Stated thread depth never works for me.

[Jim GardnerParticipant @jimgardner97734]

Posting in Beginners questions because I feel like the whole planet knows the answer to this except me.

It is not that often that I have to make a nut or bolt, but today I had to made a 1/2" Whitworth bolt with left handed thread. Obviously I turned the OD to .500, changed the gears in lathe to enable 12 tpi, looked up the thread depth (and Googled it twice) and cut to a depth of 0.055" which is 0.001" under size. The compound was at 29 degrees, depth was measured using a dial gauge. Everything seems to work perfectly.

I took the new bolt out of the lathe and,,,,it wasn't even close to having enough thread depth (at a guess I would say I should have cut another 8 thou. This seems to be the result I get time after time. I do believe I cut to the "correct" depth, but it is never enough. Even my 80 year old engineer mate who has been thread cutting all his life tells me to always go deeper.

Can any body tell me why>

[Jim GardnerParticipant @jimgardner97734]

[David AmbroseParticipant @davidambrose86182]

At the risk of insulting you – is your cross slide graduated in depth of cut, or reduction in diameter? Mine is the latter, so I would have to double the value to achieve the correct depth.

[JasonBModerator @jasonb]

Most likely your cutting tool does not have the correct radius ground on the end so to compensate for that you need to have a deeper infeed so that the flanks end up in the correct position

[Jim GardnerParticipant @jimgardner97734]

David,

It's reduction of diameter. The error isn't 50% but it is enough to be a problem.

[Howard LewisParticipant @howardlewis46836]

Firstly, a Whitworth thread has an thread form angle of 55 degrees, (Not 60 ) so the Cross Slide should have been set to 27.5 degrees, and geometric allowance made for the angular offset, to cut gradually to the full depth.

This maybe one reason why you cut the thread to an incorrect depth.

60 degrees is for Unified or Metric threads.

47.5 if you want to cut BA, 29 for Acme.

Howard

[Jim GardnerParticipant @jimgardner97734]

Jason, It's a commercially bought/made 55 degree tool insert so I had assumed it would be the correct radius. If not, how would I ever know what my required depth of cut should be?

[JasonBModerator @jasonb]

Pic of why a pointed tool (green) or one with incorrect radius when fed in the set dimension doe snot reach the flanks of the thread

[Jim GardnerParticipant @jimgardner97734]

Howard, good point, but I don't think 2 degrees would make any difference to cutting depth would it?

[JasonBModerator @jasonb]

Jim is that a full form or partial generic 55deg insert, only full form will work to the dimenson

[Jim GardnerParticipant @jimgardner97734]

Jason, you may have fund the problem. I looked it up and it is "partial profile". How then, would I work out depth of cut I need?

[Jim GardnerParticipant @jimgardner97734]

And more to the point, what are "partial profile" used for?

[JasonBModerator @jasonb]

A lot of people make the famale part first and then cut the male to fit.

Trial and error measuring with thread wires as you cut deeper will get the flanks right if you don't have a female part.

Your insert may give a tpi range, it will have the radius for the smallest pitch in that range, a fag packet sketch or better still a CAD sketch will probably be the easiest way to work out how much deeper you need to go

Edit partial are used bymodel engineers etc who don't want to spend out on a whole range of specific tpi insert. They will cut a specific range say 16-12tpi or 12-8tpi and work out cheaper if you are only doing the odd thread

 

Edited By JasonB on 02/08/2023 17:13:08

[mgnbukParticipant @mgnbuk]

Edit partial are used bymodel engineers etc who don't want to spend out on a whole range of specific tpi insert.

Not just model engineers, my former employer used PP iinserts most of the time. A 60 degree PP insert threading tool lived permenantly on one tool station on all the CNC lathes and would cut the majority of metric or unified threads. Thread specific inserts were only used for "out of the ordinary" situations or where the thread pitch was outside the range covered by the PP insert. It was standard practice to use go-no go thread gauges on the first off to determine the infeed required to get the desired fit.

Nigel B.

[Jim GardnerParticipant @jimgardner97734]

Thinking about it some more, it seems to me that depth of cut, is depth of cut regardless of full or partial profile inserts.

This is not my usual field so I am learning as I go, but it does seem strange that with all the tables/charts/equations etc. available, most people seem to measure thread depth by trial and error.

Unfortunately the female the bolt is needed for, is a spindle on a large electric motor that is far to big and heavy to offer up to the lathe.

[SillyOldDufferModerator @sillyoldduffer]

I think Jason is on the ball. Jim's 0.055" is about right for a 0.64 x pitch depth of cut but that's only valid for a full-form thread cutter. I bet Jim has a partial form cutter. For these, depth needs to be more like that of a V cutter, where Whitworth depth of cut is 0.8 x pitch, roughly 0.068" (13 thou deeper than Jim)

Thread inserts are often 'Partial form', that is they don't finish the top of the thread. The advantage is the same insert can be used to cut different pitches, but for them 0.64p isn't deep enough unless the outside diameter is reduced to compensate. Or the insert is pushed a little deeper.

'Full form' inserts cut the thread correctly top and bottom, so 0.64 pitch is the correct depth for them. Unfortunately a full form insert only cuts one pitch, and owning a full set of them costs a bomb. A waste of money unless exact specification threads are essential.

Rather than do hard sums when using partial form inserts, I cut the thread to a middle depth, and then finish to fit by testing with a nut. I'm a metric shop, where full form is 0.625P and V form is 0.72p. I normally use partial form carbide inserts. For ordinary work I generally jump straight in by going 0.66 x pitch deep. If a close fit is needed, I only go in about 0.63 pitch and carefully fine tune to fit the nut from there. When using a V HSS cutter, I go straight to 0.72p, and don't worry about the result not being perfect metric – it only has to fit. Or I take a little off the rod diameter before starting, about 0.1xpitch, and then cut to theoretical depth.

if I have one the right size I always finish lathe cut threads with a die. Dies do a good job of getting the form correct.

Dave

[Jeff DaymanParticipant @jeffdayman43397]

If you want to check the thread accurately you could google "thread measurement over Van Keuren wires" which is a method that measures the thread geometry at or near the actual pitch diameter of the thread and takes away variables of tool radius, profile, etc. Assuming thread flank angle is correct and root radius is not too large, if your thread measures correct "to the tables dimension" using correct sized wires and a micrometer, it's very likely to fit a "to spec" nut. Just food for thought.

[Anonymous]

Posted by Jim Gardner on 02/08/2023 17:44:42:

Thinking about it some more, it seems to me that depth of cut, is depth of cut regardless of full or partial profile inserts.

Wrong!

As SoD says a full form insert will finish the crests of the thread and also have the correct profile for the root for the thread pitch to be cut. A partial insert has a very small radius for the root. For coarser threads in the range covered by the partial form insert the depth of cut will need to be increased over the theoretical value.

I use full form inserts; can't be ^^^^d to faff about with partial form inserts. In practice I find that the theoretical thread depths are fairly accurate. One might have to cut a thou or two deeper to get a nice fit. But there's nothing to say that the female thread is accurate.

Andrew

[JasonBModerator @jasonb]

Posted by Jim Gardner on 02/08/2023 17:44:42:

Thinking about it some more, it seems to me that depth of cut, is depth of cut regardless of full or partial profile inserts.

Couple of image sthat may show why some depths of cut are more correct than others. Nothing to scale but just to show the differences

First shows three 55 degree tools, on the left is what a partial would look like with a small radius, in the middle a simple vee form straight off the bench grinder and on the right a full form with the larger radius to suit it's particular pitch If we touch off on the side of the shaft and then infeed 3mm the width of the Vee at the edge of the bar varies, the two sides of the vee are the thread flanks.

If we now try and cut a thread with the middle pointed form and the right hand full form both to the same 4.337 pitch as the full form you can then see that the full form has gone in and cut the correct thread but the pointed form has not gone in enough and the crest is flat and measuring with thread wires wire would measure a larger value

Now if the infeed of the vee tool is increased from 3mm to 4.116mm so that the flank distance becomes the same as the full form

You will then get a thread form that has the correct flank distance and if measured by wires would equal the 3mm infeed of the full form insert

[DC31kParticipant @dc31k]

Given that a partial profile insert has a tip radius to suit the finest thread in its range, the infeed for that fine pitch should be the same as for a full profile insert.

There is a mathematical relationship between pitch and tip radius.

So for any other pitch within the insert's range, it should be possible to derive a correction factor (a number greater than 1.0) by which the full form infeed should be multiplied to obtain the partial profile infeed.

Every manufacturer's tables that I have seen are based on full form inserts so it would be a useful exercise to derive (and possibly publish) these correction factors.

—

Now to flies in the ointment…

There is a slight complication when it comes to 60 degree partial pitch inserts – what tip radius do they actually have – the minimum tpi stated for the insert or the minimum metric pitch stated for the insert? The two numbers will vary slightly, but maybe not enough to be significant.

Partial profile inserts generally come as Axx, AGxx and Gxx (and Nxx and Qxx for really coarse stuff). The minimum pitch of A and AG is the same, but G starts at a coarser pitch, so there would need to be two sets of factors for each vee angle (correction factor for G60 on 1,75mm pitch would be 1.0) .

[MadMikeParticipant @madmike]

Sorry chaps but I am slightly confused. Setting the compound slide at 29 instead of 27.5 degrees was an error but if you were using a full form threading tool why did you set the slide at all? If you use the compound slide to set the tool to its correct depth then you need to calculate the length along the flank of he thread as it will greater than the actual final thread depth. Simple schoolboy geometry will give you the flank depth. From memory you need OHMS… Opposite over hypotenuse equals Sine (in this case Sine 27.5 degrees).

[Martin KyteParticipant @martinkyte99762]

Forget half set over angle method for insert tooling when threading.

This is what I do.

Set compound slide parallel and the tool correctly perpendicular to the work.

Using the cross slide to put on the cut, cut the thread to the correct depth.

Using a pre-made gauge or nut increase the width of the thread by advancing the compound slide a couple of thou.

When the gauge or nut starts to go on for a few threads reduce the compound advance to a thou each time until you can fully engage the nut.

You will have produced a thread with correct angles, correct depth but with a flat bottom with slight radii generated by the partial form insert.

Simples.

regards Martin

[old martParticipant @oldmart]

If you are cutting a male thread, it helps greatly to have a matching nut to use as a gauge. Start checking after getting to the theoretical size and then creep at 0.001" cut per pass until the nut almost screws on and then take a couple of spring cuts. I always use tapping oil when singlepointing on a lathe for steel and aluminium, but usually cut brass, cast iron and bronze dry. I have just cut a 10tpi ACME leadscrew and nut and had to do a lot of small cuts before they would fit together, fortunately without any noticable loosness. I never use any offset on the compound as the lathe is heavy enough not to need it.

Edited By old mart on 02/08/2023 21:23:10

[Nigel Graham 2Participant @nigelgraham2]

Food for thought in that lot.

I must admit I'd often wondered the same problem, such as only yesterday when cutting a 1/4" X 19tpi BSP thread and had to creep up on the last bit beyond the theoretical feed depth for the commercially-made union nut to fit.

I was using an insert tool, squared to the chuck face, and direct in-feed, starting from and allowing for a slightly under-diameter surface so as to give a little flat along the crest.

Unusually for me I'd also set the cross-slide to count up to 0, rather than starting from 0 and counting up to depth value. This clearly showed a considerable discrepancy which I put down to accumulations of slight setting error, springing and wear in the lathe (a Myford ML7)… and wear in me.

It had just not occurred to me to consider the geometry. Usually I close-cut a thread then finish with a die but of all the dies I lack or cannot find, 1/4" BSP is one.

So Thank-you, Jim, for asking!

:::::::

I have only once had to analyse thread geometry to any extent.

This was to tap the hold-down holes in the four feet on my Harrison L5 lathe cabinet for the levelling-screws the Manual reckons are there, at 1" BSW. Unable to drill the holes that little bit larger through the 5/8" thick steel, I resorted to partially tapping the holes M24, easing the developing thread to Whitworth, taking another bit out at M24… Took all day, lying on my side in a very cramped position, turning the taps barely a 1/4 turn at a time…. I won't make a habit of such antics.

To prepare for it I drew the thread profiles, ignoring the roots and crests, to establish the interferences and errors. I forget if I drew them completely or in fact plotted them as a pair of traces on an "Excel" graph, but it worked.

[duncan webster 1Participant @duncanwebster1]

Posted by Martin Kyte on 02/08/2023 20:58:48:

Forget half set over angle method for insert tooling when threading.

This is what I do.

Set compound slide parallel and the tool correctly perpendicular to the work.

Using the cross slide to put on the cut, cut the thread to the correct depth.

Using a pre-made gauge or nut increase the width of the thread by advancing the compound slide a couple of thou.

When the gauge or nut starts to go on for a few threads reduce the compound advance to a thou each time until you can fully engage the nut.

You will have produced a thread with correct angles, correct depth but with a flat bottom with slight radii generated by the partial form insert.

Simples.

regards Martin

as above, but for every 20 thou cross slide movement, advance the top slide 10 thou. This means the tool is only cutting on one side (OK it's scraping on the other) and the chips can get out more easily. The angle generated is 26.5 degrees. I sometimes wonder if this is why Mr Whitworth came up with 55 degrees (half angle 27.5).

If you zero the dials when the tool just touches, it means the topslide is keeping track of where you are rather than having to remember every time you withdraw the tool

If you're grinding your own tools, Geo H Thomas came up with a table of how big a flat to put on the end for different tpi rather than trying to get an exact radius. OK it should be rounded, but unless you're into high stresses and fatigue it won't matter

[Author]

Posts