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

[dgfoster]

A friend of mine is a wood turner, and wanted to make special calipers to determine the wall thickness of bowls that he turns. The calipers have two legs each about 10 inches in length, and are comprised of sweeping but non-circular curves along their length. He brought Masonite templates to my shop and asked me to help him make calipers out of 1/8 inch steel plate to match the templates.



We did the usual transfer of the templates to the plate via scribing and then cut the contours roughly out of the plate using a vertical bandsaw faring. The convex portions of the curves is relatively straightforward, using my belt grinder in horizontal mode and portions of the slack belt. But faring the inside curves has been less straightforward since one of the two legs is more or less C-shaped and I cannot get into that leg even with small wheel extensions on my belt grinder. So I’ve been using a die grinder with up to 2 inch stones on it as well as flap wheels and files to attempt to get those curves fair with some patience. We’ve got them pretty good, but it is very difficult to get them to be a completely smoothly transitioning curve so that running your fingers over them feels like there are no local irregularities.

It seems like there should be some flexible material that I could use as a sanding block on those inside curves and mark the curves with sharpie or something similar and use that sanding block to find the high spots and actually gradually just sand them down. The problem is that I don’t know of any good material to use as a back up block that would follow those contours. I am considering contact gluing 80 grit Si-C paper to pieces of metal banding hoping this tool would "find" the high spots. Does anyone have experience with a material used in this fashion or a better technique used to provide faring? Obviously, this is somewhat of an aesthetic rather than a totally functional concern, but it is one that comes up from time to time in the shop and I have not really found a good solution for the problem. Help in this regard would be much appreciated.

The two legs will be rivetted together with a neatly hand-peened rivet and domed polished washers to provide just the right amount of friction so they hold position adequately but are easy to adjust.

He is pickling the legs now in anticipation of providing them with a satin finish.

TIA

Denis

 

[mixdenny]

I would use a 3" drum sander on a drill press. Make a base of wood with a hole large enough for the drum to fit into. They make oscillating drum sanders for wood workers.

 

[Kingbob]

I'd blue it, scribe lines and go at it with a half round file and round files for the inside curves. You should be able to get it smooth enough that all that is needed is a flap wheel after filing. Grab it in a vise with soft jaws or a roll of rags and continually reposition so that you are always able to bear down on it with your body weight. If you are going for beautiful then finish it off with some 400 grit wrapped around a wood dowel.

 

[dgfoster]

Thanks, Mixdenny and Kingbob. I have more or less used both of those methods and they work “ok.” But, contrary to something like the springy flexible pad used by autobody guys on convex surfaces, none of those techniques inherently lower high spots while sparing low spots. That is what I am hoping to learn about. I will be trying a springy steel backing plate for Si-C paper at the next work session when I work on this project.

The other thought that occurs to me is to perhaps 3-D print a couple of 1/2” thick vertical-faced “French curves” to use as backers choosing an area of the curve that approximates the area of steel part to sand that segment

Denis

 

[Kingbob]

Unless you are doing this for fun/skill development, I'd look at send cut send.

 

[dgfoster]

Something like this with the overall dimensions being about 4" by 2" and the knobs to be maybe 1.5 high and serve as grip points. The sand paper is red---I guess it must be ceramic. ;-)




Denis

 

[dgfoster]

Unless you are doing this for fun/skill development, I'd look at send cut send.

This is a for fun endeavor---certainly not for pay. The purpose is to spend some good time with my friend, help him learn a bit about metal working, and finish what is already 95% complete with the pieces already send-cut-sent (in my shop.) I get your point that a drawing could have been made with fair curves and then could have been water jetted or lasered. But a good part of this whole thing is to just have some fun in the shop.

And, while finishing this project I would also like to learn some tips and tricks for fairing inside curves. I have an idea somebody knows some clever approach.

Denis

 

[stephen thomas]

Spring steel, or some of the very thin woodworking scrapers.
I've actually surface ground a scraper to make it more flexible, though my purpose was drilling holes in it, to use as a filing and sanding mask over metallic details in wood so that when the finish was on, the metal parts could be sanded bright and be flush with the surrounding finish. The features were on curved surfaces.

For actual workpieces like you show, i tend to file. But have used the oscillating spindle sander, or stones on an arbor in a DP.

You can also make curved, close fences for whatever rotary abrasive, i've done that for work larger than what is contemplated here. It is still hand-eye-touch coordination, but sometimes adds some stability and control to the process.

If you want to get complex, i've contemplated making a ring fence insert for the oscillating spindle sander, or using grinding bobs in the router table with a ring fence, Work is mounted to patterns for either. I have not wanted to face the mess on those machines to try it yet. & it still requires a somewhat hard/durable pattern to be made fair.



smt

 

[Limy Sami]

For one offs, pin them down to shaped pieces of wood you can grip in a vise and files & emery cloth, or a slack of belt grinder.

 

[Metalurgent]

Dynafile.

 

[Metalurgent]

On the Mead belt sanders and clones , the belt can be routed to grind inside curves; the platen modified to match the work .

 

[boslab]

Inside, half round file and draw file
Outside flat file and draw file.
Round dowel Emery cloth, draw file action
Flat strip and Emery on the outside.
Finish round slip and flat slip stone, geshwein is ideal
No machines needed
Mark

 

[dgfoster]

On the Mead belt sanders and clones , the belt can be routed to grind inside curves; the platen modified to match the work .

That sounds interesting. I did a little searching on Mead belt grinders and did not find one with this feature. Would you mind providing a link or suggesting a model name etc. to look for? It sounds to me like the belt would be run over rollers that would bring the front and rear of the loop fairly close to each other. To get inside an opening the loop would be dismounted and threaded through the opening in the work piece. Then the belt would be mounted on the upper and lower pulleys?

Denis

 

[GregSY]

Oscillating spindle sander. Your woodworking friend probably has one.

 

[dgfoster]

Inside, half round file and draw file
Outside flat file and draw file.
Round dowel Emery cloth, draw file action
Flat strip and Emery on the outside.
Finish round slip and flat slip stone, geshwein is ideal
No machines needed
Mark

I'll admit to sort of forgetting about draw filing. I think it would be a very good way to deal with the inside curves and, if I did not have a good belt grinder for the outside curves, would be a good way to go there as well. Unfortunately, I do not have the partially-finished parts at my shop right now. So, I can not try those techniques until the parts return. The advantage of draw filing in addition to more rapid stock removal than cross filling would be that I think the file would be providing perceptible tactile evidence of unfair segments of the curve as the file passes over it.

I did print up a couple of the "French curve" sanding blocks, I figure that best-fitting them to the curve that has been blacked with Sharpie pen will also help find high spots for sanding or filing.

Overall, my biggest concern with this has not been having powerful enough stock removal methods so much as applying stock removal methods to the right areas to provide a fair curve.

Denis

 

[memphisjed]

Draw or scribe a fair curve on the inside. Freehand to cad trace to drafting spline/ whatever you like.
Draw filing with normal file stroke (like 45 x/y) with half round. In less curve sections the file should lean more parred to work peice, effectively making a larger curve file.
More filing motion in high spots than draw motion.

The drawing on the part is key. Every spot looks like a high spot with file in your hand.

 

[jtaylor]

You need a profile sander:

 

[dgfoster]

You need a profile sander:

Since the software glitch beginning a few days ago, myself and a fair number of other folks are unable to se embedded media. Could you please just post the URL without using the media embedding feature? Thx.

I guess IT is fixing the problem???

Denis

 

[dgfoster]

Well, out of the blue the link is visible. Thank you IT.

Yes, the profiler would get into the inside of the “C” piece pretty well. So it would remove material rapidly. And, with some practice, I might be able to get the curves fair. That’s the tricky part.

Denis

 

[EPAIII]

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