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Seneca Falls Lathe

stude_1ton

Plastic
Joined
Jan 2, 2002
Location
NJ
I recently bought a small 10 x 24 Seneca Falls Lathe. I am mounting a 3 jaw chuck and am turning the new back plate for the same. My confusion is in the thread size for the spindle nose. It is definitly 12 tpi. However I measure the nose to be 1.44 diameter which from what I can find could be a 1 3/8" or 1 1/2". My plan is to by a tap to actually thread the plate with since I do not have the knowledge/skill yet to cut my own threads or the right gearing. Obviously at about $50 a tap I want to get the right size the first time.
Thanks
Ted
 
I recommend against using a tap to thread this backplate.

You have two problems. The first you've already identified in that the thread size may not match *exactly* (more on this below)

The second is one of "perpendicular" orientation of the tap relative to the hole in the backplate. This you may be able to overcome nicely if you use the lathe boring tool to bore the hole and the tailstock to align the tap, but there may be other problems.

I'm assuming your Seneca Falls lathe is from the period from 1880 to about 1910? If this is the case you're very unlikely to find the nose thread to be a standard size. Very common in this period to form the spindle thread to match the backplate or visa versa.

Also, you may find there is a bit of a "shoulder" where the backplate sets up against the "stop collar" when it's in position. This unthreaded portion of the spindle acts as an "alignment dowel" and keeps the two parts concentric when you're in the middle of a cut. A tap will not duplicate this matching recess on the backplate and by necessity you'll have to follow up your tap threading with some work with the boring bar to cut this recess.

You'll also encounter the fact that most taps are designed to cut teeth that are only 75 percent of the normal "theoretical" height. You usually start with a hole that is a bit "oversized." This is done to make turning the tap easier and in no way impedes the threaded connection action or tightness in bolting service. For a lathe backplate to maintain concentricity you may want to hold your tolerances to a little tighter dimensions and holding a tapped hole to 100 percent tooth height leads to broken taps.

Also taps are by design created with a certain amount of "slop" or fit. You buy a very high quality tap (and pay high quality money) and you get a tighter tap.

It pretty much sounds like you'd be best to either set up your Seneca lathe to do some threading (you're likely confronting a problem with a "lost" loose change gear) or better yet get another lathe to do it. (Recommended)

The reason I recommend this is that you'll want the Seneca spindle nose in your hand to try in the threaded hole without taking the backplate from the lathe if you're going to try to match the backplate thread and get it "really nice."

I've done it a couple of times. It's not impossible, but you've got to take your time and have good measuring tools to keep track of where you are until you get down to the point where you can work by "feel."

If you're really afraid of threading in a lathe using "half nuts," do like it was likely done during the Seneca lathe period. Simply don't undo the half nuts. Relieve the cut after it's taken and then "back" the lathe down from the end of it's cut to the beginning by running the whole thing backwards. Releasing the half nuts and using a threading dial is a relatively recent (perhaps 1920's) convention. In the "good old days" they didn't need no "steenkin" thread dials. (And production was not the essential it is today.)

Hope this helps. Been there done that with these old machines.

Best regards,
Joe
"Yankee Mechanic"
 
Joe is as usual correct
tap is WRONG way to go ...bastard thread ,& clearance for "register " behind thread critical +.0005- .001 ...also backplate must be trued up & finished to spec. on ur lathe after thrd fitted...register in chk must also be critically fitted..... possible solutions ..1..make an EXACT copy of spindle nose/thrd/register & cut or have cut the backplate to fit copy....check copy with another chuck or backplate if available...2..set up backplate on light faceplate or 4 jaw in another lathe ,, remove BOTH to check fit on lathe , thereby keeping correct gear relationship to continue taking cuts if needed..."more sure than first methood"........bottom line is u need a friend!!!!!or set up on centers/dog/f/plate or 4/jaw practice thrdg & start reading abt depth/double depth of thrds/major/minor d......3 jaws are only accurate to .002-.003 when set up CORRECTLY ,& then,not for too long..(excluding zero-set chks).....good news is ,...if u screw up ,bore the thrds out & drive/shrink in a bushing......have retrofitted more than a dozen f/plates & backplates this way....shoot for abt .0015 max. interference fit for 2 in.diam.....cast iron has its limitations...

keep in touch
best wishes
docn8as
 








 
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