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Machining an "Astronomy tap" (M48 x 0.75)

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#1 nateman_doo

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Posted 28 November 2021 - 04:22 PM

As fellow hobbyist builders, I am sure you guys could appreciate how often you just wish you had an M48 x 0.75 tap to build stuff.  unfortunately no one on earth makes one.  So I decided to take a shot and try to make one.  I have no idea how successful I will be, but it can't hurt to try right?  

 

Worst case I have a helical boring operation on my homemade CNC to cut M48 threads regardless, but sometimes I just want to use a tap.

 

The material I am attempting this with is 4140 steel.  It is easily hardenable. and soft enough (and cheap) to try it out and see if I can make a successful prototype.

 

Here I am turning a chunk down to 1" just as a shaft:

douoVETl.jpg

(I have been known to dabble in precision)

 

Here are a few attempts to cut threads and a 3° taper at the end of the tap.  (This is just a scrap piece of the steel)

6o8BJcMl.jpg

 

The idea is once the threads are cut, to mill out slots on the rotary table, but I think I have to do that operation first due to the amount of burs when slot milling:

cdq2S61l.jpg

They are cut every 40° to give me some hella rake angles on the each of the cutting passes

 

Also milled a 0.375 square end on the back to put in a tap.

89mw7w0l.jpg

 

 

Now is where it sits, ready to actually cut the threads and put the chamfer. 

eqwUZyil.jpg

 

 

Will update as it goes.  I would like to cut another one first, before I touch this and ruin all the work that went into those slots.

 


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#2 Couder

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Posted 28 November 2021 - 04:45 PM

I have an old International truck. The brakes have a left hand thread bolt, but the threads in the housing were stripped. I made a tap, like you. I made it from cold rolled steel, threaded on the lathe and then cut the slots with the milling machine. I did not, however, make the taper on the end like yours, I simply tapered the end and where the tool started touching the tap is where it started. Hardened it, and it worked just fine. 

So I can certainly appreciate what you're doing!  What you did must be a lot harder.



#3 Beeham

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Posted 28 November 2021 - 07:26 PM

Here's another thing to consider - metric fine thread for M6 through M8 is a 0.75 pitch.  A standard metric tap for M6x0.75 therefore has the correct thread depth and pitch you're looking for, it's just the diameter is too small. (6mm instead of 48mm)

 

Yuu can put the M6 tap in the tool-holder of your lathe, and use the tap itself as a boring tool to create your M48x0.75 female thread.  

 

There's a nice video about using this technique on YouTube.  https://www.youtube....h?v=f-YWsgAHR6A

 

Please post some pics when you finish your project, I'm interested to see how it turns out.

 

Cheers!


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#4 nateman_doo

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Posted 28 November 2021 - 07:31 PM

Here's another thing to consider - metric fine thread for M6 through M8 is a 0.75 pitch.  A standard metric tap for M6x0.75 therefore has the correct thread depth and pitch you're looking for, it's just the diameter is too small. (6mm instead of 48mm)

 

Yuu can put the M6 tap in the tool-holder of your lathe, and use the tap itself as a boring tool to create your M48x0.75 female thread.  

 

There's a nice video about using this technique on YouTube.  https://www.youtube....h?v=f-YWsgAHR6A

 

Please post some pics when you finish your project, I'm interested to see how it turns out.

 

Cheers!

I keep debating on doing that, but when making a tap from scratch, i feel like the teeth have to be slightly oversized so the filters don't bind on the threads.  

 

If I cant do it with the single point thread, I may resort to that as my last ditch effort.  The hardest part is the thread depth.  I cant even physically handle the wire thread gauges to measure if the threads are cut to depth.  Its like holding pencil leads.  

 

I do have 2 other chunks of steel I'm going to try it on before I use this particular notched steel.


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#5 don clement

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Posted 29 November 2021 - 12:56 PM

I typically just cut  M48-0.75 female threads on the lathe. 

 

 

IMG_3667Web.jpg


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#6 555aaa

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Posted 29 November 2021 - 01:35 PM

It’s interesting but like Don I’d just single point it on the lathe because it would be a bugger to get square. But it might be handy as a chasing tap for damaged threads. I think I’ve seen an expandable tap where you slot the grooves and then you have a central bolt and taper so as you tighten the bolt you get a larger thread pitch diameter.
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#7 PETER DREW

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Posted 29 November 2021 - 03:51 PM

A standard thread chaser used as a boring bar at the correct feed also works.  It will be interesting to see what the intermittent cut across the slot gaps will turn out like. 



#8 nateman_doo

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Posted 29 November 2021 - 03:53 PM

I can already machine the threads on my CNC,  Just looks like fun to actually make a tap, and yes, to fix any kind of buggered threads.  Getting taps square is very easy.  Just have to make a tap guide.

 

You can see machined helical bored threads, and how easy it is to make a tap guide out of aluminum here:

https://www.cloudyni...r-drawer/page-2



#9 Tom Stock

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Posted 29 November 2021 - 04:14 PM

Here's another thing to consider - metric fine thread for M6 through M8 is a 0.75 pitch. A standard metric tap for M6x0.75 therefore has the correct thread depth and pitch you're looking for, it's just the diameter is too small. (6mm instead of 48mm)

Yuu can put the M6 tap in the tool-holder of your lathe, and use the tap itself as a boring tool to create your M48x0.75 female thread.


What a great idea!! Thanks for sharing that.
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#10 don clement

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Posted 29 November 2021 - 05:34 PM

I can already machine the threads on my CNC,  Just looks like fun to actually make a tap, and yes, to fix any kind of buggered threads.  Getting taps square is very easy.  Just have to make a tap guide.

 

You can see machined helical bored threads, and how easy it is to make a tap guide out of aluminum here:

https://www.cloudyni...r-drawer/page-2

I use a fisher microtap guide on practical sized taps as a tap guide usually with a T-handle tap wrench on both mill and lathe.http://www.cartertools.com/fmpdtg.html

Attached Thumbnails

  • Helicoil3719_zpspv5hfv2x.jpg

Edited by don clement, 29 November 2021 - 05:35 PM.

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#11 nateman_doo

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Posted 29 November 2021 - 08:33 PM

The real question is how to measure the threads once the tap is complete.  My standard for success is to start with a 47mm hole, use this tap, and see if my filters fit nicely.  

 

I cant even physically hold the 3 wire gauges to measure the thread depth.  I plan to have a few 1/8 sheets with holes on standby so I can test the tap while it is still engaged to the half-nut.  The minute I disengage it, the threading operation is done.  Sucks, but im just happy it cuts the thread regardless.  

 

I did measure the OD of a specific filter and that was under 48mm OD, so the tap can be 48mm OD.



#12 don clement

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Posted 29 November 2021 - 08:54 PM

The real question is how to measure the threads once the tap is complete.  My standard for success is to start with a 47mm hole, use this tap, and see if my filters fit nicely.  

 

I cant even physically hold the 3 wire gauges to measure the thread depth.  I plan to have a few 1/8 sheets with holes on standby so I can test the tap while it is still engaged to the half-nut.  The minute I disengage it, the threading operation is done.  Sucks, but im just happy it cuts the thread regardless.  

 

I did measure the OD of a specific filter and that was under 48mm OD, so the tap can be 48mm OD.

For female threads I turn a male thread gage with some extra room like a few mils oversize to make up for tolerance variation of commercial threads. To hold thread gage wire on male threads these  holders from Flexbar are handy https://www.travers....CoaAvduEALw_wcB


Edited by don clement, 29 November 2021 - 08:55 PM.


#13 bigbangbaby

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Posted 29 November 2021 - 09:31 PM

I use a fisher microtap guide on practical sized taps as a tap guide usually with a T-handle tap wrench on both mill and lathe.http://www.cartertools.com/fmpdtg.html

A manual tapping machine works too. The collets are kind of fiddly but it taps straight every time and gives good feedback, especially important for small taps.



#14 don clement

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Posted 29 November 2021 - 09:56 PM

For small taps I have a Procunier tap head that I use on the manual mill or  Tormach CNC mill for taps smaller than M6 or 1/4". I have tapped 10s of thousands of blind 4-40 holes at 1200 rpm in aluminum with good effect. The Procunier has a double cushioned cone clutch and is especially good with really small taps and blind holes. My Procunier has the Pro-quik quick change collet so no futzing around with wrenches.https://createch-des...Pro-QuikTap.pdf

 

ProcunierQuickPro.jpg


Edited by don clement, 29 November 2021 - 09:58 PM.

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#15 nateman_doo

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Posted 29 November 2021 - 10:21 PM

For female threads I turn a male thread gage with some extra room like a few mils oversize to make up for tolerance variation of commercial threads. To hold thread gage wire on male threads these  holders from Flexbar are handy https://www.travers....CoaAvduEALw_wcB

Ironic I forgot my wire gauges came with those plastic holders.  Since I only have a standard caliper, not metric all these numbers have to be converted from metric to standard.

 

The conversion data for metric threads for 0.75 pitch says: 

Wire size:  0.018

Add:   0.00925 (what even is this?)

Constant:  0.02843

 

The measurement of the 0.018 wires in between my micrometers should be this formula:

Measurement = pitch + constant

Measurement = 1.88976 + 0.02843

measurement =  1.91819

 

Maximum and minimum pitch diameter can be found in the Machinery's handbook.  I dont own one at home, and everywhere I looked online, I see M48 with much coarser threads.  

This is the closest I could find: 

https://www.engineer...read-sizes2.htm

Shows M48x5,4,3,2,1.5 pitches.  

 

So ultimately my only course of action is to get as close to 1.91819 as I possibly can without the upper and lower limits. 



#16 nateman_doo

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Posted 29 November 2021 - 10:24 PM

For small taps I have a Procunier tap head that I use on the manual mill or  Tormach CNC mill for taps smaller than M6 or 1/4". I have tapped 10s of thousands of blind 4-40 holes at 1200 rpm in aluminum with good effect. The Procunier has a double cushioned cone clutch and is especially good with really small taps and blind holes. My Procunier has the Pro-quik quick change collet so no futzing around with wrenches.https://createch-des...Pro-QuikTap.pdf

 

attachicon.gifProcunierQuickPro.jpg

I have the tapmatic that I used to use for most of my 4-40 holes.  Honestly I prefer to hand tap blind holes.



#17 don clement

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Posted 29 November 2021 - 11:25 PM

I have the tapmatic that I used to use for most of my 4-40 holes.  Honestly I prefer to hand tap blind holes.

I prefer to tap all of my 4-40 blind holes  in 6061-T6 aluminum using the Procunier tap  self reversing head and Balax form tap. There are no chips with a form tap. The Procunier disengages the cushioned cone clutch within 1/3 rev. When using the Tormach CNC mill (no servo spindle) tapping 4-40 blind holes with the Procunier tap head: Spindle speed 1200 rpm, Down feed 30 IPM, NO Dwell, retract 60 IPM. Relton A9 cutting fluid is used. I almost never break a tap with this setup. BTW I also have a Tapmatic and the Procunier is way better when tapping blind holes even way better that tapping by hand.

 

Procunier tap head in my Tormach CNC mill

procuniertaphead.jpg


Edited by don clement, 29 November 2021 - 11:30 PM.


#18 nateman_doo

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Posted 29 November 2021 - 11:50 PM

Since my rotary table is not as sturdy in the vertical position, and I need to make room for milling slots, I machined a dead center to give the end of the tool some support for chatter.  Its ghetto fabulous, and I will find out tomorrow if it works when I attempt some ball mill slots, thinner than the ones above.  trying different things since I have 3 blanks to experiment with

 

t46Me2Yl.jpg


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#19 nateman_doo

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Posted 30 November 2021 - 12:41 AM

So I found a calculator online which seems to help:

https://amesweb.info...calculator.aspx

 

I can select "custom use" and choose Nominal Diameter of 48 with a pitch of 0.75, grade 6, with G deviations (just left the grade and deviations as default)

Maximum Major diameter: 47.978

Minimum Major diameter: 47.838

Max Pitch Diameter:  47.491

Min Pitch Diameter:  47.373

(and some other root diameters and what not)

 

So I will pick the pitch diameter right in the center of 47.491 & 47.373 which is 47.432 (1.867")

 

So Caliper Measurement = Pitch Diameter + Constant

Or Pitch Diameter = Caliper Measurement - Constant

Constant = 0.2843

 

Measurement = 1.867 + 0.02843

Measurement of calipers w/3 wires must be = 1.895

this is just here for me to read when I reference this later when I am actually threading.  

 

Step 1, turn OD to middle Major Diameter: 47.978

Step 2, start cutting threads (don't disengage the half-nut)

Step 3, measure with 3 wires until 1.895 is read on the 1-2" micrometer.

Step 4, case harden slightly with torch on the lathe

Step 5, run tap through pre-bored 1/8" aluminum plate

Step 6, test fit filter



#20 Lakec

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Posted 30 November 2021 - 07:41 AM

The real question is how to measure the threads once the tap is complete.

With all the optics experts on this forum someone please start a thread on building an optical comparator!

Jeff



#21 don clement

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Posted 30 November 2021 - 11:30 AM

With all the optics experts on this forum someone please start a thread on building an optical comparator!

Jeff

Might start by looking at the  James Hartness/ Russell Porter optical comparator. 

 

The first commercial comparator was developed by James Hartness and Russell W. Porter. Hartness' long-continuing work as the Chairman of the U.S.'s National Screw-Thread Commission led him to apply his familiarity with optics (from his avocations of astronomy and telescope-building) to the problem of screw thread inspection. The Hartness Screw-Thread Comparator was for many years a profitable product for the Jones and Lamson Machine Company, of which he was president.

 

 

HartnessComparator.jpg


Edited by don clement, 30 November 2021 - 11:38 AM.

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#22 nateman_doo

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Posted 30 November 2021 - 01:07 PM

This is just a hobby for me.  I wouldnt mind making a few dollars on the side here and there, but its just a hobby.  Not going to drop money on an optical comparator so long as the tap does its job and a filter fits nicely, and I can repair any marred threads in the future.

 

Back on the progress bandwagon:

Milling out 0.25" ball end mill slots on this one:

kAt0o31l.jpg

fBjVxbQl.jpg

 

Lots of burs:

nyv9F4Zl.jpg

This is why I am opting to cut the grooves first, so cutting the threads last I can just take sandpaper and something like a parallel to hone the cutting edge of threading burs.

 

re-surfaced the shaft end and the front.  

SWlLh9il.jpg

Ready to be turned down to final diameter (above measurements) and cut the threads.


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#23 nateman_doo

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Posted 30 November 2021 - 04:49 PM

mjBk6l5l.jpg

 

Measured 1.8955  



#24 nateman_doo

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Posted 30 November 2021 - 04:51 PM

And a quick case-harden:

 

fr487Eeh.jpg

y2kGiH6h.jpg


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#25 nateman_doo

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Posted 02 December 2021 - 11:19 AM

I have just welded a "cup" made from tubular steel and a bottom, that will be filled with argon, and borax to coat the tap during the heat treating process to keep the scale down.  More on that later.  In other news, I am going to shift gears and attempt to machine some rake angle into the next prototype:

 

TPyrkbTl.jpg

kUUp2GRl.jpg

 

Single point contact with the teeth, positive rake for very sharp cuts.  An alternate with negative rake will be designed next.  Also moving on to more appropriate tap steel.  W1, (or silver steel as some Europeans call it)  is a drill rod, and designed for cutting tools, high hardness etc.  All the things you need in a tap.  Not the "best" steel on earth, but for what I need it to do, it should be more than adequate 


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