How Low Can I Go

[EdK]

Remove the compound and make a plinth that bolts to the cross slide in place of the compound for the QCTP to sit on that is the correct height to get the insert on center.

[dgoddard]

ML's suggestion of looking at the Shar's catalog gave a lot of dimensions and helped me identify that their comparable blade holder would be even worse, but the exercise in computation and my own last post helped me focus on one thing that was bugging me.

I have been an engineer long enough that my instincts were telling me that a calculation based on measurements made under awkward circumstances has plenty of chance for error, And the Tinius Olsen company's motto, "One good test is worth 100 expert opinions." came into play. I had commented in my last post that the most I could lower the cutting edge would be what ever caused the blade holder to bottom on the compound. It struck me that I had a simple test at my disposal to assess what lowering effect I could have before having to make any cuts.

I simply removed the tool post and set the blade holder directly on top of the compound. Then using a flat held against the work piece by the parting tool edge, I could accurately detect if I could get substantially below the centerline of the workpiece. My "flat" of choice is my "fish tail" thread cutting gage. The results look like this:

With that much tilt on the "flat" it is obvious that I can get well below centerline by using a 0.100 cut per option 1 or 2.

The "more complete" tool dimensions in the Shar's catalog convinced me that accomplishing this with a different blade holder to solve the problem (which does not come in a kit but must be purchased separately) would likely force me into using a blade of 0.750 height. Since rigidity varies as the cube of the height of the blade, staying with the present 1 inch blade height gives me a blade that is 2.4 times as rigid. and rigidity is pretty important in a parting tool.

My experimenting also corrected one other error in my thinking, as it became evident that the tool holder positions the blade holder far enough from the tool post that it is nearly impossible for the blade holder bottom to hit the compound, so I can get it even lower if I want but judging from the angle of my flat, I doubt that will ever be necessary.

[ML]

If you look at the Shars list, there is a blade that is "shorter". The 19 blade is only .75" tall as the 26 blade is 1" tall. Look at the h and h1.

Michael

[dgoddard]

If you look at the Shars list, there is a blade that is "shorter". The 19 blade is only .75" tall as the 26 blade is 1" tall. Look at the h and h1.l

Yes, I saw that however as I noted that blade is far less rigid (by a factor of 2.4 as noted in my last post) for that reason et al. I am willing to do some customizing of the tool holder in order to keep the rigidity of the 1 inch blade.

But thank you very much for the Shars link it was very infomative and the prices look good, has their quality been good also?

[dgoddard]

And the solution I chose was to take nominally 1/8 of an inch (per option 1) off of the bottom side of the slot in the tool holder leaving me with 3/8 instead of 1/2 inch of wall on the lower side. With the parts subsequently mounted on the lathe and adjusted for height the blade holder was very slightly below the tool holder block on the bottom side, and the bottom of the tool holder cleared the compound by 0.070 as checked with feeler gages. A test cut was made on 1 inch diameter 1018 Cold Rolled bar. The previous HSS cut showed smeared metal but with this carbide insert cutter, I got a clean cut all the way to the center at 430 RPM with small tits left on the stock and cut off piece. I expect some power feeding of the cross slide would have given me even better surface finish on the cut but it was very good as it was, and much cleaner and more uniform than what I am used to. There was also much less burr at the outside diameter of the cut, barely enough to require deburring.

The only problem I ran into was that the Enco clamp bar that presses down on the top of the blade has 3 screws. and a slight boss at the center one. Despite tightening all 3 carefully the blade was pushing back into the holder, this was corrected by loosening and re-tightening the bolts, center one first, the screw closest to the work piece second and only snugging the screw farthest from the work piece. Previous experimentation showed that this technique clamped the blade best at even modest torques. But otherwise getting a tight grip was difficult.

This set up was not low cost but the results are nice, it remains to be seen how quickly I go through inserts but I have been assured by others that these cut off inserts are relatively long lived. I hope so.