Turning a cylindrical bar into a semi circular cross section bar?

I have a basic model engine kit of parts and plans and one of the tasks is to machine a cylindrical brass bar down to form a semi circular cross section bar. . This seems quite wasteful but getting over this hurdle can people please give me some advice on how i might achieve this using only a lathe?

The finished item will have a slot cut along its length so I am able to drill the bar across its diameter a couple of times to aid mounting on a face plate and i have a 4 jaw chuck which might help in holding the piece although i have significant doubts about the stability of doing this.

The supplied cylindrical bar stock is only 1/8" longer than the finished piece needs to be.

Many thanks

Depending on length and diameter I would probably face the two ends down to final length to make them square to the cylindrical face and give you two surfaces that can be clamped.

Then you can take off the toolpost and stand the cylinder on end where the toolpost fitted, pack up if needed and then clamp it down with just over half overhanging the edge of the topslide.

You can now flycut the side of the cylinder making a number of passes with the cross slide.

Nearest video I have showing the basic method

4-jaw would have worked if you were removing less than half but not in this case. If you have a vertical slide with vice that is another good option.

If it is very thin, as to be used for decorative capping, you could solder it to a flat bar to hold it, maybe using a very low melting point solder to minimise heat warp.

Also consider using brass screws if you do use screws so that they can be machined into if necessary/in error.

Edited By Bazyle on 03/03/2023 10:21:01

How big is the bar and what size lathe do you have? And what is the part going to be? Something precision or something decorative?

Which model kit is it from? Possibly others on here have built the same engine and can share their experience. Or sharing a picture of the finished part/engine might help too.

Quick piccy may help with suggested setup. Sounds a bit like a crankshaft balance weight to me. You could saw off most of the "waste" material and save it for a rainy day 

Edited By JasonB on 03/03/2023 11:02:23

Thanks for the reply @Hopper, its the 'small brass wobbler' engine parts and plans by 'Geoff Andrews', Ebay purchase. I thought it would make a good starter project.

The piece in question is 1 3/4" long and 5/8" dia round bar and requires machining into a half round bar. It also need a slot 1/8" wide full depth 1 1/4" long centred along the bar length.

I have an Amadeal 210 lathe 8" x 16"

Thank you for your help :0)

Paul

Link to ebay page and images

SMALL BRASS WOBBLER ENGINE MACHINISTS KIT | eBay

Base for the standard

Could still be done as I suggested but it is getting a bit long and slender for that.

I'd be tempted to drill & tap a hole at either end of what will be the slot say M3. Then take a scrap of square material 8 or 10mm will do and drill two 3mm holes through that at the same ctr distance. You ca then screw the bar to the square block, hold in the toolpost and then mill or flycut half the bar away. Finally mill the slot

Edited By JasonB on 03/03/2023 11:22:11

Angle grinder just over half way would leave you 2 bits

In a lathe with a decent 4 jaw and a sharp tool you can face it down and turn the extra into swarf

Edited By Ady1 on 03/03/2023 11:30:29

Thank you for your quick replies, Whilst I do have experience I am coming back to this after nearly 40 years away plus i don't have access to the facilities I had back then.

I was tempted to screw (using brass screws) a block on the side of the 5/8" dia cylinder and clamp it in a 4 jaw chuck to face off, or clamp the same in to the tool post and use an end mill in the lathe chuck to machine it.

I think facing off the cylinder with block on the side in the 4 jaw chuck might be easier to align. What do you think?

Rats! Mad that this might seem I have just realised I don't have any scrap square section bar to bolt it to, I might as well buy some half round bar!

Edited By Paul McDonough on 03/03/2023 11:31:18

Edited By Paul McDonough on 03/03/2023 11:37:51

If I want a really accurate square face I use the lathe and face it down

Make sure its well secured andproperly flat, take your time

Thanks Ady1, yes securing it is my fear, small cuts are in order i guess. I would be happier if the stock bar was a bit longer.

The other question is how are you going to machine the slot in it to attach the upright piece? Clamp it in the toolpost with a milling cutter held in the lathe chuck?

It seems like a needlessly tricky little bit of machining all up for such a simple engine. I might be tempted to use a bit of square brass bar and make life easier. Just mill the slot in it and be done.

Or could you mill the slot in the round bar first, then attach the upright piece to the round bar and hold the upright in the four jaw chuck while machining the round bar down half-round? Can't tell from the pics how that is secured though. Might not be strong enough?

Re the cylinder: That one is only a small flat. Yes holding in the four jaw and facing with a sharp tool will do that one easy enough.

Edited By Hopper on 03/03/2023 12:01:23

Further to Hoppers suggestion of using square bar maybe consider rectangular bar.

5/8" x 3/8" can be got at not too silly prices.

If you get a piece maybe an inch longer than you need the half round, or whatever variant shape you find pleasing, can be done in the lathe. Mount in four jaw with the flat on centre and turn down the other side to half round. I'd consider a flat on top the same width as the slot. File off the corners first to reduce the risk of bending during turning and keep the knock-knock-knock noises down.

Doing the most of the work with only short sections sticking out of the chuck would be wise to reduce stresses. Finish with a couple of very fine cuts full length.

Clive

Edited By Clive Foster on 03/03/2023 13:16:09

I can see me having to buy some off cuts to try this.

The slot is going to be drill and file very slowly. I know not very precsion engineering, the part need to be soldered to the upright so unlikely to show too badly. I am used to taking the hard way. i should have read the advert properly before buying this little kit.

Edited By Paul McDonough on 03/03/2023 14:02:47

If you are doing the slot that way then drill a series of holes first in the round bar at 1/8" except the two at the end which can be tapped M3 or 5ba and drilled tapping size to suit. then screw to a block and hold block in 4 jaw. After turning half away open these holes up to 1/8 and file away.

Do you have any spare round stock to hand as that can be machined to give flat surfaces for screwing to and griping by holding in the chuck

It seems like a needlessly tricky little bit of machining all up for such a simple engine.

From the ebay listing:

“YOU WILL NEED THE USE OF A SMALL LATHE AND MILLING MACHINE AND BE ABLE TO SILVER SOLDER.”

So maybe not such an easy starter project with just a lathe?

I expect these days a lot of people would say you need a mill to make a Minnie traction engine but I managed without one, same for many of the old designs they were all done without mills but a vertical slide did make it easier.

However if the beginner is going to change designs just to simplify things then they won't learn any new skills or techniques

Surprized we have not yet had the comment "back in my day we would have used chisel and finished with a file"

File? That were luxury that were. We would have rubbed it on concrete floor of workshop until it were beautiful and smooth.

On the other hand, an important part of engineering is finding the best/easiest/most efficient/most effective way of doing a job.

But to stick with using the original material to hand and stick to the plan for the sake of learning, I reckon you could grab the round bar in the four jaw chuck and face it down to almost halfway, or close-ish to it. Then drill and file your slot. Then silver solder, or even soft solder, the upright standard into the slot. Then you could grip the standard in the four jaw and face off the last bit from the half round bar to bring the flat down to the centreline. Very fine cuts with a very sharp tool would be required, I should think.

But as the kit's advertisement says it is designed to be done with a milling machine, I think I would compromise if using only a lathe and go with a piece of square bar that is easier to deal with and will look just fine. You could even file the ends to 45 degrees (or rub them on the concrete floor!) to dress it up a bit.

PS, thinking about the file comment more seriously, you could take a lo-tech approach and file it down to size. It is only a small piece of brass and a 10 inch flat bastard file would make short work of it. Face the ends in the lathe to size first, then blue up the bar and mark a line around it at the halfway point by using a piece of flat bar or lathe toolbit etc that is half the height of the bar to guide your scriber with both sat on a flat surface.

Then hold the bar by its ends in the bench vice, with the line just on the jaws. Then file away until the file comes down to the vice jaws. Finish off with a nice smooth 10" single cut file such as mill saw file. Easy peasy and accurate enough for the job the piece does. If you want it dead accurate, you can measure the thickness of the bar as you go at both ends and the middle with a digital caliper.

Edited By Hopper on 04/03/2023 08:05:08