Cutting Radii

[rmac]

How would everyone round off the ends of a part like this:

Milling with a rotary table comes to mind, but it's a pain to set up.
Artful grinding and/or filing by hand comes to mind, but it's hard to get perfect.

Are there other ways that might work?

-- Russell Mac

[pete]

If the part is short enough or a bit less than the height of your vise jaws you can use a loose fitting pin in the hole, clamp the part as normal in the vise with the top edge of the hole down firm on the pin and the pin down on the tops of your vise jaws, make a cut across the top to your target dimension, rotate a fraction (5-10 degrees) and clamp the part on as many cuts as you feel necessary and since the parts rotating each time on that pin, the radius generated will be tangent to that through hole. A few strokes with a file will then turn that series of very short flats into a constant radius. It's obviously a slow method and mostly good for one off parts. If these are parts for your reels? Then it's too slow to work. Better would be reamed to size holes, a shop built bolt with an unthreaded area that fits the hole, stack up the parts and use that custom bolt to clamp them together. Make sure there all parallel in a stack, set in the vise in an exact vertical orientation and use a corner rounding end mill that matches your radius. If this will be a repeat job with those multiple parts? Record your dro numbers in Z and X once you get a fully formed radius without any uncut spots on the radius or over cut areas on the part sides. Corner rounding end mills are touchy about there exact Z height to the part and infeed on X to get that fully formed radius. I'd still expect to do some very light hand finishing just to blend all the surfaces in and remove any tool marks though.

If I was a jerk and wanted to brag, I'd probably use that Volstro Rotary Head Rich was kind enough to sell me, so I won't mention that.

[liveaboard]

I did it with a grinder before I got my mill.
I put a little 6mm post on my bench grinder rest, that works sort of.
I etched and cut with a handheld angle grinder, actually came out quite ok for thin material.


Here's a couple of linkages like your illustration I made recently on the mill. 60 x 20 mm with 30mm bore.
I made a little helper jig that fits on my rotary table, with a center to fit the linkage eye into.

[Harold_V]

Unless there's need for high precision, I use a belt sander for rounding. Mine is one of the commonly found 6" X 48" models. You'd be amazed at the high degree of precision one can achieve. A set of radius gauges helps when necessary. A cosmetic rounding with a rotab is just too time consuming to make it worth the effort.

If one is doing really small work, it's easy to have the piece dragged through the gap between the belt and table. To avoid that problem, I use a thin sheet of metal on top of the table surface, which I keep close to the belt. Works very well and prevents the part from being damaged.

H

[Technical Ted]

Does your mill have a DRO? My DROs have a function to plot moves so you can plunge cut a radius. Works pretty well. If you select very small incremental steps and use a large end mill the radius turns out very smooth.

Ted

[Bill Shields]

Amen on the belt sander

I make buttons with pilots that allow me to scribe the desired radius.

Done it 100's of times..

[liveaboard]

Would you guys use a belt sander for heavy work like my 20mm thick linkages?

[rmac]

If the part is short enough or a bit less than the height of your vise jaws you can use a loose fitting pin in the hole, clamp the part as normal in the vise with the top edge of the hole down firm on the pin and the pin down on the tops of your vise jaws, make a cut across the top to your target dimension, rotate a fraction (5-10 degrees) and clamp the part on as many cuts as you feel necessary and since the parts rotating each time on that pin, the radius generated will be tangent to that through hole. A few strokes with a file will then turn that series of very short flats into a constant radius. It's obviously a slow method and mostly good for one off parts.

I like this idea. Right now I'm worried about a one-off part that's small enough for this to work. I guess even for somewhat larger parts, you could rest the pin on spacers set on top of the vise jaws.

If these are parts for your reels? Then it's too slow to work.

Nope, RSG is the reel guy. I'm the hot air engine guy.

I did it with a grinder before I got my mill. I put a little 6mm post on my bench grinder rest, that works sort of.

I guess the post idea would also work with a sander. One of Joe Pie's videos even shows him doing something similar with a milling machine. That seemed a little scary to me, though. One accidental bit of climb cutting would make the cutter dig in, try to spin the part around, and probably result in a disaster of some degree.

A cosmetic rounding with a rotab is just too time consuming to make it worth the effort.

Indeed.

Does your mill have a DRO? My DROs have a function to plot moves so you can plunge cut a radius. Works pretty well. If you select very small incremental steps and use a large end mill the radius turns out very smooth.

Kind of "CNC-lite", I guess. No DRO here, but I do have a calculator, dials on the table, and a lot of patience. This general idea sounds interesting for all kinds of things, not just radii.

Thanks, everyone.

-- Russell Mac

[pete]

Lol, busted but good. Sorry and my apology's Russ, I didn't pay enough attention to the posters name to start with. A longer part well above the vise jaws with the pin resting on spacers will obviously work. Chatter is going to be an issue unless you add more clamps for the spacers on either side of the part and hold downs clamping the spacers down to the vise top. If the parts long enough to give you good leverage, then it can be pivoted on a well fitting pin by hand with LIGHT cuts of a few thou against the side of an end mill. Climb cutting while doing so would be a mistake you'd only make once though. George Thomas in one of his books made a 4" diameter hand turned with a lever non geared rotary table for doing work exactly like this. Setting up a couple of hard stops so you can't rotate the part too far and cut into the part sides would also be a good idea.

[Steggy]

I do those sort of things in my belt sander. I lay out the radius with a compass and bluing and grind to the scribed radius. It is very rare that the end radii of a part such as was illustrated are critical.

[Harold_V]

Would you guys use a belt sander for heavy work like my 20mm thick linkages?

Yes, I would, although I'd also get my saw involved, removing excess material. Unless you have a tight tolerance in the radius, you can sand it faster than you can make a setup with a rotab. Might not be as fast for multiple parts, especially if you have to remove much material. When you have the option, sawing very near the line is advisable. Removing excess material by saw is usually faster, regardless of how you do the finishing.

When you have material to move on a large surface (thick material), a coarse belt (60 grit or even coarser) is in order. Belts move metal quite fast if you have enough power on the sander.

I contracted (from Univac) to make some thin timing cams with .005" tolerance on the profile (think French curve). They were sanded to profile (after a proper layout) and accepted by their QC department. I make mention because sanding is very much an acceptable way to do fine work (when necessary). Good eyes help.

H

[Technical Ted]

Does your mill have a DRO? My DROs have a function to plot moves so you can plunge cut a radius. Works pretty well. If you select very small incremental steps and use a large end mill the radius turns out very smooth.

Kind of "CNC-lite", I guess. No DRO here, but I do have a calculator, dials on the table, and a lot of patience. This general idea sounds interesting for all kinds of things, not just radii.

Thanks, everyone.

-- Russell Mac

You use Fusion 360 so you could use that to help with plotting the X/Y moves... no need for a calculator.

Ted