Chuck backplate question

[BenW]

Hello. I want to mount a 4 jaw chuck to this backplate:

The backplate is for another chuck but I can fit both bolthole patterns. The 4 jaw bolts from the front with the threads in the backplate, unlike the other chuck. My question is basically whether I should thread directly into the cast iron backplate or drill oversize holes and make steel plugs with the threads in, that insert from the back of the plate, to increase the strength of the thread? The chuck weighs about 20kg and came with 4 m12 bolts with around 20mm of thread sticking out behind the chuck. The register on the backplate is turned to match the register on the 4 jaw so it should take most of the sideload off the bolts, but I still thought it might be a good idea.
Any input?

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[mikey]

The cast iron back plate is more than strong enough provided the fit of the register in the back of the chuck is close. Drill and tap carefully and use quality screws and you should do fine. By that, I mean use a H1 or H2 tap and something like Unbrako screws.

 

[benmychree]

I agree with Mikey.

 

[bfd]

find a new place to drill a new hole pattern and use some good allenhead bolts a few extra holes will not hurt your backplate bill

 

[BenW]

Got a couple hours in the shop yesterday, the chuck is now mounted:

I used the 5-minute-toolpost-drill (more like 5 hours at the pace I work) in the picture to drill the holes, and everything went smooth. Thanks for the help!

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