How to make a form tool?

[Sanjay F]

I have asked this question before and have tried searching for both the question & the answer, but can't find the thread 

I need to make a form tool as I'm thinking of taking on the Stuart Williamson engine as the next project and the column machining requires one. I would like to make one similar to the picture below from one of Jason's builds. As can be seen from the drawing I need 1/8" radius. I vaguely remember I could use mild steel, use an 1/4" end mill to get a 1/8" radius, then angle in a slope (wrong terminology, I know) and then somehow harden and temper. I guess the distance of the sticky out bits of the tool are important as the will be cutting either side of the curved section?

As you can tell I need to fill the large gaps in my knowledge before attempting this, all assistance gratefully received

[Jasonb]

Mild steel is not really going to cut the mustard. You either want gauge plate (ground flat stock) or Silver steel ( Drill Rod)

To make the cutter you have a couple of options, you can drill a 1/4" hole and then remove half of it. Or just mill a half round into the end/edge. Both can be done with the material tilted a few degrees which will give some front clearance that applies to teh half round and the edge either side.

Due to the higher carbon content of gauge plate and silver steel they can be hardened by heating to cherry red and then quenching. I don't bother with any further tempering. You only need get the business end to that heat so the shank does not become hard or brittle.

If using silver steel that is round mill it to half thickness first, then tilt and mill the end and finally do the half round with a few plunge cuts stepping over say 1/32" for 4No plunge cuts to get your 1/8"

[Jasonb]

Just check that bead is a full half round it may be a bit shallower in which case less stepovers.

Edit, looking at the bottom bead it is indeed only 3/32" high so your form tool only need be cut that deep with the 1/4" milling cutter or if using a drill machine back beyond the ctr of the hole.

[Jo]

Before going any further: I seem to recall the column on the later Williamsons was made of aluminium. What's yours made of?

It would be lovely for all of us to make ourselves a set of form tools for every possible diameter but there are other ways.

Jo

[Jasonb]

I thought they were all produced in alloy except the pre production prototypes.

[Sanjay F]

Mine is most definitley Ali, it looks like the set is still in its original packaging; unnecessarily huge box with parts sealed in the top, bottom and side.

@Jason - just checking, from the drawing, is the bead a full half round, or am I missing something?

Also:
How many degrees is a 'few' degrees for the tilt? 
Is the quenching in oil?

[Jo]

Have a look at router bits I suspect you will be able to find the profile you need for not a lot.

Making a tool out of hardened silver steel or case hardening steel is easy enough if need be.

Jo

[Jo]

Water cools quicker than oil. So I normally use oil for tempering and water for hardening.

Have you got any square HSS? And a 1/4" grinding bit? If so you can use your dremel to make a good tool that will last a life time.

Hand turning a one off small diameter is also an option 

Jo

[Jasonb]

See my edit. up a couple of posts

The bottom diameters are only 3/16" different so the bead is only 3/32" high I would say the top is the same.

Due to this difference you are unlikely to find a suitable router bit and much like rounding over milling cutters they tend to have a length of straight cut at the end so even less suitable.

5degrees at the most. I always Quench in water.

Also rough it out first much like you can see on those beads I'm cutting around that cylinder. So face the plain round bands ot overall diameter, reduce each side by 3/32" deep and then you can just form the curve with the form tool, run slowly to avoid chatter and lubricate with paraffin or WD40

As there are two beads to produce a form tool makes more sense once you have more than one item to form so they match unless you have a cNC lathe..

[Jasonb]

I just had a look at the drawing for my Big Willy (amson) and you can see that the ctr of the bead is inset from the flat areas each side

[john mills]

the ground stock and silver steel if it has its original wrapping it will have the heat treatment it has if it is water
or oil hardening   some is water hardening and some is oil  hardening 

[Sanjay F]

Thanks Jo, Jason & John

I haven't got any square HSS, but have a 1/4" grinding bit.

I think I've got it, from my understanding -  I only need the curved slot in the tool to be 3/32" deep;  I cut above and below the bead first, bringing them to size, then use the form tool to create the curve in the bead. I think this calls for an experimental piece before diving into the casting!

[Jo]

I think this calls for an experimental piece before diving into the casting!

It is always better to practise on a bit of scrap first   

Jo

[Charles Lamont]

If I remember correctly, gauge plate is mostly oil hardening, while siiver steel is water hardening. If you quench an oil hardening steel part in water the thermal shock can crack the material, as I know from experience.

[Jasonb]

Yes generally what is supplied as gauge plate here in the UK is O1 oil hardening. I must have been lucky as all the bits made from it and quenched in water have been OK. If it were a part like an expansion link with a lot of time invested then I'd probably go with oil but for simple form tools and small engine parts water has worked fine for me.