Calculating Diameter

Hi Guys,

I'm a lathe and threading newbie with a Myford ML S7 an am trying to learn how to better use it, but I have a question.....

I wonder if someone can kindly explain to me how to to calculate the diameter of the internal female thread when you have male and female components that screw together. For example if making a small round box, 30mm in diameter. A thread is cut on the male part, what would be the dimensions of the lid of the box that fits this thread? Is there an easy method of calculating the size of the clearance hole in the top and the depth of thread?

Thanks in anticipation....

Cheers David

I guess you want to know the minor diameter. If it's metric just subtract the pitch. Pocket Ref

I think this would involve High Level Maths, way beyond my pay grade.

On the other hand, I seem to remember Sparey giving some useful guidelines about this.

The boy bit has to be a bit bigger than the girl bit

Depends on the type of thread you want to make; Metric coarse/fine, BSW, BSF, BA, UNF, AF et al. Do you have a ZEUS book ? if not I suggest you get one they are cheap as chips. In it you'll find all the thread types you need with all the relevant core diameters, ID & OD, thread pitches / depth etc plus a load more handy information. Well worth it at todays price.

Amazon.co.uk : zeus book ...have one at £5.20

George.

First you need to decide the thread form whitworth or metric

then you need to determine the pitch if metric or tpi in imperial eg 10 tpi or 2 mm

then get some tables which will show you the proportion and the maths eg, pitch depth is .64 x pitch

then calculate the numbers from the thread profile data

too many unknowns at the moment except your stated diameter, but in wood, you will need a coarse thread to five any real strength

I suggest that you invest in a little book by Martin Cleeve, called I think "Thread Cutting in the Lathe", which has simple and clear formulae for 55 and 60 degree threads and much other wisdom besides.

Does the Zeus book cover Model Engineering threads or are they in the Pocket book mentioned above ?

Geo.

Edited By mechman48 on 12/06/2021 18:06:18

As David says 30mm diameter, lets assume Metric.

Usual to work from the male diameter, which in metric is given by the M designation. M30 means the male thread is cut into a 30mm diameter rod.

From a table, book or internet, M30 bolts can be had in 3 pitches:

• Coarse = 3mm,
• Fine = 2mm,
• Extra Fine = 1.5mm

Coarse for quick fitting or soft materials. Fine threads for strength.

The female thread is cut into a smaller hole. For roughly 80% engagement, it's simply rod diameter less Pitch. So for M30 coarse: 30 - 3 = 27mm Bit smaller for a tighter fit, bit bigger for looser. Model Engineers tend to go for loose rather than tight fits because cutting them causes less wear on taps and dies without significant loss of strength. They're also less fussy about alignment.

As my maths is terrible, there are plenty of online calculators about. This example does standard and non-standard imperial & metric sizes for any degree of fit.

Dave

I keep a bookmark of the Motalia website, they have a thread chart.

For instance, 1/2 UNF female thread. Thread depth 0.0271" Multiply that by 2 because the thread is right round the circumference, and take it away from the nominal size.

0.500" - 0.0271" - 0.0271" = 0.4458" . The 0.4458"is the start size of the bore of the female thread. These figures can be rounded down to 3 places of decimals, ie, 0.446".

**LINK**

Thank you guys for all of you helpful inputs....things are becoming more clear. I'm looking to cut mainly metric threads to start with (although the lathe is imperial and fitted with a gearbox) but I have the T33 and T34 gears for the mandrel to convert.

I have downloaded some metric course and fine thread tables which I think will help. I need to study them and to make some practice cuts, and I've ordered a Zeus book.

I appreciate all of you help....thanks....David

Hi David. I believe you will need to make a New thingy. (cannot remember what they are called.) to use the 33-34 gear set. I have a drawing of such an item. So hold on while i find it. Ok so it is called a drop arm & here is the drawing for it. Your welcome.

Steve.

Hi Steve, thank you for the info and the drawing; its the first I've heard that I needed to make/fit this drop arm, although I have the gears I haven't so far tried to fit them to the lathe, but I did have a quick look and saw the diameter of the T33 and T34 gears is somewhat larger than the existing 24T gear currently on the mandrel, and 'assumed' (shouldn't have done that) that the difference in diameter could be adjusted for on the quadrant clamp stud.

Looks like a drop arm is my next project....

Cheers

David

Short answer:

Look at Zeus chart for thread concerned., to find the OD, and the depth of thread. ( d )

Hole size = OD - (2 x d )

If no Zeus chart (You really need as a ready reference )

Metric is easy. Hole is Nominal diameter - pitch

M20 ( Coarse ) is 20 x 2.5 mm pitch

Hole diameter is 20 - 2.5 = 17.5 mm This provides truncation to prevent Crest / Root interference.

For Imperial threads, Whit, or Unified, (And BA ) you need to look up the depth.

Many charts give the Tapping size.

For smaller sizes, (Unless a non standard pitch, or you don't have the Tap ) you just drill /bore to the tapping Size, and use Taps.

For odd ball pitches, you may well have to screwcut. The you are into calculating changewheel set ups, which an different ball game.

As a beginner, avoid that that if you can, until you have more experience, or a friend who will stand with you and guide. The experience will always stand in good stead.

Biting off a little bit more than you can chew is how you gain experience! We all do!

Howard