Member postings for Andrew Johnston

I'd argue that it's more satisfying and higher up on the willy waving scale. After all in order to produce the part one needs to create the design and be able to use 3D CAD. Then one needs to create the G-code and cutting schemes which requires a proper understanding of speeds and feeds versus depth and width of cut. Fixtures also need to be created that allow the part to be machined without getting in the way of the cutter or chuck.

Andrew

It's impossible to write for every level of audience, some a priori state of knowledge has to be assumed. As it happens I know what all the acronyms mean, as it was radio that got me interested enough in electronics to pursue it professionally. But if I'm interested but don't understand I'll either do an online search or ask for clarification - it's called curiosity.

While this is nominally a model engineering forum it covers a far wider range of interests and techniques than a man in his shed building a steam loco, and long may it continue.

Andrew

1. Traditionally an endmill would not be able to plunge vertically, but is designed to cut on the side. A slot dril normally has two flutes, and on the end one edge goes to the centre, so the cutter can be plunged straight down. For reasons of cutting geometry a 2 flute slot drill should cut a more accurate width on a slot than a multi-flute end mill. The distinctions are somewhat muddied now as 3 and 4 flute centre cutting endmills are very common.

2. I suspect it's simply what individual manufacturers specify rather than some fundamental difference between 3 and 4 jaw chucks.

3. Don't know; as someone has "helpfully" pointed out recently I don't have a Myford lathe.

Andrew

If I were to do a rough calculation I'd work out the diameter of the pitch circle, add the addendum and a bit more to account for the curved nature of the worm wheel. And then watch the tap cut the number of teeth it wants to, irrespective. The worm wheel illustrated was for a "speed" control on a Pickering governor, so the number of teeth was unimportant. I simply decided what OD I wanted to give the appropriate centre to centre distance and then cut steadily deeper until the teeth on the outer edges were fully formed. The precise centre to centre distance of each worm and wheel set was measured on the job rather than relying on the theory. I suspect that gashing the worm wheel first would help, but it's not something I've tried.

However, for the steering gear worms and worm wheels in the other pictures the worm wheel was gashed beforehand using a convenient involute gear cutter and the dividing head, plus swivelling the table on the mill to the appropriate helix angle, like this:

In this case the number of teeth was important as there was no latitude for adjusting positions on the job, so gashing was essential. When free hobbing the worm wheel the dividing head plays no part, it's simply a convenient way of holding the arbor on which the worm wheel freely rotates.

Andrew

In the UK it is definitely illegal to transmit on VHF aviation frequencies without a licence, with two exceptions. One, there are a number of frequencies set aside purely for gliding and a private glider pilot does not need a licence to operate on these frequencies. Two, if operating under the instruction of the licence holder, ie, a student pilot. From a practical viewpoint most ATC units say they would much rather glider pilots contact them, even if technically illegal. When I was getting my power licence I also did a course and took the exam to get radio operators licence, which is for life. The radios themselves also need to be licenced. So I pay £15 every 3 years to Ofcom for the handheld transceiver I use in my glider.

Andrew

If you use a standard V-thread it's easy enough to screwcut the worm and use a commercial tap in the lathe to make the worm wheel:

A spiral flute tap helps, but isn't essential.

Alternatively if you can screwcut an Acme worm then you can also make a hob for free hobbing the worm wheel, although you will need to gash the worm wheel first. Here's a home made hob cutting a worm wheel:

And the final worms and worm wheels:

Andrew

Not something I'm going to lose any sleep over though; there are things that matter and those that don't.

Andrew

EVs are quiet, so the people you run over don't even hear you coming. The cost of insurance isn't particularly tied to write off value. It's much more to do with potential costs if you injure people and they require lifetime care. And you can just as easily do that in a £100 banger as a £300k supercar.

Andrew

Oddly we had a meeting today at a gas sensor company for whom we're designing an updated PCB and signal processing for their NDIR CO2 sensor. They also sell optical particulate sensors, often used for air quality monitoring. At lunch one of the scientists was saying just that, modern diesel engines are less polluting than petrol for some gases and especially for particulates.

Whatever we do there'll be some "expert" or government "adviser" thinking up a new pollution source or new target to hit. Even if we all walked people will be worrying about particulates from shoe soles. The only permanent solution to pollution is to get rid of everybody. Then at least there'll be nobody to worry about cows farting.

Andrew

At an angle; I find a couple of beers beforehand helps with getting the optimum angle.

My lathe uses Gamet super precision opposed roller bearings. The manual says nothing about lubrication, and there are no oil points for the bearings. The geared headstock is filled with an ISO68 hydraulic oil, which splash lubricates the spindle gears. I assume the same oil must also lubricate the spindle bearings. The manual gives the spindle torque needed for correct preload and how to measure it, and then strongly advises against even thinking about it. I've never touched the bearings in the last 20 odd years.

Andrew

That might be the problem. First, you should be able to get the VFD to show the nominal output frequency which would give a fair estimate of motor speed, given the number of poles in the motor. If the speed of the motor has simply been reduced by reducing the frequency, and any belt or gear drive not changed, then you may well have a lot less than 1.5hp at the tool. If anything I'd reduce spindle speed a bit and stay with the feedrate. Provided of course that you're getting full power at the tool.

Andrew

This is a theoretical answer; while I've got a fair selection of T-slot cutters I've never had the need to use one yet. Feeds and speeds seem reasonable, albeit quite a low chip load. I've got two comments. One, I suspect there's a fair amount of flex in the tool spindle; not good as the cutting edge is long and forces will be hgh. Second, as an estimate I reckon the cutter is removing about 1.2 cubic inches of material per minute. I don't know what mill or drive arrangement is being used, but that removal rate will require comfortably over 1hp at the tool.

Another couple of thoughts. If the material is cold drawn BMS has it gone slightly banana shaped due to the initial machining, thus causing the T-slot cutter to jam? It's normal to cut the initial slot slightly deeper than the T-slot so that the T-slot cutter only cuts on the sides and doesn't rub on the bottom. I can't see if that is the case here.

Andrew

That's the way I read it, and I'd also start with 6mm plate, but I already have some in stock, left over from the traction engine hornplates. A sketch, or confirmation that I haven't gone off at a tangent, from the OP would be good.

Andrew

Edited By Andrew Johnston on 07/07/2019 12:35:18

The way I read the OP is that the strip would need rolling edge on, so you end up with a ring 132mm OD, ~5mm thick and with a 102mm hole.

Andrew

Cut the lawns front and back this morning. Finished just gone 11am and by 1pm it was raining. Fortunately the light rain has now stopped so the pizzas and drinkies do at the neighbours this evening is on, and outside.

This afternoon I tweaked the CAD model, and created a CAM file, to cut the 1/8" by 3/8" slots in the anchor bolts for the traction engines. Having cut one I updated the CAD and CAM as I didn't think the slot was in quite the right place. The machining used two stages. First stage used a carbide 3mm drill to create 3 holes to remove most of the material, time about 6 seconds:

And then a carbide 3-flute slot drill to pocket out and profile, time about 2 minutes:

I've machined four slots this afternoon on spare bolts. Now I'm happy I can machine the 18 proper bolts and then tweak the program for the 36 frost spikes; same size slot but in a different position.

The slot is reasonably accurate, a 0.125" slip gauge will go in top and bottom but a 0.126" slip gauge will just fit in the top but not the bottom.

Once all the bolts and spikes are done I need to slightly thin my 1/8" keyway broach and make some shims to create square ends to the slots.

Andrew

Dunno, could have been a Sea Fury. The aeroplane was definitely silver and my impression was it was too dumpy to be a Sea Fury. Didn't sound quite right either. The jury is out, but on balance I might stick with a Thunderbolt? I can't say for sure if there were invasion strips.

Andrew

Not a problem. This is a picture of milling a high helix angle helical gear. It wouldn't be difficult to extend it to a leadscrew:

Thread milling is described in many old school books on milling.

Andrew

Yes it can, use your imagination!

A horizontal mill can be used to turn short, large diameter parts that would otherwise be too large. See here:

**LINK**

Andrew

Oh dear, we seem to have a CAD catastrophe. If the line is half a thou long then the circle at the end cannot be 1000 microns, as that is one millimetre, which is definitely bigger than half a thou. The scaling factor is 25.4, so more, but not much, much more?

Andrew

Yebbut, on a mill you can mill and turn, so a mill could be used to reproduce itself too.

Andrew