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Member postings for Andrew Johnston

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

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Thread: Shine a Light
04/10/2010 23:24:33
Wow, some interesting stories here guys. As the football manager said, 'the boys done good'.
 
To pick up on a few of the points made. I'm certainly not going to be retiring with a good pension. I've missed the generation that did so. My grandfather didn't retire until 70, and I expect I will be too. Mind you he bought his first house at 60, so he had to keep working. I do save a bit towards a pension, but I am starting to spend a bit too. At the beginning of the year I underwent a potentially life changing operation on my eyes. If it hadn't worked I'd have probably lost my sight. That made me rethink a few things. Like I'm not getting any younger, and I wouldn't want to be twenty years older and thinking, I wish I had done this, or that. I've started this process by rushing out and buying a share in an open class glider with an engine, if that isn't an oxymoron.
 
We had a metalwork shop at school, but, thinking back it was not terribly well equipped. All new kit, just not a good range. There were three lathes (Boxfords), only one screwcutting, and only the sixth form were allowed to use that. There were two pillar drills and a shaper. There were definitely no milling machines and not a great range of accessories, as I remember. I guess the powers that be thought it was a bit down market to have a metalwork shop in a grammer school. It did have a really good town gas forge and lots of facilities for enamelling, so I suppose they justified it on the grounds of it being useful for arty things. And I don't think the metalwork teacher actually liked kids. He just lurked in the back of the room and did his own projects.
 
It's an interesting point about the apparent lack of skills these days. I think there are several reasons. One, there is no doubt some things are more complex. Just about anybody with a smattering of technical knowledge could understand the idea of a Strowger exchange. But to understand the principles behind the modern mobile phone you need a pretty good understanding of number theory and signal processing. The same is true of cars. Years ago anybody could do a quick de-coke on the drive. These days even the professionals can't do much more than plug a computer in and look at the diagnostic output. However, I think a more serious problem is that the man in the street simply doesn't know, or care, about technology any more. If it breaks he buys another one, or calls a man out. Some years ago I put an ad in the local village magazine offering to make bits to help mend appliances and the like. I didn't get a single response in a whole year. I reached the conclusion that either people knew how to mend it, and did so themselves, or they were so lacking in knowledge that they had no idea, and just threw it away.
 
Right, that's enough rambling for one night!
 
Regards,
 
Andrew
Thread: Chronos silver steel, water or oil quench?
04/10/2010 22:13:02
A rule of thumb I use for hardening carbon steel is to maintain the hardening temperature for an hour per inch of thickness.
 
Ideally one should quench in brine, rather than plain water, as this inhibits the formation of bubbles at the interface with the hot metal and hence improves the cooling.
 
Regards,
 
Andrew
Thread: spindle run out
04/10/2010 19:50:32
Wow, sounds like they need to be feather bedded!
 
Regards,
 
Andrew
Thread: Machinable Ceramics
04/10/2010 11:19:51
Hi all,
 
Has anybody had experience of turning the machinable ceramics? In the UK Macor seems to be the most common trade name. The professionals don't seem that enamoured with it:
 
 
I was planning to design and make my own spark plug for my hit 'n' miss engine, and this seemed like a good opportunity to try machining ceramics. Ever since the stuff first appeared in the RS catalogue years ago I've been looking for a reason to use it. However, the stuff is expensive, and if it's going to be a PITA to machine I might just buy a spark plug instead. It's not like I'm short of projects!
 
Regards,
 
Andrew
Thread: Shine a Light
03/10/2010 19:37:31
Sid,

I sent you a PM.
 
Regards,
 
Andrew
Thread: Using an Imperial Ainjest Threading Unit for Metric Threads
03/10/2010 12:18:08
Hi all,
 
I have a background project to fit an Ainjest high speed threading unit to my lathe. Both the lathe and the Ainjest unit are imperial, so cutting imperial threads should be no problem. Just follow the instructions in the manual!
 
Within the QC gearbox on my lathe there are conversion gears for cutting metric threads. Nowhere can I find a 127 tooth gear in the system, so I am assuming that the ratio used is not exact, but is one of the very close approximations. As is normal, the advice for cutting metric threads on an imperial lathe is to leave the half nuts engaged and reverse the lathe back for the following cut. This is not possible with the Ainjest unit, as it includes an automatic knock off at the end of the thread.
 
There is a piece of equipment, called a Metradial, that can be used in conjunction with the imperial Ainjest unit to cut metric threads. I assume that this is just a fancy thread dial indicator that allows you to keep track of the starting position. The company in the UK that sells the Metradial seem to be useless, no response to emails and no 'phone number. Clearly they don't want my business, so they're not going to get it.
 
In essence the problem is how to get the lathe spindle and leadscrew back into the alignment they were originally in, for subsequent cuts. I'm not going to bore you with the number theory behind this, but in essence I think that for cutting a 1mm thread (with a 1/4" pitch leadscrew) the alignment should be exact for every 127 revolutions of the spindle.
 
But, there's a catch here. The conversion factor in the gearing is not exact, so the result will in reality be close, but not perfect. This is hinted at in the Metradial instructions, where there are correction factors to be applied if you are a bit slow in re-engaging the half nuts.
 
So, my idea is to return the lathe to exactly the point it was for the first cut. There are two things that need to be set. One the position of the tool, and second the alignment of the spindle and leadscrew.
 
Positioning the tool is easy, using a bedstop against the saddle. For aligning the spindle and leadscrew I propose fitting a rotary encoder to the spindle and counting the number the number of revolutions. If I then reverse the lathe and take the count back to zero, the spindle and leadscrew should be in the original alignment.
 
So, I have a couple of questions:
 
Has anybody used an Ainjest unit, and can give me some general pointers as to their use?
 
For cutting metric threads, will my idea work, and if not, where is the flaw in my logic?
 
Thanks,
 
Andrew
Thread: spindle run out
03/10/2010 11:05:13
Hi KWIL,
 
I have a basic appreciation of the accuracies needed for precision bearings. Far beyond anything I can achieve at home, and probably explains why precision bearings cost well into the hundreds of pounds. There's an interesting note on the Gamat website about fitting bearings. They do not recommend mechanical methods, but gentle heating of the inner race.
 
At one place I worked, there was a need to build a test rig for electric motor systems. This was basically back to back electric motors, one the custom test motor and the other a commercial item. Once running, the system was very noisy. I refused to work in the same room unless I got ear defenders. Then the bearings failed. Being nosy I insisted on being involved in the post-mortem. Turns out the mechanical designers had fitted the ball bearings onto the shaft by hitting the outer ring, doooh! I took one of the failed bearings home and cut it open. It was easy to see where the balls had left small indentations in the rings and the subsequent break up of the bearing surface.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 03/10/2010 11:17:02

Edited By Andrew Johnston on 03/10/2010 11:18:13

Thread: Rear mounted tool post
03/10/2010 10:35:30
Chris, oooops, sorry about that. Note to self: must ask for some anti-self-deprecation pills next time I go to the quack.
 
I read all the previous postings on parting off last night. It has certainly cleared up a few things in my mind.
 
I have had the odd dig-in when parting off using HSS tools. Usually when I've been in a hurry, so most likely due to operator error. Since buying a 'cheap' industrial insert style parting off system, I have had no problems (fingers crossed and touch wood). I generally part off at a few hundred rpm with a feedrate of between 2 and 4 thou per rev, depending upon the material. I use power crossfeed for parting off. I wonder if hand feeding may be the cause of some problems, as it is difficult to maintain a small but consistent feedrate?
 
I part off brass, bronze and cast iron dry. There is no problem with jamming as the swarf is granular. For steels (plain and stainless) I use coolant. The design of the insert seems to produce steel swarf in the form of a tight coil, like a clock spring, and a U cross section, so it is narrower than the groove.
 
Mind you I do still keep a hand close to the power feed knock off when parting off, just in case.
 
Regards,
 
Andrew
Thread: spindle run out
03/10/2010 09:55:43
Nick,
 
Thanks for the sketch. That's just how I imagined the bearings in my lathe would be orientated. It always amazes me that the lathe spindle and bearings are subjected to all sorts of cutting forces, yet retain an accuracy measured in tenths.
 
Regards,
 
Andrew
Thread: Classifieds
03/10/2010 09:47:49
I use Firefox, and that seems to work fine. Just tried IE8, and that seemed fine too. I agree with John, sounds like a browser problem. Have a look at the settings to see if anything is blocked that shouldn't be.
 
Regards,
 
Andrew
Thread: Rear mounted tool post
02/10/2010 10:37:33
Chris,
 
Pray enlighten this ignoramus (me) as to the myths and debunking thereof.
 
My gut feel is that there is no inherent difference in parting off with a front or rear toolpost. The differences that might exist are due to the way the rest of the lathe handles the cutting forces.
 
Regards,
 
Andrew
Thread: Drill chuck keys
01/10/2010 10:16:56
A quick search on the Companies House website reveals a company called Machine Shop Equipment Ltd., which was dissolved in 1995. Unfortunately the data has been archived, so location and business area are not available on the website. There was 'phone number to call, but I'm not that curious when there's paid work that needs to be done instead!
 
Regards,
 
Andrew
Thread: spindle run out
30/09/2010 23:10:56
Ian,
 
That definitely isn't right. I assume that low speed without backgear is about 150-200rpm. At those sorts of speeds the faceplate must be wildly out of balance to be rocking the lathe. Or there is another, as yet undetected, fault with the spindle. Does the lathe do the same with a normal 3 or 4 jaw chuck, or just the faceplate?
 
Nick,
 
Thanks for the information. I hadn't thought about the axial and radial expansions cancelling out. The headstock itself gets mildy warm (about 30°C) after a few hours of running at 1200rpm plus. So I assume that tends to counter the expansion of the spindle too. I've made the assumption that the Harrison design engineers knew what they were doing and I'm going to leave well alone. I have never had any issues with accuracy or vibration that could be attributed to the spindle, so no need to fiddle. Not the real engineers' way I know, but I believe that if it ain't bust don't bugger about with it!
 
Regards,
 
Andrew
Thread: Learning the art of model engineering.
30/09/2010 22:58:30
Hi Damien,
 
It's a bit difficult to know whether we can help you when we have no idea where you are?
 
Regards,
 
Andrew
Thread: Shine a Light
30/09/2010 20:08:56
Hi Peter,
 
I'm always a little cynical when the teaching profession demands that child assessments are left to them, as they know best. As you have discovered they know nothing of the sort.
 
Performance at school is no indicator of success in life. A point I made rather forcefully to my form master in the sixth form when he supported my chemistry teacher, who had suggested I didn't bother applying to university, as she didn't think I was up to it. Had I wanted to read chemistry I might have listened, but since she knew diddly squat about electronics I ignored her advice.
 
One of the things King's College is known for is it's music and particularly the chapel choir. While I have been in chapel, in the choir stools, I only listened, as I can't sing for toffee.
 
Regards,
 
Andrew
Thread: spindle run out
30/09/2010 19:51:18
Ian,
 
Definitely doesn't sound right. The faceplate, by itself, should be reasonably well balanced, certainly up to hundreds of  rpm. What speed were you running at?
 
Nick,
 
Yes, didn't explain myself very well. My lathe spindle has two 'Super Precision' Gamat bearings, one at each end of the spindle. It's not clear from the manual, but I presume that the bearings are arranged such that, as the spindle warms up and expands, the axial load on the bearings increases. The manual has a section on how to test the torque necessary to turn the spindle once it is warm, and how to adjust the preload in small steps. It also contains dire warnings about doing so, with strict instructions to contact the factory beforehand. Unless I have an unforeseen problem I have no intention of touching the bearings.
 
Regards,
 
Andrew
29/09/2010 22:40:11
My gut feel is that 2 thou runout is quite a lot. If I understand it correctly from www.lathes.co.uk the Raglan 'Five' has taper roller bearings. Just out of interest I've been out in the workshop to measure the runout on my lathe nose spindle. It's a D1-4 fitting, and the spindle runs in opposed taper roller bearings.
 
Measurements were made using a Mitutoyo 0.01mm DTI. Measurements are:
 
Inner MT5 taper: 0.005mm
 
Short Outer Camlock Taper: 0.007mm
 
Outer Diameter (plays no part in locating the chucks): 0.01mm
 
Front Face of Spindle: 0.01mm
 
These measurements are from a 30 year old lathe.
 
However, before we panic and start stripping the spindle it might be worthwhile discussing the measurement technique, so as to be sure that the runout you are seeing is genuine. For instance did the needle move smoothly as the spindle was rotated, or did it move in jumps? If the needle jumped is there any corrosion or pitting on the spindle. Were the readings repeatable from one rotation to the next?
 
Regards,
 
Andrew
Thread: Slitting Saws
29/09/2010 20:57:38
It's not a case of deflecting the spindle bearings. It's a case of the arbor deflecting. Unless you have an infinitely stiff arbor and/or zero cutting forces the arbor will deflect. Now I will admit that the deflection is very small, in order of a tenth or two of a thou according to my calculations. I expect in this case that it is negligible compared to the other effects. It was a slightly tongue in cheek comment.
 
Graeme,
 
Sorry I didn't explain myself clearly. As Tony says the cutter rpm is determined by the diameter of the cutter and the recommended surface cutting speed of the material. I agree with his figures. The feedrate is determined by the rpm AND the number of teeth. To take Tony's example of a cutter doing 100rpm. Suppose we want each tooth to cut about 2 thou of material. If the cutter has 4 teeth then the cutter will need to advance 8 thou per revolution. This gives a feed per minute of 0.008" times 100, ie, about 0.8"/minute. However, suppose our slitting saw has 40 teeth. Then the feed per revolution will need to be 80 thou for the same cut per tooth. This gives a feedrate of 0.08" times 100, about 8" per minute. The number of teeth doesn't affect the rpm of the cutter, but it does affect the feedrate.
 
I've added a picture of a slitting saw in action in my Odds 'n' Sods album.
 
Regards,
 
Andrew
Thread: Shine a Light
29/09/2010 20:40:15
Neil,
 
Thanks for the erudite explanation. Ah, the Beano. Actually my brother and I weren't allowed to have the Beano, unless we bought it ourselves. Our parents would only buy the more educational 'Treasure' and 'Look and Learn'.
 
Peter,
 
I did pass 'O' level English language and literature (just) but I failed French, with a grade 9. At least that's a proper fail; if you're going to do something, do it properly. Mind you the teaching wasn't marvellous. If you got something wrong our French teacher made you stand on your chair in front of the class and say it again. We did get out revenge though when it was discovered that he was a Morris dancer!
 
Regards,
 
Andrew
Thread: Boring problems
29/09/2010 11:39:28
If I understand correctly the part is supplied with a 3/32" bore already existing? Any drill will tend to follow the existing hole. I'd start with a stub drill (ideally carbide), or a D bit,  to provide a true start to the hole. Then a normal drill. Drilling from both ends should be ok, and saves buying a long series drill. However, it will depend upon the accuracy of your chucking arrangements. If, upon turning the part end for end, it doesn't run on the same axis, one cannot expect the hole to then be true. Running at 700rpm is fine, although personally I'd probably run double that.
 
Use quality drills, I use Dormer; if the drill isn't ground correctly in the first place it isn't going to drill true.
 
Regards,
 
Andrew
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