Backplate Material Selection

[Willy]

http://cgi.ebay.ca/6-4-JAW-INDEPENDE...QQcmdZViewItem

Is this it?
After having previously copied the the url from the page you want to link to, go to the reply page and click on the "insert link" icon, then all you have to do is paste it in the url field.
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[J Tiers]

JTiers, I watched a used 6" Bison 3 jaw go by at over 2 bills on E-Bay last week. I'll bet it was a nice chuck but new couldn't have been much more. I swear E-Bay's getting harder all the time.

That's nutty...... I bought mine for possibly $120 new, several years ago. Good size for a 10" (which swings almost 11").

Used chucks are a crapshoot. Used 4 jaw chucks are a bigger crapshoot.

The 4 jaw is normally a sturdy item, but is also often used to put a part in a deathgrip, so it is not immune to being worn bellmouthed, sprung, whatever.

My original one (came with the lathe) was a big tough 6", huge jaws. Trouble is, it was worn so that each jaw needed different shims, AND one of the adjusting screws had almost completely stripped the jaw threads. It was also a "knucklebuster" with square ended screws sticking out.

Nobody needs to pay $200 for something only to find that out.


Pasting the link is easy, copy the line up at the top of the page that has the site address you want. Then, paste it into the text here, on its own line. Like this:

http://www.lathes.co.uk/page21.html

I copied that while I was at that page of the site itself, and pasted it in here.

Some catalogs will not allow that (deep linking), you can only paste the top page, all the others are shown in a way that cannot be linked to.

Supposedly that pasting internal page links is really poor "net etiquette", but everyone does it. If the page setup at the target site changes it may later go somewhere unusual, but that's the risk.

[pntrbl]

Yeah, that's the one Willy. That's it! No Canadian dollars on my page and I also had a shipping price. Let's see if I can link it ...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=014&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=330108897577&rd=1&rd=1

Ya know, I've been linking all along but never had enough sense to check the preview.

SP

[abn]

Backing plate costs more than a chuck LOL...

Yeah, that's the one Willy. That's it! No Canadian dollars on my page and I also had a shipping price. Let's see if I can link it ...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=014&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=330108897577&rd=1&rd=1

Ya know, I've been linking all along but never had enough sense to check the preview.

SP

[pcarpenter]

If I'm not mistaken, Discount Machine is Shars. I've bought items from them in the past and have been pleased. The language barrier is difficult sometimes at order time, but they try hard and I get items in stock right away (they are located a few hours away, near Chicago).

The good news is that if you buy from them, you will get a print catalog...which is especially important since thier web site is usually horrible and often completely broken.

Paul

[DR]

Cast iron would be my last choice of material. Cutting that will make a gawd awful mess of your lathe.

7075 aluminum would be my first choice, except for the price.

Use 1018.

[lazlo]

Aluminum rings like crazy, which is why it's not used in machine tools.

Gray cast iron has outstanding vibration damping, which is why most machine tools are made from gray cast iron, instead of ductile cast iron or steel, which are stronger.

[GRH]

Without thing twice definately Cast Iron as a retired machine tool designe
engineer (Cincy Milacron, G & L and DeVlieg ) same reason as lazlo.
C.I. also will not gawl on the spindle and machining is not really that bad, clean all oil off the machine before you start and when finished vac up the dust etc before re-oiling.

Graeme

[Willy]

I have that same chuck, although I have not used it yet.
For a cheap chuck it doesn't appear that bad, the jaws fit the body of the chuck nice and snug, and the gripping portion of the jaws are square to the face of the chuck. Not having used it I can't say how it will stand up in service, but for that kind of money you can't go to far wrong.
I'm sure we all have more expensive door stops than that.
Having said all that, I can say that I lucked out about a year ago and scored a well used Cushman 4 jaw for fifty dollars that is still a whole lot nicer than the new one, but I'm sure it will be okay when I get around to using it.From what I've seen I sure don't feel bad about it.

By the way I have dealt with Shars/Discount Machine on several occasions as well and what Paul said is true, they do try though. So if you have to communicate with them try e-mail first, thus avoiding the language barrier.

As the others have said cast iron would be my first choice as well, it's not that bad to work with as long as you take some steps before you start in order to keep the dust down. I usually cover everything with paper towels that have been sprayed with WD40 and keep a vacuum cleaner nozzle near the where I'm working...it's usually a non issue. And it does machine very nicely.

[Evan]

Aluminum rings like crazy, which is why it's not used in machine tools.

Only partly true. In the case of a backplate it won't be able to ring with a chuck bolted to it. In order to ring the material must be free to resonate. As soon as two parts are fastened securely together with no free sections ringing become much less likely, especially if the masses differ significantly. For ringing to occur both pieces would have to share a resonant fundamental frequency or a close harmonic. The real reason for not using aluminum in machine tools is that for the same section aluminum has one third the Young's modulus of steel and about 75% that of gray cast iron. This means that aluminum is more "flexible" under elastic loading and that to achieve the same rigidity thicker sections are required. Any issues with ringing are fairly easy to deal with using correct design. It doesn't usually pose a problem in aircraft where a tendency of a structure to ring could be disastrous.

In the case of a backplate this isn't a problem and the extra material to gain sufficient strength isn't an issue either. Nor is differential linear expansion as the components are radially symmetrical.

[lazlo]

The real reason for not using aluminum in machine tools is that for the same section aluminum has one third the Young's modulus of steel and about 75% that of gray cast iron. This means that aluminum is more "flexible" under elastic loading

It's not about stiffness (Young' Modulus): cast iron is less rigid than steel. Like Graeme confirmed -- it's about vibration damping/management.

Gray Cast iron damps vibration 25 - 125 times better than steel. Steel, in turn, damps vibration 10 times better than aluminum.
In other words, gray cast iron damps vibration 1200 times better than aluminum:



If you go to a local DuraBar distributor, they like to demonstrate the superior damping characteristics of gray cast iron by hanging a 1 x 6" bar of steel, aluminum and gray cast iron by strings, and whacking them with a ball peen hammer.

The aluminum vibrates like a tuning fork. The steel rings a lot less. And the gray cast iron just makes a dull thud.

If you've ever switched from a steel to an aluminum frame mountain bike, the aluminum frame is a lot more jarring than the steel frame.
This is also why Harley-Davidson made their knucklehead engines from gray cast iron, instead of ductile cast iron.

This also why toolroom lathes have change gears made from gray cast iron instead of steel -- the cast iron gears absorb the geartrain
vibration and reduce the "phonograph" chatter marks in the work:

http://www.dura-bar.com/downloads/upload/gearBrochure.pdf

[Evan]

Gray Cast iron damps vibration 25 - 125 times better than steel. Steel, in turn, damps vibration 10 times better than aluminum.
In other words, gray cast iron damps vibration 1200 times better than aluminum:

I don't dispute that. However, the ringing effect can be damped by proper design and a material cannot ring unless it is free to resonate. Ringing is a resonance phenomenon. All it takes to make a bell stop ringing is a small amount of extra mass placed in the right spot or simply picking a shape that has no resonance modes that reinforce (cracked bell for instance). There are many ways to dampen ringing. In the design of my milling machine I used a variety of methods including the use of dimensions for various parts that are different prime numbers, the use of golden sections and the use of multiple layers of material of different masses. By doing this excellent damping can be achieved regardless of the material.

The difference in Young's modulus is much harder to deal with in machinery design as it requires greater bulk to achieve the same rigidity with aluminum. There aren't any shortcuts to achieving a greater modulus of elasticity. This is shown by the fact that all steel alloys have nearly the same modulus.

[lazlo]

Another pictoral showing the amazing vibration absorbing capabilities of gray cast iron:

[Evan]

Two layers of aluminum with an adhesive between is better. The point is that the material must be free to vibrate for it to matter. Even if it is free doesn't mean it will vibrate. Cast iron makes a lousy tuning fork. So does aluminum if the forks are different prime lengths.