Stupid question from non machinist.

[bottlefed]

Hello,

I am new to the CNC zone and have basically xero machining experience so bear with me. I have a friend who has a mill and a lathe ( both non CNC )who helps me machine parts for my race car from time to time.

The other night we were turning a part trying to reduce the diameter from 3.650" to 3.630 My friend who we will call "John Doe" had taken off 10 thou and he was going to take another 10 thou using the dial to move the tool inward. When I looked down the cutting bit was about a half an inch above the centerline()axis of the lathe and the tool was running vertical ( not indexed with the centerline of the lathe ) I stated that I did not believe advancing the tool by using the scale on the dial would remove the correct amount of material unless the bit was aligned with the centerline()axis of the lathe.

He told me it would not make any difference that the cutter would remove the same amount from the diameter of the piece when you advanced it 5 thou regardless of whether the bit was indexed with the lathes axis or an inch above it.

Could some one please address this NASA level machining question since I am unable to reach my 7th grade shop teacher.

Thanks in advance,

Richard Gavle

[Al_The_Man]

[QUOTE=bottlefed; When I looked down the cutting bit was about a half an inch above the centerline()axis of the lathe and the tool was running vertical ( not indexed with the centerline of the lathe )[/QUOTE]

What is running vertical?
..........and what was the outcome after the cut?
Al.

[kdhBOSS5VRAM]

Hi,
I think you are correct. You'd have to trig it out to find out the exact amount but I'm sure it would be less than the dial reading.

Kevin

[bottlefed]

Parallel to the floor of the shop as well as parallel to the ways (I think this is the term) of the lathe. The result was that the amount removed was not the amount intended but I was assured that it was for some other reason.

[RICHARD ZASTROW]

Yes there is a trig equation as to the tool advance relative to the diameter. My question would be "did your freind not realize that a tool point that far above centerline would cause extreme problems in the cutting action?".

Dick Z

[bottlefed]

When he said that it would not make any difference mounting the tool 1 inch off center he was trying to give me an extreme example. I don't think he would ever mount his bit ( I really don't know the if I am using the correct names for parts of the lathe) 1 inch off center on a 3 inch piece.

That said the reason I am posting is he routinely uses his lathe in the fasion stated, that is he mikes the part and then advances the bit using the readout to try to reach final dimension. For example when reducing the diameter of a part that is 3.00" to 2.80" he would advance the bit .005 steps to reach the final diameter. If the bit was not on center with the lathe then he would of course not get the disired results.

I have been unable to convince him that this would make a difference so I asked him if I posted it on here would he belive the machinists that replied, to which he said yes.

I am hoping to save him a lot of aggravation when turning materials in this fasion.

[Al_The_Man]

If you have a CAD program, it would be simple to show him pictorially.
Al.

[DareBee]

It is very simple to prove out with a hand sketch as well as simple math formula.

And

as mentioned above cutting tools are specifically engineered and ground to be run "parallel" to the floor as well as on the centerline.

John Doe maybe needs an apprenticeship - but don't send him here.