Member postings for Andrew Johnston

Bother, I can assure you it's purely coincidental.

Andrew

I don't know what the answer should be, or even if there is a 'correct' answer. Superficially one would say that an angle greater than the following drill is correct so that the tip starts first. But my spotting drills have a chisel point so the centre isn't going to be clean nor will it constrain the following drill. For a 4-facet drill it might even be worse than starting on a flat surface.

Using a 90° spotting drill seems to work; but that's anecdotal rather than proved by measurement.

Andrew

Here's a link to the NRL article I mentioned:

**LINK**

I think the summary is that stress relieving works but should be done with short, high temperature, cycles. Experiments showed that long term low temperature cycles didn't have much of an effect. This would fit in with the chart shown above by Nicholas. Note that the article is from the US, so temperatures are in Fahrenheit.

I don't stress relieve the iron castings for my traction engines. The larger ones won't fit in my electric furnace for a start. I believe that the castings are made in a small old school foundry using traditional sand casting methods. Thus far I've never knowingly had a problem with movement after machining.

As an aside I stress relieve cold drawn steel and brass sections that are going to be machined asymmetrically. I've had them go banana shaped if not. I've not seen the problem with hot rolled steel, and don't stress relieve, although a while back some posters said they had seen distortion in hot rolled sections.

Andrew

I rarely mark out and centre pop. In general only for sheet metalwork where I'll be using a hand drill. For probably 80% of my drilling on flat surfaces I don't use anything, just drill direct with a 4-facet drill. For holes up to about 8mm I use one drill, no point in faffing about going up in small steps.

I have 2, 3, 6 and 10mm 90° carbide spot drills. I use them in three circumstances. One, for non-flat, mostly round, surfaces. Two, for rough (cast) surfaces or three, where I want more accurate location of the holes.

I don't know what accuracy I achieve by spot drilling first, but anecdotally probably a thou or two. When I made this differential consisting of two ring gears and a centre casting:

I drilled the circle of holes holding the assembly together by spotting and then drilling. Unfortunately I had the centre back to front. I turned it over and all the holes still lined up perfectly. I drill 6mm for a 6mm bolt, so I estimate my location accuracy must have been a thou or two.

The last time I used a centre drill before a twist drill was on this special dividing head plate:

And that was primarily because the existing countersunk screws had 60° included heads.

Ideally I'd do some experiments, but even in lockdown, with no work available, I seem to be busier than ever and the round tuit list is expanding exponentially.

Andrew

Edited By Andrew Johnston on 27/04/2020 11:28:31

I rarely use small cutters on the Bridgeport. It's too darn easy to get the feedrate slightly wrong and ping goes the cutter, even at several thousand rpm. On the CNC mill I'd be running a 2mm cutter in steel at 5100rpm, and that's limited by the machine. For really small cutters (<1mm) I have a high speed spindle on the CNC mill which goes up to 24000rpm. One of the big changes due to improved cutter materials and coatings is the rise in spindle speeds.

On a manual mill a slow top speed would be a PITA for me. Of course the cutter will work, but you're very reliant on being able to create a steady feed. Overdo it with a bigger cutter and you'll stall the machine. With a small cutter it's ping and a trip to the local tool emporium.

Andrew

No idea, but I doubt it's due to the cast iron. I wouldn't have said that cast iron had a super smooth surface when painted. I wouldn't think it's due to the graphite either. Graphite is a lubricant because the structure consists of layers of atoms arranged in a hexagonal pattern. Each layer is strong in X & Y but the layers can slip over each other fairly easily. There's nothing to say that the paint can't stick to a layer.

As for iron castings being stress relieved outside I'm afraid that's something of a myth. In the US the NRL (Naval Research Laboratory) did some tests that indicated that stress relief due to small temperature changes and corrosion over months or years were small compared to short higher temperature cycles.

Andrew

But surely if the tip of the drill needs to seat in the spotted location the angle of the spotting drill needs to be larger than the following drill. Which is the opposite of saying it doesn't matter, and also contrary to JasonB.

Andrew

Don't think I'll bother, they'll be advocating burning 5G basestations next.

I've painted a lot of bare cast iron with smooth Hammerite in the past. All the tooling/machines are in regular use and have chips and scratches, but the paint has never flaked of it's own accord.

Andrew

Personally I wouldn't use either as they're not really prototypical. The ideal would be a tapered gib as per full size.

Andrew

For ordinary work, and always for Z, I just touch off the tool with a fag paper trapped between work and tool as an indicator. For more reliable accuracy I use one of these:

To find the middle of work I use the above in conjunction with the 1/2 function on the DRO. On the CNC mill I do the same, but just divide the readout value by 2 with a calcuator. I have broken a couple of probes on the Haimer in 10 years, both due to finger trouble. The golden rule is don't rush the job and stay calm!

For what I call centre finding a hole or spigot I use one of these:

Prior to getting the Centro I used Osborne's maneuver as publicised by Guy Lautard, It works well, but is tedious and gets trickier as the hole gets smaller.

I hate centre drills for anything other than use on the lathe. And even then I don't like 'em. So I've been using spotting drills for many years. Having said that for most of my drilling on the vertical mill I don't need to use a spotting drill. On a flat surface a 4-facet drill will start on it's own with sufficient accuracy for most purposes. I only use the spotting drill where I need greater accuracy when multiple holes need to mate with another part, and/or I haven't got flat surface to start, like round bar or the holes in this casting:

When using spotting drills the big endian versus little endian argument translates to do you use a spotting drill with an included angle bigger or smaller than the drill to follow?

Andrew

Blimey, that's a blast from the past. Welcome back and great to see you.

Andrew

Makes sense. Model engineers love to buy cheap, but if you steal it then it costs nothing. That's a bargain!

Andrew

The short answer is that it isn't going to tell you anything particularly useful. At best It might give an indication of the quality of the parallel, although given the narrowness of the parallel and overhang of the indicator I doubt even that. I'd agree that the condition of the ways is likely to be the least of your problems. The ways don't wear like those on a lathe for instance for several reasons:

The cutting forces are very low - on both my surface and cylindrical grinders the tables just rest on the ways, there's nothing holding them down

The ways have a large bearing area compared to the forces

They should be continuously lubricated

The ways are well protected from grinding dust

The ways are in contact over the full area most of the time, unlike the saddle on a lathe bed

Ways can be fixed but it's an involved process. IIRC there was a thread a while back on 'practicalmachinist' about scraping the ways on a cylindrical grinder. Pretty darn involved.

I'd agree with some of the points above. It's normal to grind the top of mag chuck in situ, so that removes many of the issues. On the older grinders problems are much more likely to be due to spindle bearings, especially if they're plain bearings. While the finish I get on my 1940s surface grinder isn't the best (pretty sure the spindle bearings are shot) it still grinds flat and parallel to better than I can measure with micrometers. Same is true of my 1960s cylindrical grinder; with some tweaking I can get a tenth or better on diameter over 10".

Andrew

As far as I'm aware the Morse taper was mostly used for drills, where the loads are predominantly axial. Some old school milling machines used it, but not many of the professional mills; they preferred the INT series. Some smaller manufacturers used special standards such as Brown & Sharpe or Jarno. But I suspect that was more to do with forcing the customer to buy your collets rather than any technical advantage. In the model engineering world I suspect it was lazyness rather than any technical reason to use Morse tapers on milling machines; plus repeating the mantra that you can use the same tooling as the lathe. Personally I don't see that. The only common tooling I'd have is drill chucks, and I'd rather have chucks for each machine than faff about sharing them. Once in a blue moon I use Morse taper drills or reamers on the vertical mill, for which I have an adapter. For anything other than drills you need a different style anyway, tang versus drawbar. So sharing tooling is even less useful.

My vertical and CNC mills have R8 tapers, although for most work on both I use the Tormach quick change system. This uses tool holders with a 3/4" shank which fit into a modified 3/4" collet, which normally stays in place. On the CNC mill I can't remember the last time I removed it. It gets removed more often on the vertical mill, last time was to trial the ARC insert milling cutters.

Given a choice I'd use the INT40 taper I have on the horizontal mill. It's rigid and can transmit real power while being self releasing. Power is transmitted via dogs and slots in the arbor rather than friction. It knocks both Morse taper and R8 into a cocked hat.

Andrew

Some pointers:

Heatsink paste tends to separate in the tube, just expel the liquid on some scrap and bin it

Forget lead free solder and use tin/lead solder

In this application forget solder paste and liquid flux

To an extent I agree with the procedure set out by Martin. I'm not sure of the size of the LED or board, but is a heatplate or pre-heating needed? For soldering I use an 80W iron running at 350C.

Unfortunately LED markings are often not clear. Worse, while the mark normally indicates the cathode it's not universal.

Andrew

No point in having a CNC mill if you don't let the computer drive it. Some 'conversions' from manual mills still have handles but those designed as CNC from the start do not. There are no handles on my CNC mill. Manual milling and ballscrews is not a good combination, as ballscrews are poor at self-locking so axes can move when least expected. Some controllers have 'wizards' which allow simple operations, such as facing or pocketing, to be done by entering parameters on screen which the computer then follows without explicitly generating a G-code file. My CNC mill has several wizards, although I've never needed to use them so far.

Andrew

That wasn't the question I was asking. We need to work the other way round. To start with, what do you think you can sell the part for? Then take off net profit, packing and delivery costs and any other costs not directly associated with manufacture, like advertising for instance. The figure left is what you need the part to be made for. Once you have that figure only then do discussions about materials and manufacturing techniques make sense. You also need to make some decisions about form versus function. Clearly the original was cast. Making a part that will have same functionality will be a lot cheaper than making a part that has the same functionality but also has all the draft angles included so that it mimics a casting. It might be an idea to check the thread; 9/32" Whitworth isn't listed in my old copies of Machinery's Handbook, but available taps seem to be both 18 and 20 tpi.

Andrew

The helix value is a red herring. When hobbing a spur gear, or ratchet wheel, the hob is set over at the helix angle of the teeth on the hob so that the tooth forms cut on the work are parallel to the axis

Andrew

I didn't, it chose me. When I bought my lathe and vertical mill from a dealer he threw in the collet chuck and collets as a deal sweetener. I didn't know any better, but I sure wasn't going to say no. As it turns out it was a good deal as I use the collet chuck by default.

Andrew

Yes it does. Now we know the part is commercial we can ask the single most important question. What is the target price to put the part into stock?

Andrew