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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.

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Thread: Notes on CNC Milling of 316 Stainless Steel
14/02/2011 22:02:25
I've recently had the need to make some parts on my CNC mill from 316 stainless steel. This has been rather a steep learning curve, so I thought I'd write a few notes on what I have learnt; the hard way of course.
 
The parts concerned are from 1.5mm 316 stainless steel sheet, and have complex outlines and internal cutouts. I had three off, of two parts, to make, and didn't fancy doing it on a manual mill. The S/S used is 316 because, after machining and bending, the parts need welding.
 
I have turned a fair amount of 303 and 316 stainless steel with few problems and have always got an excellent finish using carbide insert tooling. I have done very little milling of stainless steel and no CNC milling of it, until now.
 
I used two cutters; a 6mm three flute centre cutting uncoated endmill, and a 2mm four flute centre cutting TiALN coated endmill, both from Garr Tool.
 
Looking up Machinery's Handbook gave a surface speed of 200fpm for carbide in 316. Being cautious, so I thought, I downgraded that to 120fpm, corresponding to 2000rpm for the 6mm cutter. Choosing a chipload per tooth of 0.05mm gave a feedrate of 300mm/min. After a couple of broken cutters and another one worn out, I did what I should have done in the first place; put the kettle on and then look up the Garr Tool machining data. For 316 they give 20-35m/min and a feedrate of 0.01-0.025mm/tooth for a 6mm cutter. I chose 22.6m/min (1200rpm) and a chipload 0.025mm/tooth for a feedrate of 90mm/min. What a difference! The cutter ploughed through the cuts slowly but smoothly; yippee!
 
Figures for the 2mm cutter were 3500rpm and 70mm/min feedrate. That seems a very high rpm, but the diameter is three times smaller, so for the same surface speed the rpm needs to be three times higher. The feedrate equates to a chipload 0.005mm/tooth, at the lower end of the manufacturers recommendations.
 
I have added a few pictures in an album showing the parts and few snapshots of the machining in progress.
 
A few other random snippets. I used flood coolant at all times, mainly to flush away the swarf. The depth of cut with the 6mm cutter was 1.7mm, to ensure complete break through. With the 2mm cutter the same total depth was achieved in four passes, each of 0.45mm. Typically, despite breaking several 6mm cutters, I didn't break a single 2mm one. The aluminium subplate is sacrificial. Better to cut into it than the table! Once it gets too badly mangled I'll either recycle it or give it a facing cut. It's scrap anyway; it was cut from a larger sheet that, due to a brain fart on my part, was ordered to the wrong size for a previous project. In retrospect I am amazed that the 6mm cutter actually worked with the original speeds and feeds; it just wasn't sustainable for even one part. The local professional tool shop sold me some 6mm cutters with a small radius (0.3mm) on the tips as specially for stainless steel. I haven't tried them, but can see that it makes sense, as the cutters wear initially on the sharp tips; if you don't break them first of course!
 
One other thing, I also mangled a HSS drill on the manual mill too, by hesitating, and then getting it red hot trying to finish the hole. It was a vivid way to relearn the lesson of never, ever let the tool dwell when machining stainless steel. It'll work harden before you know it. Then only carbide will touch it, if you're lucky.
 
I hope this is of use to somebody, if only to prevent wallet damage. Mind is still sulking.
 
Regards,
 
Andrew
 
PS: Sorry about the mixed units, that's just the way it goes, and I'm comfortable speaking both imperial and metric so it doesn't bother me.

Edited By Andrew Johnston on 14/02/2011 22:04:37

Thread: Bridgeport jammed knee
13/02/2011 21:19:30
Hi Chris,
 
You may find the following link informative:
 
 
Regards,
 
Andrew
Thread: MT2 collet slipping
09/02/2011 11:06:55
Slightly OT, but the set screw in an R8 taper spindle prevents the collet turning when tightening the drawbar; it is not intended to be used as a drive key. In most Bridgeport mills I have seen, including mine, it has been removed, or sheared off. I have never had a collet or chuck slip in the R8 taper despite no set screw.
 
Regards,
 
Andrew
Thread: Smart and Brown Sabel
08/02/2011 22:40:13
If you add 2 to the number of teeth before dividing by the OD of a gear, then you get the DP exactly; no need for approximations.
 
Regards,
 
Andrew
Thread: Square peg in a square hole
04/02/2011 08:46:54
If the part can be mounted in a lathe (which I guess a clock key could be) try a rotary broach:
 
 
Caveat: Never used them myself!
 
Regards,
 
Andrew
Thread: Backlash - Steppers, Toothed Belts & Ballscrews
27/01/2011 21:11:21
Bear in mind that most commercial CNC machines use servo motors in a closed loop system, ie, the position of the ways are measured independently. Hence any play in the drive belt system is of less importance. Most hobby style CNC machines use stepper motors in an open loop system, ie, the ways are assumed to be in a certain position from knowing the number of steps made and the mechanical gearing. In this case play anywhere in the system increases the positional uncertainty.
 
A good start would be decide what you wish to make, and hence what positional accuracy you can live with.
 
Regards,
 
Andrew
Thread: steam turbine and generator
19/01/2011 15:53:08
Les,
Is it me, or is your link self-referencing? That could lead to infinite recursion.
Regards,
Andrew
Thread: Bearing running clearance - rear axle
17/01/2011 09:35:06
Hi John,
 
I'll be interested to read the replies you get. I was advised not to bother with separate bronze bearings in my rear axle shaft. Given the price of bronze I eagerly accepted the advice! I'll be running steel (EN8) directly in cast iron. Given the relatively slow speeds (see my recent thread asking about this) I don't suppose the axle/bearings will get particularly hot, or even warm.
 
Regards,
 
Andrew
 
PS: Must drop over and have a nose round your workshop sometime!
Thread: Scale Traction Engine Speeds
16/01/2011 20:25:51
Hi Jason,
 
Thanks, and yes I am aware of Conrad's site; some very interesting techniques therein. From a business perspective it's not really my problem to sort out the drawings. The supplier has chosen to depart from the LSM drawings; I don't have a problem with that. I assume that it leads to a cheaper cylinder casting, as it requires fewer cores. However, having gone that route he has an obligation to tell me how to machine the supplied castings and liners. The next time I call in, on my way back from up north to retrieve my glider, I'm planning on picking up the remaining castings, the boilers, and I'm going to sit down with the LSM drawings and mark them up so I understand exactly what to do. I'm not going to mess up an expensive casting because I didn't have the proper information.
 
Given the recommended clearances for aluminium alloy pistons and cast iron liners it'll be interesting to see how I get on running my model hit 'n' miss engine, as that has an alloy piston and cast iron liner. The clearance as I've machined it is much less than 3 thou per inch. Presumably it will seize in fairly short order?
 
Regards,
 
Andrew

Edited By Andrew Johnston on 16/01/2011 20:27:08

Thread: Tormach
13/01/2011 17:51:43
I might admit to the being the owner of a Tormach CNC mill, and in the UK too.
 
Jim: Send me a PM and I'll reply with contact details.
 
Oh, and I promise not to say it's crap; 'cause it isn't!
 
Regards,
 
Andrew
Thread: Scale Traction Engine Speeds
13/01/2011 11:29:01
Hi Diane/Jason,
 
Great, thanks for the information. In due course I'll give Clupet a call. If he's going to retire I might order some spare rings!
 
I have read that Clupet rings can be prone to breaking if they cross the steam ports. I'm building SCC (yes, I know I said DCC in the original post, that was a boo-boo 'cause it was late) Burrells to the LSM drawings, but using castings from John Rex. His cylinder/liners are significantly different to the drawings, since the cylinder castings have no steam ports cast in, but use thick liners with the steam passages milled on the outside of same. I'm having a bit of difficulty getting details of the passages out of John, so I'll have to check clearances when I do get details.
 
I'm also confused about whether to use cast iron or aluminium alloy pistons. Obviously the alloy pistons will be lighter, but the recommended clearances seem pretty large. A rattling good fit actually. Decisions, decisions.
 
Regards,
 
Andrew
Thread: Thread Depth vs Strength
12/01/2011 23:14:33
It's all laid out in Machinery's Handbook, including various formulae to calculate exactly what you require.
 
The factors at work are essentially the tensile strength of the bolt versus the shear strength of the threads. So the materials used and their properties are as important as the thread engagement.
 
I have a vague recollection that once the first 2 or 3 threads are fully engaged further engagement becomes less important, as the loads are not carried uniformly over multiple threads. In plain English, the bolt stretches!
 
Regards,
 
Andrew
 
Blast: too slow on the keyboard!

Edited By Andrew Johnston on 12/01/2011 23:15:26

Thread: Scale Traction Engine Speeds
12/01/2011 21:43:15
Hi Jason,
 
Thanks for the info, when I get time I'll spend a while searching through the TE forum. I had thought about Clupet rings, but assumed they'd be all but impossible to get in small quantities. I am building two identical engines, but quantities required are still small! Any ideas as to where to purchase them, other than 'phoning the manufacturer?
 
Regards,
 
Andrew
Thread: Hints and tips
12/01/2011 12:18:48
ChrisH: Err, I don't think so; the cable can't 'magic' current out of nothing. The current in the wire is determined by the source and the load. Anyway, the cable coiled on the holder is not a simple inductor. Think about it; assuming that we're talking about mains cable, the current flows out along one wire, and back along an adjacent wire. What is the effect of that? It minimises the inductance, as the magnetic fields around each wire tend to cancel.
 
In practice the cable coiled up will probably have a slightly higher inductance than a straight piece of cable, as the cancellation is not perfect and there will be stray magnetic coupling between turns.
 
The current rating of mains cable is generally set by a specified temperature rise in free air. If the cable has restricted cooling, buried in a wall or wound up in a tight coil, it will get hotter for a given current. The advice to unwind an extension cable before use at higher currents is sound, but it based on simple I^2R heating and temperature rise. Nothing to do with 'magic' inductance.
 
Regards,
 
Andrew
Thread: Small CNC milling machines - any hints/tips
11/01/2011 20:54:48
David,
 
I sent you a PM.
 
Regards,
 
Andrew
Thread: Scale Traction Engine Speeds
11/01/2011 20:49:39
Ah, my 267th post! Which will mean nothing to anybody but me, as 267 is the competition number on one of my gliders!
 
John: Actually no, I'm interested in what sort of rpm the rear wheel might do, not the speed of the engine in mph per se. Although of course the two are connected, and you're spot on, the rear wheels are 2 feet in diameter.
 
Jason: Thanks for the information, and for reminding me about the TE forum. I am a member there, but don't post much. I don't know enough to make intelligent posts! The consensus seems to be about 500rpm for the engine and about 50rpm for the rear wheels, give or take.
 
The reason I ask is because I've boo-boo'd on machining the final gear castings, which are 14.8" in diameter. The periphery and bore are concentric to within a thou or two, but the side of the gear has a slight wobble, about 20thou. This won't affect the mesh of the gears, but I wondered if it will be noticable as the wheels rotate, and should I try and do something about it.
 
Also, in true parliamentary fashion I feel entitled to ask a supplementary question. I am intending to use cast iron pistons, and was going to make cast iron rings, but John Rex recommends bronze rings. What do you think?
 
Regards,
 
Andrew

Edited By Andrew Johnston on 11/01/2011 20:51:07

Thread: New print and digital subscriptions
11/01/2011 11:29:25
Phew, that's a relief! Thanks David.
 
Regards,
 
Andrew
11/01/2011 10:53:18
Wot? No print only subscription? Would I have to pay more for the combined print & digital subscription, even though I don't want the digital part?
 
Regards,
 
Andrew
Thread: Scale Traction Engine Speeds
10/01/2011 22:58:23
I'm trying to get a feel for the maximum rate at which the rear wheels of my 4" scale DCC Burrell traction engine will be rotating.
 
I assume that the crankshaft will be running at a maximum of about 160rpm? The lowest ratio to the second shaft is 2.739, and the final drive ratio is 3.6. This gives a total reduction of 9.86, and hence a rear wheel rotation rate of a little over 16 rpm. Does this seem reasonable, or have I made a false assumption somewhere?
 
Regards,
 
Andrew
Thread: Miniature welding
09/01/2011 12:38:55
I would have thought it was perfectly possible to achieve what the OP is asking; given enough skill and the right equipment. See this website:
 
 
I certainly don't have the skill, but that doesn't mean others don't either.
 
Regards,
 
Andrew
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