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Member postings for Robert Atkinson 2

Here is a list of all the postings Robert Atkinson 2 has made in our forums. Click on a thread name to jump to the thread.

Thread: Loctite
12/11/2019 18:17:22

Did I religiously check the expiry dates on "consumables" when I was quality manager for an aerospace company - of course I did.

Did I then take it home a few days before it expired - well if I'd waited until it expired the company woyld have to pay to dispose of it as chemical wastedevil

Edited By Robert Atkinson 2 on 12/11/2019 18:21:40

Thread: Inverters and stop switches
11/11/2019 07:37:12

Those who say they cannot see how disconnecting a running motor will damage a VFD do not understand high power electronics. When the current though a coil (inductor) changes there is a voltage developed that is proportional to the rate of change. For disconnecting a motor, that large motor inductance can cause enough voltage to case arcing at the disconnect contacts (and possibly feed back into the drive causing issues) but more importantly for drive damage the inductance of wiring and even transistor internal connections on the drive side can cause voltage spikes high enough to cause damage. Yes you can design to account for these but don't bet on a cheap no name inverter having such protection. This does not mean every drive will fail the first time you disconnect it the magnitude of the spike and ability to cause damage depends on (among other things) the mains voltage, load, speed, temperature and most importantly the exact timing of the disconnect in relation to the load current.

I note that the drives direct advert that was linked to says they are "special" in respect of switching loads and also that they need to be rated at FiIVE to EIGHT TIMES the actual load. So for a 1hp motor you need 5 to 8 HP inverter with presumably a suitable input supply. Most electronics will take mose abuse if you only run them at 20% of their design rating.

Robert G8RPI.

Thread: Drilling big holes (in tiles)
09/11/2019 16:39:43

For cutting I'd suggest a diamond core drill rather than a hole saw. Difficulty is holding it in position. Make sure the drill has a clutch, if the bit snatches it will break your arm without.

As an alternative could you cut a slot in a toothed pulley, slip it over and clamp on and then use a belt to turn the pulley? To be fancy a slot half way through on one side could engage with one side of the valve handle. maybe even a shaft and drve pulley across the top of the "wall"

Robert G8RPI.

Edited By Robert Atkinson 2 on 09/11/2019 16:40:19

Thread: Inverters and stop switches
08/11/2019 21:57:00

That application makes some sense, being a secondary interlock after the motor has stopped and VFD output is off. It's not putting an E-Stop in series with the motor. Probably more cost effictive than using a safety rated VFD.

Robert G8RPI.

08/11/2019 20:17:39

I've never seen a switch between a VFD and motor on an industrial installation. Can you give an example?

08/11/2019 18:19:22

As stated putting any switch between the VFD and motor risks damaging the drive. Apart from this most E-Stops are not adequately rated for this.

Havig a faulty system that starts when it should not is not a got reason for fitting a switch in the motor. The root cause should be identified and fixed.

Robert G8RPI.

Thread: Lathe chuck guards - how many folk use them?
08/11/2019 12:45:11
Double post

Edited By Robert Atkinson 2 on 08/11/2019 12:45:39

08/11/2019 12:45:05
Posted by ChrisH on 07/11/2019 00:05:13:

<BIG SNIP>

Remember also, whether in your own shed on your own or in the firms shop, an accident only ever happens due to there being an unsafe action taken or unsafe condition existing. It therefore the responsibility of all of us to ensure unsafe conditions do not occur, neither do we take unsafe actions, however we operate our machines or provide for our own safety.

Have a nice day!

Chris

The problem is that you say you have taken that first "Unsafe action" - removing the guard- in your workshop. You also seem to be advocating removing guards to others.
Assessing the suitability of guards should be part of the machine selection process. If it's not suitable get the vendor to fix it or look elsewhere.

Generally in industry it is insurance assessors that drive good behaviour regarding guards and the like. If they refuse cover you can't legally work (at least in the UK if you have employees or public access to your business) Note that even in the home workshop removing guards could have accident or life insurance implications if the worst happens and they find guards were removed. Not very likely but some insurance companies seem to be looking for any excuse to reduce a settlement or increase a premium.

Robert G8RPI.

Thread: How does someone gauge the power of a DC or AC motor?
08/11/2019 07:28:40

OP has said they are cutting discs, not grinding. Still need to know what is being cut and speed expectations and why 400RPM. If it to reduce heat then faster cutter speed with slower feed might still be a better choice. The heat put into the work piece (opposed to at the cutter interface) is proportional to the rate of material removal, which includes depth of cut, cutter speed and feed rate. If work piece heating is the issue and it's a dry cut, look at it's mass and thermal capacity of the materiel and the maximum temperature you want. Basic physics (how long does it take to boil a litre of water?) will tell you the maximum time you can cut it for a given power input (torgue and speed) This will give an idea of the maximum motor power and feed rate. It will be conservative because of losses etc but will give an idea if the motor is too big for your limits. It the feed rate turns out to bee far too slow you need to rethink e.g. wet cutting.

Robert G8RPI.

07/11/2019 21:44:43

Hmm,

You don't need to know the power of the motor, you need to know the torque required to turn the disk. This will depend on what you are cutting and how fast you want to cut it.

More information please.

Robert G8RPI.

Thread: new computer
07/11/2019 17:19:10

I suggested just a replacement SSD as I got the impression from the original post that Doglas was not a big PC user. I addd the caveat "if you are not planning to buy any new software". It's easy for computer "nerds" to forgetf a machine is otherwise OK and powerful enough to run the existing software does not have to replaced. A faster disk and "tune-up" can work wonders.

Robert G8RPI

Thread: Inverters and stop switches
07/11/2019 07:39:31

The wiring regulation (BS7671) are not applicable to this (or the law, you can in theory show alternative means of compliance, but good luck trying). The quoted text applies to electrical isolation, not machine E-Stops. The mention of machines is to allow the input to motor drive to NOT be isolated from the supply so it can stop the machine more quickly.

If using a VFD (inverter) with a separate E-Stop input (and you should be if the machine needs an E-Stop) that is what you should use for the E-Stop as it stops the machine in the shortest possible time (loose chucks are a separate hazard and should be dealt with accordingly). Use the Stop / Start inputs of the VFD for just that and connect a proper isolator switch (Rotary red and yellow with provision for a padlock in the OFF position) at the VFD supply input.

A No Volt Release (NVR) should not be required with a VFD because you should set it up not to power the motor on power up or when the E-Stop is released. (This is normally the default but can be bypassed when the drive is used under automatic control)

E-stops should not be used for routine stopping of the machine but should be tested regularly.

If you want to get legal, then there are a host of assessments to do and all safety components must be designed and approved for the application. Most Hobby machine tool safety provisions, though CE marked, would not pass a through assessment. The NVRs with flip-over E-Stops are a particular example.

Any E-stop is better than none and you should NEVER EVER use the mains plug as the only means to start or stop any machine.

Robert G8RPI.
(CEng and have done this stuff for a living on machines to CE / UL & C-Tick).

06/11/2019 20:42:26

You should use the E-Stop function of the inverter if it has one. Almost all of them do. This will switch off the invertor output instantly and with most units provide at least some dynamic braking where inverter "shorts" the motor. On simple inveters this is limited due but some have positive braking either by dumping the energy or injecting DC into the motor.
There is one big problem with putting the E-Stop in the input supply - stored energy. The inverters convert the incoming AC to DC before inverting it back to AC. As part of the AC to DC conversion there is a filter capacitor that stores energy. Sometimes this is called a DC bus or link capacitor. If you just cut the supply the motor will continue to run on the energy stored in the capacitor. If the inverter is lightly loaded compared to it's maximum rating the machine can continue to run for a significant time on this stored energy. It will certainly run longer than if you use the E-Stop function.

Robert G8RPI.

Edited By Robert Atkinson 2 on 06/11/2019 20:43:32

Thread: Lathe chuck guards - how many folk use them?
04/11/2019 08:47:51

It should be noted that while (in the UK at least) you can do what you like in your own home workshop, if you are in business or an employer (even casual) you have to comply with the regulations which includes guarding.
Guards may be inconvienent at times but not as musch as missing part of a finger or worse.

Thread: new computer
03/11/2019 16:56:26

If you are happy with the machine you have and don't intend putting any significant new software on it in the near future why not just replace the disk? If you fit a solid state (SSD) one you won't have to worry about mechanical failures. Most "retail pack" SSD's come with software to transfer your operating system, programs and data with minimal fuss.

Robert G8RPI.

Thread: Sourcing an elusive Rubber-Ring!
02/11/2019 08:59:36

Hi,

Sounds like a brake caliper piston seal e.g.
www.ebay.co.uk/itm/for-Vauxhall-Astra-Meriva-Rear-Brake-Caliper-Repair-seal-Kit-Lucas-36mm-3627/274056153625
Holden Astra down under.
Try talking to the ebay seller BigRed, I've found them very helpful.

Robert G8RPI

Thread: Guarding for laser cutter
31/10/2019 12:26:55
Posted by Trevorh on 31/10/2019 08:41:39:

Many thanks to all of you,

The ref to orange plastic was my feeble attempt to understand what my son was asking about as he did quote wave length and the like but it went over my head

know where he should start with obtaining such safety equipment?

I will mention about the venting of the vapours/fumes as this is quite a good point to be taken into consideration as well as the eye protection both on and off the equipment

Once again thank you

Trevor

Hi,

Where you start looking is the details of the wavelength and power output of the laser(s) he is using. You cannot do anything without these details. I have 4 different pairs of laser safety glasses just for the lasers I play with at home and none of those are capable of cutting. well maybe a thin piece of paper if focused properly.

Robert G8RPI.

30/10/2019 21:38:51

I second what Jeff says. You cannot go by the visual colour or density of plastic to determine light blocking. It is highly dependent on the base material, pigments and wavelengths that need to be blocked. For example black acrylic (Perspex) while appearing completely opaque to visible light will pass infra-red light with virtually no attenuation. Virtually all of the coloured plastic sold for filtering lights also pass IR light. If they didn't they would melt in front of the lamp. Conversely some grades of plastic which look crystal clear will totally block UV light. Then you have to make sure the laser can't burn through the plastic or degrade the dye(s) in the plastic. Overall solid metal is the safe option. Even then you have to make sure all the joints and seams are light tight. This is easier said than done. One check is to put a bright light under the cover. close it, and then inspect in a totally dark room for any leaks of light. Don't forget to wait in the dark for at least 10 minutes to let your eyes become sensitive. Some black anodised finishes are IR reflective too.
I've done laser safety design as part of my day job for instruments with high power UV lights and various lasers.

A related problem with some lasers like some blue or green ones is that the lightis generated by frequency doubling a infrared laser and with low quality ones they don't fit filters for the IR on the output so you can get dangerous levels of invisible IR light as well as the visible light.

You only get one pair of eyes

Robert G8RPI.

Thread: Surface Mount Switch
17/10/2019 12:38:58

The KEDU NVR unit seems a good cost / safety compromise between a simple on/off switch and a full direct on line or contactor set-up. It is better than the "KJD" type NVR's with the flip-over E-stop that just presses the stop button. This is because the KEDU E-Stop is a separate switch so can still cut power even if the NVR contacts stick shut. If not using fully qualified industrial safety parts having a second means of stopping is good insurance

Robert G8RPI.

Thread: Limiting pressure to a gauge
17/10/2019 07:34:03

First, if you have data for the pressure gauge, check what it can withstand. It may be OK with 70 PSI. A restrictor will only help if there is sufficient air or other compressible fluid or resilient expansion between the gauge and restriction or in the gauge itself. If the gauge is mounted above the oil level then air in the pipework and gauge my provide this if the restrictor is at the engine end of the connection The ratio between the restrictor flow and compressible volume must be high enough that the oil pressure has reduced to below the maximum non-damage pressure rating of the gauge before enough has flowed through the restrictor to compress the volume of air to the limit. If the gauge and pipework can fill with oil and not drain you could use a small spring loaded accumulator (cylinder with a spring loaded piston designed for full compression of the spring at a bit more than the gauge max pressure rating) and restrictor.

Robert G8RPI.

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