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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: 63-tooth change wheel for Portass PD5
04/07/2020 18:22:54

Pete beat me to it!

CI = cast iron


The formula is: outside diameter = (number of teeth + 2)/DP

Thread: Old School Drawing Exercises and 2D CAD
02/07/2020 07:47:23
Posted by SillyOldDuffer on 01/07/2020 17:40:18:

Another Geometrical Drawing. What's the radius of the largest circle that can be drawn inside this isosceles triangle?

Should be able to solve it analytically, although that will involve maths. smile o


01/07/2020 18:54:01
Posted by SillyOldDuffer on 01/07/2020 17:23:12:

Not to us geeks : In computer programming, orthogonality means that operations change just one thing without affecting others.

No wonder the softies miserably failed to create a working track 'n' trace application. It used to be said that the ideal was an English-like programming language; to which the riposte was that all it would prove was that softies couldn't write English.


Thread: What Did You Do Today 2020
01/07/2020 16:11:18
Posted by Keith Wyles on 01/07/2020 15:22:09:

A mm3 to m3 is a big jump, and I wonder how many can visualise either.

Simples, it's a factor of 10^9. Easy to visualise too, a cubic metre is what my garden supplies are delivered in, and a cubic millimetre is about four 0402 capacitors stacked two across and two high. Given that the world, and the parts we make, are inherently 3D, I would have thought that any self-respecting engineer would have no trouble mentally visualising in 3D. smile


Thread: Old School Drawing Exercises and 2D CAD
01/07/2020 12:17:23

Good grief, SoD must be really bored!


Thread: Mill unexpectedly cutting crooked
29/06/2020 22:56:03
Posted by blowlamp on 29/06/2020 22:14:32:

It sounds like the workpiece is lifting as it's being cut.........

I wondered about that too, or if the work was being curved upwards due to pressure from the vice. But the OP states he had the same effect on a 40x6 part, so presumably not?


29/06/2020 21:35:42

Well, that's an interesting conundrum. Never come across it on any of my milling machines. Tool deflection does happen, but it's small, especially with the light cuts being taken. And, as the OP says, tool deflection would cause the bottom of the work to stick out, not vice versa.

Feeds and speeds don't seem unreasonable, although we don't know if the cutter is HSS or carbide. If the cut is unstable then it should generate noise/vibration - did it?

Where's SoD with his vibration monitor when you need him? smile


Thread: Copper boiler plate flanging, or not?
29/06/2020 14:28:03
Posted by SillyOldDuffer on 29/06/2020 12:52:30:
'Wire drawing' covers accidental throttling which can be considerable and is well worth reducing as much as possible.

Not necessarily. Wire drawing occurs when steam is throttled and the input and output velocities are similar. This means that the expansion is isenthalpic, ie, enathalpy stays constant although the pressure drops. Since the pressure has dropped but the enthalpy stays the same the output steam is drier, or even becomes superheated. That may be an advantage as condensation in the cylinder will be reduced and hence less energy lost due to condensation and re-evaporation.

With Stephenson's valve gear, at short cut offs, the input ports are only partially open so wire drawing is almost certain to take place, which may, or may not, be an advantage.

Losses due to poor passage design and sharp changes of direction are different.


Thread: fusion 360 cam
29/06/2020 10:31:41

I use option 2.


28/06/2020 22:25:46
Posted by Nick Hughes on 28/06/2020 19:57:20:

Using 2 Axis (2D) Facing in AlibreCAM............

That is interesting, you learn something every day; thanks very much. thumbs up

It works fine in my old version of VisualMill:


I had in mind that using a facing operation with bosses would overcut the boss; but obviously that's not the case. I think it arose from my experiments with pockets that are open on one side. Pocketing doesn't properly finish the corners that meet the open edge. But facing overcuts the three sides that are not open. I'd better go back and do some tests.


28/06/2020 11:07:33
Posted by JasonB on 28/06/2020 07:27:51:

Andrew, I wonder if it is because you are selecting two boundries and not a surface? F360 won't do it if I select the outer edge and edge of the circle.

Definitely selecting a surface and the selection is called a FlatArea. But the system still highlights boundaries, so may not be really selecting a surface. Same result on a much newer demo version. Looks like it's a limitation of my CAM.


27/06/2020 20:17:12

Pocketing doesn't work in my CAM software, which is why I didn't suggest it:


It sort of does what is sensible, ie, keeps within the outer boundary and outside of the inner boundary, even if it isn't what is wanted. Presumably Fusion360 has extra knowledge of the part geometry and knows it can go outside the outer boundary?


27/06/2020 13:13:17

It's one of those seemingly simple jobs which are a PITA. If you trying facing the system will machine the outside perimeter with no problem but will overcut the boss. Conversely if you profile between the boss and outline it won't completely machine the corners of the outline.

A simple but very inefficient solution is to profile the boss but have a number of 8mm stepovers such that the whole of the blue area is covered.

Alternatively one could create a rectangular outline more than 10mm from the outside of the blue area and profile between the rectangular outline and the boss.


Thread: 2mm endmill help
27/06/2020 08:21:29

There seem to be three fundamental issues. Roughly in order of importance:

The new cutter shown seems to be poorly ground. The used cutter shown is badly worn with the cutting edges rounded. It would seem that these cutters are simply not up to machining steel.

Runout on the collets - it's way bigger than the chip load. So one tooth may not be cutting while the next will be taking double or more the expected cut. So the loads on the cutter are much higher than expected from simply looking at the rpm and feedrate. Varying chip load is also a recipe for vibration and chatter.

There is still quite a lot of cutter stick out. even with the shank cut down.

One and three could be ameliorated by buying quality cutters intended for cutting steel. Two is more difficult, short of acquiring a different mill. A good clean and check for any small burrs may help. Reducing the stick out of the cutter would also help.


Thread: How does one scale a worm gear?
25/06/2020 22:30:27
Posted by Michael Gilligan on 25/06/2020 21:26:20:
Here are some photos that confirm the table has a throated worm-wheel:

It's single enveloping, ie, the worm wheel wraps partly around the worm giving line contact. Double enveloping is where the worm also wraps around the worm wheel. Which means that the thread on the worm is cut on a circular path on a plane which also contains the axis of the worm.


Thread: 2mm endmill help
25/06/2020 20:48:36
Posted by old mart on 25/06/2020 14:47:38:

.........I blame the heat, 33.8C at the moment.

Man, that's hot. Higher than Heathrow reported today I think. One normally expects airports to be slightly hotter than the surrounding areas due to all the engines pumping out hot gases. But not at the moment. I went down to Stansted yesterday to renew my Civil Aviation Authority medical. Never seen it so quiet. Very few cars and no buses around the terminal building and the long term car park was empty.


Thread: How does one scale a worm gear?
25/06/2020 20:40:41

Posted by William S on 25/06/2020 19:01:41:

A quick question about hobbing is it always necessary to gash a gear prior to hobbing

Assuming free hobbing (hob and blank are not geared together externally) then it depends upon if you need a set number of teeth. One can free hob using a tap and a blank with no gashing:

hobbing worm wheel me.jpg

But it's a crap shoot as to how many teeth you end up with; in the case above it didn't matter. If a set number of teeth is important then you need to gash the blank at the helix angle of the worm -

worm wheel gashing.jpg

- to give the hob something to engage with and drive the embryo worm wheel:

worm wheel hobbing.jpg

The DP of the worm wheel shown was approximately 6.281, although I used a commercial 6DP involute cutter for the gashing. The hob is home made.


Thread: 2mm endmill help
25/06/2020 14:11:09
Posted by old mart on 25/06/2020 13:32:14:

A J, your picture shows a 2mm slot drill, but the shank is al least 4mm.

A quick visual comparison with the rule in the picture will show that the shank is 3mm.

My apologies for not reading the thread sufficiently well to realise that an expert had already expounded. At least crawling back into my cave will be some relief from the heat. smile


Thread: How does one scale a worm gear?
25/06/2020 10:07:10

Posted by William S on 25/06/2020 00:09:16:

......(is DP for worm gears? cant seem to find a definitive answer)......

The basic rule for worms and worm wheels is that anything goes. But it is common to specify the worm wheel in terms of a standard DP or Mod number. The downside is that the tpi/pitch of the worm is then an irrational number. On my lathe one can modify the change gears so that the gearbox will screwcut worms to approximately the correct tpi or pitch for standard DP and Mod values.


Thread: 2mm endmill help
25/06/2020 09:54:08

Here's a picture of a 2mm diameter 2-flute uncoated slotdrill:


Note that the working length of the cutter is quite short. I'd be running this with the collet closing at the top of the flutes, so less than 10mm stick out. I used this style of cutter to form a series of fins on a liquid cooled heatsink for an experimental high power inverter. The gap between each fin was 2mm and they were 4mm deep. I ran at 4000rpm, full width, 1mm stepdown and 150mm/min feed in 6082 aluminium. Each heatsink had 18 gaps and I machined 8 heatsinks, all with the same cutter.



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