Line Boring Rig

It occurred to me that the loader project might pose some fitting problems when it came to aligning all the pins and bushes.  Whilst I can machine the bosses accurately, the process of welding them in place is almost certainly going to lead to distortion.  I’d considered machining the bosses slightly undersize and then running a reamer through to ensure accuracy.  I’ve done this before and it’s worked.  However, whilst it might be acceptable as a field repair bodge is really isn’t the way to go.  Given the number of pins in the project, and that I’m aiming for mechanical perfection line boring is the only acceptable solution.

I cannot justify the expense of purchasing a line boring set-up, and to get a specialist in will be costly too, so I’ve arrived at the idea of constructing my own rig.  If I think it through properly, and develop a comprehensive kit of parts, it’ll be another service I can offer in addition to being a useful addition to the workshop.

Initially, I plan to use a Rotabroach magnetic drill as the power source, although if successful, a 3 phase motor running through a VFD may be a better solution as it will allow fine control of the cutting speed.  Another compelling reason for opting to use a mag drill, is that the cut can be applied using the drills’ feed handles, and it makes for a simpler set-up too.  However, for bench use, a dedicated motor set-up would offer the potential to incorporate power feed, so I’ll look into this option too.

  I began with the idea of using a 15mm plate, drilled to accept a UCF self-aligning bearing unit.  These would be set-up either side of the boring operation and support the boring bar.  It seems that  bits of bar stock are commonly used to support the bearing plates, these being tacked between the bearing plate and the job.

I thought I’d incorporate bolt-on tack plates, shown grey in the image, with the idea being that a range of plates could be developed to suit a variety of boring jobs.  By going down this route, the opportunity to fixture-up the bearing plates on the bench if needed becomes a possibility.


I’ve shown the assembly here  with a UCF208 (40mm bore) bearing unit.

I’ve incorporated four sets of mounting holes into the bearing plate, these will enable arrange of different bearing units to be used with the rig to accommodate a range of boring bar diameters from 20 – 40mm.

The flanged UCF bearing units will self-align up to 5 degrees, and so in practice the set-up will not need to be tacked-up onto the job 100% accurately to achieve a satisfactory result.


This image shows how two of the plates would be mounted with a boring bar mounted between them.

I’ve incorporated the drive shank to match the Rotabroach spindle, and modelled a cutter fixed into the bar.  A selection of 12mm broached holes provides for a variety of cutter locations, and I’m planning on using a grub screw to fix the cutter.  I have an idea for mounting the cutter in a fine pitched opposing-threaded arrangement which will enable the depth of cut to be adjusted precisely.  I’m still to work this concept out.  In practice it may just be easier to use a micrometer, but I shall keep thinking through the concept, and see if the internet can offer any ideas.


So, here’s how I think the set-up might work in practice.  This is just an example, clearly the bearing plates are not ideally located, (this is just for illustrative purposes).

The next step will be to devise a method of mounting the mag drill, incorporating some feature such that it wont become unstable if the power supply is cut.

But for now, that’s where I’m at with this idea.


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