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Can You Help Me With Fairing an Inside Non-Circular Curve in 1/8" Soft steel Plate?

The only way inherent in process I can think of is a blanishing on fat curved stake. Using wood or nylon hammer. Sand the lips back to flush as you go. This would also open the curves some in areas you do not want opened, counter is on same stake you can open low areas by walking them dead center over stake.
I think drawing and removal is easiest given the parts you have.
 
I use sanding sponges for a number of apps.
They would not be useful for shaping steel or other hard metals, but can be good sometimes for breaking edges and preliminary polishing.
Flat papers/fabrics with a hard backer (whether flexible or shaped) are necessary for shaping. Though again, i prefer files.
Yes, in some ways the foam-backed abrasives are the opposite of what I think would help me in fairing since the foam tends to cause the abrasive face to be pressed against high spots in the curve and and low areas more or less equally. That may be good for putting the final bright finish on all surfaces of the caliper legs. I am not sure what my friend may prefer in terms of just how shiny he wants the parts to be. I have shown him some Starrett calipers as my preference with their "satin" finish as a good workman-like look. Taking them to a higher polish would not be that difficult, but just is a bit too fussy (actually cheap looking IMHO) for an item intended to be used in a wood-turning environment.

For such a simple project there has been a fair bit of learning associated with it.

Denis

Personal Aside: Grampa (that would be me) is making his first knife (I've made lots of chisels, gouges, and special cutting tools in the past) along with my very capable-learner 10-yr-old grandson. That has also been a big learning curve and also hugely rewarding to be able to see the look of amazement on his face as we took annealed 1095 and hardened and tempered it hitting our goal of 58 Rc, It was priceless to see the ear-to-ear smile on his face as the shiny ground metal started emerging as facets on the rough-ground black faces after heat treat. Taking a small scrap of 1095 and hardening it without tempering it and snapping it in a vice really brought home to him that this business about hardening and tempering is "real" and almost magical. He's coming over this afternoon to work on fairing the curves on that blade and to make a guard. Both of these projects are very rewarding.
 
Hi dgfoster:
How fair do you need it?
I ask because there are several ways to approach this...each has a different set of challenges.
If you need it to be "eye sweet" and "hand fair" you can just sight along the edges pick off the high spots with a Sharpie and then just draw file it with a flat file if the curves are large and with a half round file if they are small.
Ideally the curve on the profile of the file is larger than the radius of the bit you're filing...if it's smaller it's much easier to put ripples or gouges into your surface.
It's a learned art...something like learning to use a spokeshave, and you can get very nice results pretty easily once you master it, but the secret to success is to learn to feel the high spots with an exploratory file stroke and then to concentrate there without dwelling so long as to gouge the part.

A flat file, even for a concave curve (counterintuitive I know) helps you, but you do need to develop the finesse to make it work.

It's much harder to use a rotary tool to get a fair curve...if you try to do this with something like a die grinder it will be lumpy even if you are very careful.

A drum sander or a spindle sander is better but only if you use the very largest drum you can stuff into the curve...a skilled woodworker with a file can still do far better and it's because woodworkers need to get good at fairing things in a way that metalworkers mostly don't learn in the modern era.

Another way forward is to use a flexible spring steel strip with PSA abrasive cloth on it...you can get it very fair that way because the spring steel backing will follow the curve without following the smaller dips.
You can buy what you need from McMaster Carr and it's not very expensive.

The car body guy downstairs has a whole collection of spring steel strips in various weights for just this purpose, and his car bodies are just about perfect, but he's been doing this his whole life so he's very very good at picking up the subtle areas where it's not yet perfect and addressing them without trashing everything else.

The one and only pinstriper I've ever seen working has a similar skill...what that man can do with nothing more than a paintbrush and a bit of tape is astonishing, but he's been at it for decades too.

Next in complexity is to make a convex template that you can fair more easily and then blue in your concave curve using the template.
Double the work because you have to make both

Finally, you can go high tech and nibble out the profiles on the CNC mill, but that's cheating.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
Hi dgfoster:
How fair do you need it?
I ask because there are several ways to approach this...each has a different set of challenges.
If you need it to be "eye sweet" and "hand fair" you can just sight along the edges pick off the high spots with a Sharpie and then just draw file it with a flat file if the curves are large and with a half round file if they are small.
Ideally the curve on the profile of the file is larger than the radius of the bit you're filing...if it's smaller it's much easier to put ripples or gouges into your surface.
It's a learned art...something like learning to use a spokeshave, and you can get very nice results pretty easily once you master it, but the secret to success is to learn to feel the high spots with an exploratory file stroke and then to concentrate there without dwelling so long as to gouge the part.

A flat file, even for a concave curve (counterintuitive I know) helps you, but you do need to develop the finesse to make it work.

It's much harder to use a rotary tool to get a fair curve...if you try to do this with something like a die grinder it will be lumpy even if you are very careful.

A drum sander or a spindle sander is better but only if you use the very largest drum you can stuff into the curve...a skilled woodworker with a file can still do far better and it's because woodworkers need to get good at fairing things in a way that metalworkers mostly don't learn in the modern era.

Another way forward is to use a flexible spring steel strip with PSA abrasive cloth on it...you can get it very fair that way because the spring steel backing will follow the curve without following the smaller dips.
You can buy what you need from McMaster Carr and it's not very expensive.

The car body guy downstairs has a whole collection of spring steel strips in various weights for just this purpose, and his car bodies are just about perfect, but he's been doing this his whole life so he's very very good at picking up the subtle areas where it's not yet perfect and addressing them without trashing everything else.

The one and only pinstriper I've ever seen working has a similar skill...what that man can do with nothing more than a paintbrush and a bit of tape is astonishing, but he's been at it for decades too.

Next in complexity is to make a convex template that you can fair more easily and then blue in your concave curve using the template.
Double the work because you have to make both

Finally, you can go high tech and nibble out the profiles on the CNC mill, but that's cheating.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
Thanks for the nice synopsis, Marcus.

This device only has to be sweet to the hand and eye, and certainly a high-tech perfect fairness is not required from a practical point of view. We are taking it further than it needs to be taken by a longshot. But then pride gets involved and wanting to learn some new techniques is a factor as well.

Along the lines of using flexible sheet, I mentioned above that I was, considering using steel banding as back up for silicon carbide paper. And indeed I did get some banding and have attached silicon carbide paper to it using contact cement. Because the banding itself is not truly spring steel and is fairly lightweight. I found that making “Leaf springs” like you see, and various cars and trucks to be way to make varying stiffness backers. To make the leaf springs, I simply silver soldered one end of a stack of two or three leaving the other end free to move as the radius of the backer changes. This seems to be a pretty good work-around, though true spring steel would be better.

Here’s a pic of a couple of the stacks with bits of paper between the leaves to make it easier to see the individual leaves.
IMG_0757.jpeg
 
My friend came by the shop this morning and we finished up his wood-turning calipers. He had completed the fairing of the curves to his satisfaction. So we focused on pinning the two parts together and stamping them with initials and increment numbers.

You cans see that I made a couple of polished domed washers and then used some 12L14 dowel to pin the legs together. He did the peening and made a nice dome on each side and got the tension just right to his hand. I did not get to use the special tools for fairing on this project. But, we both learned a fair bit for future reference. And he does have a truly handsome tool. (I really was not eager to put my initials on the tool as it really was his creation. But he insisted.) He did the initials as well and was pleased to find out how easy stamps are to use and how nicely they make a permanent mark.

1711050491966.png

1711050445723.png

Thanks to all for the ideas and help.

Denis
 
Several major makers use do make special factory modified bandsaws. Instead of a blade they use a chain of files riveted together into a band.
BilL D
 








 
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