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Fly-cutting flat surfaces

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Tony Martyr28/12/2011 17:25:15
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226 forum posts
45 photos
For the first time I am using a tipped tool mounted in a fly-cutter on a vertical milling m/c to face the bronze cylinder block of a marine steam engine model. The casting is too large for my Myford lathe.
I have taken care to ensure, by adjusting the position of the tool, that the sweep of the cutting tool is such that one traverse of the head covers the whole width of the work-piece; thus avoiding several cuts or a continuously intermittent cut.
In spite of this the finished surfaces are always slightly dished (the 'valley' being in line with the traverse line of the head).
I can't see why this should be the case although I know that bronze can deflect the tool when facing on a lathe with an unlocked saddle. I have tried different cutting speeds and depths of cut but the effect remains.
It is not a great problem, in the present case, because I have to bed the working faces on a surface plate using a hand scraper but I would like to know the cure.
Any ideas?
Tony
Tony Pratt 128/12/2011 17:30:22
2319 forum posts
13 photos
Hi, this situation is very likely due to the spindle not being exactly 90 degrees[or square] to the table. What method do you use for checking that the head is exactly "square" to the table?
Tony
mgj28/12/2011 18:04:48
1017 forum posts
14 photos
It mans that the head is tilted along the line of traverse.
 
If you are traversing on the x axis and you have a tilting head then you'l need to check that. If its on the y axis you may need to reset the column or shim it a smidge.
 
If those things check out, another possible culprit would be the vice or whatever packing you are using, but theoretically that should not be so, because the only way to get a dished, as opposed to an inclined, surface is for the disc to be tilted with respect to the line of travel.
 
There is a sneaky one which allows the disc to deflect under load. ie your checks pan out, but the disc effectively tilts under load - flexing or in the bearings?
 
Theoretically, flycutting does generate a truly flat surface. In fact its very difficult, and you may find that you need to rub it on a plate to get it genuinely flat. (I use the term flat meaning truly flat)

Edited By mgj on 28/12/2011 18:08:53

Ramon Wilson28/12/2011 18:06:06
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1655 forum posts
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Hi Tony,
Tony P is quite correct in that the head is not exactly square if it is exhibiting this tendency. Fly cutting a 'flat' surface is not usually the best way to achieve one. 'Tubal Cain' at one time wrote an article covering this very subject though how far back I'm not sure
Personally since reading that when such a surface is required I have always milled it using a small cutter - usually 6mm or a 1/4 and with constant over laps. If the head is slightly out then this minimises the effect you are getting and finishing it off to a fine finish is relatively easy with scraping or filing with a very smooth flat file. Sometimes just stoning will take out the tooling marks.
 
All the port faces I have done so far have used this method, both on bronze or cast iron - those tooling marks, which I agree do seem far more prominent than those produced by flycutting, soon disapppear if the cutter, end mill or slot drill is very sharp, using higher revs than normal and a very fine last cut - .005" max is taken.
 
Hope that helps a little
 
Regards - Ramon
 
 
mgj28/12/2011 18:17:40
1017 forum posts
14 photos
What Ramon has raised is a very interesting point - the difference between smooth and flat.
 
Which is more useful - a smooth but slightly dished surface, or one which is a series of very small dogs teeth. It may not be (quite) so smooth, but it may actually be flatter. (CLA)
Stub Mandrel28/12/2011 18:24:13
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4318 forum posts
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As Ramon says, Tubal Cain was insistent that a sharp milling cutter did a better job and was faster, even though it meant more traversing back and forth. But his catch-phrase was 'keep up the feed rate'.
 
Neil
Tony Martyr28/12/2011 20:39:05
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226 forum posts
45 photos
Of course my mill-head is out of true to the table because it is tilted slightly on the swivel that allows 45 degrees movement either side of vertical - a glimpse of the B-obvoius after reading your replies!
Since I have never used the swivel adjustment I have never had occasion to check its setting and only using a 75mm dia fly-cutting tool showed up the problem.
I now have a job for the next raining day - thanks to you all and complements of the season to all users and experts on this forum.
Tony
Brian Price29/12/2011 10:25:39
8 forum posts
Hi Tony
 
When using a tipped cutter it is sometimes advisable to tip the head over a few thou low in the direction of the feed cut. Tipped fly cutters have a tendency to drag on the back side of the cutting sweep and make the finish look horrible.
The small amount of tip of the head does not make a lot of difference to the flatness of the part.
I would be tempted to use a nice sharp HSS tool bit in a fly cutter and keep the head exactly at 90deg. With a good set up and a good machine you should be able to achieve that lovely cross hatching finish.
Another thing that can cause this is a badly worn table and the table can actually dip as it approaches the ends of the travel. Caused by the weight of the table tipping the table down at one end.
 
Brian
www.eBolt.co.uk
Brian Price29/12/2011 10:26:31
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