Tapping an internal M4 thread in an M6 brass threaded rod,

[murrmacParticipant @murrmac]

Hi, I think this is my first thread …so hello everybody.

I have just purchased my first mini lathe (( 7" -14" from Amadeal ) and one of the first jobs I want to do on it is to tap an M4 internal thread in a M6 brass threaded rod.

The length of the M6 threaded rod will be 2 1/4" and the depth of the M4 internal thread ( blind hole) will be 1".

Question is …will there be enough wall thickness left to do this, and what size hole should I drill?

I anticipate doing the tapping manually, …I suppose that a skilled machinist could do it with the tap in the tailstock chuck …but I am not that skilled machinist .

 

All advice gratefully received.

 

Edited By murrmac on 20/12/2013 23:13:20

[murrmacParticipant @murrmac]

[Oompa LumpaParticipant @oompalumpa34302]

You are going to end up with a wall thickness of .5 mm (or thereabouts) so it will work, but it will be quite fragile.

graham.

[murrmacParticipant @murrmac]

Thanks Graham.

This brass rod is going to be embedded into wood, with an epoxy glue, so as long as I tap out the wood first to an M6 thread , then the fragility should not be an issue, as the wood and the glue will provide support …leastways I surmise …

[Paul LousickParticipant @paullousick59116]

Do a web search for drill size for tapping. There are heaps of tables to download which will specify the required drill size.

[FMESParticipant @fmes]

Hello,

This might help with the tapping sizes **LINK**

As Graham says, the resultant 0.5mm thickness between the two threads will leave a very weak joint, and you run a very big chance of running offline so drilling the tapping hole is going to be a bit of a challenge on its own.

I would suggest making a blind holed jig to hold in the lathe chuck and tapped to the 6mm size and screw your brass rod into this (lock it with a nut) then drill the M4 tapping size (3.5mm) and then using the tailstock chuck gently hold the tap to get it started square.

Turn the main chuck by hand while feeding the tap in, and remember to reverse as per the norm when hand tapping.

When the tap is well started, remove the tailstock chuck and fit a tap wrench and continue until the tap bottoms or remove before if swarf builds up.

Repeat for second and plug taps.

Lofty

[murrmacParticipant @murrmac]

Ok, you guys have convinced me that maybe it isn't such a great idea, but how's about if I reduced the internal thread to M3.5 ?…(I really don't want to go as low as M3 , and I can't really go as high as M8 for the external size of brass rod )

[Michael GilliganParticipant @michaelgilligan61133]

Posted by murrmac on 20/12/2013 23:10:05:

The length of the M6 threaded rod will be 2 1/4" and the depth of the M4 internal thread ( blind hole) will be 1".

.

murrmac,

These do seem rather strange proportions.

Could you please tell us what the insert is intended to do? … there may be a better way to achieve what you need.

MichaelG.

[JasonBModerator @jasonb]

One problem you will have is tapping 1" deep as the average M4 tap will not be long enough You will either have to have a shorter internal thread or find a long series tap but that may still not have the flute length.

You could go to metric fine which will give you a bit more metal between the two threads.

Is the M6 to give a key for the glue or to provide some adjustment before gluing. If its just for a key you could knurl the part of teh rod that will be in teh wood. Again I assume the wood is thinner than 2.25" as you will have a very hard time tapping that length.

EDIT, Michael is thinking like me

J

Edited By JasonB on 21/12/2013 07:34:36

[GoCreateParticipant @gocreate]

Hi

Actually you need to consider the M6 rod thread root diameter not the tapping drill size. The thread root diameter has a tolerance of 4.891 to 4,596 for most standard M6 threads.

You could end up with a wall thickness of 0,445 to 0.298. However, because the thread is spiral this is not a constant wall thickness, for a large proportion of the length of the M4 thread there is considerably more thickness available.

With your M6 rod epoxied into the wood I would go ahead and try the M4 thread on a trial piece, once set fit in a screw and see how difficult it is to pull out with a pair of pliers or similar. I think it's worth a trial test.

I am sure M3.5 will be OK.

Nigel

Edit – my post crossed with Jason's, as he says, a finer M6 thread would add strength.

Edited By tractionengine42 on 21/12/2013 07:38:34

[GoCreateParticipant @gocreate]

Hi

Maybe 1/4" UNF rod may be the way to go? Plenty material left for an M4 tapped hole.

A finer thread may reduce the strength of the bond with the wood, but if epoxied in I think won't be an issue.

Nigel

[Oompa LumpaParticipant @oompalumpa34302]

The job is a non starter. If you are thinking of screwing 21/4" of 6mm .5mm wall tube into wood. Forget it. Unless the wood is something like rosewood or lignum vitae or such, you are not going to get a correctly formed 6mm thread into any wood, not 21/4" of it anyway. As for putting it into and out of jigs, that isn't going to work either, the slightest bit of moisture add a bit of heat with drilling and tapping and it's all over.

You are on the edge of everything with this and it would be very useful if you could give us some idea of what you want to achieve, there are a great many experienced people on here and they are full of great ideas. I look upon engineering as (mostly) a problem solving exercise. What you propose is a real problem.

graham.

[jason udallParticipant @jasonudall57142]

First. Runnout of hole 3 mm for 25 mm…tricky
Two rod is probally thread rolled..so brass might be work hardened
Three itis normal in small threads to use the upper value for tapping drill ( makes tapping easier ) further reducing your wall thickness. .

Have you considered “heli coils” or simular ?

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Just design the problem away :

Internal thread will develop all the strength it’s ever going to with about 15 mm full threads – probably less .

External thread does not have to be full length . Some of it where the internal thread is can be unthreaded .

Make the whole thing shorter .

AND/OR

Use a Rawlplug or drive plug type profile on outside – a row of shallow grooves would probably do .

OR

Large numbers of different ready made plugs are available for putting tappings into wood . Metal and plastic ones usually come with a drive plug type external profile ready made .

MikeW

[Clive HartlandParticipant @clivehartland94829]

A simpler method is Knurl the 6mm brass rod and then drill and tap the 4mm thread. The knurled rod is then epoxied into a 6.1 mm hole and will be much stronger.

Clive

[EmgeeParticipant @emgee]

Lots of good advice given but only the Link answers the question re hole size for the M4 tap. For metric tapping drill sizes deduct the rate of thread from the diameter, this gives tapping drill size for adequate thread engagement in most cases. M4 is 4×0.7mm so 4.0 minus 0.7=3.3mm drill.

Rgds, Emgee

[murrmacParticipant @murrmac]

Thanks to all who have replied.

Basically , I want to make a fitting to attach a strap to the neck heel of my beautiful handmade acoustic guitar.

The normal method of doing this is to screw a strap button into the side of the neck heel but for many reasons, not least of them aesthetic, I don't want to do that.

A much more elegant solution is to have the fixing in the base of the heel, but the problem with that is that it throws the balance of the guitar off …it wants to cant away from the player when it is held in the strap, and what you want to occur is the exact opposite…the guitar should want to lean back into the player's body.

So what I have come up with is a solution whereby I will have a Z shaped stainless steel bar, 3mm thick and 12mm wide, with two holes drilled in each of the Z ends. One hole will accommodate the bolt which will fix the Z bar to the underside of the heel,( by screwing into the threaded insert) and the other hole will accommodate the clip on the end of the guitar strap. The length of the Z will be around 2 1/2" …this will give the correct balance, so that the guitar doesn't want to pull away.

I don't think there will be any problem tapping the wood …the neck is made of Cuban mahogany, which is close grained enough to take a thread, but I am constrained by the fact that the base of the heel, which is triangular in shape has quite a small area, so I don't want to remove too much wood.

Thinking about it, there isn't any reason why the internal threaded  hole has to be 1" deep…it only needs to be deep enough to accept the M4 bolt, so probably 15mm depth would be ample.

The whole object of the exercise is to enable the fixture to be removed when not in use, and for the fitting to be as discreet as possible.

I am thinking now, in view of the advice above, that there are only two solutions …either tap out the M6 insert to to 3mm or else do as Clive suggested and use solid 6mm rod and knurl it (I would actually file grooves, I don't have a knurling tool).

Hope I have explained clearly enough what I am aiming for , and thanks to everybody again for their help.

 

 

.

Edited By murrmac on 21/12/2013 10:51:45

Edited By murrmac on 21/12/2013 10:52:22

Edited By murrmac on 21/12/2013 11:20:23

[Tony Pratt 1Participant @tonypratt1]

An M4 tapped hole x 15mm deep will be more than adequate, you will not get any more strength by going deeper.

Tony

[JasonBModerator @jasonb]

You could possibly counterbore the hole in the wood just over 6mm and leave the end of the rod where you have the tapped hole plain.

J

[julian atkinsParticipant @julianatkins58923]

my eldest daughter who is a very good guitarist says no way should you even consider drilling the neck heel, instead tie a shoe lace around the head just above the nut like everybody else does. and depending on how your neck is fixed to the body plus if there is a truss rod there may be an awful lot that mustnt be disturbed in the neck heel.

cheers,

julian

Edited By julian atkins on 21/12/2013 13:20:04

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Tony Pratt 1 on 21/12/2013 12:01:25:

An M4 tapped hole x 15mm deep will be more than adequate, you will not get any more strength by going deeper.

Tony

In fact it can be a lot less than that. A good rule of thumb is 4 threads engaged or, as the link says 1.5 x diameter. So, for an M4 thread then a 6mm depth will give pretty much maximum strength.

Link

Rod

[Les Jones 1Participant @lesjones1]

You could cut the grooves in the lathe. mount a "V" shaped tool on its side with the point of the "V" at centre height.
Traverse the carriage back and forth to cut one V rotate the chuck a small amount. repeat this until you have rotated the chuck 360 Deg. You could use a marker against the teeth of one of the change wheels to space the grooves evenly.

Les.

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by julian atkins on 21/12/2013 13:14:36:

my eldest daughter who is a very good guitarist says no way should you even consider drilling the neck heel, instead tie a shoe lace around the head just above the nut like everybody else does. and depending on how your neck is fixed to the body plus if there is a truss rod there may be an awful lot that mustnt be disturbed in the neck heel.

Well, by hanging the strap between the nut and the end block the player is putting a side strain on the neck and the place where this will fail will be at the body/neck join. By having the strap between the bottom of the neck (the heel) and the end block all the strain is taken by the body which is a nice strong box construction. Whatever the type of neck/body joint is used (and there are dozens of designs) there should be plenty of meat to screw a strap button into.

Murrmac, any chance of a picture of your guitar?

cheers,

Rod

[Gray62Participant @gray62]

I'm gonna chip in here as an amateur luthier.

drilling the heel of the neck should not cause you any issues, I could better advise on this if you could supply a dimensioned drawing of the heel, but mahogany, being very close grained hardwood will not take harm from some intrusion.

You really need to knurl or similar on the outside of the insert to give the adhesive a good bonding surface.

What type of neck joint construction has been used, is the heel dovetailed into the block or, as Taylor guitars sometimes use, is the neck bolted into the body block?

Graeme

Edited By CoalBurner on 21/12/2013 15:14:05

[Ed DuffnerParticipant @edduffner79357]

Hi Murrmac,

Just had a look at a couple of my guitars for ideas. They pretty much use the standard strap button or nothing at all, requires a lace or something attached to the neck.

I think adding a 'Z' shape bar (basically a lever) might introduce radial and lateral forces on the mounting and over time it could loosen and fall out.

Have you considered a harness as an alternative to a strap.?

**LINK** …a bit pricy though (IMHO).

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