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How to calculate the strain of the machine?

How to calculate the strain of the machine?

(OP)
Hello

thank you for entering in my post

I'd like to reduce the strain of a 125 face-mill (250mm long) on cast-iron the pass is 5mm x 70mm, this tool is mounted on a CNC milling which is already working in its maximum capacity and it causes a lot of vibrations, I could make this tool 50mm shorter and 45mm smaller
1) How could I calculate the strain of my new tool, 80 face-mill, on the same pass?

2) How much vibrations could I reduce by building my tool 50mm shorter?
3) Could I give a good improvement on the health of my machine and especially on the column/head by building this new tool?
It is already working in its maximum capacity (one day I had to stop the machine because of the vibration as it was loosing bolts from its column/head)...


Any suggestion according this matter will be very appreciated

Thank you in advance for your help

RE: How to calculate the strain of the machine?

Hoezap;
What is you are seeing with your machine is not normal. one of two :
A. Spindle problem
B. inadequate cutting conditions
A-Spindle problem of your machine (bearings)
1. did you make any diagnostic of your spindle
2. How many hours was the machine running (see the spindle hour counter, to find the number of hours, generally it is inside the electrical cabinet)
3. Please, go to your receiving documents and check the following information: what is the maximum allowed displacement of the spindle head at your operating speed (generally it is curve or tabulated data, it can be found in the maintenance document).
4. You can start by the spindle oil analysis to find out to see the <status> of your bearings.
B-Cutting conditions
1. What materials are you machining
2. Are you using maximum spindle speed
3. What feed and speed are you using
4. What is the chips form (curled, straight black blue…)




RE: How to calculate the strain of the machine?

(OP)
Thank you very much indeed Zinov


I will find out

what is the maximum allowed displacement of the spindle head at your operating speed (generally it is curve or tabulated data, it can be found in the maintenance document).

I have no idea what you are talking about, may you explain better please?

however I know already that tooling is not the best solution on that machine

what I'd like to know is, if I build a 80 facemill instead of 125, which improvement could i achive?
I know already that if could reduce the strain of the machine if I use the same feed

Could I say exacty with which kind of percentage I will reduce this strain?

I'm seeing very often service maintenance to repair the spindle but i don't know exactly if it was a bearings problem

This is a very good question I will definitely ask about it

I will give you more details very soon

thank you very much again for your help

RE: How to calculate the strain of the machine?

Hi Hoezap:

<what is the maximum allowed displacement of the spindle head at your operating speed (generally it is curve or tabulated data, it can be found in the maintenance document)>
I will reformulate :
For each spindle there is curve : displacement vs speed.
In the x axis of the curve you have the spindle speed in rpm
in the y axis of the same curve you have the displacement or velocity in inch or inch/s

If you go higher than that allowed displacement or velocity, you will cause damage to your spindle bearing.
Thus, it is important to consult that curve. Generally, maintenance personnel do this check to verify the state of the spindle bearings.

Please explain what you mean by <Machine Strain> and if you are measuring this <strain>
Please indicate units 125? 80?

Regards,
Zinov

RE: How to calculate the strain of the machine?

There are several things you can change to reduce the strain the machine is seeing but there is a limit on what you can do.

The cutter you are using could be changed to one with reduced cutting forces. If you are using a negative rake cutter you could change to negative/positive rake cutter or to a total positive rake cutter. The lead angle of the mill could be increased thus thinning the chip and reducing loading.

One other situation which I have experienced is the materials used to construct the machine tool. If the machine is constructed of gray iron castings versus a steel weldment there is reduction of vibrations which occur in the machine tool. Gray iron absorbs the vibrations while the steel structure will resonate and conduct the vibration.

Direction of cut could also impact the cutting performance. Try to always have the cutter put the loads into the solid locators instead of against the clamps.

Good Luck
Bill

RE: How to calculate the strain of the machine?

(OP)
Thank you so much Zinov

the unit is in millimetres

I'm sure we are not going too high about velocity, however thank you so much for your lesson about displacement very interesting and I will try to find out anyway even though I think only maintenance can give such a answer

we are using a 125mm facemill (s=383rpm) (f=2450mm per revolution) the pass is 2mm x 63mm
we are machining cast iron
the chips form is curled and black

“Please explain what you mean by <Machine Strain> and if you are measuring this <strain>”

we are using this tool mentioned above that is 250mm long to do a roughing-out on the capfaces that is where we fit the caps for the crank: our engine case is upsidedown and we are machining with this long tool between the two sump-rails

thus, we are using this 125mm facemill (250mm length) from the diameter to machine our 63mm pass so we are using only half of our facemill, so for this reason I believe that reducing the radius of our facemill we could reduce also the strain of the machine, I don't undestand why they are using a so big tool to do this operation, the machine we are using for this operation is not strong enough already

thank you so much again Zinov for your help

RE: How to calculate the strain of the machine?

Hi hoezap,
If you want to play with the cutting parameters to reduce vibrations here some clues :
1-You can reduce the number of inserts on your tool ;
the machine runs@383rpm (6.38Hz), let's assume that the face mill has 12 inserts--the insert passing frequency
is about 75.56 Hz which is the 'equivalent' of running the facemill@4600rpm.
If you reduce the number inserts, one you will reduce the inserts passing frequency two you will stay in the low frequency excitations.
The low frequencies can be absorbed more easily by the machine-structure. The machine structure is intended to do that : 'absorb' the low frequency excitations. The high freq. will be near to the machine resonance frequencies. For machine-tools, these resonance are above 400 Hz or more (generally).
2-You can reduce the engagement of your tool in the metal, : avoid as possible as you can full engagement of the tool. low radial engagement is the equivalent of reducing the number of teeth cutting simultaneously.
3-Reduce your depth of cut.
4-Reduce the tool length, equivalent of reducing the hangover.
5-Adjust your tightening torque as per vendor's suggestion concerning the recommended torque.
6-Move to 'shrink-fit' tool holders or similar, if possible, if not
7-Change your tool holder , some tool makers, provide anti-vibrations tool holders.
8-Play with the tool geometry (as suggested above)
9-Play with the rpm (just increase or decrease the % potentiometer when tool is engaged in the metal and listen to the noise, select the percentage where you feel that the machine is more comfortable)
10-There are other methods, but more sophisticated , time consuming and more expensive. If, you need I can give you you an insight about these advanced methods.
Good luck,
Zinov,








RE: How to calculate the strain of the machine?

(OP)
thank you very indeed Zinov wonderful lesson

“low radial engagement is the equivalent of reducing the number of teeth cutting simultaneously”
so following your example if the radial engagement is half mill the number of inserts will 6 (12/2), Is it right?

If my reasoning above is right I've already calculated that with a 80 chasefeed mill I could reduce the inserts passing frequence of 30%

the 80 chasefeed mill is similar to this on this link with the only difference that the inserts are a bit bigger
http://www.taegutec.cz/ttcat/strana.php?fnum=1145&...

we are using this 80 chasefeed mill for a quite unusual operation that is:
we are doing a roughing out on a profile through the Z axis, from the top to the bottom, leaving an undulating surface, the reason why we are using this system is because the machine is not powerful enough and if we use a normal profile mill the machine stops

Important question:
Is it possible to do with this mill also a milling on a flat face as well as this unusual roughing out?

Thank you so much again Zinov anything you type according this matter is always very appreciated

RE: How to calculate the strain of the machine?

Hi hoezap,
The good thing about the face mill ,in the link, that it has an odd teeth number!
In my opinion this will help to reduce vibrations. Personally I prefer face mills with odd teeth numbers.

the maximum number of teeth cutting simultaneously can be
estimated using the following equation m=R*(φb+φs)/(R*φt)
In which m is raised to its nearest whole number, and where
φt angle between two consecutive teeth of the cutter = 2π/N
N total number of teeth in the cutter
φb effective angle between the leading and trailing edges
φs tooth engagement angle
R radius of the cutter
(hope I did not miss any variable :)

For the Important question:
Flat face milling: yes it will work.
<unusual roughing out> : may be done if the tool approach and the cutting parameters are well selected.

Good luck
Regards,
Zinov,

RE: How to calculate the strain of the machine?

(OP)
Thank you so much again Zinov

“The good thing about the face mill ,in the link, that it has an odd teeth number!
In my opinion this will help to reduce vibrations.”

in fact following your 1st suggestion I've already calculated that I could reduce the inserts passing frequence of 30%

question:
Could I say that reducing the inserts passing frequence of 30% I can also reduce the vibration of 30%?
or to calculate the vibration there is a different formula?

Your equation is interesting however I can't see the relationship with what I looking for

I was just trying to understand the relationship between the radial engagement and the number of the inserts are they directly proportional for instance if I use a radial engagement of half mill could I say that the number of inserts is halved too?

Thank you so much again Zinov for all your lesson

RE: How to calculate the strain of the machine?

(OP)
Dear Zinov

What is the chips form (curled, straight black blue…)?

You asked this very interesting question a few posts ago

I took a sample of chips I have here I could show it to you but unfortunately I can't show pictures on this website it doesn't work, it will there be a technical problem on the forum

However the colour is black and the shape is curled

may you give me more information regarding how to analyse chips

this a very very interesting topic

thank you again for all your lessons

RE: How to calculate the strain of the machine?

Hi hoezap,
Could I say that reducing the inserts passing frequence of 30% I can also reduce the vibration of 30%? When you reduce the tooth passing frequency you lower your exciting frequencies. By lowering the exciting frequencies, you are allowing the machine-tool-structure <to do its job>
or to calculate the vibration there is a different formula? It can be calculated, but will be a lost of time!


I was just trying to understand the relationship between the radial engagement and the number of the inserts are they directly proportional for instance if I use a radial engagement of half mill could I say that the number of inserts is halved too?

The equation will provide you with the maximum number of teeth cutting simultaneously.
For example at 12 teeth with a half immersion the number of teeth cutting simultaneously , will not be 6.

φt = 2π/N=2π/12=π/6
φb =let's assume that it is null
φs =π/2

So you will have m=(π/2)/(π/6)=3, meaning that, at the maximum, you will have 3 teeth cutting simultaneously.


The tooth passing frequency is 'INDEPENDANT OF m', it depends only on the number of inserts and the operating rpm.

Zinov,
P.S : the best way to thank is to start the post...:)



RE: How to calculate the strain of the machine?

Hi Hoezap, concerning your last question :
Curled and black, in my opinion this means high vibrations (I expect there will be strips on the back of the chips).
In addition, the tool should be under high temperature. Use coolant or MQL. Take care with cast iron and check your pumping filters
otherwise they will be blocked with <dust+coolant>.
There is an ISO standard concerning chips classification, sorry I don<t have the ref. with me. But I can find it out for you, if needed.
Chips of good cutting process : when they heat up very much taking all the heat flow (tending to dark blue without being burned out).

Zinov,
P.S :
You can download pictures in the forum w/o any pb.
the best way to thank is to star the post...:)

RE: How to calculate the strain of the machine?

(OP)
Thank you so much Bill

“The cutter you are using could be changed to one with reduced cutting forces”

I used to use many inserts with low cutting forces on steel, but now I'm machining cast-iron and I know that this kind of inserts have a cutter with a reduced angle, and I don't know if it is strong enough to machine cast-iron, however I've never tried, do you suggest to me to try?

“One other situation which I have experienced is the materials used to construct the machine tool. If the machine is constructed of gray iron castings versus a steel weldment there is reduction of vibrations which occur in the machine tool. Gray iron absorbs the vibrations while the steel structure will resonate and conduct the vibration.”

Yes I know about it, however “repetita iuvant” (repetition is useful) smile

thank you so much again Bill

RE: How to calculate the strain of the machine?

(OP)
Thank you so much Zinov


“The tooth passing frequency is 'INDEPENDANT OF m', it depends only on the number of inserts and the operating rpm.”

so by saying so I think the tooth passing frequency should be independent also of the radial engagement, is my reasoning right?

“the maximum number of teeth cutting simultaneously can be
estimated using the following equation m=R*(φb+φs)/(R*φt)
In which m is raised to its nearest whole number, and where
φt angle between two consecutive teeth of the cutter = 2π/N
N total number of teeth in the cutter
φb effective angle between the leading and trailing edges
φs tooth engagement angle
R radius of the cutter”

where could I find these variable? Like “φb effective angle between the leading and trailing edges” and “φs tooth engagement angle”

in addition how can the “tooth engagement” be an angle?

Thank you so much again Zinov

RE: How to calculate the strain of the machine?

(OP)
Thank you so much Zinov

“Curled and black, in my opinion this means high vibrations (I expect there will be strips on the back of the chips).”

this is a very very interesting... yes there are strips on the back of the chips

I've never heard about MQL before What is it? Is refrigeration by compressed air?

“Take care with cast iron and check your pumping filters otherwise they will be blocked with <dust+coolant>.”

this is a very good suggestion thank you

“There is an ISO standard concerning chips classification”

I made a research about but I've found only something for turning and not for milling, if you have something for milling I will be very appreciated if you may send me a link

Thank you so much again for all your time

ps: I've just stared all your posts

RE: How to calculate the strain of the machine?

Hoezap, If I am not wrong ; ISO 8688-2:1989 (Tool life testing in milling -- Part 2: End milling).
MQL, stands for minimum quantity lubricant or in machinist language --Mist
It is a low quantity of oil ejected in the tool nose (go to utube and see lot of shots about it).
It was a pleasure to talk to you.
Thanks,
Zinov

RE: How to calculate the strain of the machine?

(OP)
Thank you so much Zinov

however I've just seen on Amazon and "ISO 8688-2:1989 (Tool life testing in milling -- Part 2: End milling" is out of stock

Thank you very much indeed for letting me know the MQL system, I've found several videos on youtube about it and they are very interesting

thank you very much indeed for all your fantastic lessons

I hope to meet you again with my next thread

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