Amount Of Material Removal On Spring Cut

[gearbuilder]

Hi, I take two matching finish cuts. If I want to take a 0.025" finish cut I rough the part to +0.050" then I take the first 0.025" finish cut noting the number on the dial. I measure the part. The remaining stock may not 0.025". Adjust the cutter for the remaining stock using the first cut dial number as your starting point.

The amount I leave for finish is based in the part. I pay attention to the amount of deflection I'm getting while roughing. The more rigid the part is the less I need to worry about deflection and the bigger finish cut I can take.

 

[kingmt01]

Wow! .025" outs a heavy cut for me. I have piled off more but I usually rough .020" at a time. Then I try to leave .010 after a spring cut & try to finish with two .005"cuts or. 005,.003,&.002". I'm starting to feel like my 14x40 it's just a toy.

 

[stupoty]

I find the soft hot rolled steel's don't like a cut much less that 5-10 thousands I always have some surface finish issues if I try to cut just a couple of thousands. I use the tangental shear cutter if I need to shave a couple of 0.001's of a piece of steel thats been a bit gummy.

Lower tolerance items like replacement bolts I have good results taking a couple of hevey roughing cuts and a finish cut of 10 - 25th (DOC), (edit , I left a step out, opps.) measure then do another 10-25th cut to finish,



I like to do a test part or just some test cuts in mystery metals to see if anything out of the ordinary happens.

Stuart

 

[epanzella]

You may be getting some of that from your spindle, when you are measuring 12" out you are amplifying everything the spindle does wrong. I noticed on the 1144 stress proof I cut at about 12" out there was the ability to move it about .003 and it was 2-1/16"diameter, don't get me wrong I was pushing and pulling hard to move it, however I think the results would be different if it was sticking out of concrete 1'

The reason I did this in the first place was to test my spindle bearings with the test bar I just got. I had another indicator at the spindle and it wasn't moving at all.

 

[joshua43214]

How do you calculate the exact amount of set down to apply that will compensate for the fact that you are now not transversing along the radius of the workpiece.

This would be a very complicated equation that would be a function of the materials elasticity, the depth of cut and the carriage position relative to the axis of revolution. It would very well be a exponential curve

Further you would then need to have the tool height increase while the carriage is being fed in,further complicating the calculations.

Basically having your tool below centre will ensure that you always cut less than what your dials/DRO are indicating regardless of flexing of the workpiece (purely on geometry). This will mean that you are simply guessing how much you are taking off the workpiece with every pass.

Sent from my SM-N920C using Tapatalk

It is just the Pythagorean Theorem.

It's not something really worth worrying about, the effect is pretty marginal.
Here are some diagrams for some different tool heights.



Here is the Python code snippet

a = .0625 #Tool height below center line
b = [z/1000.0 for z in range(2000,-1,-1)] #Tool distance from center
i = 0
R = [[],b,[]] #[actual radius, expected radius, error]
for x in b:
    R[0].append(sqrt((a)**2 +(x)**2)) #actual radius
    R[2].append(abs(R[0][i] - R[1][i])) #error
    i += 1

You can see that actual vs expected is so close to being linear that we can treat it as being linear until it gets very small.
Error decays at an exponential rate, and quickly drops below the tolerance of our tools.

I am not sure why we would ever need a tool holder that would correct the error. All it would do is artificially change the x-axis - in other words it would just have the tool feed up at an angle. Might as well just put the tool on center and feed straight in. The tool would just advance up a wedge that is the correct angle to have the change in radius constant with respect to the change in x.
I can imagine some special applications where the rake would need to be changed as the tool advances though. It would be some material that is really fussy over changing rake angle at different surface speeds, or machines that have tools that swing into the cut rather than push into it. Probably all these problems have been solved with the advent of CNC though.
Aside from figuring out a mechanical method for doing this, figuring out the right path for the tool tip to follow would be no big deal. It is just the derivative of the radius at any given point plus what ever rake angle you want. Provided the tool is on center, the solution is always 0 + rake angle.

 

[joshua43214]

Wow! .025" outs a heavy cut for me. I have piled off more but I usually rough .020" at a time. Then I try to leave .010 after a spring cut & try to finish with two .005"cuts or. 005,.003,&.002". I'm starting to feel like my 14x40 it's just a toy.

lol, I think your just being too easy on the machine.
I was making 0.200" deep cuts in stressproof on my 13x40 the other day, it just hummed into it. Made some really beautiful cobalt blue chips

 

[kingmt01]

Maybe but it makes a racket if I push it harder. Still cuts. I've taken harder cuts maybe 4x that but it doesn't seem right. Maybe I'll slow the spindle down & try some deeper ones but looks good at what I'm doing.

 

[epanzella]

Maybe but it makes a racket if I push it harder. Still cuts. I've taken harder cuts maybe 4x that but it doesn't seem right. Maybe I'll slow the spindle down & try some deeper ones but looks good at what I'm doing.

Are you using carbide or did you grind your own HSS? If a 14 x 40 lathe is unhappy making cuts over .025 deep there is something wrong.

 

[kingmt01]

I use both. Usually cheap caribide.

 

[Deerslayer]

Well here is my finished first part.
Just a backhoe pin nothing fancy!
Part has .003 of taper in the 12" length, not sure if it's in machine or my final pass and lack of proper spring cut.

One thing that has me slightly concerned is I drilled the hole through the center of pin with a 7/16 bit, I doubt it's big enough to be a structural issue but hindsight has me thinking the next one will be smaller. I was concerned about a narrow deep hole breaking my bit, any suggestions for a deep 1/4" hole preferably without drill breakage.