Member postings for Andrew Johnston

If you're going to do it you might as well do it right. This is the stand I built for my CNC mill, complete with coolant tank:

Made from 2" square ERW tube gas welded. The top is 6mm plate and the mill sits on 18mm thick machined blocks. The verticals were checked for buckling and diagonals added for rigidity. I'd disagree with SoD that it's over-engineered, it's called proper engineering. Mind you my CNC mill does weigh 500kg.

Andrew

This type of hole is common on electrical components, such as connectors, that fit in panels. Panel holes are normally punched, although I use the CNC mill, or handraulic methods, for small volumes. That leaves a fillet in the corners, but it only needs a couple of licks with a file to remove the fillet.

Andrew

That's a valuable lesson. One doesn't need to run at industrial speeds and feeds but it is surprising what can be done. Being too cautious can be worse than being gung-ho as the tool rubs, and that's a surefire way to damage a cutter. Of course some people (no names. no pack drill) will say my advice is irrelevant since my manual vertical mill is a Bridgeport. But consider this - it's only 1.5hp, or 1100W. That's on a par with many of the mid-size hobby mills. Another important lesson is that cutters are consumables. Pussyfooting them to extend life often has the opposite effect.

On another matter when quoting speeds and feeds for slotting in softer materials such as cast iron and aluminium and copper alloys manufacturers often use a depth of cut of D, reducing the depth to 0.5D for harder materials such as steels.

Andrew

Those are fairly conservative numbers for aluminium in industry. The feedrate for slotting works out at 0.24mm per rev, ie, a chip load of 0.12mm. The feedrate for side milling is higher because the width of cut will be smaller, usually 0.1D in examples, and chip thinning will apply.

Running at a slower rpm and with a proportional feedrate will be fine. At high speeds and feeds the biggest problem will be getting rid of the swarf and stopping it sticking to the cutter.

Andrew

Thanks for the elucidation. Given that 99% of the threads I screwcut are imperial I'll stick with the Ainjest.

Andrew

A Class D extinguisher is what you need. Types that are water based just make things worse.

Andrew

Does a dog clutch have to go on the leadscrew or can it go before a QC gearbox?

Andrew

+1

While I still use reamers for sizing, the primary goal is better surface finish. On my traction engines I reckon 95% or more of holes are plain drilled.

Andrew

I knew my Dormer metric 4-facet drills had an undersize shank, but didn't know about the imperial 2-facet drills. Time for measurement. A 10mm drill measures 9.95mm on the shank and a 1/2" drill measures 0.497" on the shank. So both sets are slightly tapered. Paying a premium price doesn't ensure a premium product, but paying a bargain basement price definitely ensures a bargain basement product.

Andrew

As it happens my lathe has a separate feedshaft, but I don't see why an Ainjest unit wouldn't work as a fine feed with trip on a lathe with only a leadscrew.

This shows the internals of an imperial Ainjest unit, looking from the back and with the unit upside down:

As stated it uses one half nut, working against a plain guide to counteract the radial forces inherent in Acme threads. Being an imperial unit it has a single gear meshing with the leadscrew - metric units have a cluster of gears which need to be changed according to the thread pitch. Here is the unit installed, with the trip bar annotated:

The unit was dead easy to fit as all the holes in the bed and saddle were pre-drilled and tapped at manufacture. A little known advantage of secondhand industrial over new far eastern.

The trip mechanism is very repeatable, for internal threading I let the unit trip, move the tool into the hole to be threaded until it bottoms, only using the topslide, and then back off 4-5 thou and lock the topslide. Fingers crossed I haven't had a crash yet.

Andrew

I'd say they were pretty poor. Out of interest I've just measured a 1/2" hole I drilled in hot rolled steel last week. Using a Mitutoyo hole gauge:

it measures 0.502". Not sure I believe that; one bush that measures 0.500" just slides in, the other, at 0.501", doesn't. My drills are Dormer, the 1/2" one is old school 2 facet.

For 16mm depth I wouldn't be pecking, just drill straight through. Neither would I start with a centre drill, I use spotting drills instead. At 10mm I'd be tempted to drill through 10mm to start with. If I did use stepped sizes I'd drill one smaller hole around 5mm.

A 10mm hole through 16mm bar doesn't leave much metal at the sides. For a good proportion of the depth the drill is cutting intermittently which may well cause it to go walkies. After spotting and pilot drilling I'd probably use a 10mm centre cutting carbide endmill as it'll be much stiffer and less likely to wander. It's my standard size, so I've got lots of them. I'd also be drilling on the vertical mill with the work securely held in a machine vice bolted to the table.

Andrew

Trepan 6mm deep on one side, turn over and repeat. Tool can simply be freehand ground from a piece of HSS.

Andrew

Spot on analysis.

Andrew

That's what I do, never bothered to set the top slide at an angle. Feeding from the cross slide allows depth of thread to be read directly.

Andrew

+1

I've got a dozen or more V-blocks, mostly made by people as part of their apprenticeships, plus a couple of commercial sets by reputable makers. But like JohnH I rarely use them; the Eclipse set are still in their original box and anti-rust protection. I couldn't immediately find any photo in my albums where I'm using V-blocks.

Andrew

Probably wise, as those types of contest usually end up with someone getting wet shoes.

Andrew

As you no doubt know the centre slot is cut a bit deeper than the T-slot, so that the T-slot cutter only cuts on the periphery, not the middle.

Andrew

Keep the cutter running otherwise you run the risk of damaging the edges. When making the cut, on one side of the cutter a tooth is conventional milling, as the tooth moves round to the other side it is climb milling. When you reverse the feed direction all that happens is that the conventional and climb milling sides swap over.

Continuously extruded cast iron machines beautifully, if a little messy.

Andrew

A few more notes:

There's no fundamental reason not to use high flute count cutters on aluminium. The advice to use 2, or possibly 3, flute cutters arises from the fact that aluminium alloys are easy to machine and so feedrates can be much larger than those used for steel. So there is more swarf. Using fewer flutes helps prevent jamming of the swarf in the flutes. For instance the manufacturer of my cutters recommends, for a 6mm cutter, 3800rpm and 210mm/min in steel cutting full width and 0.5D deep. But for aluminium the recommendations are 7600rpm and 440mm/min, cutting full width but also 1D deep, so roughly four times the removal rate. Interestingly the feed per tooth is only slightly higher for aluminium. This is what happens when aluminium swarf clogs flutes:

No idea why SoD says don't use suds on aluminium. On the manual mill I machine aluminium dry, or with the odd squirt of WD40. The WD40 doesn't lubricate, or do much cooling, but it does help prevent swarf from gumming up the cutter. On the CNC mill I use suds flood coolant, mainly to wash away the swarf. Recutting swarf is a quick way to kill a cutter. I've never used paraffin on anything.

You don't need long flutes to cut deep pockets. It's unlikely you'll be doing a full depth pass, and if you do the cutter will deflect. A cutter with short flutes and large stickout will be stiffer than a cutter with long flutes and the same stickout. So less tool deflection. This pocket is 48mm deep and was machined with a 2.5mm stepdown and standard length flutes:

I start with selecting a width and depth of cut and then select an appropriate chipload. Then a spindle speed appropriate to the cutter and material, and that then allows the feedrate to be calculated. The critical parameter is chipload, in mm/tooth, rather than feedrate. It's counterintuitive but increasing chipload is often a cure for chatter. When I first started using my horizontal mill I was cautious. Result, the whole machine shook, which was impressive given it weighs 3500lbs. Double the feedrate, and hence chipload, and it cut perfectly with no chatter or vibration.

Andrew

Not me. On the Bridgeport and CNC mill I mostly use the Tormach TTS system. My two main holders are 6mm and 10mm side lock, used with carbide cutters with no flat. They're simple and have a short overhang so one can run the cutters hard, note the blue swarf:

For cutters other than 6 or 10mm I use ER collets, this one is 4mm:

I have a selection of Autolock style chucks, mainly used with HSS cutters, imperial and metric:

I also use sidelock holders with larger cutters (1" slot drill in this case) for convenience:

I have been known to use a milling cutter in a drill chuck. Personally I use whatever holder I think is suitable for the job in hand and don't worry what other people do.

Modern carbide cutters with coatings are pretty darn sharp, and will slice a finger just like a scalpel - I speak from bloody experience. For aluminium I use uncoated and polished cutters, as it reduces the tendency for the swarf to stick to the cutter. For other materials I use mid-price cutters with TiAlN, or similar, coatings. I suspect my mills will not run fast enough to use the more esoteric coatings - so I don't waste money on them. Neither do I machine the really tough materials and 'orrid materials, like inconel. The coatings generally provide a thin, but very hard, surface to counteract wear and pitting from the swarf. But as always the devil is in the detail. With TiAlN the aluminium helps with lubricity meaning the swarf slips more easily. but the coating needs to be very hot for the aluminium to do its job. So running a TiAlN cutter at slow speeds doesn't make use of the coating.

Andrew