mobile machining - boring bar sizing 2

[frans]

I am in an argument with a mobile machining company at the moment. We stopped the job because we felt the equipment they were using was not sturdy enough and severe chatter was visible during the inithial cut. They claim the equipment was suitable and the final cut would have provided a reasonable finish. The flange to be faced was id 770mm (30.31") and od 930mm (36.61"), mild steel. They were using a boring bar set-up to do the facing, boring bar approx 2". The bar was supported by two bearing holders approx 700 mm apart and the cutting tool catilevered off the one end.

I am not familiar with machining mechanics - can anybody help me with the kind of torque and forces involved and how suitable this kind of arrangement is for this purpose.

Thanks

 

[EdDanzer]

If I understand correctly the tool sticks out 18+ inches from the bearings. Not good.
A good rule of thumb for machining is the best performance of steel tools is a 3:1 or less (2:1) length to diameter ratio. So a 2” tool with more than 6” of stick out from a holder will deflect enough to cause problems. After 6:1 you can only tickle material off and if there are hardness differences the surface will not be flat or smooth.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[frans]

Thanks Ed. I will try to explain in more detail. If you imagine facing the end flange of a pipe-like structure. The bearings are supported in the "pipe" which has an approx id=31,5". The bearings are spaced 27.56" apart to support the the center shaft of 2" dia. Where the central shaft sticks out of the pipe it has the toolholder cantilevered from it. The actual tool points parallel to the central shaft to face the end flange. The flange to be faced are the diameters as mentioned above, 30 to 36" roughly.

I will post a photo on our website and provide a link to it, may be easier that way.

Cheers

 

[MikeHalloran]

I wish I could have been there to watch you walk up to a guy who does this for a living, and tell him that he's doing it wrong.


To partially answer your question, the forces on a normal cutting tool in a rough cut are best measured in tons or kips. A search will probably bring up some equations to help you get a handle on it, and some tables to help you guesstimate the feeds and speeds that are normally used.

.. which will do you little good in this instance, because the field machinist's setup probably can't reach the optimum speed, and surely isn't stiff enough to take a normal chip. Which doesn't mean that he can't do a nice job.





Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

Here is an example of a large diameter facing machine.

http://www.cpmt.com/tools_videos.php

Here is boring bar that would handle your requirements that uses a 3.5" bar. As you reach out, facing in your case, you might have to go to a larger tool carrier.

There is some data concerning the power or this size unit.

http://www.cpmt.com/docs/bm/BB6000.pdf

You will notice the counterweight arrangement on the flange facer that help to minimize chatter and save tool bits. The one time we used our boring bar for facing we found that we had to add some counterweights to help it out machining SS. The next job we rented the large flange facer.

 

[frans]

Thanks for the feedback so far. The machinist was & still is not happy! Then again the machine in question is of such value and the implications of an unsatisfactory job such that the price of the unhappiness kind of fades.

If you guys look at it and tell me I was wrong I will have to eat humble pie.

I've posted a couple of pics on my website, here is the link:

http://www.belcher.co.nz/Machining.htm

 

[unclesyd]

I will have to agree with you as the setup and robustness of the machine are a little suspect.
In my opinion the machine boring bar is too small in diameter for cutting radius as you state.

From what I can see the boring bar and reach required the tool carrier will have to be rather large, both is diameter and length.

In the photos it looks like you are using temporary diaphragm to hold the boring bar driving head.

The question is how rigid is this diaphragm?

Not related to the machining problem it appears that this is a boiler of some sorts and the question arises as to what's all the support steel welded to?

MikeHollran,
It sure is nice though when you do catch them with the right question.

 

[EdDanzer]

The system in the picture will probably machine the face ok if being flat and square ±.005" is an ok tolerance and time to machine is not important. The interrupted cut will cause problems for this type of system even with a very light cut. One way to settle the argument is to do a stiffness test. Place a dial indicator on the back of the tool and apply a load as though the tool is pushing from cutting load. If the deflection is greater than.001” with 50 lbs tool force the machining results will be substandard at best.

http://www.danzcoinc.com/html/machining_center_stiffness_tes.html

Is a link to a tool and test we did on some VMC’s and more information.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[frans]

Hi Mike - it is a tunnel boring machine. You can see some pics of previous machines we have produced here :

http://www.belcher.co.nz/Projects.htm

Hi Ed, thanks for the valuable feedback. Do you know where I can find published data that supports the type of figures you mention?

Does anybody recognize the gear in the picture - it would help if I knew who the manufacturer was to get some recommendation from them. Unfortunately I have no exact measurements at the moment which would allow me to calculate stiffness.

If the machinist agrees to it we could do a mock set-up, but I would like to avoid that if possible - the amount of money in question would not warrant the time. At this stage to me it really is about the principle of the matter - he had the opportunity to inspect the machine and tell us if he is confident he could do the job. Then when he turns up he loses our confidence because there was severe chatter and his gear just looked to light. I stopped him because the implications of a stuff-up was serious - we could not afford to experiment. He claims he would have delivered a fine job if we let him continue.

 

[MikeHalloran]

The photos make it appear that the CG of the cutting head is cantilevered out from the nearest bearing by nearly a metre, which seems a lot. I guess that would bother me, too.

Still, a good machinist can do amazing things with suboptimal tools. Right now, I think I'd be checking the guy's track record instead of trying to analyze his tools.





Mike Halloran
Pembroke Pines, FL, USA

 

[EdDanzer]

Frans,

The load deflection value is something I derived experimentally. The machine tool industry does not have any stiffness standards established and probably does not want any kind if standard. The chatter is from tool deflection. You could modify the setup to use a much larger pair of bearings and shaft the couple the power head he has to the more robust system for torque and feed. We have used large bore flange bearings on ground and polished shaft with fair success in boring boom joints. With portable boring the torque and feed portion of the tool can be marginal if the bearings and shaft are robust if all you are doing is boring a hole. If you need to face and bore the interrupted cut shown in the picture that is significantly more difficult task.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[CSLufkin]

I'm with Mr. Holloran, Talk to some of of his other customers (Not just the ones he suggests you talk to) and see what they have to say.
A good machinist can do great things with pretty sub-par tooling. A poor machinist can screw things up with the best tooling on the market. I've seen it over and again.

 

[ncprogrammer]

I tend to agree that a good machinist can make almost any setup work correctly if they understand cutting forces, speed/feed chip load etc.

I'm guessing he was cutting heavier to remove bulk material intially but intended to make several small finish passes to eliminate tool pressure and improve finish.

Personally if the machinist seems competent I would be inclined to let him continue doing what he is trained for.

 

[dicer]

Having many years of machinist experience, I agree it is a pretty weak looking set-up. And I would be concerned about the powering head weight causing it to droop, that will cause the face that is cut not to be square. So when he set it up, did he sweep that face to make sure that it was square? And another thing, how bad is that face that he would be taking such a large cut as to make it chatter?
I would have asked him to demonstrate what the finish cut is going to look like chatter wise.
I have not seen them in a long time, but I remember a portable horizontal boring mill called I think it was "master mill" and that would be the way to go if you had the floor space for it.
If on that setup they were using, and it has no way to adjust the alinement to that face to make it perpendicular to the bore then you don't need them doing it.
Good luck.

 

[frans]

The plates were pre-machined and assembled in a machined jig which we were hopeful would keep things square enough for our purposes - we only needed the centers to be in-line and squareness were of less importance since we were using spherical roller bearings. Unfortunately a wrong welding procedure was followed and machining was required. The centers were OK but the plates were too far off-square.

Initially I actually thought they were using the light setup only to measure and was surprised to see machining had started. A huge amount of chatter was visible and it was clear the interrupt cut was a problem. Unfoirtunately we did not take pictures of the chattered surface.

Due to the value of the equipment at stake I made the call to stop the job, got further advice in and in the end shipped the unit off to a company who had a proper stationary machining setup - and got a top job back.

The original crowd claimed they could have completed the job properly, that they were only taking roughing cuts and the final cut followed a different procedure. Based on the non-conclusive feedback in this forum and the lack of photographic evidence I have since paid their bill, but I still think their set-up was too light.

Thanks for all the advice received here, it was great to be able to turn to the experts via this medium.