Boring tools and the final cut

G'day. I was boring a hole in some 2024 aluminium. The desired final result was 0.865". Was taking .010" cuts with a sharp HSS bit in a 3/8" steel boring bar.

When the hole reached 0.863" (I had previously turned a plug gauge) I backed off the tool, withdrew it and took another cut at the same X-slide setting (after to backing off another full turn to account for X-slide backlash).

Well, surprise, surprise, the 'spring cut' took off enough to result in a bore of 0.868"

Bore depth was 1.75" and the boring bar was set to a few thou above center height with an overhang of <2".

I can only surmise that the deflection on the finishing 'spring' cut was less than when taking the 0.010" cuts so the tool cut a bit deeper than I anticipated.

Any Ideas?

* Danny M *

Edited By Danny M2Z on 09/03/2016 10:25:46

I tend to start reducing the depth of cut earlier than that and would go from 10 to say six then 4 then 2 and finally sneak up with a thou or less by which time all the spring should have worked its way out of the tool.

Danny, I tend to do the same as Jason, but on the last but one cut I feed the bar in, and reverse feed and let it run back at the same setting, it will usually take a little out, but when it has finished you are pretty sure the spring is out, so your measurement for the last cut with be right. I then make the final adjustment and go in and out again for the final cut. this also helps to get a parallel bore if the cut is long and the boring bar slender. If the job is important a few extra minutes does not matter.

Chris Gunn

Reverse cutting can catch you out if gibs on the saddle are slightly loose and you end up with more taken off than you expect. The last thou can also be uncertain if the tool is not really sharp and rubs rather than cuts. You then put a little more depth of cut on and this time it cuts but then takes too much off.

Doug

Reverse cutting has caught me out a couple of times, once the extra depth of cut was obvious but to late to do anything. Had a very good finish tho'. Gibs etc. all seemed ok, I put it down to the rear of the tool being less worn and hence sharper. I was taught to not take small finishing cuts, but to put the last cut on at a "sensible " depth, say 10 or 20 thou'. I know this does not work so well for small model lathes like mine

The other problem is that the spring builds up as each cut is taken so does need to be run out now and again. Often run through several times to get rid of it completely.

One saving grace is that if some depth of cut is taken and then reduced to hit size there will be less spring so it wont reach it. The bore can then be picked up again with the tool, get close and then take trial cuts of some order until one is actually taken. It's a bit like the usual idea of getting a 1 thou clear fit. If you get close and then switch to 1/2 thou cuts and at some point a gauge goes in the fit has to be 1 thou or better.

Another way is to have say 1/4" of the bore that doesn't matter too much. Take a cut for that depth measure it and adjust if needed. Done correctly even the bit that doesn't matter will come out correct even accounting for a certatin amount of spring if the adjustment is small.

Loose gibs can cause problems working the normal way if the tool isn't wound back far enough away from the work.

John

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I always try to remember to take a practice 'spring cut' when I'm a good mm/40 thou short of final size. That lets me have a pretty good idea how that tool and material combination will behave.

Neil

Danny

Been there; done that; worn out the T-shirt.

For critical dimensional boring I utilize my Myford top slide fitted with an Armstrong boring tool holder and gibs a little on the snug side; tool set to a few thou above center. I set the top slide scale so the handle is between 1/2 to 1 degree on the Y+ side. I bore to within 8-10 thou using the cross slide. I repeat the final pass 4-5 times to hopefully eliminate any residual spring. I then lock up the cross slide gibs and finish with the top slide. After locking up the cross slide, I always check, by advancing the apron, that the tool is still just barely touching the work. With the top slide set at 1/2 to 1 degree, an advance of 1 increment (.002 at 90 deg), I can feed a few ten thou at a pass. Works most times unless Mr. Murphy comes to call. For less critical applications, I use my tailstock boring tool holder.

Cheers

Bill

A 3/8" boring in a 0.86" hole seems a little on the small side. The "stiffness" of a bar is proportional to D to 4 power. i.e. a 0.75" dia bar would be twice as big as a 3/8" dia bar and 16 times "stiffer" ( 2 to the 4th power). Thus it would have made sense to change the boring bar to a much larger one before getter too close to the finished size.