Machining a convex surface (engine buffers)

This forum is dedicated to the Live Steam Hobbyist Community.

Moderators: cbrew, Harold_V

User avatar
Benjamin Maggi
Posts: 1409
Joined: Wed Apr 25, 2007 10:38 pm
Location: Albany, NY

Machining a convex surface (engine buffers)

Post by Benjamin Maggi »

For my British-based Sweet William engine, I am about to start machining the buffer heads. I will make them as three pieces: (a) the buffer casting that bolts to the bufferbeam, (b) the shaft that fits into the buffer casting, and the (c) striker plate or "head." I plan to machine the head with a recess in the back that the the shaft will be Loctited into. These will not actually engage with other car's buffers but they may be called upon sometime to help rerail the engine (hopefully not!) or used to push the engine by hand so I want to actually make them of metal. I have seen ones offered of plastic/resin but they just seem wrong on a live steam locomotive.

I have never machined a convex surface before like on the outer surface of the buffers. There are thousands of British and European model locomotives out there with them so someone must know how. Here in the States, we dodge the issue by using couplers (which are no easy machining feat either)!

I asked on the Model Engineering board (http://modeleng.proboards.com/thread/91 ... ffer-heads) and received a mixed bag of ideas. I don't want to just use a file and do every one by hand. The heads are 3.5" wide and have a constant 7.3125" radius across the front. The edges are 3/16" thick, and slowly increase to about 25/64" at the center part.

While I don't need to machine them exactly to spec, having all four similar would be desirable.

How do you guys machine a surface like this?

Thanks.
"One cannot learn to swim without getting his feet wet." - Benjamin Maggi
- Building: 7.25" gauge "Sweet Pea" named "Catherine"
UKlocoNut
Posts: 38
Joined: Sun Oct 12, 2008 7:45 pm
Location: Montreal QC

Re: Machining a convex surface (engine buffers)

Post by UKlocoNut »

Benjamin

I normally machine a flat face in the lathe then "carefully" use a hand file to generate the profile by eye.

Finer hand file for finishing then abrasive paper.

Regards.

Keith Wood
MLS
RET
Posts: 960
Joined: Wed Jun 07, 2006 8:36 am
Location: Toronto, Canada

Re: Machining a convex surface (engine buffers)

Post by RET »

Hi Benjamin,

With CNC it would be easy, but failing that you might try using a radius bar of 7 5/16" between centers. You would have to figure out how to attach it to your lathe with one end fastened to the cross slide and the other attached to the lathe bed. You would use the compound rest to advance the cut and the power cross feed to do the facing. Since the buffer faces are often oval, you would cut/file the oval shape last.

Just a suggestion. I'm sure there are other methods, but this could be relatively simple.

Richard Trounce.
Kimball McGinley
Posts: 809
Joined: Wed May 31, 2006 2:13 pm
Location: Laguna Niguel CA

Re: Machining a convex surface (engine buffers)

Post by Kimball McGinley »

My neighbor used to do this on a milling machine to make lenses. A flycutter at an angle makes the cut, while the part is turned on a rotary table. I am sure somebody could come up with a formula?
SteveM
Posts: 7763
Joined: Mon Jun 27, 2005 6:18 pm
Location: Wisconsin

Re: Machining a convex surface (engine buffers)

Post by SteveM »

You'd have to do some math (or draw it out and measure it), but you could do the machining in steps and then only finish with a file.

If you made a template out of sheet metal, you can use that as a guide to make sure they all come out the same.

Steve
User avatar
baggo
Posts: 161
Joined: Mon Jan 12, 2009 6:25 am
Location: Derby, UK

Re: Machining a convex surface (engine buffers)

Post by baggo »

I've machined buffer heads in a very similar way to that which Richard describes. I just used a length of silver steel between the headstock and the cross slide to give the radius. I got the idea from an article by Tim Coles in Model Engineer.

http://www.modeng.johnbaguley.info/loco ... helen7.htm

The method is described about halfway down the above page.

John
Attachments
buffer2.jpg
buffer3.jpg
Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME

http://www.modeng.johnbaguley.info
User avatar
cbrew
Posts: 3161
Joined: Sat Jan 04, 2003 12:17 pm
Location: Vancouver Wa

Re: Machining a convex surface (engine buffers)

Post by cbrew »

Now that is clever! i will have to remember this !
If it is not live steam. its not worth it.
NYCRRson
Posts: 24
Joined: Fri Jun 03, 2011 10:33 pm

Re: Machining a convex surface (engine buffers)

Post by NYCRRson »

They make proper tools for that, a radius/ball turning adapter for a lathe. Lots of varieties and prices;

http://www.bing.com/images/search?q=lat ... ORM=IQFRBA

Or you can make your own. Depends on your time and dollar budgets.

Since these are probably just cast surfaces on the prototype I suspect you could just "eyeball" them with a file and they will probably look fine.

I have the radius/ball turning tool for the Sherline lathe, works pretty good, takes some adjusting to get the exact results you want. I always test it first on a scrap piece.

Cheers, Kevin
NYCRRson
Posts: 24
Joined: Fri Jun 03, 2011 10:33 pm

Re: Machining a convex surface (engine buffers)

Post by NYCRRson »

Benjamin,

Also, if your lathe has a compound tool slide (capable of rotating around a pivot point), and it is in good shape, you could try;

Adjusting the tool point to the pivot point distance to the radius you want, you probably need to back off the compound slide all the way.

Loosen (slightly) the nuts that lock the compound angle. You need a compound tool rest slide that has something to hold if down
against the cross slide. Some of them have a V shaped groove with set screws that force the compound slide down against the cross slide. Others have a "T" shaped center section and setscrews that force the compound slide against the bottom of the "T". If you loosen the retaining nuts/screws and can lift the compound off of the cross feed this technique will probably not work.

Then feeding the carriage in a small amount and rotate the compound (by hand, don't let go) to "round over" the outside edges of your part. Move the carriage closer to your part (a few thousands) and repeat. Lock the carriage and cross feed (if possible) between passes. And you need the pivot point of the compound slide centered on the axis of the part.

This might work for aluminum (and maybe brass), but may not be stiff enough for steel parts.

Depends on what machine you have available. Again, I advise trying on a scrap piece first.

Good Luck, Kevin.
User avatar
Bill Shields
Posts: 10459
Joined: Fri Dec 21, 2007 4:57 am
Location: 39.367, -75.765
Contact:

Re: Machining a convex surface (engine buffers)

Post by Bill Shields »

there are add-ons called BALL TURNERS that can be used.

Jim and I just put the buffer in a SPINDEX which was bolted to the table of our belt sander and gently rotated the spindex...got a very nice looking surface.

Is it mathematically correct...eh....probably not, but it looks just fine...
Too many things going on to bother listing them.
User avatar
Fred_V
Posts: 4370
Joined: Sun Jan 12, 2003 3:26 pm

Re: Machining a convex surface (engine buffers)

Post by Fred_V »

He is talking about a 7-5/16" radius. you aren't going to get that with a ball turner or a compound.
Fred V
Pensacola, Fl.
User avatar
Bill Shields
Posts: 10459
Joined: Fri Dec 21, 2007 4:57 am
Location: 39.367, -75.765
Contact:

Re: Machining a convex surface (engine buffers)

Post by Bill Shields »

Actually, I remember seeing something like a ball turner that did large radii....think it was here, years ago.

but you are correct...that is a bit for an off the shelf ball turner....

but if you offset the tool back off the center of the compound, you could easily get that radius...
Too many things going on to bother listing them.
Post Reply