Thread: One of those 'how would you do it' questions

I have a project where I need to cut a radius on the mill that is around R600, through an angle of maybe 10 degrees. The exact value does not matter as it is mainly an aesthetic thing. Way too big for my rotary table.
I did think of making up a sector that I could anchor on one end of the mill table (for a pivot location) and then feed the part into a cutter with a worm or maybe a cable device.

If anyone has seen large radii cut on mills (and can say how it was done) or has any bright ideas on how it could be done (especially if it means a device that will allow different radii to be cut through adjustment), I would be keen to know.

Michael

what's the material?

Hi Michael, how thick is the project that you need to have the radii on?
Asking, as Toumas, made an attachment for an angle grinder to cut circles.
Found this on Google, https://www.youtube.com/watch?v=nyNAMuePsL0
HTH
Kryn

This is the edge of a bit of cast iron, approximately 3/4" thick (well, in most places). I want to try to match it to an existing leg, so a uniform smooth finish rather than hacking in with a grinding disc that might result in a slightly mis-shapen appearance.

I can work out how to do this particular bit but was more interested in if anyone had a general way of doing this that could cope with varying radii or angle etc that would be a useful piece of tooling to make up for doing this or other similar tasks. Every so often there is something that needs a large radius curve, so it would get repeat business eventually. I find trying to do these on a rotary table gets a bit dodgy when the radius of the cut is significantly larger than the diameter of the table.

I suppose I could set up a boring head with a tool 600 out from the centre but that whizzing around at 40rpm (slowest the mill will go) sounds a little hazardous (and perhaps not quite practical?)

Michael

Originally Posted by Michael G

This is the edge of a bit of cast iron, approximately 3/4" thick

Oh, I though it might be Ali. I was going to suggest a router with a carbide bit, but I guess not!

Kryn, that video was interesting and similar to what I was thinking although I did not like the way he secured the end of the board to the R/T. During the cut you could see the board shaking. While not too awful for wood, I suspect that on metal there would be chatter & gouging. Interesting approach though.

(My thought was to mount some form of gearing directly on effectively what would be the end of the board he uses. I'd get better leverage from it as well as avoiding the shakes)

Michael

Attach the item to a suitable piece of timber ply or mdf, create a pivot point in the wood substance to provide the 600R, attach the pivot point to the mill table.

Ten degrees of rotation on 600R is a 105mm chord. To keep control of the item I would be feeding the job past the cutter with a threaded rod screw arrangement that provides say 120mm of movement. Attach one and of the screw to the wood substance somewhere near the 600R mark, attach the other end to the table.

Michael
Could you set the piece up on your rotary table at a radius of say 200 and then use a combination of X feed and rotary feed to give the desired shape. Only working in my head but something like X feed at 3 times the rotary feed?

Tony

Lots of hand/mechanical ways that would work for a one-off but if it was something that was going to happen a lot, IMO there's only one decent solution.

CNC VMC.

I could do 450R (900 diameter) or maybe a bit bigger on my HBM but not 600R.

PDW

Yes, that's the rub. A one off can be bodged together but having something that will allow flexibility with multiple radii is tricky. A VMC would solve the issue but I haven't got one of those sitting around.
Can't just swing it on a lathe either, as one of the parts has the radius leading into a boss, so the cutting device has to stop when it gets to a particular position before it obliterates the boss.

Michael

Hi Michael, you need to post a drawing or even a picture of the item as it is not easy to design a machining/holding jig without a bit more detail.

Originally Posted by PDW

I could do 450R (900 diameter) or maybe a bit bigger on my HBM but not 600R.
PDW

I am sure that everyone here would appreciate it if you would put up a thread on any work that you have done on this ornament, I sure would.

cheers, shed

If I had to do it, I would give it to someone more clever then me to do it, like Michael G.

I think Brian Bloc (Bcbloc02) did some work that required machining a circular pattern on a piece of metal that was too big for his rotary table.

Michael

Originally Posted by shedhappens

... you need to post a drawing or even a picture of the item as it is not easy to design a machining/holding jig without a bit more detail.

One of the things I'm doing while between jobs is look through the pile of magazine articles and PDF's that I have collected for 'one day' (and we all have one of those piles right?). I found this in that pile and thought I may as well see whether I can do anything with it. I was not planning to run it with steam - perhaps air once or twice but it would mainly sit around for interest's sake. However, the original used castings and as the article is now 72 years old (and published in the US) I doubt they are available, so if I did make it, I'd have to machine from solid.
Engine.jpg
The legs that hold up the cylinder have a slight radius on them - but too big for my 8" RT, so I was trying to think how to get the radius in a controlled fashion. The other day I had something else with a larger radius that I had to bodge, hence the request for ideas on an adaptable fixture that could do up to R600 as greater than that is something I am unlikely to need. I know that pre CNC machinists had all sorts of ways of getting results and so had hoped that someone had seen something in a book or a historical site that using a cunning assemblage of levers, gears and wires would allow large radii to be machined on a small mill.

Michael

Originally Posted by tony_A

Could you set the piece up on your rotary table at a radius of say 200 and then use a combination of X feed and rotary feed to give the desired shape?

Thanks for having a crack Tony. I know it's sometimes not easy to jump into a conversation in these forums.

Not sure I understand what you mean there, but I think you are suggesting indexing both an axis and a RT to give what would be an elongated radius. For small angles of arc it would probably be a reasonable approximation. Need to think about that. I was hoping that someone would point me at a device used in days of yore that would not require me to synchronize movement in two axis at once.

Michael