Machining a concave radius on the end of a round bar

I have a need to machine a concave shape (with a radius of 11mm) on the end of a 12mm dia brass bar, ie at 90deg to the bar's axis, not IN the end, and am not sure of the best way of doing so.

If I had a boring head, I imagine it would be relatively easy to rig something on my mill but as I don't, I'm trying to figure out if it's possible on a lathe.

I've come up with the following idea...

1) Fasten a sacrificial plate (wood would appear to adequate) to the face plate

2) Using the crossslide (with the spindle locked), cut a horizontal groove in the sacrificial plate at centre height.

3) Clamp the bar in the groove (this ensures that the bar is aligned with the centre line of the lathe

4) Using a boring bar in the toolholder, cut the radius.

Simples.... as per the diagram (clamps omitted for the sake of clarity)

The question is whether there is any fundamental problem with this solution and/or am I over-thinking it & the there is a better/simpler way of doing it?

Brian

Don't forget to balance it

It would work but a simpler way may be to mount your brass bar in the toolpost at ctr height which saves making a holder and then use a simple boring bar in the lathe chuck to cut the radius. Boring bar only needs to be a bit of steel bar drilled through for a cutting bit and cross drilled for a grub screw.

J

Why not use a 22mm bullnose cutter?

As the 11mm radius cut is a straight cut thru/across the end of the bar, any 22mm dia cutter could be used. Either plunge cut or come in from the end on the side of the cutter on the mill.

Or clamp it to a face plate on a lathe and use a standard boring tool. If you do not have a face plate, make a jig to hold your bar in the lathe chuck.

Paul.

Easy job on the mill with a boring head or tiny fly cutter style boring bar. Pick up the edge of a block or your vice jaw, move away 11mm and set the boring tool in the fly cutter thing until it contacts the block/jaw and tighten the tool.

Yes, absolutely you can do that. I did something similar when making the fabricated versatile dividing head, carving a radius notch out of one piece of round bar to fit it up to the adjoining round bar. Round bar but the same principle you are imagining: the job rotates off centre around a boring bar that is set to the desired radius you want to cut.

Trickiest bit was setting the job and the boring bar at the right position. I used a piece of the required radius held in the tailstock and set the marked up work to match it, then bored to the marked up line.

Instead of using a piece of sacrificial wood, i used a piece of sacrificial paper and proceeded with caution. Ended up losing my bottle at the end and left a few thou unmachined and removed it with a file afterwards!

Edited By Hopper on 07/02/2017 09:22:57

Edited By Hopper on 07/02/2017 09:28:19

I have used the same technique as Hopper showed many times. One difficulty that prevails is how to measure the actual radius?

Trial and error with the intended mating part is one option but the part might not be amenable to offering up to the lathe. Doing a test bore on some scrap material is better as one can 'calibrate' the DRO or feedscrew index wheel.

My preferred method is to mount a second dummy 'workpiece' opposite the part being machined so that one has diametrically opposed concave surfaces which can them be measured withe an internal mic or caliper.

Ian P

"If I had a boring head" whoops did not read the post properly again ! Covered myself with suggesting a simple fly bar type tool set to the radius from a known edge of a block/vice jaw etc.

Sounds like a very expensive method for possibly a one time use of the cutter when other simple, cheaper options have been suggested. But, I suppose it is an option.

Dave
The Emerald Isle

My tailstock piece in the last pic (added a bit later) worked OK for not so accurate purposes. For more accurate I suppose you could bore a hole in a scrap piece to the exact size to match the desired radius and then set the cross slide to zero so you can machine to that exact point again. Then set up the job on the faceplate and position it so the boring bar, still in the finished size position, matches where you want the radius to be located on the job.

I should have used the word 'sacrificial' rather than 'dummy'. Since in all probability a counterweight would be used it could be positioned so that the boring bar took a shave off it as the correct diameter (radius) was reached. Then would then be 'bore' to measure.

Ian P

Gents, thanks for all of the comments, and apologies for not replying earlier; work (and life in general) got in the way!

Roy - reminder about balancing duly noted, although I had thought of that. I just didn't mention it for the sake of brevity.

Jason - I had thought of mounting the bar in the toolpost with a boring bar in the lathe chuck but unless I'm missing something, wouldn't the tool bit would have to be adjustable to the correct radius? The only one i have is one with an indexable tip & I don't the equipment, knowledge or experience to grind a tool.

Regarding the comments about using a 22mm cutter, it seems a bit of an expensive option as steamdave pointed out. Being the cheapskate that I am, it's an option I discounted.

I don't when I'll get an opportunity to try it but I'll report back on the results.

Brian

I'm going to do a similar job, hoping to use a sort -of fly cutter and mount the job on the cross slide. Adjust the radius of the cutter using a caliper and a bit of maths. put the cut on using the cross-slide. Well that's my hope.

For anyone interested, this is the result of my endeavours. Everything worked out more or less as planned, with the exception that I centralised the face plate under the mill in order to cut the radial groove on the backing piece.
Having said that, I used the same technique to cut a radius on a larger diameter bar but hit a bit of a snag when I found that the clamping bar now hit the top-slide when the faceplate rotated. Easily overcome by swinging the faceplate back & forth whilst nudging the carriage along like a shaper. It took a while!

I do not know what accuracy is required for the radius but I have used a hole saw in the drill press and the piece mounted in the drill vice to do something similar. Very quick, cheap and straightforward.

Mike

I have machined similar parts clamped to a faceplate when creating concave surfaces that have to closely fit a cylindrical part. One of the difficulties is accurately knowing what the radius is. Whilst its easy to measure the mating part its much harder to measure a short segment of a concave radius.

I have turned concave radii to sit against parts varying in diameter from about 20mm to 200mm and found the simplest and most accurate way of measuring the workpiece is to clamp a sacrificial bit of scrap on the opposite side of the faceplate, positioned so that at the cutter machines two concave surfaces each rotation. The job then turns into a simple boring type operation as knowing the radius is just a case of measuring the internal 'diameter'

Ian P

I also dont know what this part is for, but I reckon I could have filed that brass concave surface in less than 5 minutes very accurately.

Cheers,

Julian

You might be able to file that accurately but I know I can't, which is why I asked the original question of how to machine the radius. As such, your comment is completely irrelevant.

Brian