What type of Cast Iron is best suitable for dovetail slides

Hello to all,

I recently acquired an ALBA 2S shaper, to discover that the ram head dovetails have been smashed. Repair does not seem possible, so I need to machine out the existing and bolt in place a cast iron block into which I can hopeful machine new dovetails.

It is not easy to obtain Cast iron materials here in Namibia, but seems I can get some continuous cast SG Cast iron. However, from what I understand this Spheroidal Graphite grade iron is not easily machined, and not great for slides.

I have not found a source for a nice block of grey cast iron, if there is such a thing..I find many grades and specs for both SG cast and grey cast, and have no idea which would be best, or in fact what I am looking at!

The SG iron I have access to is :

Ductile iron Grade 400-15

heat treated so the hardness is between 180 – 200 HB

Tensile strength is 400 – 425

Elongation is min 15%

I have no idea how to compare this in terms of machinability to grey cast iron, some of which seem to have similar specs, eg:

Can anyone give me some guidance?

Regards

Joe

Joe

SG cast cast-iron is beautiful to machine (aside from the pervasive dust). The grey colour is due to the precipitated graphite and this also makes it low-friction when rubbing against itself, ideal for slideways.

It has all the benefits of grey cast iron, but is less brittle. What's not to like?

Neil

Strange Neil, I have heard of quite a few people that don't like to machine SG Iron, it is far more like machining steel and also looks like steel and not like the usual grey iron and not dusty. At least the SG castings that I have machined have been

This bit looks like steel and I would not class it as looking grey

Small curls of swarf, not dust on another crank

J

Edited By JasonB on 26/06/2017 18:37:02

Neil, I have no idea what's not to like..That's the problem - this is a completely new tack for me - I have never machined any cast iron at all. You indicate that the SG stuff machines well, etc, but in digging on the sites of cast iron suppliers, their recommendation seems to be use of Grey cast - This seems to be the gist of it:

Ductile Cast Iron or Spheroidal Graphite cast iron, where the free Graphite is in nodules, does not give the same 'slip' or lubricity in Slide use as does Grey cast - where the free graphite is in flakes. Machinability is also closer to soft steels compared to Grey cast iron.

So, I am baffled!

Since this is for the vertical slide on the Shaper, the slide does not do as much work as say a lathe slide, but I am not sure that fact guides my choice..What I need is maximum ease of machining, as its quite a lump I have to machine, and my mills are small...

Joe

I've machined (turned and milled) a lot of grey cast iron, grade GR17, superceded by BS1452 grade 250. It is quite soft and machines beautifully, if rather messy due to the fine black film that settles on everything in the workshop. On t'other hand SG iron is rather more of a pain, at least for me. It may be that the SG items I turned were slender, front axle and crankshaft, but they had a propensity to chatter, even more than an Islington dinner party.

Andrew

Thanks Andrew. I think I will try find some plain grey cast iron blocks. I really do not feel like experimenting with stuff I know nothing about, especially as all I want to do is get the shaper working!

Thought a lot more folk would have useful opinions on cast irons..

Joe

Well I sure lots of folk have opinions Joe - the problem is whether they would be useful to you or not.

I've machined a fair bit of cast iron but it's generally either been a commercially sourced casting (mostly cylinder and wheel castings) or an existing (e.g. old) machine part. Having said that, I have recently modified a Chinese cross vice and was quite surprised how 'soft' the casting actually felt when filing and drilling it.

The only piece of 'raw' cast iron I've actually purchased as such was a large chunk of Meehanite (because it was specified on the plans) and that was very nice to machine - but unfortunately (after paying for it) I now have only one arm and part of a leg left.... so machining cast iron yes - but choosing a suitable type, then others are best advising you I think.

Sorry to hear that the Shaper is damaged btw but I'm sure you can sort it.

Regards,

IanT

As a rule of thumb grey cast iron is good in compression, but weak in tension; and fails without warning. In contrast SG iron is better in tension and impact resistance. It is also more ductile, so it elongates before failure. Grey cast iron will be fine for dovetails, and has the advantage of being self-lubricating to some degree.

Andrew

As cast iron solidifies lots of crystals of steel form and cannot retain all the dissolved carbon. So the carbon is pushed out in front of the growing crystal until it meets another crystal growing in the other direction. The carbon gets left along the boundary which is flatish so you get flakes of carbon. Carbon like this has zero strength and so is like a ready made crack weakening the steel.

SG iron starts off the same but has special additives so by suitable heat treatment the carbon forms blobs, Spheroidal Graphite, instead of flakes. As we know round holes are less stress raising than sharp slots so the material is stronger, more like steel with loads of round holes - Aero chocolate instead of Cadbury's flake. Also depending on the process the steel part is more like case hardened steel than soft mild steel.

I recently acquired an ALBA 2S shaper, to discover that the ram head dovetails have been smashed.

If you put a shaper in front of a concrete wall it either punches through that wall or shoves the entire shaper backwards.

With a shaper when an irresistible force meets an immovable object it just keeps right on going, it's really not a machine you can set up wrongly and get away with it, you get one shot.

I tend to turn my small one over by hand before getting it going under power

Once you get it fixed then you'll be fine

Edited By Ady1 on 27/06/2017 00:34:43

If you are talking about the small dovetail slide that you adjust to take a deeper cut etc, you could probably make it out of mild steel, or use a couple strips of mild steel bolted and doweled to the original casting.

Photos of what you have and what the damage consists of would be helpful.

See here:

http://www.model-engineer.co.uk/forums/postings.asp?th=128212

Edited By David Standing 1 on 27/06/2017 02:24:13

I had the two throw crank for my steam launch engine cast in SG iron and it machined fine. A bit messy but if I wanted a clean hobby I would do embroidery. The roughing out was all done on the mill between centres and then it was ground to the finish size by an automotive shop. Once I had finished all the machining I had the left over throw pieces and was able to test the iron by cutting off a nice slice bout an inch wide by quarter inch thick and hammering it into a right angle bend in the vice. It passed the test!

I would be equally happy to use either the SG or the ordinary continuous cast grey iron for a job like this. The loads are not great unless you have a crash, in which case something is going to break anyway.

John

As Hopper has suggested, does it need to be cast iron? Reading Joe's other post the cracked dovetail is on the vertical slide, it's not the reciprocating ram, I'd use mild steel. Aren't a lot of these machine parts made of cast iron because you can easily & cheaply produce complicated castings which will just need a few faces cleaning up rather than it's low friction properties?

John

In that case, I suspect the chunk of SG iron I bought was actually grey cast...

Neil

I am pleased that there are other options rather than the cast iron - I am finding it almost impossible to obtain cast iron. 'Cloggs' even suggested Bronze, and I think that's a jolly good idea! I am going to try source a chunk and do it that way - if it does not work I can always try again with cast iron...(!).

I am going to try machine out entirely between the two dovetails, to a depth that is sensible and leaves enough meat on the head, behind the dovetails, and locktite and bolt/screw in a T Block of bronze, and then machine the dovetails to match the remaining part.

The Actual Break:

What I hope to do:

The whole thing is fraught with much pain...

From South Africa I can only get 45deg and 60deg dovetail cutters - these dovetails are 55deg..

Also, the dovetail is 17.5mm tall, so I need a big cutter. I see a manufactured cutter in my future..

Joe

Hi Joe,

Something like this is fairly easy to make ,i used some old carbide 1/8 inch
cutter shanks ,drilled 12 holes at the appropriate angle then silver soldered the blanks
in place .the lands that keep the position are machined or ground away,then it is
just a matter of grinding the cutting edges.

Hss would probably be easier to grind than the carbide that i used .
The cutter head is bolted on to the 2 morse taper arbor.
The cutter is 41.5 mm diameter and 18.5 mm deep.

Used to cut the front dovetail seen here of this home built shaper.

John

Looking at your red outline you don't even need a dovetail cutter to make that part. Just angle the work or mill head

The bugbear will be that while it should work fine, if you have a crash it may split at the screw holes.

55 degree dovetails? Easy if you had a shaper...

Neil

Neil;

I see no other way of fixing the replacement dovetails in place, short of making a whole new head.

Jason;

I thought of tangling the part and using an end mill - problem is getting the new part to align properly with the remaining old part. Dovetailing in situ would allow correction of any linear misalignment of the new block.

John;

I am intrigued by the dovetail cutter you made. Can you explain a bit more for me please? How did you do the cone before slotting in the carbide rods? Was it just a , say, 45mm diameter 55deg cone , holes drilled at 55deg very close to the periphery, almost breaking out but not quite, and then machine the cone down to expose the holes, ie, remove half a hole, slot in the rods and silver solder and then grind the cutting edge open? I think you did differently, since in the first photo I see the soldered length of the rods are already almost fully exposed, not siting in 'half-holes'.

I have a quantity of 4mm square HSS , about 150mm long each. I could cut those and mill slots in the cones sides to fit, but would silver soldering the HSS not ruin the temper/hardness? Re-hardening/tempering in-situ would not be easy.

I like the idea and am interested to try something along these lines. I was looking at making a cone, milling flat faces to take the rhombus shaped carbide inserts, the smaller included angle is about 52 deg. I was thinking of silver soldering the inserts on to give more rigidity to the backing. The inserts are small, about 12mm long x 7mm wide, so I would have to work down the dovetail sides - could not cut the hole dovetail length. But your implementation is very attractive. Please tell me more!

Regards

Joe