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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: Fusion 360, Mach4 and first test of cnc router
19/01/2021 20:48:57

I suspect your post-processor may not be set up correctly. How are you setting up G02 and G03? I've seen the odd wobbly in generating G02/G03 commands, but never on every one in a design.


Thread: Advice for surface finishing
19/01/2021 20:06:23
Posted by Stuart Cox 3 on 19/01/2021 19:45:33:

Its ok and will probably do but I'm wondering what the technique is to achieve a shiny smooth finish?

The finish looks to be poor on all faces and the spigot at the left doesn't look parallel? So I suspect the problem is generic. Unknown material is one potential problem. Where did the insert come from and what scale is the part? On some materials you might get away with a shallow depth of cut, on others you will need to increase speed and depth of cut to get a decent finish.


Thread: Fusion 360, Mach4 and first test of cnc router
19/01/2021 19:04:11

The toolpath that the CAM program simulates and what the G-code actually cuts are not always the same. I use NCPlot to backplot the toolpath from the G-code as a sanity check on the real toolpath. It also calculates maximum excursions in all axes, which is another useful sanity check.


19/01/2021 14:57:20

There are two ways to program a cutter to go round a corner. One, let the cutter go one radius beyond the corner as it decelerates. Then accelerate in the perpendicular direction. That way you get a sharp corner unaffected by acceleration/deceleration effects. The alternative is to pivot the cutter around the corner point, which means the centre of the cutter follows an arc. The latter is most common as it saves time and leaves an acceptable corner.


Thread: Digital Height Gauge Recommendations?
16/01/2021 11:28:47
Posted by SillyOldDuffer on 16/01/2021 11:06:42:

..............because electronics are more reliable than their mechanical equivalents.

So why are there so many threads on here regarding motors and controllers that have stopped working?


16/01/2021 09:34:51

Another vote for mechanical. I use a 12" Etalon bought secondhand on Ebay.


Thread: Spur gear help.
15/01/2021 17:17:01

Accurately measure the OD of the 34 tooth gear and tell us the make and model of milling machine.


Thread: Machining Brass
14/01/2021 15:28:36

We all make mistakes. I'm building two traction engines, but I've nearly made enough parts for three.

I'd agree that a mill/flycutter is the wrong setup. The most common brass (CZ121) is easy-peasy to machine. So if it isn't behaving there are several possibilities. The brass isn't free cutting, or may not even be brass, for instance bronze can be horrible to machine. The cutting tools are blunt/broken or have an incorrect geometry. Something is loose. The measurement system is telling porkie pies or doesn't agree with the dials on the machine.


Thread: What type of cutting tool is this?
14/01/2021 13:14:45

Not entirely convinced, but back in the 60s and 70s there were multislot milling cutters that used serrated HSS inserts similar to that shown. The inserts were held by friction, and designed so that the cutting forces pushed the insert into the pocket. They were presumably introduced to avoid the expense of a large lump of HSS, only a small part of which was used for cutting. Easier to sharpen as well. They were soon superceded by carbide insert cutters.

Holders are still advertised on Ebay at silly prices as they're totally obsolete and the inserts are long since unobtanium.


Thread: capacitor droppers and power factor
12/01/2021 22:33:16

If the circuit includes a capacitor dropper it'll be the brown device that is doing it. The electrolytic will not like AC up it (see note below). I'd hazard a guess that the brown component is dropping some AC voltage, the bridge rectifier is operating at a low AC voltage and the electrolytic is smoothing the bridge output. Although I wouldn't have thought that the electrolytic was really needed. Any flicker will be at 100Hz and probably not be visible.


Note: When I was in the apprentice hostel at RAE Farnborough one of the other students had the bright idea of seeing what would happen if you put a large metal can electrolytic across the mains. The answer is that it goes bang in a big way and spews its guts out. As a precaution the perpetrator put his motorcycle helmet over the capacitor. The paper and electrolyte that came out made rather a mess of the inside of his helmet.

12/01/2021 11:00:33

The brown rectangular component is a film capacitor. I'd agree that the 4 legged IC is a low current bridge rectifier.


Thread: Total cost + import for Tormach PCNC 440, and alternatives?
11/01/2021 20:23:11
Posted by Steve Dunthorne 1 on 10/01/2021 11:55:58:
Please can I have your honest opinion on the merits of both and whether this is a factor I should bear in mind.

I think that most commercial CNC machines use linear ways. There are several reasons. One, they're cheaper to install. On any moving parts there's stiction and friction. Stiction is how much force is needed to get the parts moving as opposed to keeping them moving. Linear ways have very low, if any, stiction. So they're good for professional machines with high feedrates and even higher rapids. Not sure there's much advantage at the lower speeds on hobby machines.

I don't know anything about Syil machines, although they don't seem to get good reviews on CNCZone. I've always found Tormach tech support to be helpful and reasonably responsive, although I haven't used them recently.


10/01/2021 11:07:38

When I bought my Tormach I didn't get any of the tool setting equipment. I started by touching off each tool as I used it and on subsequent parts altering the Z value at each tool change (I don't think you can do that in PathPilot at a toolchange). Until I forgot to do so, and the machine drove an M3 tap 20mm deep before starting the spindle. At that point I started measuring each tool as before but then entered the values into the Mach3 tool table and adjusted the post-processing file to use the tool table. I use mostly carbide cutters and got fed up of chipping them when touching off. I also found that I was avoiding the use of multiple tools in one program as it was a pain sorting out the tool table.

So I splashed out and bought the electronic tool height setter. I also made a master tool (number 0) which is longer than most tools I am likely to use. A master tool is bit old school as the modern method is to use the spindle nose. But I feel that is a bit inconvenient on the Tormach, especially with small work pieces. I also bought a Haimer Zero Master and Centro for setting X/Y work offsets. Of course they also get used on the Bridgeport and I have used the Centro on the horizontal mill.

The sequence I use now is one, zero the master tool in Z on the table. Plug in the tool setter and measure each tool in turn. With Mach3 you had to remember to save the tool table before powering off. But PathPilot saves automatically. After measuring tool heights, back to the master tool for the work Z offset and then one or other of the Haimers for X/Y work offsets.


09/01/2021 22:50:33

I'll try and add to some of the points previously made. Plus one for the TTS system. I bought a second special collet and use the system for about 80% of the milling and drilling on my Bridgeport.

At the time I bought my Tormach (2009) there wasn't an ATC. I have looked at it since. But it's very expensive and my gut feel was that it isn't robust or reliable. When I was looking at buying the Tormach I also considered Haas. The ATC option was almost as much as the base machine. It's not sufficiently painful to change tools manually to make me look at buying the ATC.

I didn't initially buy a power drawbar but have done so since. Although I'm embarrassed to say that I haven't fitted it yet.

I designed a tooling plate and bought a large (expensive) piece of tooling plate. But I never got around to making the plate and thus far I don't miss it. I'm not sure I'll ever make it.

I didn't buy any books specifically on CNC mills - I just worked it out as I went. The only book I bought was "CNC Programming Handbook" by Peter Smid. It is a professional level book aimed at CNC lathes and mills. I use it as my go to guide for G-code.

My CAM program will only do 4th axis round and round or up and down the axis. For the worm ideally one needs a helical toolpath. Turns out a helical toolpath is quite simple, one line of code:

G01 X-11.450 A2160.000 F0.400

This goes from the current position to the specified value of X and a specified number of degrees in A. So the code is simply a series of the above commands with appropriate starting values of X and Z. I used three tools, 6mm and 4mm endmills for roughing and a tapered end mill to finish the flanks. The feedrate looks odd. That is because I used inverse time feedrates (G93). I had also sorts of trouble getting the original Tormach version of Mach3 to use conventional feedrates for helical paths. In short they never worked. A look at this post will illustrate the whole sorry saga:


The tailstock is pretty much set in Y by tenons on the underside of the body. To set in Z I used the arbor on which the part sat and tweaked the adjustment until the tip sat nicely in the corresponding countersink. A quick check along the arbor with a DTI will confirm that the arbor is parallel to the table.


Thread: help identifying gear module or dp
09/01/2021 22:03:07

I've been using Alibre for 12+ years. I looked at Fusion360 (due to the built-in CAM) a few years ago. I'm glad I didn't swap over. Free access is only good while the current CEO is in place. While I use 3D CAD for work it's not my main source of income and I wouldn't want to pay for full commercial access.


09/01/2021 09:57:57
Posted by Richard Cox on 08/01/2021 16:52:45:

I admire what some of you lads can do with cad I can only dream, I keep toying with the idea of getting into it but not really got the need bit of a catch 22 really to get good at something you need to always do it....

I'd vote for 3D CAD, it's a darn sight easier than some 2D CAD packages which are are just glorified electronic drawing boards. Practice makes perfect:

Bevel Gear Pinion 3D Model

One big advantage of 3D CAD is the ability to create assemblies and sectional views so that form and fit can be checked before cutting metal:

Drive Train Assembly

It is also possible to move assemblies to check clearances at the extremes. On this model of a water pump the minimum clearance between the drive rod and the inside of the ram is 15 thou:

water pump assembly 2-11-2014.jpg

And so it proved to be in reality once I'd made the parts. Another plus for 3D CAD is that 2D drawings are simple to create.


Thread: Total cost + import for Tormach PCNC 440, and alternatives?
09/01/2021 09:40:48
Posted by Steve Dunthorne 1 on 08/01/2021 15:54:24:

I am planning on teaching myself 4th axis CNC coming from an engineering background but no CAM experience.

When I bought my Tormach I also ordered the 4th axis kit. The reasoning was that I wanted the kit and it was cheaper to ship everything at once rather than have to deal with specialist shippers again at a later date.

The key to 4/5 axis is the CAM software. It starts to get pretty expensive for true 4 and 5 axis control. It was some years before I used my 4th axis. The first item I made was a 2tpi worm:

worm setup.jpg

Finished worms:

final worms.jpg

Despite having a CAM package with some 4th axis capability the code for the worms was hand written. Subsequently I've used 4th axis indexing to machine bevel gear pinions:

After Final Cut

Finished gears:

Cast Iron Gears

I'd highly recommend getting the 4th axis capability, even if you don't use it initially.


09/01/2021 09:29:17
Posted by Steve Dunthorne 1 on 08/01/2021 15:03:27:

Andrew, you have a 3 phase supply, so I can see what you are doing, but for the normal domestic single phase garage setup, I still feel there is a problem. Your setup is slightly more industrial if I may say so.

When I moved in the electricity supply was 60A and had a wind yer own fuses distribution box. So it needed updating and it seemed sensible to install a 3-phase supply rather than just an updated single phase. I'm glad that I did as all my other machine tools are ex-industrial, and some would be difficult to run from a VFD while maintaining functionality.


Thread: Worm wanted for lathe
08/01/2021 21:50:08
Posted by AJAX on 08/01/2021 21:33:47:

* Harrison M300

That's the lathe that will cut an 18DP worm, provided you have the optional 56 tooth gear wheel for the drive train. I bought my gear on Ebay some years ago.


08/01/2021 20:31:03

Can we assume that the worm is actually 18DP? I make that equivalent to 5.73tpi, so presumably the 5.75tpi is an approximation? According to the manual, with appropriate change gears, my lathe will cut an 18DP thread.


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