Unimat3 vs. Sherline lathes

[david bennett 8Participant @davidbennett8]

Dave, my scooter motor i rated at 120 watt. It is a brushed pm motor. Naturally, being Chinese , I suspect that is the wattage going in, and less coming out. Its more than the original, but not by too much.

dave8

[david bennett 8Participant @davidbennett8]

double post

dave8

[david bennett 8Participant @davidbennett8]

ps -my drive belt is 4mm diameter green fusible.

dave8

[SillyOldDuffer]

On SillyOldDuffer Said:

On david bennett 8 Said:

Perhaps I should have mentioned that my u3 is powered by a variable drive scooter motor and has vee pullies and round fusible belts. …

 

 

dave8

The motor and drive train are important because they decide how power and torque are available at the  cutter.   In principle, putting a more powerful motor on a machine will enable it to remove more metal faster, but overdoing it may not work out that way.   Belts slip, bearings wear, the carriage lifts, the tool-post twists, the cutter bends, and so do the head and ways.  Small lathes aren’t particularly strong or rigid.

How powerful is the scooter motor?  The U3’s designer deliberately specified a rather small motor: probably proportioned to match the rest of the machine and improve precision and longevity.  Maybe the scooter motor has taken the U3 out of it’s comfort zone, so counter-intuitively it doesn’t compare well with a factory Sherline?

Dave

Hi Dave,

All good points. The motor attached to my U3 before reconditioning was a 1/4 HP Capacitor Start Induction motor. This was hung on the existing Motor Bracket. There is no doubt this was inducing a twisting motion on the bedway.

The Hoover Belt was so tight it had distorted the plastic pulley. The motor was absolutely full with brass swarf despite all the cooling vents being taped over with Parcel Tape.

No doubt the previous owner had thought this was an improvement.

The U3 I have also sports a Scooter Motor, but I have set my PWM to mimic the output of the original two speed U3 Motor. This way I can change the belts to get the same speeds as the Speed Advice Plate on the Headstock. This works for me 99 times out of a 100.

Regards

Gray,

Generally,

When it comes to Turning Hardened steel, this is something I would never contemplate on the U3. It would be far better to grind the part using something like a Dremel. In my experience of turning Hardened tool steel there are three requirements. The first of these is rigidity, then the correct insert followed by a decent amount of power.

Sadly the Unimat is lacking in some of these departments and I doubt the designer never imagined this in his or her wildest dreams. At the time this machine was designed Carbide Insert tooling was only just coming into use in Industry. A lot of companies were still using inserted HSS face mills etc or Brazed Carbide tip tools. Whilst this type of Brazed tooling is widely seen to be used on the Unimat, this machine was none the less designed to use HSS.

There used to be a programme on the BBC called “Engineering Craft Studies”. As first year apprentices we would watch this every Friday morning. Slow speed footage of tools cutting is of great value as to what is happening at the cutting edge. A dull edge will require more power to remove metal.

One good thing the Unimat taught me from early on is when the tool needs re-sharpening.

Carbide inserts have far from a sharp edge. Under a microscope the standard edge is rounded, (that is not the Tip radius). That is unless they have been subsequently ground prior to sale, and these inserts are expensive.

Regards

Gray,

[david bennett 8Participant @davidbennett8]

Gray, the carbide boring bar I used was sharp, but the low doc. was almost certainly caused by an over- large radius. I take your points, but don’t forget that I am looking into why the Sherline e.g appears to perform better. This has led to an examination of the spring washers (the only substantial difference) Why were they used? No other maker that I know of uses them. A simple extra thread on the spindle would have allowed the conventional system to be applied. It seems to me that a large pressure on the spindle may overcome some of the springiness of the washers and let the spindle thrust vary, causing chatter. An increase in the thrust on the front bearing may not be matched by an increase on the rear bearing, as the rear bearing is relatively free to move in the headstock.

dave8

[JasonBModerator @jasonb]

If it were an insert bar rather than solid then at a minimum you would want an insert designed for the difficult metal alloys, better still a CBN insert to stand a half decent chance with a bearing that could be upto 62 Rockwell

Trying to cut with a lesser general purpose insert even if new will not really work, even a **GT will struggle.

As Gray says grinding would be another option.

[david bennett 8Participant @davidbennett8]

Luckily, when I tried this and achieved a cut, I didn’t know it was impossible!

dave8

[Graham Meek]

On Graham Meek Said:

[…] There used to be a programme on the BBC called “Engineering Craft Studies”. As first year apprentices we would watch this every Friday morning. […]

Allegedly still available, on a “reassuringly expensive” DVD

http://bufvc.ac.uk/dvdfind/index.php/title/6762

MichaelG.

[david bennett 8]

On david bennett 8 Said:

… but don’t forget that I am looking into why the Sherline e.g appears to perform better. This has led to an examination of the spring washers (the only substantial difference) …

But the spring washers aren’t the only substantial difference!

Your EMCO is considerably modified, not necessarily for the better, and is considerably older than a Sherline.   Nor are the Sherline and EMCO set up with the same cutter or the same material.

The spring washers might be limiting the EMCO’s performance as a design fault,  but if so the lathe was successful for decades without anyone else noticing.  (As far as I know.)   At the moment, nothing has been proved. and the evidence inconclusive.

Any chance of setting up a Sherline and your U3 side-by-side to do an identical test run?

In terms of motor power, Sherline’s website makes this interesting comment comparing their current 60W DC motor with the 1/2HP AC motor supplied before 1994:

The 90-volt DC motor provides far more than the 1/2 HP AC/DC motor we offered before 1994. A DC motor provides its maximum torque at low RPM where you need it most, so the full speed horsepower rating of an AC/DC motor can be a little deceiving as far as offering practical, usable power.

They’re saying that an electronic speed controlled 60W DC motor on their lathe outperforms a 370W AC/DC motor on their lathe.  Could be true, because the power, torque and efficiency curves of a universal motor peak at wildly different places, so you only get full value from that 370W at one operating point.   Everywhere else across a universal motor’s speed, power & torque range the output will be inferior.

I’d expect a 120W scooter motor to do a good job, but could there be something wrong with it or the controller?  Could you lash up a dynamometer and measure how much power is actually reaching the chuck?   Might be easier to replace the washers temporarily and see if that fixes the problem.   If it does you were right all along!

Dave

 

[david bennett 8Participant @davidbennett8]

S.O.D., no chance of a side-by-side lathe comparisom. I have never even seen a Sherline in real life. I think the motor and controller are ok. – it did achieve the cut. As you say, the only way is to replace the washers, but that would mean permanently altering the original spindle. Before doing that, I thought I would ask here for comments. I don’t think my lathe has any meaningful faults.

dave8

 

[david bennett 8]

On david bennett 8 Said:

Gray, the carbide boring bar I used was sharp, but the low doc. was almost certainly caused by an over- large radius. I take your points, but don’t forget that I am looking into why the Sherline e.g appears to perform better. This has led to an examination of the spring washers (the only substantial difference) Why were they used? No other maker that I know of uses them. A simple extra thread on the spindle would have allowed the conventional system to be applied. It seems to me that a large pressure on the spindle may overcome some of the springiness of the washers and let the spindle thrust vary, causing chatter. An increase in the thrust on the front bearing may not be matched by an increase on the rear bearing, as the rear bearing is relatively free to move in the headstock.

dave8

Emco used the Belleville washers as they provide a constant loading on the bearings. As I have pointed out already, under a load of 28 lb the pre-load on the bearings did not budge. Emco used this system as it does not require someone setting a preload on the bearings and having to check that this setting meets the design specification, during assembly. From a manufacturers stand point it is tamper proof, (almost). This is not the case when it comes to manually adjustable bearings. Which from experience I have found are usually overtightened.

Professor Chaddock who designed the Quorn Cutter grinder and a much respected member of the Model Engineering fraternity. Used Spring Preloading on the Wheel Head Spindle of the Quorn. His design was based on advice from Hoffman Mfg Co, (the bearing manufacturers, later to become RHP). This system is found on many such spindles.

I would have expected such advice to be given to Emco by SKF, FAG, etc. This would not be something Emco would do without consultation.

I think you are confused with the Cutting Edge Radius and the Nose radius of an insert.

 

The above insert shows quite clearly a radius running the full perimeter of the insert, on the cutting edge, it even goes around the Nose Radius. This is always present to a lesser or greater degree on all moulded inserts. The only time it is not there is when the insert has been finished ground.

Your insert might have been new, but it will have lost this edge as soon as it made initial contact with the Hardened material.

Regards

Gray,

Hi Michael,

Thanks for the link, but I have seen them on YouTube from time to time.

Regards

Gray,

 

[david bennett 8Participant @davidbennett8]

Graham I was not using an insert cutter. It was a solid carbide bar ,sharp on the cutting edge and with a large nose radius ( this was to go in an 8mm hole)

As for belleville washers presenting a constant force on the bearings I think that is open to question for reasons already given ( the rear bearing outer shell is relatively free to move within the headstock)

dave8

[david bennett 8Participant @davidbennett8]

I have just had a thought that I would like to add. Consider the Unimal db/sl. It ight be simpler to modify this sprung spindle to the more conventional style. After all, the watchmakers spindle does not use belleville washers.  If it proves to be advantageous it would add to the evidence.

dave8

[david bennett 8]

On david bennett 8 Said:

Graham I was not using an insert cutter. It was a solid carbide bar ,sharp on the cutting edge and with a large nose radius ( this was to go in an 8mm hole)

As for belleville washers presenting a constant force on the bearings I think that is open to question for reasons already given ( the rear bearing outer shell is relatively free to move within the headstock)

dave8

Until this latest post I for one had no idea what size the bore was. However my smallest Sumitomo Insert boring Bar will enter an 8 mm hole. The inserts for this bar are finished ground. You could probably by several boxes of inserts for the cost of one of these inserts.

The rear bearing needs to slide in the housing or how else will the spring pack pre-load the bearings and take out the play between the two bearings.

The tool cutting forces applied are additional to the the load this spring pack is generating.

A negative load is only generated when the tool is cutting towards the tailstock, (which was the direction of my loading, ie against the spring pack).

The Emco construction is after all a standard bearing configuration, which is widely used throughout industry.

Clearly you are not a fan of this construction.

My 40 years in Toolmaking tells me there is nothing wrong with this set-up. I think it best if we agree to disagree.

Regards

Gray,

[John HaineParticipant @johnhaine32865]

I have a U3 and have looked at Sherlines at exhibitions and IMHO the Sherline is far better made.  The Unimat just doesn’t have a nice precision feel.  I bought the U3 before I fitted a 3ph motor and inverter to my S7 for small diameter high speed turning, but having upgraded the S7 I never use the unimat.  One day I’ll pop it back on eBay…

[david bennett 8Participant @davidbennett8]

John, thanks for your comments on the Sherline. That is a surprise. They don’t look too well made in photos.(I am using the u3 more as I get less mobile – at least it is small, light, and versatile, as I imagine the Sherline to be )

dave8

[david bennett 8Participant @davidbennett8]

I recently bought a Chinese solid state luggage scale. I used it to measure the compression on a u3 spring washer stack and got a figure of about 13 lbs. So this is the constant preload on the u3, and considered good for the bearing life. If a constant preload of 13 lbs. was applied to the sherline spindle, it would be considered too high, and bad for the bearings. Who is right?

dave8

[david bennett 8]

On david bennett 8 Said:

… If a constant preload of 13 lbs. was applied to the sherline spindle, it would be considered too high, and bad for the bearings. Who is right?

dave8

Um, depends on the bearing!  In theory, the preloaded bearing used by the U3 should be superior to Sherline’s plain ball-bearing, but this might not be the case.   Bearing technology has moved on since the U3 was built, so a modern well-made ball-bearing could be equivalent to Emco’s choice.  Dunno!

Then its starts to get complicated.  Sherline’s website says their headstock can be had with:

• cheaper ball-bearings that don’t require pre-load, ‘The C3 bearings have been standard in all of our headstocks for years and have proven reliable over that time.‘
• Or, a more expensive bearing that’s preloaded for distance, not pressure.   The pre-loaded bearing gives better performance up to 4000rpm, but Sherline recommend allowing more end-play for operation up to 10000rpm.  Note that some bearings are preloaded by weight, whilst others are preloaded for distance – not the same.

Engineering is more often about compromise than rights and wrongs.   Unless the designer explains, as Sherline do, it’s hard for an outsider to suss out what compromises have been made.   So an innocent Sherline owner might cough up for costly preloaded bearings,  and then be disappointed due to not realising there’s a 4000rpm recommendation.   Unless the manual explains, the U3 owner has no idea what the machine’s sweet spot is.  He has to find out by using it.

Comparing Sherline with a U3 is interesting, but not simple.   They’re not identical, even to there being two distinctly different Sherline headstocks.  Their relative performance is a combination of many factors, not just the bearings.  Both machines have a good reputation for the class of work they do, and I suspect the difference is largely academic.   The opinion of someone who has used both lathes in anger would be much more valuable than my theorising.   Any takers?

Dave

 

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Perhaps … just perhaps … this might inform the discussion:

https://glockcnc.com/sherline-spindle-headstock-upgrades/

I saw this page a while ago, and have at last found it again.

”Get thee behind me Satan”

MichaelG.

[Graham MeekParticipant @grahammeek88282]

The loading of 13 lb supplied has confirmed my initial thoughts that the Spring Pack has not been assembled correctly.

My Spring pack easily exceeds 28 lb and is assembled as Emco intended. To compress this spring pack when I re-assembled the 8 spring washers used. I needed two pieces of M6 studding in the motor mounting bracket holes, a U shaped bridging piece to allow access to the circlip and two M6 nuts to wind down the bridging piece.

At 13 lb I could have easily pushed the assembly together with my bare hands.

I am all for comparing like with like but there needs to be a level playing field with machines in as manufactured condition. There is also the difference in the length of the cross-slides of the two lathes to factor in.

Regards

Gray,

[david bennett 8Participant @davidbennett8]

Gray, that makes the comparable sherline even more over-preloaded.

dave8

[Graham MeekParticipant @grahammeek88282]

I have just set-up a Dummy Spring pack from the spare washers that I have available. The Spring pack in the free state is 6 mm overall height.

Under a static load of 14 lb this overall height was reduced to 5 mm. Thus the Spring pack has a rate of 14lb/mm.

If memory serves me correctly there was more than 1 mm of movement before I could get the circlip into the groove.

Thus my 28 lb is in the right ball park if this movement was, as I suspect 2 mm. If this movement was only 1.5 mm then it would still be 21 lb.

Therefore to move the spindle against the Spring pack initial preload would need a loading greater than either of these figures.

Regards

Gray,

 

[david bennett 8Participant @davidbennett8]

Gray, I can accept that. I was guarding against the stack bottoming out and giving a false reading. I am confident the pack is assembled correctly.

[SillyOldDuffer]

On SillyOldDuffer Said:

.  Both machines have a good reputation for the class of work they do, and I suspect the difference is largely academic.   The opinion of someone who has used both lathes in anger would be much more valuable than my theorising.   Any takers?

 

I have both. The Sherline is better. I’m not sure why but I would put it down to the overall build quality being a bit more sturdy. The tolerances are probably better on the Sherline too, being churned out on a CNC.

 

 

[Michael GilliganParticipant @michaelgilligan61133]

My third attempt at posting this

Another Glock link:

https://www.youtube.com/watch?v=8SKFKwAs_f0

 

About 9 minutes of explanation about their bearing options.

MichaelG.

[Author]

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