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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.

Thread: Milling a profile - help with technique please
08/04/2020 11:32:44

A few more notes:

It helps greatly to use quality cutters - currently I use YG K2 and ALU-POWER cutters from Cutwel (open and shipping) and premium cutters from Arc (closed).

I wouldn't use MDF as a base with, presumably wood screws? If I need a base I always use steel or aluminium; although I do have fairly large offcuts piles for both materials

The most widely available aluminium alloy is 6082 (aka HE30) which machines very well

WD40 is good to prevent the swarf building up on the tool but doesn't do much for cooling. But for aluminium you don't need to worry about cooling. I use flood coolant on my CNC mill - for aluminium it's primarily for washing away swarf rather than cooling. For slotting I wouldn't use squirts of ED40, it'll just cause the swarf to stay in the slot. If the worst comes to the worst I machine dry and follow the cutter with a bendy straw or the vacuum cleaner to remove swarf. If you have a compressor you might be able to setup an airblast. But beware of flying swarf - a trip to A&E to get swarf out of the eye may be a problem at the moment. Of course you wear eye protection?

Andrew

07/04/2020 20:36:42

I'd agree with Jason. I'd drill the two holes on the manual mill, screw to a fixture on the CNC mill and then profile the outside with a 3-flute 6mm cutter, full depth, 7000rpm, 400mm/min and 3mm width of cut. I normally leave around 0.5mm when roughing and then a full depth finish pass. For roughing I use climb and conventional milling, for finishing mostly climb. If the small concave radii can't be designed out I'd re-machine them after profiling with a smaller cutter. Make the cutter smaller than the radius to be machined or the cutter will chatter.

Generally I avoid slotting where possible. Even with flood coolant it's a PITA to clear the swarf and nothing fudges the cutter and finish quicker than recutting swarf. Another issue with slotting is the use of bridges. When I have to use them I make them 0.5mm high maximum. But they're still a pain to clean up. It can be easier to start with material slightly thicker than needed, profile the part, turn over and face off the unwanted material.

These parts are steel, but show the sort of finish that is possible with a hobby CNC mill:

spectacle1.jpg

Andrew

Thread: Overwhelmed!
07/04/2020 14:14:12

Posted by Jon Cameron on 07/04/2020 13:27:32:

The messy ones are where a lot of things seem to get built.

That's a relief; I was beginning to think I might be giving Pigpen a run for his money.

Andrew

Thread: DC-DC converter
07/04/2020 14:08:44

SoD: Ok as far as it goes. thumbs up But I'd expect the problem to be due to noise direct from the DC-DC converter, not external pickup.

Andrew

07/04/2020 10:49:35

Resistor in series or parallel? Putting the capacitor at the battery terminals creates a low pass filter with the resistance of the wire to the converter output, which may be just emough to reduce switching noise.

Andrew

Thread: Todays news -- well done
06/04/2020 12:40:48
Posted by Mick B1 on 06/04/2020 09:57:24:

..............some of the rule-breakers probably have PhDs..........

At least I'm not ignorant.

Andrew

Thread: Homemade Lathe Tools
06/04/2020 12:33:33

In 20 odd years of having my current centre lathe I've made very little tooling; mainly haven't had the need. What I have made would be irrelevant to the OP anyway. I've bought plenty of tooling, often secondhand, and also made many fixtures that get used for the job in hand and then put aside, or re-purposed.

For the mills I have made simple tooling that get used on a regular basis, but that wasn't the original question.

Andrew

Thread: Wow, what a battery
04/04/2020 11:30:23
Posted by SillyOldDuffer on 04/04/2020 11:00:35:

Anyone else up to checking the maths? What's 900000mAh at 5V from a 280g battery compared with 46MJ from 1kg of petrol?

A very simplistic calculation says the battery is better per unit mass. But in reality that's nonsense since I don't believe the battery is anywhere near 900Ah.

Battery chemistries have made big strides over the last few years, but the fundamental energy capacity hasn't kept pace. Lithium batteries compare well on Wh/kg since lithium is so light. A lot of the work on batteries has gone into reliability and charge/discharge cycles. It's no good having a sooper dooper electric vehicle if the battery dies after a few hundred cycles, or even several thousand.

Andrew

Thread: Square thread cutting
04/04/2020 10:58:41
Posted by Simon Collier on 04/04/2020 10:35:29:

Would you use carbide cutter and high revs?

A carbide cutter is a must. General rule of thumb is run at the same speeds as ordinary steel, but with small DOC and high feedrate. Ideally the shear zone needs to be red hot. Just to prove it works here's a toolbit being milled:

embryo cutter.jpg

In this particular case the milling was to achieve an accurate involute profile rather than bulk material removal, but the principle is the same.

Andrew

04/04/2020 09:18:34
Posted by Simon Collier on 04/04/2020 02:04:16:

I wish they would grab a HSS blank and have a go at grinding a tool or two. They might be surprised how easy it is.

There's no need to waste time grinding to shape. If a lot of material needs to be removed (like the tool shown) use a mill to shape and then grind to add reliefs.

Andrew

Thread: Gear Cutting - Pressure angle.
03/04/2020 18:57:45
Posted by Steve Crow on 03/04/2020 17:32:51:

.............I didn't know that the minimum tooth count was so high.

That's what happens when formulae are quoted without understanding how they are derived. The formula given relates to undercutting when a gear is hobbed. The undercutting is a consequence of the hobbing, not a neccessity for the resulting gears to mate.

I have made 13 tooth 5DP pinions with an involute cutter that mesh properly without needing undercutting. On the other hand these 6DP bevel gears are 10 teeth, and do need an undercut:

Bevel Gear Pinions

In both cases the gears were 20 degrees PA.

Andrew

Thread: Wow, what a battery
03/04/2020 17:26:42

m = micro = marketing b*****ks

The spec, for what it's worth, states the voltage is 5V.

Andrew

Thread: Model of an epicyclic gear made by apprentices
02/04/2020 22:44:12
Posted by Speedy Builder5 on 02/04/2020 08:37:20:

I too lived in Bedford and went around Allens in the 70's - super works visit. Very nice work by those apprentices.

That's interesting, I had a factory tour in 1971. At the time it looked like my school career was going to be a major car crash so my parents were looking for an alternative to the academic path. My father knew the Allens apprentice master through the IMechE. So we got a factory tour and I informally took, and passed, the apprentice entrance exam. Three things stick in my mind, watching a turner working on a rotor forging for a steam turbine and wondering if he was more important to the company than the MD, large wooden patterns for pump bodies in the woodwork shop and a massive 25+ foot planer machining engine blocks for the diesel engines.

Andrew

Postscript: My school career was a car crash but fortunately not too serious a one as it turned out.

Thread: A few Thou Under?
02/04/2020 22:32:04

I normally use nominal size; but I'm not a fan of split dies and avoid using them where possible. Obtaining a rattle free fit is a bit of a crap shoot. I prefer to screwcut or use Coventry die heads, where it is easy to finely adjust the finished thread diameter.

Andrew

Thread: Model of an epicyclic gear made by apprentices
01/04/2020 21:43:55
Posted by Henry Brown on 01/04/2020 21:24:31:

............apprentices at W H Allen, latterly Allen Gears, of Pershore, Worcestershire back in the 60's.

Any association with W H Allen in Bedford who made steam turbines, diesel engines and pumps?

Andrew

Addendum: Apparently the answer is yes, a management buy out in the 90s, now seems to have been sold on

Thread: Gear Cutting - Pressure angle.
01/04/2020 20:19:24
Posted by DC31k on 01/04/2020 19:58:27:

The method is one well-recognised in industry and is by no means an approximation. It is a method of _generating_ gear teeth, the same as hobbing or gear shaping. Have a look for Sunderland gear planer.

I haven't bothered to read the helicon method but hobbing and planing are approximations. They 'generate' the tooth form in the sense of a creating a series of straight lines rather than relying upon the cutter profile.

For many applications the tooth form as hobbed is fine. But for precision and/or high speed gears (as in a lathe headstock) the gears are often shaved and ground after hobbing to refine the profile.

Andrew

Thread: Magnetism in Stainless steel
01/04/2020 11:48:13

Austenitic stainless steels are paramagnetic, ie, weakly attracted to a magnet but cannot be magnetised. Cold working austenitic steels can induce martensite, due to deformation, which is ferromagnetic. The effect can be reduced, or eliminated, by using a high nickel steel and by full annealing after manufacture.

Personally I wouldn't use stainless steel, but if one must use it then 316 will be better than 304 as the nickel content is higher.

Andrew

Thread: Sharpening Coventry die chasers
31/03/2020 22:24:40

You could try Wiseman Threading Tools:

**LINK**

I've bought spares and chaser sets from them. They will grind specials although they don't mention standard resharpening. I've got the wherewithal to sharpen some sizes of chaser, but have never had the need to try it.

Andrew

Thread: DC-DC converter
31/03/2020 13:46:07
Posted by Martin Kyte on 30/03/2020 13:56:31:

My thrust was it's the wrong module for the job.

I'd agree with that.

One of the problems when designing switch-mode power supplies is that magnetic components have all sorts of limitations compared to the ideal. When designing a buck converter in continuous mode the key parameters for the inductor are the delta current and the peak current. The delta current is a compromise between output ripple and inductor value as well as determining at what output current the converter will go into discontinuous operation. Values are commonly in the range 20-50%. Peak current is what determines the choice of inductor. Many small inductor datasheets quote a current rating based on an inductance decrease of 20%, ie, starting to go into saturation. Given the wide manufacturing tolerances I'd normally keep the peak current to about 70-80% of rated current.

Another consideration is the curent limit within the IC. This is a last resort and is poorly characterised: worst case more than a 2 to 1 variation in the datasheet. It's a moot point as to whether the inductor should be sized to cope with the worst case over current. It would depend upon the inductor and IC capabilities and heatsinking. As far as I can see the LM2596 goes into cycle by cycle current limiting if there's a problem, so both the inductor and IC might get hotter than desirable.

Time to test the lockdown by going for a cycle ride - less than an hour though!

Andrew

Thread: Taper on connecting rod
30/03/2020 22:56:16
Posted by Richard Brickwood on 30/03/2020 20:38:33:

Hi Andrew - things to do when not gliding!

Sounds like you've got the taper issue sorted. Could be a lack of currency? smile

I'm probably the most current pilot at Gransden as I flew my DG200 the Thursday before last (20th) at Milfield where I was retrieving it, fortunately just before the lockdown kicked in.

Andrew

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