Problem drilling holes on the centre line of items

[J Tiers]

Just for grins, try doing things asif you had backlash..... coming back past center and then going forward to the setting.

While I assume your DRO is reading the sliding motion,as any real one* will, if a bracket is too flexible, or screws are loose, or if the table gib is loose, etc, there can be wrong display that does not reflect the actual motion. Then reversing motion can "cross the effective backlash" due to the loosely-mounted DRO.

While that is often true, you almost never have a situation where the drills are all the same size and don't require moving either the head or knee to change the tools or to get the depth that you need for the hole.


Dan

Spotting drill, followed by screw machine length drill. Just as in the part (of my post) you failed to quote

[danlb]

I trimmed out the stuff that does not matter. I won't bother addressing why.

Back to Loose Nut's latest post:

You are dealing with really small sizes. When you are off .010 on a 3/32 drill, that's a lot. It sounds like you are saying that the spot drill is always correct but that the drill is not making it's hole within the dimple left by the spotting drill.

The center drill is likely to have a 120 degree tip. Per https://www.destinytool.com/spot-drills.html you may need to change to a spot drill for a smaller angle or a twist drill with a 135 degree angle. Per the above link, if the spot drill angle is less than the twist drill will result it the flutes digging into the sides of the dimple.

Dan

[J Tiers]

I trimmed out the stuff that does not matter. I won't bother addressing why.
....

Dan

Dan, it's a wonder your own spit does not burn your throat. You are the nastiest poster on the forum.

oh, yeah.... go ahead and report this post.

[machinejack]

When I want a hole on location I get the boring head out. Changing work heights will get you off if the mill is not true the spindle may be parrellel to the work, but if the post is not you will be off. If everything is trammed in you should be spot on.

[David Powell]

Everything matters.

I spent the last 7 years or so of my working career running a " First " mill ( and a Clausing lathe). A large proportion of my work was checked, ( BY a Chinese lady named Ying with a vastly expensive co_ ordinate measuring machine )and reports of my work were filed so that any discrepancies could be followed through. What we found was simply that everything mattered. I was in the land of small items, generally from 1/32 holes up to about 1/4". Certainly, if the maintenance gang used "MY" mill on the weekend or night shift I checked the tram before starting my day. We knew of, and tried spotting drills but found equally good results with tiny centre drills. We bought good quality drills, mainly from McMaster Carr, and once they lost their initial sharpness replaced them rather than even trying to sharpen them .Drilling fibreglass pallets generally meant drilling only 10 or so holes before the drills began to dull. We normally worked from an imaginary centre point on flat items, and when allowable I marked that with a small centre drilled hole so that if later modifications were needed we knew that was how it had been made. We began to be concerned if any hole was more than 2 thous off position. I hope this is encouraging and helpful. Regards David Powell.

[loose nut]

I trimmed out the stuff that does not matter. I won't bother addressing why.

Back to Loose Nut's latest post:

You are dealing with really small sizes. When you are off .010 on a 3/32 drill, that's a lot. It sounds like you are saying that the spot drill is always correct but that the drill is not making it's hole within the dimple left by the spotting drill.

That is how it appears to me. Yesterday I drilled a couple of 3/32" holes and they where OK. .000 run out one way and 2 or 3 thou the other which is acceptable to me for a drilled hole. The next time it could be way off and there is no change in the machine tram or any other set up IE: the vise isn't moved etc. ???

When I want a hole on location I get the boring head out. Changing work heights will get you off if the mill is not true the spindle may be parrellel to the work, but if the post is not you will be off. If everything is trammed in you should be spot on.

Boring a small hole (3/32 or 1/16) is a bit impossible for me.

Me thinks the drills must be jumping around some of the time??

[J Tiers]

If you are sure the spot drill is in the right place, then please tell us if you are moving the head when you change drills, or if you are just retracting the quill.

On your mill, both the head and possibly the column might have a tram problem.

[MattiJ]

For tiny holes and parts I would try spotting with PCB carbide drill. Very sharp, reasonably stiff and cheap.
No scribing or punch marks, clean surface and just mill DRO.