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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: In need of a steel ring 132mm dia
07/07/2019 12:34:53
Posted by not done it yet on 07/07/2019 12:16:28:

As in a 5mm washer? I think a piece of 6mm plate, as a starting point, would suffice for that.

That's the way I read it, and I'd also start with 6mm plate, but I already have some in stock, left over from the traction engine hornplates. A sketch, or confirmation that I haven't gone off at a tangent, from the OP would be good.


Edited By Andrew Johnston on 07/07/2019 12:35:18

07/07/2019 11:17:56
Posted by paul rushmer on 07/07/2019 10:43:31:

Terry This is a classic case of where are you, my slip rolls will handle 6 by 20 strip.

The way I read the OP is that the strip would need rolling edge on, so you end up with a ring 132mm OD, ~5mm thick and with a 102mm hole.


Thread: What Did You Do Today 2019
06/07/2019 17:24:20

Cut the lawns front and back this morning. Finished just gone 11am and by 1pm it was raining. Fortunately the light rain has now stopped so the pizzas and drinkies do at the neighbours this evening is on, and outside.

This afternoon I tweaked the CAD model, and created a CAM file, to cut the 1/8" by 3/8" slots in the anchor bolts for the traction engines. Having cut one I updated the CAD and CAM as I didn't think the slot was in quite the right place. The machining used two stages. First stage used a carbide 3mm drill to create 3 holes to remove most of the material, time about 6 seconds:


And then a carbide 3-flute slot drill to pocket out and profile, time about 2 minutes:


I've machined four slots this afternoon on spare bolts. Now I'm happy I can machine the 18 proper bolts and then tweak the program for the 36 frost spikes; same size slot but in a different position.

The slot is reasonably accurate, a 0.125" slip gauge will go in top and bottom but a 0.126" slip gauge will just fit in the top but not the bottom.

Once all the bolts and spikes are done I need to slightly thin my 1/8" keyway broach and make some shims to create square ends to the slots.


Thread: Aircraft General Discussion
06/07/2019 17:07:09

Dunno, could have been a Sea Fury. The aeroplane was definitely silver and my impression was it was too dumpy to be a Sea Fury. Didn't sound quite right either. The jury is out, but on balance I might stick with a Thunderbolt? I can't say for sure if there were invasion strips.


Thread: What do you use your lathe for?
06/07/2019 10:13:56
Posted by Richard brown 1 on 06/07/2019 09:28:42:

I had seen something like that in ME but I'd like to see you make a leadscrew on a mill dont know

Not a problem. This is a picture of milling a high helix angle helical gear. It wouldn't be difficult to extend it to a leadscrew:

high helix gear cutting.jpg

Thread milling is described in many old school books on milling.


05/07/2019 19:06:34
Posted by Richard brown 1 on 05/07/2019 18:36:35:

A mill cant make a mill because how do you make the spindles and circular bits with a mill?

Yes it can, use your imagination! smile

A horizontal mill can be used to turn short, large diameter parts that would otherwise be too large. See here:



Thread: Microns ...
05/07/2019 16:56:07

Oh dear, we seem to have a CAD catastrophe. If the line is half a thou long then the circle at the end cannot be 1000 microns, as that is one millimetre, which is definitely bigger than half a thou. The scaling factor is 25.4, so more, but not much, much more?


Thread: What do you use your lathe for?
05/07/2019 16:47:25
Posted by Howard Lewis on 05/07/2019 16:38:22:

With a lathe you can turn or mill. The only limits on what you can do are set by the machine and your ingenuity.

Yebbut, on a mill you can mill and turn, so a mill could be used to reproduce itself too.


Thread: Cast iron - 160mm dia
05/07/2019 16:43:14
Posted by Hopper on 05/07/2019 12:12:21:

That's where theory deviates from practice. Maybe it's because Young's Modulus measures deformation under tension whereas cast iron's greatest strength is compressive?

While Young's modulus is normally measured in tension the concept is equally applicable to compression. I'd agree that cast iron is better in compression than tension. But that doesn't imply that cast iron is necessarily better than other materials in compression. The fact that cast iron breaks without bending much simply means that it is brittle, not that it is necessarily stiffer than other materials.

I'd also agree that cast iron is good at damping vibration. But that is to do with the internal structure of the material, and in particular the flakes of free carbon. These absorb energy as the material lattice vibrates and thus reduce the amplitude of the vibration. Values in the link below imply that larger flakes are significantly better than small flakes at energy absorbtion:



Thread: What do you use your lathe for?
05/07/2019 15:38:13
Posted by Howard Lewis on 05/07/2019 15:14:52:

The Centre Lathe is the king of machine tools, being the only one that can reproduce itself.

This keeps getting quoted, but I'm at a loss as to how one would create a lathe using only a lathe? Anyone care to enlighten the masses?


Thread: Cast iron - 160mm dia
05/07/2019 11:16:44
Posted by Hopper on 05/07/2019 07:47:10:

Cast iron is more rigid than mild steel.

That's odd, because cast iron seems to have a rather lower Young's modulus than low carbon steel.


Thread: Cutting a Mod 1 worm
04/07/2019 22:42:09

If the wanted pitch is 3.14159mm and one can achieve 3.165mm, I make that an error of 0.75%? Furthermore the pitch error is 0.0234mm, or less than a thou. I doubt that will be a problem. Either that, or I shouldn't do maths after two glasses of wine. smile

My lathe will cut 1 Mod threads with an error which think is 0.06%. PM me if you feel you need to use it.


Thread: Further Adventures with the Sieg KX3 & KX1
04/07/2019 21:04:14

Looks good!

As always I might have done a few things differently, but who's to say I'm right or wrong. smile

I'm either going to have to upgrade my CAM program or start using Fusion360 so I can get going with adaptive toolpaths.


Thread: Aircraft General Discussion
04/07/2019 17:23:34

Seen lots of Spitfires and Dakotas recently but sitting in the garden this afternoon I heard something different. Heavy, fast and a throaty radial engine. The aircraft flew over at about 1000ft, and he was shifting; hope he was below 250kts. smile Definitely a WWII fighter, with a distinctive curve to the trailing edge of the wing. It came from the direction of Duxford, did a circuit of Bourn airfield and headed back to Duxford. After a bit of digging on the net I'm pretty sure it was a P47 Thunderbolt.


Thread: How to use a round column mill
04/07/2019 15:06:42
Posted by AdrianR on 04/07/2019 12:06:01:

I am also learning how diffrent cutters can be.

That's an important lesson to learn, and one that is not always understood. smile

I don't buy many new small HSS cutters (I use carbide as standard), but when I do I use Drill Service in Horley.


Edited By Andrew Johnston on 04/07/2019 15:15:37

Thread: Treppaning a flywheel
03/07/2019 20:01:40

I'd agree that it would have been better to trepan first and drill holes later. For the trepanning I'd use a multi tool approach.

First I'd grind a short "parting" tool from square HSS with the necessary clearance on the outside, say a 6mm deep blade and 3mm wide. Then I'd plunge in 5mm near the outer diameter, move across 3mm and plange again and so on until the inner diameter is reached. Then repeat twice to get near the 15mm depth. Of course you won't be able to get right into the inner diameter on the second and third cuts.

Finally I'd clean up the inner and outer diameters and the bottom of the groove using L& R knife tools, or L&R boring bars in my case as I don't have matching knife tools.

For cast iron I'd add a little top rake to the tool, a few degrees.


Thread: What do you use your lathe for?
03/07/2019 14:48:01
Posted by Blue Heeler on 03/07/2019 08:08:07:

What's the main thing you use your lathe for?

Varies according to which one. smile


Thread: Sieg KX1 next step
03/07/2019 12:00:35

When I first got my CNC mill I had a foray into speed and feed calculators (won't say which one to protect the guilty) but it gave silly numbers, even for the professional user.

However, the arithmetic isn't complicated. There are two basic parameters, surface speed (which gives spindle speed) and feedrate. There are charts available for surface speeds depending upon material and cutter type. I don't stick to surface speeds exactly, I may be a little lower for materials like cast iron. If the nominal speed is above my maximum spindle speed I simply use the maximum. Feedrate depends upon spindle speed and chip load per tooth. Cutter manufacturers give recommended chip loads for various diameters of cutter and the type of cut in terms of depth and width. I use these as a guide, slightly biased to the lower end. Feedrates are probably more important the spindle speed. If the chip load is too low then the cutter will rub rather than cut especially when conventional milling. The old saying that published charts are for production doesn't apply to chip load. If it's too low the cutter will rub and be damaged irrespective of amateur or professional..

Width of cut and depth of cut are set by experience and the feedrate may be adjusted accordingly. A full depth but shallow width of cut can be run at a higher feedrate than full width at a shallow depth of cut.

Ultimately Jim is right, the numbers give you ballpark figures which you can then adjust by experience. Once you've got that experience one can refer back to it and not bother with the calculations.


Thread: How to use a round column mill
02/07/2019 19:00:22
Posted by AdrianR on 02/07/2019 18:38:31:

So either the mill does not like 2 flute cutters, or the cutters I have bought from RDG are a problem.

I'd place a small bet on the latter.


Edited By Andrew Johnston on 02/07/2019 19:00:35

01/07/2019 12:26:56

1. No it isn't normal, but on any mill play in the quill, bearings or work clamping, or a poorly ground cutter, will lead to vibration and the slot drill possibly cutting oversize during plunging, even with a centre hole

2. Feedrate seems way too slow although spindle speed isn't quoted. I normally plunge at about half the normal feedrate in deference to only one edge cutting all the way to the centre. Assuming a HSS cutter and steel workpiece I'd be running at 1000rpm and a plunge rate of 80mm/min

3. Personally I'd use Z, but I have a mill with dovetails on Z so that makes life easier

4. Not for me to say

It really comes down to machine mass and flexibility. On my vertical mill an 8mm cutter would be fine, but I have seen the same issues when using 20mm, or larger, slot drills.


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