Member postings for Ajohnw

I'm having problems with 125mm 20mm bore wheels. I grade available and others on 6 weeks.

Anyone know the German for grinding wheel ? The off had type of course. The correct word might even contain that.

John

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I agree Ian - it is a bit of a yawn.

John

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Recharge isn't 100% efficient Duncan. There are losses so in principle it's always possible to put back in some of what is taken out.

I can't reliably remember how the field control manages regen but field weakening rings a bell and that sort of makes sense. As some one mentioned permanent magnet motors don't make good generators. With zero field current all that would be left would be residual magnetism. I've worked on too many different things to remember everything so that could be entirely incorrect.

Motors in the extreme are a very specialised subject. I had thoughts about a variable speed lathe a long time ago and talked to a specialist. He happened to have an aircraft generator about that he had been bought from a scrap yard. He gave it to me along with suggestion about armature and field voltages. They were way higher than the volts it was meant to generate. I doubt if this was for a laugh at my expense. As it had a decent sized spindle and bearings I intended to try and use it as a head stock but stuck it in a cupboard at work while using power supplies to control it to see what it would do and some one nicked it. It just goes to show that the optimums for generation aren't optimums for drive as from the power supply trials the numbers he gave me were sensible.

John

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I was in a bit of a rush with the other post. In some ways it's easier to explain from motor speed and torque control using dc motors and the sep ex I mentioned. The usual way of controlling motor speed is some form of pulse width modulation with the field windings connected to the supply. I hope I get this the right way round - fairly sure I will but it's been a long time.

As the speed of a motor increases the back emf it's armature generates increases and at some point it gets close to the field voltage and at that point it can't go any faster. These are average voltages when things are driven with pulses even when it's the field. They are actually setting mean currents, usually across a diode when the power is off so that the current decay rate is slow. So say the field normally has half of the supply voltage on it. That will limit the speed it can produce and also limit the voltage that it can generate when the armature is driven by say braking but the field can also be changed so the motor can be made to go faster or slower purely on that. This is how the motor is made to produce more volts than the battery that is driving it for regen braking. In practice the whole thing is regulated by armature current both ways - providing power or regenerative braking. Most of the regen will be applied when the accelerator isn't depressed but it can also be blended in with the brakes.

Sep ex is not exactly a safe thing to play around with. Do the wrong thing with the field and the motor may rev so fast that it explodes due to centrifugal force. The other aspect is that field currents have an effect on torque.

I aught to remember which way round things go as I replaced a fair bit of a controller with a micro processor. The states were interrupt driven and noise caused some problems. If I hadn't fitted a switch to make 100% sure thing shut down properly a rather large motor would have exploded showering bits all over the place. That had happened in the past while the original controller was being worked on but the guy working on it ran out of the room.

The armature emf thing I mentioned is a model Motorola have used to describe speed control circuits. I aught to get the book out and check that but it's time for bed. They don't consider sep ex at all. Few do. In this case the motor was designed to be driven like that from a 200 odd voltage battery pack.

John

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A dc motors armature voltage forms a back emf against the field Neil so when used as a dynamo the field control can be used to set the voltage it can generate. Field control can also extend the speed range the the motor can run over.

It's usually called separate excitation in relationship to DC electric motor control. The same principles can be used to control regenerative braking - and have been. I did work on Lucas EV's.

John

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Basically one aspect - there is current running about in a the motor when it's driving something. When it's turned off the current is still there and must be given some where to go. A resistor might be used to do that and the time taken to get rid of the current that is stored is proportional to the voltage across it. Often it's simply a reversed diode but if a resistance is added to that the voltage across the motor when it's turned off will be added to the diode voltage so it will decay more quickly.

2nd aspect if a DC motor is being driven by something it will generate power and in simple terms that has to go some where

That's ever so basics but back emf and regenative braking are one and the same thing really. It's just a case of where the power goes. The average voltage on field windings on the motor have an effect on the average voltage the armature develops when it's being driven by some force and also the power the motor can generate. The power it generates that is then absorbed by something determines the braking force that is developed. On electric vehicles it's generally merged in with the normal brakes so that the user is unaware of it. I used the term average because it's usually actually varied mark space ratio pulses of the supply voltage.

John

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Some lathe have points to grease the bearings so some is added every now and again to sort of refresh it. Wabeco suggest every 8 working hours for one of their lathes with a MT3 spindle. They include all of the bearings with rotating parts in that take loads such as the rack feed and main lead screw bearings. Oil on slides etc same period but as usual no oil points for the cross and compound slides.

It's probably a good idea to use grease on lead screws but not if swarf can get on them as it collects the stuff a lot more than oil does and makes it difficult to get off.

I meant to add some are sealed for life which basically means they avoided the cost of putting them in. On the other hand bearings should be run mostly empty of grease so if some is added now and again .............................

My Boxford ME10 is sealed for life so I remove the bearing covers and poke a bit in. AUD's etc same basic head stock usually have grease points.

John

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Edited By John W1 on 20/09/2015 20:33:48

I've been wondering about cyclone separators Ian. Not down to Dyson really. I worked at a lead acid battery factory for a while that had several huge ones. long before he looked into a saw mill filter to see how they work. Good ones let nothing out. There is a bit about them on the wiki. They should still work if air is sucked through them. Trouble is I have a lot of other things to do.

John

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There isn't much help in fixing the problems in that John other than trying to design them out and no how to do that. I found the comment about bearings that look after themselves interesting. A lot of work was done on that area in the mid 60's but I don't know if it was ever fully worked out. It was noticed because some bearings by accident went on and on and on where as some didn't. It had also been noticed on some lathes, same make and model etc. Probably even constructed during the same phase of the moon but the reasons not clearly understood.

He does mention all of the problems but I suspect those will have been mentioned in this thread already but perhaps not so clearly.

There is plenty of info on some bearing manufacturers web sites as well complete with solutions but then comes the problem of implementing them at home. One other aspect is load - just what that is with various cut depths and material variations, weight of chucks and etc.

The problem with that bearing head stock arrangement is wear. Taper rollers tend to be better than angular contact but like most things compromises come into it.

John

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It's down to the pressure of the slide down onto the part it runs on. Dovetail slides usually have clearance at the top so as the gibs are adjusted tighter more pressure is generated in a downward direction as the angle gets more acute. On the other hand there has to be some angle to prevent lifting so it's a compromise. The other aspect is side forces. The more acute the angle the greater the force on the male part of the dovetail as well as downwards. I'm sure I have seen the maths in the past some where but in real terms I doubt if there is much difference between 60 and 55 and believe 55 degree dovetail cutters have been about anyway. 45 would give bigger wear areas and if the area below the slide was larger the psi would drop so there are other ways of compensating but it would require more general rigidity.

There are some books on the archive on lathe design. The best one that even mentioned harmonic oscillations on the spindle has gone. Also if I remember correctly head length - long so that bearing alignment errors produce less errors on the work which is pretty obvious anyway. One that is still there mentions using what is basically an attachment to finally size plain spindle bearings. That was common once. It even mentions just removing rather tiny amounts, more like scraping. It doesn't mention the complications with taper plain bearings, shows several bed styles but also the common 4 rail prismatic bed. Generally the saddle guides will pass either side of the head and tail stock.

Most high end lathes that are precision such as DSG and others use a prismatic bed with 4 V's. The inner 2 are for head alignment and the tail stiock. They are also massively built which is part of the reason for the finish that they can achieve even with shallow cuts. The other is bearings.

This is a drawing of the sort of bearing arrangements this ilk of lathes use that is adjustable. There are others

They run in pumped oil and the short distance between the ones at the front help with temperature effects. There are other arrangements with 2 at the front.

I was amazed when I saw a break down of one colchester head - just 2 taper rollers. It explains one or two things I have noticed about others using lathes in tool rooms. The students I have used have hardly been used at all and I'd guess those use that arrangement too. Talk about cheap.

John

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Wont last - it's done a lot of grinding going on the dust caked inside the covers yet the bearings are still fine. It's stamped with the motor output power not the input. No mention of running times before it over heats and etc. The motor will also have been designed specifically for 240v 50Hz and if one was made these days I would guess it would come in at well over a hundred quid retail, probably 2+. If only the wheels that seem to be about these days were so good.

It is a pity it uses 125mm wheels though. They did use 4in on some, 6in is more available. Imperial wheels are much more common than metric - could be that everyone in the USA must have on in their house some where. The size of the EU market should have fixed that. This seems to be an area that has escaped metrification.

I believe they also did some with universal motors but this one is capacitor run.

I'm wondering if I can make use of the extraction points maybe with a PC fan - not with the dust going through it of course.

John

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I expected to see comments about less flex, parting off and etc. They are interesting tool holders because if set up correctly they can set tools to precise angles reliably. That aspect is about it really - angles. The weakest aspect of any qctp is the base of the holder and they don't really differ from other types of similar size in that respect..

They do have one interesting fix of a problem home made qctp can suffer from so might be of interest to some. The pillar the entire holder sits on has a relatively low surface area at the bottom compared with other types suggesting that they would need tightening down more firmly than others. They have added a few sharp shallow rings / serrations to the underside of the pillar which actually dig into the surface they are mounted on. If they didn't do that they probably would slip/rotate under high loads. I had wondered how they get round that. The only aspect I was not clear on is compatibility between different makes.

A disappointment for me is that the holder has to be removed to set an angle as the cam action that pulls them back firmly hasn't sufficient movement to clear the serrations. This is fiddly compared with what I do with a Dickson type. According to mood I gauge by eye, use the face of the chuck to set a parting off tools square or even a square or protractor against a face of the holder using the outside of the compound slide as a reference for any angle.

They are a nice cheap alternative to the ratchet indexing that has been fitted to the bottom of some tool holders in the past and give more angle settings as well. The tool holder doesn't need rotating in one direction like those did either.

For me, as I mentioned I run 2 lathes and common holders make more sense than having 2 types. The only way I will get a 16mm boring bar on this machine is to make a holder myself, just like my Boxford due to it coming with a lot of Myford holders. It would cost way too much to replace those and just why should I remove a hefty clamp on knurling tool from it's holder to fit into a multifix one for instance. A number of other things too.

It turns out that it isn't genuine Multifix anyway. According to lathes co they are stamped with that if they are even in the USA where they may have Emco on the side. They were probably supplied by Wabeco but not marked as such. The price of the body is £178, 5 piece set (3 holders) £383, that's what the lathe came with. The lathe chucks are by Bison.

I'm still hoping for an answer on myford 7 ml10 / speed 10 aprox tool height above the compound slide top and fixing bolt diameters In case one looks more sensible for adaptation.

John

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There's been some debate in the past about which is best 45 or 60 degrees or some other angle. Pass - i suspect 60 is often used because 45 is wider for the same thickness. The argument usually is that 45 provides more downwards leverage which might make a difference to say facing with the cross slide as 60 will provide less.

John

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I asked the seller about the wheel size as I knew B&D have been known to go rather small. He said 6in but they are metric wheels 125mm dia 20mm bore. I think I have found a supplier that can do these in al ox grade 30 to 120, maybe green grit too but if not the seller is going to have to have it back.

I stripped it yesterday to start cleaning it and will phone the supplier on Monday. It even has dowels through the spindle that mate with the flanges so that the drive is positive.

John

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Some one would need to do some geometry but I suspect that the longer outer guide has the same effect as have an inner guide that was longer - a sort of virtual guide that has a length some where between the two. Don't quote me I'm guessing but it probably works out that way because the length between corners that catch when twisted is longer than if both were the same length as the inner.

Personally I wonder why they didn't fit wear strips. They have been around for some time and I wouldn't have thought the cost of adding them would be that significant.

I've suggested using slideway oil to several people - it helps, even the stuff off ebay which actually is a good one. I recently bought some Lubron Waylube 100 which is a bit thicker as I suspect this MAY be better as it should take more load before breaking down. Most people that have switched to slideway oil admit that the slides feel smoother. It may mean that they are adjusting them as little tighter as well. All it takes is a smear.

John

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Ball etc bearing speeds are pretty simple. They have a limiting speed and going anywhere near that will cause problems. Then the faster they go the quicker they wear out. Thanks to carbide I think many industrial lathes will run a speeds that really aren't good for bearing life. Can't imagine how I can manage to do most of my turning at around 500rpm but I'm not in a rush but do have to size in one go if say a part has an M2 thread on the end.

Mind you if people want air bearings and tiny cuts perhaps super high speeds would be a good idea due to the depth of cut limitations they will probably have.

I meant realistic home production by the way with a good chance of excellent results. I can't see my home ever having some of the things that guy has.

John

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Thanks for the info. As it's on a Wabeco I'd guess that it's probably a genuine one. The Newall dro that was on it wasn't included. I like the look of those, very neat - then I saw the price of them.

While they may be desirable I'm sorely tempted to replace it with a dickson type as I feel they have some advantages for me anyway as I could use some of my Boxford tools on it. That's fitted with an extended height branded Myford one. Good and bad, well made and easy to get holders for but a larger size without need for the height extension could hold bigger tools.

I'm not sure if it would work out. Does anyone know what the S7 and ML10 tool heights are above the compound slide and the dia of the fixing bolts?

John

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Edited By John W1 on 19/09/2015 19:46:24

There is plenty of info about on work surface defects from ball and roller bearings. Hence air bearings - plain bearings that "don't wear out". I don't think this has changed really. If it has point me at some independent info.

Some roller / ball bearing designs have other problems anyway - thermal effects altering the preload. Some lathes get round this by using 2 bearings at the head stock end sometimes just plain roller not taking thrust at all. Disk / preload springs can help especially as they damp out vibrations if there is slight variations in bearing alignment when taper, ball or angular types are used. Ball? - the unimat has been using them for years. Cheap and easy to replace when needed. They appear to deliberately leave the rear bearing as a precise sliding fit as the springs work against the outer on the race. The springs help maintain preload even due to thermal effects.

Tapered bronze sounds great. I had a plain precision lathe once that used a gear arrangement to adjust the bearings. The gears adjusted the usual nuts. It didn't work too well. These bearings don't wear round. The outer shells on races don't either. Not much lost as no one on ebay wanted it, just £33. I just kept the slides. Pultra use them as well but the bearings are tightened down on shims so everything is locked solid but isn't easy to adjust. The fit on the spindle cause them to run hot according to the manual.

I guess I feel there is no point making one unless it's aimed at perfection and it needs to be made at home.

I suspect some cnc machines use needle roller bearings - rather long ones made up of several several shorter rollers in a long cage. Seen lying next to a rather large one once.

John

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I will probably need a couple more tool holders for this style of tool holder and am not sure about interchangeability with clones etc.

Does anyone know and a good source of them etc?

John

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That seems to be happening on some others from time to time of late. It can sometimes happen because people don't pay or for some reason find collecting too much of a pain - or maybe not as expected. I always wonder.

John

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