rsser
15th December 2011, 01:59 PM
I was sceptical about the Colt Maxicut Forstner bits as posted in the original group buy thread. But having ordered a couple with the MT2 Rotastop adaptor and 150mm extension, Brett (FenceFurniture) told me to get off my backside and try them.
So to test one at a demanding task I used a dense and cranky grained lump of redgum mounted on the lathe and bored endgrain with a 35mm bit down to 220mm. 1000 RPM. Colt suggest a speed range from 750 to 3500 and normally I'd run that size bit at about 250 in hardwood. The speed range for the Rotastop and extension is 500 - 1400 RPM.
I started without the extension and got nice chips to maybe 50mm depth. Then added the extension and proceeded apace. The result soon turned to dust but with cleaning out repeatedly the cut continued fairly quickly to the final depth.
Upsides: even with the extension which is fairly spindly (10mm OD cp. 3/4" on my custom-made job) the bit tracked straight and continued boring without apparent drama. The Rotastop and extension couplings entail an eccentric shaft and so are self locking. My existing MT extension fixes the bit with a grub screw and when bits approaching 50mm are used, slippage happens.
Downsides:
1. This test produced a big wear bevel on the chipping edges. Sharpening was required.
2. To clear shavings or dust in this application you either retract the bit or blow them out or use a wooden chopstick 'scoop' with the piece spinning (yeah, sounds odd but it works and is my preferred method). The retraction option was tried before adding the extension and the jam lock of the coupling failed leaving the bit to rotate in the work piece. Perhaps with more vigorous winding back, opposite lock would kick in. Whatever; after that I used the chopstick and blowout methods.
3. Half the edge, at the trailing end, of the two teeth were blued. Hadn't checked this before starting but don't expect they were there. And looking with a 30 X mag. loupe on the unused bit, the chipping edges are on the ragged side and of course there's no bluing. I could've just been hamfisted given the ease of progress.
Final judgement waits on more use. Certainly this was a tough test.
(And when I talked with Brett about the performance he told me I'd ordered HC steel bits, not HSS as I'd assumed :doh:. Well, I dips me lid to Colt cos at that it was punching above its weight. This test would likely have had a better outcome with slower speed and feed rate).
Past experience is shallow and deep endgrain boring on the lathe with sawtooth HCS and HSS bits from wherever. And some testing of a C/tec TCT bit.
With silica-rich or dense timbers, I wouldn't go back to HCS bits. Easy enough to sharpen but need it often. Conventional HSS lasts better but can generate of lot of heat depending on other factors as well. TCT puts up a good fight but leaves a poor finish which is not a problem boring pepper grinders but is a drawback doing stuff like napkin rings.
One big variable is the lathe tailstock quality. Affects bit tracking and so heat generation. Minimal play is best obviously. With experience of four brands of lathe, the Vicmarc tops the list and was used here.
After sharpening the bit I tried a second test on English Elm, a timber within the expected scope of HCS. The bit kept cutting chips down to the end at 160 mm. Clearance of chips through the flutes was efficient. RPM of 500. The bit got very warm but not hot and developed wear bevels just visible to my reading-glass assisted eyes. The self-locking couplers seemed to have seated somewhat and held more firmly this time around; getting them apart took a bit of effort.
Sharpening
It was about half an hour's work with diamond paddles to get the bit back in condition after the first test, working the chipping edge tops and putting some effort into the flutes. Starting with a medium grit paddle and then fine and super-fine. The nice feature about the bit is that the top of the chipping edge is co-planar with the dropped section of the rim so good registration of the paddle is easy to get. The flute on the other hand obviously involves a lot of steel, and it's not all that easy to keep a paddle flat so I ended up settling for an adequate rather than a good result there.
I haven't touched up the 'wings' (the raised part of the rim). Leonard Lee's sharpening book shows a jig for doing this to maintain the bevel angle. I'll have a shot at doing the bevel with a bit in a Dremel, inking the bevel first.
The included bevel angle (chipper top to flute) is about 50 degrees.
Summary
The cutting and chip ejection are better than garden variety saw tooth bits. Sharpening is easier. The quality of the HC steel is impressive.
For deep boring I favour an MT mount over a Jacobs chuck as there's less play. The Rotastop MT2 and the extension work well.
Heat is the enemy of Forstner bits and they have to be kept sharp. Leonard Lee recommends keeping them free of resin and applying dry lubricant before use.
I've put in an order for several Colt MaxiCut plus HSS Forstners. A 35mm bit will be about double the price of the same size unit from Carba-tec and that's quite a premium so I'm only ordering the sizes that will get regular use.
The geometry of the HSS bits differs from the HCS jobs so sharpening the chipping edge may not be as easy. They also have a pilot drill instead of a brad centre and that will be a disadvantage where a relatively flat bore bottom is needed.
Many thanks to Brett.
So to test one at a demanding task I used a dense and cranky grained lump of redgum mounted on the lathe and bored endgrain with a 35mm bit down to 220mm. 1000 RPM. Colt suggest a speed range from 750 to 3500 and normally I'd run that size bit at about 250 in hardwood. The speed range for the Rotastop and extension is 500 - 1400 RPM.
I started without the extension and got nice chips to maybe 50mm depth. Then added the extension and proceeded apace. The result soon turned to dust but with cleaning out repeatedly the cut continued fairly quickly to the final depth.
Upsides: even with the extension which is fairly spindly (10mm OD cp. 3/4" on my custom-made job) the bit tracked straight and continued boring without apparent drama. The Rotastop and extension couplings entail an eccentric shaft and so are self locking. My existing MT extension fixes the bit with a grub screw and when bits approaching 50mm are used, slippage happens.
Downsides:
1. This test produced a big wear bevel on the chipping edges. Sharpening was required.
2. To clear shavings or dust in this application you either retract the bit or blow them out or use a wooden chopstick 'scoop' with the piece spinning (yeah, sounds odd but it works and is my preferred method). The retraction option was tried before adding the extension and the jam lock of the coupling failed leaving the bit to rotate in the work piece. Perhaps with more vigorous winding back, opposite lock would kick in. Whatever; after that I used the chopstick and blowout methods.
3. Half the edge, at the trailing end, of the two teeth were blued. Hadn't checked this before starting but don't expect they were there. And looking with a 30 X mag. loupe on the unused bit, the chipping edges are on the ragged side and of course there's no bluing. I could've just been hamfisted given the ease of progress.
Final judgement waits on more use. Certainly this was a tough test.
(And when I talked with Brett about the performance he told me I'd ordered HC steel bits, not HSS as I'd assumed :doh:. Well, I dips me lid to Colt cos at that it was punching above its weight. This test would likely have had a better outcome with slower speed and feed rate).
Past experience is shallow and deep endgrain boring on the lathe with sawtooth HCS and HSS bits from wherever. And some testing of a C/tec TCT bit.
With silica-rich or dense timbers, I wouldn't go back to HCS bits. Easy enough to sharpen but need it often. Conventional HSS lasts better but can generate of lot of heat depending on other factors as well. TCT puts up a good fight but leaves a poor finish which is not a problem boring pepper grinders but is a drawback doing stuff like napkin rings.
One big variable is the lathe tailstock quality. Affects bit tracking and so heat generation. Minimal play is best obviously. With experience of four brands of lathe, the Vicmarc tops the list and was used here.
After sharpening the bit I tried a second test on English Elm, a timber within the expected scope of HCS. The bit kept cutting chips down to the end at 160 mm. Clearance of chips through the flutes was efficient. RPM of 500. The bit got very warm but not hot and developed wear bevels just visible to my reading-glass assisted eyes. The self-locking couplers seemed to have seated somewhat and held more firmly this time around; getting them apart took a bit of effort.
Sharpening
It was about half an hour's work with diamond paddles to get the bit back in condition after the first test, working the chipping edge tops and putting some effort into the flutes. Starting with a medium grit paddle and then fine and super-fine. The nice feature about the bit is that the top of the chipping edge is co-planar with the dropped section of the rim so good registration of the paddle is easy to get. The flute on the other hand obviously involves a lot of steel, and it's not all that easy to keep a paddle flat so I ended up settling for an adequate rather than a good result there.
I haven't touched up the 'wings' (the raised part of the rim). Leonard Lee's sharpening book shows a jig for doing this to maintain the bevel angle. I'll have a shot at doing the bevel with a bit in a Dremel, inking the bevel first.
The included bevel angle (chipper top to flute) is about 50 degrees.
Summary
The cutting and chip ejection are better than garden variety saw tooth bits. Sharpening is easier. The quality of the HC steel is impressive.
For deep boring I favour an MT mount over a Jacobs chuck as there's less play. The Rotastop MT2 and the extension work well.
Heat is the enemy of Forstner bits and they have to be kept sharp. Leonard Lee recommends keeping them free of resin and applying dry lubricant before use.
I've put in an order for several Colt MaxiCut plus HSS Forstners. A 35mm bit will be about double the price of the same size unit from Carba-tec and that's quite a premium so I'm only ordering the sizes that will get regular use.
The geometry of the HSS bits differs from the HCS jobs so sharpening the chipping edge may not be as easy. They also have a pilot drill instead of a brad centre and that will be a disadvantage where a relatively flat bore bottom is needed.
Many thanks to Brett.