BEST WAY TO MARK HOLE CENTRES

[beefy]

I've just been doing a little job that required 6 holes at 9mm diameter each. All I normally do for things like that is get the plasma torch to give a "blip" to create a "centre punch" mark.

Problem is the blip is just too long, even though I have the arc on time set at 0.1 sec. I have read before it is something in Mach that doesn't allow a dwell time less than a certain amount. The resulting mark from the torch is not exactly accurate and can even elongate out to the side.

Does anyone use any other means for marking hole centres, something like an engraver or an automatic centre punch ??

Keith.

[I Lean]

I just cut a hole like normal, but smaller than the finished size. For this one, I'd do something like a 1/4" hole, then drill to the finished size later. The cut hole seems to be more accurate than the pierce mark, for the same reasons you've stated already.

I don't have an engraver, but it seems like that would be an awesome way to go.

[beefy]

I'm trying to get away from using the plasma to do a small hole. The accuracy just isn't there.

I'm going to play with a couple of ideas to improve the crater made by the torch. One is to use what's called a "one-shot" electronic circuit. This gives out a signal for a fixed time even if the input signal is longer. I'll have a change over switch to put this in the circuit to the torch on relay. Another idea which I was going to test today before my cnc computer died on me is to not have the ground lead connected so I get a lost arc condition. This idea came because I forgot to connect the ground clamp earlier and some hole centres got a very short "blip" of the torch and made a perfect tiny crater, just like a centre punch mark. I'm wondering if that was because of the ground clamp not making contact (it was just lying on the slats). Can't hurt to try

Keith.

[trucutcnc]

The pilot arc will still pierce, but with much less power. That could work but would require disconnecting the ground prior to marking holes...which could be automated with relays I suppose. Another option would be to just turn your plasma down all the way.

When we need perfect holes, we just pierce, which is less that a second burst, then punch the hole on the iron worker using the pierce as a pilot. drilling works fine as long as you're finished holes are around 3/8" or better. Smaller than that and the hardened edge from the plasma pierce with take out the bit rather quickly.

[beefy]

Thanks very much Trucut,

If I ever have a problem with normal HSS drills, I've got 3 options that can deal with the nitrided mark. One is a centre drill which has some cobalt added, or just standard style drills with cobalt, and then there's these multipurpose "masonary" drills with carbide tips (I never use them on masonary). I've used them to drill through case hardening on my excavator pins. My 2nd smallest centre drill punched through the crater marks no problem but I normally get through with just basic drills. I think because all the plasma gave was a blip on the surface I'm not dealing with a lot of hard nitriding, as will be the case when piercing all the way through. I've never even tried piercing all the way through for drilling holes because I was never happy with the accuracy of the pierce hole.

I'm aiming for just the tiniest mark on the surface for accuracy. This way I get a nice round crater about 1mm in diameter, perfect for accurate drilling. Any more than this then the pierce mark randomly blows out to the side, completely taking away any hope of accurately starting a drill. I normally drill a pilot hole of about 1/8" (when the crater is just right).

I had my plasma down at 25 amps yesterday but when I get more than the shortest blip the crater is a blown out mess, so the real secret seems to be in getting nothing more than a short "crack" from the torch.

Keith.

[plain ol Bill]

Marking holes is another use I have found for an engraver. Cross hairs mark the spot great and then you can center punch for drilling.

[beefy]

Thanks Bill,

would an engraver be able to make a little indentation so a centre punch would be "self locating" as opposed to lining it up by eye on the crosshairs.

Keith.

[Larry83301]

Thanks Bill,

would an engraver be able to make a little indentation so a centre punch would be "self locating" as opposed to lining it up by eye on the crosshairs.

Keith.

I would think an engraver could make just a center punch mark, but on a large piece they would be very hard to find. Perhaps just dwell at the center and mark them all with a crosshair so you can find them.

Larry

Something like this could replace plasma and mark your holes. http://www.harborfreight.com/micro-engraver-98227.html

[beefy]

Thanks Larry,

that would give a laugh if I made a Youtube video of my "perfect" hole marking system, then couldn't find the marks afterwards "I assure you viewers, the marks are all there ................"

I'll have to check if Harbour Freight sends to Australia. Like many others here, I get quite a few things from the US because Australian prices can be simply ridiculous at times.

Keith.

[Larry83301]

Yes, that would be an OOPS!!

Larry

[planetxfred]

I turn the power down to minimum (20 Amps on my HT 1000) and use a new finecut nozzle. I have a secondary pushbutton switch that I use, I am able to make a much shorter duration "blip" with it,
Fred

[beefy]

Hi Fred,

I could do that but I'd have to get the torch to stop / pause at the hole centre point, then I'd have to touch the button during the pause period, for each hole. It's a good easy solution for a few marks but I'd like to automate the process.

Cheers,

Keith.

[dgeorgester]

For Plasmacam users;
Let's assume a flange with a bolt pattern. On my cad drawing, I would mark the bolt holes either using a circles, or center marks or even just a line. These marks will not be converted to cut paths. Before cutting the actual flange, I would position the torch using the MOVE (under Machine menu or V on keyboard) with to CENTER SNAP for circle, to INTERSECTION SNAP for center mark or ENDPONT SNAP for line.

I'd set the amps all the way down, adjust the height to about 1/4" and just tap TEST on the control pad.

I always mark the holes before cutting the actual part, because there's sometimes a bit of movement because of thermal expansion, especially near the end of a cut on a circular part.

If you're using an engraver, the trick is to have good quality bracket and not having to re-home the machine once the pattern has been marked. Otherwise, your bolt pattern might not be very concentric.

My 2 cents,
Georges

[bhamer62]

just a thought... cut a template piece first and make the holes in it 1/2 dia. then use a 1/2 dia transfer punch to mark the hole centers onto your parts.

[trucutcnc]

Hi Fred,

I could do that but I'd have to get the torch to stop / pause at the hole centre point, then I'd have to touch the button during the pause period, for each hole. It's a good easy solution for a few marks but I'd like to automate the process.

Cheers,

Keith.

If you use SheetCAM, just use a drill operation with a plasma tool. The Z will touch off, pierce with a short burst and repeat. The burst is so short, at 40A sometimes it doesn't even pierce all the way through 12ga. The code will look like this.

---Touch Off---
M03
M05
---Rapid Up---
---Move to next pierce---
---Repeat---

If you're getting any code between the M03 and M04, you need to edit your post so a drill operation only posts an M03 followed by an M05.

[beefy]

Thanks Trucut,

I'll give that a try today.

I've been using the drill operation to generate the hole marking code. The code I was using (starting at a pierce height of 3.8mm) was:

M03
G01 Z3.79 F3500
M05

So after firing the torch just had to move 1/100 mm down at 3500 mm/min then switch the torch off. This is the best results I've had so far but still not satisfactory (for me at least).

I had my amps down at 25 (Finecut nozzle) yet the crater was still a bit big. Hopefully it will make a difference without the G01 code in the middle.

One interesting (or should I say irritating) problem is that on the first hole mark the torch stays on longer. I did two runs of 10 hole marks and got the same results. The torch stayed on long enough on the first hole to pierce through 3mm steel. Today I'll have to find out if that is Mach giving a longer torch on signal on the first hole or if it's the plasma cutter itself staying on longer even after the torch on signal has gone.

I've heard quite a few times that Mach introduces a pause when a macro is operated and I've actually seen it in action when using M101 to turn THC off during cutting at corners. I'm wondering if the M03/M05 macros are doing the same thing. I'm after the shortest possible "crack" to give me a nice "indentation" in the top of the metal, just like a centre punch mark. I find any more than this and I'm lucky if the partial pierce mark is round. Very often the hole errs to one side as the molten metal is being blown out.

I'll let you know how things go.

Cheers,

Keith.

[trucutcnc]

The Z move is adding a lot of dwell. We never try to run a macro while the machine is in motion. Most of the time, motion will pause while the macro executes. If it's a fast executing macro, it works out to be a slight jitter in the cut. Your THC should have an anti-dive feature and that should be handling corner, slow speed and end of cut dive issues. There should be no need to actually turn off the THC during a cut.

Regarding the M03...You can't do away with the macro if you want to automate the process. You could modify it so instead of calling DoSpinCW(), which may carry some baggage, it just calls ActivateSignal(OUTPUT1), which may run faster. You would then change the M05 to call DeactivateSignal(OUTPUT1) rather than DoSpinStop(). One issue with doing it that way is that Mach's THC functions won't work unless you call DoSpinCW(), so if you have CandCNC, SoundLogic, etc. THC, that may not work for you.

I think the best way to go would be to not use M03 at all but make a custom macro that looks like this....

ActivateSignal(OUTPUT1)
DeactivateSignal(OUTPUT1)

This will execute so fast that you may actually need to put a Sleep() statement between the two. To use the macro, edit your SC post onPenDown() function and check if a drill operation is active. If it is, use the custom macro instead of M03. Also edit onPenUp() and if a drill operation is active, do not call M05.

This should do away with all the overhead of DoSpinCW() and not mess with your THC.

Here is a video demonstrating a similar process.

http://youtu.be/ZLHkV8C0DsQ

[beefy]

Hi Trucut,

thanks a lot for that info, you got me quite excited with that latest suggestion.

There was a post on the Candcnc Yahoo forum just 2 or 3 days ago about using the Activate/Deactivate code in the M03 and M05 macros, and yesterday I played with it but it made no difference. However your suggestion of putting the Activate and Deactivate in one macro may give completely different results. We're taking our lads out bowling this morning, now I'll be thinking about wanting to try out your idea

I've been designing a little electronic circuit to give a very short output signal time regardless of input signal time, but if your idea works then that saves me a lot of hassle. It's also easy to modify Sheetcams post processor and replace the M03 with the custom macro number. The rest of the code can just stay the same, it won't affect anything although I could get rid of the M05 line too.

I don't know if you think this is a bad idea but I've got very good results by leaving the ground clamp completely off. The difference is night and day. Cute little "centre punch mark" with ground clamp disconnected, or an oversized crater or hole with the ground clamp on. Now if your idea works then that will reduce the time interval the torch is in pilot arc mode (ie with no ground connected).

Bit of a different matter but my Hypertherm was missing a torch fire every second hole. Jim Colt told me that the post cut cooling air has to stop flowing before the torch can fire IMMEDIATELY upon getting the torch on signal. He said if the air is still flowing the torch can still fire but it has to stop the airflow, turn on the torch, then re-establish the airflow, all introducing a delay which results in a misfire if the torch on signal is very short. I had to put some delay code after each M05 to get around this. I wish I could disable the post cut airflow for this type of operation but it's "hardwired" in the Hypertherm.

Thanks again,

Keith.

[trucutcnc]

Activate followed by deactivate does work. You'll need a sleep() between the two to slow it down or it will execute so fast that the output will not trigger. We use 200ms. If the Hypertherm won't fire immediately with air flowing, this may work. We run Thermal Dynamics in the shop and they fire immediately with air flowing. The video I posted is an A120. We'll be setting up a couple of PM85's next week. I'll give it a try.

[beefy]

I haven't tried any other plasma cutters but I'm quite biased towards Hypertherm.

Guess I have now found my very first disadvantage, with this post cut airflow situation. This basically means I need to wait about 10 seconds between hole marking. No big deal with just a few holes but the extra seconds will add up when there's more than a few.

Hope that little bit of info about the Hypertherms not being able to fire instantaneously (with air flowing) comes in handy for you.

Well I've modified a Sheetcam post processor, made a macro and am getting closer to testing your method.

You'll be my hero Trucut is this finalises my adventures for the perfect plasma hole marking method

Keith.

[trucutcnc]

Guess I have now found my very first disadvantage, with this post cut airflow situation. This basically means I need to wait about 10 seconds between hole marking. No big deal with just a few holes but the extra seconds will add up when there's more than a few.

I think it has something to do with the air valve. On TD handheld and Hypertherm machines the air valve is inside the plasma. When it opens, it takes a second or so for pressure to build in the torch lead and get to the nozzle. TD handheld machines also have a delay from the time the torch is triggered to when the arc lights.

The TD mechanized torch puts the air valve out on the torch which means the torch lead stays pressurized and a delay is not needed. Cutting pressure to the nozzle is almost instantaneous when the valve opens.

Each manufacturer has their advantages and disadvantages.

[beefy]

From what literature I've came across, and the way Jim Colt described it, this is how I think it works.

My understanding of my particular Hypertherm torch is that the electrode is pulled away from the nozzle when the air is switched on. In doing so an arc is drawn between electrode and nozzle, igniting the plasma arc. Therefore with air flowing the arc cannot ignite, so the airflow has to stop, the spring loaded electrode pushes back down and contacts the nozzle, then the ignition process can start.

I would never have dreamed having some understanding of how the nozzle works would affect what gcode I needed

Keith.

[trucutcnc]

...And the reason there is a delay is because it takes a second or so for the torch lead to release air then re-pressurize. With the A120 air valve being on the torch, the torch lead is already pressurized so there is no delay. The A series machines light instantly where the manual CutMasters have a delay....like the PowerMax.

[beefy]

Well Trucut, it's official.

You are now my hero

Just tested your method and PERFECT, thankyou again. The lowest I could get the sleep command was 100. At 80 or less I didn't even get the Torch On led on the screen.

It's also been a good eduction for me, the idea of putting all the code in one macro, as opposed to a macro for turn on, and a macro for turn off. I seem to be better at thinking of complicated solutions than simple ones.

This 10 seconds post cut air flow is a damn pain when doing this hole center marking. That means a minimum of 10 secs per hole mark, but a small price to pay for the ultimate hole marking system.

Cheers,

Keith.

[robertw413]

How about using a die grinder or an electric drill with a center drill. Lower the z axis to make the mark or just go ahead and drill the hole.