There are many parameters to consider when choosing a turning insert. Carefully select insert geometry, insert grade, insert shape (nose angle), insert size, nose radius and entering (lead) angle, to achieve good chip control and machining performance.
If you are looking for more details, kindly visit Guangzhou Ruiyi Technology Co., Ltd..
l = cutting edge length (insert size)
RE = nose radius
Nose angle
Turning geometries can be divided into three basic styles that are optimized for finishing, medium and roughing operations. The diagram shows the working area for each geometry based on acceptable chip breaking in relation to feed and depth of cut.
High depth of cut and feed rate combinations. Operations requiring the highest edge security.
Medium operations to light roughing. Wide range of depth of cut and feed rate combinations.
Operations at light depths of cut and low feed rates. Operations requiring low cutting forces.
ap
inchmm Feed fn mm inchThe above example illustrates the offer for steel—there are other options available for all material groups.
Use wiper inserts for improved surface finish with standard cutting data, or, maintained surface finish at substantially higher feed rate.
The -WMX wiper geometry is First Choice, and is a good starting point for most applications. When conditions change, there is always a productive alternative.
Choose a positive wiper geometry to lower forces and maintain productivity in case of vibration problems.
Choose wiper geometry as follows:
-WL: For improved chip control when moving to a lower fn/ap.
-WF: Improves chip control at a lower fn/ap. Also for lower cutting forces when vibrations occur.
-WMX: Always First Choice in the wide chip application area. Provides maximum productivity, versatility and the best results.
-WR: When a stronger edge line is needed—for example, for interrupted cuts.
Are you looking for tool recommendations?
Find our cutting tools here chevron_right
Need advice?
Ask us a question chevron_right
Want to learn more about the basics of metal cutting?
Register for our free e-learning chevron_right
The insert grade is primarily selected according to:
The insert geometry and insert grade complement each other. For example, the toughness of a grade can compensate for lack of strength in an insert geometry.
The insert shape should be selected relative to the entering angle accessibility required for the tool. The largest possible nose angle should be selected to provide insert strength and reliability. However, this has to be balanced against the variation of cuts that need to be performed.
A large nose angle is strong, but requires more machine power and has a higher tendency for vibration.
A small nose angle is weaker and has a small cutting edge engagement, both of which can make it more sensitive to the effects of heat.
Cutting edge strength (Large nose angle)
Less vibration tendency (Small nose angle)
Select insert size depending on the application demands and the space for the cutting tool in the application.
With a larger insert size, the stability is better. For heavy machining, the insert size is normally above IC 25 mm (1 inch).
When finishing, in many cases the size can be reduced.
The nose radius, RE, is a key factor in turning operations. Inserts are available in several sizes of nose radius. The selection depends on depth of cut and feed, and influences the surface finish, chip breaking and insert strength.
Small nose radiusLarge nose radiusThe relationship between nose radius and depth of cut affects vibration tendencies. The radial forces that push the insert away from the cutting surface become more axial as the depth of cut increases.
It is preferable to have more axial forces than radial. High radial forces can have a negative effect on the cutting action, which can lead to vibration and bad surface finish.
As a general rule of thumb, choose a nose radius that is equal to or smaller than the depth of cut.
A negative insert has an angle of 90° (0° clearance angle), while a positive insert has an angle of less than 90° (for example, 7° clearance angle). The illustration of the negative style insert shows how the insert is assembled and tilted in the holder. Some characteristics of the two insert types are listed below:
Clearance angle
Clearance angle
The entering angle, KAPR (or lead angle, PISR), is the angle between the cutting edge and the feed direction. It is important to choose the correct entering/lead angle for a successful turning operation. The entering/lead angle influences:
Are you looking for tool recommendations?
Find our cutting tools here chevron_right
Need advice?
Ask us a question chevron_right
Want to learn more about the basics of metal cutting?
Register for our free e-learning chevron_right
Related informationAchieving Good Turning Quality
Learn to achieve high-quality turned components by mastering chip control and cutting data. chevron_right
Discover how to choose the right thread turning insert and shim for optimal performance. chevron_right
Internal Turning Techniques
Internal turning involves machining the inner diameter of workpieces, focusing on tool choice and angle for optimal performance. chevron_right
External Turning
External turning shapes the outer workpiece diameter with high demands on process quality. chevron_right
As Andrew says the great thing about using full profile inserts is that the thread cut is exactly correct so the depth of cut needed can be taken directly from standard tables. The perfectly formed crest is, to me, merely a minor advantage. I tend to trim the crests slightly on customer jobs as the small flat is more durable when the fitter has "shade tree mechanic Bubba" tendencies. I have almost full sets of Johansson / SKF / Dormer chasers that were commonly used on capstan lathes which produce the full form and can, in home workshop terms, be re- sharpened infinitely.
If you do decide to use partial profile or sharp point inserts you will need to calculate the extra depth of cut needed over book value for the thread you intend to make to accommodate the longer tip. A decent CAD program makes dealing with the geometry much easier. The basic dimensions and geometry of threads can be found in most handbooks but handbook writer union rules seem to demand that the diagrams are too small for easy reading. A decent size one for 60° UNF (American) threads can be found here :-
**LINK**
UNIFIED SCREW THREADS
By setting out a "nest" of diagrams in a CAD program its fairly easy to see what you need to change. For all practical purposes metric threads are the same.
I strongly advise using the "Zero-to-Zero" thread cutting method if you plan to use partial profile or sharp point inserts because it makes it very easy to reliably set the required changes in thread depth. So much so that its practical to work by trial and error slowly increasing depth of cut over book until a satisfactory fit is obtained using a good quality nut or accurate test piece made using a good quality tap.
Having found the correct in-feed for that thread and that insert you should make a note of it so you can immediately make the requisite changes in set up when doing another thread of that size.
The reliability of the Zero-to-Zero method means that the thread will come out right first time, every time if you have the correct in-feed values.
The Zero-to-Zero method is classically explained as a way of getting the lathe to calculate the correct in-feed when the top slide is set over at, or close to, half the thread angle. This avoids trigonometrical calculations. Generally considered a good thing. However the method works just fine when the top slide is set parallel to the cross side too.
Because all the setting up is done before you start the job goes easier as you only have to keep an eye on the process rather than worrying about feeds, sizes and things. Setting up at the start means what you set is what you get so if the thread doesn't fit you know what you have and have a solid starting point for working out what …… went wrong (this time!).
In simple terms you start, as usual, with the job cut to the required diameter and tool tip exactly perpendicular to the job.
1) Touch the tool tip to the work then set both top slide and cross slide dials to zero.
2) Pull the tool back a bit using the cross slide, traverse it past the end of the job then feed the cross slide forward past zero by the depth of thread you intend to cut.
Want more information on Carbide Inserts? Feel free to contact us.
If using a sharp point or partial profile insert and intending to find the correct depth by trial and error use the book value for thread depth. If you have already figured out what the actual depth needed is, use that.
3) Re-set the cross slide dial to zero and pull back the top slide to just clear the job.
4) Make the cutting passes with the cross slide at zero, pull it back for clearance when returning ready for the next cut then move it forward to zero for the next pass.
5) Apply cuts as appropriate using the top slide.
6) When all spring cuts have been worked out with both cross and top slide at zero what you cut is what you set.
If using book values with a sharp point or partial profile insert this will not be deep enough.
7) Make extra cuts by feeding the cross slide past zero leaving the top slide set on zero.
When the fit is correct the cross slide dial will read the extra depth of cut needed beyond book values. Add this to the book value and make a note of the result to give the correct depth of cut to set at step 2 when you next have to cut that thread with that insert.
Note that any damage to the tip will upset the calculations and results. Errors will almost invariably be too shallow a cut.
Partial profile inserts from different makers may not have exactly the same shape so if you change insert breeds you may need to alter your personal table of results.
Full profile is lots less faff. But spendy.
Clive
Edited By Clive Foster on 04/11/ 13:57:34
Peter
You don't say whether the inserts used were full, partial or sharp triangle profile? The type makes a big difference as to how easy it is to get a nice fitting pair of threads.
Once you have a decent understanding of the thread profiles its pretty easy to see whats going on and how to modify the starting bore and cut to get a nice tight pair.
Link in my previous post shows the diagram for 60° threads. There are equivalent ones for 55° Whitworth form. In some ways its easier with Whitworth because the thread crest shortening relative to the theoretical sharp triangle is symmetrical. Assuming full profile inserts simply cutting one or other thread a touch deeper will give any clearance needed. In other ways its harder due to the existence of flat top and truncated versions.
Shortening on 60° threads is asymmetrical which can cause issues.
The proper calculations are not friendly but can be found in decent books and, presumably, on the internet. My reference book, should I ever need such things is "Guide to World Screw Threads" by P.A.Sidders. Machinery's Handbook and other references also have the data but Sidders is easiest to handle.
Never having needed to work to that level of precision with properly controlled allowances I simply cut the external thread dead to size, bore for the nominal crest position of the internal thread and cut the internal thread just enough deeper that it all screws together. Using a test piece if need be to get it right.
Initially you will probably find it helpful to make a short plain land on the outer end of each thread at a diameter corresponding to the bottom of each thread. This will give you something to measure and confirm exactly how deep a cut you have made.
Clive
Edited By Clive Foster on 05/11/ 22:51:13
Peter
As it was a partial form tip you clearly went too deep to get the necessary root clearance.
Unfortunately I can't find a good diagram of the Whitworth thread profile explaining the shortening of a real profile relative to a theoretical sharp pointed one. Best I've found is this pretty crappy example :-
**LINK**
https://www.unikgaugesindia.com/bsw-bsf-whits-thread-gauges.html
Basically "H" is the full depth of a theoretically sharp pointed thread
and
"h" is the depth of the real thing cut with rounded crests and valleys instead of sharp points.
"h" is the thread depth given in the data charts.
To get "h" from "H" you divide H by 6 and subtract that from both top and bottom of the theoretical sharp thread. This is called shortening.
The crests of the thread are at the nominal internal or external diameter so effectively a real thread is cut 1/6 th of "H" shallower than the theoretical sharp pointed one.
With a full profile its all worked out for you. Cutting to book depth gives the correct thread with the crests shortened by the correct amount.
With a partial profile you have to go a little deeper to make room for the longer, smaller radius, tip. The shortening now has to be calculated for the finest thread that tool will manage.
Summarising :-
Theoretical sharp thread depth is 5/6 of H where H = pitch multiplied by 0.
Shortening for real thread depths is H/6 = pitch multiplied by 0.
To get depth of cut calculate 5/6 of H for the pitch you intend to cut and subtract the shortening, H/6, for the finest thread that the partial profile can manage.
Hopefully that helps.
Truncated Whitworth forms have flat rather than radius tips so the shortening is different. Basically the rounded crests are removed reducing the depth by pitch multiplied by 0.. So the depth of cut is is reduced by that amount.
The gotcha with truncated Whitworth is that the outer crests are also removed so the starting diameter for an outside thread has to be pitch multiplied by 0. smaller and the bore diameter that much larger.
Calculating depth of cut for partial profile truncated Whitworth is painful!
I avoid such maths like the plague and, if I have to always draw it out on the CAD.
You see why I advise the Zero-to-Zero technique. It makes keeping track of this stuff easier.
Practical folk just make the bore a touch too small, so they know which direction the starting error is in then take fine cuts until things fit!
Clive
PS Don't even think about allowances and tolerances!
Edited By Clive Foster on 06/11/ 10:48:14
Jon
The great advantage of Zero-to-Zero is that the thread depth cut is exactly, within the limitations of machine and operator setting accuracy, what you set when feeding the cross-slide past its initial zero position.
If the tool shape is correct and the thread doesn't fit you have set the wrong depth or not fully taken out backlash in the feed screws when first touching off the work before zeroing the dials for the first time.
If the tool point is too sharp then Zero-to-Zero will indeed create an over deep thread with too small a minor diameter.
No way round that.
But if that is a worry the extra feed past zero on the cross slide needed to make the thread fit tells you exactly how much to grind off the end to bring it back to the correct width. Generally with hand ground tooling I would err on the over sharp side. If I needed less than 5 thou extra feed I'd call it good. Now I have the full profile Johanesson / SKF / Dormer chasers things just come out right.
I would only use Martin Cleeves method when opening up an Acme, Trapazoidal or Square thread for minimum backlash after first cutting it over-tight.
I dislike the uncontrolled nature of the parallel movement correction. You have no way of knowing if the error is due to fundamentally too little depth of cut or having an incorrectly shaped tool with too sharp a point. If the depth of cut is correct and the tool too sharp the method, theoretically, works fine. If there is any other error everything is somewhat up in the air an you cannot be sure of exactly what happened or be confident in the accuracy of fit.
None of which stops Martins method from producing a decently functional thread in reasonably skilled hands.
But its not something that should be resorted to if you have access to full profile inserts.
Martins method is probably most appropriate when using the part circle shaped HSS tools ground to either 60° or 55° that fit it Andycraft, Denford and other holders sized for small lathes. These are inevitably too sharp in the point and adjust in for each new thread wasteful. I imagine you could calibrate the side feed needed for different threads.
Clive
The chuck side flank will be formed by the tool tip, the opposite flank will be formed by the topslide infeed and angle at which it was set. If the tool is not square on to the work and is say angled back towards the tailstock the chuckside flank will be at too shallow an angle and the other flank whatever the topslide was set to and composed of series of 5 thou steps. For set over method which helps, when using dodgy home ground tools and no TC grinder, a slightly more pointed tool could be used with the front flank set at the correct angle and then the rear flank will be generated correctly with the topslide infeed.
If you are using tips then the tool gemetry should be correct so if you wish to use the set over then an angle of slightly less than the half angle will ensure that a scrape is taken on the rear flank and the thread angle will be correct.
However you either have to resort to geometry and advance the cross slide whilst retracting the topslide a little to acheive the correct thread width or go overly deep.
If you are using tipped tools then there is no point and as Andrew has said you may as well go straight in to depth and then you can use the topslide to widen the thread untill it fits.
regards Martin
Edited By Martin Kyte on 08/11/ 15:29:05
Edited By Martin Kyte on 08/11/ 15:30:23