We use dowel construction on a nested router. We bore 8mm holes with carbide tipped bits. In our brief experience when the bits get dull they tend to wander and the hole becomes too large and inaccurate. Our solution is to swap out the bits based on time. We selected 1 month as that interval. We also found sharpening the bits was as expensive as buying new.
This can get expensive. Does anyone have a better approach?
I would first ask how many holes you are drilling in that one month period. If it is tens of thousands you are probably getting good life out of the bit if the change is only once a month.
Also what material you are drilling. Particle board will be much harder on bits that MDF. Plywood might be somewhere in the middle.
Then what feed and RPM rates you are using. You could be feeding too slow for the spindle RPM rate. Carbide doesn't hold up well to heat.
We actually do most of our hardwood cutting with high speed steel bits. This is because the way we need to hold down our solid wood parts would lead to failure at a high feed rate. As such we have to go at a slow feed rate with a higher than recommended RPM for the spindle. This makes for over heated bits. Since HSS stands up to overheating better than carbide we get just as much life from HSS bits as carbide bits at about 1/3 the cost.
Finally if all the above are all being properly handled then you could consider going to brad point instead of standard point bits. While more expensive, and no longer lasting than a standard bit, the points should keep the bit from wandering when it begins to dull.
You will get better life from solid carbide bits. That said I suspect you are using too slow of a Z-axis move or your machine is hesitating at the bottom of the cut. We also nest for dowel construction and get much longer life out of standard tipped dowel bits. Our machine runs 8 to 10 hours a day.
Using some fancy math we use 10,000 dowels a month. So we drill 10,000 vertical holes a month. We have 7 - 8mm drills in the drill head so each drill is boring around 1,500 holes a month. That is in theory, the drill head does not use the bits evenly.
We are using brad point bits @ 4500 rpm penetration of 2.5 m/min and exit of 2 M/min with 0 dwell.
We drill in one motion no pecking.
Our monthly change is fine, I believe we are well within the life of the bits. To find out the tipping point I would have to stretch it out until the holes get sloppy which is a problem when it happens.
Solid carbide is pricey we run those in our bore and Insert machine.
The material we are boring has a 1/8" MDF outer layer and a fir ply core. I think it is relatively hard on bits.
When I read your first post, my thought was tolerance in the drilling head. Are you able to tell if there is wear in the gearbox? I had a machine years ago that had drilling block issues. It was a bad design, and we changed out 3 or 4 of them. MP27 might ring a bell for those unlucky Busellato owners that had this particular multispindle unit. First thing I would do is check for any runout on each individual spindle in the extended position. Obviously the tools won't be under load, but it is a good place to start. Also, some of the drill spindles could have more wear than others. Good luck.
I checked to see about what we were getting for life on 8mm dowel bit. Our router has a small drill block so we only use one 8mm bit. Dowel useage is between 3 & 4000/ month. We process quite a lot of HPL laminated board and went to a kind of slow Z-move of only 50"/min. On melamine it is run @ 100"/min....4500 rpm, no dwell at bottom. Since the holes don't show the bit is run until it starts to chip the entry hole quite a bit. 3 months +- So we are getting around 10,000 holes per bit change. No exact records have been kept but I can look up how many dowels are bought and how many bits are bought and the results are similar. The bore and insert machine gets even longer life, but it doesn't have to drill through much melamine & no HPL. It also drills a lot faster, less than a second per hole.
4000 rpm for us. You really need to verify if all bits are being calibrated @ 70mm or 57mm or whatever all at the same time, then inserted into the head and verifying that all are drilling the same depth. I have 13 spindle head and a 20 spindle on our router. We change about every quarter and run up to 3 shifts when swamped including weekends.
We do very carefully calibrate the length of this bits. I picked one month after we ran into problems. I think I will move it to two and make sure the assemblers know we did that. If the holes start to become "loose" we will have to dial it back.
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