I was cutting some T-slots today with a 3/8" cutter in some cold rolled. It took quite a bit of time and I had some major problems with chatter. That meant that I had to keep my feed extremely slow. I varried the cutter speed all over the place and and couldn't find ant that made thing better. I was wondering if my technique was bad. What is the proper technique and speed for a T-slot cutter?
I went in first with an sharp hogging end mill and cut to almost full depth. I then went in next with new T-slot cutter and cut the slots. I noticed that taking the cutter right down the center line of the hogged slot. Upon finishing, noticed that one side of the T was a little wider than the other. What am I doing wrong?
Hi Jake, Sounds like your machine isn't very rigid, or it has considerable wear in lots of places. Keep in mind that when you feed a cutter by its full diameter that one side is climb milling, so the direction of travel will always influence the cutter, trying to push to one side or the other. The more rigid your machine, the better it can resist. How you cut the slot was in keeping with good practice, so if you aren't happy with the results, about the only thing you can do is to take the cut in multiple steps, assuming you can buy the right cutters.. Start with one that will cut a slot that is smaller in width and diameter than you desire, then open it up with successive cutters. The less each tooth has to take the better, lowering the amount of deflection. You can also benefit by using a thinner cutter and stepping up or down to increase the height of the slot. If you do that, always feed the same direction each time so you don't get steps in the width of the cuts. The last thing you can do is open the slot, then offset the saddle an equal amount and take finishing cuts on each side, blending any irregularities you might get from the first pass. If your mill is rigid enough, climb milling is the best way to get a good finish, but only you can tell if that will work in your particular case.
A little experience cutting with Woodruff cutters will help you better understand what will work for you. Play around with it on some scrap material when you have nothing better to do. It will all come right in focus with a little experience.
Always lubricate well with the appropriate fluid for the job at hand. This type of cutter doesn't lend itself to dry operation very well.
Always remove as much metal as you can with your rougher, and use the Tslot cutter to make your finish passes if at all possible. Cut to full depth, cut the slot to full width (or leave a few thous for a finish cut) then use the T-slot to go up the middle, then finish mill each side seperatly. Ive been told that cutter deflection will never give you a true one shot cut of a t-slot, so you are supposed to make it in three passes.
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To help eliminate some of these proplems with T-slot cutters, I had our special size production use ones ground on the bottom to provide cutting teeth. This helps eliminate the cutter trying to ride up in the slot. We cut to within .005 to .010 of the bottom with the proper cutter to obtain the width of the upper part of the slot, then follow with the T-slot. The deflection or (torque-thrust, as I like to call it). will get you most of the time. We just compensate sideways for it, since we are doing multiple parts, and have a handle on how much it will deflect. I added a vertical spindle to a horizontal mill so that we could mill the slot with a horizontal type cutter, then follow directly behind it, within 6 inches, with my added vertical spindle. Less variances in setup and tool changes that way, and it tripled production. We do two sizes of slots on this machine, an ancient Brown & Sharpe, and have the other cutters mounted in arbors and adaptors, which have been preset to run by simply swapping out the arbors.
Good in theory to a point, but not generally practical. You don't always have the right combination of shank and cutter diameter. When you happen to find the right combination, you have the problem of chips getting wedged between the shank and the slot, screwing up the finish or galling to the shank. Spinning shanks are not very good guides.
The big problem with any kind of slot that is larger than the slot opening is that you are limited by the amount of clearance you have from the shank to each edge of the slot. Only in very large slots do you have the option to cut one side at a time with the first pass.. That's why it's nice to have smaller and larger cutters, so you can start the slot slightly undersized, on location, and *rough* the slot, eliminating the majority of the metal, then going back using various procedures (job specific, can't say which one would work until you know the circumstances) to finish the slot.
Roughing, a necessary evil in most instances, seems to be lacking from many of the conversations on this NG. Roughing before finishing machined parts is considered good practice in machining.
Harold, you are probably right about the rigidity. My 30 year old Sharp
3/4 copy of a Bridgeport is a little past its prime. By default I did follow Gunner's approach in order to even things up. In the end everything turned out well, but the chatter was bothersome.I did tighten down everything that wasn't supposed to move, but it didn't help much. I found that starting the cut with the T-slot did take some extra care. Once the cutter advanced past the mid-point things were a little better. The final passes were a breeze, but the first was a bear.
The Woodruff cutters with which I am familiar have no side clearance. OK in a shallow key slot, but do use them for T-slots as well? Always thought you had to use the staggered tooth or other side cutters for those.
I'll enthusiastically vouch for that. It was some time ago that I used my first roughing mill cutter. A real delight. While it does take extra time to change the cutter for the finishing cut, the time saved milling down to the desired size more than compensates. Also, roughing cutters are significantly more expensive than regular cutters -- but again, you break, butcher, and dull far fewer finish cutters when you use a rougher. If you're patient, they'll come on sale and then you can pick them up for a bargain price.
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