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How to countersink on a Mill?

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AdrianR11/05/2021 15:58:34
534 forum posts
36 photos

I have a Sieg SX3 and recently bought a set of single flute countersinks from ArcEuroTrade.

Today I tried to use them for the first time and I am unsure if the problem I had is me, the mill or the countersink.

I was making a 14mm diameter 90-degree countersink on a 6mm hole in BMS. I selected a 16.5mm countersink, mounted it in an R8 collet and lowered the head till the cuter was just starting to shave the hole corner.

From here I calculated that I need to cut 4mm deep to achieve a 14mm countersink.

I ran the cutter at 150RPM and with plenty of cutting oil started to lower the head. It cut easily till about 2mm deep ( measured 10mm dia) It then started to get a little more difficult so I raised the speed to 300RPM. It then was cutting better to 3mm deep (measured 12mm dia) After this it did not want to go deeper, I had to give more feed to get it to start cutting again. Like I was having to drive the head down, not just lower it.

Then cut quickly to 4mm deep. But the hole diameter was only 13.5mm dia. I had to give it another 0.5mm feed and it again suddenly started cutting with a thick swarf curl. This time it took the countersink to 14.5mm dia and the finish was very rough.

I have looked at the cutter, it appears as good as new, with no sign of wear. So I am wondering is it me? Have I approached it the wrong way?

I was trying to feed it down not too slowly, probably about 0.5mm/second. But I did lift it to check the cut and re-oil.

In the end, I was defiantly getting the impression the mill was not rigid enough, the head was visibly moving side to side, and I suspect was lifting relative to the feed screw. Could it be the Z dovetail is too loose? Just how do I check the play on the Z axis?

Is single flute the wrong sort of cutter?

Any ideas?

Tony Pratt 111/05/2021 16:10:17
1544 forum posts
8 photos

I actually like the single flute countersinks but as it is one flute the cutting action is a bit unbalanced so maybe on a small mill this can cause a orbiting motion. I don't know the Sieg SX3 personally but it will have a dovetail adjustment to reduce any side to side slop in the head.

Tony

AdrianR11/05/2021 16:22:28
534 forum posts
36 photos

Yes the SX3 has a tapered gib. I thought I had adjusted it right but I am unsure. The X/Y are easy as you can use a DTI to measure the 'wiggle'. With the Z, having the weight hanging makes it hard for me to tell if the gib is adjusted correctly.

Vic11/05/2021 16:40:15
2781 forum posts
1 photos

I’ve not had any problems countersinking on my mill. I don’t normally use single flute countersinks for metal though, only wood. All my metalwork countersinks are three flute apart from a couple of the snail type with a hole.

Bill Phinn11/05/2021 18:58:09
512 forum posts
81 photos

I have a similar sized mill to yours, always use the quill [it gives invaluable feedback up the hand] rather than the head for countersinking, and get excellent results on mild steel using three flute, single flute and zero flute csks.

It's a little difficult to tell where things are going wrong without seeing you actually doing the work. To help narrow down the cause of your problem you could try locking the head and using the quill. See whether this improves things.

AdrianR11/05/2021 19:45:20
534 forum posts
36 photos

Bill,

I have plenty more csks to do, I will give the quill a go.

not done it yet11/05/2021 19:57:50
5946 forum posts
20 photos

If you have a stop on the quill, once set you should not need to tarry once cutting.

Ian P11/05/2021 22:11:52
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2506 forum posts
102 photos

In my experience, single 'flute' countersinks (crossdrilled hole type?) perform best if they are allowed to 'float' and centre themselves, so are good if the machine is not rigid.

For countersunk recesses use a countersink ground to the just over screwhead diameter so there is a small parallel counterbore to hide the edge of the screwhead.

Ian P

bernard towers11/05/2021 22:20:43
188 forum posts
71 photos

I believe single hole demurrers/ counter sinks are elliptically sharpened

Ian P11/05/2021 22:31:31
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2506 forum posts
102 photos

Demurrers might be, but cutters are more 'snail' shapesmiley

not done it yet12/05/2021 08:00:09
5946 forum posts
20 photos

Ian P,

Demurrers

Is that a typo or are you making an objection (or more than one)?

JasonB12/05/2021 08:30:05
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Moderator
20637 forum posts
2296 photos
1 articles

As said a single flute will be slower cutting than say a 3 flute countersink but they tend to chatter less so you don't get a "corrugated" edge like you can with multi-flute CSK bits. I just run them slowly with steady pressure and a few drops of cutting fluid.

Also like Ian says if these holes are for M6 CSK screws then the 12.4mm size driven just below the surface gives a neater looking hole as the screws have a short cylindrical edge and not a sharp one.

This is on the similar sized X3 using quill feed and about 200rpm. with the two sizes of ARC bits, sorry about the measurement, not easy to do looking through the camera when zoomed in rather than at the callipers

Michael Gilligan12/05/2021 09:10:18
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18080 forum posts
845 photos
Posted by not done it yet on 12/05/2021 08:00:09:

Ian P,

Demurrers

Is that a typo or are you making an objection (or more than one)?

.

I think you will find that Ian has already done the joke

[ the typo was Bernard’s ]

MichaelG.

AdrianR12/05/2021 09:13:52
534 forum posts
36 photos

Jason,

Thanks for the video, I actually searched your youtube to see if you had an example of countersinking. Being able to see and hear the machine defiantly makes a huge difference. It was watching your videos that finally made me choose the SX3.

The 12.4 looks much better, and of course, is not so obvious when you make it too deep. I was aiming for 14mm as that is what is on Howard Halls plans and I am trying to push myself to work accurately.

I guess using the quill instead of the Z feed, has the advantage that pulling the handle down is also pulling the head down too.

Ian P12/05/2021 09:40:25
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2506 forum posts
102 photos
Posted by AdrianR on 12/05/2021 09:13:52:

Jason,

Thanks for the video, I actually searched your youtube to see if you had an example of countersinking. Being able to see and hear the machine defiantly makes a huge difference. It was watching your videos that finally made me choose the SX3.

The 12.4 looks much better, and of course, is not so obvious when you make it too deep. I was aiming for 14mm as that is what is on Howard Halls plans and I am trying to push myself to work accurately.

I guess using the quill instead of the Z feed, has the advantage that pulling the handle down is also pulling the head down too.

The head may move down (an almost immeasurable amount) until the point that the countersinking cutter touches the job and then the head will (try to) move upwards as a reaction against the downward movement of the quill.

Ian P

Andrew Johnston12/05/2021 09:48:02
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6055 forum posts
671 photos

I mostly use 3 flute countersinks. I would agree with using the quill and quite a high force is needed. The cutting edges are long and require a significant force to cut rather than rub. I run smaller countersinks at 5-600rpm as I'm too idle to change in to backgear on the mill. However, for this 1" countersink I dropped to 200rpm:

flypress_plate_2.jpg

The 1" countersink was in a job lot of tooling I bought from a fellow member of the local gliding club but seemed to cut ok.

When countersinking for screws I don't faff about with a parallel section. For a start it's not a defined diameter. What is defined are a minimum head diameter and a theoretical sharp head diameter. For M6 the numbers are 11.73mm and 13.44mm respectively. I simply countersink to 13.5mm, but always with a sanity check using the screw, as the tolerance on head depth for M6 is 0.2mm.

Andrew

AdrianR12/05/2021 12:24:20
534 forum posts
36 photos
Posted by Andrew Johnston on 12/05/2021 09:48:02:

The 1" countersink was in a job lot of tooling I bought from a fellow member of the local gliding club but seemed to cut ok.

Blimy they sure do have tough gliders where you fly if those are the countersinks for the skin rivets cheeky

Ron Laden12/05/2021 13:21:43
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2207 forum posts
439 photos

Just my own taste but I much prefer csk screws to finish looking like Jason shows on the second hole in his video, I think it looks a lot neater than having the band around the screw head. I have a set of 3 flute csk bits which work well enough but for M3 to M8 I mainly use a set of stub drills which I reground to 90 degree and are sized to the screw head diameter plus a tad. They work really well, always cut clean, no chatter and leave a good finish.

AdrianR15/05/2021 15:33:48
534 forum posts
36 photos

Well, I checked out the Z-axis and found I had more play than I thought on the dovetail. The mill now feels 'better'.

Using the quill was a success too, 12.4mm cutter 200 RPM and 3.5mm depth, perfect flush screw heads.

Thanks for the advice.

Adrian

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