Boring accuracy when feed with the knee

[marka12161]

I have a wonderful old kempsmith #1 gear driven universal horizontal milling machine. I have the heavy duty vertical head which is a marvelously rigid assembly. Clearly, when boring in the vertical (z) direction, the knee lock needs to be free. I ran an experiment where i bored about a 1.300 hole and measured the longitudinal (x) and cross table deflections while boring. With a .015 depth of cut (change in bore diameter?), the max to min oscillations in both the x and y directions was about .0003 when measured with .001 DIs. When depth of cut was increased to .020, the max to min oscillations increased to something like .0008 which is no surprise. After removing the part, i measured the bore diameter with snap gages across 8 compas points and it measured round to withing .001 which is about the limit of my ability to measure. My questions are:

1. What kind of boring accuracy (roundness) can be expected when feeding with the knee on a beefy mill like a K&T or Cincinnati.

2. Do the x an y oscillations increase with bore diameter? It "feels" like they would.

3. Is a vertical mill with a locked knee and feeding with a sliding head superior to feeding with the knee when boring? Again, it fee;s like it would be.

These are not just academic questions, I'm going to be freeing up some room in the shop for a larger mill and i'm trying to figure out what to keep my eye open for.

Thanks

 

[john.k]

There is a lot of weight and drag raising the knee,and rocking of the knee is to be expected ...........Id say the difficulty may be in the alignment of the bore,not the accuracy.

 

[Gary Main]

i have not tried, but if i want to bore a cylinder i have to use the powerfeed in the ring area, and then use the knee to finish as i only have 3.75 power down

 

[marka12161]

After giving this some thought, i don't believe the bore diameter will effect accuracy. The tool pressure, which is what's causing the table deflections, will remain the same and is tangential to the bore surface. I think the difference will be in the frequency of the oscillations which is purely a function of the spindle rpm. Presumably with a larger bore, you would slow the spindle down to maintain proper surface FPM and therefore the frequency of oscillation would decrease. These frequencies are low enough that i doubt they would set up a resonant condition which would result in chatter.

 

[Clive603]

The knee and table assembly is heavy, much much heavier than a quill, so the dynamics will be different.

I'd be unsurprised to discover that any force variations during normal boring will be too small to affect the position of the knee and thus the workpiece shape. Such force changes will be cyclic at a relatively high frequency relative to the resonance frequency of the knee so rapid cancellation as the spindle turns will occur leaving no net force for long enough to shift things.

So long as the guide surfaces are in good order and the gibs well adjusted the knee movement is unlikely to be subject to extraneous effects.

Clive

 

[marka12161]

The knee and table assembly is heavy, much much heavier than a quill, so the dynamics will be different.

I'd be unsurprised to discover that any force variations during normal boring will be too small to affect the position of the knee and thus the workpiece shape. Such force changes will be cyclic at a relatively high frequency relative to the resonance frequency of the knee so rapid cancellation as the spindle turns will occur leaving no net force for long enough to shift things.

So long as the guide surfaces are in good order and the gibs well adjusted the knee movement is unlikely to be subject to extraneous effects.

Clive

The odd thing is, the old Kempsmith doesn't appear to have a gib for the knee. the machine was purchased new in 1952 but the column/knee design is a hold over from the flat belt machines. I wouldn't be surprised if it were a 1930's machine that sat in surplus for 20 years.

 

[ir9074]

The odd thing is, the old Kempsmith doesn't appear to have a gib for the knee. the machine was purchased new in 1952 but the column/knee design is a hold over from the flat belt machines. I wouldn't be surprised if it were a 1930's machine that sat in surplus for 20 years.

I know Im very late to the party here, but the kempsmith brochure shows the knee having a strange style of taper gib. The entire rear part of the dovetail is mounted on a tapered surface and slides up and down in a tenon to provide adjustment. They claim it to be more rigid, which I can believe...

 

[marka12161]

I know Im very late to the party here, but the kempsmith brochure shows the knee having a strange style of taper gib. The entire rear part of the dovetail is mounted on a tapered surface and slides up and down in a tenon to provide adjustment. They claim it to be more rigid, which I can believe...

Yes, I've seen that since i originally posted this. I love the Kempsmith. Mine is a #1G universal and is in very good condition. Only a 3 HP motor but with NMTB 50 taper makes it the perfect machine for a hobby guy like me. You can run really big tooling cheap as long as you don't get greedy with the size of cuts.