Member postings for Andrew Johnston

The real question is how accurate does "exact" imply? Until that is quantified then what kit one may, or may not, have is irrelevant.

Andrew

People say that, bit I've never understood the logic? The recommended side rake for a lathe tool on brass is zero, or even negative. And the recommendation for drills is to stone a flat edge on the cutting face. Neither of those seem sharp, at least in the conventional sense. The cutting action is purely shear?

Andrew

Probably not. If by smearing you mean a burr, almost as if the metal has flowed, that's a characteristic of gunmetal and bronze. It can happen on brass but is less common. A wavy finish implies that something is moving. Could be the work not held properly, or the tool not rigid, or too slow a feed causing the tool to cut and rub alternately.

I'd be running a 3/8" cutter at 2500rpm on brass - could easily be higher but my manual mill starts to get really noisy over that speed.

Andrew

Remember that ploughing engines are large. So a 2" scale ploughing engine will probably require machines used for an average 3" scale engine. They also contain a lot of gears, including many bevel gears, which will be more difficult to get off the shelf.

Andrew

Fingers crossed I haven't broken a cutter yet when climb milling, even when the cutter does grab the table. The X axis on my mill has about 0.6mm backlash, so it's pretty obvious when the cutter grabs.

A rule of thumb when climb milling is to make the width of cut less than one third of the cutter diameter, or more than two thirds. Cutting at around 50% of cutter diameter creates high shock loads on the teeth as they enter the cut. When cutting at more than two thirds of cutter diameter the initial cut trajectory is away from the direction of table movement. So there is no grabbing. For small widths of cut I usually climb mill with a maximum width of cut of 10% of cutter diameter. Chip thinning means that the actual chip load is less than the feedrate implies. So even if the cutter grabs it isn't going to break.

For small cutters, say less than 6mm, which are most likely to break the cutting forces simply are not large enough to grab the table on my mill, even when climb milling. With larger cutters if grabbing becomes an annoyance I tweak the table lock to add a little drag.

Andrew

Not accurately without some form of tool and cutter grinder. But if the damage isn't too severe one modification is to hand grind a chamfer on each tip. Of course the endmill can't then be used to create internal right angles, but the finish when facing should be better. Larger secondhand cutters for sale often have the tips modified to be chamfered or rounded. One can buy new endmills with radii on the tips, albeit at a fair premium over a square endmill.

Andrew

Despite the name end mills are intended to be used on the side flutes. So the larger the depth of cut the more use you get for the money. Here's an example:

Central is a 10mm carbide endmill - material is hot rolled steel, depth of cut is 20mm and width of cut was 1mm conventional milling and 0.5mm climb milling. One is paying for the flutes so one might as well use them.

Andrew

Likewise I use a large bucket. For expediency I use brine for quenching both silver steel and gauge plate. I'd agree that agitation is vital. To achieve maximum hardness, mid sixties Rc, the agitation needs to be vigorous. Anything less will result in a measured hardness around the mid forties Rc. I always temper, but don't worry about keeping the part above a certain temperature after quenching. I temper within an hour or two of hardening; basically the time taken for the electric furnace to cool down to tempering temperature. These are what I use to measure hardness:

Andrew

Insert blades have a propensity to chatter unless the feed per rev is above a minimum value. I use inserts for parting off with a minimum feedrate of 4 thou per rev. But for grooving I grind a HSS tool bit which will give a better finish and no chatter even when sneaking up on a dimension.

Andrew

Actually he's playing AITA, and the answer is yes. As a result of his unhelpful response to my honest question he has been promoted to a very exclusive club.

Andrew

Where does 0.00004432951" come from?

Incidentally the official designation for a millionth of a metre is micrometre although, of course, micron is in common usage.

Andrew

It depends upon what is being used to make the measurement. Here are two instruments:

On the left is a dial indicator, on the right a dial test indicator (DTI). The dial indicator should be accurate, assuming that the shaft is aligned with the direction of movement. The DTI is not intended to give accurate readings but simply an indication of movement from an arbitrary zero.

I've just checked the dial indicator on the left with a 1mm slip gauge and surface plate, the reading was 1.006mm. It would probably be more accurate if I'd cleaned the surface plate first.

So a dial indicator should give accurate measurements of distance. If it doesn't then something is wrong with it or the setup.

Andrew

Edited By Andrew Johnston on 16/03/2021 11:29:58

It's counter-intuitive but chatter when milling can often be eliminated by increasing the feedrate, and hence chip load.

Andrew

Now there's something else that is voltage dependent.

Andrew

It is interesting to note section 1x.2.3 of the "The Art of Electronics - The X Chapters" where non-linearity (voltage coefficient) of resistors is discussed and illustrated with measurements on a variety of fixed resistors. Of course the effects are often small, but they do exist and it helps to be aware of them when designing.

Another useless fact is that Zener diodes over about 5V are dominated by avalanche breakdown rather than the Zener effect. Avalanche breakdown gives a 'sharper' knee than the Zener effect. I avoid using low voltage Zener diodes for that reason. Although it isn't a resistor an important design criteria when using a Zener diode is the dynamic resistance.

Andrew

Voltage dependent resistor? There's one on the board in question - the blue disc above the rectangular yellow capacitor.

Andrew

If I've understood correctly you are drilling across a round bar, not along the axis? If so then the larger drills will tend to chatter as the initial surface is not flat. I wouldn't pussyfoot about. Going up in small increments means the driils are more like to be rubbing than cutting. Clamp the work in the vice and then clamp the vice to the drill table. For a conventional drill with a chisel edge I'd spot drill at 10mm, drill though at 3-4mm and then drill 12mm. If the drill is 4-facet, so the end is a point, I'd miss out the smaller drill. Ideal would be to use a slow helix drill to reduce snatching at breakthough, but going up in a large step can also help to reduce snatching.

Drilling brass is generally ok dimension wise. It's bronze and gunmetal that move, close round the drill/reamer and deform.

Andrew

Andrew

That's surprising, steam is sooooo 19th century.

Andrew

From the same book:

Used for roofing, oil paint and varnish drums and in the motor-car industry.

Andrew