Member postings for Andrew Johnston

Woohoo! Did some silver soldering this afternoon, and it went waaaaaay better than it has done in the past:

They're a bit untidy but I hope to improve on that. I centre popped the bronze plate before assembly so that there would be a gap. In retrospect the gap was larger than it needed to be, which was wasteful of solder. Eventually these parts will become the piston rod glands on my traction engines. Before anyone points it out the boss is not centred on the plate. The final elliptical outline has one side cut off to clear the slidebars.

Andrew

Good grief, that's excellent work, and fast too. I'm now embarrassed to have posted pictures of my MG12 with its tatty paint.

Andrew

Edited By Andrew Johnston on 26/05/2020 22:23:14

Pretty much par for the course. Doing similar I'm pleased if I have a variation in thickness of less than 3 thou. If you want to get down to a thou or so then everything needs to be scrupulously clean. Even the tiniest bit of swarf can throw things off. Also tap the work back when the jaws are reasonably tight. If you tap too early the work just hits the back of the jaw and bounces back slightly. The same effect happens when tapping down in a machine vice which isn't tight enough.

Andew

Remind me, I seem to have forgotten, if I ever knew in the first place.

Andrew

I've got some knife edge parallels that I used to balance my grinding wheels. The cylindrical grinder works fine, whereas on the surface grinder I always get a poor finish, mostly rippled as illustrated by John. Manual versus power feed and different step overs seem to make little difference. The wheel gets glazed quite quickly despite being a 46 grit. Either I need to use coolant to prevent the glazing or the wheel spindle bearings are fudged. I'm inclined to the latter. My surface grinder is from the 1940s and the bearings are plain taper. The manufacturer recommends a spindle oil made from white water paraffin. An oil supplier translated that as an ISO3 oil. It's just like water and runs out as quick as I can pour it in. Thicker hydraulic oils also run straight out. It could be that any internal springing pushing the bearings slightly outwards are broken or that the bearings are worn. It's on the roundtuit list to strip the spindle and see what's going on. I should have plenty of time to do that this summer since it doesn't look like I'm going to be working again, at least in the near future.

Andrew

It's pushing it a bit. The single biggest error is forming the gutter before screwcutting. It seriously reduces the resistance to bending of the part. Ideally one would add also a centre. It doesn't need to be fancy; a bit of silver steel with an appropriate chamfer and held in a drill chuck will be fine. It's not taking significant loads, just enough to stiffen the work in respect of bending. Even at small depths of cut as you approach the final thread size the insert will be cutting over quite a length so the forces will be correspondingly higher than the first few passes.

Andrew

There are two separate issues; one machining to an accurate diameter, and two making consistent measurements of the diameter. What happens if you measure with a mechanical micrometer, and how does it compare with the digital micrometer. Of course we assume that both the work and micrometer anvils are clean and free of fine particles of swarf.

Andrew

The direction of the cutting forces when milling are not always along the feed axis. Consider an endmill cutting conventionally along the x-axis. If the width of cut is shallow (say less than 10% of cutter diameter) then the forces will be mostly along the x-axis. But if we now increase the width of cut to 100% of the cutter diameter then the direction of the cutting force for a single tooth will start along the x-axis, move round to the y-axis and then back to the x-axis but in the opposite direction. So how should we orientate the machine vice?

I've never had a problem with work moving in the machine vice on the vertical mill. I have had a couple of wobblies on the horizontal mill with ambitious depths of cut. A 5hp 4-pole motor geared down to 60rpm produces a torque of 596.8Nm. At the edge of a 100mm diameter cutter that translates to a force of 11936N. You need a pretty large clamping force to resist that!

Andrew

It's probably safe to assume that the material is not changing dimension, so that leaves measuring equipment and technique. Looking at external measurements first I suspect the equipment. I don't tend to use the ratchet on my micrometers but go by feel. Even so the difference between barely touching and acting as a G-clamp is normally less than the differences seen by the OP. The alarm bell for me is the phrase 'digital micrometer'. Even worse if it's based on electronics. A cheap mechanical micrometer may not be accurate, but is still likely to be repeatable. With cheap electronics anything goes. It would be helpful to see a picture of the micrometer in question.

Measuring holes is a whole different ball game. Personally I've never got on with telescopic bore gauges. For holes up to 13mm I use these Mitutoyo gauges in conjunction with a micrometer:

Above 1" I use internal micrometers. For the 'here be dragons' area between 13mm and 1" I use the mating part, or a home made plug gauge, and machine to fit if tolerances are important. If not I just drill/ream and leave it at that.

Andew

A machine vice needs to be sized appropriately. My main vice overhangs by about a third:

Personally I wouldn't bother with a swivel base. On the few occasions where it might have been useful I just clamp the vice at the appropriate angle. I've never used my machine vice parallel to the table. If I need to clamp something parallel I clamp direct to the table, or there are vices designed for the purpose:

I reckon 75-80% of my machining on the vertical and CNC mills uses the machine vice. The remaining percentage is with the work in the dividing head, rotary table, clamped to angle plates:

Or clamped to the table:

On the horizontal mill it's the other way round, I rarely use the machine vice:

I have a smaller toolmakers style vice, although I've never used it.

Like all workshop tooling, buy the best you can afford - it makes life easier. And that's important for lazy people like me.

Andrew

I'm worse than that. At school we were supposed to learn poems off by heart for recital in class. Our English teacher said we'd remember them for the rest of our lives. Not me, I can't even remember which poems we were supposed to learn. Not that I learnt them. I find it difficult to remember word sequences, but pictures are no problem. When I'm working on a circuit design I can keep the whole schematic, including component values, in my head with no problem.

Andrew

I'm interested in making parts. Cutting tools, jigs and fixtures get made as I need them, by the quickest means possible. My judgement was that it was quickest to buy the hexagons and screws and make T-nuts to fit the mill. They're not conventional T-nuts. They have a tapped hole for the offset head screw that moves the hexagon. But they also have another tapped hole for a socket head grub screw that locks the T-nut in place. In use the reaction from tightening the hexagon creates a torque on the T-nut that forces the grub screw more tightly into the T-slot.

The design of the hexagon has some subtleties. It's not a perfect hexagon, the corners are rounded to prevent them digging into the work. They also have a small lip at the bottom to stop them sliding over the screw head. Both the hexagons and screws are hardened. Of course none of those features would stop me making them but at £50 for 10 it simply didn't make sense.

The T-slots on the mill shown are 5/8" (16mm), same as my other mills.

Andrew

Oil nipples:

The nipples are colour coded, although I'm not sure if that relates to the oil type or simply the size of the nipple. If you PM me an email address I can scan the relevant pages in the manual and the sheet I have from Myford listing lubricants for their machine tools. There are some inconsistencies between the manual and sheet. I bought ISO15, ISO32 and ISO68 slideway oils from Myford when I first got my grinder.

Andrew

Thank you for the compliment. It's the result of hard work. When I create a post I read through at least twice to ensure that the flow is logical. I often check facts or recalculate numbers before posting. I also aim to catch most spelling mistakes. Before hitting the post button I usually ask myself a number of questions:

Have I done what I'm describing, or if an opinion is it based on experience?

Is the post technically accurate?

Have I read the complete thread? Not always needed but it helps avoid putting one's foot in it.

Am I adding anything to the thread? No point in just repeating what others have already said.

If I'm challenging a previous poster, or pointing out an error, is it fair and am I confident that I'm correct?

I don't claim to get it right all the time. And I'm well aware that I don't know everything. There are some categories of thread that I rarely post in as I either have no interest, no knowledge or they're sterile subjects that repeat time and again. It's not unknown for me to delete my typing upon reflection and not post at all.

I passed English language and literature at O-level, albeit with mediocre grades. But I failed O-level French with the lowest grade possible.

Andrew

To expand on the reply from Martin in contrast to ordinary bevel gears parallel depth bevel gears are designed using the tooth parameters at the inner face of the teeth.

Andrew

It seems a bit odd to have a low profile clamp with a bolt head sticking out. I prefer these gold coloured hexagon style clamps:

I bought the hexagons and offset head screws but made my own T-nuts. Height is less than 7mm.

Andrew

I'm building two 4" Burrell single crank compound engines. Wow, designing from scratch is certainly a labour of love. I sure wouldn't want to do it without the aid of 3D CAD.

My engines are based on fullsize, they're never going to be exact replicas. There are some major differences which I can't correct without starting a new design from scratch, such as the brake being on the wrong side. Now that is interesting; I thought I'd better just check that statement. The SCC in the Burrell museum has the brake on the left. But in pictures of another engine supplied by a member of the TT forum the brake is on the right, as it is on my drawings. So it looks like anything goes!

Naturally there are some compromises (and even socket head cap screws) internally but externally I'm trying to stick to at least the spirit of the design, staying mostly with imperial fasteners, many of which I'm making myself.

Andrew

Some, but as you'd expect from a 1960s manual it's all rather vague. I've also got a sheet from the original Myford company listing oil types and application. My machine has oil reservoirs rather than the sight glasses on the MG9. My internal spindle is greased for life, so there are no lubrication points.

Andrew

I've got an early MG12 and a manual for it. But I suspect that's not much help as the designs seem quite different, certainly in the detail. The arrangement of the internal grinder is similar though.

Andrew

That's an interesting experiment.

I expect the odd multiple to get the frequency axis correct is due to incorrect scaling in the time domain to start with. Caveat, see next paragraph.

If the chuck is rotating at 21Hz why is 400Hz (and the times 19) important? I'd expect a fundamental at 21Hz and multiples thereof depending upon the number of teeth on gears.

It might be a bit more difficult to implement but I expect that measurements from a vibration sensor will tell you a lot more than a microphone. Looks like cheap accelerometers are available for the popular single board computers. Quite small vibration levels may well affect surface finish, but won't produce much sound. You definitely don't need resonance to spoil a finish. Machine tool makers will design to avoid obvious resonances. If something is creating a regular impulse the item being impulsed will vibrate, resonant or not.

Andrew