Member postings for Andrew Johnston

I don't see why aluminium wouldn't work, but one would have to choose the grade carefully. As far as I can see 7075 is about the same hardness as CZ120 brass, so should be ok, and machines well.

Ideally for a really hard and resistant finish one would use the Keronite process:

http://www.keronite.com/

But, as always, the problem would be actually getting it done.

Regards,

Andrew

Ideally one would use a slide potentiometer; but quality ones are expensive and they're vulnerable to getting crap in the works. From a practical point of view I'd use a sealed cermet rotary potentiometer.

I wouldn't have thought that 255 steps would be a problem vis-a-vis response time. At any decent repetition rate one should be able to keep up with the potentiometer. You don't have to use all the steps in sequence. One could always use a 'look-ahead' term to anticipate fast movement of the potentiometer - a sort of electronic turbo charger.

Regards,

Andrew

I haven't tried freehand grinding, but I have now tried a couple of new grinding wheels. Both wheels are 150x13x31.75mm. These are smaller than the previous wheel, so probably running nearer 5000 sfpm rather than the 6000 sfpm before. When I get round to it I'll borrow a tacho and get an accurate wheel speed.

Both wheels are white aluminium oxide, one is 46KV and the other 60KV. After mounting and dressing each wheel I tried the same experiment on a block of hot rolled steel. The parameters were 0.5 thou depth of cut, stepover of 1/8" and 3 three sparkout passes, all manual feed. I took Ra (in µm) measurements with the direction of feed and across it. For the 46 grit wheel:

with across

0.01" 0.16 0.65

0.03" 0.44 0.67

0.1" 0.54 1.00

and for the 60 grit wheel:

with across

0.01" 0.16 1.11

0.03" 0.36 1.24

0.1" 0.54 1.15

Oddly enough the 60 grit wheel gave the worst finish in terms of Ra. This may be because the wheel loaded up quite quickly, whereas the 46 grit wheel hardly loaded at all. In both cases the finish looked pretty good and felt smooth. A faint pattern was just about discernable, but not spaced or regular as before. I'd say it was more a natural consequence of the grit size rather than a problem. With the light at the correct angle some faint iridescence could be seen, indicating that the surface roughness is less than the wavelength of visible light.

I also did a similar experiment with the 46 grit wheel under power feed, except that the stepover was 20 thou:

with across

0.01" 0.11 0.96

0.03" 0.47 1.01

0.1" 0.71 0.85

The surface feels smooth but has clearly visible striations perpendicular to the wheel travel. These striations are irregularly spaced, but are consistent across the width of the block. I am sure that these are a consequence of the power feed to the table. One can see and feel the whole machine vibrate as the table moves. Given the movement of the machine I'm amazed that the effect is so small.

Once again thanks one and all for the help and suggestions.

Regards,

Andrew

I suspect it's a dimensionless number, relative to a smidgen - Andrew

Geoff,

If the corners of end mills are chipping on mild steel and cast iron then something is wrong. What feeds, speeds, depth of cut and stepovers are you using? And what about the endmill, 2,3 or 4 flutes, HSS or carbide, coating and make?

Regards,

Andrew

As far as I'm aware the early Mosquitos were made using casein glue, based on a protein from milk. This glue was widely used prior to WWII, but had problems when exposed to tropical conditions. As a result new glues based on urea formaldehyde were developed for the Mosquito. The two glues that I've used for aircraft are Aerolite (urea formaldehyde) and Aerodux (phenol resorcinol), both of which are still available.

If you browse the beginning of this thread you'll find a more detailed discussion on aircraft glues.

Regards,

Andrew

Errrr, I make 1 thou equal to 25.4µm - Andrew

Thanks one and all for the suggestions. There's plenty of ideas to mull over. I've done a couple of extra tests this evening.

FIrst I tried grinding a piece of HSS tool steel. All my parallels are soft, and I don't have any hardened carbon steel parts that I'd be prepared to experiment on. The results are similar, a fine wavy pattern. After the first trial I gave the wheel a good dressing down, at least 20 thou off the diameter, but the subsequent results were similar. Surface roughness measurements were:

0.01": 0.17um

0.03": 0.20um

0.1": 0.54um

For the second experiment I reverted to the original soft steel block and tried a pass across using the table power feed and auto cross feed. Superficially the finish looked good, without the obvious patterning of before. However, with the light at the correct angle it was clear that the surface was uneven, with an irregular waviness, but with the crests much further apart, about 0.3" rather than 0.05". I think this is because the table moves much faster in power mode than I can move it by hand. The hand wheel is almost a blur, if you got in the way it'd take your hand off.

I like the idea of wheel bounce, even after dressing the wheel gets loaded quite quickly. The question is why is the wheel bouncing? Quite possibly because I've got the combination of wheel type, speed and feed incorrect. However, I wouldn't rule out badgered spindle bearings. The spindle bearings are of the plain opposed taper type, with springs in the middle holding the taper bearing apart. The machine has a plate on it specifying a strange concoction of 'water white kerosene' and a light mineral oil for the spindle. In discussion with Hallett Oils we decided this was equivalent to an ISO3 oil. It's just like water. If you pour it into the sight glass it just pisses straight out of the front of the spindle! The same is true for the ISO15 and ISO32 oils that I use on my cylindrical grinder. That seems to imply that the bearing clearances are rather larger than they should be?

Tomorrow I'll try freehand grinding. By the way the wheel spindle has no balance weights.

Regards,

Andrew

Hmmmm, what a vulgar discussion, let us hope it stays rational and proper, and doesn't veer into the improper.

Regards,

Andrew

Don't get me wrong; I'm not anti-Bridgeport. I'm pleased with mine, it's probably the machine I use most. It's just that if I was looking now I'd widen the search. Bridgeports do have the advantage of being ubiquitous, so spares are readily available, albeit at a price.

I'm sorry to hear about your travails with the Tormach. It certainly is a steep learning curve. Despite hundreds of parts and many hundreds of hours of machining I'm still making dumb mistakes on mine.

Regards,

Andrew

Edited By Andrew Johnston on 24/04/2012 10:53:12

Martin,

If I understand correctly, for a thread pitch of say 0.2mm across a 13mm wheel that means one traverse of the wheel should be the equivalent of 65 rotations of the wheel. The wheel should be doing about 3200rpm, so it takes about 1.2 seconds to do 65 rotations. I'm turning the cross feed wheel as fast as I can, I estimate between 1 and 2 seconds for a complete traverse,so that seems about right? Coolant would be nice. The machine is not, and never had been, set up for coolant, although it was originally available as a cost extra. It wouldn't be difficult to add.

Michael,

Simple things first; no the wheel doesn't have a nylon centre bush, the spindle is 1-1/4" and that's what hole in the wheel is. The grinder has fully automatic table feed and variable auto cross feed capability, although all the above tests were done manually. The table drive is rack and pinion and all the feed mechanisms are purely mechanical.

Now, onto the question of frequency domain. I assume you're not suggesting I do Fourier Transform analysis, but see how things vary with table speed. I didn't do this initially as I had in my mind that I'd tried this a while back and it didn't make any difference. So I was wrong, not for the first time, nor the last I suspect. I've just done a quick experiment across one face of the block. The depth of cut was about 1 thou, one traverse across the face, and a return sparkout traverse. The stepover per pass was 1/4". The first half of the traverses were done as fast as I could turn the table handle, the second half much slower, a few seconds per pass. The results are rather interesting. The first half of the block is as before, a very obvious regular wavy pattern spaced at about 1/16". For the second half there is still a regular wavy pattern, but the spacing is closer, about 1/32", and the amplitude is much smaller. In fact you have to get the light on the block at the correct angle to see the waviness clearly. So it appears that both the amplitude and frequency of the wavy pattern is related to table speed. Now as to what that means is another story. I'll have to cogitate on it

Regards,

Andrew

That may well be true, but a significant proportion of what I make in the workshop has nothing to do with model engineering. I have managed to stall my Bridgeport by being over-ambitious; now if I think I might be near the power limits I do a quick 'power required' calculation before switching on.

There was a thread on this forum recently where, as I recall, some people reckoned that a Bridgeport would lose tram under some cutting loads, and thus it was always worth checking tram if it was important.

By the way, how are you getting on with your Tormach?

Regards,

Andrew

PS: I'm not planning to change my Bridgeport any time soon, if only because I'd have to move a power guillotine and CNC mill to get the Bridgeport out of the garage.

Martin: Hmmm, didn't think of that, could be, thanks. I am winding the cross feed as fast as I can when truing the wheel. Say less than a couple of seconds across a 13mm thick wheel. How fast should I be traversing it?

I could also try it under table power feed, but as far as I remember it has always done this, manual or fully automatic feeds. More experiments needed!

Regards,

Andrew

I've finally found time to write up the results of some experiments with my surface grinder! The machine is a Brown & Sharpe No.2; it has a mechanical table and cross feed drive, but all the results to follow were obtained by hand feeding. The current wheel is 180x13x31.75mm, aluminium oxide, 46 grit. All surface roughness measurements are Ra, and are in micrometres. Values of 0.01", 0.03" and 0.1" refer to the distance over which the surface roughness gauge takes its average. The test block is hot rolled steel, rough machined on a horizontal mill. Here's the block before grinding:

I tried two strategies, on one side I used one turn of the cross feed (1/4" between table feeds and for the other side I used a quarter turn (1/16" per table feed. For each side I took cuts of about 1thou until the surfaces were clear of milling marks. I then took a final cut of half a thou plus a sparkout traverse. Here is the surface after grinding with a stepover of 0.25":

and here it is with a 1/16" stepover, it looks worse, with some evidence of wheel burn(?):

In both cases one can see a regular striation perpendicular to the direction of grinding. It doesn't show up on a 'tenths' dial gauge with a surface plate, but is clearly visible. Here are the surface roughness measurements for the 1/4" stepover:

0.01": 0.17um

0.03": 0.54um

 0.1": 0.62um

and for the 1/16" stepover:

0.01": 0.10um

0.03": 0.36um

 0.1": 0.46um

Oddly enough, the smaller stepover side seems to have a better finish, despite looking the worst. As far as I could tell the block is parallel to within a couple of tenths.

Both sides of the block were then rubbed on 800 wet 'n' dry until the striations disappeared. The surface roughness was then re-measured. Results for the 1/4" stepover are:

0.01": 0.06um

0.03": 0.07um

 0.1": 0.14um

and for the 1/16" stepover:

0.01": 0.06um

0.03": 0.33um

 0.1": 1.26um

Here's the finish on the 1/4" stepover side after wet 'n' dry:

Clearly there's something odd about the measurements on the 1/16" stepover side after wet 'n' dry; presumably finger trouble on my part.

The measurement parameter Ra is an average roughness, unlike the more modern parameter Rz, which is a peak to peak measurement, and so possibly more useful. There is no direct relationship between Ra and Rz, but a rule of thumb is that Rz is about an order of magnitude bigger than Ra. For the results on the 'as ground' 1/4" stepover (0.1" the peak to peak value is about 6.2 micrometres and after wet 'n' dry it is about 1.4 micrometres. The difference is 4.8 micrometres. A tenth of a thou is about 2.5 micrometres, so about two tenths. This rough 'n' ready estimate is borne out by the block now being thinner by around two tenths of a thou, according to the micrometer, in a comparative measurement.

A final word on the striations. I guess they could be wheel bounce, or the wheel bearings are badgered. I lean towards the latter, but in due course I will try different wheels and materials to see if it makes any difference. In the short term if I have to use a bit of wet 'n' dry to get rid of the marks, so what, I'm building traction engines, not a rocket!

Regards,

Andrew

Edited By Andrew Johnston on 22/04/2012 21:05:58

Here's my take on the above:

1) A Bridgeport can be very versatile, especially if you have all the accessories. However, partly because of it's versatility, it is a bit floppy. So you're not going to be taking heavy cuts. I do have a right-angle drive, but apart from running it to check it worked I've never used it in anger. I wouldn't want to take heavy cuts with it. If I was in the market again I'd consider something like the Beaver, similar versatility but someines with an INT40 taper, so a bit more rigid.

2) Here I disagree with David; I think there is a place in the home workshop for a horizontal mill. If nothing else they make a brick built outhouse look wimpy, so you can take heavy cuts, which doesn't half speed things up. All the spur gears for my traction engines have been cut on a horizontal mill. The gears are 6DP and 5DP, some in cast iron and some in EN24T. Each tooth was cut full depth in one pass with no problem. Another area where I find a horizontal mill useful is slitting saws. For some reason on the Bridgeport I always find slitting saws go walkies and wander when trying to cut deep slots. On the horizontal mill I'm happy taking a 1" depth of cut in one go with a slitting saw, with no fear of it wandering.

3) Just to balance things up, here I agree with David; I hardly ever use my pillar drill. Most of the holes I drill need to be accurate, so if I have a part in the vice on the milling machine undergoing machining why would I want to take it out in order to drill an imprecise hole before putting it back?

4) I do have a shaper, but rarely use it, for keyways and the like I find the slotting head on the Bridgeport generally meets my needs.

Mind you when all is said and done, you can't have too many machine tools, so my advice would be keep what you have and buy a Bridgeport as well.

Regards,

Andrew

Edited By Andrew Johnston on 22/04/2012 16:42:02

Chris: Thanks for the links; very informative, you've saved me the time I would have spent searching for the information. The Sandvik link in particular seems to confirm my intuitive understanding of the cutting process, both in terms of tip radius and radial cutting forces. I assume that in order to turn a part to size it's a good idea to minimise radial cutting forces.

Regards,

Andrew

See here for the results of some trials using carbide inserts:

http://www.model-engineer.co.uk/forums/postings.asp?th=51900

If you're only going to be taking light cuts I wouldn't bother with carbide inserts, you won't get any of the benefits of them. I've never met Graham or Chris, but I have met KWIL a couple of times, and I can tell you, he is an expert on using carbide inserts.

Picking up on another point from an earlier post I seem to do things backwards, in that I use a 0.8mm radius insert for roughing and 0.2mm for finishing. I think there's a rule when using inserts that the depth of cut should be greater than the tip radius. I'll have to look into this, I also feel an experiment coming on!

Regards,

Andrew

Neil: Wow, those are pretty nifty gears, I'm impressed!

Regards,

Andrew

Hi Nobby,

Looks like it gives really impressive results. Presumably one just feeds automatically along the lathe axis and aims to keep the dial gauge reading the same by manipulation of the cross slide?

With regards to the shape of the follower, the manual for my hydraulic copy unit recommends that the follower be the same shape as the tool. Obviously that requires careful selection of the tool shape with regards to the shape of the master to ensure accurate copies.

Regards,

Andrew

David: Now I reread your post I see that I mis-understood. I'll probably buy a silicon carbide wheel anyway, as I need to clean up the top of my magnetic chuck.

Regards,

Andrew