Making a boring bar for line boring

[John TV]

Finally getting around to attempting a line bore on my counter shaft on Logan 820. (Post In Logan Forum) First I have to make the boring bar for my mill.
My counter shaft has a original hole size of 3/4 inch but is wobbled out by almost 1/8th of an inch on one side. I will be attempting to line bore to 1 inch.
My questions are these:

Size of Bar? I need to have a length of about 14 inches which will allow 2 cutters. For rigidity I was thinking of making the bar 3/4 inch to keep maximum rigidity but that will leave very little room for chip evacuation in the first few cuts. Should I reduce the size of the bar to a smaller diameter in the areas the cutters will be used to allow for chip clearance? How much clearance do I need?

Cutters perpendicular to par or at an angle? As stated, I need two cutters. It would be easier to drill perpendicular to the bar and use a small grub screw to lock the cutters in place. If I do this there will be two holes on the very same plane making the bar less rigid? Or should I set cutters at an 45 degree angel with 90 degree offset grub screw. It would be more difficult to make but would it be stronger?

In 3/4 inch boring bar I was thinking of using 1/4 inch round drill rod for cutters with grub screws. It will be boring in cast Iron and may only get one use.

Thanks for your thoughts and suggestions.

 

[BtoVin83]

There are round sleeves with a square hole you can press in a bar. They have a screw at the base to advance the tool more precisely by using dial indicator on the tool point. You used the term line boring so I assume the bar is supported at both ends. Your first cut can remove a fair amount so after that chip clearance should not be an issue. Having the tools in the same plane should not be an issue either.

 

[BtoVin83]

 

[John TV]

Thanks for the suggestion. Who is the supplier of the inserts?


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[BtoVin83]

google "square hole tool bit sleeves". the drawing came from bld.com

 

[Technical Ted]

If I was attempting this, I would not try cutting both bores at the same time. Way too much tool pressure on a small bar unsupported on the end. Does your mill have an out board bearing support (is it a horizontal mill?)?

If no out board support to use...

I would start working the far end first and only put whatever holes and/or chip clearance reliefs for the tool you decide to put in on that end only. Keep the bar as large and solid as possible. If you need to turn it down for chip clearance, only do so on that far end. Depending on how bad the first (close) bore is, you might even be able to fit a temporary bearing in that bore to help support the bar if needed. For a quick and dirty bearing support you could even use steel, just keep it oiled well.

Once the far end is done, remove the bar and put the tool holding hole and chip clearance in the near end and finish things up.

I thinking the far hole is going to give you the most challenge and this approach will offer some advantages.

Ted

 

[Technical Ted]

After thinking about this some more, here's another approach that would work well. Pick the method that works best for you and your equipment.

Do the closest bore first and open that one up to size. That way you can use a larger diameter bar to work on the far bore. With this approach, if needed, you could make a bushing to fit the first bore to help support your boring bar, or leave the bar as big as possible and only turn down the very end for the required clearance for opening up the far end. Of coarse, you would need two different bars this way, but it's nice to build up your collection of boring bars...

Just throwing ideas out there!
Ted

 

[jcp]

Here's a pic of a bar and the set up I made for a line boring job on a Millers Falls power hacksaw project.
I bored the holes 1/8th oversize and machined cast iron bushings to return to original size. The i.d. of the hole did not have to be any exact size since I was machining bushings to fit. Worked out just fine. You should be able to rig something similar.

 

[John TV]

All good suggestions. I do have a VanNorman#12 mill which is universal and does have horizontal capabilities and an over arm support so both ends of the bar will be supported.

I will try and space the cutters so that only one cutter engages at a time. However I only have limited space in y axis to move the part into the cutters, it may be close. I’m thinking the set up will look something like the pic below but the cutter is not made yet so not in picture. A bar represents where the cutter will run.

Really appreciate your thoughts. I learn from all the suggestions.

John


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