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Metric bridgeport leadscrews - acme or trapezoidal?

StrayAlien

Cast Iron
Joined
Aug 18, 2014
Location
Melbourne, Australia
Hi all,

Greetings from Melbourne, Australia. This is my first post in this forum. I've been looking for a BP for some time now that wasn't a silly price and wasn't 1000+ klms away. I recently scored a 'not working, condition unknown' 1981 BP from auction. Turns out the main belt was broken so I was lucky there. Nevertheless it is getting a complete teardown and refurb where needed (and affordable). Top end rebuild kit from HM is on the list.

The machine is metric with 5mm pitch leadscrews. The metric nuts are silly prices so I'll likely make them.

Does anyone know if the stock 5mm pitch leadscrews are 29deg acme or are actually 30deg trapezoidal?

Failing actually knowing that I am guessing grinding a threading tool to match an unworn portion of a screw is a good, if maybe inexact, solution.

Many thanks,

Greg.
 
Hi all,

Greetings from Melbourne, Australia. This is my first post in this forum. I've been looking for a BP for some time now that wasn't a silly price and wasn't 1000+ klms away. I recently scored a 'not working, condition unknown' 1981 BP from auction. Turns out the main belt was broken so I was lucky there. Nevertheless it is getting a complete teardown and refurb where needed (and affordable). Top end rebuild kit from HM is on the list.

The machine is metric with 5mm pitch leadscrews. The metric nuts are silly prices so I'll likely make them.

Does anyone know if the stock 5mm pitch leadscrews are 29deg acme or are actually 30deg trapezoidal?

Failing actually knowing that I am guessing grinding a threading tool to match an unworn portion of a screw is a good, if maybe inexact, solution.

Many thanks,

Greg.

Good question..........I own a Taiwan Chinese metric BP clone. I'll be watching this thread, but I don't think it will make a difference either way.
 
Greg,
Are you reusing the existing leadscrew, or have you found spares at a decent price? If you are in the conditions of replacing the nuts, probably the screws don't look much better and you'd be better off buying good quality trapezoidal screws and matching nuts, sleeve the new one to fit the holder, and graft the new leadscrew to the end of the old one, unless you can machine it to proper profile.

Paolo
 
Does anyone know if the stock 5mm pitch leadscrews are 29deg acme or are actually 30deg trapezoidal?

AFAIK, metric machines were not an option in the 80s for the US machines. I'm going to bet that you have a UK built machine. I'm reasonably certain they were all DIN compliant leadscrews, IE trap 30.

There might be a source in the UK for nuts. If not, there's a business oppurtunity. :D
JR
 
Paolo, thanks. I'll be use the existing screw. Replacing the nuts is more of a 'while I have it apart' thing. It'll get a DRO as well.

JR - thanks. Yes, it is a UK machine. Mind, I bought it thinking it was a US machine (it doesn't have the big 'Adcock Shipley' cast into the ram and also has the smaller US style power cabinet).

Re standards compliance, I kind of suspected that might be the case, but as long as the pitch is metric it doesn't really matter, so my thinking was the same tooling could be used to make either metric or imperial. I currently have a taiwanese mill that is an oddball of some parts being metric and some parts otherwise - even down to the bolts used to hang it together). But trap 30deg makes sense for a 'proper' metric machine.

There is a UK source for the nuts ... but they are about 3 times as expensive as the US ones. :-) .. Hardinge does sell metric nuts but they're not stocked atm and only the z axis nut is shown - and it is $250+ US. Cripes!

Thanks again. If I find out for certain on the 29 vs 30 thing I'll post back here.

Greg.
 
Assuming the measurements I made on mine are correct the metric screws on Adcock Shipley built Bridgeport are ACME form cut to a metric pitch on the same size blanks as the imperial screws.

Shortly after getting mine an E-Bay seller offered a new longitudinal metric screw and nuts at a price to good to miss. There was also flurry of nuts both metric and imperial around the same time. Screw turned out to be imperial and nuts metric. Had a high old time sorting things out. 5 mm pitch and 5 TPI are almost too close to call. Unless you know the cunning method that makes separating nearly the same metric and imperial screws easy. Ended up buying several sets of nuts to get spares for mine and imperial pair so I could trade on the screw. Need to look and see whats still in stock. I'll PM you if there is a spare metric set in the box.

Clive

PS If you decide to cut your own consider buying a carbide threading insert of the correct ACME form for the job. You know the form will be right. Grinding a machine tool feed accurate full form ACME internal thread cutter is a pain. Largely because its a job most of us only do once or twice so have neither the practice or the set-up guides that make it easy(er). Really need two tools because the bronze will probably take the edge off the first one by the time the nuts are nearly done. Switching to a really sharp tool for the final cut or three makes for a nicer, easier, job.
 
If you're going to make new nuts, is it worth using cast iron rather than bronze? Bronze is a piss poor choice unless there's constant lubrication. That's probably why so many bridgeports have clapped out feed screws...
 
Clive thanks. Very interesting if they are, in fact, ACME form. If you dig up some metric nuts there I'd be interested in them. Please do PM me if you find something.


I am dying to know what the "cunning method" is for establishing the diff between metric and imperial!


dpcwright. Yes, that UK site is the place .. but rather expensive eh!


Re the nuts. I did actually find them on the hardinge site (thanks for the other link cfranks):


ShopHardinge - BP 12673
ShopHardinge - BP 126731
https://shophardinge.com/search.aspx?str=12060170


.. and excluding the z-axis which is like $250 US, the other prices are not too bad - in the $60 US range.


They are not listed with the rest of the parts but tucked away at the end of their parts list under 'metric conversion kits'. But ... who knows if those will fit on a UK-made leadscrew.


Mark: interesting re CI vs bronze. That might explain why I've seen so many CI nuts. I thought it was cost cutting, but I guess CI wears well and holds oil.

EDIT: Apols also for the late reply - I have been getting 'database errors' trying to post this reply. It seems it was the links to the hardinge site causing issues as text so I have to make 'links' of them
 
StrayAlien

Looked through my Bridgeport parts stash and, unfortunately, the extra set of feed nuts is imperial so of no use to you. Sorry.

The cunning method for establishing the difference between very similar imperial TPI and metric pitch feed screws relies on measuring the length of thread needed for half a turn of difference. With the coarse pitches used on feedscrews a simple ruler does the job just fine.

For example 6" covers 30 turns of 5 TPI thread and (for all practical purposes) 30.5 turns of 5 mm pitch thread.

Going the other way 150 mm covers 31 turns of at 5 mm pitch but only 30.5 turns of 5 TPI.

No need for micrometer accuracy. Only need to see if measured distance goes between same flanks or opposite flanks of the thread in question. Works fine on worn threads too.

Somewhere on my computer there should be a spreadsheet listing the measurement lengths for the common range of threads but darned if I can find it. Posted the results on PM a few years back (5?) but blowed if I can find them either.

Clive
 
Clive, thanks for looking and thank for the tip. I have received an offer of assistance to use an optical comparator to measure the flank angle. So, as long as I have some good thread left at the leadscrew ends we ought to be able to get a definitive answer. Hopefully! .... :)

Greg.
 
the "big reveal" ... the winner is:

The optical comparator was inconclusive on a worn leadscrew but a fun exercise nonetheless.

BUT ... I contacted Hardinge .. who contacted their "UK friends" to see if we could get an answer. It took a couple of weeks of emails .. but ... dignitaries, ladies and gentlemen, boy and girls. It is 100% ACME. Here is a snippet of their manufacturing spec.

leadscrew_spec.jpg

Case closed. :)

Greg.
 








 
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