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Member postings for Bob Jepp

Here is a list of all the postings Bob Jepp has made in our forums. Click on a thread name to jump to the thread.

Thread: Blacking engraved lines
10/02/2019 19:08:02

The method used at Colchester Lathes was to black anodise the whole dial and then skim the dial surface, removing a small amount of material and leaving the blacked engravings.


Thread: Kempes yearbook, worth getting?
14/08/2018 19:40:07

Mine is Kempe's - 1984.


Thread: Inverter Drives for Motors
03/08/2018 15:27:48

The use of inverter drives in industry has been very common for many years - I think I've been using them for 25 years or so.

Whilst the integration of inverter drives requires some degree of expertise, it is not beyond the reasonably skilled engineer ( legally known as a competent person ) assuming that they read and can interpret the supplied documentation, which is usually quite sizeable ( most inverter drives also come with a quick start document to allow knowledgeable people to short-cut reading the whole manual ).

Integration of the controls for an inverter drive will be covered within the manuals, but beware that one will have to change some parameters to get these controls working.

The primary issue - in industrial and domestic installations, is to make sure that the device is safely installed. Electrical installation must be carried out by a competent person who will understand the wiring requirements although from my experience, it is unlikely that they will understand the implications of guard closed limit switches and emergency stop push buttons. It is insufficient to connect these safety devices to normal digital inputs to the inverter drive since they pass through the software which runs the drive and are therefore considered to be UN-SAFE.

As discussed on several threads, it is possible to implement the standard no-volt method for removing power from the supply terminals to the drive and in some cases ( see the manufacturers data book ), it may be permitted to place the disconnect between the inverter output terminals and the motor. One should note that it may be necessary to feed-back an additional contact of the disconnect to shut down the drive.

Generally, disconnection of the infeed power will shut down the drive, but one must understand that there will be some stored energy within the drive and therefore the motor may run-on for a time even though the emergency stop has been actuated - an unsafe condition. Disconnecting the output of the drive from the motor can cause premature failure of the output electronics of the inverter rendering it faulty.

Since these issues have become really important in industrial installations, inverter manufacturers has added safety rated inputs to the inverter which disconnect the output power from the motor within the inverter rather than externally - such inverters allow repeated safe stopping through emergency stops and guard safety switches without risk to the user or the device.

Many inverter manufacturers provide units with the safe stopping facility and anyone looking to implement an inverter drive on a machine should contact the manufacturers for advice ( I would add that inverter manufacturers who are active within Europe will provide CE marked products - those from more doubtful sources might suggest that CE stands for China Export ).

Thread: Drilling Holes
07/07/2018 20:23:57

Drilled some holes in the cylinder support of an Elmers Open Column engine - 7BA clearance last weekend. When deburring the holes, I noticed that they looked quite large on one side of the component ( the top side whilst drilling ), and a lot smaller on the other side, Closer inspection of the holes showed that they were considerably oversize except for the last fraction of a millimetre where the drill broke through.

Looking at the drill point through an eye loupe revealed that the geometry was far from normal ( couldn't see enough detail without the loupe ). Although the point was in the centre of the drill and the point angle was similar on both sides, the Lip Clearance Angle ( according to the Presto cutting tool pocket book ) was wildly different one side to the other, in fact on one side there was more than sufficient clearance but on the other side, the cutting edge was below the flank causing that side to rub and, according to the book, causes exactly the symptoms I've seen in my holes.

Looking at all the drills in the set ( a budget set of number drills - long since forgotten the origin ) gave me a tally of roughly a third with incorrect lip clearance. Whilst I can probably just about re-grind the points on drills down to about No. 31 or 32, I find it too difficult to go any smaller, so a new set of Dormer drills were procured ( and inspected through the eye loupe - perfect ).

The other issue I've been having is with the actual drilling machine. Whilst I use a spotting drill to locate the holes from the readout on the Chester 626 mill, I find the spindle control has too little 'feel' with smallish drills, so I invested in a Sieg X0. The first real model engineering task for the X0 was drilling the hornstays / pedestal ties of my O gauge Josie clearance for 10BA. I found the speed of the drill very difficult to adjust - even on the low speed pulleys, which caused the tiny parts to become far too hot to handle. In addition, the drill seemed to struggle - only about a 1.8mm drill. I checked the speed with a tacho - should have been 0-3600 rpm was actually 0-7500 - no wonder the components got hot and there was very little power to make swarf.

I stripped down the drive motor and pulleys to investigate and the motor legend plate says 230V ac 150W and 18,000Rrpm - I wonder if the variable speed drive is only a half wave unit which may explain the top speed and the poor speed / power control. I spent some time looking to see if I could track down a lower speed motor as a swap-out, or maybe add a second stage pulley reduction but it doesn't seem that practical a solution.

Has anyone else on the forum got one of these machines and experienced similar issues and hopefully got some ideas ?

Thread: Show and Tell Event for Forum Members?
18/05/2018 03:53:54

I’m up for a meet up - Rugby is quite near Neil I think.

Thread: Drawing board v CAD
18/03/2018 19:35:18

Neil /

Quite agree - I've done it both ways around !

18/03/2018 19:06:19

I think that there is a more important issue here than drawing board versus CAD - having worked in industrial automation for the last 25 years, I fear that the understanding of what is drawn has slid away from us.

At school, I started using the board and tee square at 11 years old, being taught by Mr Turbin - an experienced draughtsman turned teacher. I started work as an apprentice and wasn't allowed to go on the board until the third year, having completed the training schools first year and some time on the shop floor.

I've just completed a project with one of my customers whose 3D CAD operators, whilst very capable on the computer, don't have the practical experience and therefore make silly errors claiming that 'it looked alright on the CAD' !

Any method of producing drawings is great, as long as one can pass over the intended information - what do I do, well a lot of scribbly sketches on scrap paper, some pencil drawings in 3D and 2D & 3D CAD - using the appropriate method for the job - as long as the information is easy to follow. My favourite - a quick 3D pencil sketch passes over ones ideas as quickly as anything.


Thread: What did you do Today 2018
31/01/2018 21:26:12

Thanks guys. I've manages two ME's today so far, and read the forum 3 or 4 times !


31/01/2018 20:53:16

Collapsed in the workshop on Sunday - no warning, just my wife asking me what was wrong ! Sitting in hospital with all the fantastic medical attention is good - at least I'll get chance to catch up on the ME reading backlog !


Thread: The diesel controversy
04/05/2017 00:34:24
Posted by Neil Wyatt on 03/05/2017 21:26:43:

Proper testing of diesel cars in the answer, it will mean they bring in decent particulate filters and adblue - but if they siphon off a fraction of the budget for heated steering wheels and 'apps on your dashboard'...

Trying to decide what vehicle to go for - it seems that petrol is more expensive to run and higher carbon emissions, diesel cheaper to run ( it seems that recently designed vehicles have particulate filters and adblue ) and hybrid cars are extremely expensive and nowhere near as economical as the manufacturers claim.

Quite agree about the apps - can you believe that you need a smartphone app to fold down the rear seats in a Land Rover - BUT - heated steering wheels - the best thing since sliced bread ( had mine on tonight driving back from Tesco's ) !

Thread: Motor connection
02/04/2017 12:39:17

Thanks Dave, now I'll have to let you into a secret - I have risk assessed my lathe and reverse engineered the circuit diagram only to find out that the 'so called Emergency Stop' in no way conforms to the legislation.

The stop side of the inverter, which included the guard safety switches, are open contacts which close when the Emergency Stop is pressed - a serious no-no in control systems. Needless to say, it's on the list of jobs to address - but I haven't as yet come up with a solution as the inverter is Chinese and I'm not understanding their manual too well !

Just a little explanation of the no-no :

Safety contacts should be normally closed and open when the device is actuated - Why ? - well several reasons :

  • Firstly, should the wire fall off, the circuit will shut down,
  • Secondly, should the switch fall apart, failing in the open mode, the circuit will also shut down and
  • Thirdly, should the switch contacts weld closed, the actuation of the mechanical part of the switch will forcibly disconnect the circuit ( albeit leaving the device permanently damaged, but better than an injury - eh ).

Using standard European Approved Emergency Stop push buttons will provide these safeguards, but, limit switches for guard interlocks are not so easy to specify correctly as the safe operation of the switch depends upon the correct mechanical application of the switch as well as the electrical aspects. Just take a moment to think about a limit switch applied to a safety guard - on my milling machine, there is a belt guard safety switch which is depressed when the belt guard is closed, relying on the spring inside the switch to operate the mechanism when the guard is opened - what happens if the spring fails or the switch jams ?


01/04/2017 22:51:16

The original posts regarding the starting and stopping of an electric motor on machinery - the machine will need the following :

1. A disconnection device which will default to OFF should the power fail and,

2. An overload detection device to switch off the motor in case of overloading,

The terms used in the previous posts should be understood as follows :


The terms DOL ( Direct On Line ) refers to a device which starts the motor directly from the supply - i.e. no other device controlling the motor current.


No Volt Release refers to the function whereupon the starting device will automatically drop out should the power supply fail and will remain off until the start control is re-actuated.

Normally, the disconnection device relies on an electromagnet to hold in a ( or several ) switch contact(s). The electromagnet is powered from the supply via a Start push button and is held in by a closing contact of the device together with a Stop push button.

These devices may be found at a number of our suppliers, I personally have used some devices from Axminster ( just because they are the most conveniently located supplier - no other reasoning ) which incorporate the Start and Stop push buttons as well as the NVR. Regarding overload protection, a correctly rated fuse is the absolute minimum requirement although various other specialist motor overload devices are also available.

Make sure that you do a risk assessment ( sorry - had to mention that ). You should consider the following and, in addition, any other pertinent points :

1. Conveniently located start and stop button ?

2. Once started, does the motor turn off AND STAY OFF if the supply is switched off and then on again ( NVR ) ?

3. Do you require any additional Stop devices such as guard safety limit switches of extra Stop buttons ( must be normally closed contacts in series with the Stop push button?

4. Have you incorporated an overload device which will shut down the NVR when it trips ?

OK, sorry about that, on to the inverter controlled motor - Lets be clear about the meaning of an Emergency Stop. Emergency Stops must safely remove energy from driven devices. With inverters, there are several ways to do this :

1. A disconnection device between the power source and the inverter,

2. A disconnection device between the inverter and the motor,

3. An inverter with built in safety outputs,

Considering each option :

1. A disconnection device ( NVR ) between the supply and the inverter is generally considered to be the safe option, but, the inverter will retain some charge at power down allowing the motor to continue running for a short time. Your risk assessment will tell you whether this is acceptable - industrially this method is going out of favour,

2. A disconnection device between the inverter and the motor potentially has the capability to remove the load from the inverter whilst running at 'full whack' causing a catastrophic failure of the inverter. This is obviously not good although some inverters are capable of withstanding this and others may or may not have a special input to feed back the status of the disconnect device to shut down the inverter when the NVR switches off,

3. Inverters with safety outputs are becoming increasingly the way to go. They incorporate two dedicated inputs whose job it is to shut down the outputs of the inverters in a guaranteed safe manner. Inverters incorporating these safe outputs need to be specifically selected,

So, the difference between Stop and Emergency Stop is that Emergency Stop must remove power from the motor in an approved safe fashion - again ( sorry ) your risk assessment should ask the question "When is Stop safe enough ?". If you are putting your hands into a lathe, maybe holding a chuck key, do you want to rely on the inverter to stop the rotation of the chuck ?

Just bear in mind that all modern inverters rely upon software to drive the motor - the basic principle of safe design is that software can fail and Murphy's Law says that it will always fail in the most un-safe way possible.



Edited By Bob Jepp on 01/04/2017 22:56:02

Thread: Beginners lathe
20/01/2017 18:11:23

Lets get back to the topic !

Many years ago ( I'd hazard a guess at 35 years ), I bought a Toyo ML210 micro-lathe - it's very similar to the C0 machine in many respects except for one major point - its Japanese. I used the machine for many years making lots of small parts for a Tich, a 2' scale Clayton Steam Lorry and a plethora of other items and, I can say that the engineering of the machine was typically Japanese ( very high quality ).

A couple of years ago I decided that I needed some additional tooling for the machine, but due to the fact it has been out of production for many years, I couldn't find what I needed. I decided to buy a new small lathe and having visited both Arc and Axminster, decided on the Axminster C0 ( I liked the colour ).

On a previous forum post, I commented on the engineering standards of the Axminster C0 - it was dreadful - I didn't need another project, so I returned it and got my money back ( I didn't even get to cutting metal ).

I've since bought a new Cowells ME90 - I spent some time talking to Colin Childs at Cowells - he said all the right things about the quality standards he works to, and I am so impressed with the machine.

So :-

Toyo - was about £60 when I bought it - really good value for money - still got it, still works well,

Axminster C0 - enough said, got my money back on it,

Cowells, fantastic machine, but it should be at just over £3000.

Why the Cowells, well, apart from the quality, it has back gear, a full screwcutting capability and a common design M14 threaded spindle nose. I have one criticism - the MT0 headstock and tailstock tapers - very difficult to get tooling to fit this unless you go back to Cowells.

Thread: what did santa bring?
30/12/2016 15:12:57

My two boys bought me a selection of imperial sized end mills to replenish my stocks ( from Arc ), wife bought me a solid gold lens hood for the SLR ( must be solid gold at the price - surely it can't just be plastic ! ), a magnifier / lamp from the Midland ME Exhibition and a MeArm robot kit.

Although programming robots is my trade, I am fascinated by the underlying motion control software which I know almost nothing about, so, as the MeArm can be controlled with an Arduino i'll be able to experiment.

My oldest lad ( he's over 30 year old ) had a Lego Technic Bucket Wheel Excavator kit - a huge model and so complicated, loads of gears and shafts - a fully working model ( kept him quiet for two days ! ).

Had a day out shopping on Wednesday - dropped in to Arc as my brother-in-law ( visiting for Christmas ) wanted some bits and pieces and was greeted cordially by Ketan, his good lady wife and had the usual lengthy chat during which we put the world to rights - came away with a bag full of assorted stuff.

Thread: 3D mouse
01/11/2016 00:32:14

I don't do Fusion either - it's Autodesk Inventor for me.

I use a Logitech something or other ( legend long since worn off ! ) with a scroll wheel for zooming etc. and forward and back buttons.

Absolutely essential - I find as I haven't mastered the standard mouse in 3d yet - a 3DConnexion Space Navigator ( the basic one - it was cheap at about £40 when they first came out ) for pitching, rolling, yawing and XYZ panning - once you get the axis set as you like them it becomes so intuitive.


Thread: Brushless DC motor for machine tool power
16/07/2016 14:53:31


Had a Google at LiPoly batteries and it would appear that a 6s battery, as specified as maximum in the ad, gives 22.2 volts - therefore we are expecting 1600 rpm/v x 22.2v = 35,520 rpm. Sorry, but that still sounds impractical when compared to an industrial 24,000 rpm spindle.

Putting the motor design to one side, the practicality of running a 9:1 worm at 35,500 rpm also sound impossible - maybe an epicyclic gear could cope, but I would expect that cooling and lubricating will be an issue here also.

Motors in CNC machine spindles are usually direct drive, axis drives are usually via ball-screws giving a reduction ratio of the pitch / circumference of the ball pcd and robots tend to use Harmonic gearboxes, Cyclo-drives or RV reducers all of which are driven with servo motors running at max 4000 rpm.

16/07/2016 14:03:46

I've read the ad several times and so far, I can't see the RPM quoted. Working backwards 4000 rpm spindle through a 9:1 worm would require 4000 x 9 = 36,000 rpm at the motor spindle - I can't see that happening, industrial routers running at 24,000 rpm have bearing issues and they are totally enclosed, built in semi-clean room conditions.

Have a look on Arc Euro Trade\s web site at the high speed spindles to see the difference. I've also looked at Zapp Automation for ideas.

Thread: Parting off
11/07/2016 01:18:36

Funnily enough, I've not had any trouble parting off ( that's it - I've done it now - next time will be a wrecked parting tool ! ). I use a couple of Glanze 2mm wide throw away tip parting tools - for small diameter stuff, the tool-holder with a maximum capacity of about 25mm diameter and larger, one of the blade type.

I reduce my RPM, then plough straight in and it all seems to work out. With steel or stainless, a few drops of thin oil every few seconds, aluminium a squirt of WD40 and, brass and plastics dry.

I think that confidence has a lot to do with it - we were taught in the training school to 'get stuck in'. I remember parting off rectangular section bar about 50mm x 25mm for some EITB ( Engineering Industry Training Board ) exercise or other ( now I'm going to have to go and look in my apprenticeship made toolbox to find out what we used that rectangular bar for .

Thread: Knurling Tools
10/07/2016 17:05:26

When I was in the apprentice training school in 1974, the machines were much larger than those in the usual home workshop. The standard issue knurling tool was the rotating disc with three sets of knurls, just pushed into the job with the cross-slide.

Although I have a reasonable size lathe - a Warco VS1224 12" swing - I went for the clamping type knurling tool as I thought it should be best since, as we all agree, the forces are contained within the tool rather than being transferred to the machine.

When knurling, I firstly set the clamping screw to make the knurls touch the diameter and for them to start rotating without making much of a mark. I withdrew the cross-slide and added a bit more to the clamp screw then fed in the cross-slid again taking the knurls just over centre.

As I said, the first knurl I did was really good, but adopting the same process on the second part left me with a knurl with the peaks broken away and the knurls covered with the remnants.

I took in posters suggestions on knurling speed, maybe I was over-egging it bit at 150 rpm. Maybe too much depth of knurl, maybe feeding in the cross-slide does something to the geometry of the knurl and causes it to break up, or, maybe a combination of all those things ?

I have a much smaller Soba knurling tool as well - I'll try that when I get in the workshop again - it's much more rigid side-to-side than the one I used this time. There must be some logic somewhere in getting a good knurl !

09/07/2016 17:26:30

I've just come in from the workshop where I tried making a couple of small thumbscrews in brass - 9mm od.

I turned the outside diameter and applied the medium diamond knurl using a proprietary clamp type knurling tool. The first thumbscrew knurl was pretty good, so I finished the screw and parted off.

When applying the knurl to the second screw ( making sure that the diameters were the same within 0.01mm, without altering the setting of the knurling tool and using the same speed of 150 rpm ), the knurl as terrible. I did clean the knurling wheels between screws but I didn't use any lubricant either time - maybe I should have ?

I notice that the knurling wheels don't line up - the top arm as skewed relative to the bottom arm by about 1/2 a knurling wheels width and the fit between the pivots and the arms seem very loose.

Any thoughts ?


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