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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: Leveling machines
04/08/2021 18:40:14

Confuse would have been a better word.


Thread: Tapping Drill sizes
03/08/2021 18:47:35
Posted by Peter Greene on 03/08/2021 18:35:41:

My "bible" for all this is Tubal Cain's "Drills Taps and Dies" (Workshop Practice Series No. 12)

Mine too.


03/08/2021 16:17:35

Depending on material I'd use 3.9 or 4mm, with 75% or 60% engagement respectively.


Thread: self catering v independent
03/08/2021 14:39:10
Posted by not done it yet on 03/08/2021 14:02:42:

How fast were you turning and how thick were those ‘curly band saw blades’..........

Probably running at 1000rpm. I don't have any swarf left (that I can find) but thickess is likely around 8-10 thou. Without measuring I don't know the exact feedrate, as the lathe in question only has an uncalibrated hydraulic feed - you adjust it to give the required finish.


Thread: Centec 2B - New arrival and Q&A
03/08/2021 14:31:05
Posted by not done it yet on 03/08/2021 14:20:18:

.............Andrew had 3 (or maybe 5)HP!

My horizontal is 5hp in the high speed range, 4hp in low speed. That's why I've never made it cough whereas I've stalled the other mills (and lathe) by being over-ambitious.


Thread: self catering v independent
03/08/2021 13:30:47

It would be better to take one cut rather than lots of small cuts. The 2" bolts at the bottom of this photo were turned from 1/2" square steel down to 1/4" diameter in one pass with no tailstock support:

square head bolts.jpg

And the resultant swarf:

square swarf.jpg

To be fair the work was held in a 1/2" square collet which doesn't work loose, but still no support needed.


Thread: Centec 2B - New arrival and Q&A
03/08/2021 09:15:37
Posted by William Ayerst on 03/08/2021 08:52:45:

I'd not considered a slab cutter but that does make alot of sense. My widest horizontal cutter is 5/8" - other than a 1" bore is there anything I should be considering with regard to teeth/etc. on a cutter like this?

Do everything up tight, and then tighten some more, and ensure the cutter is well supported. Make sure the work is firmly held; I've had a slab mill lift work out of a Kurt vice. Spindle speed the same as any other cutter of the same diameter. Feeds based on number of teeth and number of thou per tooth. I usually start at 4 thou per tooth. From a practical viewpoint I suspect the lack of arbor keyway and lack of power will be the limiting factors. A slab mill can remove serious amounts of metal; this is pussyfooting as the mill didn't even notice as the cut started:



Thread: Allchin 1 1/2” scale Traction Engine Shaft Manufacture
01/08/2021 17:13:18

When it came to machining the 6-sided splines on my Burrell SCC crankshafts I considered the slot 'n' key method, or using an offset endmill to form the sides of the spines and then nibbling out the space in between the splines. Neither seemed satisfactory, or prototypical. It is possible to buy commercial spline cutters, but none were available in the size needed. So I designed and made my own, from gauge plate which was hardened and tempered:

spline cutter.jpg

The splines were cut on a horizontal mill plus dividing head, the same setup as for gear cutting. Finished splines:


The internal splines were cut using a HSS toolbit and a slotting head with rotary table:


A length of HSS was ground on a cylindrical grinder to get the correct end radius (by measuring the diameter) and then cut in half and clearances added by hand:


The internal HSS toolbit is a couple of thou under width so final fitting of the gears to the splines was done with files. I made a test piece on scrap bar, and a messed up gear blank, to check out the process beforehand:


These are the finished splines, and outer gear:


Note that there is clearance on the inner and outer diameters, so the splines drive purely on the sides. The gears fit in all six orientations, although the shake varies from nothing to a little in the different orientations.

I made a few mistakes along the way. I didn't support the middle of the crankshaft when cutting the splines. Consequently the splines vary in width by a thou or two along their length. Each gear only works on half the spline length, so they are hand fitted to that length. The inner gear goes all the way along the splines, while the outer gear will only go half way down the spline. Originally i made a set of gears with a lot of inner clearance and with a straight ended slotting tool. But they didn't look right, so I binned them and made another set. In addition I hadn't got to grips with machining EN8 at the time, so the finish on the first set of gears was poor - another reason to recycle them.


Thread: Metric thread dimension wanted.
31/07/2021 15:17:08
Posted by JasonB on 31/07/2021 14:28:25:

..........what would be the reason that Machinery's has it as 0.541 and Zeuz etc have it at 0.613mm?

No idea. The ISO standard doesn't distinguish between internal and external thread depths. Clearance is achieved by a tolerance class which specifies the amount above, or below, a reference line for the internal and external threads. The thread depth is the same for internal and external, simply moved up, or down, a bit.

I've got a set of Zeus tables but haven't used them in over 40 years because they don't explain where their numbers come from, or why, so I don't trust them.


31/07/2021 14:15:54

All the information needed is in Machinery's Handbook. The theoretical depth of the full triangle (H) for a given pitch (P) is:

H=0.866P aka H=sin(60)P

By definition the thread depth of an ISO metric thread is:

0.625H or 0.541P

So for a 1mm pitch thread the thread depth will be 0.541mm or 21.3 thou. Simples!


30/07/2021 19:56:59
Posted by mark costello 1 on 30/07/2021 19:35:21:

....There is no data available in Imperial measurements.....

Err, there won't be, it's a metric thread. It's seem so obvious that this must be a trick question - I'd look up the thread depth in metric and divide by 25.4 to get imperial?


Thread: New Chuck won’t screw on
30/07/2021 10:56:30
Posted by JasonB on 30/07/2021 07:14:52:

...........but they almost all tend to use the common pitches

Good thing the word 'almost' was used. smile

I've just made some parts for a guitar for a friend of a friend. One part had an internal thread - M3 no problem, except an M3 tap didn't fit. Try 4-40UNC, also didn't fit. A clue is that the guitar was made in Japan. Turns out the thread was M3x0.6; an old French standard, but is also a current Japanese JIS standard. Fortunately I was able to buy a NOS tap on Ebay.

Last year I did a small PCB layout including a D-type connector. The mating equipment (also made in Japan) used M2.6 for the D-type screwlocks rather than the near universal 4-40UNC, or less common M3. So I had to make a set of M2.6 to 4-40UNC adaptors.


Thread: Help with surface plates in Derby
27/07/2021 16:06:21

My 36"x24" Windley Grade B (workshop) surface plate doesn't look like it's been ground or scraped:


There are a series of horizontal lines about 5/16" apart, almost as if it's been planed. Unless you're rebuilding machine tools part by part I'm not sure how a surface plate helps with machine tool setup.


Thread: Wohlhaupter UPA4 Boring and Facing Head Operation
25/07/2021 21:45:15

Thought I'd finally bring this thread to a conclusion. I recently got around to making the small 'tophat' detent for the coarse feed. I made some careful measurements, with a depth micrometer, of the depth of the housing to the shaft and the depth of the housing to the ball fully home. That enabled me to design the 'tophat' and then make one in silver steel. I bought the compression spring rather than make one. Once the parts were fitted the coarse/fine feed works exactly as advertised above.

This weekend I've been boring the HP liners for my traction engine cylinders:


Now that the boring head works properly it's a joy to use. Easy to rough out using the coarse feed and then swap to the fine feed for finishing. The feeds are spot on; for one liner I put on 4½ thou (on diameter) according to the fine feed knob divisions, and as far as I can tell with an internal micrometer the diameter increased by thou.

The bores measure round and are parallel, measured at each end, to within 2-3 tenths, and the bores on the two liners are the same to within 2-3 tenths. The bores are 4-5 tenths undersize to allow for honing.

One happy bunny here! Now celebrating with a glass of wine. It's just so nice to be able to use quality kit that does what it says on the tin.


NB: Thanks to SoD for the link to the program for generating fractions that he posted recently in another thread.

Thread: Centre Drill Leaves a “Pip” - Sometimes
25/07/2021 14:45:58
Posted by Tony Pratt 1 on 25/07/2021 12:28:18:

Pray tell us what is this wonderful technique..........

Skill. smile

Using a 4-facet drill helps. Look at the photo I posted earlier in this thread with four holes drilled No.71 (0.71mm) with no previous marking or drilling. The holes mate with 4 pins soldered into a small PCB. The pins are 0.64mm and the PCB plated holes are 0.68mm, so 0.04mm to play with, in theory. The pins on all the boards I tried fitted without a problem. It only works on flat surfaces, machined or extruded. Castings and round surfaces need spot drilling first.


25/07/2021 10:22:03

If small drills are wandering without centre drilling first then either the drill, or technique, are in need of revision.


Thread: Is there a chart for drill hole dia vs. boring bar size?
24/07/2021 22:02:42
Posted by Rod Renshaw on 24/07/2021 15:25:25

I was told about tapered bores 40 years ago by an apparently authorititive source and believed him without thinking it through. Once you spelled it out there is clearly no reason for it to happen.

It's unfortunate that in many areas (not just model engineering) there are statements that may have been true, but the specific circumstances have been lost in the mists of time and they then become generic. I was aware of the tapered bore issue, but was always puzzled that I never seemed to produce one, even with significant overhang. Chatter, yes, but not tapered.

I'd agree that it makes sense to use the largest diameter and stiffest boring bar available in any given situation.


Thread: Centre Drill Leaves a “Pip” - Sometimes
24/07/2021 18:24:24
Posted by Dr_GMJN on 24/07/2021 18:13:43:

......on YouTube that said the spotting drill should have a 130 degree tip, when used with a 118 degree hole drill. The reason being the tip of the hole drill will hit the apex of the spotted hole first, and not get deflected by the edge. Seems legit?

It's the subject of some controversy, even on professional machining forums. Like Jason I use 90° without any apparent problem.


Thread: Is there a chart for drill hole dia vs. boring bar size?
24/07/2021 12:42:42

Posted by Rod Renshaw on 24/07/2021 12:02:51:

Stiffness is important in single ended boring bars to avoid one end of the bore being bigger than the other.

This was discussed a while back and concluded that it is a myth. The bore may not be the diameter expected, hence the need for spring passes, but it won't be tapered. If the cutting conditions remain constant why should a boring bar cut differently at one end of the bore compared to the other?

The deflection for a given boring bar with constant load is proportional to the third power of the cantilever length and inversely proportional to the fourth power of the diameter. So doubling the length increases the deflection by a factor of 8. Doubling the diameter decreases the deflection by a factor of 16.


24/07/2021 09:42:53

Posted by William Ayerst on 24/07/2021 09:16:39:

Is there some kind of formula or reference chart I'm just missing?

There is no magic formula. Insert boring bars and carbide bars from professional tool suppliers will have a minimum bore specified. Bars from other sources will be pot luck. Les is correct; although these are internal threading bars the principle is the same, the cutting area is on the centreline of the bar, not at the top:


Likewise with this home ground HSS bar:


On insert boring bars the insert area is cut away, and slopes down, for the same reason.


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