Adjustable Live Centre

[Joachim Steinke]

Today I want to tell you about my latest activities on tool making for my small lathe.

A well and true running live centre is a must have on every lathe, although most of us will not need this accessory every day. And some jobs like miniature camshaft or poppet valve making require a smaller and more slender live centre than the big ones that come with our lathes normally.

And actually there is no real need to build ones own live centre. You can buy special slender models or even complete sets with exchangeable, slender cone inserts for nearly every morse taper size from your industrial suppliers. And for low budgets, the cheaper sets from Vertex can be an acceptable alternative.

Unfortunately all the commercial sets don’t fit for my lathe, my PD360 has a shortened MT2 inside its tailstock. You can realize this on the photo: on the left side and in the middle are two tapers that fit in my tailstock, a normal standard MT is shown on the right side.

And shortening the MT with a saw or grinder is no good idea, the secondary bearings are located directly near the end of the taper, so the whole thing will be definitely damaged afterwards this operation.

Additionally I have a slight misalignment of the tailstock, turning really precise fits, especially at shaft lengths over half an inch, is not possible between centres. To make things worse my tailstock is fixed, there is no sliding base for any adjustment.

So I had to design and build something that fits in this special short taper and will help me with the misalignment too:

The bearing house is slightly smaller in diameter than the corresponding boring in the main house, so it can be moved a little bit in all directions on its flange plane until it is fixed with the four frontal screws. And to make adjustments easier four headless screws are placed on the circumference.

The taper base is a commercial MT2 shaft with a cylindrical front area for self machining fixtures. As the shafts are soft inside you have no problems with drilling and boring until you stay away from the hardened surface zone. But I would have no chance to bring the taps in the surface area, what is the reason for the additional component which connects the MT with the bearing house.

The other components are made of high strength steel and the exchangeable cone tips are from silver steel and then hardened afterwards.

As the cone tips should run as true as possible I had to grind the bearing seats after hardening. I did this on my new cam grinder which can be well used as a cylinder grinder for such small parts.

Some pictures of the single components

and the complete centre.

A first rough alignment can be done by pressing the inverted cone insert onto a precise centre located in the main spindle. Afterwards you can tighten the flange screws and set the four adjustment screws on contact to the inner cylinder.

The final alignment was made by turning a testing shaft. After two or three attempts I already managed to fix it well enough.

I would say the results speaks for themselves, the last digit of the reading is a thousandth of a millimetre, which is equal to 0.00004in in your dimensions.

Of course the MT will need a small mark at its circumference to reinsert the whole thing precisely at the same position than before, otherwise all the adjustments are in vain.

Finally some pictures of the new centre in use on my lathe. Oh boy, it’s really nice to have enough working space in front and beside the tool holder at last….ha ha ha….

And turning a well fitting shaft of such small poppet valve would be barely possible without a well aligned tailstock centre.

End of part one

 

[Joachim Steinke]

Part two

Okay, switching over to the next theme which is also related to small turning jobs …..

Even for us hobbyists, working with carbide inserts had become very common over the last years. Beyond controversy, indexable tools do a great job even on our small lathes today.

As most of you, I too have my drawers filled with the little boxes containing treasures of CCGTs, VCGTs, DCGTs, GTNs and so on and on….and with the suitable tool holders as well…as I need small shafts (8x8mm, 0.3x0.3in) which are sometimes hard to get I make most of them by my self…..

And for real small boring jobs this sophisticated stuff here is unbeatable:

PH-Horn boring inserts, completely made of solid carbide. The smallest types they sell are build for starting holes of only 0.2mm diameter (0.008in), something for the watchmakers between us...ha ha ha…but very useful for all of us model builders too! They are really efficient on all sorts of material and if you have used them once you will never miss them, believe me.

Nevertheless there are still a couple of jobs where we still need the old fashioned turning tools made of high speed steel. Whether it’s for achieving a really nice surface or for making complex form tools and special and very small cutting tools etc. etc., without the opportunity of grinding our owns we are not able to build the model engines we intend to.

But grinding a finally rather small cutting blade out of a big blank is an exhausting and dirty job, especially if you have to do it with a bench grinder without flood cooling and in your home workshop. For nearly most of our tools we don’t need more than 2 ore 3mm (0.08in / 0.12in) of blank size for getting a suitable cutting edge, so why grinding a lot of material away?

Small bars from high speed steel (and solid carbide too) are available at all good tool shops. The only things we need are some holders that will clamp and support the small blanks. The idea is not new and you can by such holders at RGD Tools for instance. I had the chance to hold some of them in my hands a couple of month ago.

They come from China and India and all I can say, a bit clumsy and of very poor quality. Sorry, that’s not the kind of tooling I want to use…..okay, you already know me, I started to make my own ones…..ha ha ha….

The shafts measures 8x8mm, shafts and clamps are made from carbon steel, they are hardened and tempered. The heat treatment is not really necessary, but the clamps and the blade seats will be more durable that way.

The two straight ones can hold cutting blades as well as small chamfering and form tools etc. The bended holder is for right corner tools. With the 20deg bend you always have enough clearance to work on deep shoulders without the need of grinding this clearance out of the blanks side face.

Depending on the small blank size and the slender form of the holder tip you can work in limited spaces and near the tailstock centre without any problems.

And replacing the inserts is quickly done, so in principle it is sufficient to install three or four of the holders in your changer cartridges and only change the single inserts on demand.

Have a nice weekend

Achim

 

[ttrikalin]

Achim,
you are an inspiration!!!

Thank you, Sir.

take care,

tom in MA

 

[SBWHART]

Some very very nice tooling Achim.

Thank you for sharing with us.

Stew

 

[ttrikalin]

Achim,

do you buy the inserts? Grind them your self? are they carbides?

tom in MA

 

[arnoldb]

Very nice indeed Achim. I love your adjustable center Thm:

Regards, Arnold

 

[lee9966]

Such beautiful work Achim, as with all the things you have posted. Maybe someday my feeble efforts will come out so well.

Lee

 

[Joachim Steinke]

Hi Tom

The inserts had to be cut from bar stock. Square bars are available from 3x3mm (sometimes even 2x2) up to 10x10mm, the length is normally about 80 to 100mm. Cutting is easy, you can use a Dremel with a small grinding wheel for this. And the tiny ones can even be broken away from bar after grooving it with the Dremel.

If you want excellent cutting and less wearing material you should buy them at your professional tool suppliers. They have really high quality alloys like HSS-Co10 up to HSS-Co19 and you are sure to get the quality you have ordered. Some of the ever so popular tool discounters from the internet sell that bars too, but normally this is cheap stuff containing only poor quality alloys. The Chinese producer may print HSS-Co10 on the bars, but that doesn’t mean anything….ha ha ha….

I have a good amount of indexable tools for my normal lathe work, so this new holders are only intended for a variety of special jobs. For that reason I mainly use HSS inserts with them, but I have made a couple of inserts from carbide too. You can see one of them on the last picture (the straight holder in the middle), that insert is made from 3x2mm carbide bar.

Normally all the grinding has to be done by yourself, I for one don’t know suppliers who will sell ready to use inserts of this form. But tool grinding has to be done anyway, and as I wrote, one of the advantages of the small bars is the fast and easy way to get a functional cutting face without removing that much of unnecessary material.

I forgot to tell, cutting steel bars are available as round material as well. And building holders for the round ones is a lot easier, you only have to drill a hole and make a slit with a saw to get a clamping fist. But round inserts have one disadvantage, once removed from the holder they can not be relocated precisely. That means I can’t change them during work really quick and sharpening after use is a problem too. It becomes really difficult to find the former face angles on the grinder as the round insert is not referring to a base plane anymore.

So for round inserts you may grind a locating face on the side of each shaft and use an additional clamping screw from the side. Or you must leave them in the holders, but that means you will need a lot of holders then.

Achim

 

[ttrikalin]

thank you for your advice. I am inspired to follow your lead!

tom