What's the secret to drilling accurate holes?

[gordon s]

I'm fairly adept at most mechanical processes but this one has me beat, so would be grateful for some advice. I'm trying to make a sliding jig from two pieces of 25mm square aluminium. They will slide together on two 10mm stainless steel rods. The two faces have to come together parallel and square, so the holes to take the rods have to be parallel/square and perpendicular. The final hole size will be made with a 10mm reamer.

 

I've tried opening up the holes in progressive steps and tried using pillar drills and the milling attachment on my Proxxon lathe, but trying to maintain accuracy and perpendicularity seems nigh on impossible. I think the machines are up to it and should drill square and vertical without problem, so what am I doing wrong?

[Guest baldrick25]

Drill 'stiffness' itself is a major cause of drills 'drifting' off the intended course of drilling. Use the shortest length of drill possible , and keep the length protruding from the chuck to an absolute minimum.

Use 'stiff' drills, like a centre point drill as much as possible , and 'quality' drills rather than the cheapo versions , which are often a couple of grades more flexible (The flexible ones don't break, they bend. Ever seen one of these cheap drills bent at 90 degrees?).

Check the pillar drill doesn't have some side-play in the up-down mechanism, and hold the work with clamps rather than fingers.

And the final point is drill slowly , and with some lubrication.

[gordon s]

Thanks for that. Should I stack both 25mm square sections and drill right through? The problem then is having 50mm protrusion. I just feel drilling them independently just adds to the problems. I have tried drilling one hole in each and then using the 10mm rod to hold them in alignment whilst I drill the second hole, but wander in one or other direction, still prevents a smooth sliding action.

[smokebox]

I don't have any Proxxon equipment, I think it's too small for me. I have a Myford speed 10 lathe. I think you may be pushing the capabilities of the Proxxon equipment.

 

Have you checked with an engineers square or dial gauge that the drill/mill is actually set up square. Don't rely on any markings that the manufacturer has inscribed on the the machine.

 

Aluminium can be difficult to work with as it can weld itself to the cutting edge of any tools. A lubricant such as paraffin is recommended. When you are drilling, the amount of force required is possibly bending the machine slightly. Try clamping both pieces together and drilling both in one operation That would normally ensure all holes line up properly but not necessarily "square" in all directions.

[PhilH]

How are you holding the pieces to be drilled?

[gordon s]

OK. Several questions. I've used several methods, so I'll try to deal with them in turn. I have two pillars drills. One is a good quality, but small Proxxon and the other a full size floor standing unit.

 

The milling attachment on the Proxxon is fitted into a precision fixing bolted to the lathe bed. It has a 36mm hole into which the milling attachment slides. It is rigid once bolted up. The drills are fitted into colletts and I've been drilling at a fast speed, but feeding slowly into the aluminium block. I haven't used a lubricant so may give that a try on the next attempt.....

 

Here's some pics from Proxxon..... http://www.proxxon.com/eng/html/24004.html

 

I used the Proxxon machine vice which is bolted to the X-Y table that holds the toolpost. I tightened up all the slides, before drilling.

 

When using the large pillar drill, I used the same machine vice, but hand held it on the pillar drill table.

 

I haven't checked the machines but feel the drill is wandering in the material, rather than a machine inaccuracy.

 

Is drilling a succession of larger holes the way to go? If drilling a 10mm hole, what size would you start with as the smaller you go, the more flexible the drill bit will be.

 

Drill one block at a time or both together?

[Guest baldrick25]

Is the drill iteself resharpened and a bit off-centre.? Perhaps try a new ( perhaps just a different one) , or give it a go at resharpening again.

[Neil]

How about cutting the aluminium overlength, drill a pair of slightly oversize holes at the outer edges and bolt the bars together at each end, the slight slop in the holes giving slop for adjustment. You can then drill through knowing that the alignment between pieces will be taken care of and that you can concentrate on getting the holes perpendicular.Finally trim to length discarding the end sections with the clamping holes.

[mikeg]

When drilling holes of this size in alloy it is best to put say a 5mm through first using a mediun speed and plenty of lubricant, I agree parafin is best but soapy water will also help. Make sure that the final drill is sharp and has both faces the same length as an off centre drill will run out of line or drill an over size hole. If you have one I would drill the final hole 9.75mm and then ream the final .25mm to get the holes in line. Using this size bar it may pay to bolt them to the table of the pillar drill rarther than using a vice which will move as you drill unless bolted down. If the distance between the holes is important start the holes with a centre drill as this will run true to start the hole and this hole makes it easier to line up the following drill.

 

Just a thought a 3/8th drill will give a hole that can be reamed out to 10mm so the final drill could be a 3/8th one!

 

hope this helps

 

mike g

[gordon s]

Thanks for all your suggestions guys. Went back to basics and PhilH suggested drilling the holes on the lathe using the four jaw chuck. First hole in the two pieces is spot on, but can't do the second as the radial dimension is too great and the work piece hits the lathe bed, so back to the milling machine.

 

Thanks Smokebox and Baldrick for suggestions to check the mill. Just done that with a square and was surprised to find the manufacturer markings were fractionally out. Not by much, but sufficient to show once the drill length increased. So off to bolt up the pieces and hoping all will be OK.

[smokebox]

Glad to here you've made some progress Perhaps you could clamp the pieces to the cross slide, with packing to adjust the height, and drill and ream from the lathe chuck?

 

Another thought that has just occurred to me is that the aluminium may not slide very well on the stainless bars. You may want to drill the aluminium out to a larger size and fit some bushes of brass or bronze that should (I think) reduce the friction

[Peter Bedding]

If I may add a half-penn'orth.

 

Start with scribed crosshairs. Then centre punch on target. If possible, align and hold your centre punch vertically in a vee-block. Whatever follows will not be better than these two steps.

 

Then, with the machine tool of choice, start with a small diameter pilot drill, correctly ground, and work the diameter upwards. That should produce a result of 0.005" accuracy. (which will be the best outside a toolroom.)

 

HTH

 

PB

 

 

Self Edit. Tolerance decimal point corrected. 5 thou accuracy, not 0.5 thou.