Joining the Pratt & Whitney 3C family

[discodan]

Here is my "new" mill that I bought out of Virginia. Has the war production tag. Motor is a variable speed Craftsman that runs from 500 to 5000 rpm. Has one 3/4" 4pn collet but I retrofit R8 collets with an easily made draw bar and an aluminum adapter out back. One oddity is the handle that operates the table instead of the normal screw based handle on the end. I will fix that eventually since it gets in the way of the non-original vise. This thing is quiet and tight. This old stuff was made right!

[SteveM]

Interesting that it would have been converted to a hand mill when hand mills were readily available (today, you can't give them away).

How much of the table mechanism do you have? Is the leadscrew there?

Are you just using the R8 collets directly in the spindle without an adapter?

Steve

[discodan]

Steve, yes, I use the R8 collets directly in the spindle with no adapter and it still appears to hold firmly. As you probably know already, the R8 is pretty close in diameter to the 4pn, so it made the most sense to me since it is readily available. Last night I went out and used some blue magic marker (cheapskate Dykem!) on both the 4pn and R8 collets so I could see where they both contact the spindle. The 4pn collet fits into the spindle almost all the way and it looks like the spindle only touches in about an 1/8" ring just under the widest part of the taper. On the R8 some of the tapered portion sticks out of the spindle and you can see a very thing contact ring about 1/3 of the way from the narrow end of the taper. But as I tighten the draw bar you can see the slots closing. I haven't cut anything yet but will probably do a test this weekend.

As far as the table mechanism, it didn't come with any pieces since my guess is that the hand mill handle must have been from the factory. I did find a guy on ebay that has what looks like some P&W lead screws with handles for X and Y that might work but the guy won't respond to my emailed questions and I don't want to drop $75 unless I know the screw size is the same. I can make the mounting plate if I can get the screw and the handles.

If you have one can you share a pic or two?

[John Hasler]

The back end of an R8 is .9495" . The back end of a 4PN is 1.000" . This diameter is critical to locating the collet parallel to the spindle axis. I don't think the angles match, either.

[discodan]

John, the draw-bar should help keep the R8 parallel, just like the draw-tube does for the 4pn. I haven't check accuracy so I can't promise anything. You are correct that the angles don't match, which is why I checked the contact area of both and they both have a very narrow contact patch compared to the whole angled section.

[SteveM]

the draw-bar should help keep the R8 parallel, just like the draw-tube does for the 4pn.

The 4PN collet is located by the fact that it fits tight in the spindle all the way from the front to the back, not because the drawtube does it (although the tube fits snug in the tube and is VERY stiff, so I'm sure it helps).

I'll bet that if you stuck a long rod in your collet and pushed on it, you'd be shocked at how much it will move as the collet pivots. While you can't get a dial indicator on the drawbar, I'll bet it will be flexing all over the place.

You should see if you can make a sleeve that will slip over the back of the collet to make it 1.000" diameter. That will probably make a HUGE difference. it would be very thin, less than 30 thou. You would need a lathe to do that.

The R8 collet goes into a spindle that is sized for the back end of the collet. That part of the collet is so important that it actually has a name - "Back Bearing".

they both have a very narrow contact patch compared to the whole angled section.

Did you test that with something that is very accurately 3/4" in the collet and the collet pulled tight? If you were slightly over or under sized, then the taper won't match.

You could make a collet adapter to go from 4PN to R8, although it would need to stick out a bit to get the R8 to fit inside. That would be a big step up from using an ill-fitting collet.

As far as the table mechanism, it didn't come with any pieces since my guess is that the hand mill handle must have been from the factory. I did find a guy on ebay that has what looks like some P&W lead screws with handles for X and Y that might work but the guy won't respond to my emailed questions and I don't want to drop $75 unless I know the screw size is the same. I can make the mounting plate if I can get the screw and the handles.

If you have one can you share a pic or two?

I will see about getting you some pics of mine.

Steve

[SteveM]

What accessories did you get with it?

Looks like an original drawbar and overarm. I owned mine for five years before I got those. I made a homemade drawbar.

The vise is not the original.

You can read up on the 3C and see some pictures of one of the most complete 3C's here:
http://www.lathes.co.uk/prattwhitneymiller/page4.html

That picture shows just about all of the accessories that were available.

Steve

[discodan]

The only thing I got was the one collet and a boring bar. The vise I just bought while at Hershey last weekend. I saw an original vise on ebay but it was almost $500. Here is a link to the x-axis screw assembly https://www.ebay.com/itm/pratt-whitney- ... 2562216159. The dial and handle don't look exactly the same but may work. I just can't get the dude to respond to me with the dimensions of the screw. I looked at the 4pn collet again and you can see the dark ring at the widest part of the taper where the contact with the spindle is. There is almost no wear on the rest of the taper. I get what you are saying about the R8 so I will try to test it out and see how much flex or deflection there may be.

[SteveM]

Put your location in your profile so that it shows up on your posts.

Steve

[SteveM]

BTW, I just picked up the matching lathe.



You can see my post on it here:
viewtopic.php?f=42&t=107314&p=398513

Steve

[discodan]

I just added my location. Nice lathe. Does that use the 4pn collets? I saw a P&W lathe online somewhere recently but don't recall now. I will have to see if I can find it. I don't need another lathe but who knows.

[SteveM]

OK, got some pics and some measurements.

Here's the view of the screw from under the table:

Here's the handle end:

Here's an attempt to add dimensions to that:

Here's a closeup of the end plate:

The leadscrew is 5/8 x 10tpi, and from the outside end of the endplate to the end of the leadscrew is 12-1/4".

Hope all of that helps.

I sold one of those to Jim Rozen over on practicalmachinist.com. I tried searching for his rebuild thread but couldn't find it.

Steve