Generalized solution to machining 2D complex curves by hand?

[dbb-the-bruce]

This was done free hand on my Logan.

Cool. I've also done some free hand turning of metal (I was originally a wood turner).
So similar idea but flat work.

 

[dbb-the-bruce]

A shaper is definitely on my list. Already looking for one. And yes that's the kind of operation I'm looking for. I've done stuff like that for trapped circular faces on my mill. Use a pin or something to locate the center and clamp in a vise. Then take lots of flat cuts rotating each time.

Could also work for concave profile on the mill but locating the part becomes very tedious.

 

[rwm]

Maybe I am missing something. Isn't this just what a CNC machine does from a CAD file?
Bridgeport made a mill that had a head that could move to follow a pre-made pattern. Is that what you mean?

Edit: called the true trace

 

[dbb-the-bruce]

Maybe I am missing something. Isn't this just what a CNC machine does from a CAD file?
Bridgeport made a mill that had a head that could move to follow a pre-made pattern. Is that what you mean?

Edit: called the true trace

You are not missing anything.

Most of us are constrained in someway by time/space/money.
I have a small desktop CNC machine that is quite capable but limited in scale as to what can be done with it (I've made a number of small things from brass and aluminum and even some small steel stuff with it). I have invested a significant amount of time learning CAD and CNC. So I have the skills to do the type of operation I am describing - but the machine I have is the most that I can currently justify based on space and money.

Of the three (time/space/money) - time is what I have the most of. So figuring out the how to make something that would be relatively easy on a large CNC machine in a manual, slower way with the equipment I have is worth the effort. I don't have to pay myself by the hour, if I did I'd go broke! I'm slowly learning to machine stuff manually more efficiently.

I'm sure plenty people have figured out how to one-off make parts like this manually. I'm trying to get a sense of what other people have done and learn new techniques and expand on my own efforts to date.

I was aware of pre-CNC specialized machines that could follow trace outlines. I believe there was one that would optically follow printed outline and drive X & Y. I mentioned this in an earlier comment.

So in regards to this discussion, one potential possibility might be setting up some sort of trace or pattern following assist on my manual bench top mill. Even something as simple as site over printed plan could maybe work. Seems like it would be a challenge to accurately follow. The closest I've come to that is visually following a CNC cut trench. I'd managed to cut a shallow profile on my small CNC machine and was trying to complete it on my mill that has a much more powerful spindle. The results were less than satisfactory.

Or maybe something even as crazy as putting a ball bearing on the end mill like one would do with a router.

Thanks for the vid of the True-Trace

 

[ARC-170]

I have on more than one occasion wanted to machine a complex 2d curve profile. The most general case is a curve that has both convex and concave components. But either concave or convex non-circular curves also apply.

These are the kinds of things you would do with a pin router or following a template with a router if you were working with wood.

I have done convex curves with lots of straight cuts to approximate the curve. This works but is slow and time consuming, perhaps there is a better way?

For convex curves, I can imagine using a boring head or fly cutter and determining a number of circular cuts that would approximate the internal curve.

In both cases the result is something that will still need some hand work to smooth out or something that is approximate depending on how accurate you need to match a given curve and much fussing you are willing to do to get there.

One could also just make a long list of points to hit in order and do a painfully slow manual CNC job. I imagine that there are / were specialized machines that could do this prior to everything being CNC.

My question is: is there a general solution to this kind of task that's (relatively) efficient or some affordable or relatively easy to make shop fixture that can accomplish this?

If I'm understanding this correctly, you want something that is more precise than just getting it close with various cutters then using a file to clean it up so it's smooth, correct?

For example, I used a ball turner I made to make a ball for the handle on the ball turner. I use a file and sandpaper to clean it up. I wasn't concerned with the exact diameter of the ball, only that it was about 1" and smooth. I'm not sure what I would do if I need a ball that was an exact dimension.

It will be interesting to see what people come up with.

 

[rwm]

When I need a one-off like you describe, I draw it in CAD and then print it on label paper. I stick that to my raw material and use a vertical bandsaw to cut it out. Then finish on sanders including a spindle sander with zirconia abrasive.
Edit: with a 1/2" blade, my bandsaw is limited to about a 5" diameter circle so there's that...

 

[RJSakowski]

I dealt the problem of making complex curves a dozen years ago. As you mentioned, you can approximate nd complex path that can be mathematically defined with a series of straight line cuts, For a CNC mill, this is trivial but manually, it is torture. Another method of manually cutting complex curves would be to cut a series of arcs, each with different radii and/or centers. I first explored this 20 years ago when I needed to cut an Archimedes spiral in some 10mm steel plate with no more than .1mm deviation from the true path. We were looking having this done with a diamond wire saw but the hop was limited in the number of steps that could be taken. Cutting a series of arcs rather than straight significantly reduced the number of steps.

My personal need was to cut a series of complex curves for under cabinet lighting fixtures. The curves were offsets from ellipses which woould be difficult to even describe mathematically. My idea was to mount an x-y table on my RT. The x-y table would be used to precisely move the arc center to the axis of the RT. The mill x and y would determine the radius of the and the RT would determine the start end end of the arc. The math can get hairy really quickly but with a good CAD program, it would be manageable.

Of course, a simpler solution was to buy a CNC mill, which I did. By the way, The crves that I needed to cut were larger than my CNC could machine. I resolved that bu breaking the parts into multiple segments and by making a tooling plate which allowed me to machine the part in multiple setups without losing registration. Something you might consider for your too small CNC.

 

[francist]

I’m not sure which of these best applies: “great minds think alike”, or “enquiring minds want to know”.
But, I’ve been sitting on a similar egg for a few months now but for one of my lathes and not a milling machine.

Essentially I want a faster and more repeatable way to turn a fairly gentle curve on a piece (actually a series of half a dozen pieces, maybe). The best I can come up with so far is a template in the back that would afford the desired shape for a stylus to follow. Said stylus would be attached to the back of my lathe cross slide, so as I advance the cross slide in to take the next cut it would only go as far as the template allowed, thereby creating a loose reference frame for achieving the rough curve shape.

Yes there would still be a lot of hands-on feel to get the thing to work and of course still a lot of traversing as I nibble down to the final shape, but it should limit the risk of overshooting the curve and hopefully take some of the uncertainty of how much to advance at a time.

Alas no drawings other than my typical thumbnail sketch that I knock together on a coffee break, but it’s good enough to get the idea down. Maybe you’ll see where I’m going with it? At any rate, I haven’t had a chance to put it into practise yet, I need to dig out from under some other things first, but it’s still on the list to see if it’ll work. The lathe will be either my 618 in the shop or the Craftsman equivalent upstairs, both of which have flat ways which lend themselves to fitting things fairly easily.



-frank

 

[Ischgl99]

Joe Pie has a video describing how to do something like that with a rotary table. The video shows two convex curves of different radii, but the principle should also work for concave-convex profiles.

 

[RJSakowski]

I’m not sure which of these best applies: “great minds think alike”, or “enquiring minds want to know”.
But, I’ve been sitting on a similar egg for a few months now but for one of my lathes and not a milling machine.

Essentially I want a faster and more repeatable way to turn a fairly gentle curve on a piece (actually a series of half a dozen pieces, maybe). The best I can come up with so far is a template in the back that would afford the desired shape for a stylus to follow. Said stylus would be attached to the back of my lathe cross slide, so as I advance the cross slide in to take the next cut it would only go as far as the template allowed, thereby creating a loose reference frame for achieving the rough curve shape.

Yes there would still be a lot of hands-on feel to get the thing to work and of course still a lot of traversing as I nibble down to the final shape, but it should limit the risk of overshooting the curve and hopefully take some of the uncertainty of how much to advance at a time.

Alas no drawings other than my typical thumbnail sketch that I knock together on a coffee break, but it’s good enough to get the idea down. Maybe you’ll see where I’m going with it? At any rate, I haven’t had a chance to put it into practise yet, I need to dig out from under some other things first, but it’s still on the list to see if it’ll work. The lathe will be either my 618 in the shop or the Craftsman equivalent upstairs, both of which have flat ways which lend themselves to fitting things fairly easily.

View attachment 428045

-frank

Keith Fenner did a You Tube video of turning a 2" ball on the end of a shaft using a stylus to trace a pattern. While he was making a simple ball, the attachment could be used to make any desired shape.