ACME THREAD QUESTION

[CC]

So our old ROCKFORD COMPACT PLANER MILL has a badly worn vertical ACME 2.500" - 4 TPI feed screw nut.
I'm going to measure the existing screw using thread wire method and make a external threaded plug gage
that matches the existing screw to use when cutting the nut.

We want to make a new feed nut that fits the existing screw and while re-learning my ACME knowledge I ran across a question.
According to the MACHINERY'S Handbook formulas, the BASIC PITCH DIAMETER of a 2.5"-4TPI is 2.375" for the INTERNAL THREADED NUT.
The formulas also give the PITCH DIAMETER for the EXTERNAL THREAD as 2.3624" MAX.

I did layout of internal and external thread and found that the BASIC PITCH DIAMETER for the NUT is 2.375"
And the MAX PITCH DIAMETER for the SCREW is 2.3624" as the formula states due to the ALLOWANCES..
I'm going to make the nut fit the screw so I'm not worried about the pitch diameter for this job BUT.......

Here's the question: If I were making a new 2.5"-4TPI screw and didn't have the nut, would I use the 2.3624" PITCH DIAMETER to calculate the
BEST WIRE SIZE and DIMENSION OVER WIRES?

 

[4GSR]

Here's all the information you need to know for making gages and cutting the thread for a 3G fit. A 3C fit is similar. If it was me, I think on such a large thread as that is, a 2G fit would be better.

 

[Phil in Montana]

Why rebuild everthing? chase the lead screw so it is stright and true... not worn, and make a nut to fit...Phil

 

[Scottl]

Or you could buy one from these people.

Acme Threaded Mount Nut, 2 1/2 - 4, RH, Bronze - Roton Products, Inc.

Shop Acme Threaded Mount Nut, 2 1/2 - 4, RH, Bronze from Roton Products, Inc. View all of our products and shop online for your power transmission needs.

www.roton.com

They also sell matching lead screws.

Acme Lead Screw, 2 1/2 - 4, RH, Steel - Roton Products, Inc.

Shop Acme Lead Screw, 2 1/2 - 4, RH, Steel from Roton Products, Inc. View all of our products and shop online for your power transmission needs.

www.roton.com

 

[farmersamm]

I hope this helps, but it's probably wrong.

I had a shaft that had cross threading damage. The thread was totally ruined.

I turned it down to the next imperial size, and threaded it, using the correct pitch diameter for a 2G I think......been a while, so I'm not totally up on remembering.

Then, because the nut to fit the smaller diameter thread does not exist, (yeah, really), I had to make a nut.

I made a plug gauge to the specs for the external 2G thread, with the proper pitch diameter.

Then I made the nut to fit the plug gauge.

The reason I did this, was because the shaft was too heavy, and cumbersome to test fit the thread I could cut in a nut. The plug gauge was light, and easily managed. Everything fit very well after it was all done.

My takeaway............if you work to the called pitch diameter, things will fit. I know it's pretty mundane, but it was a big victory for me. The Bush Hog is back in service, and the world is once again running on greeeezed grooves.

 

[farmersamm]

for me at least, the whole thing was sized off of the new threads on the shaft. New threads, plug gauge made to the same specs as new threads, nut made to fit the plug gauge.

 

[Clive603]

CC
I reckon you are overthinking this.

As its a repair job I'd clean the screw up to remove any wear and make my plug gauge to match the cleaned up screw rather than worry about whether its to book specification.

When cutting your plug do remember that there is a tendency for the first turn to cut a bit thick due to clearances being fully taken up on the relatively heavy cuts. My late friend John who used to make feed nuts and such for the CVA lathe folk advocted removing the first thread of any turned plug gauge for that reason. I'm not gonna argue with a guy who casually demonstrated the ability to produce a nut to match a farmed out by another mate, incorrectly made, acme screw with better than a tenth backlash in about half an hour from opening the workshop door.

When the thick first turn bit me I ended up with a couple of thou longitudinal slop rather than the thou or so I was aiming for. Didn't really matter but annoying when trying for a real nice job rather than OK.

Clive

 

[stephen thomas]

random comments:
I file the snot out of the first thread, flanks and peak, lathe in reverse.. But turning it down would work, too; once close enough that you would not them be creating a new first thread just beyond it.

If it matters, Roton products are rolled thread. Lead seems pretty accurate. But multiple requests for statement of tolerance or limits after buying one were never answered. They are making "leadscrews" to transmit motion or translate position, but not with the accuracy or fit one expects from a precision screw that is accurate enough to measure position. Backlash (clearance) with one of their supplied bronze nuts is generous.

I like the suggestions about re-machining the old screw and making a new close fit nut.

smt

 

[gbent]

Green Bay Manufacturing states the accuracy of their acme thread lead screws.