cross slide screw replacement

[liveaboard]

I need to replace my crossfeed screw and nut; the slack is just ridiculous and the nut threads are worn so thin you can almost see through them.
I can get a trapezoidal screw and nut by mail from the UK [I've given up trying to source things here in Portugal].
It's left-hand, 3mm pitch, and 14mm diameter.
It's not expensive, around $15. for both parts.

They say the tolerance is "e7" Rolled thread - Manufactured from C35E
I have no idea what any of that means.
I'll need to work on it a bit to fit it to my machine of course.

Thoughts?

[Harold_V]

First, is your machine metric or is it Imperial? If you replace the screw with the wrong system, your dial won't yield proper readings, so make that the standard by which you make your decision. I make mention because the size you specified is very close to Imperial 5/8- 8 pitch, which I would assume to be Acme. If the machine is Imperial and you use a metric screw replacement, you'll have nearly seven thou error per revolution.

Harold

[GlennW]

e7 is a metric thread tolerance call out, but could apply to either trapezoidal or 60° ISO threads.

[liveaboard]

My lathe is mostly metric; the cross slide and compound slide dials and screws are metric [dials are 0-30 for 3mm of movement]. Most of the fasteners are metric, but the lead screw and the resistance adjustment screws on the slides are imperial.
The adjustment screws may have been changed [ALL of them?] but the lead screw has to be original as the gear chart is correct. I find this very odd.

Anyway, yes the screw in question is the right pitch. I'm just wondering if there's anything particular I need to know about tolerances, slack, wear rates, or anything else.

I am American and started my life in inches and fractions; I'm a totally metric man now and I hope I never have to return to inches again.

[WesHowe]

The only bad thing about either metric or imperial is having two standards used together. If I didn't have tools and materials that came in Imperial sizes I could be quite content to flip my calipers from inch to mm, get a new micrometer and adopt metric. I always convert fractions to thousandths anyway, so I can work in base10.

But as to your problem: I have a chinese-made lathe, it has a lot of metric parts, gear sets for both metric and inch threads on it, dials with both inch and mm markings, and my leadscrew is in inches. I feel there is quite a likelihood that the design was copied from another older design, and now that they had all those gears designed they are not going to do it over again for a different screw.

Perhaps this is the backstory there also.

[atunguyd]

I may be wrong, but I have always been under the impression that ACME threads biggest advantage in a crossfeed screw is that it suffers less from wear. Would it then be fair so say that replacing this with a standard ISO thread would work for the home machinist who is probably not going to wear it out in any hurry?

[SteveM]

The only bad thing about either metric or imperial is having two standards used together.

I remember working on a friends late 1980's Camaro. Half the car was metric, half was imperial.

They should have hung the guy that OK'd that. Better yet, give him a bomb to defuse and tell him that it's either 12mm or 1/2" bolts on the fuse and he has only one shot to get it right or it blows.

Steve

[RSG]

LOL@Steve! good one!

[liveaboard]

Really, you should be able to know a 1/2" thread from a 12mm by eye; the thread pitch gives it away.
except for the rare metric fine threads, which are very similar to imperial fine threads.
and 12mm hex bolt heads can be seperated from 1/2" by eye too [with a little practice].
But 13mm is just too close. Why some choose the 13 and 19mm format for hex heads beats me.

Acme threads only come in imperial sizes and pitches, the trapezoidal is pretty much the same but metric I gather.
I'm about as low budget a machinist as you'll find anywhere, but at less than $20 for the proper screw and nut, you'd have to be a nut to put a hunk of fastener grade threaded rod in there.

[Downwindtracker2]

Liveaboard, I'm from Canada, we work in both. North American autos are mostly metric now. They are using 12mm,15mm and 18mm hex head sizes. Which by the rule of thumb, 1+ 50%, is what metric should have been .

[liveaboard]

No one said there was anything wrong with it, so I've ordered the trapezoidal screw and nut.
The postage cost was a bit high...
[Grumble, moan]

[Downwindtracker2]

I bought a mapbook from Australia for our trip, $39 AUD. That's higher than what we pay for our local mapbooks, but not too far out of line. Shipping $52AUD !!