Thread: making small boring bars

What length is the bore?

Bore length would be about 20mm.

If the bore is a critical dimension you need to keep the bar as short as possable.

If you have some square or round material to use to make the bar would be good.

It would be best to use the largest size material that will fit in your tool post.

You would machine the boreing bar piece of your tool (the section going into the bore ) as close to 10mm as you can.This will also be limited by the size of your starting hole.

You only need enough clearance for you cutting tool,maybe .5mm to 1mm max.

The length of the boreing bar would be determined by the type of finished hole.

If its a through hole the bar may only have to be 21mm to 22mm long.

If its a blind hole you may have to make the diameter of the boring bar a little smaller say maybe 8mm to allow for swarf to pass back past the bar.

The length of the bar could also depend on how you mount your cutting tool,whether square to the bar or angled across the bar.

Ideally you want the bar as short as possable but the shank of the bar or the bit in the toolpost as big as possable to give ridgidity and hopefully reduce vibration.

If you dont have much to come out of the bore I would rough first at the normal cutting speed and depth of cut and then finish with the lathe RPM slow and fed rate slow and shallow depth of cut.

If you are going to do it in one pass then slow from the start.

Just a tip if you rough first and then measure I would advise to do the finish bore in 2 passes.

The reason being the measurement you get with the roughing cut will be just that a roughing cut finish size,but when you slow the machine down for the finish cut the slower feed rate will clean up all your high and low spots probably making your hole oversize.

Thanks, Pipe. Glad you made those points before I started.

What about the actual making of the boring bar? Assuming I have only basic equipment to use, how should one fabricate small boring bar(s)? My lathe is the basic 9 x 20 size.

If you have a few 10mm holes to do it might be worth buying a chucking reamer. something like these.
Standard Length Chucking Reamers - Metric Diameters Straight Shank - Straight Flutes - High Speed Steel

You'd get out of it for around $20.00.

bollie7

You have a couple of choices if you dont use the reamer as suggested.

You can as has been talked about in earlier threads attach your piece of HSS to your bar by welding,brazeing,silver sloder or by drilling a suitable hole and either file it square to suit the size of your HSS or if practical size wise use one of your broken centre drills,you will have to tap and drill a hole if you use the clamp method for your HSS for a grub screw so it can hold the HSS in place.

The holes your drill or make will have to take into account the size of the HSS,if you dont you may have trouble getting the tool on centre.

If the opening in your tool post was say 20mm I would try and go for a piece to suit of around 18mm but no less than 16,you still need the strength of that bar to help eliminate any flex and vibration.

The slightly swmaller size of the bar will allow you some adjustment for getting the HSS on centre.

You will have to do your calculations based on thecentre height of your lathe and the size of the HSS you are useing.

If you drill your hole in the centre of the 10mm bar the centre of your HSS will be on centre,you want the cutting edge to be on centre.

If possable use a piece of 1/8" HSS or if you use the centre drill sharpen the tool for boreing first then work your drill calculations out.

The hole will be offset probably a couple of mm leaveing not much as support under the cutting tool,no reason why this want do the job for you as long as you take cuts suited to the bar.

If you were to drill to 9mm ,I would take a roughing cut of .010" and 2 finish cuts of around .005" to size.

This is all based on what you desire at the end,it dosent take into consideration any other factors like material,your lathes speed and feed ranges,the type of cutting tool,the type of coolant if any or the way you sharpen your tool.

I turn my small boring bars out of silver steel of different diameters and harden them myself .
It fairly easy to do ,you just set up the round stock in the three jaw chuck and put a piece of 1/16th shim on one jaw this will give you an off set .Leave a piece unturned on the end and then grind the end to make your cutting edge.
Other option is to make a D bit out of a piece of 10mm silver steel ,harden it and drill the hole out undersize as close to 10mm as you can ,then run the d bit through and it will cut the hole to 10mm.

Kev.

I have a few bits of HSS steel lying around and I can make the boring bar. I need a way to join the HSS steel to the bar. I have an arc welder does it weld ok with standard equipment and welding sticks?

I will also need a smaller boring bar approx 8 mm for another project so probably make 2 if I can.

I do it using round pieces of HSS, grind a tip on the end shaped like a boring bar. Put a piece of square bar in the tool holder and a drill chuck in the headstock. Drill a hole into the tool holder held square bar the same diameter as the round hss. I'd use 5/16 " for 10mm holes. drill a hole perpendicular to the main hole for a grub screw. Bore away - a picture would be easier. If you want one I can go take a pic...

Your choice if you make 2 boreing bars or 1 to do your jobs.

If you have a few of these to do you would probably be better off useing a reamer,either M/T or chucking.

By the time you drill your hole,measure, take your cut or cuts,measure again you could have 2 or 3 done.

What are you useing to check your bore size?

G'day Ben, I think I can see what you're getting at but a picture would be good, particularly as to how you shape the cutting edge.

Pipe, I'll need to make a few boring bars, in the past I've shied away from making projects because I didn't have small enough boring bars particularly those that could do blind holes.

Tiger, pic below.

Shown is a holder for a 1/4" round HSS bit. On top of that is a 3/16 bit with a reasonable profile. It can be a little harder to grind accurate profiles on mall diameter bits but it'll always be better than just drilling a hole. The 3/16 bit was used to put 10mm diameter housings for very small stainless bearings in aluminium.

Attachment 136030

Hello Tiger,
I have made boring bars similar to Ben's but I drilled the hole in the tool holder across the end of the bar(90 deg. to the way Ben did it). If you only have square HSS just drill the hole large enough for the HSS to slide in and locate the HSS in position with a couple of grub screws or cap screws. The size of the hole is not important (within reason) as the screws will locate the HSS securely against the bottom of the hole. If you have a broken tap you could grind that to shape and use it instead of the HSS.
Regards,
Russell

Thanks Ben and Russ , The picture explains everything clearly. I've got some discarded drill bits that I can use.