Screw loose

[Richard V-D]

I have a bit of a wobbly problem at the start of my Sig Spacewalker build.

Instructions say drill and tap 4-40, but I'm doing M3, in an ally wing joiner piece. Harking back to school boy metal working, I drilled the 2.5mm pilot hole and tapped with a M3 second tap. Trouble is the screw is loose in the hole. Questions are

1) Is this a function of the pilot hole being too big?

2) Is this liveable with?

I've put a video on youtube so you can see.

[J V R]

Hi Richard VD probably the bolt is under size, try some other bolts with a bit of luck you might find one that's a better fit.

[Gary Manuel]

Is the tap still a tight fit in the threaded hole or is this also loose?

This will tell you whether there's a problem with the M3 threaded hole or not.

[Richard V-D]

Hi JVR, Tried a selection and all exhibit the same wobble.

The thread is strong enough to hold the screw when it's clamped down, so even though it's wrong, perhaps it's not too wrong.

[Richard V-D]

Interesting Gary. The tap is also loose in the hole. Problems with the hole then?

[Shaunie]

At risk of being rude, how do you screw? Many still advocate the half a turn and back it off method which is only necessary if the tap is well past it's best and creates a rough and baggy thread. If the tap is OK it's far better to use a little lube and tap the whole way going forward only. If you are brave enough tap using a cordless drill, make absolutely sure you have it square, tap in until the tap unloads, reverse and back out without wobbling the drill. You will get a clean thread cut to size. Trust me, tapping machines in engineering shops don't go in a bit at a time! Looks scary but when I was designing electrical control panels the Wiremen would tap hundreds of holes a day and rarely break a tap.

Shaunie. 

Edited By Shaunie on 29/09/2014 23:22:58

[Gary Manuel]

Looks like it.

Did you screw backwards regularly to release the swarf?

If you don't do this, the swarf sometimes gets trapped between the cutting edge and the metal and affects the cut. Particularly bad with soft metals like aluminium as I recall.

 

Edit - or listen to Shaunie who beat me to it - he sounds like he knows what he's talking about.

Edited By Gary Manuel on 29/09/2014 23:26:41

Edited By Gary Manuel on 29/09/2014 23:27:12

[Martin Harris - Moderator]

2.5 mm is the correct tapping size for M3 x 0.5 (coarse) - with metric threads you just deduct the pitch value from the diameter...

Is it possible that the drill bit used is badly ground? If the lands are unequal then any drill will make an oversized hole. Another possibility for drilling oversize is a wobbly hand held drill in relatively thin material - or did you use a drill press?

Edited By Martin Harris on 29/09/2014 23:33:31

[Richard V-D]

I'm one of the tap a bit, back off a bit etc etc.

[Richard V-D]

Drill press.

[Shaunie]

Looks like we have opposite opinions. The lubricant is there to help the award flow. Every time you back out the tap there is an interruption to the cut and you create a step in the helix of the thread. WD40 works quite well on aluminum, something heavier like oil for steels. Speed helps the swarf to flow hence the advantage with the cordless drill.

Shaunie.

[Richard V-D]

I'll get a new drill bit and practise some more. Luckily the clamp holes are only on one side, so I have one more go at this bit of ally!

[Shaunie]

I often use a drill press manually turned for tapping holes square but I would not use that for anything less than about 5mm as it is to easy to force the tap to cut out of pitch. Did you use a standard drill press or a miniature modellers one?

Shaunie.

[Martin Harris - Moderator]

Shaunie and Gary beat me to the post while I was writing my previous one. A lot has happened since my metalwork lessons but aren't machine taps designed rather differently?

[Richard V-D]

Standard drill press.

[Martin Whybrow]

One thing no-one has asked, how thick is the metal you're tapping?

[Martin Harris - Moderator]

Looks about 6mm on the video...

[kevin b]

I agree with Shaunie on this one.

However I would advise you practice on another piece of scrap ally first before having another go on your U/C.

[J V R]

Richard,

Do you have access to some die's Richard, if so make a new bolt with a split die & open it up as much as the holder will allow, this will cut an over large thread which will be a tighter fit in the tapped hole. You can make the bolt with a piece of 3mm rod with a short thread on the top and screw on a nut using high strength thread lock to make the head, then when its set cut the bottom section of thread using the opened up die.

Edited By J V R on 30/09/2014 10:58:11

[kc]

It would seem worthwhile examining the drill bit carefully to avoid future problems. Could it be mis-marked or could you have misread 2.6 or 2.8 for 2.5? Usually the problem is the otther way round - a worn drill produces an undersize hole and the tap then breaks!

Is the tap a quality brand?

 

Cheap drilling machines often have terrible chucks which dont centre the drill properly or have poor spindles which wobble when fully extended.  This might easily cause an oversized hole when the work is clamped fimly.  Check!

Could turning the component over and end to end produce a new place to drill correctly?

 

Edited By kc on 30/09/2014 11:32:12

[Simon Chaddock]

Richard

It depends on the type of ally but it can 'pick up' badly when you are cutting the thread. The high pressure at the actual cutting point causes the ally to bond to and build up on the tool cutting edge. Obviously the built up edge is not as strong as the tool steel so it breaks away regularly during the cut and can cause a ragged over deep cut, hence a loose thread.

As the tool edge wears the cutting forces increase and the pick effect gets worse so ally is best cut with a really sharp tool preferably one that has not been used on steel. Obviously sharpening a tap is not easy.

A 'wobbly' thread has only a fraction of the strength of a correct depth one so beware. Is it a steel bolt?

If steel then the problem is more serious as a 'loose' thread raises the pressure on the thread faces and as steel is much harder than ally guess which will wear fastest!

Enough of the Workshop Technology!

Unless you can get hold of an oversize thread bolt the best 'engineering' solution would be to cut a new bigger diameter thread and use a bigger bolt.

[Levanter]

That is a seriously wobbly thread and not to be trusted!

The size of the tapping hole is a bit of a red herring. The tap cuts the maximum bolt diameter and until the drilled hole exceeds the maximum diameter (ie becomes clearance) it will make little or no difference to the fit. In reality and taking tolerances into account the movement will increase as the hole nears the size of the thread but unless the tapping hole is grossly oversize there should be no increased movement. What an oversize tapping hole does however is reduced the amount of thread engagement and that of course can seriously weaken the connection.

I used to run an engineering company and light alloy foundry. We tapped tens of thousands of holes in aluminium every day on automatic and semi-automatic machines. The taps always went through in one pass but they were high quality, sharp and had copious lubricant. Backing off the tap, especially by hand is likely to make things worse. Some of the alloys were heat treated and quite hard in which case taps were used. Most of the alloys were quite ductile and therefore Roll taps were used. These do not have flutes but have three lobes and a continuous thread. They cold form the metal and produce excellent mechanical properties. The tapping drill size is larger because some of the material is pushed up into the tap. Most of the alloy we modellers use including Dural of the thicknesses used for undercarriages will accept roll taps very well. Available from good tool merchants.

As an aside, fastenings were generally speaking only fitted directly into tapped holes in aluminium where the assembly was permanent or semi-permanent.. Any items intended to be regularly disassembled were fitted with Helicoils from a wear point of view and also to eliminate the risk of cross threading.

Like Simon says, if you can re-tap for a bigger bolt but don't repeat the problem and has been said before do a dummy run first.

[Cliff Bastow]

Sorry Shaunie but I totally disagree, I was tought that becuse unlike a drill, a tap has no flutes to clear away swarf, it must be rotated a bit and backed off. I was taught 60/40 and this always works for me.

This is especially important when tapping blind holes. as if not the tap can easily be snapped by a build up of swarf.

[Richard V-D]

@Martin The material is 5mm.

@kc Measured with a micrometer, the drill bit is 2.47mm, and is a couple of years old (not much use though). The tap is from ModelFixings, as are the socket head screws.

Just making up a shopping list for MF, which includes a couple of new 2.5mm bits. Then a few test holes and taps.

Thanks for the help guys.

ps  Well, as I took ages to actually post this one, more came in!    Going up a size might be viable, although 3.5mm would be the max.  As for assembly/dis-assembly, the channel forms part of the wing attachment, and so would be cycled each flying session.  An aluminium bar slides into the channel and held/clamped in place by the 3mm screws.  There are four of these assy's, one left and right and one front and back.

 

Edited By Richard V-D on 30/09/2014 13:54:53

[Shaunie]

Sorry cliff but I stand by what I said, unless it's a roll tap it will have flutes although unless a machine tap they will be straight. When tapping a deep or blind hole it may be necessary to back out the tap and clear the flutes but just backing up a bit does not really do that. Filling a blind hole with lubricant helps as it is displaced up the flutes helping clear the swarf. Bear in mind my Dad now gone for twenty years was an old school engineer and taught me the old way of tapping, it was working as a design engineer in a high volume engineering company that taught me new methods and their value in improving the quality.

Shaunie.