# tips on how to machine a large arc



## dansawyer (Aug 3, 2019)

The project is to add a balancing mechanism to a telescope. The telescope is about 24 inches long and about 10 inches in diameter. My plan is to make 4 aluminum pads about 2 inches long with one side of each pad machined to match the diameter of the scope. The pads will be made from 1/2 x 1 inch aluminum bar stock. They will be mounted by 2 x 5/16 bolts on each pad into the telescope. 
I plan to mount two at each end separated by a an inch. A 1/2 inch hole will be drilled in each pad parallel to the telescope. All together these will support two 1/2 inch rods, about 3 inches apart. 
I can then mount counter weights to the rods and slide them along the rods to balance the scope. 
My lathe is a 9 inch South Bend 9a. Is there any way to machine a 10 inch arc in aluminum?


----------



## markba633csi (Aug 3, 2019)

I think you would need a larger lathe or even have the pieces cut by a CNC machine
Is there a way to rethink the design so that the pieces could be bent to shape rather than machined? Just a thought
Perhaps have rubber strips in between to compensate for any gaps if the bend was not perfect?
Does the telescope already have threaded holes?
Mark
ps a drawing would be helpful


----------



## C-Bag (Aug 3, 2019)

This came up somewhere before so you might want to search the archives. There were some interesting solutions like putting an endmill in the chuck and then rig an arm holding the work to swing off the carriage and pivot at the radius you want.


----------



## derf (Aug 3, 2019)

For your purpose, the arc doesn't need to be an air tight fit. I would lay it out and bandsaw the arc and then finish it up with a drum sander.


----------



## Illinoyance (Aug 3, 2019)

Send me a plate large enough to contain the 4 pads.  I can bore to 10" diameter and send it back for you to cut your parts from the plate.  My 16" lathe should handle it.


----------



## Mitch Alsup (Aug 3, 2019)

The arc you need is neither precision nor necessary.
Cut the blocks and then either:
a) machine some depth into the center of each block
or
b) use a (brass) rod to provide support at the edges of the block.


----------



## dansawyer (Aug 3, 2019)

Illinoyance said:


> Send me a plate large enough to contain the 4 pads.  I can bore to 10" diameter and send it back for you to cut your parts from the plate.  My 16" lathe should handle it.


Thank you for the offer. I am calculating the depth to see of my design will work. I have 3/4 by 1 inch bar stock. I was going to cut that into 2 inch lengths and machine the 3/4 inch side to fit the telescope.


----------



## dansawyer (Aug 3, 2019)

Mitch Alsup said:


> The arc you need is neither precision nor necessary.
> Cut the blocks and then either:
> a) machine some depth into the center of each block
> or
> b) use a (brass) rod to provide support at the edges of the block.


Are you saying that effectively creating an edge on each side is sufficient to hold the blocks in place? That seems like a reasonable assumption.


----------



## cathead (Aug 4, 2019)

It sounds like a job for a rotary table on the mill to me.


----------



## Downunder Bob (Aug 4, 2019)

dansawyer said:


> The project is to add a balancing mechanism to a telescope. The telescope is about 24 inches long and about 10 inches in diameter. My plan is to make 4 aluminum pads about 2 inches long with one side of each pad machined to match the diameter of the scope. The pads will be made from 1/2 x 1 inch aluminum bar stock. They will be mounted by 2 x 5/16 bolts on each pad into the telescope.
> I plan to mount two at each end separated by a an inch. A 1/2 inch hole will be drilled in each pad parallel to the telescope. All together these will support two 1/2 inch rods, about 3 inches apart.
> I can then mount counter weights to the rods and slide them along the rods to balance the scope.
> My lathe is a 9 inch South Bend 9a. Is there any way to machine a 10 inch arc in aluminum?




I suggest you have  a chat with Savarin he has made some pretty large stuff on his 9 x 20 lathe He comes up with some ideas from out left field somewhere, but they seem to work.


----------



## rwm (Aug 4, 2019)

If I understand you correctly, you want to machine an arc in the 1.00 x .750 x 2,00 pad so that it matches the cylinder of the telescope body? I would do this on the bandsaw as suggested previously. A 5" radius arc should be very easy to cut. Clean it up with a drum sander, or sanding drum mounted in the drill press.
Illinoyance idea would work but would require a large expensive plate with a lot of waste. I assume you already have the stock? A similar idea on a larger lathe would be to fix each 2" piece onto the perimeter of a face plate and make the inside cut that way. Given the dimensions of the part you would need a lathe with a swing of about 12+".

Robert


----------



## C-Bag (Aug 4, 2019)

dansawyer said:


> The project is to add a balancing mechanism to a telescope. The telescope is about 24 inches long and about 10 inches in diameter. My plan is to make 4 aluminum pads about 2 inches long with one side of each pad machined to match the diameter of the scope. The pads will be made from 1/2 x 1 inch aluminum bar stock. They will be mounted by 2 x 5/16 bolts on each pad into the telescope.
> I plan to mount two at each end separated by a an inch. A 1/2 inch hole will be drilled in each pad parallel to the telescope. All together these will support two 1/2 inch rods, about 3 inches apart.
> I can then mount counter weights to the rods and slide them along the rods to balance the scope.
> My lathe is a 9 inch South Bend 9a. Is there any way to machine a 10 inch arc in aluminum?


I do custom metal press forming and had to come up with an adjustable radii cutter to cut concave and convex shapes.  Most of what I use is UHMW, but I've also cut aluminum with this as it has a carbide router bit and does a really nice job. You obviously don't want to go through all thisand I don't use it everyday, but when I do it does radii faster and easier than my 9x20 or my RF30 and 10" RT. It can do up to 22" radii.


----------



## Flyinfool (Aug 4, 2019)

Think how well a v-block holds round parts with 2 points of contact, There you go.

If you cut a radius to fit the telescope, if your radii are just a few thou undersized you will still only have 2 lines of contact, if your radii are a few thou oversize you will have a single line of contact and the part will always be rocking and moving around. I'll bet that telescope is not perfectly round which will make it near impossible to match exactly.


----------



## Illinoyance (Aug 4, 2019)

dansawyer said:


> Thank you for the offer. I am calculating the depth to see of my design will work. I have 3/4 by 1 inch bar stock. I was going to cut that into 2 inch lengths and machine the 3/4 inch side to fit the telescope.


Then I would do it on the Bridgeport.  Rotary table, boring head, and Volstro rotary milling attachment are my options.


----------



## savarin (Aug 4, 2019)

Use aluminium channel, the two open sides bolt to telescope tubes with no problems.
You can also use two bits of channel to make a dovetail by using two different sizes that fit inside each other.
Bend the sides of the inside length out and the sides of the outer length inwards so they fit .


----------



## Illinoyance (Aug 4, 2019)

Joe Pie on You Tube showed how to set a boring head to cut a desired diameter without having to bore a hole.  Using his method to set the dividing head I could set it for 5" radius and cut one edge of a block held in the mill vise.  I do have a dividing head large enough.  The alternative is to use my Volstro rotary milling attachment.  My offer is still open.

Your only cost is postage both ways.


----------



## Flyinfool (Aug 5, 2019)

If you are going to make it as V-Blocks, then for a 2 inch long part, I would use a 160° included angle. This will sit nice on the Ø10" tube.


----------



## rwm (Aug 6, 2019)

And you have to post pics!
Robert


----------



## MrWhoopee (Aug 6, 2019)

Decades ago, Fred taught be  how to closely approximate a large radius using a flycutter on a vertical mill by tilting the head of the mill.

Divide the diameter of the cutter by the diameter to be cut = sine of the angle to tilt the head.


----------



## Janderso (Aug 6, 2019)

MrWhoopee said:


> Decades ago, Fred taught be  how to closely approximate a large radius using a flycutter on a vertical mill by tilting the head of the mill.
> 
> Divide the diameter of the cutter by the diameter to be cut = sine of the angle to tilt the head.


Mr. Whoopee, you introduced an idea that sounds revolutionary.
Could you please explain it a bit more for us hobby guys?
I appreciate it.
Thank you.


----------



## Illinoyance (Aug 6, 2019)

Cabinet makers often use a similar technique to cut coves.  The work is fed diagonally across a table saw blade.


----------



## francist (Aug 7, 2019)

Illinoyance said:


> Cabinet makers often use a similar technique to cut coves.  The work is fed diagonally across a table saw blade.



Yes, I've done that a few times although not for many years. Works well. Start with a low exposure to the blade, run a pass, nudge the blade up a sixteenth or so, take another pass, etc, etc. If the heighth increments are kept small the side loading to the blade is manageable. For the fence, a board is clamped to the table at the desired angle and the workpiece run against that.

-frank


----------



## Illinoyance (Aug 7, 2019)

So a 10" cutter mounted in the spindle could do the job in a series of light plunge cuts.
or
 might be simplest to rough out the curve with a band saw and use a sanding drum to finish to the scribed line.


----------



## RJSakowski (Aug 7, 2019)

What is missing from this thread is what machining options are available to the OP.  From the original post, I assumed that no mill was available.

If a mill is available, I would make up a jig consistying of a base plate with a pivot pin  and a clamping system for the part.  The part would be fastened to the jig and the jig rotated into the cutter to cut the arc.  The plate could be a piece of plywood.  Given the lack of precise control, I would use conventional milling to make the cut.

I used this technique many years ago to cut curved slots.  For better control, I attached a lead screw to move the plate through the arc but for the production of a few parts, it wouldn't be necessary.  Just take light cuts to control the forces.


----------



## MrWhoopee (Aug 7, 2019)

Janderso said:


> Mr. Whoopee, you introduced an idea that sounds revolutionary.
> Could you please explain it a bit more for us hobby guys?
> I appreciate it.
> Thank you.



I'm afraid I can't explain the math, it's one of those things that I was taught without an explanation of the underlying calculations. Try to envision the concept. With the spindle perpendicular to the table, the cutter produces a flat surface, which can be considered an arc of infinite radius. With the head rotated to 90 deg. (spindle parallel to table) it will cut an arc equal to that of the cutter. In between, going from parallel to perp, the radius gets progressively larger. I don't believe it generates a true radius, but it's close enough for the girls I date.

I'm trying to get a clarification of the underlying math, will post if I find it.


----------



## Choiliefan (Aug 7, 2019)

^^  Fred who, Colvin?  ^^


----------



## Illinoyance (Aug 7, 2019)

I could hold the parts in the mill vise.  Mount the right angle head to my BP and use a flycutter with a 5" radius.

By now re have probably exhaustedall the ways to make the radius





'


----------



## Flyinfool (Aug 7, 2019)

I have used the tilted head method of making an arc. You are actually making an ellipse. and not a true arc. As long as you are less than 90° of arc needed this will usually be close enough for non precision fits.

I generally lay it out in CAD so that I can draw the actual arc that I need and then play with the ellipse until I find the best (or at least acceptable) fit to the arc. I can then use those numbers to set it up. The drawing will also let me visualize just what the error will look like before I commit to making chips.


----------



## brino (Aug 7, 2019)

Choiliefan said:


> ^^ Fred who, Colvin? ^^



Maybe, Flintstone? 

(Sorry, @MrWhoopee, I just couldn't resist the silly comment......no dis-respect intended)

-brino


----------



## RJSakowski (Aug 7, 2019)

MrWhoopee said:


> I'm afraid I can't explain the math, it's one of those things that I was taught without an explanation of the underlying calculations. Try to envision the concept. With the spindle perpendicular to the table, the cutter produces a flat surface, which can be considered an arc of infinite radius. With the head rotated to 90 deg. (spindle parallel to table) it will cut an arc equal to that of the cutter. In between, going from parallel to perp, the radius gets progressively larger. I don't believe it generates a true radius, but it's close enough for the girls I date.
> 
> I'm trying to get a clarification of the underlying math, will post if I find it.


I believe that the surface cut will be an elliptical cylinder rather than circular and only then if  the cutter has zero width in the direction of the axis of rotation.  For a non zero width, there will be a small flat at the bottom of the cut and anomalies at either extreme.


----------



## MrWhoopee (Aug 7, 2019)

Choiliefan said:


> ^^  Fred who, Colvin?  ^^



Fred was my first and only employer in the trade. He was my mentor, he taught me the trade and the business. He allowed me to promote myself to shop foreman, draftsman, purchasing agent and IT tech. When he no longer required my services, he arranged a loan so that I could take some of his best employees and go into business for myself. More recently, he gave me a nearly complete machine shop so that I could do that which drew me to machining originally, make cool sh*t for myself.

I owe all that I have and all that I am to Fred.


----------



## Choiliefan (Aug 7, 2019)

> Choiliefan said:
> ^^ Fred who, Colvin? ^^


Maybe, Flintstone? 

(Sorry, @MrWhoopee, I just couldn't resist the silly comment......no dis-respect intended)

-brino

Fred Colvin would ALMOST be that old...


----------



## Choiliefan (Aug 7, 2019)

Fred Colvin would ALMOST be that old...  




__





						Fred H. Colvin - Wikipedia
					






					en.wikipedia.org


----------



## savarin (Aug 7, 2019)

I have heard of a diamond disk in a tilted head mill being used to hog out a mirro to the correct radius of curvature.
At the time I thought it was an excellent idea but I've been unable to find it again after 4 days of searching.
I will cheat and swing a diamond disk on a pendulum over the spinning blank.


----------



## Dabbler (Aug 7, 2019)

I wouldn't bother with my rotary table.  I'd figure out my depths using a spreadsheet and using a 1/4" cutter cut the exact depths by stepping down in the centre...  There would only be a small amount to take off with the sander.  Much more accurate that doing the whole thing freehand and only cutting to the layout...

p.s. the lowest point the cut is slightly under .180 (i.e. the centre)

10 minutes in the spreadsheet, and 10 minutes on the mill., and easily 15 minutes on the sander.

-Because a telescope is only 'nominal' dimensions, I'd then use chalk like bluing compound, and hand finish it to fit.


----------

