reamed hole vs. drill rod size

[IanN]

...... Regular drill tips can drift off just a bit especially on longer (depth:diameter) holes & deviation can feel like a slightly tighter fit to the length of a valve stem. A reamer will basically follow this pilot hole so you are chasing your tail. ....

To drill deep holes without the drill wandering, rotate the work piece and keep the drill stationary - if the drill starts to wander, the drilling forces push it back on to the centre of rotation of the work piece

The easiest way to set this up is to mount the work piece on a faceplate in the lathe and drill from the tailstock. Holding the work in a four-jaw independent chuck is an option, but the faceplate is usually quicker and easier

All the best,
Ian

 

[petertha]

Not quite sure I follow you Ian. In my case the bronze valve cage was set in a rotating collet chuck. The spot drill & drill & reamer were held stationary in the tail stock chuck. All of the 'hole' operations, including the larger ID cup counter-boring were done from one side to avoid flipping the part & re-chucking (to potentially a different run-out axis).

But I've had drills wander both in my mill (rotating drill) or lathe (rotating part) if the conditions are right... or should I say wrong lol. Usually with sticky grabby materials or slightly mismatched cutting edge or high depth:diamter ratio. Bronze & brass seem to be my troublemakers, but a combination of the right drill, dubbed tip if necessary, peck drilling etc. seems to help.

 

[peterl95124]

The question then is how will you machine the valve after its been silver-soldered together. You can't say it won't need any machining, for silver solder to flow there needs to be a couple thou clearance so it won't end up concentric, and even a shoulder to "keep things square" won't actually end up square after the silver capillaries unevenly under it and then cools unevenly afterwards. The stem itself is very unlikely to remain straight after being unevenly heated and cooled. Sure it may look like a valve, but it isn't likely to function as a valve.

if you're really going for a no-slop stem-to-guide fit, then that's about 0.0001" to 0.0002" clearance, and the head needs to be more accurately concentric than that and like wise the guide and seat need to be more accurately concentric than that or the thing just won't seal. you can't get that level of concentricity even chucking the stem in a collet, you'll have to chuck up the head rather than the stem, that means you actually have a long piece of rod instead of a head, and at this point you have to ask, why didn't I just start with a long piece of rod and machine down the stem and head all in one chucking so everything is guaranteed concentric. Its the same amount of machining either way, but there's no messing with silver solder in one of those ways.

 

[holmes_ca]

Yes, I plan to silver solder the stem to the valve head. I have never done this before but I think it will come out well.

Blockman, it might be worthwhile to drill a very small hole close to the bottom of the joint to release air you may find the head or stem will lift as you heat the parts,

Edmund............Alberta

 

[IanN]

Not quite sure I follow you Ian. In my case the bronze valve cage was set in a rotating collet chuck.......

Hi,

My comments were "generic" rather than specific to your case - sorry for any confusion (in general a work piece could be any shape rather than the special case of a bar where an axial hole is required)

The thing about the "self centring" inherent with a rotating work piece is that the drill must "float"

In the video links below I show the process (the clips were made to demonstrate another point about drilling small diameter holes so the context does not apply)









Al the best,
Ian

 

[willray]

To drill deep holes without the drill wandering, rotate the work piece and keep the drill stationary - if the drill starts to wander, the drilling forces push it back on to the centre of rotation of the work piece

Having nothing much better to do at the moment, I'd appreciate it if you could argue this point sufficiently well to convince me that it's true. I've certainly seen the claim before, but the physicist in me says that the cutting edges of the drill have no clue whether it's the drill, or workpiece that's turning.

Were I to "stand outside the system" of a typical drill press, and grab the chuck, such that the chuck stopped rotating and the universe (and workpiece) commenced to rotate around the bit, I fail to see how this would suddenly make the drill self-center.

Where is my thinking going wrong?

 

[Richard Hed]

Since I have one of the crappiest lathes ever made, an Enco 9-20, and it is very sloppy and does not do left hand threads (which is a big reason I bought a lathe), I am looking to buy a better lathe in the 4-5000$ range. I would say the only thing good about an Enco of this size is that any lathe is better than nothing.

I have my eye on the Grizzly G4003G, however, I browsed the Chinese stuff in "Made in China" and Alibaba and have found numerous lathes that do the same as the Grizz. We all know that the Grizz stuff is just a way for Chinese stuff to sell more easily in the USA, and that Grizz has a pretty decent reputation for fixing bad equipment that gets to the customers, but the Chinese direct sales lathes are significantly lower price even after shipping, so I am wondering if anyone can suggest a decent lathe to buy. I am so suspicious of these companies that they will not support their products and that they might have generally poor quality stuff. One can never know what each company is like. There are SCORES of these companies in China making machine lathes and mills--which ones make good stuff?

In another year or so, I will likely be looking for a small mill also. Due to the corona virus (this years horror story to keep us entertained, or under control or believing in aliens or for some agency of the government to get more $$ out of the taxpayer or whatever), I thimk the price of everything just might drop a bit. Any suggestions?

 

[IanN]

Having nothing much better to do at the moment, I'd appreciate it if you could argue this point sufficiently well to convince me that it's true. I've certainly seen the claim before, but the physicist in me says that the cutting edges of the drill have no clue whether it's the drill, or workpiece that's turning.

Were I to "stand outside the system" of a typical drill press, and grab the chuck, such that the chuck stopped rotating and the universe (and workpiece) commenced to rotate around the bit, I fail to see how this would suddenly make the drill self-center.

Where is my thinking going wrong?

Hi,

I'll try this without using pictures - if I fail let me know and I'll try and draw diagrams

First, imagine a rotating drill in a work piece:

Assuming a perfect, symmetrical drill tip with symmetrical cutting edges - the forces on each edge are equal. If the drill starts to wander, the forces on each side of the drill remain unchanged and there are no correcting forces and the drill continues to drift.

Now consider a rotating work piece:

When the drill is on the centre of the work piece rotation, the forces on each side of the drill are equal.
If the drill drifts off centre - let's say it drifts to the "left" - now the radius from centre of rotation to the left hand edge of the drill is larger than the radius from the centre to the right hand edge.

The larger radius on the "left" exerts greater forces on the drill bit than the smaller radius on the right - this results in a net force from left to right pushing the drill back towards the centre of rotation where all forces are in equilibrium

Is this clear? if not I will attempt some diagrams

All the best,
Ian

 

[peterl95124]

I'm not entired convinced by Ian's logic but note that this is how rifle barrels are drilled,
the barrel turns not the drill, though that might have more to do with that's the natural way
to do things with a lathe chuck and lathe tailstock. also remember that after the bore is
drilled the barrel is straightened with respect to the bore and the outside then turned to
match it.

on the other hand, I had to drill 1/8" holes the length of the 8" long heads of one of my model
engines for oil feed to the overhead cam shafts. I actually drilled 4" from each end. That the holes
met up in the middle was astonishing to me. I started by using a test-dial-indicator on the head
to make sure it was vertical in the mill vise in both X and Y axies. 1. spot drill the hole,
2. split-point stub-length cobalt drill for 1/2". spilt-point regular-length cobalt drill for 1-1/2",
3. PTD extra-long cobalt drill (2-1/2" flutes, 5" OAL) the rest of the way. I bought a "parabolic flute"
drill but it was too flimsy so didn't use it, the flutes are ground too deep and there's not enough drill
material left and its very flexible, instead I did a lot of "peck and retract to brush off the chips" and
an awful lot of tapping fluid to avoid chips getting gummed up and the drill breaking. It was slow
and tedious but I only had four holes (two heads, one from each end of each head). On one head
the holes were within 1/64" where they met and 1/32" in the other, plenty good for oil passages!

FWIW, YMMV.

 

[willray]

When the drill is on the centre of the work piece rotation, the forces on each side of the drill are equal.
If the drill drifts off centre - let's say it drifts to the "left" - now the radius from centre of rotation to the left hand edge of the drill is larger than the radius from the centre to the right hand edge.

The larger radius on the "left" exerts greater forces on the drill bit than the smaller radius on the right - this results in a net force from left to right pushing the drill back towards the centre of rotation where all forces are in equilibrium

This would appear to only be true if the drill is "rigid", such that the tip doesn't simply follow the offcenter-center around in a circle? (If the tip follows the offcenter-center around in a circle, both cutting edges of the drill take off identical chips, so the load on each must be the same)


But the "rotate the workpiece" strategy seems to also work for things like gun-barrel drilling, where the drill is long and flexible. so clearly I'm still missing something!

 

[Bill Lawson]

A deep hole is more than 6 diameters. A 1/2” drill drilling 3” deep. Many of us have exceeded this rule without bad results. A gundrill is designed for deep straight holes. It has1 cutting edge, 1 flute along its length, and 2 wear pads. Extreme cutting fluid pressure fed thru the drill break the chips and flushes them out of the hole. Depending on the application, the part or the tool spin.

 

[Piper]

Ian, is correct.

Although both the drill bit and the work piece are rotating the same relative to each other in both operations, if the drill bit is being rotated and the drill bit wanders the center of rotation wanders with the bit so and there are no corrective forces.

If the work piece is rotating, the center of rotation remains at the same location on the work piece and there are corrective forces that will keep the drill bit centered. However with a normal twist drill bit, the slight wandering will result in a slightly over sized hole.

-Piper

 

[willray]

Ian, is correct.

Although both the drill bit and the work piece are rotating the same relative to each other in both operations, if the drill bit is being rotated and the drill bit wanders the center of rotation wanders with the bit so and there are no corrective forces.

Unless the chuck wanders with the drill bit, I cannot see why "the center of rotation wanders with the bit" is true.

I think I understand Ian's logic as it applies to the initial "catching the center" process of landing a bit at the center of a rotation of a spinning workpiece. After that, I still fail to follow the physics.

Assuming a perfect world where my spindle aligned with my tailstock, were I to replace the tailstock quill on my lathe with a powered drill-motor, the axes of rotation of a bit held in that drill-motor, and of a workpiece held in the chuck, would be coincident.

Somehow there is an argument that locking the headstock spindle, and drilling with the drill motor, would result in holes that potentially wander, and locking the tailstock drill-motor, and drilling by spinning the work, produces different results.

I can't argue that they don't, but, if the arse-end of the drill bit is held on the same axis as the axis of rotation of the workpiece, the corrective forces _from_off_axis_flexure_ of the drill should remain the same. If there are other corrective forces, I'm still trying to understand their origin :-(

 

[Iampappabear]

A drill with any sort of point will tend to wander if it experiences variations in resistance (hard bits or voids in castings) the sharper the angle of the point, the more the tendency to wander. Flat bottom drills tend to wander less in such situation, I believe that early gun drills were in fact very long D bits with this in mind.

 

[IanN]

This would appear to only be true if the drill is "rigid", such that the tip doesn't simply follow the offcenter-center around in a circle?

Hi Willray,

I agree with this statement, but you are describing the initial drilling process on a rotating work piece with the hole centre mark placed off of the rotational centre of the work

The initial condition when drilling from the tailstock in the lathe is to mark the centre of the hole using a rigid centre drill (Slocombe drill) which ensures the married centre is truly on the centre of rotation and so the condition you describe will not ocur

So a drifting drill is held "on centre" by the material at the mouth of the hole, but the drill is following a banana shaped path so the drill tip is off centre at the bottom of the hole

Does this help?

All the best,
Ian

 

[IanN]

Unless the chuck wanders with the drill bit, I cannot see why "the center of rotation wanders with the bit" is true.

Hi Willray,

I think you are happy with the idea of a rotating work piece and fixed drill: If the drill starts to wander, the body of the drill is held on the centre of the work rotation by the material at the mouth of the hole and the drill is starting to follow a banana shaped path through the material - this means the tip of the drill is now moving in a small circle. The tip of the drill is no longer on the centre of rotation of the moving parts (the work piece)

Now consider the fixed work piece and the rotating drill: Initially, everything is running true and the intended drill path aligns with the drill axis of rotation which is the axial centre line running the length of the drill. Now when the drill starts to drift it follows a banana shaped path deviating from the desired hole centre through the work. The rotation is still around the drill axis, but the axis is now curved instead of being a straight line - imagine a flexible drive attachment on an electric drill. So the centre of rotation (at the cutting edges of the drill) will always be around the drill axis of the drill tip - "the axis of rotation wanders with the [tip of the drill] bit"

Does this help?

All the best,
Ian

 

[Steamchick]

I have been experiencing similar problems trying to drill sub-0.35 mm holes for gas jets. 1 in 3 times I get a "perfect centre" to start and the drill will drill a number of holes without breakage. Then I'll get some slight off-centre from the set-up - probably fine swarf on the bed under the tail-stock? - or something - as the height of the tailstock seems to be a thou or 2 high. Then the drill has a definite bend (Watching using a 4x jeweller's loop) and will break after drilling more than 2 mm deep. I'm simply working beyond my skill - not blaming the machine. Any advice on drilling 0.2 ~ 0.3 mm holes will be appreciated. Contrary to regular operation, I am using a 0.5 mm drill as a centre drill, as my 1/8" centre drill has a larger centre-point than the 0.5 mm drill, and being stiffer can simply mark a circle instead of providing a centre v-hole. I think the "flexible" 0.5mm drill recognises the rotational forces so picks the rotational centre without problems.
I am also looking for a reliable and simple way to gauge the jets, as if the drill has a problem it makes a slightly larger hole than intended, and at these sizes (less than 15 thou diameter) a thou makes a big difference to area and gas flow.
Incidentally, my Chinese modern lathe is more precise than an old Unimat I was using for this job. Maybe we all grow bent and less able with age?
Can anyone tell me what speed to use for 0.2mm dia?

 

[terryd]

Hi,
For centering work pieces for small drill I now never use Slocomb pattern cente drills. I use a spotting drill which give a much finer starting point, so you could try the smallest one you can find - they're not too expensive and the Chinese ones are adequate for brass and mild steel.

TerryD

 

[karlw144]

My mentor taught me that if you really want a centered hole use the most appropriate size ball end mill that you have, it will not walk off center and will leave perfect sides to guide the drill.

 

[Steamchick]

Thanks TerryD.
I have ordered a drill. I'll have a go when I get it.
Regards,
K