Author Topic: machining questions (Read 1168 times)

CU_Cannon · « **on:** July 01, 2005, 03:24:26 PM »

I am considering building a full scale coehorn mortar. I donÂ’t have the money to purchase a barrel and I want the challenge. The barrel will be machined from solid. If IÂ’m going to spend the time and money on a big project I might as well make something that is worthwhile.

The first problem is building a steady rest big enough. I plan on taking a thick steel plate milling the center out and fitting it with some fingers with ball bearings on the ends. It would work the same as a regular steady rest just bigger. I think it should work fine as long as I can build it rigid enough.

The other problem is getting the correct bore profile. I donÂ’t have access to a CNC lathe or it would be easy. I was thinking something that works the same as key cutter. It would have something that clamps to the lathe ways that would hold a template and there would be a follower on the carriage. I have seen similar setups on duplicating lathes for wood. I canÂ’t see why it wouldnÂ’t work for a metal lathe.

Any ideas and input would be great.

Double D · « **Reply #1 on:** July 01, 2005, 05:38:58 PM »

You might consider making the bore and chamber similar to the bore and chamber of the Confederate mortar.

If you just had have the round ball seat, you could do incremental radius with a boring bar, like Powder Keg does making ball moulds.

You steady rest idea sounds good to me. But one of the guys with a lot mor experience than I might not agree and will tell you why.

HotGuns · « **Reply #2 on:** July 01, 2005, 08:30:23 PM »

Building a steady rest can be done, but it'l lbe very time consuming and its better to go too heavy with it than too light. Be sure to leave some extra metal on the OD, in case it scars it up, you can take a cleanup cut on it when you are finished with the bore.When you build it, you'll want to do the bore first. As for the taper in the powder chamber, it can be done with the compound rest and a long boring bar. You'll just have to go easy with it.

When you are done with the bore, the best way to turn it would be with a pipe center in the bore, and then you can profile the outer diameter however you want. Since the mortar had a pretty good radius on it, you'll need to do it last, because you wont have enough to grip with the chuck.

Since you are gonna build a steady rest, why not build a radius cuitter ? They arent that hard to do and it'll be a tool that will last you forever if you build it right.

Forget the trace idea with the lathe. Tracers that work with metal lathes are hydraulic and arent even in the same league as wood lathe tracers. Even woth those you need a template. Tracers are pretty much obsolete these days and arent worth the trouble unless you plan on doing a whole lot of the same setup.

Cat Whisperer · « **Reply #3 on:** July 02, 2005, 01:29:52 AM »

Here are some thoughts. GOOD project.

Two ways of cutting the radius:

a) build a radius cutter: http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=53924
(this is a single bearing single arm cutting a 5+inch diameter). Next model will have TWO bearings for better support - this worked but could be better.

b) use step and offset method - there are a posts on this - external on mortars and internal on moulds.

Both methods work. Cut off the excess at a straight 45 degree angle first then the rest is minimal.

Steady rest -

I'm building mine. Still. Mine will have brass sliders but bearings are OK too.

AND, interior form - straight powder chamber is easiest to cut. Tapered is the strongest.

Put a radius on ALL interior corners to minimize the stress points.

Cat Whisperer · « **Reply #4 on:** July 02, 2005, 01:39:08 AM »

Another note - to add emphasis to what just's been said.

This is a do-able project, but it IS in a different league than wood working. Speeds are slower and leverage/torque is much greater.

4 jaw independant jaw chucks hold much better than 3 jaw scroll chucks.

When the 30 or 40 pound piece of steel pops out of the chuck just get clear of it - it will stop somewhere and you will not have any influence. (Don't ask. :eek: :-D

)

GGaskill · « **Reply #5 on:** July 02, 2005, 05:33:27 PM »

I don't know what kinds of steel are available in your area but you might consider getting a two inch wide ring cut from a suitably large piece of thick wall tubing to use as the basis of a large steady rest. It would save you the trouble of opening up the middle and the cost of the waste steel if you started from a plain flat piece. Be sure to allow for some way to open or remove the top of the ring.

There are hydraulic tracers made for lathes to cut complex profiles. You might be able to make a simple manual tracer using an index rod on the cross slide and something to hold the pattern in an appropriate place.

CU_Cannon · « **Reply #6 on:** July 03, 2005, 04:48:30 AM »

The trick with working with metal is to never be in a hurry.

I plan on using 4130. It seems to have the best properties for what I want. It should be plenty strong.

The steady rest seems like it will work. I will probably make the steady rest in sections. I could make better use of the steel. I figure something between 1Â” and 1.5Â” thick and as wide as I can manage would be sufficient. I donÂ’t think I want to find out what happens when a piece of steel this big lets loose.

Quote from: GGaskill

There are hydraulic tracers made for lathes to cut complex profiles. You might be able to make a simple manual tracer using an index rod on the cross slide and something to hold the pattern in an appropriate place.

That is the general idea for what I was going for. I may need to try a smaller version for my 9Â” south bend. The outside radius I will most likely use the incremental cut method but on the inside it would become quite cumbersome with the different radiuses and taper. It might not be too bad with a digital readout but again I donÂ’t have access to something like that.

Quote from: Cat Whisperer

4 jaw independant jaw chucks hold much better than 3 jaw scroll chucks.

4 jaw chucks are all but useless as far as accuracy is concerned. I have found it much easer to use my 4 jaw for anything that needs a degree of accuracy.

Right now IÂ’m still in the planning stage. I hope to get working on it this winter. Input is welcome.

Cannonmaker · « **Reply #7 on:** July 04, 2005, 06:07:21 PM »

With too light of a steady rest you could get chatter. The last Lathe I bought has a home made steady rest from a rear end of a 30's automobile. that part is OK, although the fingers are just too light, 5/8" allthread with Brass ends. I have a problem with chatter. If I can think of something like this that you can obtain easily I will let you know.

Both of my lathes have 4 jaw chucks on them. I have never mounted the 3 jaw. With the repair shop I am always chucking odd shaped pieces, and cannot use a 3 jaw.

Rick

CU_Cannon · « **Reply #8 on:** July 07, 2005, 05:03:23 PM »

Rigidity was my main concern. I was considering using 1" square steel bars. I'm not sure if I will use bearings for the ends of the fingers or not. I will be finishing the outside profile after the bore is finished. Any marks left from the fingers will be taken out in the final machining.

Quote

4 jaw chucks are all but useless as far as accuracy is concerned. I have found it much easer to use my 4 jaw...

That should read;
3 jaw chucks are all but useless as far as accuracy is concerned. I have found it much easer to use my 4 jaw...

Cat Whisperer · « **Reply #9 on:** July 08, 2005, 12:49:26 AM »

There are several ways of designing the support fingers. Some use round stock some flat (with long slots). You have a lot of options.

HotGuns · « **Reply #10 on:** July 09, 2005, 06:43:00 PM »

If there is any way that you can use bearings do it. You'll be alot better off in the long run and they arent near as much trouble to maintain.

I eventually modified all of the fingers on my steady rests. I had used teflon inserts in them but even that would mark a shaft. I ended up milling slots in my fingers and using high grade bearings to bear against whatever I was cutting. I didnt have near the heat problemsand you could prelaod the bearings and not damage any shafts at all. Seems to soak up the chatter a lot better.

CU_Cannon · « **Reply #11 on:** July 13, 2005, 05:32:28 PM »

The marks will not be much of a problem as I will be finishing the outside of the barrel after the bore is complete. The reduction in chatter may be a selling point for the bearings though.

One thing that occurred to me is the shear weight of a piece of steel big enough for the barrel. It figures out to be around 250 lbs. Much of that will be machined away but it will still be a plenty heavy barrel. Any thoughts on moving a piece of steel like this around with out breaking my back?

GGaskill · « **Reply #12 on:** July 13, 2005, 08:33:55 PM »

I would use an engine hoist/cherry picker and a sling. And lay 2x4's across the lathe bed while moving the iron around so a drop won't damage machine or workpiece.