Turning between Centers No Dog?

I am planning to return work on my 8 Cylinder Radial Engine. I have been gifted some very nice brass (three times what i need), and hope to use it as cylinders for the engine. I will be using UHMW for the pistons to reduce wear on the brass.

Brass being pricey as it is, I was hoping to make efficient use of it without wasting.

My plan right now would be to cut of a length just over 1.25" from the 1.5" dia bar. Then face both ends to the final length, and drill the 1/2" bore through the center. Then I plan to grab some cheap steel stock (is 1060 cheap? What should I buy from here?), put a 60 degree point (simple dead center) and just suspend the brass between the home made dead center in the chuck, and live center in the tail stock. I would then turn the preferred profile of the cylinder into the brass.

My question is, would it be a problem to have the only driving power be exerted from the dead center? (seeing as brass is soft and easy to cut the friction between the dead center and brass could be suffice)

I had thought of a possible way to make the cylinders if I were to use the 3 jaw chuck to hold the material, and that would be to have a length of brass twice the length for one cylinder, profile the first cylinder, then rotate, and be careful when clamping to line up the flat teeth of the jaws onto the cylinder fins so that they don't gouge, and then turn the other cylinder, and part in between. Worried that if there was a catastrophic problem with the tool got jammed, the cylinder set could get twisted, and the chuck would mangle the first cylinder.

Pictures soon to describe better.



Ian

I doubt very much you will be able to drive it with just a centre. Why not make up a simple expanding mandrel that way all cylinders can be held in exactly the same position.

J

If the bore through is pretty good I'd just use a regular 1/2 mandrel. They're tapered from .499 to .504 or so and usually get a pretty good grip on the bore. Failing that an expanding collet or mandrel might be used.

My lathe is a 7x10. I believe it has a MT3 Taper in the spindle. I assume I just remove the chuck, and then slap the mandrel into the spindle bore? How would I remove a mandrel or dead center from the spindle? Would I have to remove the gear cover and use some stock to knock it out like an arbor from a drill chuck?

Thanks

Ian

Ian, I keep a 12" piece of broomstick near the headstock of my lathe for removing centers. Just a light tap, and a rag in my right hand to catch the center.

Chuck

Thanks everyone, I'll look into the mandrels and see what will be best. I'll make sure to make extra cylinders so if I botch one I dont need to remount and dismount the chuck.

Ian

Update: I searched up some mandrels, but I am having trouble understanding how it is mounted to the lathe. Mandrels are just bars that have a slight taper to them, but how do I attach it to my lathe? Aren't they supposed to have a Morse Taper on them or some other custom holder?


Ian

Mandrels are usually center drilled on the ends, you mount them between centers and use a drive dog on the one end. It looks as though you have found tapered mandrels, where expanding mandrels may be what you are really looking for. Either way, they both mount the same way. You could also place one end in a three jaw chuck and support the other end with a live center, but I would support it on centers on both ends if possible, as that method would be far more accurate.

terrywerm said:

Mandrels are usually center drilled on the ends, you mount them between centers and use a drive dog on the one end. It looks as though you have found tapered mandrels, where expanding mandrels may be what you are really looking for. Either way, they both mount the same way. You could also place one end in a three jaw chuck and support the other end with a live center, but I would support it on centers on both ends if possible, as that method would be far more accurate.

Click to expand...

It all makes sense.

Thank you

Ian

Ian, I am not sure where you are located, or what sources of tooling are available to you, but this catalog page will show you several types of mandrels, both tapered and expanding. Link here

They have one type of expanding mandrel that is shorter, and is meant to be used with a three jaw chuck, and with no additional support on the end. You might find that type useful.

No affiliation with them, just an occasional customer, usually satisfied with my purchases there.

Here's an idea I used with a similar problem:



The small screw or bar is driven by the lathe jaws into which is mounted a dead center. Based on your drawing it looks as if it could be drilled in a porter that will later be milled away from the finished part.

As I do not have a milling machine just yet, I do not have a way to drill the holes in the base of the cylinders to use it as if it were a lathe dog with bent tail. I could even see the cutting forces inflecting damage on the soft brass.

My plan for machining it would be to drill the bore --> turn the profile (the square base would be a large diameter saved for later) --> mount in a chuck on the mill to mill the square base --> Drill base holes --> then mount in a 4 jaw chuck to drill and tap the cylinder head holes --> Mount in a chuck horizontally and use a dial indicator on the square base to make sure it's rotated correctly before drilling pressure release exhaust holes.

Ian

Its easy enough to make your own expanding mandrel.

Take a piece of bar the next size up from your bore and turn a length equal to the length of the bore so its about 10thou oversize. drill and tap the end part depth for a suitable bolt in your case 1/4"

Tahel the bar out of the chuck and saw it lengthways down the middle, replace in the chuck and take light cuts until the cylinder fits on the bar DO NOT REMOVE FROM THE CHUCK now until all cylinders are turned.

All you do is place a cylinder on the mandrel, tighten up the bolt which as it reaches the end of the thread expands the mandrel so it grips the cylinder. As the mandrel is held in the chuck it will drive itself.

Something like this

I've turned smaller stuff between centers with no driving dog. However, I think 1.5" diameter is too big and your driving center would just slip. You could try it, but if it starts slipping, you'll quickly wear a chamfer into the driving end of the bore.

Chuck