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Joining cast iron

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Martin Faulkner04/02/2018 20:20:43
85 forum posts
32 photos

Hi all

I have 2 100x300x12mm plates of cast iron and wanted to join them to make a 24mm thick plate. I was thinking of brazing them but I don' think it would reach the core. I am trying to machine a t slot plate for my boxford lathe.

Thank you in advance for any help

JasonB04/02/2018 20:23:13
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Thread the bottom plate and use counter bored holes in the top plate to take cap head screws placed in rows between the slots

Martin Faulkner04/02/2018 20:39:46
85 forum posts
32 photos

I was not sure if this would be a strong enough option. I do have a few plates so I will give this a go. I am planning on mounting some t slot plate over it to give me a t slot slide on the lathe.

Thank you Jason. Much appreciated.

vintagengineer04/02/2018 21:07:46
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468 forum posts
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I would use this method but in reverse with countersunk HT capscrews. You could also try to soft solder the joints as well.

Posted by JasonB on 04/02/2018 20:23:13:

Thread the bottom plate and use counter bored holes in the top plate to take cap head screws placed in rows between the slots

Clive Foster04/02/2018 22:06:17
1992 forum posts
73 photos

My practice is to glue'n screw this sort of thing. Not had a failure yet which proves absolutely nothing.

For practical purposes the tensile strength of appropriate adhesives properly bonded to suitably prepared surfaces is 1/10 that of cast iron which is around 40,000 psi depending on type and lots of other things. For example JB Weld and Loctite adhesives are approach 4,000 psi. Automotive panel bonding adhesives such as 3M 08115 are said to be a little stronger in practice although book values are similar at just over 4,000 psi.

According to this site **LINK** that is similar to the tensile strength of an ordinary hardware store 4 mm bolt and not quite half that of a 6 mm one.

The great advantage of adhesives is the sheer area of joint which adds up to considerable strength without stress raisers. Aside from the difficulty of proper surface preparation for maximum bond strength the great disadvantage of adhesives is poor resistance to peeling forces. An adhesive joint is, by definition, a crack in the material. Once you start wedging a crack apart the force per unit area at the tip becomes very large indeed so the poor glue has no chance. Generally properly sized mechanical fastener has ample strength to survive the induced stress concentration, considerably aided by the crack stopping action of the hole, so screwing and gluing gives the best of both worlds. Needs lots less screws than a simple screwed together joint.

Clive.

Chris Evans 605/02/2018 08:54:00
1564 forum posts

Glue and screw as suggested plus add some close fitting dowels to stop any fretting movement.

Trevor Drabble05/02/2018 09:54:35
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Martin , Do you have access to oxyacetylene equipment please ? Trevor.

Rod Renshaw05/02/2018 15:16:55
54 forum posts

Martin Cleeve was the pseudonym of a contributor to ME in the 1960s who designed and made workshop equipment, including some large items like a full-size drill press, from bright mild steel sections held together with homemade mild steel capscrews. Many of these projects had T -slotted tables fabricated in this way. I made one of the simpler projects, a T - slotted milling table for the lathe. This worked well as far as ease of making and strength in use was concerned.

I think it will take some serious heating system to get workpieces this size up to brazing temperature, and then there is the cleaning up and the risk of distortion. Welding may be possible, I don't really know, but might it leave hard spots which would make the T slot cutting difficult,

so +1 for the glue and screw route.

Rod

JasonB05/02/2018 15:34:00
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Or what about a series of tapped holes in a single layer plate that you could just screw your hold down studs straight into.

Martin Faulkner09/02/2018 20:32:48
85 forum posts
32 photos
Thank you to you all for your much valued input. I think the glee and screw method is a superb idea. There will not be much pressure on the joints apart from the jib strip. Most of it will be downwards force. I dont have oxy acetylene here but do have access to some. I was thinking of using dowel pins on the top plate as I would like to make it adjustable. It's only a small lathe but would like to get the maximum out of it. As for the steel comment, I was looking down that route but the to funds, I have to go with what I have access to and I have access to lots of cast plate and stainless sheets at the moment. Mainly used commercial catering equipment.

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