Flycutting issues...??

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I’m having a problem on a BP mill, flycutting any metal, where it’s cutting heavier on one side , X axis, even after tramming the mill and indicating the vise. This is on subsequent passes after making the top flat.

What could be the problem? Am I incorrectly tramming the table? Could the table be warped or the vise bed? Parallels?

The table is not in great shape but I was able to tram it, so what could I be doing wrong?
BP vari speed J head. R8 fly cutter.
Cheap Chinese lathe tool.
Parlec vise (not in good condition.

Spindle run out? (Need to check).

What can I look for? Thanks


How are you tramming the head? Is the quill lock working properly? Are you adjusting knee and if so are you locking it?
Indicate your part after you lock everything down. Then indicate after cut not touching anything.
The cuts end as if the X axis is off a bit.
I’ve trammed the mill 3x. Everything is square.
I check after all the bolts are tight. Still... ?
I’ll check for slippage.
Before you do this, lock the knee locks and take another test cut. If that fixes the problem then tighten the gib.

A large blade screwdriver and a phillips screwdriver. The gib is on the left side. Remove the top and bottom wipers, there should be screw heads under both. Loosen the bottom screw, and tighten the top screw. Make sure the the knee doesn't bind at the top and bottom. When the gib is adjusted to your satisfaction, then tighten both screws and replace the wipers.
What are you trying to surface. If the part is 1/2" and less it could be bowing from the forces of the vise. Which is what I tend to do. Confident in my vise I release pressure on the finish cut when trying to do thinner pieces. If it's a chunk disregard but it still can be moving.
Easy, clapped out Bridgeport knee mill.
If the parts are short <10" place the vice at one end of the table, as close to the end as possible. The ways will very likely be less worn at that position, do not move the knee once locked. Do the work at one end of the Y axis for the same reason.
Try it and report back, you may be surprised by the results.
Thanks to all, I appreciate the information.
This is an old refurbished BP 2J mill. I’m going to remove the knee gib so that I can inspect it, mic it. It may already have a shim.

The surface finish on the part looks amazing, so it may only need a minor adjustment.

We may just get a newer, refurb from H&W Machinery.

It’s a shame how some people abuse machinery. I’ll post back soon.
It looks like adjusting the gibs may solve the problem. I would like to remove the knee gib to inspect it. Do I need to support the knee with wood blocks? Is it safe to remove with the knee cranked up or should it be half way?
I think the gib comes out the top. So you would want the knee on the bottom. If you need it, some wood blocking and a hydraulic jack on the outboard side might be useful just to take some pressure off the knee.
Maybe some 2x6’s cut about two foot. Put them so they support the bottom of table just outside the saddle. Lower table till the weight is on 2x6’s. Say two for each side. I would clean the dovetail and oil before lowering just so she don’t get stuck. You might have to use a punch or something to lift the gibb out enough to get fingers on it so don’t bottom the knee out you might need the room. I’d probably put a strap around the knee to keep it tight against the machine. With that gibb out the knee will want to fall toward the operator. Good luck.
Is it the same in both directions? Maybe you are getting some "tool push". You said it was a "cheap Chinese lathe tool" - that really should not be a problem - so long as you have ground it nice and keen. Is it HSS or Carbide? Those brazed carbide tools out of the box are terrible. Grind it yourself to ensure clearance and a "smallish" radius. HSS with a keen grind and a bit of back rake will result in the least amount of tool push.

Let us know how you make out.
Thanks to all of you. Much appreciated information. If I do remove that gib, I’m thinking of using a hoist and also removing the table, saddle etc.

This old 2J vari head is not that bad. No table movement really. It does need a good cleaning. Old oil and chips everywhere.

I will grind a new left hand tool. Looking to buy a Tormach superFly. I will post new updates. Thanks
Just take the handles and bearing blocks off both sides of the table and slide it off. That way you can clean and inspect the whole thing. Adjust if needed. Easy to get to the lead screw nut, clean out the oil passages, etc..

It is helpful to set up a platform of some kind right next to where you slide off the table so you can slide it onto it, use the knee adjustment to make height same. Having someone there helps but if sliding to a platform maybe not needed. I had a table on wheels that was a good height and was able to slide to that and use the wheels to help side all the way off and away. Replace with reverse action.
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