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- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] Made an adjustable line boring bar
- Date: Fri, 25 Oct 2013 23:40:51 -0400
http://www.practicalmachinist.com/vb/general/made-adjustable-line-boring-bar-274124/
- Join Date
- Sep 2007
- Location
- Cairns, Qld, Australia
- Posts
- 1,641
Made an adjustable line boring bar
Thought I'd share some photos of my version of adjustable line boring bar as it worked out well. My aluminium and brass spindle castings were really difficult to bore to an accurate size for ball bearing fits. The basic line boring bar with a grub screw retaining tool steel bits, was nigh on impossible to set to a consistent size. When I first mentioned this problem, PM member TGTool helped out by posting pics of his line boring bar with it's fine adjustment screw. Anyway my unit is more or less loosely based on his unit, many thanks due to TGTools!.
The 8mm cone point grub screw fits a groove up the middle of the adjuster screw and allows easy removal of the cutting tip when setting up or measuring and also gives adjustable backlash, you can snug it up till it rubs in the groove lightly. With a Mercer 10 tenths indicator and the 0.5mm screw pitch (50.8 tpi) I can easily advance the carbide tip out a 1/3rd of a tenth at a time and bored the casting at both ends easily to size and in line with each other.
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10-25-2013, 08:29 PM #2
- Join Date
- Oct 2010
- Location
- Oregon, USA
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- 1,821
How did you machine the threads in the toolbit shank? Thread a substantially larger block eccentrically and then turn it down? Drawfile to clean up the edges?
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I was round at my Uber machinist's workshop proudly showing off the the screw part and talking about single point threading a bored hole. Without saying a word, or getting up and even looking for it, he reaches across the table and hands me the exact tap needed: a 10mm by 0.5mm pitch (arrived in the mail that morning from Ebay for the arsey so and so!).
Originally Posted by sfriedberg
How did you machine the threads in the toolbit shank? Thread a substantially larger block eccentrically and then turn it down? Drawfile to clean up the edges?
So I bore and tap some 32mm bar and then carefully split it into three equal pieces lengthwise which gives a shape that can be easily held in a three jaw chuck to turn one end round with a shoulder. That end gets locktited into a 1" thick steel block with the shoulder to roughly position it. Then I switch to the four jaw chuck and sweep the dial indicator along the thread to check it's parallel to the axis. After it's rough machined I can face off the end and leave a tiny stepped shoulder to let me measure with the calipers to each side and adjust sideways so that the threaded section is the correct depth. Several passes later and several adjustments to home in on the correct diameter with the correct threaded depth and it was done.
The edges were draw filed with a few quick strokes, oh and the partially tapped bar is used before splitting as a socket for machining the vee groove in the middle of the grubscrews and also for drilling and tapping the end so that an 8mm grubscrew can be Loctited in the end for adjustments....that screw ends up so tight in the socket that you have to Loctite in the grubscrew to unscrew it without damage.
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Nice implementation and a good variation on the idea. It's nice to see stuff work for people. The one's I made weren't my original idea either but it's good to pass things around. That's how we all grow. I might use that solution for one of mine. I did tap a full thread and machined it away on the ones I made so that's also a viable option.
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If I did it again, the groove could have a few threads either side of it machined away to allow for wear and prevent the thread from being damaged, it's far too fragile in 1214 steel. Originally Posted by TGTool
Nice implementation and a good variation on the idea. It's nice to see stuff work for people. The one's I made weren't my original idea either but it's good to pass things around. That's how we all grow. I might use that solution for one of mine. I did tap a full thread and machined it away on the ones I made so that's also a viable option.
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- [machinist] Made an adjustable line boring bar, Lawrence London, 10/25/2013
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