Machining castings in the 4-jaw - knocking?

I am embarking on my first work with castings, a Stuart 10V. I am attempting to do this as described in the 'Vertical Steam Engine' book that came with the kit, that is to say - mounting the castings in the four jaw and facing them using the slowest open gear.

I'm getting a nice finish in areas where there is broadly continuous surface, but in the areas of interrupted cuts it's all gone a bit wrong.

The four pillars which form the main bearing housing on the baseplate highlight this quite well:

https://i.imgur.com/J62yoH0.jpg

Also, one of the lugs on the base looks like this (although the other is fine):

https://i.imgur.com/4pdVvuc.jpg

There is loud knocking as the protrusions go past the tool and it looks like it's knocking rather than cutting.

I'm using a HSS bit and I've attempting to dress this with a diamond honing file, and not much seems to work. The top of the base (which is more continuous) cut well, and flat - as did the bottom of the soleplate.

This only seems to occur during interrupted cuts.

I know it is possible to hold these in a verticla slide and using a milling cutter in the spindle, but I'm trying to use 'period' methods on this and a little put out that I'm stumbling at the first hurdle.

I actually used draw filing to get the bottom of the base level and that seemed to be fine, but clearly that's more troublesome on the baseplate top!

Any ideas where I'm going wrong? I've locked the saddle, nipped up the gibs on the cross and top-slides, countered backlash in the various screws, etc.

Intermittent cutting is often a fraught operation. I can't get your images to open, but all that I can suggest is to increase rigidity as much as possible by reducing work-piece and tool overhang and reduce the speed to see the effect that has, although the back-gearing might add to the rattle. Open gear might be a tad too fast. A light cut and a sharp HSS tool is the way to go.

Best of luck.

Are they actually flat?

it's probably too much cutter stick out. It will knock especially if there's a hard patch, then the tool jumps and rubs if it's too bendy.

I often use interrupted cuts. As long as the set up is rigid, then it works well. I use carbide tips on every interrupted cut job. This will get through any hard spots on a casting and no, I have never broken a carbide tip when doing this.

I think some of the accepted wisdom is that interrupted cuts are the work of the devil, I have never understood why.

Andrew.

@Dave Halford - the areas I've NOT shown have cut 'normally' all the way across - maybe softer CI or something? but those ARE flat. These ones, not so much!

Small features on a casting can cool quicker than large mass areas and rapid cooling can result in hard patches, sometimes metal blocks are added to casting moulds to purposefully add these hard patches to a casting.

Additionally when using back gear it is possible that there is some play in the gearing resulting in the chuck bouncing back and forth in the available backlash when doing intermittent cutting.

There is a thread on setting up an album and adding pictures to the forum. This will work where your posted links do not.

Martin C

Well I can not see any pictures but it could be some parts have more metal on them and you are getting under the skin and a nice cut but were the metal is thin you are still in the skin area and it will vibrate skip and leave a mess. so deeper cuts slowly.

added pics again :

Lock the topslide and saddle to help firm things up. Check the tool if the casting was a bit hard it will have taken the edge off the HSS, underside of the box bed looks a bit chilled (shiny around the outside of the bosses) and that will blunt your tool. It's certainly a poor finish and you want to get that sorted before you start on the cylinder castings.

Maybe it's a Myford thing to run so slow with castings and HSS but I'd be going a lot faster and using inserts.

Edited By JasonB on 16/05/2021 17:22:21

I don't have any inserts, just brazed carbide and HSS - happy to give those a try at a higher speed though. And sharpen.

Generally I've found good finishes with this lathe on stainless, carbon steel, brass, aluminium, etc. so I can only assume a lack of rigidity somewhere...

Stuart Castings (well the ones I've machined) do seem a bit prone to chilled hard spots, especially in these small engines. Lookes to me like there is a harder skin in places. You could do worse than get a torch on it and heat everything to red heat then allow to cool slowly. I suspect many of your machining issues will vanish, especially if you follow the rest of the advice above regarding rigidity.

regards Martin

PS Stuarts do spare castings or at least they used to.

IF possible I avoid interrupted cuts, but beggars can't be choosers!

There will always be a hard skin on a casting where it has been chilled. Once you are through this, life will be a little easier, and you can take fine cuts to reduce the impact of the interrupted cut.

As already said, rigidity in both workpiece and tool is a necessity. Each workpiece / tool contact is an impact

So fine cuts to finish.

Howard

Sharp tools and slow speeds help, as well as locking the saddle if possible. Make sure all the gibs are adjusted well. Some of the jobs may be better using a mill if you can.

Depends on who you are buying your castings from, a lot of the US casting kit suppliers anneal their iron castings and I've never had any problems with them unlike quite a few UK sourced ones. The likes of Martin Model also run theirs through a toto blaster after annealing so you get nice clean casings.

I'm not sure what you mean exactly by "slowest open gear"? Presumably not using back gear, so about 200rpm? In which case you should try slowing down and using the fastest back gear speed, about 100rpm. It looks like the casting is about 4" long, which makes it the same as turning a 4" diameter job. So for HSS tooling you need about 100rpm absolute maximum. Then for interrupted cuts, a bit less again. So maybe even the intermediate back gear speed of about 50rpm. Slower speed = less force of impact = less flexing = happier lathe. This job is exactly the type of thing Myford provided the back gear for.

It's often best to grind the hard skin off such castings before machining. The bench grinder or a 4" angle grinder will do the job. Or even a small Dremel type grinder if you take your time. Otherwise, the lathe tool will tend to skid over the hard surface instead of cutting. And counterintuitively it's best to take a deep cut that gets in below the hardened skin area and into the softer metal below it.

Which brings us to the next point. Those are some fairly flimsy castings and holding them in the four-jaw is probably allowing them to move/flex around a fair bit under the impact of cutus interruptus. That is probably why the large pads at the end machined better than the smaller, less supported ones in the middle.

You would be better off to clamp your castings flat onto the faceplate to machine them. Or even use small screws to attach the castings to a piece of stout flat plate and hold the plate in the four jaw, or clamp the plate to the face plate. These methods will give you support behind the whole casting at the back, not just the four points where the chuck jaws are holding them.

Also, you want to keep tool overhang to a minimum and topslide extension to a minimum too. And lock the carriage. If using HSS tooling, a round nosed tool or at least one with a decent radius on it can heip stop the point getting snapped off by the impacts of interrupted cutting.

Some pics of how you held the jobs in the chuck and how you had your tooling and slides etc set up would help identify what your problems may be caused by too. But I would try using back gear low rpm and grinding the hard skin off as first steps.

Edited By Hopper on 17/05/2021 05:04:20

OK - I will take some pictures!

I've got a faceplate for the ML7 but I'm not sure what clamps would be best to use? I've got none at all.

Take a look at Harold hall's build, he shows various ways to hold the castings

I forgot to mention that if you have a faceplate to secure the castings to, it would be better than a chuck.

Agree with most comments, I would use a faceplate,cutting speeds of 80 ft per minute for HSS and 3 times faster for brazed carbide rough across the surface with brazed carbide, then finish off with HSS ,the tool tip needs a small radius to improve finish,if roughing is carried out with HSS then regrind the tool before finishing,when roughing do not bother to hone the HSS tool, When clamping a small casting to a faceplate,start by checking to see if the cast face which beds against the faceplate is flat and does not rock, use a file to remove any high spots sometimes shim is suggested buts its not so easy to hold the casting ,and clamp it at the same time, and bits of shim flying round can cause nasty cuts to your fingers.then when the second face needs machining check again to see if the casting rocks on the faceplate as it may have slightly distorted due to internal stresses being relieved,if the surface is not flat then with a casting the size of the engine boxbed lay a sheet of emery paper on a flat surface and rub it around until the surface is flat hen clamp it to the face plate,It is best to machine the lower surface of a boxbed first. At work one job that used to regularly occurr was the cast bed for a travelling microscope about a foot long ,8 inches wide and about 2 ins high, it went through a similar process,only 3 bolted to a 28 inch faceplate,100 castings total, its surprising how castings do move after machining even after being in store for some time,we used to have to file the base flat to a surface plate before the top working surface was machined,That job was full of interrupted cuts .

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