QCTP questions

[AdeV]

If I were to build a QCTP roughly the same size as the toolpost that's already on my lathe, I'll need a lump of steel or cast iron approx 5"x5"x3.5" in size.... and that's before I make any holders.

I can get steel in that dimension (not that I've priced it yet), does anyone know of a likely source of surplus cast iron? Maybe an old engine flywheel?

So, 2 questions: Could I make it from aluminium instead? It's easier (& cheaper, I suspect) to get ali in that size than steel, assuming it's generally priced on weight. In fact, I already have some which is almost big enough (only 2" thick, though). My worry would be whether Ali is rigid enough, especially around the dovetails.

2nd Q; Do I need it that big, or could I get away with a smaller one? (Not a question a man asks very often!) I don't have any T-slots on my compound slide; I'm wondering about making up a plate with a t-slot, then I can mount a smaller QCTP higher up & further forward than the current fixed position mount will allow.

In case you missed it, my lathe has a 6.5" centre height, and will swing up to 20" with the gap removed. A suitably sized bought QCTP would set me back almost as much as I paid for the whole lathe, so that's really not an option...

[bramley51]

Ade,I think you'll find aluminium is actually more expensive than cast iron.I wouldn't use ally anyway,it's simply too soft.I know there are hard grades,but then you might as well have the weight of cast iron.It's all about rigidity.
If you come across a lump of the right size steel,that 's ok.(mine is made from a piece of old railway wagon axle).
As for size,this is determined by your centre height.The ideal position,afaik,is having the cutting tip at centre height,while the holder is also roughly at the centre of it's height adjustment.
If you make the dovetails a standard size,you can benefit from cheap holders appearing here and there.
I goofed on this front,I put them the wrong way around ,e.g. the female dovetail on the post .This means I have to make extra holders up.btw,make or buy as many holders as you think you need,then add6!
Have a go,it's pretty much straightforward machining practice.Another btw,make sure you get a 60 degree dovetail cutter.Mine are 45

[bogstandard]

Ade,

Everything Bramley has said is spot on, even down to the holder count quote.

You might be able to get a lump of steel from a scrappies. Get them to flame cut you a lump off. Most of the stuff you will get this way will be rather tough and hard, and if you haven't got any heat source to soften it down any, a barbeque for a couple of hours or even a small bonfire in the garden will help. Just let it cool down naturally with the embers, and try to keep it dry. The longer you can slow down the cooling, the easier it will be to machine.

Bogs

[AdeV]

Thanks chaps  

There's a metal fabrication shop in the unit next door (we're good mates), but he rarely deals in really thick chunks of steel (and never in cast), maybe 1/2" plate is as thick as it gets - but he might know where I can get some suitable iron.

I've got a handy "wood" burning stove (it burns everything from oily rags to plastic packaging, don't think it's seen a piece of wood in years...) which will both warm the shop & allow me to soften metal.

As for dovetails: I bought a 1/2" 45-degree & 1/2" 60-degree, but I'm amazed at just how dainty the dovetails they cut are! I guess I'd need at least a 1" cutter to make decent toolpost sized dovetails?

[Majorstrain]

Hi Ade,

You'll love your tool post when It's finished
I made a QCTP a while back and I don't know how I lived with out it B4.

This is the thread about the build, It also shows a lot of tool holders from other members. http://www.homemodelenginemachinist.com/index.php?topic=5153.0

The dovetail dimensions are not standard, but you could change that with no problems. It's based on the Hardinge design.

Cheers
Phil

[NickG]

You can buy 5" square here: http://www.m-machine-metals.co.uk/Catalogue_doc_MAY_2008.pdf but does it really need to be that big?

Nick

[trevoratxtal]

AdeV
One of the reasons I come up with
http://madmodder.net/index.php?topic=2154.msg22488#msg22488
Was I did not have or could easily obtain  a block 4 inch /100mm cube.
I have since used the tool post to dress a 12 inch/300mm block of cast interrupted cut taking of a kilo of swarf.
That is without the webs added.
Trev

[AdeV]

Phil & Trev - I love both of your designs; I don't think I could do the welded one, though, my welding just isn't up to the job. Sure, I could get the welders to do it, but then I'm spending £££ again, and this is supposed to be a low cost solution 

Nick: I did wonder about whether it had to be 5"x5"; that's just the size of the current holder. I'm wondering if I'll ever use, or need to use, 2" sq tool steel (which is the capacity of the current holder), or whether 1" sq will plenty. In which case, I could go a bit smaller with the tool holder. Then again, if I can get the metal at the right price, I may as well "go large" for the extra rigidity I'll get.

[ozzie46]

  I may be wrong here but I think the large mass to the tool block is for vibration dampening more than anything else. At least on the smaller lathes.
Just my 2 Cents.


 Ron