Bridgeport Series 1: Metric Feed Nuts

Hi all,

I'm in the process of renovating my Bridgeport Series 1, and have bought some new feed nuts to install.
The old feed nuts were so worn that there was barely any thread remaining at all!

I purchased a bunch of parts from H&W in USA, including new feed nuts.
When installing the Y-axis leadscrew into the new feed nut, I noticed that it was extremely tight.
Once I got it past the first 100mm, it became much easier (where the leadscrew is somewhat worn).

I'm guessing I've got a metric leadscrew, trying to mate with an imperial nut!
It also turns out that metric nuts are hard to find/buy.

Now that the y-axis is installed, I'm inclined to leave it as is.
The leadscrew operates freely within the range that i'm likely to use it. it's only when the table is at the very back of the knee where it starts to bind.

However, with the x-axis, I'd like to try and 'open up' the threads on the nuts, so that they fit the metric leadscrew better.
Being a right-hand thread, I could do this on my lathe.
By my calcs, it should only require up to around 0.6mm of material removal from the thread, so it's nowhere near as bad as a moderately worn nut.

BTW, the nuts are split, so I'll still be able to adjust the backlash.
And I'm using a DRO, so backlash isn't critical either.

Is there any reason why I shouldn't stick my x-axis feed nuts in the lathe and re-thread them to 5mm pitch metric??


Thanks,
Hugh

I am sure that Hardinge in the UK have metric feed nuts in abundance. It seems strange to try to alter a brand new feed nut to fit something it was never designed to fit - if you can cut the thread why not make your own feed nuts?

It appears that these people will get you a metric feed nut [email protected] at a much better price than Hardinge around GBP70

dpcwright said:

It appears that these people will get you a metric feed nut [email protected] at a much better price than Hardinge around GBP70

Click to expand...

Thanks for the tip. I'll send them an email.
GBP70 is still pretty pricey in my book for such a beast (and that's the imperial version - waiting on email response to confirm price of metric version). That's 3.7 times the price of what I paid for the ones from H&W in USA.

As it turns out, my lathe can only turn up to 3.5mm pitch threads, so my plan isn't going to work anyway.
I started to measure up the thread form, with the idea of getting my local machine shop to make some, but I can't seem to work out what thread form it is!

As I understand it, ACME is an imperial leadscrew thread form, and Trapezoidal (aka DIN-103) is a metric equivalent (or close equivalent).
If the leadscrews were imperial, then it appears the appropriate ACME thread form would be 1 1/4 inch nominal diameter, which has 5 TPI.
My leadscrews measure as having roughly 32mm major diameter, and 30 pitches across 150mm, i.e. 5mm pitch. However, there is no Trapezoidal thread form (that I can find) which has a 32mm major diameter and 5mm pitch.

Can anyone confirm what thread form the metric lead screws should be??

Thanks,
Hugh.

This looks like a special as my metric feed screw measures 32.27 mm major diameter and has a pitch of 5 mm i.e. the same as yours and DIN 103 quotes 32mm diameter threads but in pitches of 3,6,and 10 mm.
Maybe Mr Bridgeport did something special when he made a metric version & perhaps why it is so expensive.

Investigating for similar reasons I concluded that Bridgeport used 5 mm pitch ACME to true metric dimensions on the same diameter rod and bores as Imperial ones. Would have made sense from production viewpoint as keeping materials and machining steps common right up to the actual threading process. ACME standards are defined by relative porportions so true metric dimensions are fine and, in priciple, any convenient diameter can be used for any thread. Conventional standard sizes are merely a convenience to make life easier.

Had similar problem some time back with E-Bay bargain NOS screws and nuts for my metric B'port. Pretty much none of the purchases were what they claimed. Even got metric nuts supplied with an imperial X feed screw.

Pretty sure that worn metric and imperial can be made to work, possibly with one component new. Had a Heinz 57 assortment of screws and nuts to hand when adapting Boxford saddle to a SouthBend. I'm pretty sure the best fit nut & screw pair were different language but which was what .....

Clive

The pitch would be off about 0.004"/inch. I don't think that it would fit. The new nuts should be very tight over the "un-used" portion of the old screw.
JR

I had that same problem once, I ended up using epoxy, devcon bronze to rebuild the nuts. I cast the nut on the screw you break them free just before they harden completely. It was better than using the wrong nuts, I have no clue how some metric screws got on an old J head BP with standard dials. I used to overhaul these things for a company and I always just slapped new nuts on and went about the job but nothing fit for some reason.

I replaced my METRIC series II up down with a new leadsrew and nut and dial if you have metric leadscrew your dials would also be in metric ! my cross feeds i just set the readouts to Eng. I do have the metric Up down screw /nut / and dial in really good shape if anyone is interested .

Hi all,

Thought I'd update this thread with my solution, in case anyone else comes across the same issue.

For the last six years, I've just lived with having the imperial feed nuts installed on the metric leadscrews. It mostly works fine (my leadscrews were quite worn in the middle of the range anyway), it's just that they get very tight at the ends of travel, meaning I don't have the full range of movement.

The reason I never purchased the proper metric feed nuts is that they worked out to be around $300AUD per axis. Which is a lot to spend solving a minor inconvenience.
I did intend on making my own nuts on the lathe, however my lathe was unable to do that 5mm pitch thread.

Fast-forward six years, and I decided to buy a 3d printer, to print some feed nuts.
I attempted some reasonably accurate measurements an unused part of the x-leadscrew.
Major Dia = 31.45mm , Minor Dia = 25.8mm.
Tip width = 1.8mm, Root width = 2.2mm.
Interestingly, that works out to be around a 20deg thread angle, which is significantly different from a 'trapezoidal' or 'ACME' thread.
I modelled my nut thread to have an extra 1mm on the major and minor diameters (knowing that the 3d print would come around 0.4mm undersize).
The nuts are a really nice fit, and I've now reassembled everything and have full range of the tables again!
Sorry I didn't take more pics.

I'd love to know if anyone knows of the 'proper' bridgeport metric leadscrew/nut dimensions. I find it strange that my measurements don't align with any standard threads.

Cheers,
Hugh.