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Member postings for Marcus Bowman

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

Thread: Tapping Cast Iron
25/01/2016 23:05:00

Interestingly, Whitworth is still a commonly used threadform for threads in cast iron, because of its relatively coarse pitch compared to its diameter; and because of its nicely rounded root and crest profile which means there are no sharp corners to act as stress risers, and no sharp vee points to crumble. Whitworth is still used in the metric zone for that specific purpose. Compared to, say, M6 x 1, 1/4 Whitworth is 20TPI and has a pitch of 1.27mm, so it is significantly more coarse than the nearest readily available ISO metric thread. 5/16 Whitworth has a pitch of 1.41mm which is greater than an M8 x 1.25 thread. If you prefer to stay with metric, on the basis that the taps will be more useful for other jobs, I would be inclined to choose M8 x 1.25 instead of M6 x 1 in cast iron. It probably depends on the amount of metal left around the threaded hole, because too little leaves the uncomfortable prospect of cracks in the main casting; so M8 might be too large for your tailstock.


Thread: Metric Acme thread question.
10/01/2016 23:41:49

Assuming this is a 26mm x 3mm pitch 30 degree Metric trapezoidal thread, 26mm is "second preference" from BS 5346:1976.

The basic pitch diameter should be 24.500mm

Fundamental deviation is zero for the internal thread

Other dimensions depend on the class of fit, but ....

On a thread of this diameter, the easiest way to cut the nut is to use a full-form carbide threading tip and use the existing screw as the gauge. That way you don't need to worry too much about measuring to achieve a particular the class of fit. If you are really picky, you will need a proper thread plug gauge (jolly expensive, but the proper way to do the job).

I'm not sure if that helps, but the full-form tip will certainly allow you to do the job accurately.


Thread: How can I tell if a carbide tip is worn?
29/12/2015 19:50:46

Les, you are of course correct. Mea culpa. 30M/min is more realistic, and what I thought I was typing....

So; for a 60mm diameter and an HSS tool the speed should be around 160rpm, and for carbide perhaps 300 to 500rpm. And hot chips.


29/12/2015 17:58:24

Use a watchmaker/jeweller's loupe or a microscope to examine the tip. That will soon tell you if it is blunt. Chipping or ridges along the tip will indicate wear, and a build up of material behind the cutting edge will also produce a rotten finish and a poor cutting action. Normal HSS cutting speed would be about 30M/s, so a carbide tip will work best at 2 to 3 times that speed. Cut depth will vary according to the design of the tip, but 0.2 is fine, and 0.5mm might be ok, but might also be a bit on the deep side. If you took twice as many cuts at 0.25mm depth you might find the tip liked it better, lasted longer, and gave you a better finish. You need to experiment a bit, I think. There is a sweet spot where the combination of speed, feed and depth of cut wqork well (a small range, actually).

Carbide tips don't last forever, though, so I would examine it closely first.


Thread: J&S 540 help
16/12/2015 19:57:35

Thanks, Chris. That echoes other statements I have seen about not needing to balance wheels of 8 inches or less. Interestingly, the three weights on the hub which was fitted to the machine were not at 120 degrees, and I assumed that was because the positions had been altered to bring the assembly into balance. The performance of the wheel suggested that was not the case, and that the wheel assembly was out of balance. I shall try the 120 degree approach, with confidence.


15/12/2015 19:53:03

I have a 540 and the manuals (skinny things, but useful for their purpose). The lubrication is listed as:

Propulsion - Mobil Vacuoline 1405 (now VACTRA 1, viscosity 32)

Oil nipples - Mobil Vacuoline 1409 (now VACTRA 2, viscosity 68

Spindle (Note: Plain bearing spindle) - Mobil Velocite 6 (equivalent is ISO 10, but I have never used that). Mix, as stated on the wheel guard, is 50% Velocite 6 and 50% kerosene. J&S can still supply this mixture (or could a few years ago), but it is easy enough to mix. I think the kerosense just thins the oil. That's a different mix to David's machine, but perhaps the difference is in the viscosity of the grade of oil.

I have just fitted a new grinding wheel and need to balance it. I need to know the slope of the taper in the wheel hub, so that I can make a balancing mandrel. Does anyone know the actual figure?

I would also like to know why there are three balance weights on each of my hubs, instead of two, The J&S manual only mentions two, and gives the simple procedure for balancing using two weights. I feel sure there is some logic behind three weights, but need to know how they are used to balance the wheel, in a practical procedure.


Thread: Aluminium swarf
05/12/2015 19:08:35

Can you fix a piece of steel just behind the cutting edge, to force the ribbon sharply upwards, so that it breaks? That might be a piece of steel taped to the toolpost, or it could be clamped in place along with the tool, by the screws that secure the tool in the tool post. Mind you; I use a polished carbide tip designed specifically for aluminium, and it is just as good at filling the place with long strips of swarf. That tip has a built-in chip breaker, but its tiny. I'm thinking of something much larger.


Thread: What is better than Shellac ?
05/12/2015 15:32:09

Loctite Detach removes Loctite (as the name suggests), and there is a similar thing for Superglue (which I have used on my fingers once or twice.....). They are commercial products, but designed for the job.

The superglue remover is supposed to live within arm's reach of my bench, but I can't see it at the moment, which is a bit worrying.


Thread: a replacement tool to cut sheet steel.
30/11/2015 12:03:59

Up to about 1.5mm I use Makita electric shears. They are my go-to tool for speed. There is a little bit of a knacjk to these, to hit the "sweet spot" where forward movement is easy, and you do need to exercise positive management of the curling waste strip on longer cuts. Electric shears remove a thin strip to create a channel which forms the cut. They are great for straight or gently curved cuts.

I also use a Makita nibbler for up to 1mm sheet, and that works well on tight curves down to about 50mm diameter. As Ian S C says, the nibbler generates millions of tiny crescent-shaped SHARP waste, and leave a SHARP edge just perfect for slicing flesh.

I also use a pair of good hand shears, up to about 2mm (tough on the wrists at that thickness. I prefer my big Gilbows for that, but tight curves need the aircraft shears which grip one edge more securely, but do leave tiny serrations on the top surface just next to the edge of the cut. The Gilbows tend to slip sideways just a little on the tight curves (Speed and old habits means I am using straight-jawed Gilbows, rather than the curved-jaw versions which might be better for curves).

Fine-toothed jigsaw blades work well. Coarse-toothed certainly do not. Support the sheet next to the edge of the cut.


Thread: Screw cutting Advice
06/11/2015 20:08:09

You may find that if you are using inserts you will get better results when threading at higher speeds. That's true of non-insert tools too, of course, but more so with carbide inserts.

If you are using a partial-form insert, the extent to which you will need to deal with the shape of the thread crests afterwards will depend on which threadform you are cutting. Whitworth crests are rounded (unless you are cutting a truncated Whitworth form) while metric threads are flat-topped (although just a smidgeon of corner rounding is both allowable and helpful).

Using a chaser as described in earlier posts does improve the flanks and crests very effectively. My personal preference is to mount the chaser in a holder in the toolpost, for accuracy and fine control. It's too easy to lose the fine control of size, fit and finish when using a hand-held chaser (or at least that's my personal experience, so probably reflects my own tendencies). I use copious lubrication when chasing.


Thread: Motorcycle Restoration
29/10/2015 23:07:45

I have had a few of those. I had one I used daily for a few years, then had to sell it. I still miss it.

Which model (250 Star; C15SS; C15T; or C15S - S was scrambles and T was trial version, so most were plain C15 - the 250 Star, although most folks didn't call it that - or the very nice C15SS Sport Star).


Thread: Best Way to Create These Radii
28/07/2015 09:54:59


This is probably no help, but I think you could produce all, or most, of that 3D shape using a small CNC vertical mill. Do you have dimensioned drawings (or even your own sketches with approximate sizes)?

You could certainly produce the curved section as you suggest, using an end mill and a rotary table, or using a boring head or boring bar, even though it is only a part-circle.

The bores should be no problem either.

The top curved sections might be possible using the bore holes as a pivot point, and the side of an end mill to get some or all of the curves. Then blend with files. Coarse files, well-chalked work best with aluminium, at least for removing bulk. As JasonB's post shows, you can get a really nice finish on aluminium.

Mind you; if you are not too fussed about the exact detail of the outside of the shape (i.e. you don't need an exact replica) it may be possible to simplify the shape and make machining easier. Anything that eliminates the filing would be a bonus.

Thread: Simple DC speed control
02/06/2015 23:42:52

To be honest, I enjoyed the article, and felt it gave a useful and interesting explanation of some aspects of the device. I've a friend who has already sent for the components, and I expect to do the same for the items I can't source from my electronic junk box. What interested me most was that I have a board which will substitute for the potentiometer to provide speed control from the computer.

I suppose its a variation on the "different strokes for different folks" theme; but this one certainly suited me.


Thread: Pictures of first attempts at cutting metal on the Tormach.
12/05/2015 22:24:18

Excellent. I'm very interested in this.

Which model is this?

Can you comment on the experience of ordering and getting this shipped to the UK?

Did you have it shipped from Wisconsin or from Quingdao Port, China?

Are you running the new Path Pilot software, or the older software?

Lots of questions...


Thread: Copper pipe bending
13/03/2015 23:44:13

Or what used to be called Cerrobend.


Thread: Need help locating bolt
19/11/2014 23:15:13

The original bolt looks like brass or similar softish metal. It may be designed so that wear in this soft component prevents wear in the parts which pivot on the bolt. That would prevent a much more awkward (and potentially expensive) repair or replacement of those other parts. If it was me, I would replace the bolt with one of a similar material.


Thread: Electronics Engineering Project
20/10/2014 23:21:20

How about a robot for exploring the sea bed? That's the sea bed and not the bottom of a paddling pool, so there would be considerable challenge in designing and making the vehicle, especially where it has to withstand reasonable pressure and go deep enough to get to the bottom of a typical harbour. It should have a steerable camera, and a way of sending video and/or telemetry back to the operator. That's a not inconsiderable challenge.

It should be steerable, of course, but it might have some intelligence of its own (it depends on the focus of your project, whether it is mechanical, electronic or computing-orientated).

Or you could assume a submersible and just focus on the command and control and telemetry and autonomous exploration, although that would miss out part of the challenge. You might need to simplify it in that way, though, for a project with a deadline.

It is easy to do this kind of thing on the land, and quite common now to do it from the air, but under water is not so easy.


Thread: Vintage motorbike
20/10/2014 23:14:04

I don't much care if its a model or a real thing; it's about the engineering and the making. And the bikes have always hit the spot. But then so has almost anything else where someone has tackled the challenge of making something which appeals to them. It could be a custom car, or a toaster (petrol or steam), I don't discriminate.

That tank is a thing of beauty, with its folds and compound bend at the top rear corner. I'd like to see an article showing how that was done. Lovely.


Thread: Cherry's Model Engines
14/10/2014 13:29:56

Well; it's a dog's life right enough...


12/10/2014 23:05:50

I have a copy of this book. In fact I've had it for a fortnight or more, so my pre-ordered copy must have been despatched quickly.

This is a beautiful book, and I am keeping it for my Christmas stocking.

There is a single chapter entitled "How its done" with photos from Cherry's workshop. The rest is a 'coffee table' book, with gorgeous photos of beautiful models. As a long time fan of the lady's work, I'm absolutely delighted with the book. The author has done a fine job; as has the publisher.

Yes; a blow-by-blow exposition, with some of the techniques and tools revealed would have been nice, but that's not really the aim of the book. So, after a heavy lunch with family and relatives, I will leave the others to the tv and chocolates, and slope off to a quiet corner on 25th December, for what I expect to be a joyous celebration of the very best in model engineering skill.

Did I say I like the look of this book...


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