Threading on a Colchester lathe with 1-piece half nut

I'm looking to purchase a Colchester Bantam lathe, but concerned about scattered reports that due to the one-piece half nut on these and other Colchester models, these machines produce threads with uneven pitch. I would appreciate hearing from anyone with experience threading on these machines, good or bad.

Mike

Same setup is found on a Chipmaster. I have zero issues with the one-piece halfnut. I'm not seeing why it would be more prone to uneven pitch than a two-piece nut. I could see it being more prone to wear, since only one half is carrying all the load. I certainly would not use the half-nut structure to guide my choice of lathe. Other stuff is more important.


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Works fine in practice. Maybe little more problematical if really, really worn than a two piece nut but by the time you get that stage you have bigger problems.

One man says its easier for dirt and rubbish to get in promoting wear other man says its easier for dirt and rubbish to get pushed out reducing wear. I say service the thing occasionally and keep the oil in the apron clean so the problem doesn't arise.

Bottom line is its a Colchester, not a Holbrook. Built and priced to be destroyed for the value of the work it did. Plenty of near destroyed ones floating around in the hands of folk who expect too much. Also, even today, still plenty of low time machines out of maintenance and training shops bought for the excellent price / capability / performance ratios. One of those is unlikely to have done enough threading for nut wear to be even the merest smidge on the horizon.

Save your worries for real issues. Like Bubba tryning to adjust the clutches and failing miserably.

Clive

I have a 1967 Colchester Bantam Mk2 that was a bit tired and used it to make a new cross slide screw and nut for itself without any problems. Close examination of an unused cross slide screw for a taper attachment showed a highly polished mirror finish on the threads with a round, radiused valley with makes me suspect the cross slide thread was rolled rather than cut. The lead screw and cast iron half nut are 1 &1/8" diameter which is quite large for a machine of this size which helps with any potential flex. Several Colchester lathes have only a half nut and I've yet to hear of any issues anywhere with threading caused by this design.

All issues were what you'd expect from a 50 year old machine: oil leaks to be tracked down and sealed, worn motor drive pulley to re-sleeve after a dodgy earlier fix (common to all Bantams), spindle bearing preload to be reset. The saddle casting was worn a bit and had about 30% contact when blued up the tailstock end of the ways so a bit of careful scraping brought it back into good contact and got rid of a lot of problems I had with chatter and parting off. The base of the compound also rocked on the cross slide and had to be machined along with the top of the cross slide (you could see oil squirt out from under the cross slide when doing a cut). Most of the issues were due to dirt/neglect and were easily fixed.

I've had a Chipmaster for over 20 years. It's cut a lot of threads including recently 16" of 8 TPI LH square thread. Never a problem I've noticed with pitch errors.

The threading system works. A badly worn one probably won't but I fail to see how this differs from anything else badly worn.

PDW

The Harrison M300 has used as one piece half nut since the early 1970's. Still made that way AFAIK. So I guess it works pretty well. I did replace the shift bellows with a silicone baby bottle nipple rather then the factory one for big money.

I have a Clausing Colchester with the single half nut and it and I don't use it for threading because of the pitch constantly changing and getting screwy threads. The problem is the guide bushings on either side of the half nut wear and this allows the half nut to rise up as it is loaded. I think most of the wear is from the swarf getting on the lead screw and being drug through the guides when using the normal feed mechanism and not the lead screw. I know of another Clausing with the same problem.

Shaybuilder said:

I have a Clausing Colchester with the single half nut and it and I don't use it for threading because of the pitch constantly changing and getting screwy threads. The problem is the guide bushings on either side of the half nut wear and this allows the half nut to rise up as it is loaded. I think most of the wear is from the swarf getting on the lead screw and being drug through the guides when using the normal feed mechanism and not the lead screw. I know of another Clausing with the same problem.

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Would it be straightforward to fabricate and install new guide bushings?

--Mike

Mike,

If you have the time to make and install new bushings that would be fine. I don't seem to have time to do it, I want something that does a good job for me when I need it.

Ken