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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: How to make an accurate internal hole
05/06/2011 11:34:36
Basic rule of thumb for reamers is half the speed and twice the feedrate that would be used for an equivalent size drill.
 
Regards,
 
Andrew
Thread: A Challenge - How Would You Machine This Part?
05/06/2011 11:28:25
Now for a brief discussion on the various methods of producing bevel gears.
 
First, thanks to Nick for the drawings, very good! I'm sure the method would work, as would the use of a hacksaw and file. But my word it would be a bit tedious. I wonder what sort of templates would be needed to ensure accuracy, and how would you make them?
 
As an aside Richard mentioned the Hiraoka books; I've heard good reports about them. I have a friend who has an obsession with Shay locomotives; I'll have to ask hin if he has the books.
 
Now onto the suggestion by ady of using a shaper. I considered this too, essentially I guess one is making a crude Gleason bevel gear planer. It seems a lot of work though, and secondly my shaper is sadly out of action at the moment, awaiting repair of a broken casting.

So finally we come to the production of the bevel gears on a milling machine. First, a statement; it is not possible to machine a true bevel gear on a conventional milling machine, with one caveat.
 
JohnS has succinctly described the two approximation methods, so I won't repeat them here, but just make some comments.
 
On the first method (that requires 'adjustment' afterwards) as I understand it the gear is designed using the DP at the outer diameter of the gear. An involute cutter is then selected that has the correct tooth form for this DP and the appropriate number of teeth. But, here's the difficulty, this cutter has to pass through the tooth gap at the small end of the gear. So you cannot use a standard involute cutter. Instead you need a special involute cutter that has the correct form but is thinner than standard. Can anyone confirm this? As far as I'm aware these cutters are no longer available; at least I couldn't find any, not even from China or India. I have made bevel gears in the past using this method, but I had the facilities of the main workshop at RAE Farnborough behind me. They lent me the special cutter required. Here are the gears prototyped in aluminium, the final gears were made in EN32 and were case hardened:
 

If you look closely at the lower gear you might be able to spot the 'deliberate' mistake.
 
The parallel depth method avoids the need for a special cutter as the design is based around the DP at the smaller end, and hence a standard involute cutter is fine.
 
Now onto the post by JohnS, really interesting. I downloaded the Gearotic demo last night and had a play. It looks very good, but I cannot output data from the demo so cannot see what the G-code looks like. Are the bevels generated true bevel gears, or do they use the parallel depth method? The other thing I couldn't see how to control was the stepover of the tool, which determines the surface finish, trading this off against run time. The cutting time JohnS got is much quicker than mine. I wonder if this is due to a coarser stepover, or a more sophisticated toolpath. My CAM system will only interpolate X and Z, The Y axis never moves and A is only used as an indexer. It would be more efficient to interpolate 3 or even 4 axes. This would also allow one to generate a 'Sturz' style cutting strategy which would be more efficient and give a better finish for a coarser stepover. Further I don't see why a tilting dividing head is needed? I didn't use one, but just used the 4th axis as an indexer, no cuts took place while the rotary table was moving. It would have been possible to cut the pinion using a 3 axis mill, but this would have required a long series end mill. A 4mm ball end cutter with a flute length of at least 22mm would be a bit delicate! I will be cutting the crown bevel gears using 3 axes, as the teeth are much flatter, so we don't need the 4th axis.
 
Now, back to the caveat. It would be possible to machine a true bevel gear on a manual mill by essentially replicating what the CNC mill does. In other words a long list of X, Y, Z and A co-ordinates that are stepped through by twiddling the handles. It wouldn't half be tedious though.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 05/06/2011 11:32:20

05/06/2011 10:45:21
JasonB: Thanks for the link to Conrad's site; an interesting read. At least I didn't fall into the trap of not having the number of teeth on the crown wheel divisible by 3 when I re-designed the differential using parallel tooth bevels.
 
Steve: Wow, nice looking gears! I may be asking you for advice in the future; there are two bevel gears needed for the governor on my traction engines. I think they're about 20DP so milling by the CNC route will need a small cutter. So I may use the parallel depth method for them. Just one comment; the space between the teeth on the two gears to the left seems quite deep in relation to the tooth width. Is this a consequence of the parallel depth method?
 
Regards,
 
Andrew
Thread: How to make an accurate internal hole
05/06/2011 10:37:37
The reason for sequential drilling is all to do with the chisel point on the drill. This is very inefficient at cutting, it has a large negative rake. Consequently it requires high axial forces to drive the drill into the work. (See the table in Appendix A of WPS12). With relatively low power machine tools it is much better to use a small drill first, so that the chisel edge on the larger drill is in the clear, and the drill only cuts on the lips, with positive rake.
 
Personally I wouldn't bother with a reamer; I'd rough drill and then bore to size. You're not looking for extreme accuracy on hole size (+/-0.001" should be fine) and by boring you will ensure concentricity.
 
Regards,
 
Andrew
Thread: The Cambridge Turning Trials
04/06/2011 21:24:03
It's what you've been waiting for; the turning trial results for EN8! Not surprisingly in many ways it was similar to EN1A and EN3B, but in other ways was sufficiently different to be interesting.
 
First, turning trials as before, same insert from Korloy, 40 thou depth of cut, 4 thou/rev feed.
 
Material: EN8, 1.625" diameter
 
Trial1: 260rpm (111fpm), no coolant
Trial2: 260rpm (111fpm), coolant
Trial3: 800rpm (340fpm), coolant
Trial4: 800rpm (340fpm), no coolant

General Appearance:
 
Trial1: Uniform finish but slightly rough, picks up fibres from a paper towel
Trial2: Uniform finish, feels smoother than 1
Trial3: Uniform finish, feels smooth, but picks up fibres from a paper towel
Trial4: Uniform finish, feels smooth

Under a Magnifying Glass:
 
Trial1: Surface is slightly torn
Trial2: Dull but not torn
Trial3: Dull but not apparently torn
Trial4: Uniform, not torn, slightly shiny
 
Swarf:
 
In all cases the swarf came off as a short (5mm) tightly curled helix; at higher speeds it was hotter!

Surface Roughness (Ra µm):
 
Trial1: 5.73 5.36 5.90 Average=5.66
Trial2: 3.95 4.02 3.52 Average=3.83
Trial3: 3.06 3.36 3.06 Average=3.16
Trial4: 3.27 3.20 2.66 Average=3.04
 
So similar to previous steels, but with two major differences. The turned surface was always very uniform axially and radially, even when slightly torn. Secondly the higher speeds didn't give as great an improvement as might be expected from previous results.
 
So, do another trial, same conditions as trial 4, except at 1200rpm (511pm). Like night and day! The finish is now almost a mirror finish, easily the best of the low carbon steels seen so far. The surface is smooth, but with slight radial grooves under the magnifying glass.
 
Surface Roughness (Ra µm)
 
1.55 1.76 1.62 Average=1.64
 
I'm gobsmacked at the difference in finish between 800rpm and 1200rpm.
 
Finally the fine depth of cut trial, insert from Greenwood Tools as before, 1200rpm, 2 thou/rev feed and no coolant.
 
The finish is smooth and uniform but not as good as for the deeper depth of cut. Under the magnifying glass the surface is smooth and uniform.
 
Surface Roughness (Ra µm)
 
1.92 1.96 1.86 Average=1.91

So there we have it, similar to EN1A and EN3B, but different too.
 
Regards,
 
Andrew
Thread: A Challenge - How Would You Machine This Part?
04/06/2011 16:01:06
Hi Steve,
 
There's nowt wrong with questions!
 
I'm using Alibre for solid modelling. I have the 'Expert' version, so including sheet metal and all the bells and whistles. I've been using it for about 6 years now. I bought it primarily for my work, but it is also very useful for model engineering. For anything other than a simple arbor, or similar, I tend to design in Alibre to ensure that parts fit together, will move together without interfering and then be able to produce 2D drawings. It has a few foibles but overall I think it is good value for money. Solidworks was outside my budget. In the distance past I've used ProEngineer, but didn't get on with it, very difficult to use.
 
That's the book! First time I used it, mumble, mumble, years ago I borrowed it from the library at RAE Farnborough, but eventually had to buy my own copy from Amazon.
 
As I understand it 12 teeth for involute gears is about the limit without introducing undercut, which weakens the teeth, I think. Certainly the standard B&S style involute cutters only go down to 12 teeth. But there's no theoretical limit, just practical ones. Well may be a one tooth gear would be a bit difficult!
 
Regards,
 
Andrew

Edited By Andrew Johnston on 04/06/2011 16:01:32

Edited By Andrew Johnston on 04/06/2011 16:02:27

04/06/2011 14:48:22
Hi Jason,
 
Yes, I will be writing this up for submission to MEW. And before there's any whinging about yet more CNC, the majority of the article will be about the design of bevel gears, as well as a summary of the machining methods, manual and CNC. The actual CNC machining part will be quite short, as it's not that interesting.
 
I'm building my SCC Burrells from the Filby drawings, as modified by LSM; but I bought my drawings, and all the castings, from John Rex in Pontefract. I've had a quick look on the LSM website and they don't mention castings for the differential pinions, just the crown wheels. Similarly John Rex sells the crown wheel castings, but not the pinions. The John Rex crown wheels are just blank castings, I assume the LSM ones are similar.
 
Regards,
 
Andrew
Thread: Living with the digital copies
04/06/2011 14:37:10
The drawings are pretty much indecipherable in the printed magazine too.
 
Regards,
 
Andrew
Thread: A Challenge - How Would You Machine This Part?
04/06/2011 12:26:31
Now to discuss how, and why, I did this machining the way I did. The whole farrago started because I didn't agree with some of the numbers given on the drawings for the bevel gears in the differential for the traction engines I am building. I'm not interesting in spending hours making parts only to find they don't fit because the drawings are wrong. I like to make sure I understand the numbers and how the part fits with its related parts before machining.
 
Having spent some time working through the design of bevel gears I concluded that the drawings are slightly wrong; 3 thou out, not much, but 3 thou is 3 thou! It is not possible to machine a true bevel gear on a horizontal mill; it needs some adjusting afterwards and needs a special cutter. So naturally I turned to the parallel depth method. I did in fact redesign the whole differential using parallel depth bevel gears. I did, of course, consider buying ready made gears from my supplier, but I wasn't terribly happy about the quality, the gears were expensive, and being awkward I wanted to machine them myself. Eventually the little light bulb illuminated (dull of course as it's an energy saving bulb) and it occurred to me I could CNC mill the gears.
 
Obviously to do this I needed a solid model. Easy enough to do in principle; I knocked up a basic gear in about 30 minutes. But as always the devil is in the detail. We all know that the tooth form is an involute curve, but how do you draw an involute curve in the CAD system. What part of the involute curve do you use, which of the infinite number of involute curves is correct, what about undercut clearances and root radii? It took me many hours of book buying, reading, drawing and internet searching to sort all this out. Plus the inevitable wobblies with the CAD software. In contrast the CAM program and G-code generation was fairly quick, apart from an annoying bug in the CAM software. For some specific operations the G-code generated didn't agree with the toolpath shown.
 
Here's the sequence of machining, first the blank, it's just a cylinder, diameter and length are easy to get from the CAD/CAM system.
 

Next, after the roughing cuts:
 

And after the final cut:
 

All cutting was done with a 4mm three flute carbide ball nose cutter running at 5000rpm and 400mm/min, giving a conservative chip load of 0.027mm per flute. Total cutting time was about 2 hours 45 minutes. The chamfers on the back and front were done on a normal lathe, using the same arbor mounted in a collet chuck.
 
How long have I spent doing this? Too long! I didn't keep a note of times, but the following are rough estimates:
 
Design calculation on the proper bevel gears: 8 hours
Designing the differential using parallel tooth bevels: 4 hours
Designing a 3D model of the bevel gear with correct tooth forms: 40+ hours
CAM software and G-code generation: 8 hours
Machining a backplate and fitting a 5C collet chuck to the mill: 6 hours
Loading, setting and cutting time for two pinions: 6 hours

It's interesting to note that by far and away the biggest task was the design phase. Of course I now know a lot more about bevel gears (!) so if I do it again I'll be a lot quicker. As an aside the cutting time for the bevel gears wasn't wasted. During the time to machine the two gears, over two nights, I cleaned up the workshop, made and ate supper twice, and did most of the turning trials that have been reported elsewhere on this forum.
 
Regards,
 
Andrew
04/06/2011 11:18:54
So, the weather is not good, we've got alto-stratus and some alto-cu here, otherwise known as crap off the North Sea, so flying has been postponed.
 
Thanks to everyone for the replies. Rather than one long post I'll do several shorter posts. First a few general notes.
 
The mating crown wheel will be 36 teeth, so John S wasn't far out. These are true bevel gears, so they cannot be cut on a horizontal mill with a dividing head. I'll discuss this in another post, as there are some things I would like to clarify on the cutting of bevel gears on a manual mill.
 
Congratulations to WALLACE for getting the answer correct in the first reply, CNC all the way.
 
JasonB is also correct, these are for the differential on a 4" scale SCC Burrell. I admit that casting was the one solution I hadn't thought of. I know that full size engines often used 'as cast' gears in the differential, presumably mainly for cost reasons. Since the full size engines often used only 6 or 8 teeth on the bevel pinions they would have also been heavily undercut and hence difficult to produce by machining. It would have been an interesting exercise to make the patterns and have the gears cast, but of course you still have the problem of how to make the pattern, chicken and egg? I'd use the CNC mill to make the pattern; it's a couple of keystrokes in the CAD program to alter the scaling of the solid model to allow for shrinkage, about 1/8" per foot or 1% for cast iron.
 
Donald, yes you could use a rat tailed file. But the radius at the bottom of the small end is just over 2mm, so may I suggest that a mouse tail file would be more appropriate. You'd probably have to ask for a wee, sleekit, cow'rin, tim'rous beastie file though.
 
On books, I do have Ivan Laws book, it's good as far as it goes, but, it seems rather weak on the theory. I haven't heard of the Marshall book, I'll have to have a look. I normally use 'Gear Design Simplified' from Industrial Press, Machinery's Handbook and the internet for design information.
 
Regards,
 
Andrew
 

Edited By Andrew Johnston on 04/06/2011 11:19:44

04/06/2011 09:21:27
Oeeeer, we can't have that; fortunately my imagination isn't that vivid!
 
The short answer is that the gears were machining on a 4-axis CNC mill. It is not possible to machine them on a manual mill, with one extremely tedious exception.
 
I have a number of comments and queries, which I'll post later today after I've been flying, or decided that the weather isn't playing ball.
 
Regards,
 
Andrew
Thread: Prams and teddies again
03/06/2011 22:51:37
So much for free speech; it's one thing saying what you think, it's another entirely, mucking up the formatting so that nobody else can post. It's enough to make one throw a teddy out of the pram!
 
Regards,
 
Andrew

Edited By Andrew Johnston on 03/06/2011 22:51:56

Thread: syil vs tormach
03/06/2011 19:34:06
Hi Robby,
 
As John and Jim say I have a Tormach 1100 PCNC. I've had it for about two and half years now. I haven't kept a log of run time, but I would suspect I have many hundreds of hours of machining on it. This has mostly been with aluminium and various plastics, but I have also cut low carbon steel and quite a lot of 316 stainless steel. In the very near future I will be cutting cast iron and gauge plate; I have no worries about the machine coping with these materials.
 
Overall I've been very pleased with my machine and Tormach have been very responsive to the few issues I have had.
 
To answer the question about using a 16mm cutter; in a roundabout way it is the wrong question. Here's why. Fairly early on in my CNC machining I made some complex heatsinks, 300x300x40mm that had a lot of pockets. I started machining these using a rather expensive 16mm rougher. As I recall depth of cut was 5mm, 80 to 100% engagement and about 2500rpm. It did work, but didn't sound nice when plunging at the beginning of the cut. Eventually as the cutter got a bit blunt I managed to stall the machine. What I now know is that CNC requires a different approach than that used on manual machines. Rather than run a relatively large cutter at slow speeds it is better to run a smaller cutter at higher speeds and feeds. For aluminium I now have three stock cutters. One, a HSS 12mm rougher, 4000rpm and 900mm/min feed, two and three are 10mm and 6mm carbide slot drills run at 5000rpm and 600mm/min feedrate. This is just as quick, or quicker, than a large cutter moving slowly. The cutters are much cheaper too!
 
I'm near, but not in, Cambridge; if you are in the area you are welcome to drop in.
 
Rather than type out lots of possibly irrelevant information I'll send you a PM so we can discuss specific issues off line.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 03/06/2011 19:35:34

Thread: Machining what am I doing wrong?
03/06/2011 12:16:22
The rule of thumb I use is: feedrate is half the tip radius, may be a little less for finishing cuts.
 
Regards,
 
Andrew
02/06/2011 21:18:17
Hmmmm, having initially suggested headstock bearings, I've now changed my mind. Unless the bearings are in very bad shape I don't think they are the cause of the poor finish. Presumably you do not see the same effects on other materials, say aluminium?
 
In the picture I can't see all that much difference in the surface cut towards the chuck and that cut outwards. I make 8mm diameter and 450rpm about 37fpm surface speed. Therein lies your problem I think. For carbide, much too slow a speed, and much too fine a feedrate. Result, a torn rather than 'cut' surface. See my thread on turning trials; for ordinary carbon steels I had very poor results at low speed with carbide. I didn't try very fine feeds, but my general experience is that they give a poor finish. I never use less than 2thou/rev and normally use 4thou/rev finishing and up to 20thou/rev roughing.
 
Over the coming weekend I will try some turning trials using EN8, and will report the results.
 
Let us know how you get on.
 
Regards,
 
Andrew
Thread: A Challenge - How Would You Machine This Part?
02/06/2011 15:58:25
Here's a tongue in cheek challenge!
 
How would you machine the part shown below? It's a true bevel gear pinion, 10 teeth, 6DP, 3/4" face width. The largest OD is just under 2".
 

As illustrated, the bevel gears are machined in aluminium; the final parts will be in cast iron. Note that. after deburring, the gears mesh together perfectly.
 
So, what machines would you use, what accessories might be needed, and what tools would you use or need?
 
Regards,
 
Andrew
Thread: Machining what am I doing wrong?
31/05/2011 23:00:57
Well I dunno, I find it difficult to believe that the headstock bearings are being affected by a cut on an 8mm diameter workpiece? Unless it's a very small lathe and, or, the bearings are shot. A question: do you get the same inconsistent results on other materials, say brass or aluminium?
 
The finish in the picture looks slightly dull and torn to me? If that was with the carbide tooling I'd say you were not running fast enough. For my own curiousity I'll experiment with some EN8 later this week and report the results in my thread on turning trials.
 
Regards,
 
Andrew
Thread: The Cambridge Turning Trials
31/05/2011 22:54:33
Thanks, I'm glad the results have been of some use. They have certainly made me think! At some point I expect I'll try the Sumitomo tips from Arc, as they seem to be reasonably priced and come recommended.
 
I would also hope to add the results of extra tests as interest and time permits. The next test will be on EN8, as this is pertinent to another thread.
 
Regards,
 
Andrew
Thread: This months MEW are 3 CNC features two too many
30/05/2011 20:44:05
Posted by David Clark 1 on 30/05/2011 20:19:48:

I got lots of complaints about the Side Lever enigne that was in Model Engineer but I also got some compliments and a toolmaker said he was part way through it.
 
 
Good grief; on what gounds? Some people seem to whinge just for the sake of it.
 
I thought it was an interesting series, nicely presented and photographed, and shows what can be done at the smaller end of the equipment range. I also assume that English is not the author's native language, which makes it all the more impressive.
 
I'll never build one of the engines, but nevertheless I always read the articles as I find them instructive.
 
Regards,
 
Andrew
Thread: The Cambridge Turning Trials
29/05/2011 21:32:35
Here are the final set of test results (stand up the boy who said hooray). It was suggested to me a while ago, on this forum, that high positive rake polished tips could be used to get a good finish on materials when using fine cuts (I'm embarrassed to say that I cannot recall who made the suggestion).
 
All tests were done with the afore mentioned tip from Greenwood Tools taking 2 thou off the diameter and at a feedrate of 2 thou/rev. All the materials are as listed in the previous posts. After each test I made a subjective assessment of the finish and measured the Ra values. Here are the results:
 
Brass CZ121 1700rpm, no coolant
 
Excellent surface finish, completely uniform
 
Ra values: 1.15 1.27 1.32 Average=1.25
 
EN3B 1200rpm, no coolant
 
Uneven finish axially

Ra values: 1.81 1.92 1.95 Average=1.89
 
EN1A 1200rpm, no coolant
 
Smooth finish, but slightly uneven axially
 
Ra values: 1.70 1.67 1.75 Average=1.71
 
Stainless Steel 303 800rpm, no coolant (I forgot to turn it on!)
 
Excellent finish, completely uniform
 
Ra values: 1.61 1.86 1.72 Average=1.73
 
Aluminium Alloy 6082T6 1700rpm, coolant
 
Excellent finish, completely uniform
 
Ra values: 1.12 1.11 1.14 Average=1.12

So, the polished tips do give an excellent finish, but not apparently on low carbon steels, at least not visually. It may be that the finer finishes are partly due to the slower feedrate.
 
Ho hum, there's a point when experiments have to stop and the real work begins. So tomorrow in the workshop I'm actually going to make parts for my traction engines.
 
Regards,
 
Andrew
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