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Machining a slight radius in gauge block

crossthread

Titanium
Joined
Aug 5, 2004
Location
Richmond,VA,USA
This is kind of an oddball project that has been handed to me. Material is 1/4" AL. One inch wide by six inches long. It is to be used as a caul for bridge work. If the caul was sitting on a surface plate the center would be 1/8" up from the plate. I have no idea what the actual radius is and it would not do me any good to know anyway because I can't think of any way to machine this. I would need a fly cutter with a ten foot radius or something like that. I thought of making sure all sides are parallel and then putting it upright in a press. At some point it is going to bend one way or the other. I could set up an indicator and see how much over-bend it takes for it to spring back to what is needed. I then thought to make it from steel and heat it on one side causing it to bow slightly. I know this sounds simple but it has to be quite accurate. Any ideas? Thanks.
 
This sounds like a real easy project for a cnc? Maybe sub it out?

If you can provide a couple heavy paper (pasteboard?) profiles that approximate the curve you need, and clearly sketch the orientation onto the Al plate, I'm sure someone here can help you out. Even me if you get desperate, but scheduling on my machines is a little up in the air at the moment so it's difficult to commit.
 
Draw it up in cad, and layout a step over grid to create the radius.

I would use a known diameter with enough reach to go across the face of your workpiece.

May be step over .100 and adjusting the second axis whatever is required. Then simply hand work the tiny peaks you create with a sanding block or a stone.

I don't think bending is going to be accurate enough to make a known radius.
 
How accurate does it need to be? Cauls are not usually all that fussy. What kind of bridge?

Being made of aluminum, aka shiny wood, woodworking techniques may suffice.
 
not sure if i understand correctly, but i dont think the radius would that large. my guess is 2 feet (too lazy to look it up). so you could maybe set it up on a mill on a sturdy arm, especially if its aluminum.
 
A lot of DROs have the radus function. It gives the coordinates for each cut to create a radius.

Just a thought: could you just bend the strip to get the radius?
 
I have no idea what the actual radius is and it would not do me any good to know anyway because I can't think of any way to machine this.

This is backwards thinking.
You certainly cant machine it if you dont know the radius, and you said it needs to be accurate.
Bending it and guessing is not accurate.

Draw it in CAD, determine the radius.
If you have CNC, this shouldnt be difficult to machine
I assume you dont, so you have to rough it out, and then swing a cutter or the work in an arc. You're lucky its aluminum, as you can do it with a router fixed to a pivot point.
Not really hard if you put some thought effort into it.

Here is a chord length calculator, I input your approximate dimensions and get a 36" radius

Circle - Radius from chord length and arc height

Also there are several youtube videos on how to mill large radii
 
If you can provide a couple heavy paper (pasteboard?) profiles that approximate the curve you need, and clearly sketch the orientation onto the Al plate, I'm sure someone here can help you out. Even me if you get desperate, but scheduling on my machines is a little up in the air at the moment so it's difficult to commit.

Im on the same page as this. I would like to help and my mill doesn’t turn on much anymore. (All mill turn stuff these days) but I just don’t want to commit and not have a chance to do it it.

If you can’t find anyone let let me know and I’ll try to help you out.
 
OK, step one is to find the radius. Your caul is six inches long and it should stand up 1/8" at the center. In geometry language, you have an arc that is six inches long and it's sagitta is 1/8". First, a bit of a simplification: this is a very shallow curve so the length of the arc and of the sagitta are just about equal or close enough for this situation.

From Wikipedia

Sagitta (geometry) - Wikipedia

the formula for the radius, given the arc length and the sagitta, is given by:

attachment.php


So your radius is 36 1/16". Your ten foot radius was a bit over so it is easier to do in what is probably an available space.

As for how to make it, I would be tempted to use wood working tools and techniques. Clear off a work bench or put a sheet of plywood on some saw horses and use a sacrificial wood piece under your aluminum. I leave how it is fastened down to you, but a couple of screws in countersunk holes would be my choice.

A 36 1/16" radius arm can be used to swing the router in the needed arc. Here is one way of doing this:

How to Rout a Large Arc | Popular Woodworking Magazine

There are numerous others, just Google "cut large radius with a router". Use a new carbide bit in the router and some WD-40 for cutting fluid.

WEAR GOGGLES! I would also wear a shop apron as the chips are going to fly.


Note: I can't believe that the word "sagitta" was not in my computer spell check dictionary. But then neither was "caul".



This is kind of an oddball project that has been handed to me. Material is 1/4" AL. One inch wide by six inches long. It is to be used as a caul for bridge work. If the caul was sitting on a surface plate the center would be 1/8" up from the plate. I have no idea what the actual radius is and it would not do me any good to know anyway because I can't think of any way to machine this. I would need a fly cutter with a ten foot radius or something like that. I thought of making sure all sides are parallel and then putting it upright in a press. At some point it is going to bend one way or the other. I could set up an indicator and see how much over-bend it takes for it to spring back to what is needed. I then thought to make it from steel and heat it on one side causing it to bow slightly. I know this sounds simple but it has to be quite accurate. Any ideas? Thanks.
 
This is kind of an oddball project that has been handed to me. Material is 1/4" AL. One inch wide by six inches long. It is to be used as a caul for bridge work. If the caul was sitting on a surface plate the center would be 1/8" up from the plate. I have no idea what the actual radius is and it would not do me any good to know anyway because I can't think of any way to machine this. I would need a fly cutter with a ten foot radius or something like that. I thought of making sure all sides are parallel and then putting it upright in a press. At some point it is going to bend one way or the other. I could set up an indicator and see how much over-bend it takes for it to spring back to what is needed. I then thought to make it from steel and heat it on one side causing it to bow slightly. I know this sounds simple but it has to be quite accurate. Any ideas? Thanks.

Not sure if I understood the problem correctly, but I was thinking "ball turning attachment in reverse" should work for cutting large radii in aluminum on a mill. I am attaching a sketch with the concept. If the radius ends up being longer than what can be attached to the table, then the point of rotation needs to be moved further like by clamping another flat bar.

radius.jpg
 
"Note: I can't believe that the word "sagitta" was not in my computer spell check dictionary. But then neither was "caul"."

Some years back, my kids had an' app' on their phones that was a word game....it would basically give you 6 letters and you had to see how many words you could spell using all or some of them. The stupidity of the game was it failed to recognize real words as being real words ('hale' was one that comes to mind) but it would accept words that weren't words. No wonder people are getting more dumberer.
 
Thanks for all the replies. No I do not have CNC capabilities. Thanks very much for the offer to help Milland but I think I can wing this. Being semi retired gives me the time I need and it's kind of a labor of love. I have made a lot of tools over the years for my oldest son who has been a luthier for over twenty five years and this would be for one of his special applications. I agree that since this is for wood, then woodworking techniques may suffice (router on a string etc.). As far as how accurate it needs to be, I am not sure since I am not the person who is going to use it. The guitar is worth about twenty thousand and needs to be right since he is an authorized technician for Gibson and Martin. It is the application of heat and pressure that will take the wonky two directional bow out of the bridge area. Again, thank you all for the replies and ideas.
 
Here is an online calculator - 500 Internal Server Error

Which way is the curve - across the length or width? I get 36.062 and 1.062 repectively

For 36", you could make a cradle that pivots and just take very light cuts feeding by hand. Could be on mill, or router table. Maybe rough out on bandsaw first. I'd add some weight to it for stability. You could also swing a router on a pivot arm.

What is the guitar, sounds interesting?
 
How about tilting the mill head to produce a curved cut using a large flycutter or face mill? Surely there's someone here with the calculations to set this up. It produces an ellipse rather than a true radius, but if the 1/8 is important and the radius is not, seems like a perfect solution.
 








 
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