How to Reduce Cutting Tool Chatter when boring a welded hole:
Are you all set up to line bore some holes but find that your carbide cutter vibrates and chatters excessively? What can you do? Prevention is the key and prevention begins during set up or during welding. Drive your boring bar at the proper RPMs for the size of the hole. Turning your boring shaft too fast is the number 1 cause of cutting tool chatter. Having a dull cutter is another common reason. Not having enough support bearings is another common reason. And welding on top of galled metal is less common but the hardest to correct. Keep reading for more info.
After figuring out where to position your bearing assemblies by referencing off what is still left of the original hole diameters, try to add more bearings next to each hole to reduce vibration and deflection of the boring shaft. Before welding any hole it is important to grind out (with a small hand grinder) or bore out the hole to remove any galled metal. If hand grinding, grind down at least 1/32 inch (0.8 mm) deep or more to get down to fresh metal. Galled metal is the metal inside the hole that has been rubbed against by a pivot pin or bushing over and over for weeks or months. The problem with welding on top of galled metal is that the weld will suck up carbon from the galled metal and become super hard. If this happens you might need to use up 20 cutters instead of the 2 cutters to line bore through the hard weld. A one day boring job can become a week long ordeal. The same problem can happen if the was oil or grease in the hole before welding since this will cause oil hardening of the weld. You want to weld on top of fresh metal.
Also be careful when welding cast steel or other steel that has the ability to transfer carbon to your weld. If you think you will have a problem with weld hardening, you can compensate by using a softer weld such as 6011 instead of 7018. If you already welded the hole and the weld is too hard to bore through, you should normalize the weld to soften it. To do this you can send out the part to a facility with a large industrial oven, or you can try to normalize the weld yourself. To normalize the weld yourself, dis-assemble the line boring equipment and only leave the standoffs tack welded in place. Then heat up the hole with your torch until it is 1250 degrees (check temp with a Welders Temperature Stick). The hole should not be cherry red. Keep the hole at 1250 degrees for a long time. Then wrap the hole with some sort of industrial insullation and allow the hole to cool down very slowly (ideally at a rate of about 150 degrees per hour). Then the weld should be soft enough to line bore using a carbide cutter.
If you don't have the optional bore welder to weld up the holes with a perfect spiral weld, the next problem is hand welded holes leave many high spots caused by welding in stripes or half-moon shapes. I suggest using a small hand grinder to grind down the larger high spots in the welded hole until the hole is nearly round. Then bore through the weld using one of the Cobalt-Steel cutters to remove some more of the high spots in the weld. Assume you may remove less than 1/64 inch per pass. The colbalt-steel cutters do not break but they get dull very fast. Then bore through the hole using the AL8 Carbide cutter (cut to length) or the Borite Indexable Carbide cutter (for larger holes). Assume you may remove about 1/64 inch (0.4mm) per pass. If boring through steel, make sure you are using C5 carbide or PC300 carbide. C2 is for castings not steel.
It is important to make sure you tack weld a bearing assembly on each side of each hole you are boring to reduce boring shaft vibration. Use the short standoffs on the side you don't need to access for hole measuring or cutter adjustment. This will get the bearing closer to the hole and make a big difference by reducing vibration and deflection.
You will need to manipulate the Speed, Feed, and Depth of Cut. So try cutting at a slower RPM. Examples: 100 max rpm for 8" dia, 160 max rpm for 5" dia, 200 rpm for 4" dia, 225 max rpm for 3.5" dia, 350 rpm max for 2" dia. The actual formula is 800 divided by the diameter (inches) equals the maximum recommended RPMs. .
portable line boring work using one of our magnetic drill presses: The
dial on the Milwaukee Magnetic Drill Press goes from 0 to 9, where 9 equals
350 rpm full speed and each change in number equals about 39 rpm of change.
It is very important to Feed with the drill handle very slowly and gently to allow the cutter time to do its work. Allow the cutter to determine the feed-rate. Do not force it. You can see this in our videos that were filmed by customers using our equipment. If you want you can try using a bunge cord to create an elastic power feed.
If you are boring a hole larger than 5.5 inches (140mm) diameter please use the optional Oversize Boring Adapter Collar to hold the carbide cutter.
Be patient. Assume you will remove about a 1/64 inch (0.4mm) per pass. Use cutting oil when boring through weld but avoid using cutting oil if you are boring before you weld because it could cause oil hardening if you weld on top of it. If you can't get any cutting fluid, try using WD40.
When you get real close to your final diameter, try using a Cobalt-Steel cutter again to avoid removing too much metal. But remember, as your cutter wears down the hole becomes tapered. The last .001 inch should be honed out using 100 grit homing stones. Standard 220 grit honing stones will take too much time.
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