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Stuart S50 (Want to cry)

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Michael Edwards 112/12/2019 09:29:26
59 forum posts
46 photos

I was giving as a Christmas present last year by my children a Stuart S50 castings kit. Due to health issues I have not been able to get on with the build. I have now managed to start working on the build again.

I have recently purchased a brand new set of ripping milling cutters from RDGTools as I have been fed up with the gold coated ones that I purchased when I got my mill. I have thrown more in the bin than I care to remember.

I started milling the valve steam chest cover last night and to my astonishment I totally wrecked the cutter and the valve steam chest cover. Is this the cutter is the steam chest cover too hard. I have attached some images. The cutter was 12mm.

Norman Billingham12/12/2019 09:53:23
45 forum posts

I had the same problem with the steam chest cover on a 10V. Some of the castings Stuart supply are chilled and glass hard. You can return it to Stuarts and they will supply a replacement. If there's enough metal left it's also possible to soften the casting by heating and then cooling slowly - but I'd just get a new one from Stuarts.

Incidentally the "gold" coating is actually vacuum deposited titanium nitride (TiN) and it can be a mark of very good but also very poor cutters

JasonB12/12/2019 10:21:25
21999 forum posts
2539 photos
1 articles

Almost certainly a chilled casting though the photos are a bit small to see clearly.

Stuarts should be able to replace it but not the cutter, BTW mot much point in using ripper/roughing cutters for a job like this as you are buying a cutter which is meant to have it's side used for profiling not just the end for facing.

If you do get a new one then try it with an old file first as it too may be chilled, if the file just skids off it will be. heating to bright red and holding it the re for 10mins then allowing to cool slowly may help or leave in an open fire or wood burner overnight. Carbide tipped tools can help but that would only cover the milling not drilling and tapping.

Final option is to get a bit of 1/8" or 3mm flat steel and mill a decorative recess into that.

Not tried the RDG cutters and doubt I ever will but the ARC ones cope fine with a decent bit of cast iron, notice I'm using the edge not the end.

Edited By JasonB on 12/12/2019 10:26:17

Andrew Johnston12/12/2019 12:36:57
6404 forum posts
682 photos

I'd agree with Jason that rippa cutters are inappropriate. They're intended for large depth of cut and shallow width of cut, where they reduce cutting forces, not vice versa.

The casting may well be chilled, but a carbide cutter should cope without a problem. They'll cut HSS!

The main issue is a poor quality cutter. The picture of the cutter is a little fuzzy but it looks like all four edges meet in the middle and are not sharply defined. So the cutter will be rubbing rather than cutting. This is what professional 3 and 4 flute centre cutting mills should look like:

centre cutting.jpg

On the 4 flute cutter note that only two edges meet, the other two are gashed so they are not centre cutting. The meeting of the edges are crisp and well defined. I'd never buy RDG milling cutters, but I have used the ARC premium mills, and very good they are too. There are two rules for buying cutting tools:

1. Never buy cheap cutting tools

2. See Rule 1


mechman4812/12/2019 12:42:39
2938 forum posts
466 photos

Had a similar problem with my S10V, the standard had a chilled surface on the diameter & the only way I had to machine the face was to use a carbide insert face mill & a brazed on carbide tipped tool for the OD...

Bright ring is the chilled iron …

s10v casting hard skin (2).jpg


Lambton12/12/2019 13:49:23
694 forum posts
2 photos


Keep on complaining to the supplier, and ask if replacement castings will be soft enough to be machines on small machines using HSS tooling. Get them to test the replacement casting with a file before despatching it.

There are age old reasons why glass hard castings happen. It is usually due to the metal being cooled too quickly which is totally within the control of the casing company. Suppliers of castings for model engineering castings charge very high prices for very small and mostly simple items. They appear to have the familiar police of allowing the customer to do their quality control and simply supply a replacement part and hope the customer will be placated.

Ransoms used to deliberately chill the working face of plough shares in order to give them longer life - a very good reason for produce deliberately hard zones.

Do not accept any excuses such as all casting have hard skins and an initial heavy cut should be take to "get below the skin

Any caster selling you glass hard castings are not doing the job properly.


Michael Edwards 112/12/2019 14:03:24
59 forum posts
46 photos

Thank you all for your replies. Lesson learnt. Ordering some from Arc.

I tried not to go so cheap on these cutters but I think they were still cheap rubbish

First set were 12 different sizes for £70 (gold coloured end mills)

This new set of rippers £60 for seven (silver)

I tried a gold end mill from RDG first but that did start to rub so went straight for the ripper out of the box.

Just had an email from Stuart Models. They are sending another Steam Chest Cover out. FOCsmiley

DMB12/12/2019 14:08:22
1230 forum posts
1 photos

Hi Michael,

Wait for a forecasted dry day and put the lot outside in the bbq and cover with dry sand to ensure a very slow cooling. Cover with scrap sheet steel to keep rain/dew off.

Still use carbide cutters.



Maurice12/12/2019 14:58:43
469 forum posts
50 photos

Hi Michael,

I have always put my steam chest covers into a four jaw Chuck, and machined them in the lathe. If you have no chuck backstop to press them against, I flind that pushing the front face of the casting against the face of my drill chuck in the tail stock, or even the face of the tail stock barrel itself, if the casting is suitable size, will get the casting “square” and then the chuck jaws can be tightened.


SillyOldDuffer12/12/2019 15:14:20
7909 forum posts
1725 photos
Posted by Michael Edwards 1 on 12/12/2019 14:03:24:


I tried not to go so cheap on these cutters but I think they were still cheap rubbish

First set were 12 different sizes for £70 (gold coloured end mills)

This new set of rippers £60 for seven (silver)

I tried a gold end mill from RDG first but that did start to rub so went straight for the ripper out of the box.

Just had an email from Stuart Models. They are sending another Steam Chest Cover out. FOCsmiley

First off, chilled cast-iron can be hard enough to give HSS a hard time. Cast-iron is on my list of difficult materials because it's anything between lovely to machine and awful. Usually the hardness is just a skin, but it can go several mm deep and a small casting might be hard throughout. As Carbide has no trouble with hard cast-iron, I always start with a carbide end-mill, only switching to HSS when through the skin.

Secondly,an important part of my learning curve was being told that tools last far longer when made to cut rather than allowed to rub. Rubbing is ruinous because it rapidly blunts the tool without removing metal. And once the edge is blunt, game over - carrying on with a blunt cutter results in poor finish, slow progress, ruined work and bad language. Rubbing is an easy trap to fall into when working with hard materials and a low powered machine. Blunt cutters due to rubbing may be the root cause behind this bad experience.

Ideally, the cutting edge should be forced deep into the material and then moved at a rate that slices steadily through the metal. Small milling machines may struggle and cause the operator to back off and blunt the tool when he should attack. After Andrew Johnson of this forum told me to stop pussy-footing, all my cutting tools suddenly stayed sharp for far longer. To avoid premature loss of edge it's also important to remove swarf and keep HSS cutters cool - flood cooling may be necessary.

Thirdly, £70 for a set of 12 end mills isn't either dirt cheap or very costly. You seem to be buying the same sort of mid-range end-mills as me, and - though I've ruined a few - mine generally give good service. I've not found it necessary to go up-market yet. Just as well, a single 20mm 4-flute end-mill from Cutwel is nearly £40 (not the dearest in the world) and expensive cutters can be blunted by rubbing too...

Glad to hear Stuart have sorted you out with a new casting. Well done them!


JasonB12/12/2019 16:15:05
21999 forum posts
2539 photos
1 articles

All very well saying force the tool in deep but when Stuarts give you a 1/64th or less machining allowance you don't have much choice.

A really chilled casting can even give carbide inserts a run for their money the good old **GT can come to your aid once again with the "sharper" edge being less likely to be pushed off of the work.

Lainchy12/12/2019 16:55:02
271 forum posts
103 photos

I had the same with my S50, AND the replacement cover, so beware when it arrives. I totalled 2 HSS cutters, but used carbide in the end... and that was fine. Interestingly, Banggood sells a set of carbide endmills for less than £20, but take a while to arrive.

Hope the rest of the build is going well though I'm still working through mine, but getting closer to the end. I felt exactly the same though when doing the valve chest cover.

Good luck and enjoy.

I.M. OUTAHERE12/12/2019 17:17:59
1468 forum posts
3 photos

Hi Michael,
Welcome to the club !

Although most won’t admit it many a model engineer has been caught out by hard spots in cast iron myself included !

I wouldn’t be too harsh on the cutters you have purchased as they may be perfectly ok for anything except cast iron and I don’t even think twice about using carbide on cast iron - HSS stays in drawer for this material !

I was luck when i built the Stuart score engine a few years back as the only hard spots were on one steam chest cover and only on one corner , a quick rub with an old blunt file picked that straight up and it was marked with a sharpie so i knew what to expect when i started machining it . I think it is good practice to give a chunk of unknown steel a rub with an old file just to ensure it is not hardened before you commit a good cutting tool to it !

Michael Edwards 112/12/2019 17:26:40
59 forum posts
46 photos

Thank you all for the replies. Can anyone recommend the cutters they use.



I.M. OUTAHERE12/12/2019 17:53:57
1468 forum posts
3 photos

And the lid on pandoras box just flung open 😂

A lot of the stuff i use is NOS off ebay - usually Sutton or P&N which are probably not available in the UK .

Ideally i look for HSS with 5% cobalt or M35 .
I can’t recall ever reading a bad word about the cutters from ARC and although I haven’t used one i would be more than happy to buy them based on Jasons recommendations.

If you want industrial grade tooling I’m sure Andrew can point you in the right direction but be prepared to dig deep into your pockets for that grade of tooling !

lfoggy12/12/2019 17:56:43
174 forum posts
17 photos

I have had your experience so many times with cast iron castings that I now only use indexable insert endmills. I have several from Greenwood tools that make short work of chilled cast iron and stay sharp for ages as well.

JasonB12/12/2019 18:28:35
21999 forum posts
2539 photos
1 articles

I'd hold fire and see what the replacement is like with an old file as I suggested earlier.

You can then resurrect that cutter by grinding right across the end at an angle of 2-4 degrees, no need for fancy jigs or cutter grinders and use that on the casing. If it is soft you will be able to cut it OK and you have saved yourself a cutter, if hard you have not risked a new cutter.

Like Foggy for flat surfaces on CI I more often than not opt for insert cutters on the mill which will show what wonders are hiding within, if OK then smaller more detailed work can be done with HSS milling cutters or if a bit dodgy then Carbide ones where the larger insert cutters won't fit such as machining a narrow slot.

old mart12/12/2019 22:32:29
3510 forum posts
216 photos

You don't mention which mill you have, or its spindle fitting. If you cannot use a lathe to face the casting with carbide inserts, then maybe this isn't too big. Much smaller cutters from 10mm diameter are also available with carbide inserts.


Maurice13/12/2019 03:08:12
469 forum posts
50 photos

If you are using a mill, why not use a fly cutter? If it is long enough you will do it all in one pass. As to the chilled iron, I had one some time ago, and got round it by giving the casting a brief touch on the side of the grinding wheel; just long enough to remove the rough surface.


JasonB13/12/2019 07:06:08
21999 forum posts
2539 photos
1 articles

Just a word of caution following Old Marts link. Do be very wary of the super cheap "face mills" on ebay etc, from personal experience they can leave a lot to be desired - poor fit on arbour leading to eccentric running, poor inserts that will shake you machine to bits and poorly seated inserts. I have posted video of the short comings here before.

Next MEW will have a review of what look to be similar items from ARC but a world of difference in quality for not a lot more cost.

If flycutting a hard casting you will want to use a left hand insert holder otherwise it will take the edge straight off the HSS bit.

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