Member postings for Tim Stevens

I think the specification 'plastic but not ABS' is too vague - sorry. And whatever you use has got to be heat resistant, too....

tim

Plastic is a wide description, - not all plastics are anything like the same in machining terms. There are 'brittle' plastics like polystyrene, and 'waxy' plastics like polythene, and 'rubbery' ones like polyurethane, but the nicest to machine include acetal, nylon, and rigid PVC. One problem is the nature of the turnings (which can be really tough) and the low melting points of plastics creating problems when parts rub together in machining, or when your tooling is the wrong shape or blunt.

Pure aluminium is a bit soft - the turning waste (or swarf) tends to push together into lumps, and it sticks to the tool, making a good finish difficult. But there are lots of light-alloys which turn nicely and some which polish nicely too. Just follow the advice - get hold of some reference books, which are often around second-hand or even seriously old (and rather dog-eared) so not very costly - and more recent ones for plastics, of course.

And practice.

Cheers, Tim

Lots to learn - but keep at it.

One thing that can catch out a beginner is trying to turn brass with a nice sharp tool. Even a new twist drill. What happens is that the tool digs in and breaks off, or jams the machine, or even bends a shaft (etc). Tools for cutting Brass need no 'back rake' on the cutting edge - without this there is no tendency for the brass to grab, pulling the tool further into the work - a problem especially bad if there is backlash in your tool feed. Which of course there always is.

Cheers, Tim

I hesitate to run counter to Clive Foster in the debate about ball-centres, but ... If you use conical centres, rotating the work or not, the exact angle to calculate when setting up requires that both conical ends sink exactly the same amount into their respective centres. And the the centres themselves are drilled using the same size of centre. Not really a problem, I suppose, as long as the hardness of each end, both points and bar, is the same. Until wear takes place, of course, on the minute slivers of steel in contact.

But a pair of ball centres does away with such problems. And blueing the balls for each end (not needed to be exactly the same diameter) and pressing them lightly in the relevant centre cone each end, will allow you to measure with some precision the relevant length for your calculation. This last is not possible using cones which bear on part of the mating cone on one side, and a different part on the other side, each end.

Its not as though these ball end things are difficult to make - although I do not understand why they are not available off the shelf. (Or are they?)

Cheers, Tim

Might finger collets refer to collets comprising sets of individual (matched) fingers - so that the collet could be used in cases where a more conventional collet would not go over an obstruction? In other words, take an ER collet for example, and grind away the connecting bars between the fingers (having first, I guess, marked each finger in number order?).

And in my view, an ER collet is a spring collet, just not a very strong spring.

But what do I know?

Cheers, Tim

A back plate has usually got plenty of thickness (when new, at least). So, if you take too much off it should be an easy matter to face right across, and then start again to cut the recess.

As the upright toy ferret says - simples.

Cheers, Tim

Itmay help to add a few basic facts:

All common metals can exist in harder and softer forms, but with many of them the difference is not great. Changing from hard to soft can usually be done with heat - not enough to melt, and then allowed to cool fairly slowly. This is called 'annealing'. Lead is an odd one as its annealing temperature is lower than normal room temperature so it is always in the soft condition, unless, as pgk pgk has discovered, further cooling is taken seriously.

Another exception is steel, in which the difference between hard and soft in the same alloy is greater than other metals, and in steel including a small % of carbon the range can be extreme. It all depends on the exact alloy, the possible addition of other metals to the alloy, and the heat treatment.

So, your 'hardness table' is going to be complicated by each metal having a range of hardnesses, some with a much wider range of hardness than others. This probably explains why such tables are not readily found.

I have found the book 'Machinery's Handbook' very useful in this regard. My edition (No 21, 1980) has 16 index references to Hardness, and 51 about Heat treatment of Steel. Don't look at the most recent versions, which will be expensive - the book has many editions back to about 1910. Hardness and what to do about it have always interested engineers.

Hope this helps

Tim

Edited By Tim Stevens on 24/08/2019 11:51:11

SoD says (above) - A 15 Ah battery can deliver 15 Amps for an hour.

Sorry, but this is based on a misunderstanding. It is true that a 15 Ah battery willl (should) deliver 1 Amp for 15 hours, but that is not the same thing. 15 Amps would overload a small battery, causing overheating, and likely damage to the plates, if continued more than a few minutes.

It is just one of life's (many) swindles, sorry.

Cheers, Tim

Chromium is added to a steel mix (along with a range of other metals) to change the characteristics of the resulting steel. Why and how are questions which take too long to answer here. If you are keen to know more, metallurgy is the name of the science (for when you are visiting a proper book shop*) but you might like to start on Wiki.

* a proper book shop is one that sells books I want to buy. Very rare, nowadays.

Cheers, Tim

If you do try this idea, you need to use shot which is whole (no cracked pieces etc) and try to ensure that the impact s are all the same 'weight'. Cracked shot will cause sharp edged notches - just what you don't want for fatigue resistance, and uneven application will ensure that some of the dents are deeper and wider than others, and this is not the 'stretched evenly all over' condition you need.

An alternative - perhaps not as effective but it depends on your application - is to clean up the surface with abrasive working in line with the major stress (so the scratches are in line, not across, the main loads), and work up to a fine polish, always working along the scratches, not across them.

Hope this helps. Of course, fatigue depends on many other factors, including the crystalline nature of the material, and the presence (or not) of repeated shock loads in use.

Cheers, tim

Can I start from first principles?

All that is needed is a device which gives the pendulum a prod as it passes. This need not happen every time, or even regularly, as long as the movement is so free of friction that it swings for several cycles without further impulse. It is easier, though, to prod every time. The actual prod can be mechanical, or electro-magnetic (etc).

It might be worth trying a pendulum supported, pivoted, at the top of an upright strut, which itself swing a small amount round a pivot at its lower end. The strut is what receives the nudge or prod, so the pivot for the pendulum moves slightly too and fro, and this keeps the swing in action.

As the strut moves to and fro it can operate microswitches, which would be more stable in operation on a two-position flip-flop strut than on the continually moving pendulum itself.

Unless, of course, you actually need an escapement wheel - which is a lot of work for no real benefit for driving a pendulum on its own.

Unless I have mis-understood (again).

Cheers, Tim

Edited By Tim Stevens on 24/07/2019 22:42:41

You need a clutch because:

The motor drive includes a reduction gear which is likely to involve slowing the ouput down by a factor of several. In other words, the motor whizzes round like a fan and the drive rotates sedately and you can count the turns. If the drive is the other way round, you wind the handle slowly but the motor has to whizz round. This makes the drive seriously hard work, if it is possible at all. So, use a clutch to separate the drive and winding the handle will be no more difficult than standard.

The clutch does not need to be fancy - a peg which can slide into a notch will do nicely.

One other thing - it reduces the load on the motor if you can counter-balance the weight of the mill head. In principle - a rope going upwards over a pulley hanging from the roof, and a weight the same as your machine head hanging on the other end. In practice, something more fancy will help to put the dangling weight out of the way. Some users prefer a pair of sliding spring-rods, as used to hold up a hatchback - but finding the right strength is not easy.

If you don't do the counterbalancing, the motor will drive downwards much more quickly than upwards, and this can be disconcerting.

Cheers, Tim

The device shown by Ian P has a vertical lead screw for the Z movement. This is fitted (originally) with a mitre gear and the handle fits in place of the motor drive shown. So, it should be easy to extend the lead screw upwards (perhaps by a more complex nut) and fit a motor there, the motor having internal gearing to reduce its rpm. This would leave the original handle in place, and it would rotate in sympathy with the motor drive.

Of course, you also need a manual drive, without turning the motor. So, you need a dog clutch between the motor and the lead screw.

The motor you need will be geared down a lot to about 50 - 100 rpm, and the torque at the output wuill need to be rather more than the torque required to turn the lead screw nut in normal conditions. The motor is likely to be specified as output torque at the motor (not including reduction gears) and so many rpm. Knowing this the reduction ratio and output torque needed can be calculated fairly simply. Halve the RPM and you nearly double the torque. 10% of the rpm gives you about 80 times the torque. [It would be 100 times if the gearbox did not add a friction load.] And it does no harmn to have a bit of torque in reserve.

Remember that you must include switches to turn off the motor automatically just before the Z movement gets to the end of its travel. Best to do this first, or you might get carried away with your efforts and try switching on before it is finished ... And another switch which cuts out the motor as you disconnect the clutch.

The voltage you need is not really a serious concern, but - a low voltage motor is much safer than anything on the mains supply. Just chose from what is on offer, bearing in mind it will need a power supply, so 12V seems favourite perhaps using a computer power box. But it must be a DC motor or you will have fun reversing it.

Hope this helps

Tim

Edited By Tim Stevens on 23/07/2019 15:00:29

It is fairly easy to check the flatness of a slab of eg granite worktop, before purchase. Hold the slab at eye level and about horizontal, in a god light, and look carefully at the reflections in the whole of the surface. As you move the plate slightly, the reflection should not distort at all as it moves across the surface. Any wavyness or hollows etc will show as the reflection changes shape and/or size as the plate is moved.

This is a useful check on any bits of worktop left over or sink cutouts etc, when it is not possible to do a proper engineering check in a builder's yard.

And although worktops can be fairly thin (12mm rather than 50mm for a 'proper job' a slab of thicker MDF glued on makes a good reinforcement.

Cheers, Tim

Whatever tool roll you make (or buy) will be wonderful until you go to the next boot sale and there is the 1/4 Whitworth obstruction spanner you have lusted after all your life, and no slot to put it in.

cheers, Tim

I have a few comments:

Plasma is not well informed about insurance (at least in the UK). If you have ordinary road insurance, the insurers are not allowed to decine their responsibility because the driver was going too fast, was drunk, was racing, was on the phone, etc etc. That applies only to the bit of road traffic insurance that is compulsory.

Jason B is not up to date on the requirements and costs for entering any speed event in a motorcar. All such events require special boots, gloves, helmet, fireproof suit, etc. And most of this stuff has to fit properly and goes out of date fairly quickly. Say £500 just to turn up, never mind getting the car to comply etc. The MSA has the sport firmly in its control, and the insurers know that any non-MSA bunch is not going to play by the recognised rules, so they will not offer insurance for the organisers, land-owners, etc.

The 'event' took place on a car park, which is different from the highway. Not for insurance, it isn't, though - a car park is regarded as a public place so road insurance is needed (along with lots of other boring stuff).

And every employed person has a contract of employment, but some jobs don't have them in writing.

I'm not against anyone having a rant at such activities, just trying to make sure they are not making claims that might lead to others saying 'We can ignore them as they obviously don't know what they are talking about).

So please don't rant at me ...

Cheers, Tim

The skin of cast iron can be nothing like the 'real' stuff underneath. The skin can be chilled - forming iron carbide, which is very similar to tungsten carbide, and just as difficult to machine. It has a silvery appearance, whereas the body of the casting is going to be dark grey, with included carbon which rubs off on a finger. This darker material can be machined using old fashioned tools of carbon steel, which is one reason why it was used so much in the 18th and 19th centuries for engineering.

It might be worth trying to remove some of the carbide by heating the casting to redness, and allowing it to cool in the ashes of a fire (such as a barbeque). This slow cooling should help to ensure that some at least of the dead-hard problem material changes back to iron, and carbon. The ashes serve as a blanket and prevent any sudden colling from drafts, etc.

Hope this helps

Tim

A feeble tool like a jewellers piercing saw can be dangerous, too. If the blade breaks at the wrong moment (and they do) you can have a blade through your index finger. Every jeweller I know has permanent blue marks on his left fingers where this has happened. So, all tools can be dangerous, even when every caution is used.

But just sitting on the sofa can be dangerous too.

Cheers, Tim

My concern about valve guides is only partly to do with strength - and others have commented on that aspect. The other part of the problem relates to heat transfer. The exposed exhaust valve stem takes in heat from the exhaust gas, and the part in contact with the guide loses heat into the cylinder head. In your design there is too much exposed and not enough in contact.

Another heat concern is the cooling of that area of head, which is exposed to a large area of exhaust port, and has no fins. You might think that modern engines don't seem to need fins here, but they are drenched in oil, which carries the heat away. Your system relies on no fins and next to no oil ...

It may also be a problem with the whole exhaust port as a separate insert of bronze. This adds an extra joint where heat must be conducted, and this relies on real firm contact at all times. The port will heat up faster than the surrounding metal (although its coeff of expansion is going to be similar), so even if everything is tight on manufacture, the joint will soon cease to fit firmly everywhere. If you are reconsidering this area, I would suggest that the sides of the port are directly in the head aluminium, with a lid comprising the guide and a flange, in bronze. If the flange extended as a fin, that would be an advantage.

But then, what do I know?

Tim Stevens
[Silver medal, motorcycle engineering C&G]

Edited By Tim Stevens on 10/07/2019 12:32:49

If you soften (anneal) the brass it should be easy to squeeze out a 50mm disc to over 53mm, using a hydraulic jack, for example.

Tim

Edited By Tim Stevens on 09/07/2019 23:12:42