Boring a round hole on a Mill

[Mark DaviesParticipant @markdavies88192]

Hi All, I bought a Sieg SX3 mill a few months ago and am teaching myself milling. I'm currently boring bronze bushes on Con Rod small ends for an engine. The bushes are 1.5’ deep and finished diameter is 1.1254’’ to 1.1262’’ (they start off at 1.1115’&rsquo. First found the holes were not round or parallel ( they were bigger as they got deeper) but have improved this a lot now since I found the head rotation when set to Zero was slightly out. I’m pretty happy the hole is nearly parallel on the X axis, but the diameter on the Y axis is always slightly larger by about .001’’ to .002’’ giving me a slightly elliptical hole. Any idea what may cause this, do I need to adjust the alignment of the head vertically over the Y axis too?

If I can get the hole Diameter the same on X and Y axis this would be a huge improvement.

Thanks

[Mark DaviesParticipant @markdavies88192]

[JasonBModerator @jasonb]

How are you feeding teh tool ? By the fine feed with quill loose & head locked or with a locked quill and lowering the whole head?

[Mark DaviesParticipant @markdavies88192]

Locked Quill and lowering whole head.

[RobCoxParticipant @robcox]

Several things to check. I assume you have a dial indicator or a dial test indicator. Mount this on the spindle so the tip describes a circle about as big a diameter as the table depth front to back. At four compass points measure the difference in table height. Any errors will cause your circular cutter motion to describe an ellipse on the part.

Second, check the head is rigid when not locked (you can use the same DI or DTI to measure how much it moves when you push/pull on the spindle) . If its not rigid, try feeding with the quill instead with the head locked.

[Ian PParticipant @ianp]

Are the X and Y axes locked too?

To produce an elliptical hole with single point tool (revolving on good bearings) maybe the cutter is springing the work on one axis.

Ian P

[DC31kParticipant @dc31k]

How are you holding the bushes?

I am quite sure you are not squeezing them in a vice, but it is instructive to look at the bad effects of doing so and see if any of them could appear in your chosen workholding method.

If one axis of the bush is compressed more by the workholding than the other, the two axes will measure differently when the clamping pressure is released.

If the bushes are being bored insitu in the con rods, maybe check the roundness of the unbushed hole and the fit of the OD of the bush.

What is the wall thickness of the bush?

You say the Y-axis is 0.001" bigger than the X-axis, but what if the X-axis is 0.001" smaller than the Y-axis? Talking in these terms is not useful as it might mislead you into looking in one place and not another. All you can say is that there is a difference between the two axes.

[JasonBModerator @jasonb]

Tram in both Y and Y is important, also make sure it does not change excessively between a locked and unlocked head.

Also check the vertical movement is true to the table in X and Y as that could be out even if things tram correctly

Also check he Z gibs are not too loose as the tool could be pushing the head and making it nod.

Are these new bushes or are you reboring old worn ones, if old and worn oval there may be some pushing off of the tool where it is taking a different depth of cut. Sharp cutters will help either well ground HSS or if using an insert bar then use **GT insert.

[Mark DaviesParticipant @markdavies88192]

Will try the check Rob suggests next.

Ian, X and Y both locked.

DC, bushes are in the con rods, Rods clamping to parrallel bars. I think Y axis is the problem as when I make several passes without making any boring head adjustments, the Y axis Dia keeps getting bigger but X stay pretty constant. Bushes are a split bush, pressed into place, wall thickness before working is 0.035''.

But I'm here to learn, thanks.

[HopperParticipant @hopper]

Tramming the mill is the first step,as described with dial indicator sweeping the table. But the fly in the ointment with some hobby mills is getting the vertical column set up to be truly square to the table in both directions. This can be done by scraping or shimming the base of the column. Can be checked putting a known good angle plate or circular square on the table and reading off a dial indicator as the head is moved up and down the column. (Circular square is just a parallel cylinder with one end square to the OD, easily turned up in the lathe.)

A workaround could be to set the tramming then use the quill to feed the cutting tool downwards rather than move the whole head, which follows the column, which may or may not be perfectly aligned.

[Howard LewisParticipant @howardlewis46836]

Since the bores are tapered, has no one suggested taking one or more "Spring cuts" until the boring bar no longer cuts?

Howard

[Nigel McBurney 1Participant @nigelmcburney1]

Sitting here with not much to do I have read the posts a few times,the critical part of Marks last post is the statement that with no adjustment to the boring tool and after making several passes the the x axis of the bush stays relatively constant yet the y axis gets bigger, this surely indicates ,assuming the table locks are on ,that the bush is moving or springing towards the tool.perhaps opposite sides of the bush are springing in or just one side. this could be checked by marking out blue or a felt tipped pen marks around the id of the bush,then see if the blue is removed by the boring tool in one place or two. Not knowing what type of engine the rod comes from it ot easy to find the problem though a split bush of the is size with a wall thickness of one mm indicates possibly a post ww2 design. In my restoration work I have found many instances where motor components do get distorted and these thin walled bushes do not make contact all around the bore, itsno good saying bore out the con rod eye to get a true bore and then fit a bigger bush as nno doubt the engine requires one of these thin wall bushes to maintain reliability and as I would think that bushes of this type would come in one stanard size, to check that this the problem ,press out the bush rotate it 90 degrees ,press it back in,set it up as you had and see if the oversize boring still occurrs on the y axis, One way to get another check would be to machine up a free cutting steel bush with a lot thicker wall say 5 mm or 6mm press that into the con rod and then try boring that out with exactly the same set up on the mill and the same cutting tool, if that machines ok then it is the con rod at fault. The position of the split in the bush relative to the two machine axis rmallow the bush to springegarding the mill you try the following, are the vertical column securing bolts tight,check that the the column is truly vertical to the table by using a good engineers square which are usually more accurate than an angle plate, attach the dial idicator via a magnetic base to the vertical head casting,NOT the spindle or quill. if the column is true then tram the spindle. Can you let readers know what engine this con rod fits and age ,it might help. Not quite relative to this probem but it may be of interest, when I was still at work,35 years ago, alloy castings with accurate bores coming off a robotic machining cell had a quality problem down in a bore there was a groove similar to a circlip groove, it was machined by a cutter similar to a T slot cutter,the cutter in a toolholder went down into the bore started spinning up to cutting speed and then moved over to get the depth of cut and then by driving the x and y axes to describe a circle of the table the groove was cut in one pass. Known at the time as circular iterpolatation,if the groove dia started to reach drawing limits an increase on the cnc controlled diameter could be dialled up quickly without adjusting the tooling by hand, well all went well for a long time until it was found that the grooves were oval, a bit of investigation revealed that a thrust bearing had failed on one table ball screw resulting in loss of table travel. That was one problem at the time with an enclosed machining cell with multiple machines,robotic work loading and cnc inspection which only at that time only measured some features, plus if that bearing was making a noise it cannot be heard by operators outside the very large cell.

[Andy AshParticipant @andyash24902]

I think there are four angles that you have to think about.

Two are the rotation of the spindle axis centreline within the head. The other two are the rotation of the linear travel of the head relative to the machine as a whole. If everything was true and square, your hole would be round and not tapered.

The ellipse comes from a misalignment of the spindle axis from the hole axis. The hole taper would probably be a complex combination of machine truth and head truth.

Some of the four angles won't easily be adjustable, but don't forget that once you understand the limitations of your machine, you can use shims to bring the work into alignment with the machine. Ideally if there is a reference face on the work that is parallel with the hole, you can use a dial gauge. Just pack the work with shim stock until it is in alignment with the undesired axis features that cannot easily be adjusted.

In the end a straight line is a straight line. If the machine is not tight, but not loose either then it is likely to produce a pretty straight line from a single axis. That's all you actually need. If you use the vertical axis instead of the quill, you have more problems because the head might be "nodding" rather than "swaying" and you can't usually correct the nodding angle.

If you can just use the quill, then you can pack the work until it is true to the quill.

I too have a Chinese mill!

Hope you get the hole you want.

Edited to add;

Actually I should also say that I'm presently using the boring head in the lathe for exactly this reason. I have less range of movement in the lathe, but the machine as a whole has better truth.

Edited By Andy Ash on 27/12/2022 18:31:59

[Bruce EdneyParticipant @bruceedney59949]

Please don't get me wrong but surely 0.001" (0.025mm) to 0.002" (0.05mm) is pretty darn close for a benchtop mill is it not? I mean I can push my Optimum BF20 column to easily produce more of an error than that.

[HopperParticipant @hopper]

Posted by Bruce Edney on 28/12/2022 02:40:20:

Please don't get me wrong but surely 0.001" (0.025mm) to 0.002" (0.05mm) is pretty darn close for a benchtop mill is it not? I mean I can push my Optimum BF20 column to easily produce more of an error than that.

One thou is pretty good for a hobby mill,yes. Two thou I would be trying to do better.

The OP would be best to set up a good solid block of steel or aluminium etc clamped solidly to the table on parallels and do some test boring to see exactly what is going on without the factors of possible distortion from thin walled bushes, vices, etc etc.

[Mark DaviesParticipant @markdavies88192]

Nigel, thanks, some good info there I will work through, currently tramming the bed. Con rod is from a Lycoming Aircraft engine, I'm an aircraft engineer and have also been overhauling these engines for 10 years, but I always subcontracted this bush replacement job out to another engine shop, I now want to be able to do this myself.

Andy, thanks for your suggestions too, it does look like head has some nod.

Bruce, I agree, normally .001'' is a good margin, but I have a finished tolerance of .0008'' to work to.

Hopper, test bores sound a good idea once I've trammed and shimmed as best I can.

I also have a Roller burnisher that I can use for final sizing if I can get bore size close to final.

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