More Chatter Check Your Overhang

I had the opportunity (and fun) of taking a technical call from a “New” customer not just to Criterion but also to boring. He was calling because the finish in his bore looked like a rasp it had some much chatter in it. He was attempting to make a 1-1/4” diameter bore 5” deep. His tolerance was +/- .002” and the material was 4140. The 3/4" diameter boring bar projected out of his boring head 8”. He had tried changing his RPM and changing his feed rates but nothing was working. He had slowed his RPM down to less than 200 hundred and his feed rate was less than .001 IPR; nothing was working for him.

We talked about some general rules of thumb for boring. First the depth to diameter ratio of a boring tool is critical to successfully boring a hole. The general rule for steel boring tools is 4:1, heavy metal boring tools is 6:1 and solid carbide tools is 8:1.  So his 8” projection with a 3/4” bar gave him a better than 10:1 ratio making it difficult if not impossible to achieve a satisfactory bore.

The second general rule is to use the largest diameter boring tool that will fit into the bore while still giving you clearance to evacuate chips from the bore. This gives maximum rigidity to your operation and helps to insure that you make the precision bore you need

The third general rule is to make sure your depth of cut is enough to clear the nose radius of the tool you are using ( I wrote about this in an earlier blog). His depth of cut was greater than the nose radius so this was not part of the problem.

The last issue we discussed was operating the tool at the correct surface feet per minute (SFPM) for the bore diameter. For his operation and the carbide he was using in steel with a hardness of about 30 R/C the starting point was 600 RPM with a feed rate of 2.0 IPM. This was based on the material he was cutting and the amount of off-set on his boring head.

The “Fun” for me was the customer had modified his program while we talked and had some else cut the bar so it only project slightly over 5” from the boring head. He asked if I could stay on the line while he tried the new program. I could hear the machine make the cut in the back ground and the customer describe it over the phone. He successfully bored the hole and was excited about what he had just learned. It is fun to help customers solve issues they have making their parts. If you would like us to try and help you with a boring problem don't hesitate to contact us.