# C-C Lathe 8014 13x40 Rack / Pinion Backlash



## pboulay (Jul 22, 2013)

Hi Guys,
I have a Clausing-Colchester Type 8014 square head 13 inch lathe from the late 1960's. (Colchester Master 2500 http://www.lathes.co.uk/colchester/page17.html). And I have been working on fixing the parts that are not functioning properly or are worn more than I want. My most recent effort is to reproduce the pinion shaft that is part of the carriage apron and runs on the rack. 
In the pictures you can see the original with the chewed up gear (below and to the right in the first 2 photos) and my replacement (new part above and to the left.) Clausing wanted $1375 or so for this part. On mine the gear bit was purchased from QTC - a division of SDP / Sterling - with induction hardened and ground tooth surfaces - for about $95. (http://www.qtcgears.com/RFQ/default.asp?Page=../KHK/newgears/KHK044.html).




The third photo has my sketch showing how the purchased pinion shaft was turned and pinned in place. I made the collar part a bit wider than the original to have increased mating with the ground reference shaft of the gear. I took the old one out and put my replacement in and after a bit of last minute deburring it all went back together just like it should. And the carriage motion is smoother than it was before. (At least I'm telling myself that...)

All that is prologue to the question I came to ask. How much backlash is appropriate in this gear when mated with the rack? I measure about 0.030 which is more that I thought a well mated gear should have. My goal would be less than 0.005. The factors that contribute to this are the wear in the ways and saddle. plus when I measured the original pinion I had to assume that the tooth profile was offset by roughly 0.012. This profile offset is often used in pinions with fewer than 12 teeth. My replacement part was not produced with this feature.  What I might be able to do about this is to move the rack down by 0.040 or so. Is it worth it? I will end up with a series of abandoned screw and alignment dowel holes and the rack will stop too soon -- by about 1 inch on the tailstock end. Not great but not the end of the world either. 

My next work item is to get the ways reground. I'm certainly not going to mess with adjusting the rack until I reestablish the turcite leveling of the carriage ways. After that the rack might have to be lowered to deal with the backlash issue. Any thoughts on this?

Paul in Campbell California.


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## Ulma Doctor (Jul 22, 2013)

Hi Paul,
that was a really nice replacement gear, not to mention the savings in parts cost.
most geartrains i have run across prefer minimal backlash somewhere between .005" and .015"
as you probably already know, the tighter the gears the more force it takes to move them and keep them moving.
since this is for the carriage it's not a big deal, the only real danger is wearing the gear or rack out faster if you are way too loose.

another approach might be to add shim stock between the rack and the lathe bed to take up the clearance
just a thought... 

i hope thing are going well for you Paul, glad to see the lathe is coning along!

mike)


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## pboulay (Jul 22, 2013)

Ulma Doctor said:


> Hi Paul,
> that was a really nice replacement gear, not to mention the savings in parts cost.
> 
> another approach might be to add shim stock between the rack and the lathe bed to take up the clearance
> ...



Thanks Mike. 
The business about moving the rack down is what I had in mind when it came to shims. But it is more than I thought could be correct. Compared to the normal location of the rack, sitting the rack on the pinion measures out to be 0.048. Now that is too tight with essentially 0.000 backlash. To get to 0.005 backlash i need to move the rack down 0.040. And 40 thousandths is a lot of shim.

I suppose there is about 0.010 wear in the carriage ways and there is another 0.012 in the profile shift in the original pinion plus the pinion shaft runs in a cast iron bore with about 0.003 clearance and finally I added a 0.005 shim between the apron and the carriage saddle to make the cross slide gear engagement work right. that leaves 19 thou un-accounted for. Unless of course there is supposed to be more than 0.005 backlash in this gear for some reason. Or I'm way off on my wear estimates. 

What sort of backlash numbers did these machines run out of the factory?

Paul


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## Ulma Doctor (Jul 22, 2013)

this might be another way, if you have 0 backlash right now, could you have the rack ground down a couple thou to create more clearance if you are too tight? 
would the guy that's doing the bed be able to work the rack as well?
just thoughts to help out...
mike


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## pboulay (Jul 22, 2013)

Mike,
Something didn't come across quite right. Now, I have in excess of 0.030 backlash. Also the rack mounts in its side to the ways casting. It is aligned with 4 dowel pins and fastened with 9 ea. 1/4 in. cap screws. So in order to move the rack relative to the pinion, I'd have to create a new hole pattern of holes drilled in the rack vertically offset from the existing holes. Which would not be pretty but it would be doable. The alternative is to start with a new chunk of rack stock. That would be $400 or so.
So my problem is simple. I want it to be perfect and I don't want to spend any money. 

Cheers,
Paul


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## Ulma Doctor (Jul 22, 2013)

i see, i was misunderstanding, i re-read and think i found where i got off track...:lmao:


i'm trying constantly to perfect things, it doesn't really come cheap:rofl:
i'll be interested to see the progression!
mike)


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