VB 36 or Serious SL2542

[Bill Boehme]

To my understanding, cast iron will have less movement than steel, due to ambient temperature variations.

The difference may be so small for a wood lathe, that the comparison will be without realistic consequences. Most of those lathes that use steel for bedway support, have many welded support braces, and in the case of the Oneway, that huge steel tube, have the bases covered pretty well.

You won't find many big industrial metal lathes, vertical mills, etc., that will use steel as a support base. In this case, only a few thousandths of variation will be problematic for precision machining......but, this isn't as much concern for the wood turner.

ooc

I agree that for precision milling nothing beats a very large mass base to stabilize the machine. There are, however, additional ways to skin that cat (apologies to Mr. Squeak, my Avatar).

There are many different types of cast iron as well as different alloys of steel. The best that can be said of the type of cast iron used on most woodturning lathes, gray iron, is that it is cheap. The quality of the cast iron also depends on the process used in casting -- usually sand casting. The quality of sand casting from Pacific Rim countries is generally not very good. The quality of a casting made from gray iron can be very good if using the Meehanite process or something similar.

The coefficient of thermal expansion of iron and steel is about the same. Some steel alloys have less thermal expansion than iron. However, that is irrelevant since the reference datum of a workpiece is not related to the bed of a lathe or mill. Three-axis positioning systems move the workpiece reference datum relative to the cutter.

Gray iron has some advantages and some disadvantages with respect to steel. The main advantage is making a complex casting at lower cost than hogging a similar structure out of steel. It is also fairly easy to machine. The disadvantages of gray iron includes low tensile strength and brittleness along with very poor impact and shock resistance (both mechanical and thermal). Anybody who grew up using woodburning heaters and stoves has probably experienced the loud bang and sudden appearance of a huge crack followed by everybody scurrying around to douse the fire. The disadvantages of a casting has to do with problems such as warping, non-uniform soundness (pouring flaws), consistent physical properties (uneven cooling), blowouts from steam being released in the mold material, and built-in stresses.

How about vibration? The graphite in gray iron helps absorb the energy of vibration. I think that many woodturners attribute too much to this property. It has nothing to do with any sort of flexing or shaking that we can either see or feel. Think of it as the sound quality if we made a bell out of the material. A lead bell might just make a bump sound while an aluminum bell might go thump, and a cast gray iron bell might go boink and steel might go clank. None of those would make particularly great bells. Malleable iron might ring better, but for a great bell use bronze or stannite.

Do industrial lathes and milling machines use cast iron? Some do and some don't. Fifty years ago most of them did, but the reasons were the technology back then and fabrication cost. The technology has changed a bit since then. One company that still uses cast iron for the base of most of their milling machines is Hardinge-Bridgeport. Their marketing brochures state that their use of cast iron is a discriminator that makes their product unique and better than their competition because of less vibration. A more honest statement might add that it saves them money. However, there is certainly nothing wrong with a well made casting. Another company, Milltronics, uses Meehanite process cast iron. Most of today's CNC milling machines are a lot like this Hyundai milling machine. I am sure that it probably has some cast iron components wherever it makes sense such as gear box housing and tailstock housing, but things like the box guideway and precision positioning components are not cast iron.

Getting to the bottom line, there is hardly any difference in the coefficient of thermal expansion and various steel alloys are better than cast iron in that respect. Bottom line: Even if there was some significant difference, it would be irrelevant for reasons mentioned above.

 

[John Lawson]

... an aluminum bell might go thump, ...

As a side note, deep bass English handbells (i.e. those used by church handbell choirs) are now made of aluminum by Malmark, a major maker of bells. The critical frequencies apparently radiate better than bronze bells, and they are also lighter, making it easier on the poor ringer.

 

[Bill Boehme]

As a side note, deep bass English handbells (i.e. those used by church handbell choirs) are now made of aluminum by Malmark, a major maker of bells. The critical frequencies apparently radiate better than bronze bells, and they are also lighter, making it easier on the poor ringer.

That's interesting. Some of the handbells seem to make more of a bump or thump sound anyway and some can barely be heard -- at least by my ears.

 

[robo hippy]

Bill,
As I heard once, "skin the cat" is a shortened version of "skin the cat fish" of which there are many methods. I like the fur on my kitties as well.

robo hippy

 

[John Lawson]

That's interesting. Some of the handbells seem to make more of a bump or thump sound anyway and some can barely be heard -- at least by my ears.

It's probably mere prejudice on my part, but I don't like the sound of bells below about C4 (the C below middle C). And don't get me started on chimes, which are an abomination. I currently play B4 and C5, my favorite position, but I've played every position from C4 to D6.