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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: Slitting Saws
29/09/2010 11:25:17
Actually there's a third reason a slitting saw might wander off on a vertical mill. It might be due to slight deflection of the arbor. My vertical mill has a weedy R8 taper, while the horizontal mill is a rather chunkier INT40 taper. In practise I suspect that the wandering I've seen is a combination of all three factors. My saws are not new, although not of far eastern origin, and one can never get the mill trammed perfectly. As far as I recall the wandering was about 0.25 mm in a 25mm deep cut.
 
Regards,
 
Andrew
29/09/2010 09:27:25
The slitting saw is a milling cutter. Calculate rpm, feeds and chip loads just like any other milling cutter. There are just more teeth to count! If things are getting hot that implies a lot of friction. Might be a feedrate problem. Feedrates for slitting saws are proportionally quite high, as there are a lot of teeth.
 
Don't bother with fine pitch slitting saws, unless you are doing a lot of shallow depth cuts such as slotting screw heads. The gullets clog up too easily.
 
Rules for coolant follow those for milling other materials, nothing special about slitting saws.
 
If the mounting arbor has a slot for a drive key, don't use one. If it all goes pear-shaped you want the cutter to slip on the arbor, not shatter.
 
My experience has been that on a stub mandrel on a vertical mill, there is a tendency for the slitting saw to wander off on cuts deeper than the few millimetres. On a big horizontal with outboard arbor support, no such problems.
 
Last but not least, whatever you do, and no matter how perfect everything is, the slitting saw will not cut evenly on all teeth. Don't worry about it.
 
Regards,
 
Andrew
Thread: Shine a Light
28/09/2010 11:46:46
Neil,
 
Ah, there you have me. You'll have to translate the Latin for me. Despite going to a grammer school that had pretensions to be a public school, I was never subjected to Latin. Probably just as well, as I have zero ability for languages. I have enough trouble with English!
 
Regards,
 
Andrew
Thread: Fly Cutting
28/09/2010 11:41:45
A cutter diameter of 60mm and 200rpm gives about 124ft/min cutting speed. About right for steel, although given the intermittent nature of the cut a slightly slower rpm might be prudent. Tool sharpness is important, it also needs a radius on the tip. Think about turning, you wouldn't necessarily get a good finish with a sharp point on the tool. Also don't feed too slowly, the tool still needs to cut, rather than rub, on each pass.
 
Flycutters always cut on both edges; it's a fact of life. Even if the mill head is perfectly trammed there are issues to do with tool and holder deflections.
 
The material itself can also have an effect on the finish obtained. On aluminium I got a mirror finish on a piece of scrap (probably 7075) and a much poorer finish on 6082.
 
Regards,
 
Andrew
Thread: Advice re: holding workpiece to cut taper
28/09/2010 11:26:38
Hi John,
 
Sounds like the chuck and tailstock are in reasonable condition. A poor finish on the centre would certainly cause issues. Sounds like you have identified the cause of the problem.
 
Regards,
 
Andrew
27/09/2010 22:29:05
Hi John,
 
Yes, 12" would be fine. It's not critical, just long enough to get a good purchase on.
 
Regards,
 
Andrew
27/09/2010 20:29:15
Hi John,
 
As a first step you don't need anything fancy to check if the chuck jaws and tailstock are worn. For the chuck, put a longish bar in the chuck and tighten moderately. Then see if you can move the bar by pushing and pulling by hand. The most likely error is that the jaws no longer grip parallel. If this is case the outer end of the bar should move.
 
I assume that you have a Barker lathe, with the round bar bed? Interesting design concept. The tailstock looks a little short, so this may be where the play is. Wind the barrel out, apply clamps and see if you can move the barrel by pushing and pulling on the end. The most likely fault is some wear in the barrel housing, and this will be most apparent when the barrel is fully extended.
 
I wouldn't even think about swivelling the headstock to turn a taper. Once it is set up to turn parallel, leave well alone. Apart from anything else, I don't think it will be that easy to set. Much easier to move the topslide. It's a right royal pain in the posterior adjusting a cylindrical grinder for Morse tapers, where it is relatively easy to adjust the table the odd thou at a time with a fine pitch screw. It'll be more difficult on the headstock. According to my calculations 1 thou over 3" gives an included angle of about 1 minute of arc. That's the sort of accuracy you will need to achieve.
 
Best Regards,
 
Andrew
27/09/2010 13:00:06
I think I'd have probably done it the other way round, ie, machine the taper first and then the shaft. Two reasons for doing this; one, if I bog up cutting the taper I haven't lost a lot of other work. Second, assuming that if the lathe has MT3 in tailstock then the headstock must be at least MT3, the part can then be held in the headstock taper and be supported by the tailstock.
 
However, that's not where we're at. Even as described I don't think the shaft should be flexing. I assume a reasonably robust lathe if it has a MT3 tailstock. So, something is not right. Could be the 3 jaw chuck is worn, the tailstock barrel might be worn if it is locked and still flexing. Not enough pressure on the tailstock centre? Is everything tight and locked?
 
Regards,
 
Andrew
Thread: Scale model Economy hit & miss engine builders wanted
27/09/2010 08:53:19
Been using Paintshop Pro for some years to edit pictures, it does all I need - Andrew
26/09/2010 19:40:58
Hi Charles,
 
As far as I can recall I made the crankshaft to the drawings, but as separate pieces to be assembled. The parts were then loctited together, and 3/16" silver steel pins were loctited though the webs where the shafts were located.
 
I made the webs from hot rolled EN3. I think the shafts were made from oversize EN8, rough turned and then ground to size.
 
Any other questions feel free to ask!
 
Regards,
 
Andrew
Thread: What are you building?
26/09/2010 11:16:50
John S,
 
Very interesting means of cutting gears. I've been working towards doing the same for the bevel gears in my traction engine differential. I've added a photo album showing the 3D model of the bevel gear pinion and the CAM system toolpath, as far as I've developed it.
 
The most difficult part of the modelling was the tooth profile. The addendum is simple, an involute curve. But what about the dedendum? Most texts skate over the problem and say it can be modelled by an arc. But what radius and centre? It was complicated in my case as the pinion is 10 teeth, and thus quite heavily undercut. In the end I found a website where I could download a DOS program that generated DXF drawings for 1DP spur gears from 10 to 200 teeth. Then is was just a case of importing the DXF into the CAD system, extracting the form of one tooth and scaling to 6DP.
 
Regards,
 
Andrew
Thread: Shine a Light
26/09/2010 10:51:39
Hi Neil,
 
Yep, got it in one! Durham is the third oldest collegiate university in England. I'm not even sure if there are any other collegiate universities in England. I assume that most of the red-bricks and white tile universities are conventional in their organisation. I vaguely remember seeing something on 'Coast' about Aberystwyth university recently. Wasn't it started by private subscription?
 
Sid,
 
So which part of Canada are you in? After all it's a big country. One of my fathers friends, from his apprenticeship days, emigrated to Canada, probably about 55 years ago. Montreal I think; he went to work for De Havilland Canada. Probably a bit late for me to emigrate now; and how would I ship several tons of machinery over! I gather than machine tools are relatively scarce on the secondhand market in Canada, at least compared to the USA?
 
Regards,
 
Andrew
Thread: Scale model Economy hit & miss engine builders wanted
26/09/2010 10:38:45
Hi Ian,
 
I did wonder if my photos were too big; I was too tired last night to work it out. I got confused because one of the first photos I uploaded was 3488x2616, but just over 2Mbytes in size. I thought there may have been a problem as the Economy photos are the same number of pixels, but are a bit over 4Mbytes. Lo and behold, this morning the first photo uploaded, no problems; then the system started sulking again. So the other two photos have been resized to be about 2.5Mbytes. Bloody software!
 
The upshot is the photos of my Economy engine are now uploaded.
 
Regards,
 
Andrew
25/09/2010 21:44:41
Hmmmm, trying to upload photos seems to cause the website to crash out. I'll try again tomorrow morning.
 
Regards,
 
Andrew
Thread: Shine a Light
25/09/2010 11:24:54
Hi Sid,
 
Funny that you should mention that MEW costs the earth in Canada. I mentioned to my friend in Vancouver that I'd just had an article published in MEW, and he bemoaned the fact that he'd need a second mortgage to buy a copy locally.
 
I wonder why it is so expensive in Canada, and how does it compare with US prices?
 
Regards,
 
Andrew
25/09/2010 11:11:31
Hi Ian,
 
No, it's not named after me! I agree that St. Andrews is the third oldest university in the UK, but it's not the third oldest collegiate university in the UK.
 
Regards,
 
Andrew
 
Thread: Newall DRO UK Suppliers?
24/09/2010 23:17:43
Hi Hugh,
 
I bought mine from Chester UK Limited, albeit about six years ago, so I don't know if they are still agents. Have you tried contacting Newall?
 
I have been very pleased with my system, two axis, Microsyn encoders, installed on a Bridgeport mill. It is probably the single most useful thing I've ever bought for the workshop.
 
I've only had one problem, where the display got itself into a funny state. Over the 'phone a chap from Newall gave me some special codes to reset the unit, which worked a treat. He was probably correct in stating that the unit had suffered a glitch on the power supply. I live in a rural area and our electricity supply is pretty unreliable. The supply had a hiccup just before the unit went funny.
 
Regards,
 
Andrew
Thread: Shine a Light
24/09/2010 22:56:57
Hi Sid,
 
I must admit that Canada is on the short list of countries I would like to visit. A friend of mine from Cambridge has just upp'd sticks and moved to Vancouver, to become technical director of a power electronics company there. It was a no holds barred move, he's sold the house here, and taken the wife, kids, and the machine tools, but I don't know about the dog.
 
He did offer me some work a few months ago, but I had to turn it down because I was too busy on other projects. In due course I hope there'll be some more work available, so I can visit Vancouver and BC, and get paid for it.
 
Regards,
 
Andrew
24/09/2010 22:50:17
Oh dear Neil, that's a real faux pas! Even though Oxford University does have some red brick buildings and sad to say the engineering department is white tile.
 
But here's a little puzzle; Oxford and Cambridge are the two oldest collegiate universities in the UK, but which is the third oldest? Hint; it's a proper red-brick university.
 
Oh, and I don't need to worry about the SO either; the dining room is the tool and measurement equipment store, the kitchen is for castings and parts for the traction engines and the entrance hall is for work in progress.
 
Regards,
 
Andrew
Thread: Scale model Economy hit & miss engine builders wanted
24/09/2010 22:35:06
'Fraid not, but I'm always willing to discuss how I made a given part, which parts of the kit I threw away, and how I've altered the design.
 
In due course (tomorrow) I'll post a picture or two of progess so far.
 
Regards,
 
Andrew
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