Thread: Help with tapping

I'm trying to put an 8mm thread into a piece of 4140 steel rod.

I've drilled a 7mm hole in the end of the rod (the tapping tables I have said a 6.8mm hole was required so I went with the next size drill I had.)
The total length of the rod is 70mm but I only have to tap about 35mm of it.
I'm doing this in the lathe, unpowered, so I can get a straight for the tap and I'm using a tapping compound.
I can get to about 28mm and then can no longer turn the tap without the rod turning in the lathe chuck. Being new to this sort of thing, I'm reluctant to force it and break the tap.
Any tricks to this or is it just a fact of life that tapping 4140 is difficult?

Are you working through the different types of taps? eg: taper, intermediate etc. And are they good quality and good condition?

Originally Posted by INVENTOR

Are you working through the different types of taps? eg: taper, intermediate etc. And are they good quality and good condition?

They're brand new (Suttons) but there's only two taps in the set. I'm assuming taper and intermediate.

If you are already in 28 mm then there is no nead to continue in the lathe the tap is already straight and will continue that way.
If for some reason the lathe chuck will not hold against the tapping force then remove it and hold it in a clamping block .
Make up something like a boring bar holder and clamp down on the shaft and then lock that block in the vice,
It seems to me that you may be putting too much force on the tap before you back it out. Tapping should be , turn in a bit and feel it cut , as soon as it gets harder , stop reverse , and if it stops again go forward a bit and then back , and once it turns a few turns back go in again for a new cut.
Only high quality tooling that is being used on purpose set up machines with perfect lubrication , speeds , feeds and geometry on the right material will just bore in all the way and tap a thread.
You should use a tapping lubricant like easy tap , tap magic or even pure coolant will work.
Back the tap right out now and then and brush the tap clean .
Also as the taper tap gets deep it may reduce turning force to put the second tap in and clean out a bit to loosen up the taper tap a bit more and work your way down.
It is possible that you have heated up the 4140 a bit during drilling and it is making the threading hard going.

I bought a set of three 6mm suttons taps a while back ,and the intermediate tap was blunt strait out of the packet.So it could be that the tap that is not very good.
Dave

Just a suggestion, read this lot and take note of the type of tap, called a GUN TAP. Keep scrolling, they are there.

http://www.sutton.com.au/IndustrialMetricTaps


Then a bit more reading as well.

http://en.wikipedia.org/wiki/Taps_and_dies

I did a fair bit of toolmaking when I was in the trade and found "Devcon Devtap" to be fantastic as a cutting lube on real hard to deal with metals when tapping,don't be fooled by it's clear water like look because it sure used to pack a good punch (no sniffing).

As I have been out the trade for many years I don't know wether or not you can still by it, but take it from me if you can get hold of a can it's magic and saved me many times.

I have many kinds of taps but when I was teaching in the USA I discovered "Gun Taps". These taps are now my favorite tap to use. They are the best ever for tapping "through" holes but need a deeper hole drilled if the hole to be tapped is a blind hole. They get their name Gun Tap because the swarf and chips are pushed ahead of the tap the same as a bullet in a gun gets pushed ahead from the case in the chamber. They are very clean cutting and the ones I have only have two flutes.
To tap the 4130 do as Retro suggested. When the tap has entered about 20 or 25 mm, Take the job out of the lathe and secure it firmly in a vice using soft jaws to protect the job. Wind the tap in with a normal tap wrench and lots of cutting oil. The two fluted taps are still easy to start and dont jamb up with the swarf like a "normal" tap does.Gun taps used on "through" holes can be screwed straight in without having to stop and reverse every 1 or 2 turns when used with mild steel. I have used them on automatic tapping machines that ran at 800 rpm.tapping 6mm thick steel.
For tapping 4130, it is permissable to have a thread engagemant of 75%. What loads are you placing on these parts?

Kody

The item I'm making is an adaptor sleeve for mounting a QCTP.

The sleeve will screw onto the existing tool post and have an 8mm threaded piece screwed into it. It's modeled on the drawing below but the threaded section is continuous i.e. the sleeve was drilled all the way through with the 7mm drill. .

Geoff,
Just a few comments, as I have down similar adapters before.
I assume the body of the toolpost holder is seating on the bottom spacer. I would suggest making a wider spacer ring, as these holders increase the lever arm action. Why not turn down the head of a shorter 10mm HT bolt or set screw, and replace the existing bolt. This would allow tapping a normal depth hole and increasing the fastening strength. The rigidity of the toolpost holder will be increased.
Peter

I made the one on Steve Bedairs' website with an additional sleeve to cover the non threaded part of the mounting stud, see here

As this was done soon after I got the lathe and this finish on that reflects that fact, I have made another recently which looks a bit better. At least I don't cringe every time I use it!

Peter,
The existing tool post is shown on the left in the photo. It's a very tight fit in the compound and has a thin (4mm) round head that sits in a recess in the bottom of the compound. There's also a locating pin to stop it rotating.
I considered making a longer replacement but thought that the current method would be easier given my skill level.

That's OK Geoff,
Take one step at a time. But can you make up a wider spacer for bearing the toolpost base? If so, it will be worthwhile.
Peter

Peter,
Not sure what you mean.
What's not clear in the photo is that the body of the QCTP has a hole in the base that will fit over the raised part on the compound. Here's another picture that shows it a bit more clearly.

You could just come in from both ends with the tap for this job and it wouldn't hurt, as there are 2 different bolts going into it from each end.
Dave