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Member postings for Andrew Johnston

Here is a list of all the postings Andrew Johnston has made in our forums. Click on a thread name to jump to the thread.

Thread: Stop ended Tee slot in Meehanite
17/04/2021 13:09:29

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

17/04/2021 12:03:23

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

Thread: Milling around with bits
17/04/2021 11:52:38

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:

clogged cutter.jpg

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:

cylinder_deep_pocketing.jpg

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

16/04/2021 23:10:50
Posted by Stuart Munro 1 on 16/04/2021 22:02:02:
If not, any thoughts on holding the bits; I use tool holders and weldon mills but most people use collets. Why?

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:

eccentric rod roughing.jpg

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

After Final Cut

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

machining pattern bar chamfer me.jpg

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

governot hole.jpg

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. smile

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

16/04/2021 20:58:54

Posted by SillyOldDuffer on 16/04/2021 19:27:06:


ps I don't think the people I buy cutters from offer different helix angles! As I understand it, they are optimised for roughing or finish, or in between.

It's more dependent upon material. Cutters intended for aluminium often have high helix flutes. But they are only advantageous if one can run at the high speeds and feeds required. Similarly for tough materials cutters can have esoteric coatings and variable helix angle and/or flute spacing to reduce chatter. Again one needs a mill that is capable of utilising the features.

I don't regard my Bridgeport as being able to make proper use of the fancy cutters, so I don't buy them, other than polished cutters for aluminium.

Andrew

Thread: What to do with old reamers
16/04/2021 16:08:32

Depends what sort of reamer. Machine reamers cut on the leading chamfer, not on the flutes. So sharpening the chamfer should restore them to good condition. If they're hand reamers with a long taper it's more difficult to re-sharpen. Irrespective of type if a reamer can't be made to cut to size I'd recycle it.

Andrew

Thread: Tapping Mode on Mill
15/04/2021 18:44:00

Too small a tapping drill, I use 3.5mm for M4 threads. Spiral point is better in the sense that it pushes the swarf ahead of the tap. But for smaller threads I prefer spiral flute as the swarf is ejected back out of the hole as a long spiral and without jamming. The long thread length doesn't help; does it need to be that long? I'd be looking at changing the design.

Andrew

Thread: Set Screws on TTS Gauge Holders
11/04/2021 12:12:35

Seems pretty straight forward for X and Y. I understand how he measures Z, but not sure why he chose the Haimer to be tool 1, or how this interacts with the tool table. I'm old hat and use a master tool, labelled 0, to set a zero for measuring tools for the tool table, and to set the work zero in Z. Modern thinking tends to use the spindle nose as a reference, but that can be a PITA to see what's going on. And in some applicastions is too large to be able to set the work zero in Z.

Andrew

10/04/2021 16:46:17
Posted by JasonB on 10/04/2021 10:15:39:
I'm not sure what Pathpilot comes with in the way of probing software.

Pathpilot has a fairly comprehensive set of probing utilities, but all reliant on an electronic probe. Can't see why I'd want to use a non-electronic probe? As far as I'm aware Renishaw probes work on conductive and non-conductive materials as does the Tormach active probe.

Andrew

10/04/2021 10:06:22
Posted by Matt Harrington on 10/04/2021 09:38:37:

Can someone explain why you would use this device over something like a 3D CNC probe and say ProbeIt addon for MACH3?

Mine gets used more often on my manual mills than the CNC mill. I can't afford a Renishaw probe solely for the CNC mill, and even if I could, I wouldn't trust an addon for Mach3.

Andrew

Thread: Tyre Guage DRO - capacitance issues?
03/04/2021 11:40:16

Some statements by SoD need clarification. smile

A single wire has inductance, so as the frequency of a signal increases the resistance of the wire stays constant, but the impedance increases. One might argue that the effective resistance increases due to skin effect, just to pre-empt the experts.

The subject of EMC covers a wide range of areas. The main ones are radiated emissions, immunity to radiation, conducted emissions and ESD. If a device is mains powered then there are also mains borne nasties to be checked.

Radiated emissions are fairly simple to control, with the exception of cables. I hate cables inside, or external to, a product. Immunity is also fairly simple, especially for some applications, such as consumer, where the unit doesn't need to continue working, but just has to recover without damage. In other applications, automotive for instance, the unit needs to work throughout immunity tests. It's no good if the ECU hiccups every time a spark plug fires. Conducted emissions are fairly simple to control with appropriate filters and/or shielding.

A big cause of EMC test failure is ESD - it has a habit of getting into places it shouldn't, through pathways that one hadn't thought about. sad

Andrew

Thread: CNC - What's the Problem?
01/04/2021 17:05:59
Posted by Martin Kyte on 01/04/2021 13:38:54:

It would be interesting to get some feel for how people got going in CNC.

I suspect my path was somewhat different to the norm.

Over the years I'd seen small CNC mills pocketing out clock wheels and similar at exhibitions. But that really didn't interest me. Similarly I have the skills to convert my own CNC, but not the interest. I'm lazy and want to make parts, not faff about with machine projects.

The impetus to go CNC came from work, where I was doing the mechanical and electronics design for a high power bi-directional AC to DC power converter. Clearly the case and liquid cooled heatsink was going to need to be CNC milled. I looked at a number of options from new, or secondhand, professional to high end hobby. In the end I settled on Tormach. The criteria were a minimum envelope of 6" cubed (which in restrospect would have been far too small) and 4-axis. Of course I could see that the CNC mill would be useful for my traction engines as well. smile

I was already conversant with 3D CAD, so it was simply a matter of learning CAM. Suffice to say I ended up with VisualMill, although looking back I'm not sure that was the right decision. OneCNC might have been better. These are commercial packages, but remember I want to make parts in a sensible time so multiple tools, automatic tool length measurement and tool tables, plus 2.5D, 3D and 4th axis capability were essential. Re-machining capability is essential for me.

Learning CAM and CNC was a very steep learning curve done under extreme time pressure from the client. Within a few weeks (full time) I was comfortable making fairly complex 2.5D and 3D parts. I can't show many of the larger parts, but here are some of the simple parts I made while feeling my way:

photo1 multiple parts_resized.jpg

Subsequently I've made many parts for work and for my engines. The engine parts range from simple but where multiple parts are needed like wheel spokes to parts that would be difficult to make in the workshop by other means:

nameplates brass me.jpg

or impossible:

governor bevel gears me.jpg

Like everything else the CNC mill is a tool to be used when appropriate. I don't have a CNC lathe, but my repetition lathe is probably quicker at turning out the multiple parts I need such as nuts, bolts and studs.

Andrew

31/03/2021 19:07:24

I suspect margins on the machines are low and the cost of support with users adding their own computers and CAM software is high. Let alone when the electronics play up.

Andrew

Thread: Milling machines - western-made s/h recommendations up to 2k
30/03/2021 11:08:46

Posted by SillyOldDuffer on 30/03/2021 10:33:23:

.......I'd get the plans of a suitable candidate and study them carefully, asking of each part, what do I need to make that?

That's what I did - bought the plans and then decided what machines to buy. The swing of the lathe was set by the flywheel and the distance between centres by the front/rear axles. To some extent it depends upon how much the OP wants to do himself. Making spur, bevel and worm gears for instance will dictate sizes and capability. I had to drill/bore the hornplates for my engines in several steps as the Bridgeport wasn't big enough, particularly in Y. I didn't turn my wheels but made them by the fudgeit and bodgeit method and hoping for the best. Most spoke/strake holes were freehand drilled, but I had to get creative for the first row of strake holes, even with a 12" rotary table:

rear rim drilling me.jpg

The biggest issue will be Z, followed by Y and then X. I've run out of Z several times and have needed to buy stub drills/mills or collets. I've also run out of Y a few times, but X only when machining the rear rims before rolling:

rear_rims.jpg

I also ran out of Z on the horizontal when cutting the final drive gears:

final drive gear cutting.jpg

i suspect size of mill will be dictated by a 2" scale traction engine - in general parts for a 5" gauge loco will be smaller. I'd also echo SoDs note about clamping. It can take up a surprising amount of space/travel, not just on the mill but also on the rotary table.

Andrew

29/03/2021 21:25:18
Posted by SillyOldDuffer on 29/03/2021 16:37:33:

..............Andrew Johnson keeps traction engines in the living room.....

Actually they're in the entrance hall. They were in the kitchen, but after a complete refit a couple of years ago i was told no way they're were going back. And you don't argue with my mum! The sitting room is for work in progress and parts waiting to be fitted:

parts_sitting_room.jpg

Andrew

Thread: 1.1 kw brushed motor Torque
29/03/2021 12:28:58
Posted by JasonB on 29/03/2021 10:46:22:

.......it will do the job and a lot faster than an old lathe in backgear would.

i suspect that's nonsense. A lathe with a proper backgear should have full power available at the spindle, whereas with a DC motor running at slower speed will not provide full power. In this case the cut is limited by the weedy boring bar:

flywheel_rim.jpg

Flywheel diameter is 16.5" and is running at 40rpm with DOC of 0.06" and feed of 0.01" per rev. That's about 1.25 cubic inches per minute, well below what the lathe can do, but limited by the boring bar. Not allowing for transmission losses the available torque is over 500Nm. Metal removal rate is almost all about available power at the spindle.

Andrew

Thread: Milling machines - western-made s/h recommendations up to 2k
29/03/2021 12:08:44
Posted by William Ayerst on 29/03/2021 11:53:10:

.....based on the aggregated advice I'll be looking for a vertical mill firstly, if it has a horizontal attachment then that's great - but if not, no harm done.

They are available for some ex-industrial vertical mills:

bh_2.jpg

But they seem to go for silly money these days and are no where near as rigid as a proper horizontal.

Andrew

29/03/2021 11:45:41

Good grief - I never thought it would happen, but I agree with SoD!

I have both vertical and horizontal mills, and in both cases the accessories to convert to the other mode. But I'd agree with SoD that the vertical mill is the more versatile. In particular for drilling the quill is all bar essential. While vertical heads for horizontal mills can have quills they seem to be pretty rare. I've just looked at the manual for my horizontal mill (Adcock & Shipley) and there is no option for a vertical head with quill.

Another point to note is that travels on a horizontal mill are small compared to the size of mill. For instance my vertical mill weighs about a ton, is 1.5hp, and has travels of 33" and 12" in X and Y respectively. Whereas the horizontal mill weighs nearly 2 tons, is 5hp, but only has travels of 23" and 8" in X and Y. Vertical travel is the same on both at 16".

The horizontal mill is way more rigid than the vertical mill and that is excellent when gear cutting and where lots of metal needs to be removed. Horizontal mills are pretty much obsolete commercially and so secondhand ones are cheap - mine was £175 compared to £2k for the vertical.

I wouldn't like to be without either, but if I had to give up one it would have to be the horizontal mill. sad

Andrew

Thread: Small Poppet Valves
26/03/2021 10:40:21

Could use a roller box.

Andrew

Thread: Tungsten Alloy Spam PMs
26/03/2021 10:36:37

Pity we can't administer some Chinese justice.

Andrew

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