Lathe mandrels

[viking]

I've got to be the worlds slowest HSM. I spent the entire afternoon yesterday making an ecentric for the Tiny Power Ajax steam engine I had started on and then set back as a project for after retirement. Since it was my first attempt at intentionally making something that was not concentric I took great pains in my layout.

When I completed the project I checked all the dimensions and was very pleased until I measured the "lift" of my cam. The offset was 3/16" which should have resulted in a 3/8" lift but when I measured it was only .325". I'm quite confident that the error was not from the layout.

My theory is that I used a cheap import mandrel of the type with a set screw in the end that you tighten to mount the eccentric while I turned the outside features and I think the orintation turned ever so gradually as I was turning the outer surface.

I have orderded some tapered mandrels from J&L and have a couple questions on there proper use.

1. How hard do you press the mandrel into the part?

2. Will it affect the size of the hole when the mandrel is pressed off?

3. Is the taper of the mandrel shallow enough that the mandrel can be held accurately in a 5C collet?

Thanks in advance for any advise on this matter.

P.S. Since the steam port is .125 wide will this .025 reduction in the valve opening really affect the operation of the engine?

[SGW]

"It's a hobby. It's supposed to take a long time." --Unidentified member of the New England Model Engineering Society.

Can't help you much on mandrel use. I bought a few used ones, eons ago, and I've never used them. I generally make a stub mandrel, in place, as the need arises. But I'd guess:

Press on only firmly enough so the work stays in place, take light cuts.

Hole size shouldn't be affected significiantly.

If you're planning to use a collet to hold the mandrel, why not make a stub mandrel?

[J Tiers]

<font face="Verdana, Arial" size="2">Originally posted by viking:

1. How hard do you press the mandrel into the part?

2. Will it affect the size of the hole when the mandrel is pressed off?

3. Is the taper of the mandrel shallow enough that the mandrel can be held accurately in a 5C collet?



Thanks in advance for any advise on this matter.

P.S. Since the steam port is .125 wide will this .025 reduction in the valve opening really affect the operation of the engine?</font>

1 Plenty hard enough to hold it, normally rather hard

2 Generally, no, but if there is "fuzz" from a boring operation, it will be squashed flat, changing the apparent size

3 They go on centers, otherwise there is little point in using them, IMO. The ends are only rough turned, that is where the dog goes to drive them.

Set up with the big end of the mandrel at the headstock. That way normal turning with tool going R to L will tighten and not loosen the part on the mandrel, by virtue of the side force of cutting shoving it on harder.

[viking]

Thanks for the replies. I will use the mandrels between centers as suggested.

Anyone care to respond on whether the reduced lift will affect to operation of the engine?

[JCHannum]

The reduction in lift may affect the engine performance somewhat. If the engine is to be run on air for display purposes only, it should not be of much consequence. If it is to be used on steam to power something, performance may be reduced.

Finish the engine and try it out. Worst case is that you will have to redo it. The second time will be easier now that you have the experience. Some of us have lots & lots of experience.

[rkepler]

<font face="Verdana, Arial" size="2"> Anyone care to respond on whether the reduced lift will affect to operation of the engine? </font>

Just make the valve face and ports proportionally narrower to accomodate the travel (assuming that you haven't cut the ports yet).

Next time just rough machine the spigot, the chuck it up offset in a 4-jaw, indicating on a dowel pin in the hole you drilled/reamed through the rough turned spigot. After the eccentric is turned you can mount it back through the hole and finish the spigot and the face of the eccentric. (The reason I know this is that I made the same eccentric & sleeve for the Ajax this past weekend).

[C. Tate]

Cannot see where you would lose .050 without there being some setup issues. I am making some assumptions about the process since you have not given a discription. I would think you could check the rise on the lathe with an indicator. Can you use a four jaw check to produce the eccentric?

CT

[viking]

rkeplr: The process you describe is basically the same as the one I used . First I turned the outside surface round. Then I found the center and marked the offset for the 3/8 hole. Mounted in a 4 jaw chuck and with a wiggler in the punch mark I indicated it to run true. Drilled and reamed the 3/8 hole. Took the part out of the chuck and mounted it on a cheap imported expanding mandrel and remounted it in the 4 jaw and indicated for the outside surface to run true. While I was turning the outside features I think this is where the problem surfaced as the part turned on the mandrel unknown to me.

Great to hear someone is building the same engine as me! I will have a million questions before this is done.

[rkepler]

<font face="Verdana, Arial" size="2">Great to hear someone is building the same engine as me! I will have a million questions before this is done.</font>

Well, different tooling will cause us to use different fixturing. Outside of that things should be done aboue the same.

As for the sequence of operations on the eccentric - I was able to do it in 3 chuckings - the first mounted the casting centering the small boss and turned the small diameter and drilled & reamed the through hole. The next chucked on the small diameter freshly turned, offsetting it the 3/16 eccentric necessary then faced off the outside of the eccentric, turned the major diameters and parted in with a 3/32 tool for the minor; the final chucking was on an expanding arbor in the 3/8 hole and finished the eccentric thickness and the minor diameter and projection. I don't think I messed with it for more than an hour.

I've still got to do the valve chest and linkage and port the cylinder, but that's about it. Oh, final diameter on the flywheels which are rough turned. I think I'm going to key them to the crank, grub screws won't hold them well enough.

Watch the location of the crosshead guides if you haven't put them in yet. You'll have to mill pretty dang close to the boss for the cylinder to get a decent center in it for the crosshead.

[viking]

I also had considered keying the flywheels but being an amateur at broaching was concerned about the fact that the hub width on these flywheels exceeds the length of the bushing for my Dumont broach kit. How is this dealt with?