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Results 1 to 15 of 23
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22nd June 2015, 11:10 PM #1
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Calculating thread depth for cutting threads on a lathe
Hi all, I've been here only a short time and only posted acouple of times. I know there's a lot of much smarter people here then me. I'vebeen trying to work out thread cutting on my hercus model A, the part I'mhaving trouble with is getting the right thread depth. The formula that I'veworked out is my own as I'm not a machinist, only a hobbyist.My formula in short is ( for imperial) 1000divided by the thread pitch divided by 2 multiplied by (for a 55 deg thread)the tangent of 62.5 degree =1.921 (say a thread of 36 tpi .277 divided by 2 =.138 x 1.920 would have a thread depth of .248)or for a 60 degree thread you would multiply it by 1.732.
Does that sound anywhere near the right way to calculate thethread depth?
I've tried it and it seems very close, the threads seem toknit together ok but maths isn't my strong point and this formula is one basedon angles and tangents.
Any thoughts?
Stephen
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23rd June 2015, 07:38 AM #2
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Hi Stephen
general rule of thumb is
55 deg= 0.64 x pitch
60 deg= 0.61 x pitch
Pitch = 1 divided by the threads per inch (TPI)
In your 55 deg example
pitch is 1/36= 0.0278
multiplied by 0.64 which equals 0.0178
which means the depth of thread would be 18 thou or 0.018" rounded up of course
If my maths is right
Phil
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23rd June 2015, 10:55 AM #3
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Thanks for that Phil, my maths isn't much good these days.Numbers start to rattle around and I lose them. I've muffed so many attempts at cutting inside and outside threads because I didn't have a clue but always believed there must be some formula to calculate it. I'm only a boilermaker and as such only ever learnt the tricks applied to that trade and that never included machining or any of it's trade tricks. I'll have a go at using your formula and let you know how I went.
thanks heaps
Stephen
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23rd June 2015, 06:56 PM #4
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I've never trusted myself to rely on formulas to do thread cutting. My hand ground HSS toolbits don't follow standard thread form rules, I just slightly round the point. So, book formulas wouldn't work for me.
A practical method is to have a matching nut to test fit, with small cuts when approaching full depth.
I cheat and finish with a die after thread cutting on the lathe first to near size.
I can now get perfect threads, with very little stress on the dies. More relaxing for me too!
Jordan
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23rd June 2015, 10:09 PM #5
Philomath in training
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See if you can find a Zeus book. It's a little booklet made of card that has some useful information in it on holes and threads in particular.
One thing that you can do is regardless of the thread OD is use the depth information for any thread provided it is the same pitch/ tpi and same thread angle. To put that another way, it doesn't matter whether it is a 1/4" 20 tpi UN thread or a 2" 20tpi UN thread, the depth is still the same*.
There are formulas and I use them myself when necessary but I find it much easier to use tabulated information like that.
Michael
*Although the tool tip radius may change that admittedly
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24th June 2015, 07:19 AM #6
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I'm pretty much the same Jordan, always use a nut wherever possible and normally stone the point for the radius as I don't have anything to correctly measure it.
I have thought many times about thread wires and am confident I will be buying a set.
Another way I have used to get the correct thread thread form is multi-point screwcutting.
This is it here https://www.woodworkforums.com/showthread.php?t=166136
There is a plethora (second time this week I have used that word) of books out there for your reading pleasure Stephen to help you understand the dark art of screwcutting.
Just as an aside, I have a set of thread gauges that has the depth of thread written/stamped on them.
PhilLast edited by Steamwhisperer; 24th June 2015 at 07:31 AM. Reason: corrections after hitting send instead of before
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26th June 2015, 10:23 AM #7
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Hi all, I tried using the formula today of dividing 1000by the tpi and multiplying it by .64 I used a fine thread of 48 tpi, (1000/48 = 20.8333 x .64 = 13.333) theformula said it would be around .0133 but it was closer to .017 so I'm stillway off and can't be clear about what allowances I should leave when attemptingto cut matching threads. So far I've only used 55 or 60 deg inserts, I thinkeven the art of grinding accurate tools is acquired over many years and ratherthen throw in yet another variable at this stage wouldn't help my cause much. It still seems to be a trial and error thing.
Stephen
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26th June 2015, 03:34 PM #8
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Not sure where you pulled the 1000 from.
Try dividing 1/48 = .020" 020"x.64 =,013"
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26th June 2015, 07:50 PM #9
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Hi, I pulled the 1000 from the 1" to calculatethousandths, it gave me 13.3 thou which is .0133 and that s what I worked onbut it didn't work out, the thread was to shallow or only partly cut I had totake it .017 before it formed a full thread. I will do some more reading aboutit because I'm not doing something right. I've used 55 degree inserts to eliminatepoor tool grinding, so far it's been like the Bermuda Triangle for me, one bigmystery, but hopefully soon the penny will drop and I'll find where I'm goingoff track.
Stephen
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26th June 2015, 09:07 PM #10
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What method are you using to cut the thread, Offset or Plunge.
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26th June 2015, 09:23 PM #11
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Hi Stephen
I am not very clued up on inserts but is the insert you are using within that thread/TPI range?
Phil
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27th June 2015, 11:22 AM #12
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27th June 2015, 11:35 AM #13
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Yes Phil, it's a standard 55 deg, I have some 60 deg tips, I might change to those and see what happens but I should imagine it would only result in a shallower cut depth. I have some more paperwork to study and I'll try things again in a few days time.
Thanks
Stephen
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27th June 2015, 09:55 PM #14
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Is the insert your using an 11R or 16R ?
For the pitch your cutting it should be an 11R.
Are you feeding at all with the compound?
Generally with plunging you will need to also advance the tool toward the headstock by using the compound, I think in total around 25% of the pitch.
This is done in ratio to your in feed of the cross slide, although you don't need to feed the compound every time you feed the cross slide.
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27th June 2015, 11:39 PM #15
Mechanical Butcher
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Is it possible to do a nice clean thread by plunging?
I tried it once and made a mess of it.
Being a lazy amateur, I'd suggest it's much easier to do a one-sided cut with top slide angled.
Jordan