Depth of cut cowells me90

[kinross1Participant @kinross1]

My first post. I have recently acquired a cowells ME 90 lathe.
I have been using Myford 7 but due to space constraints I have moved it on.
I am reducing some 18 mm silver steel. The best depth of cut I can achieve is 1 thou manual and .8 tho auto.
can any one tell me if this is correct. I am also having trouble parting the silver steel off. as soon as full width of tool is in groove machine stalls. Tool is 2.3 mm wide.
I have put zero top rake on but no better.
any help or suggestions appreciated. Mike.

[JasonBModerator @jasonb]

It should certainly be able to do better than that, My little Unimat 3 can with a fine feed or by hand. What cutting tools are you using. HSS should work fine or if using carbide then get a **GT insert with a small 0.2mm tip radus not a **MT one which are blunt in comparison.

Regarding parting off give the machine half a chance and you a thinner blade, 1.5mm would make it’s life easier. again HSS. Though personally on the small machines I would saw it off and face the cut end.

[kinross1Participant @kinross1]

Thanks Jason. The problem I have is belt slip. They are new belts. The finish I get is excellent. I did wonder about parting tool width. Will thin it down. My tools are all HSS.
I did have a Unimat SL some 50 odd years ago but cannot remember how I managed with depth of cut etc.

Mike.

 

 

[JasonBModerator @jasonb]

Make sure you have not muddled the 410 and 420 belts

I have been known to take 2.5mm (0.100″) depth of cut in steel on the U3 but would suggest you shpould be able to do 10-20 thou without problem.

[kinross1Participant @kinross1]

The belts came with machine. I am not sure what you are referring to. Is it the size of belt and do you know how I identify which I have. It is belt on headstock pulleys that slips.
Thanks again. Mike.

[FulmenParticipant @fulmen]

Have you checked the pullies for oil/grease? And are you sure there isn’t a way to tighten the belts?

[JasonBModerator @jasonb]

The Belts are MB410 and MB420. Usual way to size them is by length (circumference)

The shorter MB410 should go between layshaft and spindle

Some have a lever to alter tension, if it were the other belt then the motor is moved.

[kinross1Participant @kinross1]

I have just checked belts. The headstock is marked 410 and there is no marking on motor belt. Assume it is ok.
Have cleaned belts and pulleys. There were traces of oil.
Have just made a 5 th cut in 18mm silver steel at 500 rpm. Very good surface finish. Will thin parting tool later and try at much slower speed of course .

I do belong to Cowells Facebook forum my first two post a few weeks back went through ok. Now they get stuck in pending  have deleted them and given up. perhaps I have been cancelled

Thanks for all suggestions. Mike

 

 

[SillyOldDufferModerator @sillyoldduffer]

Rule of thumb, one HP (750W) will remove 1 cubic inch of mild-steel per minute.  A sharp cutter must be set correctly and worked at optimum RPM, Feed-Rate and Depth of Cut.

That being so a 90W Cowell will remove about 0.12 cubic inches per minute, assuming the motor can put the necessary torque on the spindle.  Expect less in practice.

The Depth of Cut, Feed Rate and RPM can be calculated accurately, but not necessary in my workshop.  Instead, rule of thumb:

• RPM = 10000/diameter in mm.   (18mm dia is 555rpm)
  • Adjust for material.   The immediate answer is right for most steels.  Halve it for cast-iron, x3 for Brass or EN1A-Pb, x5 (or faster) for Aluminium alloys.
  • Adjust for cutter: halve for Carbon Steel, x3 to x20 for Carbide.  Most lathes aren’t fast enough to cut at optimum carbide speed, but using sharp inserts as fast as the lathe will go works well.
• Then experiment with DoC and FR for best results.   Roughing out I go slower and deeper than finishing, where I speed up and cut lighter at a higher feed rate.
• Though it helps to get the RPM about right, none of it is highly critical.  But much depends on the machine, so adapt to it.  A 90W Cowells is for small delicate precision work and it won’t remove metal as fast as a Myford.

0.12 cubic inches per minute should be achievable on a Cowells with a sharp single point cutter but parting off loads the motor heavily,  hence the lathe is likely to underperform, stall, or slip the belt.

Parting off on a small lathe is a special case – everything has to be ‘just so’.  RPM should be reduced by at least half.  The width of the cut should be kept small, and watch out for friction and jambing as the cut gets deeper.  Trapped swarf and insufficient clearance cause trouble.  Parting off applies puts considerable pressure on the job causing it to bend away as the diameter reduces, and causing the side of the slot to press against the length of the tool.  Fixed by slowing down and lots of cutting fluid helps.   Worse, on my mini-lathe, the tool-post flexed enough to cause trouble.   A skilled hand applying slow steady pressure works, a light cut, not rubbing, but the best answer is a rear tool-post with an automatic drive.  Or a bigger lathe!  Always fancied a Cowells though, small lathes have many attractions…

Dave

 

[kinross1Participant @kinross1]

Hi Dave. A wealth of information. Thank you. Some of it is outside my job description but I will try carbide tipped tools to see how it goes. I suspect coming from a Myford I have become a little gung ho with my settings. Will practice the points you make.
you should change your handle to ‘not so silly old duffer’

cheers. Mike.

[not done it yetParticipant @notdoneityet]

Belts should provide adequate power transmission without slippage.  I would suggest either the belt cross section is incorrect, the adjustment is incorrect or one of the pulleys has been changed/modified.  If V belts, the drive should be on the V, not the bottom of the belt.  90Watts is not a lot of power to be dealing with.

[bernard towersParticipant @bernardtowers37738]

IMHO you are wasting your time using carbide tools on the cowells unless its nasty material, but also I think you are pushing your luck with silver steel of 18mm dia on lathe that size

[kinross1Participant @kinross1]

I have checked the belts and pulleys. All seems in order.
I am not sure about carbide tipped tools. Everything I have read says not much advantage on small machine tools. Never used them on the Myford because the need never arose.
this is a one off job I am doing. So if I take it steady the quality and accuracy of finish is fine. The main use is small parts for my models. Crankpin bushes, truing wheels etc.

I have pondered having a go at a Stuart 10V engine but in the light of experience maybe not. I do have a decent mill so might get away with it.

Thanks. Mike.

[old martParticipant @oldmart]

I got a spray tin of belt slip spray which does improve the grip after degreasing the belt and pulleys.

A lot of problems with lathe tooling is getting the exact shape and sharpness of the cutting tip.

Welcome to the forum, Kinross.

[kinross1Participant @kinross1]

I think my lathe tool geometry is not far off as I do get good finish straight off. The steel comes off in long curls which is good.
May try anti belt slip spray but will wait to see how I get on with later jobs.
thanks. Mike.

[JasonBModerator @jasonb]

I did my first steam engine a Stuart 10V on an Unimat3 would hope the Cowells was up to it. If not then they are even more uover rated and over priced than I thought.

Just been cutting some 3/4″ Silver steel on the U3, will post the video a bit later. But basically what I said above.

[kinross1Participant @kinross1]

I believe that there is a book on building the 10V by Andrew Smith who wrote the Cowells manual. Pretty sure it’s built on Cowells but have not seen book so may be wrong.
As I wrote previously I did have Unimat SL and I remember it was a good little machine. I would love a Schaubin 70 but that is expensive. At 88 years of age  it is a bit late to go for that even if I was able afford it.
looking forward to your video if you manage to post it.
Mike.

[JasonBModerator @jasonb]

I tried some 3/4″ Silversteel first with HSS then with Carbide. In both cases 0.5mm depth of cut (1mm off diameter) was about the max I would want to take and with a 0.1mm finishing cut the Carbide gave a slightly better result, probably due to the 0.2mm tip radius as the HSS was straight off the belt sander.

I did not have a suitable holder for the 1.5mm parting blade and it just chattered being held with teh toolpost screws but I doubt it would have worked in a decent holder. I did roughly regrind a 1mm Carbide parting insert to get teh tool height down and that just about did it. So best to stick with what I said first of all saw the piece off and face the cut.

The U3 is 65W in slow speeds and 95W in the faster speeds, the later was used. Belts are a bit old so there is a bit of clanking.

[SillyOldDuffer]

On SillyOldDuffer Said:

Rule of thumb, one HP (750W) will remove 1 cubic inch of mild-steel per minute.  A sharp cutter must be set correctly and worked at optimum RPM, Feed-Rate and Depth of Cut.

That being so a 90W Cowell will remove about 0.12 cubic inches per minute, assuming the motor can put the necessary torque on the spindle.  Expect less in practice.

 

 

Dave

 

I have always found this statement of 1cu.in per min when scaled down just by motor size to be flawed and theory is nothing like practice.

We don’t just put a smaller motor onto the same larger machine that could remove this fabled 1cu.in/min. We tend to also reduce the size of the machine which will amoung other things reduce the rigidity.

 

Keeping it simple if the Cowells is a quarter the size of that industrial Harrison or Colchester then the cross section of the bed is going to be something like 1/8th that of the industrial machine.

So now we have 1/10th the power  x 1/8th the rigidity and you are looking at 1/80th or 1.25% of that 1 cu.in not 1/10th or 10%

Now putting it into practice as seen in the video opening cut I was only able to remove metal at a rate of 1/333th or 0.3% of a cu.in and the machinability of the material would only have been one other factor of many when looking at small machines in comparrison to large ones.

[Michael GilliganParticipant @michaelgilligan61133]

For a reality-check on expectations … I would suggest viewing the first video on this page: https://cowells.com/

cutting demonstration starts at about 5:55

MichaelG.

[JasonBModerator @jasonb]

Which does confirm my thoughts that they are overrated. What would you say that piece of CI is 3/4″ or 1″ and he is just scratching away at the surface.

Worth noting at 8.52 the belt being slackened off, if there is any adjustment on where it wove sto when the slack is taken up that may help reduce slippage

[HollowpointParticipant @hollowpoint]

He’s using a braized carbide tipped cutter, they are crap at the best of times. Nearly useless on a small lathe, and not a good choice for CI either.

The Cowells isn’t overrated. In fact it’s probably the best of all the small micro lathes by some margin. It’s certainly the most rigid. I can say this as someone who currently has 3 Cowells lathes, a Sherline, an Emco 3 and a Unimat SL.

The OP issues will likely be gear train related. Belt slippage is common. Both the belts and pulleys need to be clean and oil free. It doesn’t take much oil to make them slip. If they have been slipping for some time then they might also become glazed, resulting in them being very smooth with low grip.

The other thing to check is pulley ratios and back gear. Obviously lower gears/ratios provide more torque.

The next thing to look at is the cutter, is it sharp? Is it on centerline? Is it held firmly?

It may seem counter intuitive but small lathes are more difficult to use than big ones. They are much less forgiving.

[kinross1Participant @kinross1]

Thanks for taking time to post video. I have cleaned my belts and sharpened tool. I can now take .5 mm cut with excellent surface finish. Parting off is still a no no so hacksaw and facing off it is.
It is true that coming from Myford to smaller Cowells is a bit of a culture shock. I must be more careful with settings and tool condition. I think the Cowells me 90 is a good machine.
much of its cost comes from being produced and assembled n this country. Up until a few years ago Brian Childs checked out and I think assembled every machine. ( think that was his first name ). They have now changed hands so do not know how much of the old philosophy still apples.
Mike.

[JasonBModerator @jasonb]

well as that is what Cowell’s choose to put on their website a san example of the machine in action then they ar enot showing it in a very good light. As I said I would have expected one to perform about the same as my similar size Unimat.

But as you have three of them perhaps you could answer the OP’s question and say what DOC he should expect in 18mm Silver steel?

I would have expected speeds and tool type/position to be beginners issues but a sthe OP has previously had an SL and Myford should know how to sharpen and mount a tool.

I don’t find the brazed tools to be much of a problem and do use them fairly ofyten, that 190mm Cast Iron “bit on the side” thread of mine last week had some of the machining done with a brazed tool. Also that Minnie in my avitar was almost all done with brazed tip tooling.

Anyway I thought as the U3 wasw still out on teh bench I would take a couple of cuts in some 25mm dia cast iron. 0.5mm DOC seemed fine and a faster feed than in the Cowells video with HSS, Inserts and brazed carbide all working well. (first cut is actually 1mm

[JasonBModerator @jasonb]

just looking at some old Cowells catalogues that I have and there is an adjustment for how much tension the belt lever will put on the belts. may just need a small adjustment to stop the slipping.

Looks like it is just thr head of an M5 or M6 Hex head screw what bears against the cam. No sign of a locknut to stop it moving.

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