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New Machine Pratt Whitney Model C, Apron Question

Mark999

Aluminum
Joined
Mar 29, 2018
Hello,

I've recently acquired a PW Model C lathe 14x54. Serial number is 656 and thanks to John Oder that puts it around late 1941. I'm guessing prewar since the badges are bronze. DSCF7595.jpgDscf7640.jpgApronTop.jpg

When I first moved the carriage, it seemed fine, but now does not move at all when I turn the hand wheel. If I help the movement with a block of wood prying on a bed web, it will move while turning the hand wheel. At first, I was assuming that a key had sheared, but I don't see any key referenced in the parts PDF. I also do not feel any interruption while rotating the wheel indicating a key or other drive structure being partially present.

I was going to just table this until the winter when I will have some time to tear into it, but my friend at work who operates a model C (16x54) said his machine will sometimes stop the carriage during a heavy feed. I went to see it and locked the carriage lock and sure enough, the carriage hand wheel just rotates with some friction without moving the carriage.

I hope the pictures attached show what I'm trying to describe. The carriage hand wheel drives a gear that also meshes with the friction feed clutch. When I turn the hand wheel, it turns with some friction but does not turn the gear meshing with the rack on the bed. If I engage the longitudinal feed, I cannot turn the hand wheel at all because I can't backfeed the worm. So how does this gear lock onto the shaft holding the bed rack gear? Is it a pressed fit that over time has relaxed causing this issue? Thanks for any help.

Mark
 

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johnoder

Diamond
Joined
Jul 16, 2004
Location
Houston, TX USA
Looks like three socket head cap screw removed would make it possible to pull the carriage hand wheel assy out

Question...are you trying to turn that wheel with the feed lever for long feed engaged?
 

Mark999

Aluminum
Joined
Mar 29, 2018
I've tried it both ways. With the longitudinal feed engaged it's locked up (of course) because then I'm trying to back feed the worm. Without the feed engaged, like just wanting to move the carriage the wheel just spins with some friction present. I finished reassembling the compound last night so tonight I'll pop off the carriage wheel and see what I can see. Will share pics too. Thanks John!
 

Illinoyance

Stainless
Joined
Aug 24, 2015
There is a knob in the center of the handwheel. It disengages the handwheel. Unfortunately I can't remember if the knob has to be in or out to engage the handwheel.
 

Mark999

Aluminum
Joined
Mar 29, 2018
I guess I should have stated it. The knob in the middle is "IN" to engage the carriage wheel. Out is free wheeling so it doesn't influence threading. My tests have been done with it "IN".
 

johnoder

Diamond
Joined
Jul 16, 2004
Location
Houston, TX USA
There is a knob in the center of the handwheel. It disengages the handwheel. Unfortunately I can't remember if the knob has to be in or out to engage the handwheel.


Out disengages hand wheel

PW C Hand Wheel.jpg

Item 6 is the pinion shaft - has double row ball bearing adjacent pinion - could be it is destroyed by overloading

PW C Apron Gearing A.jpg
 

Mark999

Aluminum
Joined
Mar 29, 2018
With the cover off, I can see item 6 try to turn until it takes up the slack in the gear rack, then stops. At that point, the hand wheel is turning the gear 5 as it rotates on the shaft 6. If I wedge a block of wood between the saddle and a bed web and pry, I can get it to move.
 

johnoder

Diamond
Joined
Jul 16, 2004
Location
Houston, TX USA
Maybe as simple as distorted / bent teeth on pinion

Never know with all the years it has piled up - maybe a PO made a new shaft and left it soft
 

Mark999

Aluminum
Joined
Mar 29, 2018
Well, I don't have a picture of the smoking gun, but I sense the trouble is a missing key. I pulled the hand wheel off and verified that the gear 5 spins on the shaft 6. The pinion looks good and is meshed with the bed rack correctly. I did see evidence of earlier work that was done in a rough manner that makes me think some parts were omitted because they were lost or "too hard" to install. The slotted nut to set bearing preload looked as if it was beat on with a cold chisel. However, there is a keyway on the shaft whether it is just for the locking ring or full length for a key, I won't know for a bit yet. Pulling the front of the apron was presenting some issues so I stopped to study some drawings before damaging an 80 year old casting. Problem is the threading dial,,,

What gives me pause, is the machine at work is presenting with the same issue just not as severe. Could it be common of early mechanics not giving a hoot about keys or something entirely else? Time will tell as getting the suspect gear out of the apron will require it sitting on a bench and that won't be for some time yet.

DSCF7641.jpgDSCF7655.jpgDSCF7645.jpgDscf7649.jpgDSCF7644.jpg
 

Mark999

Aluminum
Joined
Mar 29, 2018
Ah yes, didn't look close enough at the drawing. Since I know of 2 machines with this symptom, I wonder if the key is made of some soft metal to perform a safety/shear function if overloaded. Makes sense with machines this old to have some type of abuse in their past.

Of course, if that key is special it should have been noted in the parts list or note somewhere.
 
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Mark999

Aluminum
Joined
Mar 29, 2018
In case anyone searches this thread in the future, I'll provide some pics for closure. Indeed the key is sheared as is likely the machine at work too. I'm guessing that feeding into a hard shoulder created some kind of crash and the part that yields first is the key on the gear shaft that runs along the rack. The key itself is common steel. 0.25" x 0.375" x 1.5". Unfortunately, the apron has to come off to access it because the shaft drives out towards the back. Since it's off, I'll clean everything up and replace all the other keys that clearly need it.
Dscf7702.jpgDscf7703.jpgDscf7705.jpg
 

Salem Straub

Cast Iron
Joined
May 22, 2012
Location
WA, USA
Thanks for following up with the cause. I have a PW model C as well, threads like this will be very helpful for the PWC “community” although hopefully none of our lathes ever break in any way in the future :P
 








 
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