
This is the gist.
Get youself a peice of bar about 38mm in diameter and 180mm long ( I don't know the swing of an ML7 but I am going to assume that you can chuck a peice about that size.)
You want somewhere about 120-130mm protruding from the chuck.  DON'T HAVE ETHE MACHINE RUNNING FLAT OUT.  You want a fairly low speed to run this test. This is not to be performed between centers, it is a test of the relationship between the headstock and the carriage.
Turn two collars about 15mm long each.  To do that, move the carriage in about 15mm from the end towards the chuck and turn down by about 3mm to about 25mm from the chuck.
Return the carriage to the end and with the power feed engaged, take enough off the both collars the ensure that you are cutting all the way around the bar.  Once you have done that, set the crosslide to take a light cut and power feed it the length of both collars without interupting the cut.
Measure both collars with a micrometer.  I am betting you will find that they are of different diameters.  Which means you need to adjust the bed by using the height adjusting bolts or shim under the bed until you get it right.
It is of course made much easier by using a machine level.  It is not that the machine needs to be level as such, just that levelling it allows you so find any twist in the bed.
To use the level, clean the bed under the chuck and at the far end of the lathe and the underside of the level.  Place it under the chuck.  Note where the bubble is.  Place the level at the end of the bed and note where the bubble is there.  Adjust the bed until the bubbles are the same at both ends of the bed.
I think you will find as I did that once you have that under control, your cutting quality will improve markedly.
When I started out as a computer technician, my mate for whom I worked told me "sort out any hard ware problems first.  Anything that is left is software."
In the case of the lathe the same holds true.  If the bed is not true, nothing else will be and no amount of playing with tooling will fix it.
Buying a level is a fairly expensive undertaking, the one I bought cost me about AU$240.00 though you may find that a local club has one that they may lend you.  Or just buy one. 
My lathe is on a wooden bench and so is subject to distortion with changes in humidity so I check the lathe into summer and into winter just to make sure

Edited By Lawrie Alush-Jaggs on 29/05/2010 15:07:10

Steve Morris 329/05/2010 18:16:35
5 forum posts
Thanks for all the info guys, had the family arrive for the bank holidays but will escape and try and check out some of the sage advice given
 
Thanks again
Steve
V8Eng29/05/2010 21:00:26
1730 forum posts
6 photos
Hi Steve.
 
When I started using an ML7 somebody gave me two books, they are:-
 
The Amateurs Lathe, by L H Sparey.
 
Myford Series 7 Manual, by Ian Bradley.
 
These proved invaluable for setting up and using my lathe, and still get used years on.
 
I think they are still in print, and probably available from the My Hobbystore site,

 

Edited By V8Eng on 29/05/2010 21:04:35

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