Short drill or Spot / Centre drill first

Hi,

I am making a crossing out jig for clock wheels and need to drill a piece of aluminium I have which is 25mm thick.

I will need to drill approx 20 through holes approx 3mm dia. I intend using my mill and rotary table to find the positions of the holes but the question arises do I need to centre drill these holes first or will a short drill be rigid enough used on the mill.

My thinking is this is a job where absolute precision of the hole diameter is not crucial but I would like each hole to be within a reasonable few thou of each other. Once the holes are drilled pegs later made on the lathe have to be made to fit in the holes and each peg should be a fairly decent fit in any hole. A fairly good fit is required what is important is the holes as I said must be pretty uniform. Is a reamer required here or will a short drill likely do it in one?

Chris

If you want the holes to be within a thou of each other, I would use a spotting drill first (or a small centre drill if you do not have one). A short 3mm drill could drill flex slightly but spotting and centre drills have a bigger diameter and more rigid.

Paul.

I would make your pegs first and experiment with drills in your material to get your required fit

make your pegs out of 3mm silver steel perhaps

rather than as you said making your pegs fit the holes...make the holes fit the pegs

I would most certainly centre drill or use a spotting drill to start the holes, if you use 3mm silver steel as suggested by Paul for the pins I would ream the holes, you then stand a chance of getting them all pretty much the same size.

Thanks Guys for posts todate.

At the moment I am noting all posts before taking a view.

Doing some further research I have come across spit point or 4 facet drills which virtually eliminate wander. Still struggling to get ectly the difference in geometry but think it may come down to instead of having a complete v angle cutting the point is altered in some way. Can anyone give me or point me to a better explanation.

Chris

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Thanks Barry,

You have answered one of my reasons for asking and that was I too think drilling all the holes with a spot drill then replacing after doing all holes is likely to invite inaccuracies, so doing each one in turn is the answer if i go that route. Still researching a fully ground split point though.

Chris

Update; found the explanation hope it helps a future reader.

A Split Point drill has two additional edges ground into the chisel edge (flat centre of drill bit)

which make the entire point of the drill one long cutting edge.

Chrus

Hi Chris,

For jobs like this, I use stub length split point drills from cncpoorboy. They are very good quality. Being stub length, they don't flex much and eliminate the need for spotting.

25mm is quite deep for a 3mm drill. Do you need to go all the way through or will a blind hole do?

Steve

If you want to ensure all your pegs fit vertically, spot or centre drill as it just makes common sense to take precautions to avoid the drill ‘wandering’, even before entry.

If you are only needing a few thou accuracy then if blind I would go straight at then with a stub length split point drill, something like Dormer A022 series. If they are though holes then you will need a jobber length drill which is a bit more flexible so I would spot first, if you don't have a DRO then spot & drill at each position before moving to the next.

Lubricate drill with paraffin or WD 40 and withdraw often to clear swarf particularly if through holes.

Put some drag onto your rotary table as the large PCD of your peg holes combined with a small table will accentuate any backlash and have far more bearing on accuracy of position than what you are actually drilling with.

Edited By JasonB on 07/07/2020 13:11:37

Thanks Guys really useful.

Steve's point, do I have to go all the way through. Answer is no but Malcolm Wild advised to enable release of stuck pins. I could just as easily not cater for this as the additional work may not be necessary....will think on this??

Equally taking all posts on board to drill first using the single point to 8mm on one radius then using the rotary table reverse one step at a time manually (so I can check its position) with say a 2mm jobber to give a small through hole for the release mechanism, the accuracy of this hole should not matter??

Jason what is a DRO? I know several people use WD40 for aluminium but the clock forum has scared me stiff of it so paraffin it is which I have anyway. Good point Jason on drag on rotary I think this parameter comes down to speed on the Sherline but have only used it a little yet.

Chris

Chris

I recently bought several different tapping size split point drills and found they cut oversize in all materials, especially aluminium, so best check your drills for drilled hole size in a piece of scrap before letting an unknown drill spoil your work.

I will add that the store agreed to a full refund including postage when I return the drills.

Emgee

Chris, Is your rotary table the Sherline CNC model?

Don't reverse with your rotary table, just complete one circle and then go round again so you are always feeding in the same direction which will help to reduce backlash.

Yes Keep the WD40 away from the actual clocks as it can go sticky over time but no problem using it as a cutting fluid on aluminium

DRO = Digital Read Out. This gives exact position of the mill's table so you can be just about certain to return to the same spot which would allow you to do several hole at one angular setting and then get the ones on the next angular setting spaced out on the same radii. As you don't there will be some play (backlash) in the handwheels and leadscrews then do all at one radius winding the rotary table in a positive direction spotting and then drilling each one then move out to the next radius and go round again.

Edited By JasonB on 07/07/2020 16:25:28

Yes Steve but I have a manual one as well.

Chris

Hi Chris, the reason I ask is if you have the CNC table there is no problem with repeatability when indexing so you can go round and spot, change tools, go round and drill and then again to ream for each circle of holes.

Steve

Thanks Steve, I am still familarising myself with the instructions relating to tiny positional error and backlash issues. More reading is necessary. My concern is if you end up with a tiny error to get the division position then unless you can return exactly to the start position each time round increases the error. This is tiny but as I said more study is called for. I am sure your right but I have to have a better grasp on this yet.

Chris

Chris

As others have said only rotate the table in 1 direction, you don't then have to worry about backlash. Same with the cnc table, only rotate in 1 direction.

If when using your manual rotary table you go past where you you should have stopped, rotate backwards way past the desired stop point and then rotate slowly to the desired position, that way backlash is always taken up in the same direction.

If your table is very free to move and you have any backlash always apply at least 1 table lock before drilling. (edit)

Emgee

Edited By Emgee on 08/07/2020 18:22:12

Appreciate your post Emgee and will swot up on the instructions soon.

Chris