Circular hole pattern help

[rleete]

I have a cylinder head that I'm trying to make a hole pattern in, to bolt to the cylinder.  It's about 1.5" diameter, and roughly 3/16" thick.  I've tried both laying out the holes with a compass and making a groove using the lathe for the bolt circle.

In either case, when I transfer the part to the mill, and try using coordinates to locate the holes, I end up with at least one hole that looks off.  Not by much, but enough that it's visible, and it bugs me.  Since the holes are so small, even a couple thou off is apparent, making the part look lopsided.

So, how do I get a circular pattern of holes to be even and concentric?

[b.lindsey]

Are you adjusting for backlash on both the x and y axes? That could throw you off a little. Do you have a rotary table?

Bill

[rleete]

Re backlash:  apparently, not well enough.

Yes, I have a rotary table.  How would I hold a part like that?

[b.lindsey]

You could try super glueing it to a larger sacrificial piece that could be clamped to the rt. Center the rt, then without moving the table, nudge the mounted part till it is also cenrered with the spindle and then snug the hold down clamps.
Bill

[steamer]

Rleete
check my wallaby tbread...i us a glue chuck for a part like that

Dave

[Jo]

I agree with Bill, sounds to me like the backlash in your leadscrews is causing the problem. DRO's would solve this and do all the calculations for your cylinder head holes.

Another option is to use a chuck mounted on the rotary table.

Do you have bull wheel/indexing on your lathe? If so you could hold the cover in the chuck and think about mounting a drill on a vertical slide on the saddle and use that to drill the holes.

(I now use my DRO.... ).

Jo

[b.lindsey]

Not that it helps at the moment, but an adapter for locating your lathe chuck in the center hole of the rt can be very handy. Its a feature i use often on my sherline rt, and can transfer parts from the lathe to the mill rt without ever having to remove them from the chuck. Obviously easier to do on smaller machines.

[rleete]

Bill, that's on the list.  With the trouble I'm having doing these simple parts, it may have just moved to the top.

Nothing like ruining a few parts to really put the brakes on a build.

[Ian S C]

For 4, or 6 holes its easy to index it in the lathe, you don't need to drill it there if you are not set up for it.  Put a center punch in the tool post, set it to the required radius, and as each index is done, bring up the punch and mark the work,  remove from the lathe and drill.   Ian S C

[Mel Larsen]

For about $800 you could solve your problem, buy a DRO for your milling machine.  The hole bolt pattern thing was the first thing I praised after installing a DRO on my machine.
Mel

[rleete]

I was afraid that would be suggested.  No way I'm putting $800 DROs on a $500 mill.

Mounting the part on a rotary table seems to be my best solution.  I'll have to figure out a way to hold it down.

[Mel Larsen]

Heck, I put a $800 DRO and a $275 Table Drive on my $800 mill and think they are worth every penny,  I can do thing now that I couldn't even dream of before.  I was loosing interest before because of things I couldn't do.
Mel

[steamer]

Rleete,

one way to hold it down on a rotary table is to do a few holes and then shift the clamp, and do a few more.   When I say shift the clamp, I mean put another one on first in another rotary position, and then take the first one off.   Do a little more...ect.

Work your way around.

Dave

[steamer]

If you want Rleete....I can sketch something up .....just for food for thought ...let me know  How many holes?

Dave

[rleete]

I think I have a solution for this one.  I'm going to have to try and see if it works.