Counting TPI?

[Ms Wolffie]

How do you count the TPI on an ACME screw?
I know the diameter.
It has 3 threads.
I counted 8 TPI, however, does it make a difference when it is a triple thread?
TIA
Wolffie

[Ms Wolffie]

CORRECTION:

The screw has 4 starts,
The diameter is 10mm (3/8")
I counted 8 TPI
When I see a tap that says ' Trapezoidal Metric Thread Tap TR 10 x 2 '
I know the TR10m= 10mm but what does the 2 mean?

Cheers
Wolffie

[CarveOne]

If you trace one of the threads with a marking pen you will see that it makes two turns per inch of rod length. It has four threads spiraling around the rod in parallel. That's why you see a total of 8 threads in one inch of rod length.

CarveOne

[Ms Wolffie]

Thank you C1.
Is it called 2 TPI then?
I need to buy a new screw, the old one is bent.
I won't go into how I did it
Cheers
Wolffie

Ps it seems to me a 2mm per turn is a very fine thread?

[CarveOne]

TPI = Threads Per Inch. Your metric screws would be referred to differently since it it is Threads Per MM. TPMM?

I may need to make a correction also. I posted at about 2:38 am my time and my brain was probably not fully engaged. Here is a link to some information that should help. http://en.wikipedia.org/wiki/Trapezoidal_thread_forms

Scroll down to the chart and formula for metric threads.

I use 1/2" x 10 - 5 start on one of my two machines and was more familiar with the specifics two years ago when I built it. My old head has to purge some mechanical information to make room for so much new Aspire 3D software information.

[Leo]

Glad to see the link to the wikipedia

There is a difference between TPI and pitch.

TPI - is Threads Per Inch. If you count how many peaks on the thread in a one inch distance - THAT is threads per inch. I do not think that is whet you want.

On something like a lead screw it is talked about a little differently. It is discussed in terms of thread form, number of starts, and pitch.

The pitch is the distance from one point on the thread to the next "same" point on the next thread.

The lead is the distance one revolution will advance the screw. In other words - if your screw was installed in you machine - one revolution would advance the slide - one lead.

If you take a marker and color the top of one segment of the screw - then another color on another segment - and repeat for all the segments - it is easier to see.

[Ms Wolffie]

Thank you both for your help.
This screw is for my CNC Shark made in the USA and hence has TPI not pitch and it also has 4 starter points.
I am trying to work out how to either straighten the x-axis screw or buy a new one.
It advances 16 mm (3/8") per turn.
As fa as I know, it is an ACME screw 19 degrees, not a Trapezoidal one.
Unfortunately it looks like ebay only has metric 10mm (3/8") x 2 screws, so I am trying to work out what I can use.
2mm pitch seems a very fine thread to me but maybe I am wrong?
Looks like I can only find 3 starter points as well.
Cheers
Wolffie

[Leo]

I dunno how badly bent it is, but you should be able so straighten it.

I stand to support the screw at both ends with an indicator stand in the middle to measure runout.

You can apply pressure in the middle at the high point.

Turn the screw and measure runout.

At my place of employment we make drop indicators.

This is a method we use to straighten the spindle to within .0002"